Symposium K: Zinc Oxide and Related Materials

Chair

Juergen Christen
Inst fur Experimentelle Physik
Universitate Magdeburg
Otto-von-Guericke
Magdeburg, D-39106 Germany
49-391-67-18668 

Chennupati Jagadish
Research School of Physical Sciences
Australian National University
Canberra, Australia
61-2-6125-0363

David C. Look
Semiconductor Research Center
Wright State University
Dayton, OH 45435
937-255-1725

Takafumi Yao
Center for Interdisciplinary Research
Tohoku University
Sendai, 980-8578 Japan
81-22-215-4404

Symposium Support

  • Air Force Office of Scientific Research
  • U.S. Army Research Office

* Invited paper

SESSION K1: Devices

Chairs: David Look and Bruno Meyer
Monday Morning, November 27, 2006
Room 200 (Hynes)


8:30 AM *K1.1
Atomically Controlled Heteroepitaxy Realizing ZnO LED And 2-DEG. Masashi Kawasaki, Institute for Materials Research, Sendai, Japan.

We present pulsed laser epitaxy of heterostructures composed of ZnO and (MgZn)O. Two-dimensional growth is atomically controlled as verified by persisting RHEED oscillation and atomically flat film surfaces. Point defect density is controlled and optimized by monitoring the electron mobility, positron annihilation, and life-time of exciton emission of the resulting films. Bulk mobility over 400cm2/Vs at 300K and over 5000cm2/Vs at 100K are achieved. Conversion of ZnO into p-type can be made for such films upon appropriately doping of nitrogen. At the (MgZn)O/ZnO heterointerfaces, two dimensional electron gas can be formed and quantum Hall effect is readily observed. The key points of epitaxy control and their relations to the physical properties of the films will be presented. This work was supported by the Japanese Ministry of Education, Culture, Sports, Science and Technology in Japan, Grant-in-Aid for Creative Science Research (14GS0204).


9:00 AM K1.2
Acoustic Resonator Based on ZnO Belts. Brent A. Buchine, Georgia Institute of Technology, Georgia.
Abstract not available.


9:15 AM K1.3
Abstract Withdrawn

9:30 AM *K1.4
CdZnO/ZnO Heterostructures for Fabrication of Light Emitting DiodesAndrei Osinsky1, J. W Dong1, B. Hertog1, P. P Chow1, W. Schoenfeld2 and D. C Look3; 1SVT Associates, Eden Prairie, Minnesota; 2College of Optics and Photonics, University of Central Florida, Orlando, Florida; 3Semiconductor Research Center, Wright State University, Dayton, Ohio.

Some of the recent progress made in the development of high quality ZnCdO layers and CdZnO/ZnO heterostructures grown epitaxially by RF-plasma molecular beam epitaxy (MBE)[1,2]. Experimental results on p-type and n-type doping of ZnCdO alloys are discussed in the presentation. We also review electrical and optical characteristics of ZnO homostructures and CdZnO/MgZnO heterostructure for LED devices. Quantum efficiency is also considered. High quality single-crystal wurtzite CdxZn1-xO alloys with Cd mole fraction ranging from 0.02 to 0.78 are shown to have no phase separation in this concentration range. Strong optical emission for the fundamental interband transition in the 380 nm to 574 nm spectral range has been achieved at RT from CdxZn1-xO with various compositions. Dependence of the band gap on the composition in ternary alloys, band gap bowing for CdxZn1-xO have been identified. Crystallographic and optical properties of CdZnO/ZnO multiple quantum wells as well as their incorporation into the LED structures are presented. ZnO-based device simulations for type-II and type-I heterostructures, which incorporate strong piezoelectric and spontaneous polarization fields in ZnCdMgO and AlGaN-based materials and interfaces are discussed.


10:30 AM *K1.5
Development of Thin Film and Nanorod ZnO-Based LEDs and Sensors.Stephen Pearton1, W. T Lim1, J. S Wright1, R. Khanna1, L. Voss1, L. Stafford1, L. c Tien1, H. S Kim1, D. P Norton1, J. J Chen2, H. T Wang2, B. S Kang2, F. Ren2, J. Jun3 and Jenshan Lin3; 1MSE, University of Florida, Gainesville, Florida; 2Chem.Eng, University of Florida, Gainesville, Florida; 3ECE, University of Florida, Gainesville, Florida.

The development of new etching and contact metallurgies for n- and p-type ZnO and various approaches for realizing ZnO LEDs will be given. In addition, a comparison of the sensitivities for detecting hydrogen with Pt-coated single ZnO nanorods and thin films of various thicknesses (20-350 nm) will be discussed.The Pt-coated single nanorods show a current response approximately a factor of three larger at room temperature upon exposure to 500ppm H2 in N2 than the thin films of ZnO. The power consumption with both types of sensors can be very small (in the nW range) when using discontinuous coatings of Pt. Once the Pt coating becomes continuous, the current required to operate the sensors increases to the μW range. Both ZnO thin and nanorods cannot detect oxygen. The sensors can be integrated into a self-contained wireless hydrogen sensor system powered using ambient vibrations and light. A large number of distributed sensors are required to safely operate hydrogen production, storage, distribution, and fuel cell facilities for aviation, space, and terrestrial applications.


11:00 AM K1.6
Fabrication and Electrical Characteristics of Dual-gate ZnO Nanorod Metal-oxide Semiconductor Field-effect Transistors.Chul-Ho Lee1,2, Hyeong-Jin Kim1,2, Dong-Wook Kim3 and Gyu-Chul Yi1,2; 1National CRI Center for Semiconductor Nanorods, Pohang, South Korea; 2Department of Materials Science and Engineering, POSTECH, Pohang, South Korea; 3Department of Applied Physics, Hanyang University, Ansan, South Korea.

One-dimensional semiconductor nanorods are potentially ideal functional components for nanometer-scale electronics and optoelectronics, due to both well-controlled composition modulation and their high aspect ratio. Even though back-gate ZnO nanorod metal-oxide semiconductor field-effect transistors (MOSFETs) have been fabricated, such a nanodevice structure does not allow individual device operation necessary for many integrated circuit applications, and may show degradation of the devices due to the undesirable chemisorption of gases in ambient. Top-gate MOSFETs are highly desirable, as a nanodevice structure with local gates. Nevertheless, top-gate nanorod MOSFETs have rarely been studied and their effects of gate geometry on ZnO nanorod device characteristics have not been investigated. In this talk, we report fabrication of dual-gate ZnO nanorod MOSFETs with SiO2 films as both top- and bottom-gates and improvement in the device characteristics due to geometrically enhanced capacitance upon using a top-gate. Furthermore, fabrications and device characteristics of intentionally doped ZnO nanorod MOSFETs will be presented.


11:15 AM K1.7
Ultraviolet Photodetection Properties of ZnO mMcrotubes.Jiping Cheng, Ming Fu and Ruyan Guo; Pennsylvania State University, University Park, Pennsylvania.

Photodetectors based on wide-bandgap semiconductors are advantageous over traditional ultraviolet (UV) detectors (photomultiplier tubes and Si-based UV detectors) in terms of low power consumption, high stability, and no need of other optical filters. ZnO stands a good chance of being a candidate material for solar-blind UV detection because of its direct bandgap of 3.37eV and high photoresponse. In this work, UV photodetection properties of single crystal ZnO microtubes synthesized using a microwave-heating growth method are reported. The ZnO crystals were grown in hexagonal hollow tubular form with well facetted end and side surfaces, which have cross-sectional dimensions of 100 to 250 mm, lengths of 3-5 mm and wall thickness of 1-2 mm. The ZnO microtubes exhibited relatively fast photoresponse with a cut-off wavelength ~370 nm, which indicates their potential in applications as selective UV detectors with high sensitivity due to their very large surface areas.


11:30 AM *K1.8
Multilayer and Multifunctional ZnO DevicesYicheng Lu, Electrical and Computer Engineering, Rutgers University, Piscataway, New Jersey.

ZnO has a direct energy bandgap (Eg ~ 3.3 eV at room temperature). It can be alloyed with MgO to form the MgxZn1-xO, extending the direct energy band gap up to 4.0 eV. Through proper doping, ZnO can be made transparent and conductive, piezoelectric, or ferromagnetic. It is possible to use ZnO based multilayer and multifunctional structures to design and construct novel devices. We have grown high quality epitaxial ZnO and MgxZn1-xO films on r-plane sapphire substrates using MOCVD. The high speed MSM photoconductive type and Schottky type of UV photodetectors are demonstrated. The semiconducting/piezoelectric ZnO based multilayer structures have been used to design and build various new devices based on the interaction between surface acoustic wave (SAW) and charge carriers in the semiconductor. In a voltage tunable SAW phase shifter, piezoelectric and semiconducting ZnO layers are used for SAW excitation and electron conducting, and a SiO2 layer acts as the gate insulator. The device can be used for voltage controlled phase shifters and oscillators. In an integrated UV SAW detector, an optoelectronic-acoustic interaction is used. The device can be used as a zero-power UV wireless sensor. The selective MOCVD growth technology has been developed to fabricate single crystalline ZnO nanotips. Such ZnO nanostructures are grown on Si and glass substrates to form UV detectors.
 

 

SESSION K2: Nanostructures

Chairs: Chennupati Jagadish and Steve Pearton
Monday Afternoon, November 27, 2006
Room 200 (Hynes)
1:30 PM K2.1
Electroluminescence from an Sb-doped p-ZnO/n-Si p-type Schottky Light-emitting Diode.Faxian Xiu, Leelaprasanna J. Mandalapu, Zheng Yang and Jianlin Liu; Dept. of Electrical Engineering, University of California, Riverside, Riverside, California.

ZnO materials have received much attention due to its superior properties, such as the high exciton binding energy of 60 meV at room temperature, which may lead to a strong light emission in the UV region. Therefore, ZnO could be the potential material for the next generation solid-state lighting devices. In this presentation, we report a ZnO p-type Schottky light-emitting diode fabricated with an Sb-doped ZnO on top of a n-type Si (100). An Sb effusion cell was used to provide the Sb dopants in a plasma-assisted MBE. A good rectifying characteristic was observed with a turn-on voltage about 1.2 V at 10 K while the leakage current was close to 2×10-5A at -10 V. It is noted that the diode is a p-type Schottky diode since the rectifying characteristic comes from the forward bias on the n-Si side, instead of the p-ZnO side. The electroluminescence (EL) spectra were recorded at 10, 50, 100, and 150 K. Strong emissions appeared at about 570~630 nm for all the temperatures. Comparing with the low-temperature photoluminescence, it is found that the EL originated from the oxygen interstitials (Oi) deep level with an energy of about 2.28 eV from the conduction band. In addition, the EL intensity was observed to decrease with an increase of temperature. This is presumably due to the activation of non-radiative recombination centers at higher temperatures. Finally, the transport properties of electrons and holes were provided based on the low-temperature band diagram of this diode. This study suggests that Sb-doped p-type ZnO can be used for next-generation LED devices.


1:45 PM K2.2
Evaluation of Nanolithographic Methods for Spatial Control of the ZnO Nanowire Growth.Margit Zacharias, FWIM, FZ Rossendorf, Dresden, Germany.

World wide the synthesis of semiconductor nanowires has been studied intensively for a wide spectrum of materials. Such low-dimensional nanostructures are not only interesting for fundamental research due to their unique structural and physical properties relative to their bulk counterparts, but also offer a fascinating potential for future technological applications. A deeper understanding and sufficient control of the growth of nanowires are in the center of current research interest. Based on our recent review [1] various growth processes, especially the vapor-liquid-solid process offers an opportunity for the control of spatial positioning of nanowires. Strategies for position-controlled and nano-patterned growth of nanowire arrays are reviewed and demonstrated by selected examples based on ZnO nanowires as well as discussed in terms of larger scale realization and future prospects. [1] H.J. Fan, P. Werner, M. Zacharias, Semiconductor nanowires: from self organization to patterned growth, Small 2 (2006) 700, (invited review).


2:00 PM K2.3
Flame-made ZnO: From Quantum-dots to Nanorods.Sotiris E. Pratsinis, Particle Technology Laboratory, ETH Zurich, Zurich, Switzerland.

Recent major advances in understanding flame aerosol formation and growth allow now synthesis of sophisticated materials with controlled composition, size and morphology leading to exciting new products. Here combustion of liquid-fed Zn-containing solutions by the so-called flame spray pyrolysis (FSP) process, a single-step, continuous and scaleable process, is used to make nanostructured ZnO particles. That way stable quantum-dots of ZnO for UV filters can be made by co-precipitation with SiO2 without fractionation1. In addition selected ZnO phases can be made by combining FSP with in-situ treatment of the effluent particles2. Nanorods of ZnO with closely controlled aspect ratio from 5 to 10 can be made also that way up to 30 nm in length and up to 5 nm in diameter3. Indium and tin dopants selectively affect a specific ZnO crystal plane and are incorporated into its lattice. Nanorod formation is attributed to the higher valency and coordination of indium and tin dopants relative to zinc and the associated disruption of crystal growth within the Zn plane. In contrast, lithium with an equivalent ionic radius to these dopants but lower valency than zinc, has no effect on the ZnO texture. The formation of nanorods within the flame seems to occur by annealing crystallization during flame cooling. 1. L. Mädler, W. Stark, S.E. Pratsinis, ”Rapid Synthesis of Stable ZnO Quantum Dots”, J. Appl. Phys., 92, 6537-40 (2002). 2. T. Tani, L. Mädler, S.E. Pratsinis “Synthesis of a-Willemite Nanoparticles by Post-calcination of Flame-made ZnO/SiO2 Composites”, Part. Part. Syst. Charact. 19, 354-358 (2002). 3. M.J. Height, L. Mädler, S.E. Pratsinis, “Nanorods of ZnO made by Flame Spray Pyrolysis”, Chem. Mater., 18, 572-578 (2006).


2:15 PM K2.4
Control of ZnO Nanorod Shape, Morphology, and Composition in Highly Oriented Arrays via Aqueous Solution Synthesis.Yun-Ju Lee, Erica Fang, Dana C. Olson, David Scrymgeour, Darren R. Dunphy, Nelson S. Bell, James A. Voigt and Julia W. P. Hsu; Sandia National Laboratories, Albuquerque, New Mexico.

Highly oriented, close packed nanorod arrays have been synthesized via a two step seeding and aqueous solution growth process, which promises to produce inexpensive, large-area ordered nanostructures for sensing and photovoltaic applications. Previously, we demonstrated that the degree of orientation and uniformity of nanorod length and diameter are controlled by the ambient humidity during the seeding step. Here, we report several methods to further modify the nanostructure of oriented ZnO nanorod arrays during the seeding and growth process. For example, we show that the shape and growth rate of ZnO nanorods are varied by adding different zinc chelating agents during the growth step. The spacing between ZnO nanorods is adjusted by changing the seed anneal temperature and by incorporating a poly(styrene-block-vinylpyridine) copolymer in the seeding process. Finally, by applying a multistep growth process to a non-close packed ZnO nanorod array, we find that complex, hierarchical ZnO nanostructures can be generated. The morphologies of different ZnO nanorod arrays are compared using SEM, XRD, PL spectroscopy, and surface area measurement with a surface acoustic wave resonator. In addition, synthesis of ZnMgO, ZnAlO, and ZnGaO nanorods will be explored.


3:30 PM *K2.5
MBE Grown ZnMgO-ZnO Quantum Wells Embedded into ZnO Nanopillars. Andreas Waag and Augustin Che-Mofor, Institute of Semiconductor Technology, University of Braunschweig, Braunschweig, Germany.

ZnO based semiconductor heterostructures are potentially interesting for the fields of optoelectronics, transparent electronics, and magnetoelectronics. In addition, the tendency towards self-organisation can be used to efficiently fabricate self-organised nanostructures with a high degree of c-axis orientation. Due to the small footprint of ZnO nanopillars on the substrate, the nanopillar quality can be very high, independent of substrate used. The goal of our work related with ZnO nanopillars is to gain control on the nanopillar characteristics, and still leaving the self-organisation mechanism in operation. This relates to control on size, position, composition, and doping. We report on the fabrication of ZnO nanopillars, using various growth techniques, and leading to well aligned, c-axis oriented nanopillar systems, with typical diameters between 50 nm and 500 nm. ZnMgO-ZnO quantum well structures have been embedded into these nanopillars, with quantum well widths between 1 nm and 4 nm. For that, an optimised MBE based process using RHEED oscillation techniques has been used. The confinement is increased as a function of decreasing quantum well width, as can be seen by a corresponding blue shift of the photoluminesence peaks. The nanopillar quantum wells show bright PL, even at room temperature. Detailed TEM investigations indicate that there are no extended defects in the nanopillar, as expected for such high aspect ratios, since the footprint is small. The correlation between structural quality and photoluminescence of both thin film quantum wells and nanopillar quantum wells will be discussed. Such quantum well nanopillars as reported here can eventually be the basis for future nano-LED structures, which - due to their low defect density - should have a high efficiency.


4:00 PM K2.6
Low-temperature (~270°C) Growth of Vertically Aligned ZnO Nanorods Using Photo-assisted Metal Organic Vapor Phase Epitaxy.Kokoro Kitamura1, Takashi Yatsui2, Motoichi Ohtsu1,2, Tohru Nakamata1, Jungshik Lim3 and Gyu-Chul Yi4; 1Graduate School of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo, Japan; 2Solution-Oriented Research for Science and Technology, Japan Science and Technology Agency, Machida, Tokyo, Japan; 3Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Yokohama, Japan; 4Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang, South Korea.

ZnO nanocrystallites are a promising material for realizing nanophotonic devices at room temperature, owing to their large exciton binding energy and large oscillator strength. Furthermore, ZnO/ZnMgO nanorod heterostructures have been fabricated and the quantum confinement effect from single-quantum-well structures (SQWs) was observed. Nanophotonic device applications using ZnO quantum-well structures require a high Mg concentration in the ZnMgO layers, which results in strong localization of the photons in the quantum-well layers. To realize this, a lower growth temperature is required to avoid the interdiffusion of Mg. To obtain high-quality crystallinity at a low growth temperature, we used photo-induced metal organic vapor phase epitaxy (MOVPE). ZnO nanocrystallites were grown on Si(100) substrates using a low-pressure MOVPE system without using any metal catalysts. Diethylzinc (DEZn) and oxygen were used as the reactants and argon was used as the carrier gas. To decrease the growth temperature, we used a He-Cd laser (λ= 325 nm, 300 µW) as the light source during MOVPE growth, yielding reactive Zn and O radicals via photolysis. To examine the temperature dependence of the morphology of the deposited film, we realized nanoparticle growth inside the laser spot only, even at room temperature. Furthermore, when the growth temperature was increased to 270°C, vertically aligned nanorod structures measuring 1 µm in length and 200 nm in diameter were obtained under He-Cd laser irradiation. Although the photoluminescence (PL) spectra (measured at 5K) of the nanoparticles grown at room temperature did not show any peaks, those of the nanorods grown at 270°C resulted in strong PL emission at 3.37eV, corresponding to the emission from a neutral-donor bound exciton in ZnO. The emission peak energy of room temperature PL (3.29 eV) agreed with the reported value (3.26 eV), which corresponds to the spontaneous emission from the free exciton in high-quality ZnO nanocrystallites. The full width at half maximum of the PL spectra was about 105 meV, which is comparable with the 100 meV of high-quality ZnO nanorods grown using MOVPE at 400°C. In addition, high-resolution transmission electron microscopic images of the nanorod structure grown at 270°C revealed hexagonally packed single-crystalline growth with (0001) direction. These results imply that the ZnO nanorods grown using photo-induced MOVPE at low temperature (270°C) were composed of single-crystalline ZnO nanocrystallites. Decreasing the growth temperature using photo-assisted MOVPE should realize a higher Mg concentration in the ZnMgO layers.


4:15 PM K2.7
Carrier Dynamics of Surface-Related States in ZnO Nanorods.Tobias Voss1, Lars Wischmeier1, Ilja Rueckmann1, Juergen Gutowski1, Sandra Boerner2, Wolfgang Schade2, Augustin Che Mofor3, Andrey Bakin3 and Andreas Waag3; 1Institute of Solid State Physics, University of Bremen, Bremen, Germany; 2Institute of Physics and Physical Technologies, Clausthal University of Technology, Clausthal-Zellerfeld, Germany; 3Institute of Semiconductor Technology, Braunschweig University of Technology, Braunschweig, Germany.

ZnO nanorods and nanowires have recently attracted broad interest because of their fascinating properties as potential nanoscale optoelectronic devices. Although nanorods are mainly characterized by the well-known properties of ZnO bulk material the influence of the surface region should become more important the more the diameter of the rod is reduced. It is therefore mandatory to analyze the influence of the increased surface-to-volume ratio of nanorods compared to bulk crystals to fully understand the microscopic processes which eventually govern the characteristics of potential nanorod-based devices. We have performed systematic time-integrated and time-resolved micro-photolumincescence (PL) investigations of ZnO nanorod samples grown by vapour-phase epitaxy, and on single nanorods grown by a vapour-liquid-solid mechanism. The diameters of the single rods have been independently determined with a scanning electron microscope. We find a strong correlation of a PL peak observed in the near-band-edge region at an emission energy of about 3.367eV with the diameter of the analyzed nanorods, i.e., this peak becomes more pronounced for rods with decreasing diameter. As a function of excitation density a saturation behaviour of this peak is observed already at moderate densities. The time-decay of the corresponding PL emission shows a behaviour being approximately biexponential in contrast to the monoexponential decay of donor-bound excitons. All these observations taken together lead to the conclusion that the observed peak at 3.367eV has to be attributed to surface related states in the nanorods. We present a systematic study of the corresponding PL dynamics as a function of temperature, detection energy and excitation density. The experimental observations are explained by a phenomenological rate-equation model which takes into account a limited number of surface-related states. The results will give first insight into the nature of the surface-related PL emission which in both VLS and VPE nanorods with diameters <200nm strongly contributes to the near-band-edge PL emission at low temperatures.


4:30 PM K2.8
Low Temperature Growth of ZnO Nanorods in AAO on Si Substrate by Atomic Layer Deposition.Ching-Jung Yang1, Chih Chen1, Shun-Min Wang1, Shih-Wei Liang1, Yung-Huang Chang1 and Jia-Min Shieh2; 1Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan; 2National Nano Device Laboratories, Hsinchu, Taiwan.

We used AAO template as a template, and atomic layer deposition (ALD) to deposit ZnO nanorod arrays at 250oC on silicon substrates. With this approach, we have achieved a breakthrough to develop highly ordered ZnO nanorod arrays at 250 oC without any metal catalysts, or seed layers. The dimension of the nanorods is 400 nm long, 70 nm in diameter, and 90 nm in pitch. The PL spectrums of ZnO nanorod arrays and ZnO films located in blue and red curves respectively. For the ZnO nanorod arrays, an dense and acute distribution of UV emission accumulated at around 379 nm, and around 480nm wavelength. From the PL spectra, we can conclude that ZnO nanorod arrays have extremely low defects in the ALD process. ZnO nanorod arrays have superior quality themselves and needn’t the annealing treatment. In addition, we also observed that the ratio of the UV to blue emission of ZnO nanorod arrays is relatively lager than that of ZnO films. Nanostructure arrays are favorable to the performance of intrinsic signal of ZnO structures, and ALD process are capable of depositing superior quality of ZnO nanostructure. In addition, after AAO template was removed, the nanorod arrays exhibited excellent field emission properties with a turn-on electric field (Eto) of 4.85 Vμm-1 and a threshold electric field (Ethr) of 7.2 Vμm-1. Here, we defined the turn-on field (Eto) and the threshold field (Ethr) as the electronic fields that were required to produce a current density of 10 μAcm-2 and 10 mAcm-2, respectively. This is a promising approach to fabricate large-scale well-aligned ZnO nanorods for many important applications in the field of nanotechnology.


4:45 PM K2.9
In-situ Morphology Controlled Synthesis of ZnO Hetero-Nanostructures.Xudong Wang, Jinhui Song and Zhong Lin Wang; Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia.

ZnO is one of the most important functional materials and has been widely used as optoelectronics, piezoelectric devices and sensors. Due to its hexagonal Wurtzite structure and polar crystal surfaces, ZnO exhibits a large family of nanostructures. Those structures not only provided valuable models in understanding crystal growth mechanisms in nanometer scale, but also exhibited high potential for fabricating novel nano electronic and optical devices with enhanced performance. Different morphologies were generally induced by different vapor concentration. Thus, in-situ morphology switching can be realized by modifying the vapor concentration inside the growth chamber during the growth. By quickly cooling down the furnace, we have successfully switched the growth from hexagonal disks to nanorods/nanobelts, resulting in the formation of a “nano-castle”, a new structural configuration for ZnO. The morphology is basically composed of two parts: quasi-two-dimensional (2D) hexagonal crystal base, and quasi-one-dimensional (1D) nanostructures surrounding the base as the “walls” of the nanocastle. The two different structures appeared to grow consecutively and formed a single-crystal structure. Realizing the morphology controlling mechanism, the 1D growth of nanostructure has also been successfully restrained when the 2D growth was preferred. As a result, a large-area single-crystal ZnO mesoporous film was achieved without any unwanted nanowires extended from the surface. These new structures synthesized by in-situ morphology control could enrich our knowledge about Wurtzite crystal growth. They also present new structural configurations with potential applications. The in-situ morphology switching mechanism presented here would be a powerful technique in fabricating high quality heterostructures or interconnects in nanometer scale. <br>[1] for details: http://www.nanoscience.gatech.edu/zlwang/

SESSION K3: Defects

Chairs: Jürgen Christen and Masashi Kawasaki
Tuesday Morning, November 28, 2006
Room 200 (Hynes)
8:30 AM *K3.1
First-Principles Defect Theory for Zinc Oxide.Shengbai Zhang, National Renewable Energy Laboratory, Golden, Colorado.

In some sense, defect study is at the heart of ZnO research for the utilization of its vast potential in electronic and optoelectronic applications. Over the years, we have carried out systematic first-principles calculations on the various defect properties of ZnO ranging from native point defects [1], to the enhancement of nitrogen incorporation [2], to the behavior of elemental p-type dopants [3], to the theory of large lattice-mismatched impurities [4], and most recently to the formation of complexes for unintentional free electrons [5]. We now know that oxygen vacancy (VO) is an important deep-level defect, possibly giving rise to the green band and persistent photoconductivity. Zinc interstitial (Zni) is, on the other hand, an important shallow donor. Zinc oxide is a typical large gap semiconductor with doping asymmetry, and is hence difficult to dope p-type. To remove this obstacle, non-equilibrium doping is required. We predicted that by using metastable molecular doping sources such as NO or NO2 and a control of growth kinetics, one could significantly increase the solubility of substitutional NO in ZnO, as having been confirmed by several later experiments. We have systematically studied the various potential p-type dopants. While substitutional group-I dopants produce relatively shallow acceptors, to be successful one needs to overcome their intrinsic tendency for Jahn-Teller distortion and to form interstitial donors. We show that most of the perceived group-V acceptors (P, As, and Sb) have very low solubility and very high ionization energy. It is therefore a real puzzle why recent experiments have succeeded in using these dopants for p-type ZnO. To solve the puzzle, an antisite model was proposed, in which the dopant substitutes Zn, instead of O, and acts as a gettering center for zinc divacancies (2VZn). It is this complex that is responsible for the observed p-type conductivity. We will analyze the recent XANES study on arsenic-doped p-type ZnO, as it questions the validity of the AsZn-2VZn model. We will show that in fact their raw data support the model. Finally, we show that in ionic crystals such as ZnO the complexing of native defects with impurities could be a more general phenomenon. For example, residual NO can bind the otherwise mobile Zni during post-growth cooling, causing intrinsic n-type behavior of ZnO in good agreement with recent experiment. Work supported by the U.S. DOE/BES and EERE under contract No. DE-AC36-99GO10337. [1] S. B. Zhang, S.-H. Wei, and A. Zunger, Phys. Rev. B 63, 075205 (2001). [2] Y. Yan, S. B. Zhang, and S. T. Pantelides, Phys. Rev. Lett. 86, 5723 (2001). [3] C. H. Park, S. B. Zhang, and S.-H. Wei, Phys. Rev. B 66, 073202 (2002). [4] S. Limpijumnong, S. B. Zhang, S.-H. Wei, and C. H. Park, Phys. Rev. Lett. 92, 155504 (2004). [5] D. C. Look, G. C. Farlow, P. Reunchan, S. Limpijumnong, S. B. Zhang, and K. Nordlund, Phys. Rev. Lett. 95, 225502(2005).


9:00 AM K3.2
Electrical Characterization of Deep Acceptor States in ZnO.Holger von Wenckstern1, Heidemarie Schmidt1, Rainer Pickenhain1, Gisela Biehne1, Matthias Brandt1, Michael Lorenz1, Gerhard Brauer2, Armin Dadgar3, Alois Krost3 and Marius Grundmann1; 1Institut für Experimentelle Physik II, Semiconductor Physics, Universtät Leipzig, Leipzig, Germany; 2Institut für Ionenstrahlphysik und Materialforschung, Forschungszentrum Rossendorf, Dresden, Germany; 3Institut für Experimentelle Physik, Otto-von-Guericke-Universität Magdeburg, Magdeburg, Germany.

The p-doping of ZnO is the great challenge in order to make this multi-functional material useful for optoelectronic applications. Several deep electron traps in ZnO have already been characterized electrically. Up to today the information about deep acceptor states in ZnO is scanty. Only the nitrogen acceptor is agreed to have a binding energy of about 135 meV. We report on acceptor-like states in ZnO investigated by deep level transient spectroscopy (DLTS). One of the test structures is a ZnO p+n-homojunction realized by ion implantation. Scanning capacitance microscopy measurements on the implanted side of the sample revealed p-type conduction. Bias-dependent measurements revealed a behavior typical for MOS-structures on a p-type semiconductor. DLTS measurements confirmed the existence of the electron traps E3 and E4. Pulsing into the forward regime of the diode, we inject holes and can probe deep acceptor states in the n-type region of the diode. We were for the first time able to characterize deep acceptor states labeled A2 and A3 by DLTS [1]. The thermal activation energies of A2 and A3 are estimated to be about 150 meV and 280 meV, respectively. The states A2 and A3 have tentatively been ascribed to intrinsic defects and the Li acceptor on Zn lattice site, respectively. The level A2 may correspond to the thermal activation energy of a deep acceptor state created by proton implantation of ZnO determined by optical deep level transient spectroscopy to be about 160 meV in Ref. [2]. Our results are compared to p-GaN / n-ZnO heterostructures also enabling the injection of holes into ZnO. [1] H. von Wenckstern, M. Grundmann, G. Brauer et al., APL in press.

9:15 AM K3.3
Defect physics in ZnOAnderson Janotti and Chris G. Van de Walle; Materials Department, University of California - Santa Barbara, Santa Barbara, California.

As with any semiconductor material, understanding the role of impurities and native defects is essential for controlling the electronic and optical properties of ZnO. We have performed a comprehensive investigation of native point defects in ZnO using first-principles methods based on density functional theory. Excess zinc, manifested in the form of oxygen vacancies and zinc interstitials, has long been invoked as the source of the commonly observed unintentional n-type conductivity in ZnO. However, contrary to the conventional wisdom, we find that native point defects are very unlikely to be the cause of unintentional n-type conductivity [1]. Oxygen vacancies, which have most often been invoked as shallow donors, have high formation energies in n-type ZnO, and are deep rather than shallow donors [2]. Zinc interstitials also have high formation energies in n-type ZnO, and are fast diffusers with migration barriers as low as 0.57 eV; hence zinc interstitials are unlikely to be stable. Zinc antisites are also shallow donors, but have even higher formation energies. Zinc vacancies are the deep acceptors possibly related to the green luminescence, and act as compensating centers in n-type ZnO. Oxygen interstitials have high formation energies. They can occur in the form of electrically neutral split interstitials, but also as a double acceptor at octahedral interstitial sites in n-type ZnO. Oxygen antisites have very high formation energies and are unlikely to exist in measurable concentrations under equilibrium conditions. Our calculated migration barriers are in good agreement with experimental data where available and may provide a guide to more refined experimental studies of point defects in ZnO. We also explore possible sources of the unintentional n-type conductivity and the consequences of point-defect behavior for the control of p-type doping. [1] A. Janotti and C.G. Van de Walle. J. Cryst. Growth 287, 58 (2006) [2] A. Janotti and C.G. Van de Walle, Appl. Phys. Lett. 87, 122102 (2005).


9:30 AM K3.4
Persistent Photoinduced Changes in Charge States of Donors and Acceptors in Hydrothermally Grown ZnO.Nancy C. Giles, Yongquan Jiang and Larry E. Halliburton; Physics Department, West Virginia University, Morgantown, West Virginia.

Zinc oxide (ZnO) bulk single crystals grown by the hydrothermal technique are being developed for use as substrates for thin film growth. In this work, a bulk single crystal obtained from Tokyo Denpa (Japan) was investigated using electron paramagnetic resonance (EPR), photoluminescence (PL), and infrared absorption (FTIR) techniques. The sample is low n-type with a room-temperature Hall carrier concentration of about 2 x 1014 cm-3. EPR spectra from Mn, Co, Ni, Fe, shallow Group III donors (Al, Ga), and substitutional lithium acceptors were easily observed. PL revealed the presence of trace amounts of copper impurities (the characteristic spectrum associated with Cu2+ ions was present). Lithium, which is present during the hydrothermal growth process as a mineralizer, was the only major acceptor impurity detected in this material, and substitutes for zinc on the cation site. Lithium can be present in ZnO as an isolated substitutional singly ionized acceptor in n-type material. Also, as shown using the FTIR data, a significant portion of the lithium impurities are compensated by a nearby hydrogen in the form of a neutral Li-OH complex [1]. Photoinduced changes in the charge states of the different deep and shallow centers were monitored using 325 nm light from a He-Cd laser. Before illumination, all the lithium acceptors are compensated (i.e., are non-paramagnetic). After illumination, the neutral Li acceptor is detected using EPR, and the electrons have been redistributed among the various deep and shallow donor levels. All the trivalent nickel and iron centers initially present are converted to their divalent forms, and increases in divalent Co and Mn are observed. When the laser light is shuttered, the changes in charge states are persistent and stable at liquid helium temperatures. The thermal stability of the various centers is determined by monitoring the recovery of their EPR signals to the pre-illumination state. The EPR signals are monitored at a fixed temperature (which was optimum for each defect spectrum), as a function of sequential steps of heating to progressively higher temperatures and then cooling back to the measurement temperature. The thermal release of charge from the deep transition-metal donors occurs in the 100 to 150 K range. This work was supported by the National Science Foundation under Grant DMR-0508140. [1]. L. E. Halliburton et al., J. Appl. Phys. 96, 7168 (2004).


9:45 AM K3.5
Diffusion of Intrinsic Point Defects in Zinc Oxide.Paul Erhart and Karsten Albe; Institut für Materialwissenschaft, Technische Universität Darmstadt, Darmstadt, Germany.

For the fabrication of zinc oxide devices the mobilities of point defects are of crucial importance. Unfortunately, experiments have produced very scattered data for the self-diffusion coefficients in ZnO and did not succeed in resolving the mechanisms by which intrinsic defects migrate. In order to obtain a more profound understanding of self-diffusion density-functional theory calculations were carried out using the generalized gradients approximation with semi-empirical self-interaction corrections (GGA+U). The migration barriers for vacancy and interstitial motion for oxygen as well as zinc were obtained. The lowest barriers (0.2-0.3 eV) are found for zinc interstitials moving via a second neighbor interstitialcy mechanism. They become mobile at temperatures between 80 and 130 K, which conincides with the range of experimentally determined onset temperatures for annealing due to intrinsic defects. The migration enthalpies for oxygen interstitials are strongly dependent on the charge state ranging between 0.3 and 1.0 eV. The barriers for vacancy migration on the other hand are only weakly dependent on the charge state and somewhat higher. The influence of Fermi level and chemical environment on the self-diffusion coefficients is discussed. Depending on the conditions both vacancy and interstitialcy mechanisms can be operational. For conditions, however, under which oxygen diffusion experiments are carried out, interstitial migration dominates.


10:30 AM *K3.6
ZnO Near-Interface Defects and Control of Schottky Barriers.Leonard J. Brillson1,2, H. Lee Mosbacker2, Michael J. Hetzer2, Yuri Strzhemechny3, David C. Look4, Stephen A. Ringel1,2, Maria Gonzalez1, Gene Cantwell5, Jizhi Zhang5 and Jin Joo Song5,6; 1Electrical & Computer Engineering, The Ohio State University, Columbus, Ohio; 2Department of Physics, The Ohio State University, Columbus, Ohio; 3Department of Physics & Astronomy, Texas Christian University, Fort Worth, Texas; 4Semiconductor Research Center, Wright State University, Dayton, Ohio; 5ZN Technology, Inc., La Brea, California; 6Department of Electrical & Computer Engineeing, University of California, San Diego, California.

As ZnO becomes a leading candidate for next generation semiconductor electronics, the ability to control its electrical contacts with metals becomes critical. Until now, there have been no ZnO Schottky barrier studies that isolate the effects of surface contamination, lattice defects, impurity dopants, and interface chemical reactions. We have used low energy depth-resolved cathodoluminescence spectroscopy (DRCLS) at 10 K in a UHV scanning electron microscope and macroscopic current-voltage (I-V) measurements to study Schottky barrier (SB) formation at metal interfaces to clean, ordered ZnO(000-1). We fabricated sets of 30 nm-thick, 0.4 mm diameter Au, Al, Ni, Pt, Pd, Mo, Ta and Ir diodes on the same single crystal surfaces from different growers. Prior to metallization, DRCLS revealed orders-of-magnitude difference in native bulk defect densities for crystals grown by different techniques, and these defect densities varied substantially between the crystals’ bulk and surface. For all crystals, surfaces treated with a remote oxygen (20% O2/80% He) plasma created clean, ordered surfaces and reduced defect emissions in the surface region.[1] Micro-DRCLS taken through the metal diodes revealed defect transitions at 2.1, 2.5, and 3.0 eV that change dramatically with process steps and metal. These transitions are associated with native point defects predicted theoretically [2] and identified experimentally via positron annihilation spectroscopy.[3] Deep-level optical and transient spectroscopies found corresponding bulk defects at EC-2.54 eV and EC-0.53 eV. DRCLS from bulk and shallower depths reveal that these densities can increase by > 100x at the surface. Near-interface defect accumulation depends on the ZnO-metal chemical interaction and crystal quality. Plasma cleaning strongly affects which interface defects form. Metals on low defect ZnO more than double near-interface densities, while the same metals on high defect ZnO increase densities by 10-100x - indicating both defect creation and diffusion of existing defects. These metal-induced defects impact device performance. I-V characteristics changed from Ohmic to Schottky and idealities decreased for Pt, Au, Ir, and Pd with plasma treatment. SBs increased and idealities lowered as near-surface defect densities decreased by orders of magnitude with a combination of higher crystal quality and remote O2 plasma treatment. The magnitude of the metal’s influence on Schottky barriers measured electrically correlates directly to the metal reactivity and the DRCLS- measured defect densities at the surface and ZnO bulk. Similar gap state-limited SBs for other compound semiconductors indicate that the impact of near-interface native defects on ZnO Schottky barriers is more general in nature. [1] H.L. Mosbacker et al., Appl. Phys. Lett. 87, 012102 (2005). [2] A. Janotti and C.G.Van de Walle, Appl. Phys. Lett. 87, 122102 (2005). [3] F. Tuomisto et al., Phys. Rev. B 72, 085206 (2005).


11:00 AM K3.7
Field-Emitter SEM based Cathodoluminescence Characterization of ZnO Nanostructures and Films Martin Schirra, Anton Reiser, Raoul Schneider, Günther Michael Prinz, Martin Feneberg, Tobias Röder, Klaus Thonke and Rolf Sauer; Institut für Halbleiterphysik, Universität Ulm, Ulm, Germany.

The wide-gap semiconductor ZnO is a promising candidate for a new generation of optical and electronic devices including small structures down to the nanometer scale. Prior to successful application of this material system adequate preparation of both bulk material and nanostructures is mandatory requiring also a detailed characterization of such structures. Using an optimized field-emitter type electron microscope (SEM), cathodoluminescence measurements with very high spatial and spectral resolution were performed on ZnO films and ZnO nanostructures to assess material quality, doping, and strain on a scale of a few tens of nanometers. ZnO nanopillars on different substrates were investigated exhibiting intrinsic and extrinsic luminescent features. Pillars on a-plane sapphire show increasingly dominant near-band edge luminescence when cathodoluminescence spectra are mapped from the bottom close to the substrate up to the top of the pillars indicating optimum crystal quality in these top regions. There are indications that aluminum from the sapphire substrate is incorporated in the lower parts of the pillars. Also single ZnO pillars grown homoepitaxially on ZnO films deposited before pillar growth on a GaN substrate were studied. These pillars show very high luminescence intensities at narrow linewidths in the region of donor bound excitonic recombination, demonstrating very high material quality. For ZnO films grown on various substrates, the influence of the substrate material was investigated by performing line scans on cleaved samples from the ZnO/substrate interface to the ZnO film surface. In addition, monochromatic CL images were recorded from ZnO films and the results correlated with the surface morphology and strain.


11:15 AM K3.8
Electrical Characterization of Proton Irradiated n-Type ZnO.Danie Auret1, Michael Hayes1, Jackie M Nel1, Walter E. Meyer1, Werner Wesch2 and Elke Wendler2; 1Physics, University of Pretoria, Pretoria, Gauteng, South Africa; 2Institut für Festkörperphysik, Friedrich-Schiller-Universität, Jena, Germany.

Due to its direct wide bandgap of 3.37eV, Zinc oxide (ZnO) has become the focus of many studies. Devices such as detectors, lasers and diodes operating in the ultra-violet (UV) and blue regions of the spectrum have been reported. Furthermore, the large band gap of ZnO renders it suitable as window or buffer layers in the fabrication of solar cells and as a substrate or buffer layer for the group III - nitride based devices. Further practical advantages of ZnO include bulk-growth capability, amenability to conventional wet chemistry etching, which is compatible with Si technology and convenient cleavage planes. It has previously been reported by us that the defect introduction rate in ZnO by MeV protons at room temperature is almost two orders of magnitude lower than that in GaN. To investigate if this is due to a material property of the ZnO, or perhaps a case of radiation enhanced annealing, low temperature proton irradiation were performed in this study. ZnO Schottky barrier diodes (SBD’s) were fabricated with 20/80/40/80 nm Ti/Al/Pt/Au ohmic contacts on the O-face and circular 0.5 mm in diameter 500 nm thick Ru Schottky contacts in the Zn face. The carrier concentration of the ZnO prior to irradiation was approximately 5x1016 cm-3. These diodes were irradiated with 1.8 MeV protons at a temperature of -140 °C with fluences ranging from 5x1012 cm-2 to 5x1013 cm-2. Current (I) deep level transient spectroscopy (I-DLTS) was used to study the shallow level defects introduced during the proton-irradiation of the ZnO SBD’s. These measurements revealed that this implantation introduced a defect with an energy level at 0.04 eV below the conduction band and with an electron capture cross section of about 10-15 cm2. There are at least two possible origins of this defect. Firstly, it may be that it is hydrogen-related and is the consequence of the hydrogen implantation performed for this study. Theory has predicted that hydrogen forms a shallow donor in ZnO. Secondly, it was reported that electron irradiation with an energy of >1.6 MeV produces the ZnI in ZnO which is a shallow level defect at 0.03 eV below the conduction band. The defect we measure here may therefore also be the ZnI.


11:30 AM K3.9
Optical Quenching of Hydrogen Shallow Donors in Zinc Oxide.Norbert H. Nickel, Hahn-Meitner-Institut Berlin, Berlin, Germany.

Research on zinc oxide (ZnO) is driven by a strong desire for blue and ultraviolet light emitting devices. So far, however, the major shortcomming is the lack of reliable p-type doping. Interestingly, the unique properties of hydrogen have been identified as a significant problem that interferes with effective p-type doping. Hydrogen forms a complex with oxygen that gives rise to a local vibrational mode at 3326.4 cm-1. In this paper we will show that the intensity of the O-H vibrational modes is very sensitive to sub bandgap light. For this purpose single crystal ZnO samples with a c-axis orientation were hydrogenated by the following procedure. The samples were sealed into quartz ampoules with 800 mbar of hydrogen gas. Then the ampoules were put in a furnace and annealed for 2.5 hours at 830 °C. Subsequently, the ampoules were rapidly quenched to room temperature by immersion in water. The hydrogenated ZnO samples were characterized with Fourier transform infrared spectroscopy. All measurements and illumination sequences were performed at a temperature of 5 K. Upon illumination the intensity of the O-H vibrational line decreases significantly. This effect is completely reversible and annealing at temperatures above 30 K restores the O-H vibrational mode to its initial intensity. Isochronal annealing experiments indicate the presence of a second shallow donor state with larger ionization energy than that of the hydrogen donor state. The observed changes of the O-H vibrational mode will be discussed in terms of changes of the effective charge of the O-H complex due to capture and release of electrons.


11:45 AM K3.10
Defect Characterization of Zinc Oxide Bulk CrystalsGovindhan Dhanaraj1, Balaji Raghothamachar1, Michael Dudley1, Michael Callahan2, Buguo Wang2 and David Bliss2; 1Dept. of Materials Science, Stony Brook University, Stony Brook, New York; 2Sensors Directorate, Air Force Research Labs., Hanscom AFB, Massachusetts.

Zinc oxide (ZnO) is a wide band-gap semiconductor with an energy gap of 3.37eV. It has a potential for applications as emitter devices in the blue to ultraviolet region and as a substrate material for GaN based devices. It also has the highest shear modulus and stable lattice because of the very small inter-atomic distance among the II-VI semiconductors. Its large exciton binding energy is useful for efficient UV laser applications. In our research effort, ZnO crystals were grown in autoclaves made of high strength steel, with a sealed platinum liner. The mineralizer solution was a mixture of Li2CO3, KOH, and NaOH, with the fill quantity at 80%. During growth, the nutrient zone was at 355°C with a temperature gradient of 10°C. These ZnO crystals obtained from many growth runs were characterized using synchrotron x-ray topography to find the suitability of these crystals as substrates for epitaxy. ZnO crystal plates of (10-10) orientation, cut perpendicular to the seed, were prepared for X-ray topographic characterization. These crystal plates contained the seed crystal and the bulk grown from the seed to show the growth history. Also, c-cut plates were used for the defect evaluation. X-ray topographs were recorded using synchrotron white beam radiation. The systematic study revealed the presence dislocations penetrating into the bulk from the seed, dislocation refraction, growth sector boundaries, inclusions, planar inclusion defects etc. Process induced dislocations such as dislocation loops around the seed suspension as well as slip bands could be observed. The role of edge dislocations in the growth process could also be seen. The defect structures were compared with the structural quality of the ZnO crystals grown from other techniques. Details of the synchrotron X-ray topographic defect characterization will be presented.

SESSION K4: Spintronics and magnetism

Chairs: Alex Hoffmann and Andreas Waag
Tuesday Afternoon, November 28, 2006
Room 200 (Hynes)
1:30 PM *K4.1
Spin-Exchange Interaction in ZnO-based Quantum Wells.Bernard Gil1, P. Lefebvre1, T. Bretagnon1, T. Guillet1, T. Taliercio1 and C. Morhain2; 1Groupe d'etude des Semiconducteurs, University of Montpellier, Montpellier cedex 5, France; 2Centre de Recherche sur l'Hetero-Epitaxie et Applications, Valobonne, France.

Continuous-wave, time-integrated, and time-resolved photoluminescence experiments are used to study the excitonic optical recombinations in wurtzite ZnO/Zn0.78Mg0.22O quantum wells of varying widths. By comparing experimental results with a variational calculation of excitonic energies and oscillator strengths, we determine the magnitude (0.9 MV/cm) of the longitudinal electric field that is induced by both spontaneous and piezoelectric polarizations. The quantum-confined Stark effect counteracts quantum confinement effects for well widths larger than 3 nm, leading to emission energies that can lie 0.5 eV below the ZnO excitonic gap. Wurtzitic ZnO/(Zn,Mg)O quantum wells grown along the (0001) direction permit unprecedented tunability of the short-range spin exchange interaction. In the context of large exciton binding energies and electron-hole exchange interaction in ZnO, this tunability results from the competition between quantum confinement and giant quantum confined Stark effect. By using time-resolved photoluminescence we identify, for well widths under 3 nm, the redistribution of oscillator strengths between the A and B excitonic transitions, due to the enhancement of the exchange interaction. Conversely, for wider wells, the redistribution is cancelled by the dominant effect of internal electric fields, which dramatically reduce the exchange energy.


2:00 PM K4.2
Reconsidering Magnetic Exchange in Oxide-based DMS.Rebecca Janisch1,2, Priya Gopal2 and Nicola A. Spaldin2; 1Electrical and Information Engineering, Chemnitz University of Technology, Chemnitz, Germany; 2Materials Department, University of California, Santa Barbara, California.

Many transition metal (TM) doped oxides, such as TMxTi1-xO2 and TMxZn1-xO, are considered to be robust room temperature diluted magnetic semiconductors (DMS) and therefore promising materials for spintronic devices. However, for most oxide-based DMS there is still controversy over the origin of the magnetism as well as the factors influencing its magnitude and ordering temperature. In recent ab initio density-functional studies [1,2] we showed that in defect-free systems the magnetic interaction is very short ranged and strongly dependent on the TM-O-TM bond angle. In this presentation we will show our results on Co-doped TiO as well as ZnO doped with a range of transition metals. Based on our results the influence of the host crystal structure as well as of point and extended defects in the oxide host on the magnetic interaction will be discussed. Finally, we will demonstrate the extreme sensitivity of the sign and the magnitude of the magnetic interaction not only on the choice of the exchange correlation functional, but also on structural relaxations, and the convergence parameters of the calculation. [1] R. Janisch and N. A. Spaldin, PRB 73, 035201 (2006) [2] P. Gopal and N. A. Spaldin, cond-mat/0605543 (2006)


2:15 PM K4.3
Demonstration of Spin Injection into ZnO using ZnO Based Diluted Magnetic Semiconductors.Shivaraman Ramachandran1, Jonh T Prater2,1 and Jagdish Narayan1; 1NCSU, Raleigh, North Carolina; 2Materials Science Division, Army Research Office, Raleigh, North Carolina.

Spin injection using diluted magnetic semiconductors (DMS) materials has been a subject of immense scrutiny over the past few years. After the early work by Datta and Das conceptualizing a spin field effect transistor1, attempts to utilize magnetic metals in conjunction with widely used semiconductors have yielded marginal results due to spin scattering by interfacial states at the hetero-interfaces. Therefore, DMS materials play a very important role from this perspective, as efficient spin injection into semiconductors can be achieved by synthesizing a homoepitaxial interface with the same semiconductor that is used to make the DMS itself. In this regard, a few wideband gap semiconductors, (e.g. ZnO, GaN) have captured the imagination of many scientists in the past few years. It has been shown by several groups that these semiconductors can be made ferromagnetic up to room temperature by doping with magnetic elements such as cobalt or manganese. From a materials perspective zinc oxide (ZnO) has been identified to be a good candidate material for producing DMS materials. ZnO is n-type in the as grown state and has a good spin coherence length1, which is an important requirement for making spintronic devices. Moreover, it can be grown as an epitaxial layer on very commonly used substrates such as Sapphire (0001). In our work, we have demonstrated spin injection across ZnO using Co-doped ZnO DMS contacting layers. We have synthesized heterostructures and have detected spin injection up to a temperature of 50K using magnetotransport measurements. The multilayer heterostructure devices were grown using the process of pulsed laser deposition technique where a non-magnetic spacer layer of ZnO has been sandwiched between two DMS layers based on Co-doped ZnO. We have grown samples with different spacer layer thicknesses to vary the transport properties. Magnetotransport properties were studied using a Physical Property Measurement System (PPMS), in the temperature range of 2K-300K. Microstructural characterization of the heterostructures has been performed using transmission electronic microscopy, including conventional TEM, high resolution TEM and Electron Energy Loss Spectroscopy (EELS). These devices also exhibit interesting magneto-transport properties at very low temperatures (around 2 K) which shed light on the mechanisms that produce ferromagnetism in such DMS materials. If harnessed fully these could pave the way for the first spintronics-based devices. 1Datta and Das, App.Phys.Lett, 56, 665 (1993) 2 S.Ghosh et al, App.Phys.Lett, 86, 232507 (2005)


2:30 PM K4.4
Room Temperature ZnO Nanostructures For Spintronics Application.Weilie Zhou, Jingjing Liu, Jiajun Chen and Minghui Yu; Advanced Materials Research Institute, University of New Orleans, New Orleans, Louisiana.

Mn and Co doped ZnO diluted magnetic semiconductor nanostructures, such as bowls, cages, nanorodes, and nanowire arrays have been successfully fabricated through chemical vapor deposition and pulsed laser deposition, respectively. The nanostructures were characterized using X-ray diffractometer (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The doped elements were detected by electron energy dispersive X-ray analysis (EDS), electron energy loss spectroscopy (EELS), and X-ray photoelectron spectroscopy (XPS). Superconducting quantum interference device (SQUID) magnetometer was employed to measure ferromagnetism of the DMS nanostructures and room temperature ferromagnetism was observed. The DMS nanostructures with room temperature ferromagnetic ordering have strong applications in spintronic nanodevices.


2:45 PM K4.5
ZnO Based Diluted Magnetic Semiconductors: Novel Materials and Devices.Shivaraman Ramachandran1, John T Prater2,1 and Jagdish Narayan1; 1NCSU, Raleigh, North Carolina; 2Materials Science Division, Army Research Office, Raleigh, North Carolina.

Spintronics is the emerging field where the spin of the charge carrier is utilized in addition to the charge to bring about novel functionalities for ultra-low power devices, non-volatile memories, magnetic recording and optical polarizers. One of the prime requirements for practical applications is that these materials be ferromagnetic above room temperature. Zinc oxide has been studied extensively to synthesize what are known as diluted magnetic semiconductors, where a transition metal dopant is used to substitute a fraction of the host atoms. We have doped various dopants such as cobalt (Co)1, manganese (Mn)2 and vanadium(V)3 in ZnO in varying concentrations and synthesized thin films of these DMS materials by pulsed laser deposition technique to study the effect on the magnetic properties. In addition we have done extensive annealing studies to correlate the properties of these thin films with the free carrier concentration that is generated due to intrinsic defects in the as grown films. Microstructural characterization of these materials is done using cross sectional transmission electron microscopy (TEM) including high resolution TEM and electron energy loss spectroscopy (EELS). We have established that ZnO-Co and ZnO-Mn systems are ferromagnetic up to room temperature whereas ZnO-V system is not. We have recently shown that ferromagnetism in ZnO-Mn can be controlled by varying free-carrier concentration2. Upon annealing in oxygen the conductivity decreases and the moments per atom of the dopant reduces considerably and in some cases the DMS ceases to be ferromagnetic. In other cases the annealed films are ferromagnetic at very low temperatures. We conclude from these results that a mixture of carrier induced ferromagnetism and the bound polaron exchange mechanisms are operative in these materials. From a materials perspective to device perspective, it is important to study the efficiency of such DMS materials to inject spin polarized electrons into a semiconductor. We have used such materials in conjunction with ZnO itself to make novel device structures to accomplish this objective. Preliminary results are very encouraging to the mark that we can achieve highly efficient spin injection at least up to temperatures of the order of 50K. These ZnO based materials could pave the way for a new generation of novel devices based on the concept of spintronics. 1S.Ramachandran et al, App.Phys.Lett, 84 , 5255-5257(2004) 2S.Ramachandran et al, App.Phys.Lett, 87, 172502 (2005) 3S.Ramachandran et al, App.Phys.Lett, 88, 242503 (2006)


3:30 PM K4.6
Electrical and Magnetic Properties of Doped ZnO Nanowires.Gennady Panin1,2, Andrey Baranov3, Tae Won Kang1, Oleg Kononenko2, Dubonos Sergey2, S. K Min4 and H. J Kim4; 1Physics, QSRC, Dongguk University, Seoul, South Korea; 2Institute of Microelectronics Technology, RAS, Chernogolovka, Moscow distr., Russian Federation; 3Chemistry, Moscow State University, Moscow, Russian Federation; 4Semiconductors, QSRC, Dongguk University, Seoul, South Korea.

Zinc oxide doped by different impurities has a unique combination of electrical, magnetic and optical properties (Eg. = 3.37 eV, Eeb = 60 meV) which can be used in a variety of novel devices such as UV light emitting diodes, lasers, spin and dipole controlled switches, chemical and biosensors. Here we report on magnetic properties of single doped ZnO nanowires and a persisting highly reproducible resistance modulation of the wires by a dc voltage at room temperature. ZnO nanowires were synthesized by heating the mixture of solution processed Zn precursor with NaCl Li2CO3 salt at 700°C [1] and were doped by different electric-active and magnetic impurities. The size of wires was in the range of 40-150 nm in diameter and 1000-5000 nm in length. The nanowires characterized by high resolution transmission microscopy showed a single crystal wurtzite structure and were free from second phase. For transport measurements the individual nanowires were configured as two terminal device with electrode-ZnO-electrode structure on silicon substrate capped with thick SiO2 layer. E-beam lithography was used to pattern aluminum electrodes contacting individual nanowires. Structure of the device was controlled by high-resolution scanning electron microscopy. Electrical transport properties of the two-terminal device were investigated by applying quasi-dc voltage with a constant sweep velocity. Magnetic susceptibility of the samples as a function of temperature demonstrated Curie-Weiss behavior. Hysteresis with the coercive field <200 Oe was clearly observed in magnetization versus field curves at 5 and 300K. Increasing the Mn concentration increases significantly the magnetic hysteresis indicating ferromagnetic properties of single nanowires. Magnetic force microscopy measurements of single nanowires with spatial resolution revealed the local magnetic polarization. The domain nanomagnet structure aligns perpendicular to the surface and can be controlled by magnetic field. The effects of ZnO surface band-bending due to formation of charged traps [2] as well as trapping effects and charge transfer process via impurity and intrinsic point defects with changes of polarization and sensitivity to oxygen are discussed. [1] A.N. Baranov, G.N. Panin T.W. Kang and Y.-J. Oh, Nanotechnology 16, 1918 (2005). [2] G.N. Panin, T.W. Kang, A. N. Aleshin, A. N. Baranov, Y.-J. Oh, I. A. Khotina, Appl. Phys. Lett. 86, 113114(2005)


3:45 PM K4.7
Structural and Magnetic Properties of Iron and Cobalt Implanted ZnO Thin Films.Wing Yan Luk3, S. P. Wong1,3, Ning Ke1 and Quan Li2,3; 1Dept of Electronic Engineering, Chinese University of Hong Kong, Shatin, Hong Kong; 2Dept of Physics, Chinese University of Hong Kong, Shatin, Hong Kong; 3Materials Science and Technology Research Centre, Chinese University of Hong Kong, Shatin, Hong Kong.

In this work, ZnO thin films were prepared by RF sputtering on thermally grown silicon dioxide layers on Si substrates. Iron and cobalt implantation was performed at an extraction voltage of 70 kV using a metal vapor vacuum arc ion source to various doses ranging from 4x1015 cm-2 to 4x1016 cm-2 with or without substrate cooling. The substrate temperature during implantation without cooling was determined to be above 150oC due to beam heating effect. With liquid nitrogen cooling during implantation, the substrate temperature was controlled to be about -100oC. Post-annealing was performed in a vacuum chamber (2-7x10-6 Torr) at temperatures ranging from 400oC to 700oC for 2 and 4 hours. The implantation dose was determined by Rutherford backscattering spectrometry. The microstructures were studied by transmission electron microscopy and x-ray diffractometry. The chemical bonding states were determined by x-ray photoelectron spectroscopy. The magnetic properties were studied by vibrating sample magnetometry. Clear room temperature ferromagnetic properties were observed in these implanted samples. It was also found that the implantation temperature has significant effects on the magnetic properties. For example, for the as-implanted sample with a Fe dose of 4x1015 cm-2, a saturation magnetization (Ms) value of 0.82 µB/Fe atom was obtained when the substrate was cooled during implantation, while for the sample without substrate cooling, a much smaller Ms value of 0.44 µB/Fe atom was obtained. Details of the dependence of the magnetic properties on the sample preparation conditions will be presented and correlated with the microstructures. Possible origins of the room temperature ferromagnetic properties will be discussed in conjunction with the structural properties. This work is supported in part by a direct grant for research from the Faculty of Engineering of CUHK.


4:00 PM K4.8
Nano-scale Spinodal Decomposition Phase in ZnO-based Dilute Magnetic Semiconductors.Masayuki Toyoda, Kazunori Sato and Hiroshi Katayama-Yoshida; The Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka, Japan.

The possibility of high-Curie-temperature (high-TC) ferromagnetism in ZnO-based dilute magnetic semiconductors (DMS) was first predicted by theoretical calculations [1]. Being encouraged by the predictions, ZnO-based DMS have been studied intensively and several groups have succeeded to fabricate ferromagnetic samples of, for example, V- or Co-doped ZnO [2]. However, there are attempts to recalculate Curie temperature of DMS by using exact Monte Carlo simulations because of the suggestion that magnetic ordering in DMS is significantly influenced by the magnetic percolation effect [3] which is ignored in the former calculations based on the mean-field approximations. The recalculations show that in wide-gap DMS, such as ZnO- and GaN-based DMS, the exchange interactions are very short-ranged and thus that we can not expect high-TC [3,4]. We are now need a theory to explain the ferromagnetic behavior of ZnO-based DMS at high temperature. As an explanation, we propose the possibility of formation of the spinodal decomposition (SD) phase in ZnO-based DMS as already proposed for the other DMS such as GaMnN [5] and ZnCrTe [6]. In the SD phase, the local fluctuation in concentration of the magnetic impurities would support the magnetic network of DMS. To verify this possibility, we present computer simulation results of the SD phase formation in ZnO-based DMS. Our simulation method consists of three steps. Firstly, We perform first-principles calculations of ZnO-based DMS by using the Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) method [7]. Then we simulate the formation of the SD phase by performing Monte Carlo simulations on an Ising-type model which describes the distribution of magnetic impurity atoms in DMS system. Finally, we perform Monte Carlo simulations on a classical Heisenberg-type model which describes the spin configuration of the SD phase in order to estimate Curie temperature. The parameters used in the simulations such as the chemical pair interactions and exchange interactions between two impurity atoms in DMS are calculated directly from the first-principles results. In the talk, we will discuss the effect of SD in ZnO-based DMS by comparing magnetic properties between the SD phase and the homogeneous phase. [1] T. Dietl et al., Science 287 (2000) 1019; K. Sato and H. Katayama-Yoshida, Physica B 308-310 (2001) 904. [2] H. Saeki et al., Solid State Commun. 120 (2000) 439; K. Ueda et al., Appl. Phys. Lett. 79 (2001) 988; K. Rode et al., J. Appl. Phys. 93 (2003) 7676. [3] L. Bergqvist et al., Phys. Rev. Lett. 93 (2004) 137202 [4] K. Sato et al., Phys. Rev. B 70 (2004) 201202. [5] K. Sato et al., Jpn. J. Appl. Phys. 44 (2005) L948. [6] T. Fukushima et al., Jpn. J. Appl. Phys. 45 (2006) L416. [7] H. Akai and P. H. Dederichs, Phys. Rev. B 47 (1993) 8739.


4:15 PM K4.9
Ferromagnetism in Cu doped ZnO Based Diluted Magnetic Semiconductors.Deepayan Chakraborti1, John T Prater1,2 and Jagdish Narayan1; 1Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina; 2Materials Science Division, Army Research Office, Research Triangle Park, North Carolina.

Recent developments in the field of spintronics has led to an extensive search for materials in which semiconducting properties can be integrated with magnetic properties to realize the objective of successful fabrication of spin-based devices.Transition metal doped ZnO has proved to be a potential candidate as a diluted magnetic semiconductor to achieve this end. There still exists a debate on whether the magnetic behavior is an intrinsic property of the film or it is due to the presence of nanoclusters of a magnetic phase or both. Here we report systematic studies on the epitaxial growth and properties of Zn1−xCuxO thin films deposited onto c-plane sapphire single crystals by pulsed laser deposition technique.Cu is a potential magnetic ion with + ½ spin in the 2+ state. Since Cu is not ferromagnetic as metallic Cu, Cu2O or CuO, Cu doped ZnO has the potential of showing magnetic behavior only due to substitution of Cu on Zn sites; not due to clustering of nanomagnetic particles. An analysis of the magnetic properties of these materials considering carrier induced and F-center mediated exchange mechanism will be discussed. Magnetic measurements including magnetization as a function of applied magnetic field and magnetization as function of temperature (field cooled and zero field cooled) have been performed in a superconducting quantum interference device (SQUID) magnetometer. Room temperature ferromagnetism was observed in the Zn1−xCuxO films with magnetic moment per Cu atom decreasing with increasing Cu content. Detailed atomic scale characterization done using transmission electron microscopy (TEM), including high-resolution TEM, STEM Z-contrast and Electron energy Loss Spectroscopy (EELS) techniques were used to determine that origin of the ferromagnetism was mainly due to Cu ions substituted into the ZnO lattice as the presence of nanoclusters of any secondary phase was ruled out. These studies coupled with optical characterization using absorption spectroscopy yield interesting insight into local environment of the dopants in the crystal field of the host.


4:30 PM K4.10
Transition Metal-doped ZnO: A Comparison of Optical, Magnetic, and Structural Properties of Bulk and Thin Films.Matthew Kane1,2, William Fenwick1, Rengarajan Varatharajan3, Martin Strassburg1, Bill Nemeth3, David Keeble4, Hassane El-Mkami5, Graham Smith5, Jeff Nause3, Christopher Summers2 and Ian Ferguson1,2; 1Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia; 2Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia; 3Cermet, Inc., Atlanta, Georgia; 4Carnegie Laboratory of Physics, Faculty of Engineering and Physical Sciences, University of Dundee, Dundee, United Kingdom; 5School of Physics and Astronomy, University of St. Andrews, St. Andrews, United Kingdom.

Following theoretical predictions of room temperature (RT) ferromagnetism in transition-metal (TM)-doped zinc oxide, extensive experimental studies have attempted to produce ferromagnetic behavior in Zn1 xMnxO and Zn1 xCoxO. The nature of ferromagnetism remains unclear in all cases, and crystalline defects may play a significant role. This work reports on the optical and magnetic properties of Co- and Mn-doped ZnO grown by a modified melt-growth technique and metalorganic chemical vapor deposition, and the effects of annealing and co-doping on the magnetic behavior. Good crystalline quality was confirmed by X-ray diffraction (XRD), which revealed that the as-grown crystals are pure single crystals with no second phases. Mn doping up to 5% results in an increase in c-axis lattice parameter (5.207 Å to 5.211 Å) , and in X-ray linewidths (78 arcsec to 252 arcsec). Structural properties were also investigated with Raman spectroscopy. The standard Raman active modes were visible in the Raman spectra, suggesting good crystalline quality. An additional Raman mode observed at 522cm-1 has been attributed to Mn incorporation in the crystal, though the dominant feature with transition metal doping is the activation of ‘silent’ Raman modes within the wurtzite lattice due to a loss of translational symmetry with Mn-doping. Optical transmission shows distinct absorption spectra related to the color of the Zn1 xTMxO sample resulting from interatomic transitions within the divalent transition metal ion in a tetrahedral crystal field. Electronic paramagnetic resonance strudies confirm the divalent nature of the substitutional transition metal atoms. Magnetization measurements reveal a paramagnetic behavior at all temperatures for both Mn- and Co-doped ZnO with the dominant exchange mechanism in both the Mn- and Co-doped ZnO single crystals as antiferromagnetic (AFM) superexchange. The results will be compared with temperature-dependent optical and magneto-optical spectroscopies, in order to examine binding energies of the dopants and defect centers and to investigate the incorporation and possible formation of spin-split electronic states leading to ferromagnetic behavior. The influence of annealing and codoping in both bulk and thin film samples within the framework of relevant current theories of ferromagnetism will also be discussed.


4:45 PM K4.11
Ferromagnetic Mn-Mn Interactions in Low Doped Zn1-xMnxO Thin Films. Aroussi Ben Mahmoud2, H.Jurgen von Bardeleben1, Alain Mauger3, Jean-Louis Cantin1 and Ekaterina Chikoidze4; 1University Paris 6, CNRS, Paris, France; 2Faculté des Sciences de Gabès, Gabès, Tunisia; 3MIPPU CNRS, Paris, France; 4GEMC CNRS, Meudon, France.

Recent experimental results on Mn-Mn interactions in moderately and highly doped single phase Zn1-xMnxO thin films (x>0.05) have shown that the nearest neighbour interactions are antiferromagnetic with an effective exchange integral of J/kB=-22K [1]. This behaviour has been quantitatively confirmed by recent calculations[2]. Such samples are paramagnetic at room temperature and transform into a spin glass phase at typically T=50K. We have further studied the magnetic interactions in low doped ZnMnO films (x=0.03) where the nearest neighbour Mn pairs are a negligeable fraction of the total Mn amount. Surprisingly, in such samples ferromagnetic interactions have been observed as deduced from Curie-Weiss plots obtained by EPR spectroscopy. We ascribe this phenomena to the interaction with moderately deep donors present in this generally n-type conductive material with carrier concentrations of 1018cm-3. [1] A.BenMahmoud et al, submitted to Phys.Rev.B (2006) [2] T.Chanier et al, Phys.Rev.B73,134418 (2006)

SESSION K5: Growth

Chairs: R.D. Vispute and Takafumi Yao
Wednesday Morning, November 29, 2006
Room 200 (Hynes)
8:30 AM *K5.1
Growth of Large Size ZnO Bulk Crystals by Hydrothermal Method.Tsuguo Fukuda1,2, Y. Kagamitani1, D. Ehrentraut1, Y. Mikawa2, K. Maeda3 and T. Ono3; 1Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Japan; 2Fukuda Crystal Laboratory, Sendai, Japan; 3Tokyo Denpa Co., LTD., Tokyo, Japan.

ZnO is a wide band-gap semiconductor (Eg = 3.37 eV) with a large exciton binding of 60 meV. ZnO has a high potential for broad applications in optoelectronics, piezoelectronics and UV-blue LED device. Since exciton luminescence of ZnO shows very short decay time of < 1 ns, it is attractive as ultra-fast scintillator. In this research, 3-inch size undoped bulk ZnO was grown by the hydrothermal method and its basic properties were characterized. Also, indium-doped ZnO crystal was grown and scintillation properties were measured. The hydrothermal method has been used for industrial growth of quartz crystals for more than 50 years. The advantages of this method are (1) growth at low temperature, (2) suitable for mass production and large size crystals by scaling up the equipment, (3) high crystal quality because of the extremely small temperature gradient at the growing interface. However, the hydrothermal method never before was applied to the growth of semiconductor crystals. The technique had to be developed to control electrical properties by impurity control and accomplish doping of ZnO for use in semiconductor devices. The growth of 3-inch ZnO crystals was achieved by the modified hydrothermal method, which is based on the conventional technique for quartz mass production. A high pressure vessel (autoclave) of 250 mm ID and 4.5m in height is used. A platinum inner container was installed inside the autoclave to prevent impurity incorporation from autoclave wall. Alkaline aqueous solution (3M KOH and 1M LiOH) is filled into the inner container whereas the outside of the container is filled with pure water for pressure balance. ZnO single crystal wafers were set in the upper half zone (growth zone) as seed crystals and ZnO poly crystals, sintered at 1000-1200°C, were placed in the lower half zone (dissolving zone) as raw materials. The inside of the container was separated by a baffle plate into the two zones. The crystals were grown at 300-400°C and 80-100MPa. The growth rate was 0.1-0.3mm/day along c-axis. The crystallinity of 3-inch undoped ZnO is characterized by X-ray rocking curve measurements and the FWHM of 0002 reflection is 18 arcsec. To our knowledge this is the best value of FWHM ever reported for large bulk ZnO. For scintillator application, indium-doped ZnO crystals were grown so as to increase the luminescence intensity at longer wavelength by avoiding the self absorption. Exciton luminescence of In:ZnO was observed at 398 nm (undoped ZnO: 387 nm) at room temperature. The decay time was measured using a femtosecond laser at λ = 260 nm. Ultrashort decay of 40 ps and 650ps was observed.


9:00 AM K5.2
Hydrothermal Growth of Various Doped ZnO Crystals.Michael Callahan1, Buguo Wang2, Lionel Bouthillette1 and David Bliss1; 1Sensor Directorate, Air Force Research Laboratory, Massachusetts; 2Solid State Scientific Corporation, Nashua, New Hampshire.

Although there are several techniques to grow bulk ZnO crystals, only the hydrothermal technique has produced low-defect, large-diameter ZnO substrates. The hydrothermal technique has several advantages over molten techniques for the growth of doped substrates due to the small temperature gradients in the growth zone and near equilibrium growth conditions at the growth interface. ZnO properties can be modified by various doping such as M:ZnO (M = Mg, Co, Mn, Fe, In, Er, Eu), with wide applications in optoelectronics, gas sensing, surface acoustic wave generation, and radiation detection (superfast scintillators). Here we present the growth of doped ZnO crystals by the hydrothermal technique and characterization of the grown crystals by photoluminescence, X-ray diffraction, Hall, and SQUID measurements. The effect of doping on the growth mechanism and morphology of the ZnO crystals will also be discussed.


9:15 AM K5.3
Growth an Characterization of Homoepitaxial ZnO Thin Films Grown by CVD.Joachim Sann1, Christian Neumann1, Bruno K. Meyer1, Frank Bertram2 and Jürgen Christen2; 11st Physics Institute, Justus Liebig University Giessen, Giessen, Germany; 2Institute for Experimental Physics, Otto-von-Guericke University Magdeburg, Magdeburg, Germany.

One of the major advantages of ZnO over GaN has been the fact that growth of ZnO single crystals for substrates and thus homoepitaxial growth of ZnO thin films is possible. Despite this fact reports on high quality ZnO thin films on ZnO substrates are quite rare. We report on high quality homoepitaxially CVD-grown ZnO thin films grown on ZnO single crystal substrates supplied by Crystec. The substrates were exposed to a high temperature annealing step prior to growth. The film was grown with a CVD using metallic Zinc and NO2 as oxygen precursor. The films where characterized by atomic force microscopy, low temperature photoluminescence and Cathodoluminescence. The AFM data shows a change in the growth mode from 3-dimensional to 2-dimensional growth with increasing growth temperature. This change can be observed in the PL-data as well. The 3-dimensionally grown films exhibit a large number of up to 11 very sharp excitonic lines, whereas the 2-dimensionally grown films show only 3 excitonic lines. An assignment of morphological and optical features via Cathodoluminescence-measurements will be presented.


9:30 AM K5.4
Orders of Magnitude Reduction in Threading Dislocations in ZnO Grown on Facet-controlled GaN.Soo Jin Chua1,2, Hai Long Zhou3, Hui Pan3, Thomas Osipowicz3 and Jian Yi Lin3; 1Institute of Materials Research and Engineering, Singapore, Singapore; 2Department of Electrical and Computer Engineering, National University of Singapore, Singapore, Singapore; 3Department of Physics, National University of Singapore, Singapore, Singapore.

GaN is grown by Metal Organic Chemical Deposition on oxide masked GaN with stripe window openings of 6μm in width. Growth conditions were selected to achieve trapezoidal shaped cross section of GaN with {11-22} sidewalls and (0001) top surface. ZnO is next epitaxially overgrown (EO) on these facets by Chemical Vapour Deposition (CVD). SiO2 is chosen as the masking material to prevent nucleation of ZnO on it. For different CVD growth parameters, the growth morphology of EO ZnO changes from long, needle-shaped nanorods to rectangular stripes with (0001) top facet and {11-20} sidewalls and finally to triangular stripes with {11-22} sidewalls. It is also demonstrated that the EO ZnO growth facets can be controlled and the overgrown region can achieve two orders of magnitude reduction in dislocation density in the entire epitaxial film as measured by the X-ray diffraction (XRD), Rutherford Backscattering (RBS), high resolution transmission electron microscopy (HRTEM) and room temperature photoluminescence (PL). The strongest PL intensity of the EO ZnO is observed from samples grown at 800oC with the full width at half maximum (FWHM) of the ZnO photoluminescence peak of about 5.5 nm. The spectra of 820C EO ZnO/GaN shows the declining crystallinity, with the broadening of ZnO peaks to around 23 nm. Laterally resolved channeling data have been extracted from the Rutherford Back Scattering spectra in near-surface regions. The random and channeled spectra of the band of the EO ZnO grown at 800C showed the greatest contrast. This is further evidence of better quality of the EO ZnO grown at 800C. This growth method can be used to fabricate ZnO/GaN heterostructures with low dislocation densities, which will find important applications in future electronic and UV laser devices.


9:45 AM K5.5
Electrochemically Induced Growth of ZnO on (0001) GaN.Thomas Loewenstein1, Joachim Sann2, Christian Neumann2, Bruno K. Meyer2, Tsukasa Yoshida3 and Derck Schlettwein1; 1Institute of Applied Physics, Justus-Liebig-University Giessen, Giessen, Germany; 2Institute of Experimental Physics I, Justus-Liebig-University Giessen, Giessen, Germany; 3Graduate School of Engineering, Gifu University, Gifu, Japan.

The sensitization of wide-bandgap semiconductors is an attractive approach to realize a photoelectrochemical photovoltaic cell. As an alternative to the established technology based on nanoparticulate TiO2, ZnO/dye hybrid materials provide some advantage. Porous, crystalline thin film electrodes can be deposited in electrochemical reactions from aqueous solution without annealing steps, compatible with a large number of conductive substrates. The structure and texture of the deposited ZnO in the hybrid materials is highly relevant for the efficiency of the electrodes. Single crystalline substrates allow to investigate this aspect to far higher extent than typical polycrystalline or amorphous substrates used in technical applications. Caused by a well- fitting crystal lattice, (0001) GaN offers the possibility of epitaxial growth of ZnO [1]. If the conductivity of GaN is chosen sufficiently high, also the electrochemical deposition of ZnO can be studied [2]. In this study, ZnO/dye hybrid materials were deposited on (0001) GaN from aqueous zinc salt solutions induced by a local pH-increase at the working electrode during the reduction of oxygen. The morphology of the deposited domains was quite comparable to those deposited on polycrystalline electrodes as detected in scanning electron microscopy (SEM). Crystalline ZnO was deposited in the hybrid thin films as proven by X-ray diffraction (XRD). Already from the intensity pattern of the observed diffraction peaks it could be concluded that the desired preferential orientation with the c- plane of ZnO parallel to GaN (0001) was established. Rocking curves were analysed and indicated a high level of in-plane orientation of the grown ZnO crystalline domains. The detailed peak positions spoke in favour of a ZnO lattice expansion in the direction of the c- axis caused by dye adsorption on ZnO (0001). Photoluminescence measurements are used to analyze consequences on the electronic structure. 1. A. Zeuner, H. Alves, D.M. Hofmann, B.K. Meyer, phys. stat. sol. (b), 229, 907 (2002). 2. Th. Pauporté, R. Cortès, M. Froment, B. Beaumont, D. Lincot, Chem. Mater., 14, 4702 (2002).


10:30 AM *K5.6
Effects of Polarity on MBE Growth of Undoped, Ga- and N-doped ZnO Films.Hiroyuki Kato1, Akio Ogawa1, Hiroshi Kotani1, Michihiro Sano1 and Takafumi Yao2; 1Stanley Electric Co., Ltd., Yokohama, Japan; 2Tohoku Univ., Sendai, Japan.

Crystal structure of ZnO is wurtzite and c-axis ZnO has two distinct polar faces: (0001) Zn- and (000-1) O-faces. The difference in polarity affects the growth mode, impurity incorporation and dislocation generation. Our growth studies in undoped ZnO revealed that the growth mode of O-polar ZnO showed two-dimensional (2D) growth independent of O/Zn flux ratio [1], whereas the growth mode of Zn-polar ZnO showed 2D growth under only O-rich flux condition [2]. Most doping studies in ZnO have been carried out using O polarity due to the difficulty of polarity control of ZnO on sapphire. Recently, we found that control of the polarity in ZnO films grown on c-plane sapphire by plasma-assisted molecular beam epitaxy (MBE) was achieved by inserting an MgO buffer layer of specific thickness [3]. In consequence of optimization of growth process, we obtained high quality O-polar ZnO films with residual carrier concentration and mobility of 3.7x1016 cm-3 and 164 cm2/Vs, and high quality Zn-polar ZnO films with insulating property [4]. In this study, we report on Ga- and N-doped ZnO films with Zn and O polarity grown by plasma-assisted MBE. Gallium-doped Zn- and O-polar ZnO films were grown on c-plane sapphire at 700°C under optimal flux conditions (i.e., O-rich for Zn polarity and slightly Zn-rich for O polarity). In O-polar ZnO, the growth mode showed 2D growth independent on Ga concentration, the carrier concentration monotonically increased and the activation ratio of Ga showed unity when the Ga concentration ranged from 1017 to 1020 cm-3. In Zn-polar ZnO, however, the growth mode was changed from 2D to 3D growth with increasing Ga concentration and the carrier concentration was smaller than the Ga concentration, namely the activation ratio of Ga was low. The low activation ratio of Ga in Zn-polar ZnO films is likely caused by the formation of GaZn-VZn complexes and /or ZnGa2O4. Nitrogen-doped Zn-polar ZnO films were grown on c-plane sapphire at 750°C. The RF-power supplied for N-plasma was varied from 0 to 300W and the growth was carried out under Zn-rich flux condition. We found that the growth rate decreased and the growth mode changed from 3D to 2D growth with increasing the N-RF power. The structural, optical properties in Ga-doped ZnO will be presented and discussed in detail, and also the mechanism of growth mode change in N-doped ZnO with Zn polarity will be discussed. [1] H. Kato, M. Sano, K. Miyamoto, T. Yao, Jpn. J. Appl. Phys. 42 (2003) 2241. [2] H. Kato, M. Sano, K. Miyamoto, T. Yao, Jpn. J. Appl. Phys. 42 (2003) L1002. [3] H. Kato, K. Miyamoto, M. Sano, T. Yao, Appl. Phys. Lett. 84 (2004) 5462. [4] H. Kato, M. Sano, K. Miyamoto, T. Yao, J. Crystal Growth 275 (2005) e2459.


11:00 AM K5.7
Solution Growth and Luminescence Characteristics of Undoped, In- and Ge-doped ZnO Thin Films.Dirk Ehrentraut1, Jan Pejchal2, Martin Nikl2, Hideto Sato3, Yuji Kagamitani1, Hiroshi Fukumura4 and Tsuguo Fukuda1; 1IMRAM, Tohoku University, Sendai, Japan; 2Institute of Physics, AS CR, Prague, Czech Republic; 3Murata Mfg. Co. Ltd., Shiga, Japan; 4Dept. Chemistry, Faculty of Sciences, Tohoku University, Sendai, Japan.

Exploitation of the very fast, sub-nanosecond excitonic emission of ZnO for superfast scintillators was recently discussed in the literature [1]. However, severe re-absorption may occur due to the small Stokes shift of Wannier exciton emission in the direct-gap ZnO structure. Efficient collection of such emission from bulky scintillation elements is therefore limited. Manipulation of the excitonic emission by shifting the excitonic band to lower energies could be the way to overcome the problem. In the case of double donor-acceptor doping also radiative recombination within the donor-acceptor pairs can be considered to obtain low-energy shifted emission. A crucial aspect is, however, if such modified luminescence centers in ZnO structure will keep the superfast character of the decay kinetics. As the doping of hydrothermal-grown bulk ZnO [2] appears rather difficult and time consuming, we have used our recently developed growth process by liquid phase epitaxy (LPE) to obtain high quality single crystal films of several micrometer thicknesses [3]. The doping by various elements can be accomplished much more easily and at shorter time scale. Moreover, the fabrication from a liquid solution delivers high crystal perfection as the growth is conducted under near thermodynamic equilibrium conditions. In this work, LPE is used as screening tool to obtain single-crystalline ZnO surfaces at reasonable processing time. Another advantage of LPE method is that the sample surface is mechanically untouched. Mechanical treatment appears as a possible source of problems in the case of polished bulk crystals as the penetration depth of the UV excitation in the region of ZnO intrinsic absorption is less than 100 nm. Homoepitaxial ZnO thin films were grown from a LiCl solution at 640°C under ambient air conditions [2]. ZnO is produced by a reaction of ZnCl2 with K2CO3, such way providing the feeding for continuous growth. Doping and formation of solid solutions with bi-, tri-, and tetravalent ions like Mg2+, Cd2+, Ga3+, In3+, and Ge4+ is enabled through employment of the relevant metal halogenides. Photo- and radioluminescence spectra were measured at the undoped, In and Ge-doped ZnO thin films using a steady-state UV and X-ray excitation sources, respectively. The decay kinetics of the exciton-based and donor-acceptor radiative recombination was studied using a femtosecond laser excitation. The effect of In and Ge concentration in the films on the decay kinetics is discussed in the context of the origin. The results of these experiments will be demonstrated and discussed in the light of possible application of ZnO-based materials in superfast scintillation detectors. 1. S. Derenzo et al, NIM A 486, 214 (2002). 2. E. Ohshima et al., JCG 260, 166 (2004). 3. D. Ehrentraut et al., JCG 287, 367 (2006).


11:15 AM K5.8
ZnCdMgO-based Quantum Well Structures Grown by Molecular Beam Epitaxy for Light-emitting Applications. Sergey Sadofev, Sylke Blumstengel, Jian Cui, Joachim Puls and Fritz Henneberger; Institut für Physik, Humboldt-Universität, Berlin, Germany.

ZnO is a promising candidate for applications in ultraviolet to green light-emitting devices. Radical-source molecular beam epitaxy on sapphire substrates is used to fabricate ZnO/ZnMgO as well as ZnCdO/ZnO quantum well structures. A specific growth procedure combining low-temperature growth and post-growth annealing at intermediate temperatures allows for layer-by-layer growth controlled by distinct RHEED oscillations. Despite of the large lattice mismatch induced by the sapphire substrate, no phase separation is found up to Mg mole fraction of 0.40. The procedure enables us to grow multiple quantum well structures with atomically smooth interfaces in a wide range of structural designs, as revealed by AFM and high-resolution TEM measurements. The quantum well structures exhibit prominent exciton emission features up to room temperature. The absorption edge can be tuned from 4.4 to 2.5 eV. Decreasing the well width down to 2.8 nm, prominent blue shifts of the exciton emission indicating robust quantum confinement are seen. Optical gain and lasing properties of ZnO/ZnMgO multiple quantum well structures (with and without separate optical confinement) are investigated in the temperature range from 5 to 290 K. The data signify that localized states are crucially involved in the laser action up to room temperature. The lasing threshold increases by about one order of magnitude and reaches 140 kW/cm2 at 290 K. The room temperature material gain is about 103 cm-1.


11:30 AM K5.9
Epitaxial growth of ZnO on (0001) 6H SiCChristian Pettenkofer1, Stefan Andres1 and Thomas Seyller2; 1SE6, HMI, Berlin, Germany; 2Institut für Technische Physik, Universität Erlangen, Erlangen, Germany.

ZnO is deposited by MOMBE on H-terminated 6H-SiC substrates. In situ XPS reveals that the interface is abrupt and non reactive. The growth mode is FvdM type (layer by layer). LEED data show a clear LEED pattern for the substrate and the grown film confirming the epitaxial growth. A detailed analysis of the LEED data show beside the (0001) plane a facetting into the (10-12) plane for thicker films. Ex situ AFM shows considerable surface roughness and supports the morphology deduced from the LEED data. The interface energetics are determined by XPS and UPS. The obtained heterojunction is of type II with a valence-band offset of 1.25 eV and an bandbending of 0.5 ev on both sides of the junction. Interesting is the observation of an interfacial dipole considerably smaller than 0.1 eV for the prepared junction.


11:45 AM K5.10
The Growth of ZnO on CrN Buffer Layer Using Surface Phase Control by Plasma Assisted Molecular-beam Epitaxy.Jinsub Park1, Tsutomu Minegishi1, Seunghwan Park1, Inho Im1, Meoungwhan Cho1,2 and Takafumi Yao1,2; 1Institute for Materials Research, Sendai, Japan; 2Center for Interdisciplinary Research, Sendai, Japan.

We propose CrN as a new buffer material for the growth of high-quality ZnO on Al2O3(0001).Since CrN has a rock salt crystal structure with lattice parameter of 0.293 nm, the epitaxy relationship for the heterostructure of ZnO/CrN/ Al2O3(0001) is expected to be as follows: ZnO(0002)//CrN(111)// Al2O3(0001). Assuming this epitaxy relationship, the 18% lattice misfit between ZnO/Al2O3(0001) can be split into two heterostructures with smaller lattice misfits: ZnO/CrN (11%) and CrN/Al2O3(6.6%)[1]. Since the thermal expansion coefficient (TEC) of CrN is 6X10-6/K, the difference in TEC between ZnO and CrN is 16.7%, while that between CrN and Al2O3(0001) is 20%. Hence ZnO will suffer from tensile stress by ZnO/CrN thermal mismatch, while CrN layers will be compressive strained by CrN/Al2O3(0001). These situations will help grow high-quality ZnO on Al2O3(0001). On top of those advantages, the surface treatments of CrN surface by oxygen plasma will produce various surface phases including Cr2O3(rhombohedral crystal structure), and CrO2(tetragonal crystal structure), which may result in combined CrxOy/CrN double buffer layers. Such novel buffers will play crucial roles in controlling crystal polarity of ZnO, since crystal polarity of ZnO layers are quite dependent on the crystal structure of buffer layers. This paper will report : (1) Although CrN buffer deposited on Al2O3 shows twinning, ZnO layers grown on CrN buffer are single crystalline without rotational twinning. (2) RHEED investigation indicates that ZnO grows in a two dimensional mode on CrxOy/CrN double buffer formed by oxygen plasma treatment on CrN. The surface of ZnO layers is smooth which is consistent with the RHEED observation. (3) ZnO layers grown directly on CrN show O-polarity with small anti-phase domain density, while those grown on CrxOy/CrN double buffer show a mixture of O-polar region and Zn-polar anti-phase domains. We are optimizing the oxygen treatment conditions to obtain complete Zn-polar ZnO layers. (4) X-ray rocking curve of ZnO layers grown on CrxOy/CrN double buffer shows a narrower FWHM value compare to ZnO directly on CrN buffer. Therefore, we believe that cubic CrN layer is an appropriate material as a buffer layer for ZnO grown on Al2O3 and that the control of surface phase of CrxOy/CrN double buffer will play a crucial role in growing high-quality ZnO layers with controlled crystal polarity.

SESSION K6: Optical Properties, Nanostructures

Chairs: Bernard Gil and Seong-Ju Park
Wednesday Afternoon, November 29, 2006
Room 200 (Hynes)
1:30 PM *K6.1
Optical Properties of ZnO Epilayers.Axel Hoffmann, Inst. f. Fstkoerperphysik, TU Berlin, Berlin, Germany.

We present time-resolved photoluminescence (PL) measurements of nitrogen implanted ZnO single crystals and lithium doped ZnO, grown by chemical vapor deposition. The samples revealed the presence of several bound exciton states as well as their two electron satellites (TES) and phonon replicas. The decay times of the observable exciton complexes and their associated emissions were studied for different excitation energies. The dynamics and energy transfer processes were analyzed by probing the free and bound exciton states, TES and phonon replicas, while varying the laser energy. Furthermore, magneto-optical PL and absorption spectroscopy has been performed. The electron and hole effective g-values are calculated from the Zeeman splitting at 5 T. Above 3 T an additional fine-splitting of peaks in magnetic fields was clearly apparent in the Li doped sample. The relaxation of selection rules by internal strain in combination with a non-zero an-isotropic hole effective g-value for B⊥c is discussed a the possible origin of the observed splitting. In this context, new evidence for the uppermost valence band having Γ7-symmetry are reasoned and clarified by angular and polarization dependent measurements. The donor or acceptor character of the bound exciton complexes is investigated through their thermalization behavior in magnetic fields. Finally, time resolved resonant and non-resonant photoluminescence measurements have been performed in externally applied magnetic fields. The interference of the coherently excited Zeeman states provides addition insight about the de-phasing mechanisms, influenced by the spin dynamics and energy relaxation processes.


2:00 PM K6.2
Recombination Kinetics of Bound Excitons in ZnO: Carrier Capture by Ionized Impurities.Frank Bertram, Juergen Christen, Armin Dadgar and Alois Krost; Insitute of Expermental Physics, University Magdeburg, Magdeburg, Germany.

The relaxation and recombination kinetics of free and bound excitons in ZnO from thermal equilibrium into true steady state excitation condition and back into thermal equilibrium is investigated by spectrally-(ps-)time-resolved cathodoluminescence (CL). The high quality 8 µm thick ZnO epi-layer under study was grown by MOVPE on an optimized low temperature ZnO/GaN/sapphire template. At T = 4 K the luminescence spectrum is dominated by the impurity bound exciton (BE) I8. The free exciton XA, the BEs I1, I2, and I6, as well as I9 are clearly resolved. Three additional peaks can be assigned as excited states of these BEs, i.e. I6*, I8*, and I9*, respectively. Spectral-time-resolved CL was performed using rectangular excitation pulses with ps rise and fall times (τ(10% - 90%) << 100 ps ), however, very large pulse lengths (Δt > 20 ns - 50 ns) assure the excitonic system reaching true quasi equilibrium during the excitation pulse. In this way, both the onset, i.e. the excitation from equilibrium into steady state as well as the transient decay from steady state can be investigated. Time delayed (td-) spectra were taken at different delay times after the excitation is switched off. No spectral shift with time is observed for all excitonic lines. However, a distinct change in intensity ratio of I8 and I9 as compared to I1 and I2 is found in the td-spectra: I8 and I9 dominate the “in-pulse-spectrum” and exhibit a perfectly constant intensity relation during decay. In contrast, I1 and I2 are less intense than I8 and I9 in the in-pulse-spectrum and rapidly drop during the initial decay. For longer decay times, however, I1 and I2 catch up with I8 and I9 and even dominate after 1.9 ns. The free exciton XA and the excited state I6/8* disappear extremely fast after the in-pulse spectrum and already completely vanish in the spectrum after 0.9 ns. The neutral impurity bound excitons I8, and I9 exhibit a delayed, however, strictly mono-exponential decay over three orders of magnitude yielding to a life time of 310 ps (I8), 270 ps (I9), and 160 ps (I8*). The same is found for XA with a very short life time of τ = 90 ps. In complete contrast, the ionized impurity bound excitons I1 and I2 show a non-exponential decay starting with a very fast initial decay (140 ps / 160 ps) followed by a persistent slow stretched-exponential decay. The fast initial drop results from the carrier capture by the ionized impurities resulting in their neutralization and thus feeding the neutral impurity bound exciton channel. Consequently, the decays of I8 / I9 are delayed with respect to the initial fast I1 / I2 decay. This capture kinetics is also observed in the CL onset.


2:15 PM K6.3
Cathodoluminescence Study of Indented ZnO Crystals. J. Mass1,5, M. Avella1, Juan Jimenez1, T. Rodríguez2, M. Callahan3, E. Grant3, K. Rakes3, D. Bliss3 and B. Wang4; 1Dpto. Física de la Materia Condensada, Universidad de Valladolid, Valladolid, Spain; 2Dpto. Tecnología Electrónica, Universidad Politécnica de Madrid, Madrid, Spain; 3Sensors Directorate, Air Force Research Laboratory, Hanscom AFB, Massachusetts; 4Solid State Scientific Corporation, Hollis, New Hampshire; 5Dpto. Matemáticas y Física, UniNorte, Barranquilla, Colombia.

ZnO is a wide band gap semiconductor with high potential for UV optoelectronics applications due to its large free exciton binding energy (60 meV). The understanding of the mechanisms governing the luminescence emission is necessary to improve the emission properties of ZnO. The luminescence spectrum of ZnO consists of a complex spectrum in the near band gap (NBG) spectral region and a broad band composed of at least two subbands in the visible (green-orange) range. The origin of some of these bands remains to be a matter of controversy; in particular, the role of intrinsic defects is far to be understood. We present herein a cathodoluminescence study of ZnO crystals in which defects were created by Vickers indentations under different conditions. The influence of the indentations in the luminescence spectrum was studied by spectrally resolved cathodoluminescence, paying special emphasis to visible luminescence and the luminescence bands in the DAP spectral region. Thermal treatments in different atmospheres are carried out in order to study the evolution of the luminescence spectra and thepossible origin of the defects created by the indentations.


2:30 PM K6.4
Valence Band Photoemission Spectroscopy of ZnO and CdO.C. F. McConville1, T. D. Veal1, P. H. Jefferson1, L. F.J. Piper1, A. Schleife2, F. Fuchs2, J. Furthmüller2, F. Bechstedt2, J. Zúñiga-Pérez3 and V. Muñoz-Sanjosé3; 1Department of Physics, University of Warwick, Coventry, United Kingdom; 2Institut für Festkörpertheorie und Theoretische Optik, Friedrich-Schiller-Universität, Jena, Germany; 3Department Física Aplicada i Electromagnetisme, Universitat de València, València, Spain.

The rediscovery of electron accumulation at ZnO surfaces [1] following its previous observation as early as the 1960s [2] has resulted in renewed interest in the surface electronic properties of II-O compound semiconductors. Understanding the surface space-charge layers of ZnO and CdO as a function of surface preparation and bulk doping level requires the application of surface sensitive spectroscopies. Additionally, the proximity of the Zn 3d and Cd 4d electrons to the O 2p-dominated valence band presents a considerable challenge for theoretical band structure calculations to appropriately describe the resultant pd-repulsion [3]. Here, high-resolution valence band x-ray photoemission spectroscopy has been used to measure the surface Fermi level as a function of surface preparation and to accurately determine the binding energy of the Zn 3d and Cd 4d shallow semi-core-levels. The 500 nm-thick ZnO(11-20) and ZnO{0001} films used in this study were grown at 420°C by metal organic vapour phase epitaxy (MOVPE) on a- and c-plane sapphire substrates, respectively, using dimethylzinc-triethylamine and tertiary-butanol as Zn and O precursors. The 900 nm-thick CdO(001) films used were grown at 383°C on r-plane sapphire dimethylcadmium and tertiary-butanol as Cd and O precursors. For the atmospherically contaminated ZnO{0001} surface, the Fermi level was found to lie at 3.55 eV above the valence band maximum (VBM) and therefore above the conduction band minimum (CBM) (Eg = 3.37 eV). After the surface contamination was desorbed by in vacuum annealing for 12 hours at 450°C, the surface Fermi level shifted to 3.25 eV above the VBM and therefore below the CBM. This shift is discussed in terms of the changes to the surface as determined by core-level XPS and the position of the surface and bulk Fermi levels with respect to the universal charge neutrality level (3.04±0.21 eV above the VBM in ZnO [4]). Meanwhile the surface Fermi level of the clean CdO(001) surface lies at 1.4 eV above the VBM, suggesting that it lies above the CBM if the indirect band gap of CdO exceeds 0.9 eV as indicated by previous absorption spectroscopy [5]. The Zn 3d and Cd 4d shallow semi-core-levels are found to lie at 7.5 and 9.3 eV above their respective valence band maxima. [1] O. Schmidt, A. Geis, P. Kiesel, C.G. Van de Walle, N.M. Johnson, A. Bakin, A. Waag, and G.H. Dohler, Superlattices and Microstructures 39 (2006) 8. [2] G. Heiland and P. Kunstmann Surf. Sci. 13 (1969) 72. [3] A. Schleife, F. Fuchs, J. Furthmüller and F. Bechstedt, Phys. Rev. B, in press. [4] W. Monch, Appl. Phys. Lett. 86 (2005) 162101. [5] J. Kocka and C. Konak, Phys. Stat. Sol. (b) 43 (1971) 731


2:45 PM K6.5
Photoluminescence and Phonon Properties of ZnO and MgZnO Nanocrystallites. John L. Morrison1, Jesse Huso1, Heather Hoeck1, Erin Casey1, Xiang-Bai Chen1, Leah Bergman1 and Tsvetanka Zheleva2; 1Department of Physics, University of Idaho, Moscow, Idaho; 2Army Research Laboratory, Adelphi, Maryland.

ZnO and MgZnO alloys are promising wide-bandgap semiconductors for optoelectronic applications, and also of considerable interest from a fundamental viewpoint. The environmentally friendly chemical composition and the deep excitonic level ~ 60 meV of ZnO make it an excellent candidate for high-efficiency next generation ultraviolet light sources. Moreover, the MgZnO solid solution has been recently realized for thin films as well as for nanopowders. These optical alloys enable the tuning of the bandgap and the luminescence at the range of ~ 3.0 for ZnO of the wurtzite structure up to ~ 7 eV for the MgO of the rocksalt structure. We present studies on the ultraviolet photoluminescence and Raman properties of wurtzite MgZnO nanopowders of average size ~ 30 nm that were synthesized via the thermal decomposition method. For the studied composition range of 0-26% Mg, the room temperature UV-PL was found to be tuned by ~ 0.3 eV towards the UV-spectral range; the extent of the blueshift found to depend on the crystallite quality. For that composition range the first-order LO Raman mode was found to exhibit a significant blueshift of ~ 33 cm-1 and the second-order LO a shift of ~ 60 cm-1 indicating that a good solid solution was achieved at the nanoscale. Results and issues concerning the properties of the nanoalloys at above 26% of Mg concentrations; where structural phase transition takes place will be presented as well.


3:30 PM *K6.6
Piezoelectric and Luminescent Properties of ZnO Nanostructures on Ag Films.Julia W. P. Hsu1, David Scrymgeour1, David R. Tallant1, Nancy A. Missert1, David C. Look2, James A. Voigt1 and Jun Liu1; 1Sandia National Laboratories, Albuquerque, New Mexico; 2Wright State University, Dayton, Ohio.

In the past decade, significant advances have been made in the synthesis of ZnO nanostructures. The next step in making these nanomaterials useful is to assemble them on surfaces in a controlled and desired fashion. In this talk, I will discuss the growth of complex ZnO nanostructures via a solution method in which organic templates are used to control assembly of these nanostructures on substrate surfaces. The low temperature aqueous growth method used in this work is an environmentally benign process, which is compatible with organic templates and modifiers, can be used to grow large area uniformly, and has potential for inexpensive manufacturing. To control the assembly of these solution grown nanostructures, we modify the substrate surfaces with patterned self-assembled monolayers, which in turn determines the final spatial organization of the ZnO nanorods. This is a bottom-up approach in which materials are deposited only where they are needed. Using this approach, we have achieved excellent control in spatial placement, selectivity, crystal orientation, and nucleation density. In addition, complex, hierarchical structures have been synthesized by controlling solution chemistry and growth conditions. Due to lack of inversion symmetry in hexagonal crystal, ZnO is a piezoelectric material with Zn (0001) polar and O (0001-) polar surfaces exhibiting drastically different physical and chemical properties. Hence, it is important to determine the orientation of the ZnO nanorods on surfaces. Using piezoelectric force microscopy (PFM), we have determined that the nanorods are [0001] oriented. We also studied the luminescent properties of these solution-grown ZnO nanorods. As-grown nanorods displayed a broad yellow-orange sub-bandgap luminescence and a small near-bandgap emission peak. Scanning cathodoluminescence experiments showed that the width of the sub-bandgap luminescence is not due to an ensemble effect. Upon reduction, the sub-bandgap luminescence disappeared and the near-bandgap emission increased. Comparing to ZnO powders that are stoichiometric and oxygen deficient, we conclude that the yellow-orange sub-bandgap luminescence most likely arises from bulk defects that are associated with excess oxygen.


4:00 PM K6.7
Fabrication of ZnO Based Multifunctional Nanostructures. Shiva S Hullavarad, R. D Vispute, R. Heng and T. Venkatesan; Center for Superconductivity Research, University of Maryland, College Park, Maryland.

Recently growth of nanostructures has attracted tremendous research interest due to their nearly one dimensional structure possessing unique electrical, thermal, optical, and mechanical properties, which can be exploited in a variety of applications such as sensors, micro and nanoelectronics, and nano-electromechanical devices. Nanomaterials based on ZnO are extremely attractive due a direct optical band-gap (Eg = 3.37 eV) material with a large exciton binding energy (60 meV), exhibiting near UV emission, and tunable for transparent conductivity and piezoelectricity. The high exciton binding energy in ZnO crystal can ensure an efficient excitonic emission at room temperature under low excitation intensity. As was predicted, the observation of room-temperature UV lasing from the ordered, nano-sized ZnO crystals prospects an important step for the development of practical blue-UV laser using ZnO. We report the growth of ZnO nanowires (12-60 nm) and nanorods (500 nm) by a method of catalysis free vapor phase growth technique. The evolution of ZnO nanowires under supersaturation of Zn metal species and subsequent desorption is proposed. The nanowires were grown on c-Al2O3 and pulsed laser deposited ZnO nucleation layer on Al2O3 substrates at 800 °C without employing any metal catalysts that are conventionally used in MOCVD or Vapor-Liquid-Solid phase techniques. The nanowires are 30 nm in diameter and 5 micron in length with a clear hexagonal shape and clear [0001], [1011] and [1010] facets. The ZnO nanowires are found to emit UV light at 386 nm with considerably lower green band emission. The mechanism of formation of nanowire on the top of nanorods has been discussed. The stoichiometry of the ZnO nanostructures is also confirmed by XPS measurements. Exploiting geometrical features of the ZnO nanostructures, sensors and integrated devices integrated with optical lasers can be fabricated. Application of our approach combined with growth of variety of nano materials with controlled organization will be discussed


4:15 PM K6.8
Piezoelectric Nanogenerators Based on Zinc Oxide Nanowire Arrays.Jinhui Song and Zhong Lin Wang; Materials Science & Engineering, Georgia Institute of Technology, Atlanta, Georgia.

Developing novel technologies for wireless nanodevices and nanosystems are of critical importance for in-situ, real-time and implantable biosensing, biomedical monitoring and biodetection. An implanted wireless biosensor requires a power source, which may be provided directly or indirectly by charging of a battery. It is highly desired for wireless devices and even required for implanted biomedical devices to be self-powered without using battery. Therefore, it is essential to explore innovative nanotechnologies for converting mechanical energy (such as body movement, muscle stretching), vibration energy (such as acoustic/ultrasonic wave), and hydraulic energy (such as body fluid and blood flow) into electric energy that will be used to power nanodevices without using battery. It also has a huge impact to miniaturizing the size of the integrated nanosystems by reducing the size of the power generator and improving its efficiency and power density. We have demonstrated an innovative approach for converting nano-scale mechanical energy into electric energy by piezoelectric zinc oxide nanowire (NW) arrays [1, 2]. By deflecting the aligned NWs using a conductive atomic force microscopy (AFM) tip in contact mode, the energy that was first created by the deflection force and later converted into electricity by piezoelectric effect has been measured for demonstrating nano-scale power generator. The operation mechanism of the electric generator relies on the unique coupling of piezoelectric and semiconducting dual properties of ZnO as well as the elegant rectifying function of the Schottky barrier formed between the metal tip and the NW. The efficiency of the NW based piezo-electric power generator is ~ 17-30%. <br>[1] Z.L. Wang and J.H. Song, Science, 312 (2006) 242-246. <br>[2] http://www.nanoscience.gatech.edu/zlwang/


4:30 PM K6.9
Investigation of Optical and Electrical Properties of ZnO Nanowires Grown by Pulsed Laser Deposition.Marius Grundmann, Andreas Rahm, Thomas Nobis, Gregor Zimmermann, Christian Czekalla, Jörg Lenzner, Holger von Wenckstern and Michael Lorenz; Institut für Experimentelle Physik II, Semiconductor Physics, Universtät Leipzig, Leipzig, Germany.

ZnO nanowires are readily fabricated with a novel and unique, high-pressure pulsed laser deposition (HP-PLD) process developed recently by us. It allows the flexible and reproducible control of the morphology (prisms, neeedles), diameter (<50 nm up to >3 µm ), and composition (e.g. ZnO, MgZnO, doping) of the nanowires [1-3]. HP-PLD works successfully on various substrates in a wide range of growth temperatures. Regular arrays have been fabricated with appropriate templates. We found that vapor liquid solid growth is of importance at best only in the very first stage of sample growth. However, transmission electron microscopy and secondary electron microscopy investigations proof that this is not the major growth process, especially for later growth stages. Geometrical properties of the wire ensemble will be given and correlated with the growth conditions.The optical properties of our nanowires are superior compared to commercially available ZnO bulk material. We present linear and non-linear optical investigations of single wire photoluminescence near the band gap. The broad "green" emission from deep levels allows the investigation of resonant optical modes (whispering gallery modes) which are modelled numerically in detail [4] and found to agree with polarization-resolved experiments without adjustable parameter./br> Selected wires were contacted using a FEM-FIB. Ohmic contacts are realized using tungsten enabling investigations of the sample resistivity as a function of temperature. Schottky contacts are produced using palladium. With that, we are able to characterize deep levels in ZnO micro- and nanowires by thermal admittance and deep level transient spectroscopy./br>/br> This work is funded by the Deutsche Forschungsgemeinschaft (DFG) within the framework of FOR 522 and by the EU within STReP NANDOS and NoE SANDiE. [1] M. Lorenz et al., Ann. Phys. (Leipzig) 13 (2004) 39. /br> [2] Th. Nobis et al., Phys. Rev. Lett. 93 (2004) 103903./br> [3] M. Lorenz et al., Appl. Phys. Lett. 86 (2005) 143113. [4] Th. Nobis, M. Grundmann, Phys. Rev. A 72, 063806 (2005).BR>

4:45 PM K6.10
Nonlinear I-V Characteristics of ZnO Nanoparticle Compacts and Nanocomposites.Simone Herth1,2,3, Xiaoping Wang1,2, Teresa Hugener1,2, Henrik Hillborg4, Tommaso Auletta4, Linda S Schadler1,2 and Richard W Siegel1,2; 1Rensselaer Nanotechnology Center, Rensselaer Polytechnic Institute, Troy, New York; 2Materials Science and Engineering Department, Rensselaer Polytechnic Institute, Troy, New York; 3Faculty of Physics, University Bielefeld, Bielefeld, Germany; 4ABB Corporate Research, Västerås, Sweden.

Materials with nonlinear I - V characteristics are commonly used as field grading materials. In many cases, the non-linearity is achieved through the addition of equiaxed fillers to a polymer matrix. These composite field grading materials are optimized in terms of non-linearity, conductivity, and breakdown strength. One limitation in designing new field grading materials is a robust understanding of the relationship between powder morphology, composition and electrical characteristics of the powder, as well as a robust understanding of the relationship between powder conductivity and non-linearity and composite non-linearity. In this work, treatment of ZnO powder with a SnF2 solution resulted in a powder that yielded highly non-linear behavior. The highest non-linearity was achieved for powders with at least two different phases and a rough surface, as indicated by transmission electron micrographs. In contrast, the non-linearity of the nanocomposite conductivity is mainly determined by the conductivity of the nanofiller. The electrical behavior of the non-linear powder can be understood by a polarization of the nanoparticles at the interfaces, whereas the nonlinearity of the nanocomposites can be explained by a tunnelling mechanism between two particles.

SESSION K7: Poster Session I

Chairs: Jürgen Christen, Chennupati Jagadish, David Look and Takafumi Yao
Wednesday Evening, November 29, 2006
8:00 PM
Exhibition Hall D (Hynes)
K7.1
Vibrational Characterization of ZnO Nanostructures Revealing Phonon Confinement.Sanju Gupta1, Rusen Yang2 and Zhang L. Wang2; 1Physics and Materials Science, Missouri State University, Springfield, Missouri; 2Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia.

Zinc oxide (ZnO) and related compound semiconductors (CdO, MgO) have drawn interest in recent years due to their suitability for visible and UV light emitters and detectors as well as high-temperature electronics. ZnO, in particular, can be utilized for technology of photonic and electronic devices including light-emitting diodes, gas, chemical, and biosensors and now spintronics (if doped with magnetic ions dilutely). ZnO nanostructures in forms like nanobelts, nanorods, and nanotubes were prepared using thermal evaporation of oxide powders inside an alumina tube in the absence of catalysts. One of the key issues of phonon/lattice dynamics of nano- and micrometer-scale is the identification of the observed Raman active modes. It is because, due to the tilted orientation of smaller crystallites and/or nanostructures, the usual Raman selection rules pertaining to the symmetry axes or in general no longer hold, and it is likely that mixed-symmetry modes need to be considered to explain the phonon properties. In addition, nanostructures in general give rise to phonon confinement which induces phonon peak shifts and the main mechanism are: (a) spatial confinement within the boundaries; (b) phonon localization by defects (oxygen deficiency, zinc excess, surface impurities etc.); or (c) laser-induced heating in nanostructure ensembles (which can be minimized or eliminated by using very low powers of excitation source). Usually, only the first mechanism, referred to as optical phonon confinement, is involved as an explanation of the phonon frequency shifts in ZnO nanostructures. Effect of confinement is investigated on optical phonons of different symmetries using nonresonant Raman spectroscopy. An optical phonon confinement model is used to calculate theoretical line shapes which exhibit different asymmetric broadening and shifts, depending upon the symmetries of phonon. *Supported by internal Funds.


K7.2
Surface Luminescence of Zinc Oxide Excited by Recombination of Hydrogen Atoms. Vladimir Tyutyunnikov1, Michael Sushchikh3 and Vladislav Styrov1,2; 1Physics Department, Azov Sea State Technical University, Mariupol, Ukraine; 2School of Physics and Mathematics, Mariupol, Ukraine; 3Chemical Engineering, UCSB, Santa Barbara, California.

Excitation of a luminescence by highly exothermic chemical reaction on the surface of a luminophore provides unique opportunity to separate surface luminescence from the bulk one. This enables studies of the electronic properties of the semiconductor surfaces even for the surfaces of complicate shapes. We have studied surface luminescence of ZnO powders, films and single crystals excited by catalytic recombination of hydrogen atoms, i.e. heterogeneous chemiluminescence (HCL), and the HCL spectra were compared to the photoluminescence (PL) spectra. The HCL spectra were sensitive to the details of the sample preparation and treatment whereas PL spectra almost did not change. The HCL spectra of powdered samples exhibited long wavelength tail (up to 800 nm) and their maximum was blue-shifted by 5nm as compared with PL spectra. Different HCL bands forming long-wavelength tail were separated by changing the temperature of the samples and by varying the treatment conditions. Additional milling of ZnO led to amplification of the HCL-specific bands. Special pure grade ZnO showed neither PL nor HCL, however we were able to observe HCL bands with max at 610 nm and 730 nm after exposure the powder to H2+H atmosphere at 570K. Such treatment did not caused appearance of the PL. The HCL can be utilized for in situ monitoring of the growth and evolution of ZnO in controlled atmosphere including its nanoforms.


K7.3
ZnO and MgZnO Nanoalloys: Optical and Structural Properties.Heather Hoeck1, John L. Morrison1, Jesse Huso1, Erin Casey1, Xiang-Bai Chen1, Leah Bergman1, Slade J. Jokela2, Matthew D. McCluskey2 and Tsvetanka Zheleva3; 1Department of Physics, University of Idaho, Moscow, Idaho; 2Department of Physics, Washington State University, Pullman, Washington; 3Army Research Laboratory, Adelphi, Maryland.

ZnO and MgZnO alloys are promising wide-bandgap semiconductors for optoelectronic applications, and also of considerable interest from a fundamental viewpoint. The environmentally friendly chemical composition and the deep excitonic level ~ 60 meV of ZnO make it an excellent candidate for high-efficiency next generation ultraviolet light sources. These optical alloys enable the tuning of the bandgap and the luminescence at the range of ~ 3.0 eV for ZnO of the wurtzite structure up to ~ 7 eV for the MgO of the rocksalt structure. The optical properties of bulk and nanoscale ZnO at ambient conditions have been extensively investigated. In contrast, less is known about ZnO bulk properties under the influence of applied pressure and still less so for ZnO nanomaterials. We present studies on the ultraviolet photoluminescence and Raman properties of wurtzite MgZnO nanopowders of average size ~ 30 nm that were synthesized via the thermal decomposition method. For the studied composition range between 0 and 26% the room temperature UV-PL was found to be tuned by ~ 0.3 eV towards the UV-spectral range; the extent of the blueshift found to depend on the crystallite quality. For that composition range the first-order LO Raman mode was found to exhibit a significant blueshift of ~ 33 cm-1 and the second-order LO a shift of ~ 60 cm-1 . Additionally we present studies on the pressure response of the ZnO and MgZnO nanocrystallites. We found that up to 6 GPa the pressure coefficients of ZnO and MgZnO are 23 and 27 meV/GPa, respectively. The pressure coefficient of the ZnO nanocrystallites is similar to that reported elsewhere for bulk ZnO material. The higher value found for MgZnO is discussed in terms of the difference in the atomic numbers of the cation constituents.


K7.4
Sonochemical Synthesis of Transition Metal-doped ZnO Nanorod Arrays on the Substrate.Eugene Oh1, Seung-Ho Jung2, Kun-Hong Lee2, Soo-Hwan Jeong1, Moon-Hyung Lee1, Tae-Yong Kim1 and Yu-Sung Jin1; 1Chemical Engineering, Kyungpook National University, Daegu, South Korea; 2Chemical Engineering, Pohang University of Science and Technology, Pohang, South Korea.

A facile sonochemical route was demonstrated for the direct fabrication of iron doped ZnO nanorod arrays on the substrate under ambient conditions. X-ray diffraction and inductively coupled plasma atomic emission spectroscopy showed that 0.9 wt% iron doped ZnO nanorod arrays were vertically well-aligned along the [0001] direction on a Si wafer. Although we have shown the in situ doping of iron on ZnO nanorod arrays, it is expected that this sonochemical technique can be easily applied to the doping of other transition metals. This novel method is expected to have great potential for spintronics, such as diluted magnetic semiconductors.


K7.5
Erbium Doped ZnO Thin Films Synthesized using Glycerol as Chelating Agent in Modified Pechini Process.Uma Choppali and Brian P Gorman; Materials Science and Engineering, University of North Texas, Denton, Texas.

Erbium doped ZnO (ZnO: Er) is considered to be a suitable candidate to fabricate the current injection optical devices. However, enhancement of Er-related emission at room temperature has been an important issue. In this work, we want to study its effect on luminescence by increasing the doping concentration of Er. Although ZnO: Er thin films have been synthesized previously by pulsed laser deposition, we present low temperature processed erbium doped ZnO thin films from polymeric precursors. ZnO nanoparticles of varied Er doping concentration, derived from the prepared polymeric solution, has been spin-coated onto surface modified substrates and annealed at different temperatures. The effect of Er doping concentration on film grain size and strain was analyzed using X-ray diffraction. XRD data reveals that doping of Er ions reduces compressive strain considerably in the films. It is observed that the presence of larger Er cations in ZnO cause tensile stress which neutralizes the inherent compressive stress observed in undoped ZnO, significantly decreasing and hence making the films stress free. Crystallite size of Er doped ZnO thin films, annealed at 600οC, was calculated to be approximately 12 nm using Scherrer’s equation. The surface morphology of the thin films was characterized by SEM and AFM. Electrical properties of the annealed thin films were measured by four point probe method. Fluorescence of different concentration of ZnO:Er have been studied by NIR absorption.


K7.6
Bulk Acoustic Resonator Based on ZnO BeltsBrent Alan Buchine1, William Larry Hughes1, Fahrettin Levent Degertekin2 and Zhong Lin Wang1; 1Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia; 2Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia.

A bulk acoustic resonator based on one-dimensional ZnO belts is demonstrated [1]. This device shows a great deal of promise in applications as an electronic filter, as well as a highly sensitive mass sensor. While the materials were synthesized using a bottom-up approach, e-beam lithography and focused ion beam deposition were employed to manufacture other components of the resonator, like the support substrate, ground pads and contact electrodes. The fabricated device was characterized using vector network analysis and both the first and third harmonics of resonance were observed at approximately 247 MHz and 754 MHz respectively. In addition, a 1-dimensional Krimholt-Leedom-Matthaei model was utilized to predict the resonant frequency of the device and confirm the observed behavior. [1] B. A. Buchine, W. L. Hughes, F. L. Degertekin and Z. L. Wang, NanoLetters, 6 (2006) 1155 [2] for details visit: www.nanoscience.gatech.edu/zlwang


K7.7
Unambiguous Identification of the PL-I9-line in Zinc Oxide. Sven Müller1, Daniel Stichtenoth1, Michael Uhrmacher1, Hans Hofsäss1, Jens Röder2 and Carsten Ronning1; 1II. Institute of Physics, University of Göttingen, Göttingen, Germany; 2Institute of Physical and Theoretical Chemistry, Technical University of Braunschweig, Braunschweig, Germany.

The intense luminescence of zinc oxide (ZnO) is usually dominated by transitions of donor bound excitons, which are commonly labelled from I1 to I11. The identity of the respective causing donors is in the most cases unknown, but some of them were assigned to specific elements like hydrogen, aluminium, gallium, or indium; mainly based on doping studies during growth and/or ion implantation [1]. However, an unambiguous identification is in both cases difficult due to the accompanying co-doping effects during growth and creation of defects during ion implantation, respectively. A clear identification can be achieved using radioactive dopants, which undergo an element transition upon decay. Thus, luminescence lines, which vary their intensities with the specific half-life of the respective isotope with increasing measuring time, can be unambiguous assigned to specific elements. In recent literature such an experiment has been described for ZnO resulting into a clear assignment of the I1 & I8-line to Ga [2]. We have implanted radioactive 111In with an ion energy of 400 keV into commercial available ZnO single crystals. The isotope 111In decays into stable 111Cd with an half-time of 2.8 days; thus, an element transition from a donor to an isoelectronic element within the ZnO crystal occurs upon time. The samples were annealed at 700°C for 15 minutes under vacuum, and the annealing process was monitored by perturbed angular-γγ-spectroscopy (PAC). Here, a recovery of the lattice and no out-diffusion of the implanted 111In atoms were observed. The samples were finally characterised by photoluminescence spectroscopy over a time period of 3 weeks. The obtained results together with additional ion implantation studies with stable 115In and varying ion fluences will be discussed in respected to the existing literature in this presentation. [1] B.K. Meyer et al.; Physica Status Solidi B, 2004 , 241 , 231-260 [2] K. Johnston et al.; Physical Review B, 2006 , 73 , 165212


K7.8
Synthesis and Growth Mechanism of Zinc Oxide Multi-needles in Arc Discharge.Vladimir Pokropivny1,3 and Mais Kasumov2; 1Frantsevich Institute for Problems of Materials Science of NASU, Kiev, Ukraine; 2Vernadsky Institute of General and Inorganic Chemistry of NASU, Kiev, Ukraine; 3Institute of Physics of Tartu University, Tartu, Estonia.

Needles, tetraneedles, tubes, and other micro- and nano-structures on base of ZnO with length c.a. 1 mk and diameter c.a. hundreds of nanometers were synthesized using in first time an arc discharge technique. By picularity of arc synthesis is a wide variety of vapour products and growing particles. Mechanisms of their formation and growth were suggested. Sheets on deposition surface roll-up into rolls and tube which transofrm into needles. Tetraneedles crystallize and grow in vapour and deposit in kind of white snow.


K7.9
Growth Behaviors of ZnO Nanostructures on SrTiO3 Substrates.Dong-Wook Kim1, Heejun Jeong1, Heung-Suk Oh1, Seo-Hyung Chang2 and Young-Jun Chang2; 1Dept. of Applied Physics, Hanyang University, Ansan, Kyunggi-do, South Korea; 2Department of Physics, Seoul National University, Seoul, South Korea.

ZnO nanostructures were grown on (100)- and (110)-oriented SrTiO3 single crystal substrates by vapor phase deposition. When 2 nm thick Au films were used as catalyst layers, randomly oriented nanostructures were obtained on both the substrates. ZnO nanostructures were also grown on (11-20)- and (0001)-oriented ZnO thin films, which were deposited on (100)- and (110)-oriented SrTiO3 substrates, respectively. On the ZnO seed layers, growth behaviors of the nanostructures were largely different from those on the Au catalyst. Well-aligned ZnO nanowires were densely grown on the ZnO(0001)/SrTiO3(110) films, but tilted nanowires were sparsely distributed on the ZnO(11-20)/SrTiO3(100) films. These results showed that anisotropic growth nature of the ZnO nanostructures and crystal-orientation dependent catalytic activity of the ZnO thin films.


K7.10
Vertically Aligned Single Crystalline ZnO Nanorods Grown by Hydrothermal Synthesis and the Theoretical Model for Predicting the Dot Density.Soo Jin Chua1,3, Hong Quang Le3 and Kian Ping Loh2; 1Institute of Materials Research and Engineering, Singapore, Singapore; 2Department of Chemistry, National University of Singapore, Singapore, Singapore; 3Singapore-MIT Alliance, Singapore, Singapore.

Uniformly distributed ZnO nanorods with diameter 80-120 nm and 2μm long have been grown at low temperatures on GaN by a catalyst free and inexpensive aqueous solution method. The formation of the ZnO nanorods and the growth parameters are controlled by reactant concentration, temperature and pH. The experimental procedure for ZnO nanorod growth consists of dissolving Zn (CH3COO)2 .2H2O in deionized water at room temperature. Then, NH4OH was added into the solution to create the alkaline environment (pH ~10). The resulting suspension was transferred into a Teflon - lined stainless steel autoclave. Next, the GaN sample, after cleaning with deionized water, was dipped into the solution and suspended in the autoclave by a tantalum wire. Finally, the autoclave was sealed and put into the oven. The hydrothermal treatments were carried out at 100oC. After the growth, the autoclave was allowed to cool down naturally. The samples were taken out, washed in the deionized water several times. The XRD and TEM studies show that the ZnO nanorods are single crystals and they grow well-oriented, along the c axis of the crystal plane. The room temperature photoluminescence measurements have shown ultraviolet peaks at 388nm with high intensity, which are comparable to those found in high quality ZnO films. From the photoluminescence spectra measured in the temperature range 5 - 250K, the excitonic emission and its multiple phonon replicas have been observed and their origins have been identified. An unprecedented high degree of c-axis orientation is achieved in this hydrothermal epitaxial growth. The effect of growth conditions on the deposition of ZnO nanorods was systematically studied. We showed that by changing the molar ratio of the reactants, we can control the growth morphology of the ZnO nanorod arrays. The optimal conditions to produce high density single crystalline ZnO nanorods with c-axis orientation have been identified. The mechanism affecting ZnO nanorod epitaxial nucleation and growth on GaN is discussed. The ZnO nanorods exhibited ultraviolet photoluminescence (PL) at 388 nm and the PL shows a decay life time of 58 picoseconds. The mechanism of the nanorod growth in the aqueous solution is also proposed. An thermodynamic model is developed which predicts successfully the nucleation density of hydrothermally-grown zinc oxide nanorods on GaN epilayer substrates. The model parameters are extracted from experimental data. The ionic equilibrium of the precursor solution was calculated and factors affecting the nucleation density were identified. Finally the nucleation density was modeled as a function of degree of supersaturation, ratio of ammonia to zinc ions and temperature. An activation energy of 39.2 meV was found for this nucleation phenomenon. The model can be used to predict the nucleation density within the temperature range of 333K to 423K and degree of saturation 3 < S < 41 where the experiments were conducted.


K7.11
Study of the Temperature Dependence of E2 and A1(LO) Modes in ZnO. Esther Alarcon-Llado1, Ramon Cusco1, Jordi Ibanez1, Luis Artus1, Juan Jimenez2 and Michael Callahan3; 1Inst. Jaume Almera, C.S.I.C., Barcelona, Spain; 2Fisica Materia Condensada, Universidad Valladolid, Valladolid, Spain; 3Air Force Research Lab, Hanscom AFB, Massachusetts.

ZnO is a wide band-gap semiconductor with great potential as an alternative to GaN for optoelectronic devices in the blue and UV spectral region, since it has a much higher free exciton binding energy (60 meV), it is more resistant to radiation damage and large native substrates are available. The possibility of growing large crystals of ZnO has renewed the interest in this material. High-quality large ZnO single crystals can now be obtained by the hydrothermal (HTT) growth method. These crystals exhibit optical properties similar to those of crystals grown by vapor-phase transport (VPT). Raman scattering provides useful information about sample quality as well as phonon interaction with free carriers, both of which have an impact on device performance. Here, we present Raman spectra of a high-quality HTT ZnO single crystal obtained over a wide temperature range, from 80 up to 750 K. All Raman-active modes are identified in polarized first-order spectra and their temperature dependence is determined. Second-order spectra show a rich structure that is discussed in the light of recently published ab-initio calculations of the ZnO phonon dispersion. The temperature dependence of the intensity of the second-order Raman modes allows us to establish the assignment of peaks that had been previously attributed to different possible combinations. The low temperature spectra reveal additional peaks that we tentatively assign to local impurity modes. On the other hand, the optical phonon lifetimes are important for the performance of ultra-fast devices because of their role in the energy relaxation of excited carriers. Raman scattering provides a way to measure phonon lifetimes as well as an insight into phonon decay mechanisms. We have carried out high resolution Raman spectra of the E2 and A1(LO) modes as a function of temperature in the 80-750 K range. The analysis of the observed line shapes allowed us to determine the phonon lifetimes as well as to discuss possible phonon decay channels for these modes. The temperature dependence of the A1(LO) mode linewidth is well accounted for by third-order anharmonic decay into a TO and a LA phonon (Ridley's channel). We find an A1(LO) lifetime of 0.5 ps at room temperature, which is one order of magnitude lower than the one reported for GaN. This value suggests that hot phonon effects should be expected to play a less relevant role in carrier relaxation in ZnO as compared to GaN.


K7.12
In situ Electrochemical Deposition and Lithography of ZnO nanocrystalsNicholas Polomoff and Bryan D Huey; Institute of Materials Science, University of Connecticut, Storrs, Connecticut.

Thin films of polycrystalline ZnO nanocrystals have been deposited electrochemically. Deposition is possible near pH=7, therefore allowing in-situ observation of film evolution and growth in a liquid-cell equipped atomic force microscope. Through careful control of the electrochemical parameters and ionic concentrations, film growth rates on the order of 30nm per hour and higher have been achieved. The morphology of films grown with a variety of conditions have further been imaged by AFM and field emission SEM, and their piezoelectric properties have been characterized using piezo force microscopy. Finally, using a conducting AFM tip as the working electrode, individual ZnO islands can be lithographically deposited according to the tip position, contact force, dwell time, and electrochemical parameters, allowing a variety of nanoscale oxide features to be prepared.


K7.13
Growth And Interface Microstructure Of Zinc Oxide Thin Film On Elinvar (Fe-Ni-Cr-Ti) Alloy By Radio Frequency Sputtering.Yukio Yoshino1 and Akira Saito2; 1R&D Division, Murata Mfg. Co., Ltd, Yasu, Shiga, Japan; 2Materials Research Division, Murata Mfg. Co., Ltd., Yasu, Shiga, Japan.

Zinc Oxide (ZnO) thin film has been grown on ELINVAR (Fe-Ni-Cr-Ti alloy) substrate by RF magnetron sputtering. ZnO thin film has piezoelectricity. ELINVAR is a good elastic material. The temperature coefficient of sound velocity in ELINVAR can be controlled by the heat annealing temperature, so the sound velocity depends on Young’s modulus that is changed by ferromagnetic transition of ELINVAR [1]. We can fabricate piezoelectric devices to combine ZnO thin film and ELINVAR if we make a ZnO thin film which had good piezoelectricity on ELINVAR substrate [2]. The ZnO thin film on ELINVAR shows c-axis orientation confirmed by x-ray diffraction. The ZnO/ELINVAR interface has been observed by high resolution cross section transmission electron microscope. A polycrystalline layer grows on the ELINVAR substrate surface, and a c-axis oriented layer grows on the polycrystalline layer. This result is similar to ZnO thin film grown on glass substrate [3]. However, the interface microstructure between ZnO and ELINVAR is complicated. The c-axis orientation of ZnO is disordered near the ZnO/ELINVAR interface. Seeing the interface microstructure of ZnO on ELIVBAR, these results are caused by the rough polished surface of the ELINVAR substrates. The c-axis orientation of the ZnO thin film on ELINVAR is strongly dependent on the surface roughness of the substrate. [1] The Japan Institute of Metals, Metals data book, 1971, p.1050[in Japanese]. [2] Y. Yoshino, M. Takeuchi, K. Inoue, T. Makino, S. Arai, T. Hata, Vacuum, 2002, Vol.66, p.467. [3] Y. Yoshino, S. Iwasa, H. Aoki, Y. Deguchi, Y. Yamamoto, K. Ohwada, 1996 Fall Meeting Proceedings, Mater. Res. Soc.1997, Vol. 441,p.241.


K7.14
ZnO-Sn bilayer Ultraviolet (UV) Photon Detector with Improved Responsitivity.Harish Kumar Yadav, Kondepudy Sreenivas and Vinay Gupta; Physics and Astrophysics, Delhi University, Delhi, Delhi, India.

Owing to the wide band gap (Eg = 3.3 eV) and short carrier life time, ZnO is an ideal candidate for high-speed ultraviolet (UV) photon detector. Its high radiation hardness renders it to use even under harsh environment. For a long time photoresponse from ZnO thin films and single crystals has generated a lot of interest for selective (~ 375 nm) UV light detection. The observed photoresponse was attributed to adsorption and desorption of the oxygen neutrals at the grain boundaries. The presence of large number of surface states and defect levels at the surface and inside the material strongly influence the dynamics of the charge carriers and the photoconductive response transients depend crucially on the density of these levels. Passivation of surface states by surface treatment is reported to affect the photoconductive response transients. However no attempt has been made to study the bilayer UV photo detector where the presence of surface states on the surface of ZnO film is expected to be influence to a great extent by the transfer of charge carriers across the interface. In the present study a novel ZnO-Sn bilayer ultraviolet (UV) photo detector has been fabricated using rf magnetron sputtering. An enhanced UV photoresponse is observed in the bilayer structure with responsivity of the order of 8.5 KV/W at a low UV intensity of 140 μW/cm2 (λ= 365 nm). The detector is found to exhibit a fast rise and fall time of around 100 ms. The surface states present on the surface of ZnO thin film is seem to be compensated by the transfer of electron from tin metal layer towards the interface, and resulting in an increase in the dark current and thereby yield an enhanced UV photo conductivity.


K7.15
Two Dimensional Well Organized ZnO Nanowire Arrays for Photonic Applications.Jingbiao Cui and Ursula Gibson; Dartmouth College, Hanover, New Hampshire.

Recently there has been a great interest in the investigation of ZnO nanowire arrays due to their potential applications in electronics, photonics and sensors. Dense randomly distributed ZnO nanowire arrays have been obtained using both catalytically activated vapor phase processes at high temperature and solution routes at low temperature. To date, however, it's still challenge to fabricate large scale well organized ZnO nanowire arrays for many applications such as photonic devices and displays, which require the ability to precisely control the placement of each individual nanowires. In this study, we developed a new technique to grow well organized ZnO nanowire arrays using a soft template created by e-beam lithography. The nanowires were grown in an aqueous solution at 90 degree using an electrochemical process, which ensures the suitability of using Polymethyl Methacrylate (PMMA) as template materials. We demonstrate that individual single crystal ZnO nanowires with diameter about 100 nm can be precisely placed in the desired location to form two dimensional ordered nanowire arrays. The structure, composition and optical diffraction of the ordered arrays were investigated. This approach provides a possibility to design and fabricate complicated structures of two dimensional ZnO nanowire arrays with variable periodicity. Importantly, the fabrication process is compatible with current micro-fabrication technique and promising for the fabrication of photonic and optoelectronic devices.


K7.16
Electrical Characterization of Isotype n-ZnO/n-GaN Heterostructures. Yahya Alivov1, Bo Xiao1, Sena Akarca-Biyikli1, Qian Fan1, Daniel Johnstone2, Cole Litton3 and Hadis Morkoc1; 1VCU, Richmond, Virginia; 2SEMETROL, Chesterfield, Virginia; 3Air Force Research Laboratory, Dayton, Ohio.

Devices capable of functioning in harsh environments are desirable. From this point of view ZnO, a wide band gap semiconductor (Eg=3.3 eV, T=300 K), is a very promising material due to e.g. large exciton binding energy (~60 meV), radiation hardness, availability of bulk crystal and ease of ZnO film growth. Although p-ZnO growth technology is still a hotly debated issue, development of ZnO heterojunction based devices is of interest nonetheless because such devices have advantages over the former owing to carrier and optical confinement, lower diffraction losses, and reduction of threshold current in injection devices. A number of reports available on the anysotype ZnO heterostructures composed of n-type ZnO films on different p-type materials. And n-ZnO/p-GaN heterostructures are of particular interest because ZnO and GaN have relatively close lattice parameters and physical properties. To study the properties of isotype (in terms of conductivity) n-ZnO/n-GaN type heterostructures is also important because, in particular, isotype heterostructures can also have strong diode-like rectifying behavior comparable to that of anysotype p-n heterjunctions; transparent highly conductive ZnO layers are very good ohmic contacts to GaN; n-ZnO/n-GaN is the part of p-GaN/n-ZnO/n-GaN double heterostructure which may potentially lead to higher efficiency optoelectronic devices. In the vein, understanding the current transport mechanisms of n-ZnO/n-GaN heterostructures is very important. Here we report on the electrical properties of n-ZnO/n-GaN heterostructures. The structure were fabricated by radio-frequency sputtering of 0.3 μm thickness ZnO layers on n-type GaN grown by metal-organic chemical vapor deposition on sapphire. 250 μm diameter size mesa structures were fabricated using conventional photolithography. Ohmic contacts to ZnO and GaN were formed using Au/Al (300/300 A) and Au/Al (300/300 A). Current-voltage (I-V) characteristics revealed well rectifying behavior of the structures with forward and reverse currents 10-6 A and 10-3 A, respectively, at 8 V bias. Using temperature dependent current-voltage (I-V-T) measurements, deep level transient spectroscopy (DLTS) methods, the current transport properties, defect structure, and kinetics of the n-ZnO/n-GaN heterostructures were studied, and the results will be reported in this presentation.


K7.17
Defect Engineering in ZnO. Victoria Anne Coleman1, Hark Hoe Tan1, Jodie E Bradby1, Manuela Buda1, C. Jagadish1, Sergei O Kucheyev2, Jin Zou3 and Matthew R Phillips4; 1Research School of Physical Sciences and Engineering, Australian National University, Canberra, Australian Capital Territory, Australia; 2Lawrence Livermore National Laboratory, Livermore, California; 3School of Engineering, Division of Materials, University of Queensland, Brisbane, Queensland, Australia; 4Microstructural Analysis Unit, University of Technology, Sydney, Sydney, New South Wales, Australia.

Recently, the wide band-gap semiconductor zinc oxide (ZnO) has emerged as a potential material for the fabrication of a range of devices including optoelectronic devices such as light-emitting diodes, laser diodes and detectors. The relative infancy of the field means that there is still much to be explored and understood about ZnO before such devices can be commercially realised. p-type doping of ZnO remains an issue, as does the origin of the native n-type conductivity. Band-gap engineering of ZnO-based systems will also be important for wavelength tuning and device integration, and will need to be addressed. Additionally, the response of ZnO to contact-induced damage which will invariably occur during device processing also needs to be investigated. These issues all require a thorough understanding of the defect structure of ZnO. In this work, these avenues have been investigated, and broad conclusions for the processing and characterisation of ZnO-based devices have been drawn. This has been achieved by focusing on the following: (i) the response of ZnO to contact-induced damage and how crystal orientation and growth influence the mechanical properties of this material; (ii) investigating the thermal stability of heavily damaged ZnO as applicable to selective area doping by ion implantation and (iii) exploring band-gap engineering in ZnO/ZnMgO multiple quantum wells by quantum well intermixing. These aims have been achieved through a number of processes including nanoindentation, ion-implantation and sulphurisation and characterised using a range of techniques including cathodoluminescence (CL) spectroscopy and monochromatic imaging, Rutherford backscattering spectrometry/channelling (RBS/C), atomic force microscopy (AFM), cross-sectional transmission electron microscopy (XTEM), optical microscopy, and current-voltage (I-V) and capacitance-voltage (C-V) profiling. The results of this study lead to a further understanding of the necessary sensitivities that need to be considered when undertaking ZnO-based device fabrication by defect engineering. Work at LLNL was performed under the auspices of the U.S. DOE by University of California, LLNL under Contract W-7405-Eng-48. This research is supported by the Australian Research Council.


K7.18
Recombination Mechanism of Green Emission: Study of ZnO Nanocrystals.Yinyan Gong1, Tamar Andelman1, Gertrude F. Neumark1, Stephen O'Brien1 and Igor L. Kuskovksy2; 1Department of Applied Physics and Applied Mathematics, Columbia Unversity, New York, New York; 2Department of Physics, Queens College of CUNY, New York, New York.

ZnO has long been regarded as a promising candidate for optoelectronics owing to its direct, wide, band gap and to its high exciton binding energy. It is known that the photoluminescence of most ZnO samples consist of a near band edge UV emission and a defect-related green emission. For many device applications, it is important to suppress the green emission, whose origin is still under debate and no consensus has been reached. Here we present results of photoluminescence (PL) and optical absorption measurements for ZnO nanocrystals of various morphologies as well as spherical nanoparticles of comparable size but with differently modified surface. All samples were prepared by a simple solution method [1, 2]. For optical characterization the nanocrystals were removed from their growth solution and redispersed in hexane. We found that although the PL from all our samples is dominated by the UV emission, the green emission is present most of the time, but it strongly depends on morphologies and sizes of the nanocrystals. Furthermore, for spherical nanoparticles we found that the green emission is largely suppressed by modifying surface states, for instance, by adding a small amount of trioctylamine or trioctylphosphine oxide to the original solvent. Based on the present study, as well as our previous results [2, 3], we suggest that the green emission is mainly due to radiative recombination between deep levels, formed by oxygen vacancies located on the surface, and free holes. Finally, we use the green luminescence to estimate the sizes of optically active spherical nanoparticles. The results are in excellent agreement with those of transmission electron microscopy. The proposed approach, estimating size based on green emission, is seemingly better than the use of excitonic emission, since it does not require the knowledge of the exciton binding energy, which depends on the degree of confinement in the nanoparticles. 1. M. Yin, et al., J. Am Chem. Soc. 126, 6206 (2004). 2. T. Andelman, et al. J. Phys. Chem. B 109, 14314 (2005). 3. Gu et al., Appl. Phys. Lett. 85, 3833 (2004).


K7.19
Acceptors in ZnO Studied by Photoluminescence.Michael A. Reshchikov1, Hadis Morkoc2,1, Bill Nemeth3 and Jeff Nause3; 1Physics Department, Virginia Commonwealth University, Richmond, Virginia; 2Electrical Engineering Dept., Virginia Commonwealth University, Richmond, Virginia; 3Cermet, Inc., Atlanta, Georgia.

Due to unique optical properties, ZnO is considered as an alternative to GaN in optoelectronic devices. While structural quality of bulk and epitaxial ZnO is commonly very good, uncontrolled point defects can notably affect the electrical and optical properties of the material and degrade the performance and reliability of devices made based on ZnO. Shallow donors in ZnO are already well studied through their manifestation in excitonic emission, whereas acceptors, including dopants and unintentionally introduced defects, are barely understood in this semiconductor. In undoped ZnO a green or yellow broad band often dominates in the photoluminescence (PL) spectrum in the visible range. The nature of the green band, appearing at about 2.5 eV in undoped ZnO, has remained controversial for decades. It is possible that two or more defects are responsible for the PL bands with similar shapes and positions. PL bands peaking in the photon range from 2.0 to 2.4 eV were also reported in literature, yet not studied in detail. We studied high-quality bulk ZnO crystals by steady-state and time-resolved PL. Several PL bands have been detected and studied in detail. These PL bands have been attributed to different acceptors unintentionally introduced in ZnO during growth. Characteristics of the acceptors and their concentrations are estimated from analysis of temperature and excitation intensity dependences of PL.


K7.20
Controllable Synthesis of ZnO nanorod Arrays via Simple Solution-Based Method.Patcharee Charoensirithavorn and Susumu Yoshikawa; Institute of Advance Energy, Kyoto University, Uji, Kyoto, Japan.

Recently, nanoscale one-dimensional (1-D) semiconductor materials have drawn a considerable amount of attentions owing to their unique properties and potentials in fabricating nanoscale optoelectronic devices [1]. Among them, ZnO semiconductor, with a wide band gap (3.2 eV) and large exciton binding energy of 60 meV at room temperature, is one of the materials gained the most intensive studies as it is a versatile material and has been considerably used for its catalytic, electrical, optoelectronic, and photochemical properties. Various synthesis methods have been developed for 1-D ZnO nanomaterial growth, including vapor-liquid-solid (VLS) growth, chemical vapor deposition (CVD), hydrothermal process and template-based methods. However, these methods involve complex procedures, sophisticated equipment and high temperature (≥890 °C [2] for VLS and 500 °C for CVD methods [3-4]). Here we present a solution-based method, which can easily fabricate highly oriented ZnO nanorods on substrate at relatively low temperatures. The as-synthesized products have been characterized by scanning electron microscopy (SEM). The results reveal that a densely packed and perpendicularly oriented single-crystalline ZnO nanorod arrays grew vertically on the fluorine-doped SnO2 transparent conducting oxide (FTO) glass substrates with narrow size distribution of average diameters and lengths up to 15 mm. An XRD result indicated that prepared ZnO consisted of wurtzite structured with high crystallinity. In addition, we found that the length of the nanorod could be freely modified by controlling the solution conditions, such as reaction temperature, reaction time, and precursor concentration. The obtained ZnO nanorod array with ordered and strongly interconnected nanoscale architecture offers the potential for improved electron transport leading to higher photoefficiencies[5]. Several nanotubular architectures have been investigated for potential enhancement of electron percolation pathways and light conversion as well as improved ion diffusion at the semiconductor-electrolyte interface. This controlled synthesis approach may open up the opportunities for fabricating ZnO nanorods on various substrates and extend the realm of ZnO based nanoscale device. Moreover, the scalability of this method has high potential to scale-up for providing a low-cost and large scale manufacturing of functional nanomaterials. References: [1] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001), 1947. [2] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo and P. Yang, Science 292 (2001), 1899. [3] J.J. Wu and S.C. Liu, Adv. Mater. 14 (2002), 215. [4] Y.C. Kong, D.P. Yu, B. Zhang, W. Fang and S.Q. Feng, Appl. Phys. Lett. 78 (2001), 407. [5] G. K. Mor, K. Shankar, M. Paulose, O. K. Varghese, C. A. Grimes, Nano Lett.(2005).


K7.21
Selective Growth of Zn- and O-polar ZnO Films by P-MBE for Fabrication of Periodically Polarity-inverted Photonic Crystals.Tsutomu Minegishi1, Takashi Hanada2, Rafal Boze3, Seunghwan Park1, Jinsub Park1, Kazushi Sumitani4, Osami Sakata4, Katsushi Fujii1, Meoungwhan Cho2 and Takafumi Yao1,2; 1Center for Interdisciplinary Research(CIR), Tohoku University, Sendai, Miyagi, Japan; 2Institute for Materials Research(IMR), Tohoku University, Sendai, Miyagi, Japan; 3Institute of Experimental Physics, Warsaw University, Warsaw, Poland; 4Japan Synchritrin Radiation Research Institute(JASRI), Sayo-gun, Hyogo, Japan.

ZnO has crystal polarity, ie Zn-polar and O-polar crystallographic directions. Crystal polarity produces many interesting properties, including piezoelectric field, spontaneous polarization field, difference in impurity incorporation during growth, and optical anisotropy. To exploit these attractive properties for real devices, the control of crystal polarity of ZnO films is prerequisite. Especially, two-dimensional nonlinear photonic crystals (NLPCs) require an in-plane array of periodically polarity inverted (PPI) structures. This paper will present a preliminary step towards the fabrication of 2D PCs of ZnO based on selective growth of Zn- and O-polar ZnO films. We have already reported the selective growth of Zn- and O-polar ZnO films on c-Al2O3 substrates varying the thickness of MgO buffer although the interface structure has not been fully elucidated yet. Detailed investigation on the interface of ZnO/MgO/c-Al2O3 by grazing incident X-ray diffraction using synchrotron radiation reveals that (1) MgO buffer thinner than 2.4nm has wurtzite (WZ) structure and ZnO grown on WZ-MgO buffer results in O-polar ZnO and that (2) MgO buffer thicker than 3nm has rocksalt (RS) structure and ZnO grown on RS-MgO buffer results in Zn-polar ZnO. It is also found that MgO buffer grown at substrate temperatures higher than 700oC results in the formation of Al2MgO4 at the MgO/c-Al2O3 interface and that O-polar ZnO grows on the Al2MgO4 layer without forming rotational domains. Based on the above findings, we propose the following process for the fabrication of 2D PPI PCs: 1) growth of templates consisting of RS-MgO buffer thicker than 3 nm on c-Al2O3 substrates followed by deposition of 5nm-thick ZnO cap layers; 2) patterning of the MgO buffer by lithography and chemical etching to produce layer-thickness modulated MgO buffer with un-etched part being thicker than 3nm and etched part thinner than 2.4 nm; 3) deposition of ZnO layers on the layer-thickness modulated MgO buffer which should yield 2D PPI PCs. Structural properties of the 1DPPI PCs were characterized using x-ray diffraction (XRD), revealing that ZnO layers have no rotational domains. The crystal polarity of the 1D PPI PCs is determined using scanning probe microscopy (SPM): Kelvin force microscopy (KFM) and piezo response microscopy (PRM). Both KFM and PRM are capable of determining crystal polarity based on the clear difference in surface potential and piezo response respectively. As a final check that the 1D PPI PCs act as nonlinear optical materials, we have performed second harmonic generation experiments. The examined 1D PPI PCs consists of 30 μm wide Zn-polar stripes and 30μm wide O-polar stripes. Although the quasi-phase matching condition is not fully satisfied at the excitation wavelength of 1.06 μm, we have observed clear difference in SHG efficiency between the two incidence directions of parallel and perpendicular to the stripes.


K7.22
Controlled Growth of ZnO micro- and Nanorod Arrays by the Wet Chemical Method.Yong-Jin Kim1,2, Chul-Ho Lee1,3, Young Joon Hong1,3 and Gyu-Chul Yi1,2,3; 1National CRI Center for semiconductor nanorods, Pohang, Gyeongbuk, South Korea; 2Environmental Science and Engineering, POSTECH, Pohang, Gyeongbuk, South Korea; 3Materials Science and Engineering, POSTECH, Pohang, Gyeongbuk, South Korea.

There has recently been much interest in the preparation and processing of semiconductor microstructures and nanostructures on Si substrates for high-temperature electronic devices and Si-based photonic devices. Submicron patterning of epitaxial films on Si substrates has been developed for device fabrications using plasma-based reactive ion etching (RIE). However, RIE often results in surface damage as well as contamination from the cathode materials. Furthermore, it is still very difficult to obtain high quality single crystalline semiconductor films on Si substrates because of large mismatches in lattice constants and thermal expansion coefficients and different chemical properties between Si substrates and semiconductor films. For example, ZnO films are polycrystalline if they are grown on Si substrates because Si surfaces are easily oxidized to form an amorphous silicon oxide layer during the film growth processes. The difficulties in preparation of semiconductor nanostructures and microstructures on Si substrates may be solved using the bottom-up approach to utilize methods of self-organization on molecular and nanocrystalline levels. Here, we report on the selective growth of ZnO micro- and nanorod arrays through the wet chemical method. Compared to previous methods including a metal catalyst assisted vapor-liquid-solid method, the wet chemical process has several advantages such as relative low growth temperature as well as large area growth, no need for use of metal catalysts and utilization of various substrates including glass and polymer. Our approach for fabricating ZnO nanorod and microrod arrays on specific positions is to grow selectively on pre-patterned substrates using a wet chemical solution method. In this solution method, the nanorod and microrod growth temperatures were so low that the organic mask layer rarely used in other methods can be employed for selective growth. Furthermore, structural and optical characteristics of ZnO nanorods will be presented.


K7.23
Photoluminescence of MgxZn1-xO/ZnO Quantum Wells Grown by Pulsed Laser Deposition. Susanne Heitsch, Gregor Zimmermann, Jörg Lenzner, Holger Hochmuth, Gabriele Benndorf, Michael Lorenz and Marius Grundmann; Institut fuer experimentelle Physik II, Universitaet Leipzig, Leipzig, Germany.

MgxZn1−xO/ZnO/MgxZn1−xO double heterostructures (DHS) have been grown on a-plane sapphire substrates with ~ 100 nm ZnO buffer layers by pulsed laser deposition (PLD). The nominal ZnO well layer thicknesses lie between 25 nm and 3 nm. Atomic force microscopy (AFM) investigations at ZnO/ MgxZn1−xO heterostructures corresponding to the DHS without the capping MgxZn1−xO barrier layer show the film-like structure of the ZnO layers. Their surface root mean square roughness of less than 0.5 nm gives information about the interface roughness in the DHS. AFM results of the MgxZn1−xO barrier layer show the same surface structure and roughness. We confirmed the lateral homogeneity of the MgxZn1−xO barrier luminescence by scanning cathodoluminescence measurements performed on MgxZn1−xO thin films. The inhomogeneous spectral broadening of the barrier luminescence is limited by alloy broadening. The DHS show a bright quantum well-related photoluminescence, suggesting good crystalline quality of the ZnO wells. A slight shift to higher energies with decreasing well width is observed down to nominally 6 nm. In the 3 nm quantum well (QW) the luminescence is shifted to higher energies by 69 meV compared to the free exciton emission in ZnO, which is a result of quantum confinement.


K7.24
Raman Scattering Characterization of Implanted ZnO. Esther Alarcon-Llado1, Ramon Cusco1, Luis Artus1, German Gonzalez-Diaz2, Ignacio Martil2, Juan Jimenez3 and Michael Callahan4; 1Inst. Jaume Almera, C.S.I.C., Barcelona, Spain; 2Fisica Aplicada III, Univ. Complutense, Madrid, Spain; 3Física Materia Condensada, Univ. Valladolid, Valladolid, Spain; 4Air Force Research Lab, Hanscom AFB, Massachusetts.

ZnO is a wide-bandgap semiconductor of great interest for integrated optoelectronic devices operating in the ultraviolet and blue spectral range because of its electronic and piezoelectric properties. One of the main challenges for the development of ZnO based devices is the difficulty in obtaining reproducible p-type doping. Since nitrogen and phosphorus are among prospective acceptors for ZnO, attempts to obtain p-type ZnO by implanting nitrogen or phosphorous ions have been undertaken. However, p-type conductivity after a subsequent annealing has not been demonstrated so far. Raman scattering is a powerful non destructive technique to assess the lattice damage caused by the implantation process as well as the lattice recovery achieved by a subsequent annealing process. The observation of some additional modes in Raman scattering experiments performed on implanted and subsequently annealed ZnO samples is at present a controversial issue. Thus, whereas some authors assign these additional modes to local modes related to the doping ions (which could then be used as an indicator of the activation degree), others assign them to disorder activated modes of a not fully recovered ZnO lattice. In the present study, we have performed Raman scattering experiments on ZnO samples implanted with different ions to assess the damage caused to the ZnO lattice and the lattice recovery after subsequent annealing, as well as to find out in which cases the additional modes are observed in the implanted samples. Double implantations of both N+ and P+ were performed at different energies and doses to achieve a doping profile with a flat homogeneous region and doping densities around 2x1019 cm-3. To investigate specific effects on the Raman spectra of implanting these acceptor ions, we have also studied ZnO samples implanted under similar conditions with Zn+ and O+ ions, the natural constituents of the ZnO lattice. The implanted samples were annealed in a rapid thermal annealing (RTA) system at 950°C for 10s under an O2 flux. Raman scattering measurements of the implanted samples were carried out before and after RTA. The line shapes of the E2 and A1(LO) modes of the ZnO as well as the appearance of disorder activated modes at frequencies with high density of phonon states indicate that lattice damage is considerably higher for Zn+ implantation. Disorder activated features present in the Raman spectra of the implanted samples are fully removed by RTA, indicating a high degree of lattice recovery by RTA even for the Zn+ implanted samples. The additional modes can be observed in samples analyzed before the RTA process and the intensity of these modes decreases after annealing, but their observation depends on the implanted species.


K7.25
Morphology and Electronic Structure of MOMBE Grown Epitaxial ZnO Surfaces.Christian Pettenkofer and Stefan Andres; SE6, HMI, Berlin, Germany.

We will report on detailed investigations on various ZnO surfaces with respect to the electronic structure and its implications on the electric properties of ZnO films. It is known that ZnO is very sensitive on irradiation with UV photons with respect to the interface energetics (band bending, interface dipols and doping). In particular for photoemission experiments time (and irradiation) dependent line shifts may be observed which are influenced by the hydroxide concentration in the film and at the surface. ZnO is deposited by MOMBE and magnetron sputtering in an UHV system on various substrates such as Si(111), Al2O3, and ZnO single crystals at tempratures between 200 and 500°C. The morphology of magnetron sputtered films depends on the deposition conditions ( Plasma-bias potential, oxygen partial pressure). In contrast to the complicated interface strucure for magnetron sputtered films on Si the interfaces of MOMBE grown ZnO are abrupt and show at the interface no intermixed phases in considerable amounts. Epitaxial films with orientation (11-20) are deposited on Al2O3 r-plane and are investigated in situ by LEED, PES and STM. For deposition with the precursor system DEZ/H2O significantly lower OH- admixtuires were found in the O1s region by MXPS in comparison to magnetron sputtered films. To model the surface reaction for the DEZ/H2O process adsorption experiments of the precursors water and DEZ (Diethylzinc) are presented. The surface electronic struture of the epitaxial films is investigated by ARUPS and will be compared to spectra obtained from ZnO single crystal surfaces. For ZnO films on Al2O3 the electrical properties (doping and resistivity) are determined ex situ and are compared to the data on the energetics obtained from the valence band spectra.


K7.26
Atomic Layer Deposition of ZnO for Display Applications.Eun Ho Kim, Chang Yeon Kim, Doo Hyeong Lee, Bo Hyun Chung, Jae Ha Jeon, Hee Soo Kim, Jun Won Hyun, Yongmin Kim and Seung Jeong Noh; Applied Physics, Dankook University, Seoul, South Korea.

ZnO has wurtzite structure exhibiting very excellent piezoelectric and optical properties. ZnO has rapidly emerged as a promising optoelectronical material due to its large band gap of 3.37 eV, low power threshold for optical pumping at room temperature, and highly efficient UV emission of a large exciton binding energy of 60 meV. Various deposition techniques, such as sputtering, pulsed laser deposition, chemical vapor deposition, spray pyrolysis, and molecular beam epitaxy have been widely employed for the growth of ZnO films. However, these processes have drawbacks of rough morphology, poor step coverage, etc. In atomic layer deposition(ALD), the deposited film has excellent conformality and uniformity down to an atomic scale thickness since surface reactions of metal precursor and reactant gas are complementary and self-limiting. In addition, ALD provides high quality films especially at low temperature. Thus, we have fabricated ZnO and ZnO:N thin films by ALD using diethylzinc(DEZn) and H2O for ZnO deposition and DEZn, H2O and N2 for ZnO:N deposition. Self-limiting growth was observed at substrate temperatures from 105 to 165 °C, in the different flow rates of DEZn, H2O and N2. The crystal growth orientation was investigated by x-ray diffraction and the characteristics were by Hall measurement, photoluminescence measurement, etc. This work was supported by Seoul R & BD Program (10555).


K7.27
ZnMgO UV Photodetectors Fabricated By A Novel Method; Linear Source Mist CVD.Yudai Kamada1, Tosiyuki Kawaharamura1, Hiroyuki Nishinaka1 and Shizuo Fujita2; 1Department of Electronic Engineering and Science, Kyoto University, Kyoto, Japan; 2International Innovation Center, Kyoto University, Kyoto, Japan.

In order to meet the increasing demand for ZnO-based devices, we need to develop ZnMgO thin films which allow various heterostructures. A lot of technologies such as MBE, MOCVD, or PLD are reported to form ZnMgO thin films, but evolution of safe and economical deposition technologies is significant especially for utilizing the unique functions of ZnO-based materials on large-area substrates, that is, for such as transparent conductors, UV absorbers, and UV image sensors. For this reason, we developed a new method to fit the demand. An attention has been given to the mist CVD method where a liquid solution of constituent elements is ultrasonically atomized and the aerozol particles hence formed are transferred to the reaction area to form ZnO. We have established an improved system by utilizing a special nozzle composed with a linear aperture to flatten the flow to guarantee the formation of uniform and high quality films. This method possesses advantages of safety, cost-effective, light load to the environment, and multiplicity of applying to a lot of materials. Following to the successful deposition of highly uniform ZnO in our earlier works, in this presentation the fabrication of ZnMgO thin films and UV photodetectors will be reported. ZnMgO films were deposited on soda-glass substrates. The sources used were zincacetate and magnesium acetate which were diluted in deionized water with the concentrations of 0.050 mol/l and 0-0.050 mol/l, respectively. The carrier gas for the aerosol particles was N2, whose flow rate was 8 l/min. The temperatures of the substrate were set at 360-520 oC. ZnMgO thin films were successfully grown on glass substrates and these transmittances in visible range was more than 90%, showing good transparency. With increasing the source concentration ratios of [Mg]/([Zn]+[Mg]), the absorption edge shifted to the shorter wavelength and the band gap energy gradually increases. It was possible to control the band gap energy up to 3.75 eV. In order to demonstrate the optoelectronic performance of the ZnMgO films, we made a planar geometry Schottky type metal-semiconductor-metal UV photodetector using the ZnMgO film and measured the dark-current and photo-current characteristics. The 50 pairs of interdigital electrodes consisted with the Au (60 nm)/Cr (20 nm) bilayer with 2 mm long, 10 µm wide, and 15 µm spacing were formed by conventional photolithography and lifting off on the ZnMgO layer whose band gap was 3.5eV. No photoresponse was seen with illuminating the 400nm light. However, by illuminating the 300nm light, the photoresponsivity of 5.0A/W was obtained. Considering the Schottky characteristics of the electrodes, the large photoresponsivity may be attributed to the variation of Schottky barrier height under the illumination. Promising optoelectronic performance of the ZnMgO has been confirmed in spite of the simple and cost-effective deposition technique.


K7.28
A Solution Method for Large-scale Selective Growth of Aligned ZnO Nanorods.Q. Ahsanulhaq, A. Umar, Sang-Hoon Kim, Yeon Ho Im and Yoon-Bong Hahn; School of Chemical Engineering and Technology and Nanomaterials Processing Research Center, Chonbuk National University, Jeonju, South Korea.

A convenient and facile aqueous route has been employed to selectively synthesize high-aligned ZnO nanorods. Field emission scanning electron microscopy showed ZnO nanorod arrays with large homogeneity were grown at 343 K on ZnO/Si substrate. The average diameter of ZnO nanorod was in between 50-60 nm. The length of each nanorod is 400-500 nm. Transmission electron microscopy and XRD analysis showed that the ZnO nanorods were single crystalline with wurtzite hexagonal phase. Room temperature photoluminescence spectra of the ZnO nanorod arrays exhibited a strong ultraviolet emission at 378 nm, but weak blue emission at 460 nm and green emission at 580 nm. In addition, selective growth of ZnO nanorods on patterned ITO glass substrate was obtained. Each nanorod had diameter of 50-70 nm and length of ~ 500 nm. XRD pattern showed that ZnO nanorods on the patterned ITO substrate were single crystalline in nature with wurtzite hexagonal phase. The room temperature photoluminescence from the aligned ZnO nanorods showed a strong ultraviolet emission at 378 nm and broad deep level visible emission at 58 0nm. This novel and efficient pathway was based on experimental monitoring of temperature, reaction time, and pH. Nucleation and growth process was discussed to elucidate the mechanism of formation of single crystal ZnO nanorods through a solution method.


K7.29
ZnO Quantum Dots in PMMA Matrix.Shanghua Li, Weiyi Zhang, Muhammet Toprak and Mamoun Muhammed; MSE, Royal Institute of Technology, Stockholm, Sweden.

ZnO, a direct wide band gap semiconductor with a large exciton binding energy of 60 meV, is an ideal material for optoelectronic applications. In addition to UV excitonic emission peak, ZnO commonly exhibits visible luminescence at different emission wavelengths which is dependent on the size of ZnO. It is well-known that, for ZnO quantum dots (QDs, ~ 1-10 nm), the photoluminescence (PL) excitation wavelength has direct relationship with the ZnO QD diameters. The UV absorption spectra and PL excitation spectra red-shift when the ZnO QDs grow larger. However, the visible emission of ZnO QDs is much more complicated than the excitation spectra and the UV absorption and the origin of these visible emissions, especially the green emission, has been controversial. According to Xiong, green emission could be observed when the diameter of ZnO QDs varied from 2 to 8 nm; while blue emission was predicted to exist when the size of ZnO QDs was around 1 nm. The blue emission from ZnO QDs was always a challenge and was not until recently observed in highly diluted solutions at around 0 0C and was not stable at room temperature. Organic-inorganic nanocomposites with a polymer matrix have attracted considerable interest since they usually combine desirable properties from inorganic and polymer components. Polymer matrix exhibits an ideal phase separation function, which may help isolate the clusters as they form, thus it plays a valuable role to tune the size of the inorganic nanoparticles in polymer matrix. In this work, polymeric nanocomposites containing ZnO QDs were synthesized by an in-situ polymerization process. ZnO QDs were fabricated through a sol-gel hydrolysis process while polymerization took place simultaneously. A double function agent, monoethanolamine, was adopted both to retard the hydrolysis process and to connect the inorganic phase to the polymeric phase. As a result, homongenous polymer-inorganic nanocomposites containing stable ZnO QDs were achieved. Moreover, the size of ZnO QDs was tunable by careful control of the synthesis conditions. Blue and green emissions from ZnO QDs were observed under UV light.


K7.30
Controllable Synthesis of ZnO Nanonails by Vapor-solid Process: Growth Mechanism and Structural and Optical Properties.Ahmad Umar, Q. Q. Ahsanulhaq,, Sang Hoon Kim, Yeon Ho Im and Yoon-Bong Hahn; School of Chemical Engineering and Technology and Nanomaterials Processing Research Center, Chonbuk National University, Jeonju, South Korea.

With the controllable morphologies, wurtzite ZnO nanonails have been synthesized on steel alloy substrate without the use of any metal catalyst or additives by the thermal evaporation of metallic zinc powder at low temperature. Morphological studies revealed that different types of ZnO nanonails in terms of their diameters and lengths of caps versus shafts were obtained at different positions in the reactor chamber depending upon the distance of the substrates from the source material in different temperature zones. The deposited ZnO nanonails exhibited well defined morphologies with the clean surfaces without any surface contaminations. High resolution transmission electron microscopy (HRTEM), X-ray and selected area electron diffraction patterns indicated that the deposited nanostructures are single crystalline and grew along the [0001] direction. A sharp, strong and dominant E2 mode in Raman spectra, for all the cases, confirms the excellent crystallinity and wurtzite hexagonal phase for the grown ZnO nanonails. Additionally, the appearance of strong and sharp UV emission with the absence of green emission observed in the photoluminescence (PL) spectra at room temperature for the deposited products gives a strong evidence for their excellent optical properties.


K7.31
Surface Preparation of Single Crystals for ZnO Homoepitaxy.Christian Neumann1, Stefan Lautenschläger1, Swen Graubner1, Joachim Sann1, Niklas Volbers1, Bruno K. Meyer1, Jürgen Bläsing2 and Alois Krost2; 1I. Phys. Institute, Justus-Liebig-University, Giessen, Germany; 2Institute of Experimental Physics, Otto-von-Guericke-University, Magdeburg, Germany.

Here we report on a thermal surface preparation of commercial ZnO crystals. Usually the surface is severely damaged by cutting and polishing. Thus, an additional high temperature treatment is necessary to improve the morphology and surface crystallinity of the delivered crystals. The ZnO crystals were treated at different temperatures and characterized by atomic force microscopy, X-ray diffraction and reflection, photoluminescence, and hall measurements. Finally, we report on first results of a successful ZnO homoepitaxy.


K7.32
Structural Properties of ZnO Nanowires Grown by Chemical Vapor Deposition on GaN/sapphire (0001). F. C. Tsao1, P. J. Huang1, C. J. Pan2, C. J. Tun2,3, C. H. Kuo4, B. J. Pong1 and G. C. Chi1,2; 1Department of Physics, National Central University, Jhongli, Taoyuan, Taiwan; 2Optical Sciences Center, National Central University, Jhongli, Taoyuan, Taiwan; 3National Synchrotron Radiation Research Center, Hsinchu, Taiwan; 4Institute of Optical Sciences, National Central University, Jhongli, Taoyuan, Taiwan.

Fabrication of low-dimensional semiconductor structures such as nanowires, nanotubes and nanorods is an interesting subject due to their optoelectronic, bio-sensor device and other technological applications. Zinc oxide (ZnO) is an attractive candidate for ultraviolet light emission since it has both a direct wide bandgap of 3.37 eV and a very large exciton binding energy of 60 meV, important for robust light emission. It has the same crystal structure type as GaN, and the lattice mismatch is 1.8% on the c-plane. The thermal expansion coefficients of ZnO are αa = 6.5×10-6 K-1 and αc = 3.0×10-6 K-1, along the a- and c-axis, respectively, close to those of GaN, which are αa = 5.6×10-6 K-1 and αc = 3.2×10-6 K-1. This opens up the applications of ZnO/GaN heterostructures. The ZnO nanowires used in this study were grown on 2-μm-thick GaN templates by thermal chemical vapor deposition (CVD) without employing any metal catalysts. The GaN template was predeposited by metalorganic chemical vapor deposition on a c-plane sapphire substrate. The diameters of the resulting nanowires are in the range 40-150 nm depending on growth time. The ZnO nanowires were vertically well-aligned with uniform length, diameter, and distribution density as revealed from electron microscopy. High-resolution x-ray diffraction (XRD) experiments were carried out on a Bede triple-axis diffractometer system, using Cu Kα1 (λ = 1.54056 Å) radiation. The XRD spectra show that ZnO grows in single c-axis orientation with the c axis normal to the GaN basal plane, indicating a heteroepitaxial relationship of (0001)ZnO||(0001)GaN. The lattice constant of the c-axis of the ZnO nanowires with the diameter of 40 nm is 5.211 Å, which is larger than that of bulk ZnO, 5.207 Å. Thus, in the ZnO nanowires there exists a residual tensile strain along the c-axis. The strain decreases with increasing the diameter of nanowires. Hence, the tensile strain might be from the surface stress of nanowires. In conclusion, the ZnO nanowires grown by catalyst-free CVD on GaN template are vertically well-aligned with the relationship of (0001)ZnO||(0001)GaN. In the ZnO nanowires there exists a residual tensile strain, which might be from the surface stress of nanowires.


K7.33
Conversion of Deep Level Emissions into Band-edge Emission from Au/ZnO by Using Plasmonic Mediation.Weihai Ni and Hock Chun Ong; Physics Dept., The Chinese Univ. of Hong Kong, Hong Kong, China.

Radiative and nonradiative defects are detrimental in semiconductor because they create unwanted emissions and reduce devices’ intrinsic quantum efficiency. Therefore, the know-how of “converting” these problematic deep level and phonon emissions into desirable band-edge emission will not only provide new science and technology in semiconductor physics but also offer new opportunity in device design and application. Recently, Lin et al [1] have reported the use of surface plasmon polaritron (SPP) to enhance the band-edge emission from ZnO nanorods by recovering the energy lost via defects. The enhancement is based on the proposition that deep level emissions can be effectively coupled to the SPPs arising from metal/ZnO interface. The energy trapped in the SPPs can then be released by exciting more electrons to the conduction band of ZnO, which thus results in 16-fold of band-edge emission enhancement. Although their results demonstrate great promise of “defect conversion”, many details are still missing in their picture. Here, with the mind that conventional thin film is more appropriate for device fabrication, we extend their study from nanostructures to heterostructure films. By studying the dependence of emission enhancement (“conversion”) on the microstructure of metal films, we have identified the presence of nanovoids within the metal film in the proximity of the interface is essential for maximizing the plasmon coupling and therefore emission enhancement. On the other hand, the surface roughness of ZnO also plays a major role in governing the SPP scattering. As a result, more than two orders of magnitude in emission enhancement from ZnO have been achieved at the optimal condition. In addition, by using photoluminescence excitation spectroscopy, surface plasmon resonance spectroscopy and time-resolved photoluminescence, we have studied the aforementioned mechanism in detail. Summarizing our results, a different but more thorough model will be discussed. [1] H.Y. Lin et al, "Enhancement of band gap emission stimulated by defect loss," Opt. Express 14, 2372 (2006).


K7.34
RF Magnetron Sputtered ZnO Films for Flexural Plate Wave Devices.Sang Hoon Yoon and Dong-Joo Kim; Materials Engineering, Auburn University, Auburn, Alabama.

ZnO film has been considered as a potential material for sensor and actuator applications, due to high piezoelectric and electromechanical coefficients. For acoustic wave device applications, good crystallinity with highly c-axis oriented texture, and smooth and dense structure are required for high piezoelectric activity and for acoustic wave traveling within acoustic wave device. The fabrication of ZnO films has been performed by RF magnetron sputtering. The film structure of ZnO has been controlled by modifying process parameters such as process gases and ratio, substrate temperature, and substrate type. Various characterization such as XRD, SEM, AFM, and Raman spectroscopy has been performed to examine the structural properties of ZnO films. Optimized ZnO films exhibit dense and uniform surface morphology, and highly textured structures. In particular, c-axis orientation is maximized at the ratio of 25% oxygen content in argon gas mixture and on Pt(111) textured silicon wafer. The arrays of flexural plate wave device have been successfully fabricated onto 4-inch Si wafer, and the relationship between the film microstructure and the electro-acoustic property of the device will be discussed in terms of device performance for the potential biosensor applications.


K7.35
Hetero-Junction Diodes Composed of Undoped p-ZnO and n-SnO2 Thin Films. Nick Brilis1, Dimitris Tsamakis1, Hasina Afroz Ali2, Soumya Krishnamoorthy2 and Agis Iliadis2,3; 1School of Electrical Engineering and Computer Science, National Technical University of Athens, Athens, Greece; 2Department of Electrical & Computer Engineering, University of Maryland, Maryland, District of Columbia; 3Department of Information and Communication Systems Engineering, University of the Aegean, Karlovasi, Samos, Samos Island, Greece.

Metal - oxide semiconductors of wide band gap give devices with potential advantages in applications such as UV detectors, LED diodes, transparent transistors as well as chemical sensors. Most of the above devices have recently been developed using p-n homo- or hetero-junctions. Hetero-junctions based on the metal-oxide semiconductors can be used in sensor devices in order to increase their sensitivities. Reported optical hetero-junction devices are based mainly on the interface properties between the oxides [1]. In particular the sensitivity and the response properties of such sensors can be highly improved by using hetero-junctions composed of SnO2 or ZnO as the most well known n-type materials and CuO or NiO as p-type materials. Nevertheless there are no reports on p-n junctions using p-ZnO as a p-type layer propably due to the difficulties in preparation of stable p-doped ZnO films. Furthermore, to the best of our knowledge no studies have been reported yet on p-n heterojunctions using intrinsic ZnO as p-type material. As-grown ZnO films exhibit usually n-type conduction. In order to form a p-n junction a high enough - electrically activated p-dopant concentration is required. Apart from the use of ZnO based p-n junctions as LED diodes, gas sensor studies with non - ideal oxide junctions have not been explored so much so far. For gas sensor applications these non ideal oxide junctions may be advantageous because the sensitivity of the device is proportional to the difference of the initial and final response values exhibiting a high sensitivity in hydrogen detection [2]. In our paper we present the growth, characterisation and H2 sensing properties of a p-n hetero-junction using a PLD grown ZnO film as p-type material and a typical CVD grown n-SnO2 layer as p-type material. In particular CVD grown n-SnO2 thin films have been deposited on PLD grown p-ZnO films, using the appropriate mask, in order to produce p-n hetero-junctions. I-V measurements at room temperature showed that there is great difference between reverse and forward currents for a bias of about 1V. This behaviour revealed a rectifying p-n junction. C-V measurements exhibited a linear part in 1/C3 vs V dependence graph which would indicate a linearly graded region in the junction. From the x-axis intercept of the linear part the diffusion potential Vd could be obtained. These devices, used for sensing trace levels of H2, have exhibited higher sensitivity compared to that of a sensor based on pure SnO2 or ZnO thin films as well as a room temperature operation, explained by the field assisted sensing capability.


K7.36
Cathodoluminescence Study of Hydrothermal Zn1-xMgxO Alloy Crystals. J. Mass1,4, M. Avella1, Juan Jimenez1, M. Callahan2, E. Grant2, K. Rakes2, D. Bliss2 and B. Wang3; 1Dpto. Física de la Materia Condensada, Universidad de Valladolid, Valladolid, Spain; 2Sensors Directorate, Air Force Research Laboratory, Hanscom, Massachusetts; 3Solid State Scientific Corporation, Hollis, New Hampshire; 4Dpto. Física y Matemáticas, UniNorte, Barranquilla, Colombia.

Band engineering of ZnO is necessary for UV optoelectronic applications. It can be achieved by alloying with MgO for increasing the band gap, and with CdO for decreasing the band gap. Alloying with MgO has been reported to reach 36% using epitaxial techniques. There are not reports about the growth of ZnMgO crystals by the hydrothermal route. The alloy composition should be limited by the solubility of MgO in wurtzite ZnO. One of the main challenges consists of obtaining crystals with homogeneous composition to be used as substrates for both ZnO and GaN based optoelectronic devices. We present herein a cathodoluminescence (CL) study of ZnMgO hydrothermal crystals. The incorporation of Mg was found to depend on the growth face using CL spectral imaging, however, the variations were very small, being the changes in the formation of defects responsible for the visible luminescence much more significant. On the other hand, varying the acceleration voltage of the excitation e-beam one observes a depth gradient in the incorporation of Mg. The Mg concentration was found to increase close to the surface. The crystals were previously etched in order to remove the thin ZnO layer grown on the surface during cooling.


K7.37
Properties of Dominant Electron Trap Center in n-type SiC Epilayers by Means of Capacitance Spectroscopy.Muhammad Asghar Hashmi1, Quamar ul Wahab1,2, Ijaz Hussain1, Hafiz Shahid Noor1, Faisal Iqbal1 and Muhammad Shahid1; 1Semiconductor Division, The Islamia University, Bahawalpur, Punjab, Pakistan; 2Physics and Measurements Technology Materials Science, Linkoping University, Linkoping, Sweden.

Striking properties of silicon carbide (SiC) make this material potential candidate for fabricating power devices, but, the material should be with significantly reduced defect density. Amongst a number of electron traps in SiC epilayers, Z1/2 center is widely recognized and mostly debated and so far its origin is an open question. In the present study, using deep level transient spectroscopy (DLTS) on the assumed material, we present detailed investigation on this trap level and compared the measured data with available standard data. The DLTS spectra from as-grown low doped n-SiC layers demonstrate two electron levels in which Z1 level is dominant trap but in high doped samples these levels are not observed. The corresponding activation energy, capture cross section (indirect) and trap concentration of Z1 level are Ec - 0.64 eV, 5.5x10-15 cm2 and 1.23x1013 cm-3 respectively. The line shape fitting and capture cross section data indicate that Z1 center is a single level. Spatial distribution discloses that this level is in fact generated in near epilayers/substrate interface and the rest of the overgrown layers are defect free. The concentration data of Z1 level with respect to back ground concentration ND of a number of as grown n-SiC samples reveal that the level decreases with increasing ND. The careful comparison with well-known data reveals that Z1 center is complex defect consisted of anticites and interstitial. PACS: 85.30.Kk, 73.30.+y, 73.40.-c, 73.20.At, 71.55.Ht, 61.50.Nw [1] T. Dalibor, G. Pensl, H. Matsunami, T. Kimoto, W. J Choyke, A. Schöner, and N. Nordell, phys. stat. sol. (a) 162, 199 (1997). [2] S. Uekusa, K. Awahara, and M. Kumagai, IEEE Trans. Electron Devices 46, 572 (1999). [3] J. Zhang, L. Storasta, J. P. Bergman, N. T. Son, and E. Janzén, J. Appl. Phys. 93, 4708 (2003).


K7.38
Low Temperature Growth and Characterization of Mg0.15Zn0.85O Thin Film by Pulsed Laser Deposition.Wei Wei1,2, Chunming Jin2, Andy Doraiswamy2, Roger Narayan2 and Jagdish Narayan1; 1Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina; 2Joint Department of Biomedical Engineering, University of North Carolina and North Carolina State University, Chapel Hill, North Carolina.

Zinc oxide and zinc oxide based alloys have several potential optoelectronic, solar cell, and sensor applications. Many research activities are focused on integration of these II-VI semiconductor materials with polymers and other substrate materials that cannot stand high temperatures. It is well known that the incorporation of magnesium into zinc oxide can form an alloy with hexagonal structure, in which bandgap increases with magnesium concentration. Although the equilibrium solubility of magnesium is only four atomic percent, the nonequilibrium nature of thin film growth is responsible for the high concentration (up to 40 %) of magnesium in hexagonal zinc oxide matrix. It remains to be determined whether the hexagonal structure is maintained in films grown at low temperatures and how the optical and electrical properties of the films change with growth temperature. In this work, we performed a systematic investigation of the effect of growth temperature on the structural, electrical, and optical properties of Mg0.15Zn0.85O thin films grown using pulsed laser deposition. All the films were grown on amorphous fused silica and unetched silicon substrates. The films were grown using substrate temperatures between room temperature and 700 °C. X-ray diffraction and transmission electron microscopy were used to investigate film structure. The surface morphology was examined using atomic force microscopy. The optical properties of the films were investigated using absorption, transmission and emission spectra measurements. X-ray diffraction demonstrated that all of the films, including those grown at room temperature, had a textured structure, with the c-axis perpendicular to the substrate surface. Excitonic absorption peaks were observed for the films grown at temperatures higher than 100 °C. The transmittance of films grown at 300 °C was found to be about 90% in the wavelength range from 1100 nm to 320 nm. The film also showed strong excitonic luminescence. These results suggest that Mg0.15Zn0.85O thin films grown at 300 °C demonstrated similar structural, electrical, and optical properties to films grown at much higher temperatures.


K7.39
Visible Luminescence from ZnO Nanostructures. Minseo Park2, An-jen Cheng1,2, Dake Wang2 and Yonhua Tzeng1; 1Electrical Engineering, Auburn University, Auburn, Alabama; 2Physics, Auburn University, Auburn, Alabama.

Abstract Room temperature photoluminescence (PL) spectra from zinc oxide (ZnO) nanostructure were studied. The samples were produced via thermal chemical vapor deposition (thermal-CVD) and a variety of ZnO nanostructures were synthesized by adjusting the oxygen flow rate during the growth process. All samples exhibit a sharp and strong ultra-violet near-band-edge (NBE) emission at about 3.18 eV. The visible emission from the sample deposited using an oxygen-deficient condition was dominated by blue-green band emission at 2.34 eV. The intensity of the blue-green band was greatly reduced to zero (so-called green band free) for the ZnO deposited on the center of the wafer while strong violet-blue emission bands and broad bands at yellow-orange-red range were collected from the ZnO grown on the edge of the wafer. We believe that the spatial inhomogeniety can be produced by turbulent gas flow in the reaction chamber. It is hypothesized that this difference may be caused by the difference in local oxygen concentration. Raman spectroscopy was also performed to study the crystal quality of the nanostructures. Origin of visible luminescence from ZnO nanostructure will be discussed and a model to explain the observed visible luminescence process will be presented.


K7.40
Magneto-optical Properties and Strain Evolution in Homoepitaxially Grown ZnO and ZnO:Li Layers.Axel Hoffmann1, U. Haboeck1, M. R. Wagner1, R. Mc Kenna1, S. Lautenschläger2, J. Sann2 and B. K. Meyer2; 1Inst. f. Fstkoerperphysik, TU Berlin, Berlin, Germany; 21. Physikalisches Institut, Justus Liebig University, Giessen, Germany.

The homoepitaxial growth of Zinc Oxide reveals some unexpected difficulties. Although there should not be a noticeable thermal or lattice mismatch we found a strong dependence on growth properties and substrate preparation. We investigated different ZnO and ZnO:Li samples as well as the pure substrates with a variety of methods in optical spectroscopy. Cross-section Raman measurements show a radical change of the spectra leading to the conclusion of different growth domains existing in the samples. This might be the reason for small internal strain gradients observable by the shift of the nonpolar E2 mode of the host lattice. With magneto photoluminescence and transmission spectroscopy a number of bound exciton lines are observable with the smallest FWHM of 80 µeV. Besides the expected Zeeman-splitting up to 580 µeV at 5 T we found a further splitting of peaks in magnetic fields stronger than 3 T which vanishes after a special treatment of the samples. Simultaneously a shift of the bound exciton lines is observed in the PL spectra. From all these findings we draw conclusions concerning the influence of sample preparation, in particular surface effects of the substrates, on the optical properties of homoepitaxially grown ZnO films.


K7.41
Influence of Buffer Layers on the Structural Properties of ZnO Grown by Plasma Assisted Molecular Beam Epitaxy.Thomas Andreas Wassner, Bernhard Laumer, Martin Stutzmann and Martin Eickhoff; Walter Schottky Institut, Technische Universitaet Muenchen, Garching, Germany.

We have investigated the influence of ZnO and MgO low temperature buffer layers on the structural properties of ZnO-films heteroepitaxially grown on (0001)- and (11-20)-sapphire substrates by plasma assisted molecular beam epitaxy. A systematic high resolution X-ray diffraction study of symmetric and asymmetric reflexes was carried out to analyze the impact of the buffer layer thickness on the structural properties such as the edge- and screw dislocation densities in the deposited ZnO-films. The effect of buffer layer annealing on the structural quality of the ZnO epilayers was also investigated. By comparison with atomic force microscopy imaging it is shown that a buffer-induced increase of the crystallite size in the nucleation stage leads to a decrease of the screw dislocation density for optimized buffer layers. In contrast, the density of edge dislocation increases due to the more pronounced accumulation of stress in larger crystallites. The influence of the structural properties on the electrical and optical data (luminescence, conductivity, carrier mobility) of the ZnO layers will also be discussed.


K7.42
Abstract Withdrawn


K7.43
In-Situ Arsenic Doping of ZnO Grown on GaN/Sapphire and ZnO Substrates by Molecular Beam Epitaxy.Weiming Wang, Emine Cagin, Willie Bowen and Jamie Phillips; University of Michigan, Ann arbor, Michigan.

Zinc oxide and related oxide semiconductor alloys are emerging as important materials for active electronic and optoelectronic devices due to their desirable growth parameters, availability of native ZnO substrates, excellent optical properties, and near lattice-matched alloy system. High quality ZnO materials have been achieved using molecular beam epitaxy (MBE) with a plasma source to provide atomic oxygen. However, many challenges remain with respect to the repeatable growth of high quality material, and the ability to achieve reliable p-type doping. In this work, the epitaxial growth of arsenic doped ZnO and resulting electronic properties are presented. ZnO was grown on GaN/sapphire (0001) substrates and on native c-plane ZnO (0001) bulk substrates. GaN/sapphire (0001) substrates consisted of several microns of GaN grown by MOCVD to provide a substrate with a near lattice match (1.8% mismatch) and semi-insulating properties. Growth on ZnO substrates were conducted on both Zn-face and O-face of high resistivity bulk ZnO for comparison. The background n-type carrier concentration for these epilayers was found to span the range of 1017 cm-3 to 1019 cm-3, and to be highly dependent on VI/II flux ratio. The incorporation and activation of acceptors related to arsenic doping were found to have a strong dependence on both the arsenic flux and the VI/II flux ratio. The electronic properties of undoped and arsenic doped ZnO will be presented. Challenges associated with the achievement of stable p-type behavior will be discussed.


K7.44
Manipulation of Chemical and Optical Properties of MgO Nanocubes via Surface Functionalization.Slavica Stankic, T. Berger, Oliver Diwald, J. Bernardi and Erich Knözinger; Institute of Materials Chemistry, Vienna University of Technology, Vienna, Austria.

We use chemical vapour deposition (CVD) for the synthesis of pure and doped MgO nanoparticles. In this presentation we demonstrate how the admixture of a second insulating metal oxide component alters the electronic surface properties of MgO nanocrystals. When MgO nanoparticles are doped with monovalent Li-ions, traces of the dopant already segregate into the surface during thermal annealing and induce dramatic changes in the optical and chemical surface properties. Isovalent Ca2+ cations can be distributed homogeneously in MgO nanocrystals although CaMgO mixtures are thermodynamically forbidden on a macroscopic scale. Subsequent thermal activation leads to calcium ion segregation into the MgO surface. These CaMgO nanocrystals represent novel materials with enhanced surface basicity and a higher thermal stability in comparison to pure MgO. Furthermore, an unexpected photonic behaviour of those mixtures is reflected by a significant increase of photoluminescence emission which is red-shifted with respect to pure MgO. On the other hand, the admixture of larger isovalent Sr2+ cations induces - irrespective of the excitation wavelength - a blue-shifted emission. In order to study the transition from insulating metal oxide to semiconducting metal oxides, binary ZnO/MgO nanoparticles were produced by CVD and subjected to UV/Vis absorption and luminescence spectroscopy. First results clearly show that - different from the nanoparticles exclusively consisting of insulating constituents - the optical properties are now determined by both surface and bulk processes. Detailed measurements are still in progress. (1) S. Stankic et al. Angew. Chem. Int. Ed. 44 (2005) 4917. (2) S. Stankic et al. Nano Letters 5 (2005) 1889.


K7.45
A Study of Nano-phased ZnO Thin Film Grown by Pulsed Laser Deposition.Wonwoo Lee, Materials Science, University of alabama at birmingham, Fultondale, Alabama.

Zinc Oxide (ZnO) is one of promising wide bandgap semiconductor materials because of their unique and novel applications in laser, piezoelectricity, and optoelectronics. Pulsed laser deposition (PLD) has been used to deposit nano-phased ZnO thin film. X-ray diffraction spectroscopy is used to confirm the crystalline orientation of nano-phased ZnO thin film. Optical properties are investigated using by photoluminescence (PL) and Raman spectroscopy. A strong PL peak located at 374 nm is attributed to the free-exciton recombination and can be estimated the bandgap energy of ZnO. A broad emission peak near at 2.8 eV may be corresponded to the defect levels, such as zinc vacancy, oxygen vacancy, or interstitials. Raman scattering spectroscopy is used to investigate the vibrational properties of the nano-phased ZnO films. In additions, Electron paramagnetic resonance spectroscopy (EPR) is performed to identify the defects in nano-phased ZnO thin fims as seen from PL measurements.


K7.46
Effects of Homogeneous Buffer and Annealing on the Low Temperature Optical Properties of ZnO Films Grown by PLDXiyao Zhang, Anuj Dhawan, Patrick Wellenius and John F Muth; North Carolina State University, Raleigh, North Carolina.

Zinc oxide (ZnO) films have been deposited by PLD with different types of homogenous buffers and annealing conditions. Optical transmission and photoluminescence spectra have been taken to study their optical properties. For both the cases of buffered and annealed ZnO films, improvements of optical quality have been observed. The optimal growth condition of buffer was found to be at low pressure (5x10-6 Torr) and low temperature (1000°C). The optical quality of the buffered ZnO film is also sensitive to the thickness of the buffer. Low temperature (4K) measurements of optical transmission and PL have also been used to further characterize the films.


K7.47
Growth of ZnO Thin Films by Metalorganic Chemical Vapor Deposition for Optoelectronic and Spintronic Applications.William Fenwick1, Tahir Zaidi1, Vincent Woods1, Nola Li1, Matthew Kane1,2, Shalini Gupta1 and Ian Ferguson1,2; 1Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia; 2Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia.

Zinc oxide (ZnO), with its wide bandgap (~3.37eV) and high exciton binding energy (~60meV), is a promising material for use in optoelectronic and spintronic devices. ZnO growth by metal organic chemical vapor deposition (MOCVD) is a useful technique because of its flexibility and scalability to larger commercial systems. However, reactor dynamics and growth kinetics of ZnO growth by MOCVD must be better understood in order to allow for intelligent design of a process that can consistently yield high-quality undoped, n-type, and p-type ZnO. The effects of transition metal incorporation on the structural and optical properties of ZnO must also be investigated in order to use these materials in spintronic devices. This work presents results obtained from an investigation into factors affecting MOCVD growth of ZnO and the effects of transition metal incorporation on structural, optical, and magnetic properties of ZnO. DEZn and O2 were used as the precursors in a vertical injection, rotating disk MOCVD growth system. Trends in materials properties were observed and correlated to changes in growth parameters in order to build an understanding of the kinetics of ZnO growth by MOCVD. Total volume flow and injection velocity were found to have a significant effect on growth kinetics and, therefore, crystal quality. Structural properties were investigated using X-ray diffraction (XRD) and Raman spectroscopy. Optical properties were investigated using room temperature (RT) photoluminescence (PL) and low temperature PL. This study has produced undoped ZnO thin films that show both strong luminescence and good crystal quality as measured by XRD and Raman spectroscopy. This allows for a correlation of both optical and structural properties to growth kinetics in order to better understand MOCVD growth of ZnO. PL spectra show a strong luminescence peak around 3.28eV, suggesting that the dominant emission peak may be due to an LO phonon replica of the band-to-band emission. Low temperature PL measurements will be done to confirm this assignment. XRD omega-2theta scans show a linewidth of about 180arcsec. Raman spectra showed the E2(high) mode at 437cm-1 while the 2nd-order phonon mode near 332cm-1 was of very low intensity. Effects of a two-step growth process on structural and optical properties of ZnO thin films will be discussed, and initial results will be presented on p-type doping and TM-doping of ZnO thin films by MOCVD.


K7.48
A Comparative Study of MOCVD Produced ZnO Films Doped with N, As, P and Sb.Gary S. Tompa, S. Sun, C. E. Rice, L. G. Provost and D. Mentel; Structured Materials Industries, Inc., Piscataway, New Jersey.

ZnO thin films are of interest for an array of applications; including: light emitters, photovoltaics, sensors and transparent contacts among others. Of particular interest is the potential to produce p-type layers from which p-n junction devices could be routinely produced. While it is fairly routine for MOCVD to produce n-type films with doping concentrations in the 10E20 cm-3 range and resistivities below 10E-3 ohm-cm; it is very difficult to produce measurable p-type ZnO. We report on our efforts with doping films p-type using N gas sources and metalorganic sources of P, As, and Sb. Films showing acceptor bands by photoluminescence have been demonstrated; however reliable electrical measurements remain difficult. Specific problems include achieving low resistance ohmic contacts, accounting for the photo-responsiveness of ZnO films and sensitivity limits in Hall measurements of low-doped and compensated materials. The presentation will review deposition parameters, produced and processed films and material characteristics.


K7.49
Electrical Properties of Solution Grown Piezoelectric ZnO Nanorods.David Scrymgeour, Dana Olson and Julia Hsu; Sandia National Laboratories, Albuquerque, New Mexico.

Nanoscale semiconducting zinc oxide has been regarded a one of the most promising materials for the next generation sensors, UV lasers, solar cells, and nanoscale electronic devices. This is because of its important physical properties that include a wide band gap (~3.44 eV) and interesting optoelectronic, pyroelectric, and piezoelectric properties. However, despite advances in the growth and fabrication of nanostructures, reliable nanoscale metal-semiconducting oxide contacts are required for utilization of ZnO nanostructures for device applications. Understanding the nature of this contact is a critical step in incorporation of nanostructures.<BR><BR> Piezoelectric zinc oxide nanocrystals are grown by solution techniques on highly textured Ag (111) films in patterned arrays. These ZnO nanocrystals form hexagonal crystal rods with diameters of 100-600 nm and heights of 400-1200 nm with their [0001] polar axis growing from the substrate. The nature of the electrical contact on a nanorod will be studied by contact atomic force microscopy (C-AFM) where the metallic coating on the probe tip forms the metal contact. This contact will be investigated on different rods as a function of tip coating work function, as well as rod surface treatments such as vacuum and UV ozone exposure. These measurements will be compared to work function measurements on assemblages of nanorod. From the results of these studies, we will discuss the mechanisms that control the contact properties and I-V characteristics of the ZnO nanorods.<BR><BR> Additionally, the piezoelectric response of the individual nanorods has been determined by piezoelectric force microscopy (PFM). The rods have a response of ~5.6 pm/V that can vary from rod to rod and is not correlated to nanorod height or radius, which indicates another factor is responsible for the variation. It is known from studies of other semiconductor piezoelectric crystals that the conductive properties can have a strong influence on the strength of the measured piezoelectric coefficient. Correlations between the conductive properties and the variation rod-to-rod of the measured piezoelectric coefficient will be presented.<BR><BR> Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.


K7.50
Photodetectors Based on Ti/Al Contacts to Sb-doped p-ZnO/n-Si.Leelaprasanna J. Mandalapu, Faxian Xiu, Zheng Yang and Jianlin Liu; Department of Electrical Engineering, University of California, Riverside, Riverside, California.

ZnO is a semiconductor material that shows great promise for optoelectronic devices, specifically in the ultraviolet region due to its wide band gap (3.37eV), large exciton binding energy, high radiation hardness, etc. ZnO is also a strong competitor for GaN towards this application because of its superior properties, such as availability of native substrates and amenability to wet etching. Recently, we have succeeded in producing reliable p-type ZnO on Si by using Sb doping in molecular-beam epitaxy. In this study, we report various photodetector devices based on an Sb-doped p-type ZnO film on n-type Si (100) substrate. Room-temperature Hall effect measurements showed that the Sb-doped ZnO layer exhibits a hole concentration, mobility, and resistivity of 1.0×1018 cm-3, 22.4 cm2V-1S-1, and 0.27 Ω cm, respectively. Ti/Al metal was evaporated on the ZnO film and appropriately annealed to form metal-semiconductor-metal (MSM), Schottky and photoconductive detectors. Heterojunction diodes were also formed by making Ti/Al contacts onto ZnO film and backside contacts on Si. Current-voltage measurements resulted in typical characteristics of these devices. MSM devices showed the presence of double barrier while Schottky detectors had clear rectification of a diode. Photoconductors with Ohmic contacts exhibited a contact resistance of about 915 Ω and heterojunction devices behaved like typical p-type Schottky diodes. Good photoresponse in the ultraviolet (UV) region was obtained from all the four devices. Response in the visible region was also observed due to the Si substrate. The results suggest that Sb-doped ZnO is a suitable p-type ZnO film for UV optoelectronic applications.


K7.51
Zinc Oxide Fiber-like and Whisker Formations by Electrospinning.Onur Sinan Yordem, Erdem Ogut, Mehmet Ali Gulgun and Melih Papila; Sabanci University, Istanbul, Turkey.

Zinc Oxide (ZnO) has been focus of many researchers for its electronic and optical properties. Its wide direct band gap value of 3.32eV, is attractive for the potential applications such as functional electrical devices, gas sensors and solar cells. Among several processing techniques such as chemical vapor deposition, thermal evaporation, electrodeposition, sputtering, and pattern growth techniques; electrospinning process is utilized in this study to form nano-scale Zinc Oxide fibers. Zinc Oxide fibers attract our attention particularly in terms of piezoelectric characteristics, actuation and sensing capabilities. Although the fiber formation with electrospinning has been in use for the last decade, control of ceramic fiber formation in order to yield single crystal structures by electrospinning is relatively a fresh field of interest. The polymer system poly(vinyl alcohol) (PVA) in an aqueous solution is mixed with the aqueous zinc acetate (Zn(CH3COO)2.2H2O) solution at 80C and gently stirred for five hours for the sol-gel reaction. The so-called precursor solution is electrospun under a high DC voltage and the precursor fibers are collected as a mat of fibers. Randomly oriented and aligned precursor fibers of PVA/(Zn(CH3COO)2) in ~600nm diameter are produced. TGA analysis of the precursor mats revealed the significant heating regime for the calcination and sintering procedure of the mats. The precursor fibers are first dried at 120C for 1hour and then calcinated at different temperatures above 500C for various durations at several heating rates. The resultant samples exhibit the significance of the process parameters in the ZnO formation. Better process schemes produced porous ZnO fibers of 100nm to 200nm in diameter. The porous fibers which are composed of hexagonal ZnO particles are further made uniform by a sintering process at temperatures higher than the calcination temperature. XRD results suggested the electrospun ZnO fibers have a hexagonal wurtzite structure which is favorable due to its piezoelectric property. Further sintering procedure revealed the micron scale single crystal ZnO whisker formation throughout the mat.


K7.52
Carrier-Dependence Photoluminescence Study of Ga-doped ZnO Thin FilmsZheng Yang, Faxian Xiu, Leelaprasanna Mandalapu and Jianlin Liu; Electrical Engineering, Univ of California, Riverside, Riverside, California.

Ga-doped ZnO has been considered as one of the potential candidates of transparent conducting oxide (TCO) thin films to substitute for the conventional TCO such as indium tin oxide. Additionally, higher electron concentrations can be achieved in ZnO through Ga doping than those in undoped n-type ZnO thin films as a result of intrinsic defects. Ga-doped ZnO films with electron concentrations in the region of 10^17 to 10^18 cm^-3 are important for UV optoelectronic applications. High carrier concentration (>10^19 cm^-3) is necessary for ZnO-based diluted magnetic semiconductors (DMS) from theoretical predictions. Therefore, it is important to systematically study the optical and electrical properties of the Ga-doped ZnO. A series of Ga-doped ZnO thin films were grown on sapphire substrates using an SVTA plasma-assisted molecular beam epitaxy (P-MBE) system. Different electron carrier concentrations were achieved by controlling Ga effusion cell temperature. Reflection high Energy Electron Diffraction (RHEED), X-ray diffraction (XRD), Hall effect, and photoluminescence (PL) measurements were performed on as-grown Ga-doped ZnO thin films. RHEED and XRD results show that these films are of good crystalline quality. Hall effect measurements show that the sample with higher Ga cell temperature has higher electron concentration, which can be controlled precisely. Well-resolved low-temperature (9K) PL spectra were obtained from these ZnO:Ga samples, the Ga-related donor-bound exciton (D^0X) peak does not appear until the Ga cell temperature reaches 550 °C. The Ga-DY^0X peak dominates in the spectra when the Ga cell temperature reaches 600 °C. The temperature-dependent and power-dependent PL spectra were used to investigate the detailed information on the donors in the ZnO:Ga samples.


K7.53
Growth in Aqueous Solution and Characterization of ZnO Whiskers Bridging Between Micron-gap Electrodes.Keisuke Kametani1, Hiroshi Imamoto2, Herve Dumot3 and Shizuo Fujita1; 1International Innovation Center, Kyoto University, Kyoto, Japan; 2Advance Device Laboratory, OMRON Co. Ltd, Kyoto, Japan; 3Venture Business Laboratory, Kyoto University, Kyoto, Japan.

In this presentation we report a novel technology to firmly bridge zinc oxide (ZnO) whiskers between micron-gap electrodes in aqueous solution. Their sensing characteristics for external stimuli such as UV light, gas, humidity, and so on will also be presented. It is one of the next-generation technologies to securely bridge a functional material between two electrodes with a narrow gap. The important features with respect to this structure are high current density and low power consumption. Especially, with the low dimensional structure of materials, a bigger ratio of the surface for the volume, in other words, an increase in the specific surface area is remarkable. Therefore, the response characteristics for external stimuli can be developed, and a highly sensitive sensor which operates at low electric power consumption can be expected as one of the applications. As the material which is bridged between two electrodes, the functional materials such as carbon nanotubes, whiskers of the semiconductor, and so on have been reported. Among those materials, ZnO has extremely been expected for optical and electronic devices as well as sensors. being supported with the wide band gap (3.37 eV), the large binding energy of excitons (60 meV), the chemical activity of surfaces, together with the strong tendency of forming whiskers. There are numbers of works for bridging ZnO whiskers between electrodes. The following technique has generally developed for this purpose; (i) collect the functional material which was produced beforehand, (ii) disperse it in the volatile solvent, (iii) fall the liquid droplet on the electrodes, (iv) a bridge between the electrodes is achieved accidentally. But, it is difficult to crave highly-reproducible with the characterization of the structure because the structure obtained by this technique cannot be stabilized from the viewpoint of mechanical strength. In this report in order to fabricate a bridge firmly connecting two electrodes, growth was directly conducted on the electrodes in the aqueous solution. The aqueous solution with zinc nitrate hexahydrate (Zn(NO3)26H2O) and hexamethylenetetramine (C6H12N4 and HMT) are heated at 90oC for 0.5-9 hours. After the synthesizing, the sample was washed with deionized water, then the whiskers which grew through homogeneous nucleation were removed. Moreover, the sample was applied by ultrasonic agitation to remove the whisker weakly attached on the electrodes. As the result the whiskers sticking firmly onto electrodes were left selectively. In this process, ZnO whiskers bridging between micron-gap electrodes was successfully fabricated. The value of electric resistance of the structure has several micron ohm in a room, while when it is irradiated by the light of 390-nm wavelength with impressing voltage 5 V, high photoconductivity with more than 20 μA can be confirmed. The response characteristics for other external stimuli have also been confirmed.


K7.54
A Photoluminance (PL) Study on LPCVD ZnO Assisted by RF sputtered Buffers.Yuneng Chang, Chemical and Materials engineering, Lunghwa university of science and technology, Taoyuan, Taiwan.

Compound semiconductor ZnO is a valuable optoelectronic material. MOCVD prepared ZnO films can be used as light source materials for white light LED applications. This presentation will address the observation of (002) ZnO films by low pressure (APCVD). CVD was performed in a horizontal hot wall LPCVD reactor, with precursor Zn(acac)2 sublimed at 110oC, and radio frequency (RF) sputtered ZnO buffer on Si(100) as substrates. FESEM show that, for ZnO LPCVD at 500-580oC, with oxygen concentration at 50-90%, continuous but rough films form. The averaged grain size depends on the deposition temperature, oxygen concentration, and buffer layers used. We also noticed that the optical properties of LPCVD ZnO depend on the microstructure of ZnO grains. To define the process window for depositing LED grade ZnO, PL spectra were taken from deposited samples using a fluorescence spectrometer (Jasco FP 6300) at room temperature. The results show that, for LPCVD on some RF sputtered buffer, at 550-600oC, 60-100 torr, and oxygen concentration of 90%, ZnO deposits exhibit a sharp PL spectral line at 380 nm, without tailing or green emission. Such facts reflect a pure ZnO band structure, with a bandgap of 3.2 eV, and non detectable deep level emissions. However, we found a weak correlation between PL and XRD findings. It seems that XRD patterns can not be used as the unique index to predict PL quality. From deposited samples examined, we have observed many ZnO films with strong (002) preferential orientations, but poor PL results like weak 380 nm peak, obvious tailing to 500 nm, and strong green emissions. PL is a more sensitive tool to reveal the properties of deposits, such as point defects, oxygen vacancies, and interstitial ions. To acquire good quality PL, it is very important to control oxygen concentration in LPCVD system.


K7.55
Low Temperature Atmospheric Pressure CVD of ZnO Films at 150oC using Zinc acetylacetonate as precursorYuneng Chang, Chemical and Materials engineering, Lunghwa university of science and technology, Taoyuan, Taiwan.

Due to efficiency, portability, and flexibility, soft electronic products has become a fast expanding area in consumer electronics such as solar cells, and flat panel displays. One key technology in flexible electronics device fabrication is depositing transparent conducting oxide films as Al: ZnO on liquid crystal substrates like polyimide (PI) or PET for circuit use. In this study, we make atmospheric pressure chemical vapor deposition (APCVD) ZnO films on polyimide substrate realized, using ZnO buffer technique. The ZnO buffers, prepared by radio frequency (RF) sputtering at 5mtorr, 50 watts, are polycrystalline and have a thickness from 100 to 300 nm, surface roughness from 3 to 8 nm. Comparison study shows such buffer can assist CVD film growth and reduces deposition temperature from 300oC to 150oC. XRD results show APCVD ZnO films are polycrystalline, at a deposition temperature of 150-250oC. Above 200oC, the films have a preferential (002) orientation. SEM and AFM results indicate these dense structured films are continuous, with averaged grain size of 200 nm and surface roughness from 10 to 30 nm. The lowest deposition temperature by APCVD we achieved is 150oC, using 15 torr H2O as co-reactant. These results prove that RF sputtered buffer can reduce the deposition temperature of ZnO CVD, and making the realization of CVD ZnO on plastics for soft electronics applications feasible.


K7.56
Gas Phase Fourier Transform Infrared Spectroscopy (Ftir) Analysis of ZnO APCVD at 150-300°C.Yuneng Chang, Chemical and Materials engineering, Lunghwa university of science and technology, Taoyuan, Taiwan.

Hexagonal wurtzite structured zinc oxide (ZnO) is a semiconductor with a band gap of 3.37 eV. ZnO can be used in piezoelectric, optoelectronic, photoconducting, and optical waveguide applications. Chemical vapor deposition (CVD) is a promising technique to prepare ZnO films. However, recent studies show that the quality of CVD zinc oxide films depends on the precursor decomposition pattern primarily. As reported in III-V compound semiconductor literatures, adduct formation and parasitic reaction are frequently encountered in near atmospheric VPE. In this study, Fourier transform infrared spectroscopy (FTIR) is used to analyze vapor phase product of zinc acetylacetonate based CVD. In an atmospheric pressure CVD system, 0.80~1.20 torr of precursor zinc acetylacetonate (Zn(acac)2, Zn(C5H7O2)2)vapor was introduced to the chamber by He/O2 or He/H2O carrier gas and reacted over buffer coated Si(100) substrate. The buffer is crystalline ZnO, pre-deposited by radio frequency sputtering, and used to catalyze reaction and reduce deposition temperature. After deposition, the effluent gas stream was collected, and analyzed by infrared spectra, from 450 to 4000 cm-1. In the temperature range from 150 to 300oC, the main IR absorption peak in APCVD ZnO observed was acetylacetone C5H8O2 at 1624 cm-1, resulted from C5H7O2* radicals. Additional H2O IR bands were found. At higher temperatures (>320oC), IR bands of CO2 were observed. As indicated by a kinetic model based on plug flow tubular reactor, the IR absorbance of acetylacetone AC5H8O2 was related to deposition rate. Based on 50 times IR data, we have compare the XRD, SEM of films with IR spectra of CVD gas, and build the correlation between AC5H8O2 and grain size, (002) intensity of films. The results show AC5H8O2 can be a very effective parameter indexing film growth. AC5H8O2 has a stronger relationship with SEM; it increases as average grain size increases. AC5H8O2 has a weaker relationship with XRD, it also increases as (002) peak intensity increases observed by XRD. As for AC5H8O2 is larger than 0.5, CVD films will be thick, dense, and continuous in most cases. If AC5H8O2 is less than 0.5, CVD films will be thin, loose structure, and discontinuous.


K7.57
Visible-Light Sensitivity for N-Doped ZnO Films Prepared by Reactive Magnetron Sputtering.Yoshitaka Nakano, Takeshi Morikawa and Takeshi Ohwaki; Toyota Central R&D Labs., Aichi, Japan.

Semiconductor photocatalysis is becoming more and more attractive and important since it has a great potential to contribute to environmental problems extensively. ZnO is one of promising materials in solar energy conversion and photocatalytic field due to its photochemical properties similar to TiO2. However, ZnO, as well as anatase crystalline TiO2, becomes active under irradiation with ultraviolet (UV) light whose energy exceeds the bandgap of 3.2eV. Therefore, ZnO-based materials capable of visible-light photocatalysis are strongly required with respect to solar energy and interior lighting applications. From this point of view, visible-light-driven photocatalytic technology has attracted much attention. The modification of ZnO with the goal of improving the optical absorption and photocatalytic performances, e.g., extending spectra response into the visible-light region and enhancing photocatalytic activity, seems to be the most important interest. As for TiO2-based materials, our group has recently demonstrated that N doping into TiO2 is effective for bandgap narrowing and visible-light photocatalysis, as determined by first-principles calculations and deep-level optical spectroscopy (DLOS) [1,2]. Thus, similar results can be also expected for ZnO-based materials, based on the same concepts in accord with our research. In this study, 1-μm-thick N-doped ZnO (ZnO:N) films were deposited on indium-tin-oxide (ITO)/quartz substrates by reactive rf magnetron sputtering using a ceramic ZnO target in a mixture of Ar and N2. In order to control N-doping concentration in the ZnO:N films, N2 concentration in the mixture gas varied from 0 to 40%. The ZnO reference sample without N-doping was colorless, whereas the color of the ZnO:N samples changed from brownish to umber brown with increasing N2 gas concentration. The colored ZnO:N samples showed enhanced polycrystalline with increasing N-doping concentration, as determined by x-ray diffraction. From XPS measurements, the N-doping concentrations in the ZnO:N samples prepared at N2 gas concentrations of 10, 20, and 40% were determined to be 3.6, 4.3, and 4.7%, respectively. In addition, the optical bandgap varied significantly from 3.1 to 2.2eV by the N-doping, as determined by optical absorption measurements. Furthermore, DLOS measurements revealed three characteristic deep levels located at ~0.98, ~1.20, and ~2.21eV below the conduction band for the ZnO:N samples. In particular, the pronounced 2.21eV band was found to be newly introduced by the N-doping and to behave as part of the valence band, resulting in bandgap narrowing of ZnO. Therefore, this deep level is probably one origin of visible-light sensitivity in ZnO:N. [1] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, and Y. Taga, Science 293, 269 (2001). [2] Y. Nakano, T. Morikawa, T. Ohwaki, and Y. Taga, Appl. Phys. Lett. 86, 132104 (2005).

SESSION K8: Doping

Chairs: Leonard Brillson and Yicheng Lu
Thursday Morning, November 30, 2006
Room 200 (Hynes)
8:30 AM *K8.1
p-type Doping in ZnO and ZnO-based LED realized by MOCVD.Zhizhen Ye, Weizhong Xu, Yujia Zeng, Liping Zhu, Haiping He and Binghui Zhao; Zhejiang University, Hangzhou, Zhejiang, China.

Metal-organic chemical vapor deposition (MOCVD) is of particular interest because it has many advantages over other growth methods, such as the feasibility of large area growth as well as simple and accurate doping and thickness control. Thus, combining the N doping technique based on ZnO with MOCVD method can be a promising way to overcome the bottleneck of p-type doping in the development of ZnO-based devices. Despite the importance of the MOCVD technique, there have been relatively few reports on N-doped, p-type ZnO by MOCVD method. This is probably due to the low solubility of N element in ZnO, especially in a CVD process. In our previous study,p-type conduction was realized in zinc oxide ZnO thin films by different doping methods, such as N-doping in NH3-O2 atmosphere, solid-source chemical vapour deposition, co-doping of Al-N using reactive magnetron sputtering, metalorganic chemical vapor deposition (MOCVD) using NO and P2O5 as the dopant source. In this work, N-doped, p-type ZnO thin films were grown by plasma-assisted MOCVD and ZnO homojunction light-emitting diode was successfully fabricated. The p-type ZnO thin films were grown on n-type bulk ZnO substrates. The as-grown films on glass substrates show hole concentration of 1.0E+16-1.0E+17 cm−3 and mobility of 1-10 cm2 V−1 s−1, and were consistently reproducible. A N-related free-to-neutral-acceptor emission and an associated phonon replica were evident in room temperature photoluminescence spectra, from which the N acceptor energy level in ZnO was estimated to be 180 meV above the valence band maximum. A typical ZnO homojunction shows rectifying behavior with a turn-on voltage of about 2.3 V. Electroluminescence at room temperature shown as Fig.1 has been demonstrated with band-to-band emission at I = 40 mA and defect-related emissions in the blue-yellow spectrum range.


9:00 AM K8.2
Systhesis of p-type ZnO Thin Films by (N,Ga) Co-doping using DMHy Dopant.Hui Wang and Ho-pui Ho; Dept. of Electronic Engineering, The Chinese University of Hong Kong, Hong Kong, China.

P-type ZnO films have been fabricated on sapphire substrates, using DMHy as nitrogen dopant by metal-organic vapor phase epitaxy. As far as we know, this is the first report of using DMHy as nitrogen dopant source for p-type ZnO films. We found that DMHy exhibits a narrow temperature window from about 500oC to 550oC for efficient nitrogen doping, and also that nitrogen incorporation is greatly enhanced by co-doping using Ga. Low temperature photoluminescence spectra show the conversion of the dominant peak from neutral donor bound excitons (DoX) to neutral acceptor bound excitons (AoX) with (N,Ga) co-doping. Pronounced donor-acceptor pair (DAP) transition with an intensity comparable to the excitonic recombination was observed at 3.235 eV, which is repeated by longitudinal optical phonon replicas with an energy of 72 meV. The nitrogen acceptor level is calculated to be about 160 meV. A hole concentration of 2.41x1018 cm-3 and hole mobility of 4.29 cm2/V-s were observed from the co-doped p-type ZnO:(N,Ga) film by Hall measurement.


9:15 AM K8.3
Electrical Characterization of Defect States in Local Conductivity Domains in ZnO:N,As Layers.Andre Krtschil, Armin Dadgar, Anette Diez and Alois Krost; Institute of Experimental Physics, Otto-von-Guericke-University of Magdeburg, Magdeburg, Germany.

In the last few years zinc oxide has been established as one of the most intensively studied semiconductor materials worldwide offering a tremendous application potential for optoelectronics because of its wide direct band gap and the large exciton binding energy. However there are still some significant challenges to solve before ZnO can replace the group-III-nitrides there. Mainly the lack of reproducible and effective acceptor doping as well as the homoepitaxial growth are still puzzling. Adressing the first point, we have already shown in a previous work [1] that simultaneous doping with arsenic and nitrogen (so-called dual doping) during metal organic vapour phase epitaxy provides long-time stable p-type ZnO. However, these layers are not homogeneously p-type, but contain largely extended p-type domains surrounded by some local n-type areas as monitored by scanning capacitance microscopy (SCM) and scanning surface potential microscopy (SSPM). A comparison with the AFM signal revealed a one-by-one correlation between conductivity type and surface morphology. In fact, smooth 2D-regions are p-type and structural defects (pits, micro-cracks) and 3D-islands are n-type. We tentatively discussed this phenomenon as selective dopant incorporation due to local defects and grains very similar to the group-III-nitrides [2]. Unfortunately at that time we had no further information on the origin of the acceptors in the p-type regions and about other defects which control the local compensation equilibrium. In the present paper we will analyze the electrically active defects in these conductivity domains in more detail. To achieve the necessary spatial resolution during the defect characterization, the samples were investigated by SSPM and SCM after a bias pulse at different temperatures (from room temperature to 75°C) or under optical excitation with monochromatic light of variable wavelength between 300 and 3000 nm, respectively. These techniques are derived from the macroscopic deep level transient and optical admittance spectroscopy approaches and allow the analysis of thermally and optically induced defect-to-band-transitions, respectively, with submicron spatial resolution. First results indicate a broad defect band around 460nm which typically appears after arsenic incorporation, whereas nitrogen effectively reduces signals in the near band gap region and increases transitions at about 400nm. Still more interesting are states which exclusively appear after simultaneous doping with both acceptor species. These states are determined from a comparison of As and N mono- and dual-doped samples and will be discussed in detail. [1] A.Krtschil, A.Dadgar, N.Oleynik, J.Bläsing, A.Diez, and A.Krost, Appl. Phys. Lett. 87, 262105 (2005) [2] A.Krtschil, D.C.Look, Z.-Q.Fang, A.Dadgar, A.Diez, and A.Krost, Physica B 376-377 (2006), p.703-706


9:30 AM K8.4
Properties of p-type ZnO Grown by Oxidation of Zn-group-V Compounds.Eliana Kaminska1, Ewa Przezdziecka2, Anna Piotrowska1, Jacek Kossut3, Piotr Boguslawski2, Elzbieta Dynowska2, Witold Dobrowolski2, Rafal Jakiela2, Iwona Pasternak1 and Elzbieta Lusakowska2; 1Institute of Electron Technology, Warsaw, Poland; 2Institute of Physics, PAS, Warsaw, Poland; 3Institute of Physics, PAS and ERATO Semiconductor Spintronics Project, Warsaw, Poland.

ZnO is well recognized for its potential applications in transparent electronics and photonics. The implementation of this material, however, has been hampered by an inability to reproducibly control p-type conductivity. In search for appropriate p-type doping procedure, large-size-mismatched As and Sb acceptors [1] have recently received much attention. In this communication we discuss preparation of p-type ZnO by oxidation of Zn-group-V compounds. We extend our recently developed method of obtaining p-ZnO by oxidation of zinc nitride or N-doped ZnTe to other Zn compounds. Zn-As and Zn-Sb layers deposited by sputtering either on the sapphire or on undoped ZnO substrates, were used as starting material. Thermal oxidation and activation of the dopants were performed by RTA in O2 and N2 atmospheres. The microstructure and composition of thin ZnO films were determined with XRD, SIMS, SEM, and AFM. Transport properties were assessed from Hall measurements. Optical characterisation involved PL and transmission measurements. The optimal conditions for formation of p-type ZnO layers without inclusions of second phases have been established. The observed doping properties of N are qualitatively different from these of As and Sb. In particular, doping stability depends on the acceptor species. While the N-doped samples are readily converted into n-type when exposed to ambient atmosphere or to high temperature annealing, the As- and Sb-doped layers are stable. PL study showed meaningful differences between samples with different acceptors. Finally, we find that a similar amount of H causes higher compensation of N than of As and Sb. The transmittance of p-ZnO films in the visible wavelength spectrum was high independent of the acceptor used. The different doping properties of N as compared to As and Sb the most likely originate from their different incorporation modes into ZnO. While N is expected to substitute O, both As and Sb were suggested to substitute Zn and form complexes with Zn vacancies [1]. 1. S. Limpijumnong et al., Phys. Rev. Lett. 92, 155504 (2004).


9:45 AM K8.5
Donors and Acceptors in Bulk ZnO Grown by Vapor-phase, Melt, and Hydrothermal Processes.David C Look, Semicondcutor Research Center, Wright State University, Dayton, Ohio; Materials and Manufacturing Directorate, Air Force Research Laboratory, Dayton, Ohio.

One of the advantages of ZnO as a material for photonic and electronic devices is the availability of large-area wafers from bulk crystals. Such crystals can be grown by vapor-phase (VP), melt, or hydrothermal processes, and each type of material has unique structural, optical, and electrical characteristics. Here we will discuss important similarities and differences in their donor and acceptor properties, as determined by low-temperature photoluminescence (PL) and temperature-dependent Hall-effect (TDH) measurements. All three growth techniques produce material with sharp (FWHM ≤ 0.3 meV), intense donor-bound-exciton (D0X) lines in the UV region (3.357 - 3.365 eV), but much broader spectra in the visible region (1.7 - 2.5 eV). However, there are interesting growth-dependent differences. For example, I4, arising from interstitial H, is usually the strongest line in VP material, whereas it is weak or nonexistent in melt or hydrothermal samples. On the other hand, the lines due to Group III elements, I6(Al), I8(Ga), and I9(In), are all found in VP and melt ZnO, but only I6 and I8 appear in hydrothermal ZnO. These donors, H, Al, Ga, and In, are all shallow, with energies in the 45 - 60 meV range. Besides the D0X transistions, there are also much weaker visible emissions. The VP samples are distinguished by a 2.5-eV green band and sometimes a small 1.7-eV red band, whereas the melt and hydrothermal samples show a strong 2.4-eV band with a much weaker shoulder at 2.2 eV. The 2.4/2.5-eV band is variously assigned to CuZn, VZn, or VO, and may have more than one origin. The 1.7-eV band is associated with an interstitial, according to a recent study. The TDH measurements in the VP and melt samples always show a dominance of shallow donors, of total concentration in the low-mid 1016-cm-3 range, while the hydrothermal samples are controlled by much deeper donors, but of similar concentrations. In VP material three donors, at energies of 30, 45, and 75 meV, and assigned to ZnI-NO, HI, and AlZn/GaZn, respectively, are sometimes necessary to accurately fit the n vs. T data. However, in melt ZnO, a single donor at about 50 meV is usually sufficient, and in hydrothermal ZnO, a deeper donor, at 200 - 400 meV, is nearly always dominant at room temperature. With regard to acceptors, the VP ZnO has the lowest concentration, ~ 2 x 1015 cm-3, and thus the highest peak mobility, up to 2500 cm2/V-s. The dominant acceptor in VP material is known to be VZn, but other acceptors, such as NO, may also be present but passivated by H. Hydrothermal ZnO has a much higher acceptor concentration, ~ 1 x 1016 cm-3, probably due to LiZn, and melt ZnO has an intermediate concentration. Further insights can be gained from electron irradiation, forming-gas anneals, and SIMS. Surface conduction will also be discussed.


10:30 AM *K8.6
Phosphorus Doped ZnO Light-emitting Diodes. Jae-Hong Lim, Kyoung-Kook Kim, Dae-Kue Hwang and Seong-Ju Park; Department of Materials Science and Engineering, Gwangju Institute of Science & Technology, Gwangju, South Korea.

Among the available wide bandgap semiconductors, zinc oxide (ZnO) of a large direct bandgap of 3.37 eV is a promising candidate for use as an efficient UV light emitter due to the characteristic features of ZnO. Homojunction ZnO light-emitting diode(LED) was successfully fabricated by using phosphorus oxide doped p-type ZnO films. Photoluminescence spectra of p-type ZnO:P thin films showed an acceptor bound excitonic and phosphorus related peaks at low temperature and strong band edge emission peak at 380nm at room temperature. A p-n homojunction ZnO LED with a structure of p-ZnO:P/n-ZnO:Ga was fabricated and it showed a clear emission peak at 380 nm which corresponds to the near band edge of ZnO. The I-V characteristics of ZnO LED showed a low threshold voltage of 3.2 V. The intensity of near bandedge emission was further increased and the deep-level emission was greatly suppressed by using Mg0.1Zn0.9O layers as energy barrier layers to confine the carriers to the high quality n-type ZnO. The fabrication and the characteristics of heterojunction ZnO light-emitting diode (LED) which consists of p-type ZnO:P and n-type GaN:Si layers are also reported. The current-voltage (I-V) and electroluminescence measurements of ZnO LED showed a threshold voltage of 5.4 V and a red-shifted band-edge emission of 409 nm due to the band offset.


11:00 AM K8.7
A Promising p-type Doping Method in ZnO Thin Film Using Li-N Dual-acceptor Dopant Source.Liping Zhu, Zhizhen Ye, Jianguo Lu, Yinzhu Zhang and Binghui Zhao; State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou, Zhejiang, China.

Due to the large exciton binding energy of 60 meV, ZnO is known to be the brightest emitter of available wide-bandgap semiconductors. However, The realization of low-resistivity, stable p-type ZnO has proven difficult due to asymmetric doping limitations. Considerable efforts have been made in creating p-type ZnO, but more recent studies revealed that the resultant p-type behavior could be unstable, and even disappeared over time. The optimal choice of acceptor species for realization of p-type ZnO remains to be determined. In this study, a Li-N dual-acceptor doping method has been developed to prepare p-type ZnO thin films by pulsed laser deposition. The lowest room-temperature resistivity is found to be 0.93 Ωcm, much lower than that of Li or N mono-doped ZnO films. The p-type conductivity of ZnO:(Li,N) films is very reproducible and stable, with acceptable crystal quality. The acceptor activation energy in ZnO:(Li,N) is about 95 meV. ZnO-based homostructural p-n junctions were fabricated by depositing a n-type ZnO:Al layer on a p-type ZnO:(Li,N) layer, confirmed by secondary ion mass spectroscopy. The current-voltage characteristics exhibit their inherent rectifying behaviors. The Li-N dual-doping mechanism is discussed, and this approach is expected as an ideal candidate in producing p-type ZnO for practical applications.


11:15 AM K8.8
Effect of Annealing Temperature and Ambient Gases on the Phosphorus Doped p-type ZnO.Dae-Kue Hwang, Mis-Suk Oh, Jae-Hong Lim, Chang-Goo Kang, Young-Seok Choi and Seong-Ju Park; Gwangju Institute of Science and Technology, Gwangju, South Korea.

It is well known that the ZnO films grown in an O2 rich condition have a better chance to show p-type conductivity compared to those grown in the Zn rich growth condition due to the decrease of native defects acting as compensation centers in ZnO films. Recently, we reported that the phosphorus doped n-type ZnO can also be converted to p-type ZnO by a post annealing process. However, there is no report on the effect of various ambient gases on the post annealing of phosphorus doped ZnO. In this study, we report on the thermal activation of phosphorus doped ZnO thin films grown by radio frequency (RF) magnetron sputtering. Phosphorus doped ZnO thin films were activated to obtain p-type ZnO in the N2, Ar, and O2 ambient at different annealing temperatures. The hole concentration of p-type ZnO in an O2 ambient showed a lower hole concentration (2.01×10^17 cm-3 at 850 °C) compared to those of samples annealed in N2 (4.8×10^18 cm-3 at 850 °C) and/or Ar (4.5×10^18 cm-3 at 850 °C) ambient. The measurement of activation energy of phosphorus dopant in the ZnO film and the study on the effect of ambient gases on the hole concentration suggested that the dissociation of Zn-O and P-O are suppressed in the O2 ambient and the phosphorus replaces oxygen atoms in the ZnO film to increase the hole concentration in the phosphorus doped ZnO film.


11:30 AM K8.9
Bipolar Phosphorus-doping of ZnO Thin Films Fabricated by Pulsed Laser Deposition. Xiaoqing Pan1, Arnold Allenic1, Guangyuan Zhao1, Yong Che2, Zhendong Hu2 and Bing Liu2; 1Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan; 2Materials Research Group, IMRA America Inc., Ann Arbor, Michigan.

Semiconductor-based light-emitting devices require n-type and p-type materials. ZnO can readily be doped n-type, however it shows strong resistance to shallow acceptor doping. In this work, epitaxial phosphorus-doped ZnO (PZO) films were grown on sapphire by pulsed laser deposition. The full width at half maximum (FWHM) value of the rocking curve of the (0002) ZnO peak is as low as 0.08°. The surface of PZO films is smooth with rms values smaller than 1 nm. The film microstructure and film/substrate interface structure were characterized by transmission electron microscopy. As-grown PZO films are n-type with n~1019 cm-3 and ρ~10-3 Ω.cm. On the other hand, PZO films grown between 400 °C and 600 °C and subsequently annealed are p-type with p~5.0×1017 cm-3 and ρ~40 Ω.cm. PL spectra at 10 K of p-type PZO show a dominant phosphorus-related peak at 3.3417 eV and a broad red band peaking at 1.93 eV.


11:45 AM K8.10
Microwave Enhanced Nitrogen Plasma Doping of ZnO Crystals.Yi-Chun Liu, Jiping Cheng and Ruyan Guo; Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania.

ZnO has attracted significant interests in optoelectronic devices research, due especially to its direct wide band gap (3.37 eV), large exciton binding energy (60 meV), and relatively low cost. Although n-type ZnO is easily attained, it has been recognized that it is very difficult to create reproducible p-type material, which thus impedes the development of various active devices. In this work, we report a new approach and the initial success in producing controlled doping in ZnO by microwave enhanced plasma deposition. Both ohmic contact, using Ti/Au, and p-type doping, by nitrogen plasma doping, are obtained. The characteristics are verified by I-V measurement. Synthesis details and the analysis on the microwave nitrogen plasma doped ZnO materials, including photoluminescence spectra, I-V curve, and Hall-effect measurements, will be presented.

SESSION K9: Processing, TFT

Chairs: F. Danie Auret and Julia Hsu
Thursday Afternoon, November 30, 2006
Room 200 (Hynes)
1:30 PM K9.1
Formation of Nanovoids and Nanocolumns in High Dose Hydrogen Implanted ZnO Bulk Crystals. Rajendra Singh1, Roland Scholz1, Ulrich Gösele1 and Silke Christiansen1,2; 1Experimental Deptt. 2, Max Planck Institute of Microstructure Physics, Halle, Sachsen Anhalt, Germany; 2Physics Department, Martin Luther University Halle-Wittenberg, Halle, Sachsen Anhalt, Germany.

ZnO(0001) bulk crystals grown by a hydrothermal synthesis method were implanted by 100 keV H2+ ions with various doses in the range of 5x1016 to 3x1017 cm-2. The ZnO crystals implanted up to a dose of 2.2x1017 cm-2 did not show any surface exfoliation after post-implantation annealing of a variety of time/temperature combinations while those crystals implanted with a dose of 2.8x1017 cm-2 or higher exhibited exfoliated surfaces already in the as-implanted state. In a narrow dose window in between, controlled exfoliation can be obtained upon annealing. Cross-sectional transmission electron microscopy (XTEM) of the implanted ZnO wafers showed that a large number of nanovoids, having dimensions of about 10 nm, formed within the implanted zone of ZnO. These nanovoids obviously serve as precursors for the formation of microcracks leading to the exfoliation of ZnO surfaces. In addition to the nanovoids, nanocolumns perpendicular to the substrate surface form, having diameters of up to about 10 nm and lengths of up to a few hundreds of nanometers. These nanocolumns are found in the ZnO lattice even well beyond the projected range of hydrogen ions. Moreover, the micro-roughness, as given by the root-mean-square (RMS) roughness, of the hydrogen implanted and exfoliated ZnO surface, as measured by atomic force microscopy (AFM) yields a particularly high value of about 24 nm in a 10x10 µm2 scan and a peak-to-valley height of about 100 nm, which is much higher compared to other hydrogen implanted and exfoliated semiconductors such as Si, InP, GaAs and GaN, where an RMS roughness value of less than 8 nm is usually found. These findings of nanostructural pecularities in ZnO are discussed with respect to successfully using a high dose hydrogen implantation to initiate ZnO layer transfer after direct wafer bonding to a variety of desired handle wafers.


1:45 PM K9.2
Effect of Cryogenic Temperature Deposition of Various Metal Contacts to Bulk, Single-Crystal n-type ZnO.Jon Wright1, L. Stafford1, B. P. Gila1, D. P. Norton1, S. J. Pearton1, Hung-Ta Wang2 and F. Ren2; 1Materials Science and Engineering, The University of Florida, Gainesville, Florida; 2Chemical Engineering, The University of Florida, Gainesville, Florida.

The development of reliable and thermally stable Ohmic and Schottky contacts to ZnO is one of the critical issues related to the fabrication of ZnO-based UV light emitters/detectors and field effect transistors. To date, a number of different metallization schemes and surface cleaning procedures prior to metal deposition have been examined for rectifying contacts on n-ZnO. While these reports have shown that low reactive metals such as Au, Ag and Pd form rectifying contacts with Schottky barrier heights in the 0.6-0.8 eV range, the thermal stability of these contacts is usually extremely poor, with degradation occurring even at 60 C for Au/n-ZnO. One approach to achieving increased barrier heights that has proven successful for GaAs, InP, InGaAs and other compound semiconductors is the use of cryogenic deposition temperatures. In this context, we report in this work on the effect of cryogenic temperature metal deposition on the contact properties of Pd, Pt, Ti, and Ni on single-crystal n-type ZnO. Deposition at both room and low temperature produced contacts with Ohmic characteristics for Ti and Ni metallizations. In comparison, both Pd and Pt contacts showed rectifying characteristics after deposition. All rectifying contacts exhibited barrier heights around 1-2 eV and idealities between 1 and 2. Low temperature deposition gave higher resistances in comparison to room temperature deposition for all cases. Larger contacts also corresponded to an increase in resistance. Changes in contact behavior were measured on Pd to anneal temperatures of ~300 C, showing an increase in barrier height along with a decrease in ideality with increasing temperature. This difference with annealing temperature is in sharp contrast to previous results for Au contacts to ZnO. There were no differences in near-surface stoichiometry for the different deposition temperatures; however low temperature contacts demonstrated some cracking in Pt and Pd, probably due to surface stress.


2:00 PM K9.3
Schottky Contact Behaviour as a Function of Metal and ZnO Surface Polarity Martin W. Allen1,2, Paul Miller3,2, Jessica Chai1,2, James B. Metson4,2, Roger J. Reeves3,2, Maan Alkaisi1,2 and Steven Durbin1,2; 1Dept. of Electrical and Computer Eng., University of Canterbury, Christchurch, New Zealand; 2MacDiarmid Institute for Advanced Materials and Nanotechnology, Christchurch, New Zealand; 3Dept. of Physics and Astronomy, University of Canterbury, Christchurch, New Zealand; 4Dept. of Chemistry, University of Auckland, Auckland, New Zealand.

Although significant advances have been made in recent years concerning the heteroepitaxial growth of high-quality single crystal ZnO, many of the best reported electrical properties are for bulk crystals, and p-type material remains elusive. For visible-blind UV detectors and related applications, Schottky as opposed to pn junction devices may prove the most desirable, anyway, although control over the electrical properties is still important in such situations. There are numerous reports of high-quality Schottky contacts to ZnO, particularly in the case of bulk crystals. Among the many metals investigated, Ag, Au, Pd and Pt are the most common, although few reports include results for each of these materials on the same samples, and there remains a need for further study of the role surfaces (in particular, surface polarity) play in determining contact properties. High-quality bulk ZnO single crystals are available commercially and have been for some time, although the electrical characteristics vary somewhat from vendor to vendor. We have recently investigated the comparative performance of Ag, Au, Pd and Pt contacts to “epi-polished” bulk ZnO single crystals from three separate vendors (Cermet, MTI, and Tokyo Denpa Co., Ltd.) for both polar and non-polar surfaces. In each case, arrays of ring-dot contacts of each metal were lithographically patterned onto the same wafer sample which was first cleaned using solvents. No etching, annealing or surface treatment of the wafers was performed. A separate piece of each wafer was characterised using photoluminescence at 4 K and 300 K, and room-temperature Hall effect measurements were conducted. Diodes were characterised at room temperature by both current-voltage and capacitance-voltage techniques, the results of which were generally found in good agreement with one another, indicating a reasonable degree of homogeneity/uniformity over the contact area. The most striking result is that the best diodes tested to date were based on Ag, with barrier heights slightly greater than 1 eV - despite the fact that Ag has the lowest workfunction of the four metals considered. No significant polarity effect was observed for these diodes, nor for corresponding devices based on Au. Intriguingly, a measurable difference in barrier height was observed for both Pd and Pt based diodes fabricated on Zn-polar (0001 oriented) and O-polar (000-1) faces, with the higher quality devices being those on O-polar surfaces. Analysis of the Ag-based devices indicated that the best rectification behaviour corresponded to contacts exhibiting clear signs of oxidation under an optical microscope, confirmed by X-ray photoelectron spectroscopy to include Ag2O - a material which has been suggested by others to have a work function as high as 7 eV. We have confirmed this correlation by intentionally fabricating Ag2O contacts via RF sputtering using an elemental Ag target in conjunction with a 50 W O2/Ar plasma.


2:15 PM K9.4
Microstructure and Electrical Property Correlations in Ga:ZnO Transparent Conducting Thin Films.Vikram Bhosle and Jagdish Narayan; Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina.

We report the correlations between processing, microstructure and electrical properties of Ga doped ZnO films. Films with varying grain size were grown on amorphous glass by changing the substrate and pulsed laser deposition variables. The results corresponding to these films were compared with those from epitaxial single crystal films grown on (0001) sapphire. Microstructural characteristics were analyzed in detail by using X-ray diffraction and transmission electron microscopy. Electrical properties were evaluated by resistivity measurements in the temperature range of 15-300K and Hall measurements at room temperature. It was observed that the grain boundaries and orientation of grains (texture characteristics) affected the carrier concentration and the mobility considerably in nanocrystalline films deposited on glass substrates. This effect is envisaged to occur as a result of trapping of electrons and build up of a potential barrier across the grain boundaries. However, the resistivity in nanocrystalline films could be decreased significantly by carefully controlling the deposition conditions. For a film deposited on glass at 2000C and 1 mtorr of oxygen partial pressure, we attained a minimum resistivity value of 1.8 x 10-4Ω-cm. The epitaxial films on sapphire substrates showed a resistivity of 1.4 x 10-4Ω-cm, deposited at 4000C and pressure of 2.4 x 10-2 torr. Role of grain boundaries and defects in controlling the carrier generation and transport is considered in detail and the possible mechanisms limiting the electrical conductivity in films with different microstructures are identified.


2:30 PM K9.5
Large Area Multi-wafer MOCVD of Transparent and Conducting ZnO Films.Gary S. Tompa1, S. Sun1, L. G. Provost1, D. Mentel1, D. Sugrim1, Philip Chan2, Keny Tong2, Raymond Wong2 and A. Lee2; 1Structured Materials Industries, Inc., Piscataway, New Jersey; 2Podium Photonics, Ltd., Kowloon, Hong Kong.

ZnO thin films are of interest for an array of applications; including: light emitters, photovoltaics, sensors and transparent contacts among others. Production routes for ZnO include sputtering, MBE and MOCVD. This paper focuses on our efforts to produce a large scale MOCVD thin film production tool and the results obtained from the reactor. Specifically, we have constructed a tool with a 16" wafer carrier that uniformly deposits ZnO films on 38x2" wafers simultaneously. The reactor operates at low pressure (<0.1 Atmosphere) and through 700oC. High quality, uniform films have been deposited on an array of substrates. Al doped films exhibited conductivities in the 1x10-3 ohm-cm range and transmissivity greater than 80%. Film morphology and crystallinity are a function of process parameters. The presentation addresses large area oxide MOCVD reactor design challenges that affect tool performance and ZnO thin film quality.


2:45 PM K9.6
One-step Non-Lithographic Microetching of Transparent Conductive Oxides and Semiconductors.Stoyan Smoukov and Bartosz Grzybowski; Chem. & Biol. Engineering, Northwestern University, Evanston, Illinois.

A benchtop, one-step, maskless-but-parallel method of etching micropatterns is described. Transparent conducting oxides (ZnO, ITO) and semiconductors (GaAs) are etched with lateral resolution down to 200nm using a reaction-diffusion process. Patterned hydrogel stamps act as a reservoir and a two-way "pump" transporting etchant onto the substrate while removing reaction products into its bulk. This low-cost method is suitable for rapid prototyping of bulk and thin film materials, minimizing the need for multistep lithographic processing.


3:30 PM *K9.7
The (R)Evolution of Thin Film Transistors.Elvira Fortunato, P. Barquinha, A. Pimentel, A. Goncalves, L. Pereira and R. Martins; Materials Science, CENIMAT, Caparica, Portugal.

Transparent electronics are nowadays an emerging technology for the next generation of optoelectronic devices. Oxide semiconductors are very interesting materials because they combine simultaneously high/low conductivity with high visual transparency and have been widely used in a variety of applications (e.g. antistatic coatings, touch display panels, solar cells, flat panel displays, heaters, defrosters, optical coatings, among others) for more than a half-century. Transparent oxide semiconductor based transistors have recently been proposed using as active channel intrinsic zinc oxide (ZnO) [1,2]. The main advantage of using ZnO deals with the fact that it is possible to growth at/near room temperature high quality polycrystalline ZnO, which is a particular advantage for electronic drivers, where the response speed is of major importance. Besides that, since ZnO is a wide band gap material (3.4 eV), it is transparent in the visible region of the spectra and therefore, also less light sensitive. In this work after a short overview about the history of TFTs, we report some of our recent results concerning the fabrication and characterization of high field-effect mobility ZnO and ZnO based-thin film transistor deposited at room temperature by rf magnetron sputtering. [1] E. Fortunato, P. Barquinha, A. Pimentel, A. Gonçalves, A. Marques, L. Pereira, R. Martins, Appl. Phys. Lett. 85, 2451 (2004). [2] E. Fortunato, P. Barquinha, A. Pimentel, A. Gonçalves, A. Marques, L. Pereira, R. Martins, Advanced Materials 17, 590 (2005).


4:00 PM K9.8
Electrical Stability of Low-Temperature Amorphous Gallium-Indium-Zinc-Oxide Thin Film Transistors under Constant Current Stress for AM-OLED Application.Ihun Song1, Chang Jung Kim1, Donghun Kang1, Jae Chul Park1, Hyuck Lim1, Sunil Kim1, Youngsoo Park1, RanJu Jung2, Jae Cheol Lee2 and Eunha Lee2; 1SDM Lab, SAIT, Suwon, South Korea; 2AE Center, SAIT, Suwon, South Korea.

The transistors which drive AM-OLED circuits should be operated under a constant current stress condition. Previous studies showed that the threshold voltage of amorphous silicon thin film transistor shifted by several volts under current stress condition due to charge trapping and dangling-bond defect state creation. Recently, there have been reports that oxide based thin film transistors could be better choice for the purpose of reducing charge trapping in the channel materials. Especially, amorphous oxide thin film transistors take advantages of the good uniformity and smooth surface morphology. In this study, we have developed very stable amorphous Gallium-Indium-Zinc-Oxide(a-GIZO) thin film transistors under constant current stress operations. The a-GIZO films were deposited by rf magnetron sputtering at room temperature and confirmed to be amorphous by X-ray diffraction. We also examined the effect of the thickness of channel layer and the chemical composition of GIZO thin films on the stability of the a-GIZO thin film transistors. The a-GIZO thin film transistors showed a high mobility of 40 cm2/Vs with a high on-to-off current ratio of 108. Gate leakage and the sub-threshold voltage swing were about 2 pA and 0.23 V/decade, respectively. Surprisingly, the a-GIZO thin film transistors exhibited the threshold voltage shifts less than 0.2 V for 100 hours at 60°C under constant current stress. These promising results indicate that the a-GIZO thin film transistors could be a candidate for driving transistors of large area AM-OLED display.


4:15 PM K9.9
Atomic Layer Deposition ZnO as an Active Channel Layer of Transparent Thin Film Transistor.Seongjoon Lim, Soonju Kwon and Hyungjoon Kim; Materials Science and Engineering, POSTECH, Pohang, Gyungbuk, South Korea.

Recently, the application of ZnO as an active channel layer of transparent thin film transistor (TTFT) has become of great interests. However, ZnO prepared by deposition methods other than sputtering has rarely been studied for this application. Atomic layer deposition (ALD) is an attractive deposition method for display device due to large area uniformity and low process temperature. In this study, ZnO thin films were deposited by ALD using diethyl Zn (DEZ) as a precursor. To optimize the electrical properties of ALD ZnO as an active layer of TTFT, the ALD ZnO thin films were deposited with various reactants. By this approach, ALD ZnO thin films with carrier concentration as low as 1014 cm-3 were obtained at low growth temperature, producing TTFT with low off current. For comparison, ZnO thin films were also prepared by RF sputtering. For both ALD and RF sputtered ZnO, key electrical properties including resistivity, mobility, and carrier concentration were characterized and the microstructure and chemical properties of were analyzed by x-ray diffraction, transmission electron microscopy, X-ray photoemission spectroscopy, and Rutherford back scattering. TTFTs with high k gate insulator, also prepared by ALD, were fabricated and the device properties were characterized. The device characteristics of TTFT will be discussed focusing on the comparison between ALD and RF sputtered ZnO active layer.


4:30 PM K9.10
A Comparison of the Stability of ZnO TFTs with Oxide and Nitrides as the Insulating Layer. R.B.M. Cross and M.M. DeSouza.
Abstract not available.


4:45 PM K9.11
Scaling and Parasitic Effects on ZnO Transparent Thin Film Transistors.Hsing-Hung Hsieh1,3 and Chung-Chih Wu1,2,3; 1Graduate Institute of Electronics Engineering, National Taiwan University, Taipei, Taiwan; 2Graduate Institute of Electro-Optical Engineering, National Taiwan University, Taipei, Taiwan; 3Department of Electrical Engineering, National Taiwan University, Taipei, Taiwan.

Recent development of transparent TFTs (TTFTs) using large bandgap (Eg>3eV) semiconductors such as ZnO and related metal oxides is of particular interest. Such TTFTs could render a nearly 100% aperture ratio or even fully transparent active-matrix displays. To meet the requirements for real high-resolution display applications, we fabricated miniaturized ZnO TTFTs by using the lithographic method and studied the scaling and parasitic effects of these devices. Inverted staggered type ZnO TTFTs with varied dimensions were fabricated at room temperature. The channel length was varied from 2 to 50 um, and the channel width was varied from 20 to 200 um. ITO was used as gate, source, and drain electrode. ZnO was used as active layer and deposited by RF sputtering in Ar/O2, with gas flow rates of 45 sccm/5 sccm, chamber pressure of 5 mtorr, and RF power of 200 W. Double layer gate insulators consisting of Al2O3 and HfO2 were used due to their large bandgap and high dielectric constant, respectively. Post-fabrication annealing at 265 C in nitrogen was performed in order to improve the crystallinity and device performances. The ZnO TTFTs operated in the n-type enhancement mode, and exhibited hard saturation in long-channel devices (> 5 um). Mobility larger than 8 cm2/Vs and on/off ratio up to 10^7 were achieved with these devices. The transmission was more than 80% in the whole visible band. The device characteristics were rather immune to ambient illumination, which implies an advantage of needing no black matrices or light-shielding structure in active-matrix applications. Our results showed that these ZnO TTFTs retain well-behaved transistor characteristics down to channel length of ~5 um, rendering possible high-resolution applications. Apparent short channel effects (e.g., lowering of threshold voltages, degradation of subthreshold slope with the decrease of the channel length and the increase of drain voltage, and loss of hard saturation, etc.) were observed in our devices when the channel length was reduced below 5 um. These short channel effects can be explained by the charge sharing or drain-induced barrier lowering. Influences of parasitic series resistance on TFT characteristics were also studied. Parasitic series resistance and channel resistance were extracted using devices of various dimensions. The parasitic series resistance Rp*W was typically on the order of 10^2 ~ 10^3 ohm-cm, depending on the gate voltages. The ratio of parasitic series resistance to channel resistance at Vg = 10 V was increased from 0.04 to 0.36, when the channel length decreased from 20 um to 2 um. This indicates that parasitic series resistance has substantial influences on device performances when the channel length is reduced, and better contact techniques may be required.

SESSION K10: Poster Session II

Chairs: Jürgen Christen, Chennupati Jagadish, David Look and Takafumi Yao
Thursday Evening, November 30, 2006
8:00 PM
Exhibition Hall D (Hynes)


K10.1
Ferromagnetic Ordering at Room Temperature in Co:ZnO Nanoparticles. Sujeet Chaudhary, Kanwalpreet Bhatti, Shankhamala Kundu, Subhash C Kashyap and Dinesh K Pandya; Thin Film Laboratory, Department of Physics, Indian Institute of Technology Delhi, New Delhi 110 016, India.

The progress in the emerging field of spintronics is inherently associated with the search for newer semiconductor materials which should exhibit ferromagnetic behavior at room temperature. Such semiconductors are proposed to add new functionalities to the existing electronic and photonic devices. The challenge is to modify the oxides of Zn, Sn, Ti, etc., so that they exhibit intrinsic and stable room temperature ferromagnetism (RTFM), and a high Curie temperature, (T_c>300K). In past few years, various II-VI semiconducting materials, such as transition metal doped ZnO exhibiting RTFM, gained worldwide research interest. The case of Co:ZnO system is particularly interesting since most of the reports on bulk samples did not corroborate RTFM, though invariably evidenced in several reports on the thin films. In the present work, we report the results of the effect of processing parameters on the magnetization (M), structural property and phase purity investigations of the cobalt substituted nanocrystalline bulk samples of ZnO. The bulk samples have been synthesized by using acetates and poly vinyl pyrrolidane as precursors. The pallets were made from calcined powders. Both the 5% and 10% Co-substituted ZnO samples exhibited RTFM and had an estimated particle size in the range of 50-60 nm. These pellets were successively sintered upto 900C in steps of 100C, after which the particle size became ~80 nm. The sintering in air ambient at 700C for 6h resulted in formation of Co_3O_4 phase causing a sizable reduction in the saturation magnetic moment (M^sat). With further sintering at 800C, CoO also started forming. Further reduction in M^sat, however, was only small. When sintered at 900C for 6h, no further change in M^sat was detected in either of the samples. No noticeable change in the intensity of diffraction peaks due to either CoO or Co_3O_4 was seen even when the samples were sintered at 900C for additional 6h. The observed M-H behavior can be understood on the bases of ferromagnetic and paramagnetic contributions. The para contribution is attributed to Co_3O_4 and CoO phases, which are known to be paramagnetic at RT. The origin of FM can definitely not be attributed to metallic Co-clustures since these were not found in our samples prepared in air ambience. Based on the detailed M-H and Reitveld refinement studies, it is found that although the RTFM in Co:ZnO system gets affected by processing conditions, it can not be totally destroyed even after the prolonged sintering of the samples at higher temperatures. The observed RTFM in Co:ZnO system is explained on the basis of defect mediated bound magnetic polaron model.


K10.2
Abstract Withdrawn


K10.3
Structure-Property Relationships in the xZnO-(1-x)alpha-Fe2O3 Nanoparticle System. Monica Sorescu1 and Lucian Constantin Diamandescu2; 1Physics, Duquesne University, Pittsburgh, Pennsylvania; 2Materials Science, National Institute for Materials Physics, Bucharest, Romania.

The xZnO-(1-x)alpha-Fe2O3 nanoparticles system has been obtained by mechanochemical activation for x=0.1, 0.3 and 0.5 and for ball milling times ranging from 2 to 24 hours. Structural and morphological characteristics of the zinc-doped hematite system were investigated by X-ray diffraction (XRD) and Mossbauer spectroscopy. The Rietveld structure refinements of the XRD spectra yielded the dependence of the particle size and lattice constant on the amount x of Zn substitutions and as function of the ball milling time. The x=0.1 XRD spectra are consistent with line broadening as Zn substitutes Fe in the hematite structure and the appearance of the zinc ferrite phase at milling times longer than 4 hours. Similar results were obtained for x=0.3, while for x=0.5 the zinc ferrite phase occurred at 2 hours and entirely dominated the spectrum at 24 hours milling time. The Mossbauer spectra corresponding to x=0.1 exhibit line broadening as the ball milling time increases, in agreement with the model of local atomic environment. Because of this reason, the Mossbauer spectrum for 12 hours of milling had to be fitted with two sextets. For x=0.3 and 12 milling hours, the Mossbauer spectrum reveals the occurrence of a quadrupole-split doublet, with the hyperfine parameters characteristics to zinc ferrite, ZnFe2O4. This doublet clearly dominates the Mossbauer spectrum for x=0.5 and 24 hours of milling, demonstrating that the entire system of nanoparticles consists finally of zinc ferrite. As ZnO is not soluble in hematite in the bulk form, the present study clearly demonstrates that the solubility limits of an immiscible system can be extended beyond the limits in the solid state by mechanochemical activation. Moreover, this synthesis route allowed us to reach nanometric particle dimensions, which would make the materials very important for gas sensing applications.


K10.4
Opto-Electronic Properties and Stability of Artificial Zinc Oxide Molecules. Liudmila A. Pozhar1 and Gail J. Brown2; 1Chemistry, Western Kentucky University, Bowling Green, Kentucky; 2Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, Ohio.

Quantum confinement provided by well-characterized pore arrays of silica and alumina membranes with pore diameters in the range of a few nanometers suggests an opportunity to increase the density of elements by up to three orders of magnitude and synthesize 3D nanoheterostructures and/or integrated circuits (IC) necessary for a new advance in electronic device development. However, experimental synthesis and studies of artificial molecules composed of a few semiconductor compound atoms confined in such nanopores are extremely demanding and case-specific. Alternatively, virtual (i.e., fundamental theory-based, computational) synthesis method can be used to provide predictions for physico-chemical properties of such artificial molecules nucleated and stabilized in quantum confinement. In this work Hartree-Fock (HF), restricted open shell HF, and multiconfiguration self-consistent field (CI/CAS/MCSCF) methods are used to study computationally the electronic properties of several zinc oxide artificial molecules whose structure and composition have been derived from those of the symmetry elements of the wurtzite bulk lattice of ZnO. Such molecules may provide realistic models of small ZnO nanostructures synthesized in quantum confinement provided by silica and alumina membranes using supercritical fluid deposition method. The corresponding “vacuum” counterparts of these molecules (i.e., the molecules whose geometry has been optimized without any spatial restrictions applied to the atomic coordinates) have also been studies. A special attention is paid to the effects of quantum confinement on the electronic energy level structure (ELS), direct optical transition energy (OTE), and charge and spin density distributions (CDD and SDD, respectively) of the artificial molecules. As compared to the ELS of the “vacuum” clusters, the ELSs of the confined clusters reflect changes caused by the confinement. The OTEs of the confined molecules appear to be several tenths (in some cases up to .5) of electronvolt smaller than those of the corresponding vacuum clusters, and their CDDs and SDDs differ dramatically from the CDDs and SDDs of the vacuum clusters. Such a significant change in electronic properties due to the confinement suggests a number of ways for manipulations of the electronic properties of the confined clusters by sophisticated design of their confinement and stoichiometry, thus providing means for synthesis of electronic materials by design.


K10.5
Analysis and Applications of ZnO Semiconductor Films Deposited by Laser and Sputtering Techniques. Tingfang Yen1, Meiya Li1, Nehal Chokshi2, Yongwoo Jeong1, Sung Jin Kim1, Alexander N. Cartwright1 and Wayne Anderson1; 1EE, SUNY-Buffalo, Amherst, New York; 2AMBP Tech, Tonawanda, New York.

ZnO was deposited by two methods.Specifically, the films were grown using i) Laser Assisted Molecular Beam Deposition (LAMBD) and ii) RF sputtering. For LAMBD, a high energy excimer laser ablates Zn atoms, from a pure Zn rod, to allow them to chemically interact with oxygen to form ZnO films. For RF sputtering, ZnO was deposited using a 1-in diameter ZnO target located 12 cm from the substrates consisting of glass, Si, oxide-coated Si and a thin film transistor (TFT) gate pattern. The deposition was done with oxygen at 15mT, argon at 5mT, RF power of 100W, with the substrate held at 400°C during deposition. Subsequent to film deposition, a subset of samples deposited using LAMBD were laser annealed using a Lambda Physik COMPex102 excimer laser at the 248 nm KrF transition with a pulse energy of up to 30mJ/Pulse; the corresponding energy density at the surface was up to 200mJ/cm2. The laser beam spot size was 2.5cm x 0.8cm. The laser beam was focused to produce the higher fluence values. An additional set of samples (from LAMBD and RF sputtering) were annealed with nitrogen or forming gas, 15% hydrogen and 85% nitrogen, at 600°C for 30mins. Ellipsometry was used to define the thickness and refractive index of ZnO films before and after annealing. Moreover, Electron Spectroscopy for Chemical Analysis (ESCA) was used to analyze chemical composition of the films. The optical properties were tested by photoluminescence (PL) and pulse response (PR).The compositional analysis at a depth 100 Å by ESCA showed ZnO films close to 1:1 Zn:O stoichiometry for the RF sputtered ZnO. The sputtered sample with nitrogen annealing had excellent 1:1 Zn:O stoichiometry of 43.7% Zn and 43.8% O. The sputtered sample without any annealing also had close to 1:1 stoichiometry with minimal carbon contamination, namely 48.4% Zn, 49.7% O, and 1.9% C. Using PL, the sputtering sample with 600°C, nitrogen, 30mins annealing and LAMBD sample with forming gas annealing had the highest intensity at the wavelength 373.6 nm indicating a 3.32eV energy gap, expected for ZnO. The full width at half maximum (FWHM) of the spectrum for the sputtered sample was less than for the LAMBD sample. An interdigitated metal deposition was used to fabricate MSM photodetectors (MSM-PD) using annealed and un-annealed films. Two parallel metal contacts with 3 mm spacing were deposited for conductivity measurement. Source and drain contacts were used to complete the TFT’s. With 100mW/cm2 solar-simulated light and 5V voltage bias, the value of photo-conductivity was 4.13x10-4(1/Ω-cm)for the sputtered ZnO and high values of responsivity were observed. The pulse response of the MSM-PD using sputtering had a rise time 18.89ns. TFT’s gave very parallel output characteristics with good gate control. The response of ZnO/Si heterojunction solar cells will also be reported.


K10.6
Investigation of Growth Mechanism of Aligned ZnO Nanowires. Sharvari Dalal1, Richard J.H. Morris2, Daniel L Baptista1, Ken Teo1, David A. Jefferson1, Rodrigo G Lacerda3, Michael G Dowsett2 and William I Milne1; 1Cambridge University, Cambridge, Cambridgeshire, United Kingdom; 2Physics, University of Warwick, Coventry, West Midlands, United Kingdom; 3Physics, Universidade Federal de Minas Gerais, Belo Horizonte, MG, Brazil.

Zinc oxide is a II-VI wide bandgap (3.37eV) semiconductor that has many interesting optical and mechanical properties making it a promising material for various optical and electronic devices. Using a simple vapor deposition process and by controlling the growth kinetics, a wide range of 1D nanostructures have been synthesized. Vertically aligned ZnO nanowires have been grown which could be used in bottom-up devices for applications in sensing, optoelectronics and field emission. The present work reports an alternate view of the mechanism involved in the catalytic growth of aligned ZnO nanowires/rods. The nanowires are grown in a horizontal tube furnace by evaporation of 1:1 ZnO:C mixture at 1000 °C onto 1 to 10 nm Au coated sapphire substrates using a N2/O2(20%) gas flow. The deposition temperature and pressure varied from 680 to 890 °C and 1 to 9 mbar respectively. Although a catalyst was used, no traces of the metal have been detected in the nanowires. HRTEM analysis showed flat 0001 ends with no metal visible or detected with EDS. Secondary ion mass spectrometry (SIMS) was used for its more sensitive detection limits (ppm-ppb) and the ability to build up a depth profile. Two types of SIMS experiments were performed: mass spectral analysis to study the composition of the nanowire tips and depth profile analysis to investigate the bulk. The surface mass spectra showed no Au peak implying Au is not present at the tips of the nanowires. Since nanowires form a non-uniform, disconnected surface, a low angle of incidence was used during depth profiling to limit sputtering to the uppermost part of the sample and reduce the detection of secondary ions from deeper in the sample. The resulting profile showed that the initial Au level remained at the background detection limit before rising to a peak, and then gradually decreasing. Due to the sample morphology, the erosion during profiling is non-uniform, hence depth interpretation is difficult, but simultaneous measurement of the O level showed a jump followed by instability indicating the possible transition into the sapphire substrate. This jump occured just after the gold peak indicating the gold remained near the nanowire-substrate interface. The VLS mechanism is often cited as being responsible for vapor phase growth using a catalyst. This mechanism involves a vapor species supersaturating a liquid catalyst and later precipitating to form part of a crystal1. Since the gold catalyst is not detected in the nanowire, the mechanism may be different from the standard VLS process necessitating an alternative growth mechanism. The gold catalyst may be necessary only in the initial stage of growth, giving place to a posterior self-catalytic growth. Further investigations are crucial to elucidate these results. The ability to control the growth mechanism is integral to the use of ZnO nanowires for many technological applications. 1R. S. Wagner, et al., Journal of Applied Physics 35 (10), 2993 (1964).


K10.7
Effect of Co-doping Co and Cu on the Properties of ZnO Based Diluted Magnetic Semiconductor Thin Films. Deepayan Chakraborti1, Shivaraman Ramachandran1, John T Prater1,2 and Jagdish Narayan1; 1Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina; 2Materials Science Division, Army Research Office, Research Triangle Park, North Carolina.

The exiting possibility of combining the charge and spin of an electron and to incorporate them into novel spintronic devices has attracted widespread attention recently. ZnO doped with transition metals like Mn,Co,Cu,etc has been proven to be a potential candidate as a dilute magnetic semiconductor showing room temperature ferromagnetism (RTFM).In this work, we report detailed characterization of ZnO thin films co doped with Co and Cu grown on c-plane sapphire by pulsed laser deposition technique. Here we have varied the dopant concentrations to study the effect of each dopant species on the magnetic and electronic properties of the thin films. An analysis of the magnetic properties of these materials considering carrier induced and F-center mediated exchange mechanism will be discussed. Magnetic measurements including magnetization as a function of applied magnetic field and magnetization as function of temperature (field cooled and zero field cooled) have been performed in a superconducting quantum interference device (SQUID) magnetometer. Co-doping Zn(Co)O thin films with Cu shows an increase in saturation magnetization initially but as the Cu concentration approaches 10% , saturation magnetic moment decreases by an order of magnitude. Detailed atomic scale characterization has been performed using transmission electron microscopy (TEM), including high resolution TEM, STEM Z-contrast and Electron energy loss spectroscopy (EELS). These studies determined that ferromagnetism was mainly due to dopant (Co,Cu) substituted into the ZnO lattice and not because of presence of nanoclusters of any magnetic phase. Optical characterization using absorption spectroscopy when combined with these results give interesting insight into local environment of the dopant atoms in the crystal field of the host.


K10.8
The Study of the Effect of Al-doped ZnO Surface Treatment for Organic Light Emitting Diode. Jong Kook Jung1, Sil-Mook Lee1, Seong Eui Lee1 and Ho Nyeon Lee2; 1Department of Advanced Materials Engineering, Korea Polytechinc Univ, kyonggi-Do, South Korea; 2Samsung Advanced Institute of Technology, Display Lab., kyonggi-do, South Korea.

TCOs (transparent conducting oxides) possess a wide range of applications in a variety of opto-electronic devices such as flat-panel displays or thin-film solar cells. By the present time, ITO is used almost electronic display devices, but its very expensive and it very high temperature to make ITO. So we made AZO (ZnO 98wt%: Al 2wt%) transparent electrode in RF magnetron sputtering method. To assess the credibility, we made sure of the electrical characteristics along the different temperature conditions. Also we wanted to fabricate high quality AZO thin films in order to apply to anode for OLED (Organic light emitting diode), and to improve OLED efficiency. Device structure of SM-OLEDs grown by vacuum thermal deposition is Anode(300nm AZO)/60nm Alq3/2nm LiF/75nm Al. The comparison of UV-Ozone and Electrolysis AZO thin films were studied, mainly, in the aspect of electrical stability and work function. The improved efficiency could be understood to be due to the change in work function at surface of AZO after UV-Ozone treatment. Additionally, the effects of surface roughness of AZO thin films were studied at various thermal conditions. We are now trying to find out the mechanism of this enhanced efficiency.


K10.9
Synthesis, Morphological Control, and Photoluminescence Properties of Zinc Oxide Nanoparticles. Tamar Andelman1, Yinyan Gong1, Igor Kuskovsky2, Dalia Yablon3, Alan Schilowitz3 and Stephen O'Brien1; 1Applied Physics and Applied Math, Columbia University, New York, New York; 2Physics, Queens College of CUNY, Queens, New York; 3ExxonMobile Research, Annandale, New Jersey.

We present here a straightforward solution method to control the morphology of zinc oxide nanoparticles. By changing the coordinating power of the solvent used in the synthesis, nanorods, nanotriangles, or spherical nanoparticles can be made via a thermal decomposition route. The effect of the morphology on the photoluminescence (PL) properties of the nanoparticles has been investigated. Solution measurements show that the intensity of the green band, the origin of which is disputed, varies according to the surface to volume ratio of the different shapes. This points to surface oxygen vacancies as the cause of the green band. Modification of the surface of the ZnO dots has been conducted, and supports the fact that surface oxygen vacancies give rise to the green band. Near field scanning optical microscopy (NSOM) is being employed to study the optical properties of individual nanoparticles, which may behave differently than large numbers of nanoparticles in solutions. Preliminary NSOM results of individual nanoparticles match the solution data.


K10.10
Ferromagnetism in Co-doped ZnON Nanocrystals. Xuefeng Wang, Jian Bin Xu, Wing Yiu Cheung and Sai Peng Wong; Department of Electronic Engineering, The Chinese University of Hong Kong, New Territories, Hong Kong.

Diluted magnetic semiconductors (DMSs) are of particular current interest since they are believed to be potentially suitable for future spintronics (spin-based electronics) applications. Although recent reports of carrier-mediated ferromagnetism in oxide-based DMSs shed the light on the practical spintronics applications, the complete understanding of the latent microscopic mechanism of high-temperature ferromagnetism in oxide-based DMSs still remains a technical challenge. In this presentation, we demonstrate a novel solution-based crystal growth strategy to clarify the ferromagnetism in oxide-based DMSs, by exploiting the n-type cobalt-doped ZnO DMS nanocrystals as an example. The observed high-temperature ferromagnetism is established to be intrinsic using a couple of characterization techniques. An intimate correlation between self-orientation crystal growth and high-temperature ferromagnetism is observed, further supporting the recent proposal. It is the shallow donorlike electrons (as the legacy of aggregation-based growth) that are strongly bound on cobalt sites, yielding bound magnetic polarons and thus activating ferromagnetism. The complicated status of shallow defects upon different treatment conditions may lead to the highly controversial reports in oxide DMSs.


K10.11
Thickness Dependent Strain Studies on Optical Properties of Low Temperature Processed Polymeric Precursor Derived ZnO Thin Films. Uma Choppali and Brian P Gorman; Materials Science and Engineering, University of North Texas, Denton, Texas.

Low temperature processing of ZnO thin films is essential to find applications as transparent conducting oxide in flexible polymeric displays. Stresses develop during deposition and annealing of thin films but have not been studied for polymeric precursor derived ZnO thin films where it is of utmost importance due to the large amount of shrinkage during processing. In this paper, we study the effect of thickness dependent strain on optical properties of ZnO thin films. We developed a novel technique to synthesize dense, nanocrystalline ZnO thin films using glycerol as chelating agent in modified Pechini process. The prepared polymeric precursor is spincoated on surface modified substrates and annealed at different temperatures in ambient atmosphere. Film thickness and refractive indices (porosity) of single and multiple layers were measured using variable angle spectroscopic ellipsometry (VASE). The effect of film thickness on strain and orientation was studied by X-ray diffraction (XRD) and interfacial reactions of the annealed ZnO thin films was characterized by high resolution transmission electron microscopy (HRTEM). XRD spectra revealed that compressive strain exists in ZnO thin films. We also observed that the strain increases with increase in annealing temperature. It was also observed that the compressive strain decreased with an increase in film thickness. Strain effects on optical properties of ZnO thin films were studied by UV - visible spectroscopy and room temperature photoluminescence.


K10.12
Optical Properties of Porous ZnO Nanorods Grown by Aqueous Solution Method. Lee Sang Hyun1, Lee Hyun Jung3, Goto Takenari1, Cho Meoung-Whan1,2 and Yao Takafumi1,2; 1Center for Interdisciplinary Research, Tohoku Univ., Sendai, Japan; 2Institute for Materials Research, Tohoku Univ., Sendai, Japan; 3Graduate School of EnVironmental Studies, Tohoku Univ., Sendai, Japan.

ZnO is an important wide-band gap semiconducting oxide material with many useful properties such as piezoelectricity, photoconductivity, and surface sensitivity with gas or organic materials. In order to take full advantage of ZnO nanostructures, it is necessary to control the shape and positioning of nanostructures on substrates. Especially, porous nanostructures have the feasibility for application, such as gas or organic included bio-materials, solar cell, due to their relatively high surface area and single crystallinity. In particular, optical emission from porous nanostructures will be greatly enhanced due to large surface area. In this presentation, we will show optical properties of porous ZnO nanorods with high surface area in the temperature range from 10K to room temperature. ZnO nanorods were grown using typical aqueous solution method that is reaction of zinc acetate dihydrate in ethanol and ammonia water. The morphology of as-grown ZnO nanorods was changed to porous structures by thermal annealing followed by chemical treatment. Optical properties of as-grown and porous ZnO nanorods were investigated by photoluminescence (PL) measurement. The excitonic emission dominates in PL of porous ZnO nanorods and much stronger compared with that of as-grown or annealed samples with the ratio of deep level to excitonic peak being decreased. This enhancement in excitonic emission intensity can be ascribed to high surface area of porous ZnO nanorods and to decreasing surface defects by etching. The internal quantum efficiency for porous ZnO nanorods is estimated to be about 21% from the ratio of PL intensities at 10K and 300K. Random lasing properties were evaluated using micro-PL with He-Cd and Nd-YAG laser sources. Interestingly, thermally annealed ZnO showed only spontaneous emission even at high optical excitation intensity. To the contrary, porous ZnO nanorods showed characteristic of laser action. Stimulated emission from porous ZnO nanorods could be explained in terms of an amplified photon by injection of a second photon during traveling inside or outside of porous ZnO nanorods before leaving porous ZnO nanorods. This mechanism is different from previous results on lasing actions of ZnO nanowires or nanorods in which Fabry-Perot laser cavities are formed by top and bottom facets. The formation mechanism and structural properties of porous ZnO nanorods will be discussed in the presentation.


K10.13
High-density Plasma Etching of Zinc-Oxide and Indium-Zinc-Oxide in Cl2/Ar and CH4/H2/Ar Chemistires. Wantae Lim1, Luc Stafford1, Rohit Khanna1, Lars Voss1, Jon Wright1, Brent Gila1, David Norton1, Stephen J Pearton1 and Fan Ren2; 1Materials Science and Engineering, University of Florida, Gainesville, Florida; 2Chemical Engineering, University of Florida, Gainesville, Florida.

The dry etching characteristics of bulk single-crystal Znic-Oxide(ZnO) and RF-sputtered indium-zinc-oxide (IZO) films have been investigated using an inductively coupled high-density plasma. The Cl2-based plasma mixture showed little enhancement over physical sputtering in a pure argon atmosphere, and the etch rate of ZnO is very similar to that of IZO, which indicates that Zinc and Indium atoms are driven by a similar plasma etching dynamics. The CH4/H2/Ar chemistry produced an increase of the IZO etch rate. The surface morphologies of bulk ZnO and IZO films after etching in Cl2/Ar discharges are smooth, whereas that of IZO after etching in CH4/H2/Ar presents particle-like features resulting from the preferential desorption of In- and O-containing products. Etching in CH4/H2/Ar also produces formation of a Zn-rich surface layer, whose thickness (~40 nm) is well-above the expected range of incident ions in the material (~1 nm). Such alteration of the IZO layer after etching in CH4/H2/Ar plasmas is expected to have a significant impact on the transparent electrode properties in optoelectronic device fabrication.


K10.14
Epitaxial Growth of ZnO Thin Films on SiC Prepared by Chemical Solution Deposition. Kyu-Seog Hwang1, Young-Sik Park2, Young-Sun Jeon2,1, Kyung-Ok Jeon2,1 and Young-Hwan Lee3,1; 1Dept. of Applied Optics and Institute of Photoelectronic Tech., Nambu University, Gwangju, South Korea; 2Camera Module Team, Korea Photonics Technology Institute, Gwangju, South Korea; 3Department of Automobile, Chunnam Techno College, Chonnam, South Korea.

Zinc oxide (ZnO) thin films have emerged as one of the most promising oxide materials owing to their optical and electrical properties, together with their high chemical and mechanical stability. Chemical solution deposition (CSD) is attractive technique for obtaining ZnO thin films and has the advantages of easy control of the film composition and easy fabrication of a larger-area thin film at low cost. In this work, epitaxial ZnO thin films on SiC substrate were prepared by using a CSD method with a zinc naphthenate precursor. Precursor films were pyrolyzed at 500°C for 10 min in air and finally annealed at 600, 700, 800 and 900°C for 30 min in air. Crystallinity and in-plane alignment of the films were investigated by X-ray diffraction θ-2θ scan and β scan(pole-figure analysis). Field emission - scanning electron microscope, scanning probe microscope, and He-Cd laser (325 nm) are used to detect the surface morphology and photoluminescence of the films. The effects of annealing temperature on crystallinity and epitaxy of the films will be fully discussed on the basis of the results of X-ray diffraction analysis.


K10.15
Improvement in Moisture Durability of ZnO Transparent Conductive Films with Ga Heavy Doping Process. Osamu Nakagawara, Yutaka Kishimoto, Hiroyuki Seto, Yoshihiro Koshido, Yukio Yoshino and Takahiro Makino; R&D Division, Murata Manufacturing Co., Ltd., Nagaokakyoshi, Kyoto, Japan.

Many researchers have investigated ZnO as a depletion-free and cost-effective substitute of ITO. For product commercialization, we assure that the improvement in reliability of ZnO films is very important as well as the progress in their electrical properties. The increase in resistivity occurs in the presence of moisture due to the H2O reaction with oxygen vacancy and grain boundary region in the ZnO film. Almost every researcher in this field recognizes this instability discourages the ZnO from practical applications for a transparent conductive oxide (TCO) material. Accordingly, we focused on the improvement of ZnO conductive films in their moisture durability. The increase in resistivity can be attributed to carrier decrease owing to re-oxidization of oxygen vacancies reacted with ambient moisture. We intend to make the site replacement play as a dominant role in supplying carriers, which motivates Ga heavy doping process. As opposed to previous papers focusing on the Ga content ranging from 2 to 4 wt% to obtain the lowest initial resistivity 1), ZnO films with heavier Ga doping up to 23.1 wt% are purposely prepared in our research. In consequence, moisture-resistant ZnO transparent conductive films were formed with Ga heavy doping by off-axis type rf magnetron sputtering. The resistivity of 12.4 wt% Ga-doped ZnO is 1.3×10-3Ωcm and changes less than 3 % over a 2000-hour reliability test at the temperature of 85 degrees Centigrade and the humidity of 85 degrees Centigrade. X-ray diffraction analysis shows the peak broadening of FWHM up to 16.9 degrees in a (002) rocking curve profile of 12.4 wt% Ga-doped ZnO and the decentering of the intensity distribution in 23.1 wt% Ga-doped ZnO in a (002) incident pole figure profile. The cross-sectional transmission electron microscope images of the heavily Ga-doped ZnO films indicate that the c-axis grows along with various directions as well as the normal line to the substrate surface. This is quite a different crystal structure from the conventional c-axis orientated growth, for example, observed at the 4.3 wt% Ga-doped ZnO films. The effect of heavy doping is discussed based on the grain boundary distribution and carrier compensation by excess Ga segregated in the film. This heavy doping process is remarkably effective for the improvement in reliability requisite for practical application of ZnO transparent conducting films. 1) T. Minami, H. Nanto, and S. Takata, Appl. Phys. Lett., 41 (1982) 958.


K10.16
Elaboration and Characterization of ZnO Transition Metal (Co, Mn, Ni, Fe) Doped Aerogel Nanoparticles. Lassaad El Mir1, Aroussi BenMahmoud1, H.Jurgen von Bardeleben2 and Jean Louis Cantin2; 1Faculté des Sciences de Gabès, Gabès, Tunisia; 2University Paris 6, CNRS, Paris, France.

Nanocrystalline transition metal (TM=Co, Mn, Ni, Fe) doped zinc oxide powders have been elaborated by a new protocol based on slow hydrolyse of zinc acetate dissolved in methanol and supercritical drying in ethyl alcohol. High doping levels of up to [TM]=0.25 have been achieved. X-ray diffraction studies showed the formation of the ZnO wurtzite phase for all dopants; secondary phases as detected by this technique were below the 1% level. The powders have a narrow size distribution with an average value of ~25nm. Electron microscopy characterization showed that the size of the ZnO:TM particles did not change significantly for the different dopants. The dopant incorporation and their magnetic properties were studied by electron paramagnetic resonance spectroscopy; high temperature ferromagnetism was detected for Ni and Co doping whereas Mn and Fe doped powders showed only antiferromagnetic interactions. The magnetic properties are very sensitive to subsequent high temperature annealings


K10.17
Magnetic Properties of ZnO:Ni Aerogel Nanopowders: Effect of Thermal Treatments. Lassaad El Mir2, H.Jurgen von Bardeleben1, M. Saadoun2, Aroussi Ben Mahmoud2 and Jean-Louis Cantin1; 1University Paris 6, CNRS, Paris, France; 2Faculté des Sciences de Gabès, Gabès, Tunisia.

Ferromagnetic phases in transition metal doped ZnO have been reported in various publications but their origin remains controversial. We report the elaboration of Ni doped ZnO nanoparticles prepared by a sol-gel processing technique. In this technique the water for hydrolyse was slowly released by esterification with methanol of the metal acetate followed by a supercritical drying in ethyl alcohol. Doping concentrations between 5 and 25 at% have been investigated. In the as-prepared state the powders with an average particle size of 30nm present ferromagnetic properties; thermal annealings in the 500°C to 700°C temperature range in air or oxygen modify the magnetic properties. We ascribe the observed ferromagnetism to the presence and transformation of Ni based secondary phases.


K10.18
Photoluminescence Characteristics of ZnO Nanorods Fabricated by Different Methods. Tobias Voss1, Chegnui Bekeny1, Lars Wischmeier1, Birgit Hilker1, Sandra Boerner2, Wolfgang Schade2, Bianca Postels3, Augustin Che Mofor3, Andrey Bakin3 and Andreas Waag3; 1Institute of Solid State Physics, University of Bremen, Bremen, Germany; 2Institute of Physics and Physical Technologies, Clausthal University of Technology, Clausthal-Zellerfeld, Germany; 3Institute of Semiconductor Technology, Braunschweig University of Technology, Braunschweig, Germany.

In the last years remarkable progress has been achieved in reproducibly fabricating ZnO nanorods and nanowires of high crystalline and optical quality. Well-established high-temperature approaches like the catalyst-mediated vapour-liquid-solid (VLS) growth and the vapour-phase epitaxy (VPE) have been successfully optimized, and new low-temperature techniques like the aqueous-chemical growth (ACG) method have been extended to yield large-scale ZnO nanorod arrays with a highly preferential c-axis orientation. To finally integrate these nanostructures into optoelectronic devices it is mandatory to carefully analyze their specific optical properties and to look for possibilities to further improve their quality. Here, we present a systematic comparison of the fundamental optical properties of ZnO nanorods fabricated by the VLS, VPE, and ACG processes. For all three growth techniques we analyze the near-band-edge emission as well as the deep-level emission and discuss the underlying microscopic processes. Photoluminescence (PL) studies in the temperature range between 4 and 300K have been carried out supplemented by time-resolved studies and high-excitation-density measurements of the near band-edge luminescence. At low temperatures, the VLS and VPE nanorods show well-resolved donor-bound-exciton (D0X) related emission lines accompanied by their phonon replica. For samples grown at different temperatures a significant shift of the room-temperature PL is observed and can be clearly attributed to different ratios of the zero-, one-, and two-LO-phonon emission replica of the free exciton. In contrast, ACG nanorods exhibit rather broad near band-edge emission lines even at low temperatures (>7meV). First experiments demonstrate that their linewidths and the ratio of the deep-level to the near-band-edge emission can be significantly decreased by annealing in argon or oxygen atmosphere at moderate temperatures (500-600°C). First high-excitation-density measurements on ACG nanorods show emission from an electron-hole plasma at elevated excitation densities. To clarify the microscopic origin of the broad near-band-edge emission lines in the ACG nanorods, temperature dependent and time-resolved PL measurements have been carried out before and after annealing of the samples. The results reveal an energy shift of the main emission peak with temperature being different from that observed for the D0X and FX in the VLS and VPE samples. Additionally, in the as-grown samples a rather short recombination time of the carriers has been found (<100ps) which distinctly increases to about 200ps after the annealing procedure. These results point to the presence of large donor densities in the as-grown ACG nanorods which could even form a band of donor states and which are distinctly reduced through the annealing procedure.


K10.19
Surfactant-assisted Alignment of ZnO Nanocryatals to Superstructures. Hao Tang1,2 and Shuit-Tong Lee1,2; 1Centre of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong, hongkong, Hong Kong; 2Physics and Materials Science, City University of Hong Kong, hongkong, Hong Kong.

Self-organization of ZnO nonspherical nanoparticles into various three-dimension semiconductor superstructures can be realized with the assistance of micelles formed by surfactant in aqueous phase under one-pot conditions. Taking advantage of structure transformation of micelles under different environments such as temperature and so on, two-stage self-organization can be used to form more complex ZnO superstructures. This use of structure transformation of micelles as shape templates can offer a potential new route to self-assembly of colloidal structures into complex three-dimensional photonic crystal.


K10.20
Characterization of ZnO Nanostructure Networks Grown on AZO/Si Substrate. Chung Ting Fung, Physics and Materials Science, City University of Hong Kong, Hong Kong, Hong Kong.

ZnO nanowall and nanowire networks are [1, 2] are simply synthesized by thermal evaporation using catalyst-free approach. The nanowalls are separated from each other and have a thickness of about 30-70 nm. Transmission electron microscope shows that the growth direction of micron size long nanowall is along [0002]. Nanowires are found to be epitaxially standing at the end of a single nanowall. But some nanowalls have clear end with no epitaxially grown nanowires. The direction of nanowire and nanowall structures are successfully obtained from the c-axis oriented AZO film. Room temperature cathodo-luminescence measurements of the networks exhibit dominant UV emission at 380 nm and broad green-blue emission. The green-blue emission is contributed by nanowalls and nanowires, while the UV emission arises from some nucleated sites on the continuous film. We suggest that the defective emission of nanostructures is related to an underlying polycrystalline ZnO template. Post-annealing treatment will be used to reveal the nature of defective emission. Furthermore, the growth mechanism of nanostructure networks will be discussed. We believe that defects and dangling-bonds of AZO film may act as nucleation sites, while the rough film surface prompts the growth of disconnected nanowalls. 1. H.T. Ng, J. Li, M.K. Smith, P. Nguyen, A. Cassell, J Han, M. Meyyappan, Growth of epitaxial nanowires at the junctions of nanowalls, Science, 300, 1249 (2003). 2. B.P. Zhang, K. Wakatsuki, N.T. Binh, Y. Segawa, N. Usami, Low-temperature growth of ZnO nanostructure networks, J. Appl. Phys., 96, 340 (2004).


K10.21
Transport Properties and Conduction Band Offset of n-ZnO/n-6H-SiC Heterostructures. Yahya Alivov1, Bo Xiao1, Qian Fan1, Daniel Johnstone2 and Hadic Morkoc1; 1Electrical Engineering, VCU, Richmond, Virginia; 2SEMETROL, Chesterfield, Virginia.

Zinc Oxide (ZnO) is being considered for a variety of optical devices owing to its large exciton binding energy (~60 meV), high emission efficiency, relative ease of bulk material growth, and the possibility of growing highly conductive transparent layers. Because high quality, reproducible p-type ZnO layers are not yet available, p-n junctions currently investigated take advantage of heteroepitaxy. Heteroepitaxial structures are also of interest on their own merits because of advantages provided by the resulting valence and conduction band offsets. One of the vital criteria in heteroepitaxial based devices is to match the lattice parameters, and from this point of view GaN and its alloys with aluminum (Al) have been considered as one of the best candidates. Another plausible candidate for ZnO based heterostructure devices is SiC, which matches the wurzite crystal structure, and has a relatively small lattice mismatch of ~4% with ZnO. To better understand heterostructure device performance, knowledge of the band offset between the materials forming the heterostructure is necessary. In this report we discuss the current transport properties and the defect structure of the n-ZnO/n-6H-SiC isotype heterostructures as well as the conduction band offset of this materials system determined by a variety of methods, namely temperature dependent current-voltage characteristics (I-V-T), photocapacitance (PC), and deep level transient spectroscopy (DLTS) measurements. The n-ZnO/n-SiC heterostructure samples were fabricated by growing of 0.3 μm thick undoped ZnO films on 300 μm commercial n-type 6H-SiC substrates. ZnO layers were deposited directly on 6H-SiC substrates at 750 °C by RF magnetron sputtering in an Ar+O2 ambient with subsequent postgrowth annealing at 950 °C for 1 hour to improve the crystal quality. The 250 μm diameter mesa structures were fabricated by conventional photolithography method. The ohmic contacts to n-ZnO and n-SiC were formed by depositing Au/Al (300/300 Α) and Au/Ti/Ni (300/300/300 Α) metal layers, respectively, with subsequent rapid temperature annealing at 800 C for 2 min. The I-V-T and DLTS measurements were performed in the temperature range of 80-700 K. The photocapacitance spectroscopy was taken at 80 K, using a xenon lamp passed through a monochromator. A strong dependence of transport properties of n-ZnO/n-6H-SiC heterostructures on measurement temperature and applied voltage was observed. Conduction band offset of n-ZnO/n-6H-SiC heterostructures measured by a variety I-V-T, PC, and DLTS measurements with values of1.2 eV, 1.1 eV, and 1.22 eV, respectively. These results will be discussed in detail.


K10.22
Low Voltage Operating ZnO Thin-film Transistors with Ni Doped BaSrTiO3 high-K gate Insulator for Transparent and Flexible Electronics. Jeong Ung Kim1, Jae-Kyu Lee1, Young-Woong Kim1, Duck-Kyun Choi1, Il-Doo Kim2 and Jae-Min Hong2; 1Ceramic engineering, Hanyang University, Seoul, South Korea; 2Optoelectronic Materials Reserch Center, KIST, Seoul, South Korea.

Transparent ZnO based thin film transistors (TFTs) have received intensive interest due to their potential of replacing hydrogenated amorphous or polycrystalline silicon (a-Si:H or poly-Si) TFTs. Zinc oxide (ZnO) is a transparent compound semiconductor with a wide band gap (3.37 eV) which can be grown as a polycrystalline film at low or even room temperature. ZnO is, therefore, considered to be an ideal material for serving as the channel layer in transparent and flexible TFTs. As an important element, gate insulators for ZnO based TFTs have received increasing attention because ZnO-TFTs switching voltage can be reduced by using high-K gate dielectrics which can lead to high capacitance value. In this presentation, we report on the role of Ni doping in markedly reducing leakage currents in Ba0.6Sr0.4TiO3 (BST) high-K gate insulator. The 1% Ni-doped BST thin films, deposited by rf magnetron sputtering at room temperature on Pt/Ti/SiO2/Si substrates, exhibited a relatively dielectric constant of ~15. The 1% Ni-doped BST films exhibited remarkably improved leakage current densities less than 6x10-9 A/cm2 as compared to that of (5x10-4 A/cm2) of undoped BST films at an applied voltage of 7 V. All room temperature processed ZnO based TFTs using the 1% Ni-doped BST gate insulator exhibited a high optical transparency (> 80%, for wavelength > 400 nm), a high field effect mobility and low voltage device performance of less than 6 V. This result demonstrates that ZnO based TFTs with 1% Ni-doped BST gate insulator will open up a promising route for a wide variety range of applications in transparent, flexible, and portable electronic devices.


K10.23
Characteristics of ZnO-TFTs with SiO2and Al2O3 Gate Dielectrics. Chan Jun Park, Jae-Kyu Lee, Jeong-Ung Kim, Young-Woong Kim and Duck-Kyun Choi; Division of Materials Science and Engineering, Hanyang university, Seoul, South Korea.

ZnO thin-film transistors (ZnO-TFT) have attracted much attention due to its optical transparency, wide band gap energy, and possibility of room temperature poly-crystalline formation. ZnO based thin-film transistors can be incorporated into next generation electronic devices such as transparent electronics, opto-electronics, and flexible electronics. ZnO-TFTs have been shown to perform better than hydrogenated amorphous silicon or organic materials based thin-film transistors. In spite of its advantages as an active layer, relatively low field effect mobility, low on/off ratio, and inadequate threshold voltage hinder the extensive usage of ZnO-TFTs. Electrical properties of a transistor are affected by various factors. Among them, the gate dielectric is the most important component because it determines the leakage current, threshold voltage, and operation voltage. In addition, unstable characteristics of a field-effect transistir are closely related to the interface between semiconductor and gate dielectric layers and trap density in the dielectrics. In this study, we investigated the dependence of gate dielectrics such as SiO2 and Al2O3 on the ZnO-transistor characteristics. All the component layers in ZnO-TFTs with bottom gate configuration were deposited by rf magnetron sputtering except for the SiO2 (100 nm) and Al2O3 (50 nm) gate dielectric layers which were deposited by the inductively coupled plasma chemical vapor deposition (ICP-CVD) and atomic layer deposition (ALD), respectively. The process temperature of ZnO-TFT on glass substrate was limited to 270 oC, for potential extension of this process to plastic substrates such as polyimide or polynorbornene. All of the fabricated ZnO-TFTs with SiO2 or Al2O3 dielectric layer showed high optical transmittance above 80 % in the visible light wavelengths between 400 and 700 nm. Field effect mobility and on/off ratio of ZnO-TFTs (W/L = 100μm/30μm) on Al2O3 were 6.98 cm2/Vs and 5×106, respectively. It is much higher than those of ZnO-TFT on SiO2. In addition, ZnO-TFT with Al2O3 showed lower threshold voltage of 4.2 V and smaller swing voltage of 0.60 V/decade compared to that with SiO2 with threshold voltage of 8 V and swing voltage of 2.13 V/decade. It is well known that ALD is a method for growing dense, pinhole-free films. Therefore, it is indicated from the current results that quality ALD Al2O3 film with higher dielectric constant induced good electrical properties such as low operation voltage and swing voltage as well as the better physical state in the ZnO channel region.


K10.24
Synthesis and Characterization of Nickel-Doped ZnO Nanocrystals. Xiao Li Zhang, Yan Li, Ru Qiao, Ri Qiu, Young Hwan Kim and Young Soo Kang; Department of Chemistry, Pukyong National University, Pusan, South Korea.

ZnO-based dilute magnetic semiconductors (DMSs) that are doped with nickel impurities via a solvothermal method in alcohol solution. Compared with those previous reported methods with the evaporation processes, the synthesis method we reported here is really facile and economical. Moreover, the percentage of doped nickel can be easily controlled. The X-ray diffraction, transmission electron micrograph, luminescence and magnetization hysteresis loops of nickel-doped ZnO nanocrystals were presented to confirm that the nickel impurities are embedded inside the nanocrystal. Optical measurements show that by exciting the nanocrystal, a green-light emission is observed, and efficient emission from Ni is not obtained, which further suggests that the nickel ions only substitutes for zinc ions sites.


K10.25
Highly Transparent and Conductive Ga-doped ZnO Electrodes for High-performance GaN-based Green Light-emitting Diode. Min-suk Oh, Dae-Kue Hwang, Jae-Hong Leem, Young-Seok Choi, Ja-Yeon Kim and Seong-Ju Park; MSE, Gwangju Institute of Science and Technology, Gwangju, South Korea.

Recently, transparent conductive oxide (TCO) thin films draw a great attention and they are broadly used in many fields such as flat-panel displays, solar cells, and optoelectronic devices. Many conventional TCOs such as In2O3:Sn, SnO2, CdO and other related-materials have been reported to meet the various requirements for those devices. However, high-cost and toxicity make them difficult to be persistently used in industry. In contrast, ZnO is a harmless and low-cost material. In this study, we report on the growth of 200 nm thick Ga-doped ZnO (GZO) thin films deposited at a low temperature of 100 °C by oxygen radical-assisted pulsed-laser deposition (RA-PLD). The structural, electrical, and optical properties of the GZO films have been investigated as a function of rf power levels for the application of GZO thin film as a transparent and highly conductive current spreading layer on the green light-emitting diode(LEDs) which should be processed at low temperatures due to the high indium contents in the InGaN active layer. The X-ray reflectometry showed that the density of the GZO films grown by RA-PLD increases up to 5.32 g/cm3. An average optical transmittance of 97 % in the long wavelength range (500~600 nm) and resistivity as low as 3.5×10-4 Ω-cm were achieved in the GZO films deposited at an rf power of 100 W. The luminescence emission intensity of the GaN-based green LEDs fabricated with an optimized GZO electrode was improved by as high as 76 % at 20 mA as compared to those fabricated with conventional Ni/Au layers.


K10.26
Preparation of Zinc Oxide Nanorods by Cost-effective Catalyst Free Chemical Spray Pyrolysis Technique. Tatjana Dedova, Malle Krunks, Olga Volobujeva, Jelena Aparina, Maarja Grossberg and Valdek Mikli; Department of Materials Science, Tallinn University of Technology, Tallinn, Harjumaa, Estonia.

Highly structured layers comprising of vertically aligned zinc oxide rods were fabricated at relatively low temperatures of 450-560 °C without using any catalyst. In this work, a novel and cost-effective deposition technique of spray pyrolysis was applied to grow zinc oxide nanorods on glass substrates covered with a seed layers of conductive transparent oxides of indium tin oxide, tin oxide or zinc oxide. Zinc chloride was used as a zinc source. The precursor solution was pulverised onto the preheated substrates with the help of compressed air as a carrier gas. The properties of ZnO obtained were examined by scanning electron microscopy, selected area electron diffraction, X-ray diffraction and low temperature photoluminescence (T = 10K) spectroscopy. Highly c-axis orientated ZnO nanorod arrays are consisting of well-developed hexagonal rods with length from 50 nm up to six microns and with diameter from some tens up to some hundred nanometres. The development of nanorods is highly sensitive to the seed layer properties and spray procedure parameters. The dimensions of rods can be controlled by adjusting the deposition parameters. The rise of both the growth temperature and solution concentration increases rod dimensions. According to the selected area electron diffraction patterns an individual rod is a single crystal. PL studies reveal that the as-grown ZnO nanorods have perfect crystal quality and high purity and thus no any post-deposition treatment is needed. It has been shown that the seed layer surface morphology is critical for subsequent ZnO rods development. Depending on the seed layer surface properties ZnO platelets or elongated crystals can be grown. Only certain ZnO flat layer morphologies facilitate the formation of ZnO nanorods. Such type structures as ZnO flat/ZnO rods might have different potential applications, particularly in solar cells.


K10.27
Influence of the Particle Size on Acoustic Phonon Modes of ZnO Nanocrystals. Harish Kumar Yadav and Vinay Gupta; Physics and Astrophysics, Delhi University, Delhi, Delhi, India.

Harish Kumar Yadav1*, Vinay Gupta1, K. Sreenivas1, S.P. Singh2, B. Sundarakannan3 and R.S. Katiyar3 1Department of Physics and Astrophysics, University of Delhi, Delhi-110007 2National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi 3Department of Physics, University of Puerto Rico, San Juan, PR00932, USA *Electronic address: harish789@rediffmail.com Recently, nanoparticles have been the subject of intensive investigation for their properties resulting from the confinement of electrons and phonons. The confinement of electron and phonon not only changes the electronic band structure of the solid but also modify the vibrational spectrum to size dependent discrete lines. The study of such systems is invaluable for developing an understanding of the electronic, optical and vibrational properties so that they can be tailored for optical, electro-optical and non-linear optical applications. In this work, we report the observation of low frequency acoustic phonon modes in ZnO nanocrystals of spherical shapes with varying sizes using Raman spectroscopy. X-ray diffraction pattern and transmission electron microscope studies confirm the synthesis of spherical nanoparticles with wurtzite phase. A red shift in the absorption band edge was observed in the UV-Visible transmission spectra of colloid after dispersing ZnO nano-particle in ethanol. Two strong phonon modes were observed (around 10 cm-1 and 20 cm-1) in the low frequency Raman spectra obtained on ZnO nanoparticles. The observed shift in the low frequency Raman peak towards higher frequency with decrease in particle size is attributed to the confinement of acoustic phonon modes. The results are analyzed in the light of Lamb’s theory and observed peaks are assigned to the spheroidal modes with l = 0 and l = 2.


K10.28
Room Temperature Ferromagnetism in Co-substituted ZnO. Kousik Samanta1, Pijush Bhattacharya1, Ram S Katiyar1, W. Iwamoto2, P. G Pagliuso2 and C. Rettori2; 1Physics, Univ. of Puerto Rico, San Juan, Puerto Rico; 2Physics, Instituto de Fisica DEQ-UNICAMP Cidade Universitaria- Barao Geraldo 13083-970, Campinas, SP, Brazil.

The field of Spintronics is generating much interest due to its potential to conserve energy in a wide range of applications, such as spin valves, non-volatile memory devices, and quantum computation. The driving parameter for these applications is the electron spin rather than the charge and that’s why spin polarization is needed. The magnetic property of dilute magnetic semiconductor comes from the cationic substitution of 3d transitional metal in tetrahedral site in the II-VI semiconductor. High band gap energy (3.34eV) and large excitonic binding energy (60 meV) makes ZnO the most promising candidate for optoelectronic devices in ultraviolet region, also the 3d transition metal ions as Co, Ni, Mn etc doped ZnO is one of the most promising candidates for spintronics applications. Room temperature ferromagnetism in Co-substituted ZnO thin films was investigated by SQUID measurements. Thin films of Zn1-xCoxO (with x = 0.01, 0.05, 0.10 and 0.15) were deposited on (0001) Al2O3 substrate by pulsed-laser deposition (PLD) technique. The maximum saturation magnetization at 300K was obtained 1.1µB per Co atom in 10% Co substituted ZnO with a high Tc >350K. The lattice constant along c axis of the wurtzite Zn1-xCoxO decreases from 5.215Å to 5.199 Å. The chemical composition of ZCO was studied by x-ray photoelectron spectroscopy. The energy difference between Co 2p3/2 and Co 2p1/2 is 15.54 eV, which indicates that the Co ions are surrounded by oxygen ions, where as if Co exists as a metal cluster then this energy difference should be 15.05eV. The Raman spectra of ZCO films were carried out using the Raman microprobe system with an Ar+ ion laser source of 514.5 nm wavelength. The E2low peak frequency shifts towards the lower frequency side due to the Co substitution in the ZnO lattice. The XPS and Raman scattering data confirm that the Co ions are occupying the Zn substitutional site and the FM property in Co substituted ZnO is intrinsic in nature.


K10.29
Abstract Withdrawn


K10.30
Self-assembled ZnO Nanostructure Characteristics in [PS]m/[PAA]n Diblock Copolymers with Varying Block Lengths on p and n-type (100) Si Substrates. Hasina Afroz Ali1, Agis Iliadis1,2, Luz Martinez-Miranda3 and Unchul Lee4; 1Department of Electrical and Computer Engineering, University of Maryland, College Park, College Park, Maryland; 2Department of Information and Communication Systems Engineering, University of the Aegean, Mytilene, Greece; 3Department of Material Science and Engineering, University of Maryland, College Park, College Park, Maryland; 4Army Research Laboratory, Adelphi, Maryland.

A comparative study of the characteristics of self-assembled ZnO nanostructures in polystyrene-acrylic acid, [PS]m/[PAA]n, diblock copolymer with varying block lengths (m, n) on both p and n-type (100) Si substrates, is reported. The nanostructures were developed in [PS]m/[PAA]n with four different block repeat unit ratios, m/n, of 159/63, 139/17,106/17, and 106/4, respectively. The ability of these particular diblock copolymers to microphase separate and form self-assembled spherical nanodomains is used to template the ZnO nanostructures of spherical morphology onto the substrates. Our methodology for the ZnO nanostructure synthesis involves incorporating the ZnCl2 precursor in the copolymer in liquid phase, spincasting the doped solution on the substrate and, then converting the precursor into ZnO. The converted samples contained self-assembled ZnO nanostructures of spherical morphology as determined by AFM. Study of these four different copolymer systems helped to determine the correlation between the particle parameters and the copolymer block lengths (m, n). From the AFM images we established that the nanoparticle average size increased linearly with minority block length n, while the average density decreased exponentially with majority block length m. The AFM images showed particles with average size and density of 250 nm and 3.5x107 /cm2, 60 nm and 7x108 /cm2, 60 nm and 1x1010 /cm2 and 20 nm and 1x1010 /cm2, which were developed with block repeat unit ratios of 159/63, 139/17, 106/17 and 106/4, respectively. The x-ray diffraction θ-2θ scans showed the nanostructures in all the copolymer systems to have a wurtzite crystal structure with the (100) diffraction peak being the dominant orientation. Room temperature I-V characteristics of the Al/ZnO nanocomposite/Si structure in both [PS]106/[PAA]17 and [PS]159/[PAA]63 on p-type (100) Si exhibited rectifying junction properties. Evaluation of the I-V characteristics for both systems showed barrier heights of 0.7 V, while the ideality factor varied between 1.7 and 2.4, depending on particle size and density. The transport mechanisms of these systems will be discussed. <br> The support of NSF through grants #ECS0302494 and #ECS0601481 is gratefully acknowledged.


K10.31
Self Assembly of Zinc Oxide Hollow Spheres on Single-walled Carbon Nanotube Templates. Cengiz Sinan Ozkan, Mechanical Engineering, University of California at Riverside, Riverside, California.

The selectivity and the possibility of assembling 3D structures along SWCNTs could provide a new avenue of using SWCNTs as conducting templates that can automatically integrate itself as a conductive probe or pathway in a nano device. ZnO hollow spheres have generated interest with potential applications such as remote coupling of electromagnetic signals into nanoscale machines or devices. We describe here the self assembly (highly selective and controlled) of ZnO hollow spheres on single walled carbon nanotubes (SWCNTs) conducting nano templates. The carboxyl groups act as electrostatic co-ordination sites for the Zn nucleation. The above was confirmed via synthesis of ZnO-SWCNT heterostructures in aqueous phase and later extended to synthesize ZnO hollow spheres on oxidized SWCNTs with high selectivity and control. SWCNTs play a dual function, as templates and as scaffolds providing structural intactness to the hollow ZnO spheres (20 nm wall thickness). The heterostructures of ZnO hollow spheres and SWCNTs were characterized by SEM, TEM, EDS and electron diffraction.


K10.32
Abstract Withdrawn


K10.33
ZnO Thin Films Prepared by a Single Step Sol-gel Process.Shane O'Brien1, Lee H.K. Koh1 and Gabriel M. Crean1,2; 1Tyndall National Institute, Cork, Ireland; 2Dept. Of Microelectronic Engineering, University College Cork, Cork, Ireland.

ZnO thin films were prepared on fused silica and borosilicate glass from a single spin-coating deposition of a sol-gel prepared with anhydrous zinc acetate [Zn(C2H3O2)2], monoethanolamine [H2NC2H4OH ] and isopropanol. Crystallization annealing was performed over the range 500 to 650oC. X-ray analysis showed that thin films were preferentially orientated along the [002] c-axis direction of the crystal. The films had a transparency of greater than 85% in the visible region for sol-gels with a zinc content of up to 0.7M and exhibited absorption edges at ~ 378nm. The optical band-gap energy was evaluated to be 3.298 - 3.306 eV. Photoluminescence showed a strong emission centered at ca. 380nm along with a broad yellow-orange emission centered at ca. 610nm. Single step sol-gel thin film deposition in the film thickness range from 80nm to 350nm was demonstrated. The effect of substrate, sol-gel zinc concentration, film thickness and crystallization temperature on film microstructure, morphology and optical transparency is detailed. A process window for single spin coating deposition of c-axis oriented ZnO discussed.


K10.34
Abstract Withdrawn


K10.35
Electrical Properties and Interface Structures of Binary-alloy Schottky Contacts on ZnO Fabricated by a Combinatorial Ion Beam Sputtering.Takahiro Nagata1,2, Janos Volk1, Michiko Yoshitake2, Ahmet Parhat2 and Chikyow Toyohiro2; 1International Center for Yong Scientists, National Institute for Material Science, Tsukuba, Japan; 2Advanced Electric Materials Center, National Institute for Material Science, Tsukuba, Japan.

ZnO has been attracting attention for use in light emitting and light detecting devices in the UV wavelength region. For these applications, high quality and thermally reliable Schottky contacts are inevitable. In this paper we have demonstrated hetero structures consisted of composition spread metal-alloys on ZnO and quick screening for their electrical properties with combinatorial alloy synthetic technique to fabricate the Schottky UV detector. The Schottky barrier height is related to the work function of metals. It is known that the work function is generally affected by a crystallogrhaic orientation of metal because each orientation of the metal has a different surface potential. Additionally, the work function is dependent on interface structures. To control the Schottky barrier height between ZnO and metals, a comprehension of crystal structures of Schottky metal layer is important. By the combination of ion beam deposition and combinatorial system, the Schottky barrier heights of binary alloys have been systematically controlled in response to the compositional fraction of the Pt-Ru binary alloy. By using the combinatorial composition spread technique, it is proved that we could control Schottky barrier heights on ZnO. Pt is a candidate for high work function value metal and Ru is a candidate for thermally stable metal with lower work function. Mixing of two metals is expected to realize the thermally stable but efficient UV detector with high Schottky barrier heights. Pt-Ru alloy metal film grew on ZnO substrates epitaxialy, and crystal structures change from Pt-phase (cubic structure) to Ru-phase (hexagonal structure) in the Pt-Ru alloy phase diagram. It was obtained that the Schottky barrier heights increased with increasing Pt content by current-voltage measurements. By employing the combinatorial composition spread technique, we can control Schottky barrier heights of Pt-Ru alloy on ZnO and investigate electric properties systematically. Results of XPS analysis will be discussed in detail in the presentation.


K10.36
Fabrication Of Textured Zinc Oxide By Electrophoretic Deposition In A Strong Magnetic Field.Tetsuo Uchikoshi, Tohru S Suzuki, Hideo Okuyama, Fumiko Kimura and Yoshio Sakka; Nano Ceramics Center, National Institute for Materials Science, Tsukuba, Ibaraki, Japan.

Many materials in asymmetric (non-cubic) crystalline structures have anisotropic magnetic susceptibilities. When a single crystal of these materials is placed in a magnetic field, the crystal is rotated and the crystallographic axis of high magnetic susceptibility is aligned in the direction of the magnetic field. Zinc oxide has a hexagonal crystal structure and anisotropic magnetic susceptibility. As this anisotropy is quite small, no magnetic orientation is observed in a conventional magnetic field generated by a permanent magnet. However, the energy of anisotropy in a strong magnetic field generated by a superconducting magnet can be higher than the energy of thermal motion at room temperature. Therefore, when zinc oxide particles are placed in a strong magnetic field as high as about 10 T, they rotate to an angle minimizing the system energy. We demonstrate in this paper that a crystalline-textured zinc oxide ceramics are fabricated using the property of magnetic alignment. Commercial zinc oxide powder was dispersed in ethanol with dispersing agents and deflocculated stable suspension was prepared. The suspension was placed in a superconducting magnet and then a strong magnetic field of 12 T was applied to rotate each particle. Consolidation of the particles was carried out by electrophoretic deposition (EPD). The magnetic field was kept on applying to the suspension during EPD at a constant voltage condition at room temperature. The angle of the substrate against the magnetic field was altered to control the crystal faces of the particles. The zinc oxide particles in the deflocculated suspension were aligned due to their anisotropic magnetic susceptibility and then deposited on a cathodic substrate. The green compacts were dried, separated from the substrate, and then sintered at fixed temperatures. The sintering of the deposits was conducted at fixed temperatures for 2 h in air out of the magnetic field. The orientation of the zinc oxide crystallites was confirmed by XRD. It was confirmed that the a- or b-axis is easily aligned along the magnetic field. Orientation of the crystallites was dependent on the angle between the directions of magnetic and electric fields applied to the particles.


K10.37
Electroplating of ZnO Using Nanohole Arrays of Anodized Aluminum Oxide.Ken-ichi Ogata1, Shoso Shingubara1, Hiromi Yorozu2 and Tadahiko Nakanishi2; 1Kansai University, Suita, Osaka, Japan; 2Shin-Chuo Kogyo, Higashihiroshima, Japan.

Much attention is paid to zinc oxide (ZnO) nanostructure since the increase of the exciton binding energy occurs. Until now, lots of works related to the growth of ZnO nanostructure were reported, however, control of the size and position is still difficult. In this contribution, electroplating of ZnO was performed using nanohole arrays of anodized aluminum oxide (AAO) toward the fabrication of ZnO nanowires or nanodots. Nanohole arrays of AAO were prepared by anodization of Al foils (5N purity) or Al films sputtered on Si (100) substrates. 0.3M of H2SO4 or H2C2O4 was utilized as electrolytes, where the average nanohole sizes can be controlled in the range of 20 - 60nm by changing applied voltage. Depths of nanohole can be also variable by anodization time and their typical values are 2μm. In order to reduce the barrier layers that exist at the bottom of nanohole arrays, AAO arrays were immersed in 5wt% H3PO4 at 30oC for 3-5min. Then, they were annealed at 500-900oC for the enhancement of the chemical resistivity. Before electroplating of ZnO using a 0.1M of Zn(NO3)2, embedment of Au at the bottom of nanoholes was carried out using a solution contained Na3[Au(SO3)2]. Embedment of Au was available in the wide range of electroplating conditions, namely, that could be conducted at various applied voltage, AC frequency and annealing condition of AAO arrays. On the contrary, embedment of ZnO was done only when AAO arrays were annealed at 900oC, otherwise ZnO nanoparticles on the top surface of AAO arrays were observed.


K10.38
Enhancement of Field Emission Current from ZnO Nanorods Fabricated by Two Step Chemical Vapor Deposition with Laser Ablation of ZnO.Takashi Hirate, Takashi Kimpara, Kazumoto Takizawa and Tomomasa Satoh; Faculty of Engineering, Kanagawa University, Yokohama, Japan.

ZnO is an attractive II-VI compound semiconductor material for various optoelectronic devices. Recently, growth of various nanostructures of ZnO such as nanorod, nanobelt, nanowall, etc. has been reported, and ZnO has been considered as a promising material for nanodevices. We have studied on fabrication of ZnO nanorods by a low-pressure thermal chemical vapor deposition (CVD) method cooperated with laser ablation of a sintered ZnO target. In this paper, we report on field emission characteristics of ZnO nanorods with thin diameter grown by two-step growth process. An electron emitter by electric field has been pointed out as an application of ZnO nanorods. We succeed in fabrication of well aligned ZnO nanorods with about 10 nm diameters by changing O2 flow rate on the way of growth, and the field emission current from the nanorods is considerably enhanced. The fabrication method is almost same method used in our previous study. Metal Zn vapor and O2 gas are used as precursors to synthesize ZnO, and N2 is used as carrier gas. A sintered ZnO target is placed near a Si(111) substrate in a deposition chamber and ablated by a pulsed Nd:YAG laser beam (wavelength =1.064 mm, pulse width = 8 ns, repetition frequency = 10 shots/sec). The conditions as ZnO nanorods are grown firstly on a substrate, first step growth, are as follows. The growth temperature is 530 C. The growth pressure is 66.5 Pa. The laser energy is 0.92 J/shot/mm2 and the laser-irradiated area on the sintered ZnO target is 0.13 mm2. O2 flow rate is 0.5 to 12 SCCM. Well aligned ZnO nanorods can be grown only with very narrow O2 flow rate region centered at 1.5 SCCM. The diameter is about 100 nm. The length is 2.7 μm for 15 min growth time. The field emission current from these nanorods, however, is very small because the nanorods’ diameter is thick. The diameter and the growth direction of nanorods can not be controlled independently in the first step growth. We developed a new process to grow well aligned thinner nanorods. Firstly, well aligned but thick nanorods are grown on substrates using the first step process described above. Then, only O2 flow rate is changed abruptly to a value between 0.1 and 0.6 SCCM, second step growth. Nanorods with about 10 nm diameter grow on a center of flat tip of thick nanorod grown by the first step process. The diameter is not almost dependent on the O2 flow rate. The directions of the nanorods grown by the second step are parallel to those of nanorods grown in the first step process. Thus well aligned nanorods with thin diameter can be grown by the two step method. The field emission current is drastically enhanced. When a metal ball with 12.7 mm diameter is used as an anode and a separation between anode and nanorods is 150 μm, field emission current of 100 μA is obtained at 27 V/μm. We consider that this two step method is promising to grow well aligned ZnO nanorods with thin diameter and excellent field emission characteristics.


K10.39
High Mobility Ga Doped ZnO Thin Films on Flexible Substrate.Ved P Verma, Do Hyun Kim and Wonbong Choi; Mechanical & Materials Eng., Florida International University, MIAMI, Florida.

Zinc oxide (ZnO) based transparent thin film transistors (TFTs) have been studied intensively as transparent FETs due to their potential of replacing hydrogenated amorphous or polycrystalline silicon (a-Si:H or poly-Si) TFTs which in present serve as the backplane for active matrix displays such as liquid crystal displays and organic light emitting diodes. ZnO is a transparent compound semiconductor with a wide band gap (3.37 eV) which can be grown as a polycrystalline film at low or even room temperature. ZnO is therefore considered to be an ideal material for serving as the channel layer in transparent and flexible FETs. The Ga doped ZnO thin film, which can be used as field-effect transistor or optoelectronics, have attracted much attention because of their potential for future nanoelectronics applications. In present study ZnO:Ga thin films were successfully deposited on flexible substrates at various temperatures ranging from room temperature to 300°C by RF plasma sputtering in pure oxygen environment. EPMA (Electron Probe Micro Analysis) and SEM analysis shows high quality Ga doped ZnO thin film can be grown on flexible polymer substrate. To enhance the electrical properties, especially for the high carrier mobility, thin film was treated by rapid thermal annealing in N2 atmosphere. ZnO: Ga thin film transistors were fabricated using 100nm ZnO/SiO2 as gate insulator by e-beam lithography and plasma sputtering. These thin films show a high carrier mobility of ~1.0 cm2/V.s and on/off ratio of ~105.


K10.40
The Impact of Hydrogen Plasma Treatments at Moderate Temperatures on Sintered Zinc Oxide Samples - Evidence for Hydrogen Induced Nano-Void Formation.Reinhart Job, Mathematics and Computer Science, University of Hagen, Hagen, Germany.

ZnO pellets were prepared from high purity ZnO powder (Merck). To stiffen the pellets, the ZnO powder was mixed with the acrylic resin Elvacite. The homogenized mixture was mortared and pressed to a pellets , which were sintered in a vacuum furnace at a pressure somewhat below 1E-4 mbar. The applied heating profile had a maximum temperature of 900 C, which was attained after conducting a linear heating ramp within 1 hour. After annealing at 900 C for 2 hours the furnace was switched off and the temperature was reduced down to 150 C within about 1.5 hours. At this temperature the samples were taken out of the furnace. After this heating procedure no remnant Elvasite was observed in the samples as proven by µ-Raman spectroscopy. Hydrogenation was applied for 1 hour in a RF plasma setup at various substrate temperatures between 250 C and 500 C. The applied hydrogen plasma was stabilized by an admixture of argon using a hydrogen flux of 50 sccm and an Ar flux of 20 sccm, a plasma frequency of 13.56 MHz and a power of 150 W. The H-plasma treated samples were mainly investigated by micro-Raman spectroscopy, supporting analyses were done by scanning electron microscopy, and cathodoluminescence. ZnO has a wurtzite crystal structure; therefore, according to group theory one can expect A1+2E2+E1 phonon modes. According to the phonon dispersion curve the Raman spectra of ZnO show the E2 (low) mode at 101 1/cm, the A1 (TO) mode at 381 1/cm, the E1 (TO) mode at 408 1/cm, the E2 (high) mode at 437 1/cm, the A1 (LO) mode at 574 1/cm, and the E1 (LO) mode at 583 1/cm. We could identify Raman modes at about 331 1/cm, 381 1/cm, 408 1/cm and 431 1/cm for our sintered and plasma hydrogenated ZnO samples. Moreover at about 4150 1/cm a Raman line was detected, which can be attributed to hydrogen molecules which are located in nano-voids. Oxygen vacancies or multi-vacancies introduced during the high temperature vacuum annealing might be the seeds for the nano-voids stuffed with hydrogen molecules. This result is interesting with regard to a nano-structuring of ZnO in a region close to the surface of the ZnO crystallites. In addition cathodoluminescence shows significant intensity variations, which can be correlated to the impact of structural damage and morphological changes of the surface regions of the ZnO samples. In summary one can state that hydrogen plasma exposures have a strong impact on some characteristic material properties of ZnO. Our investigations show a way to modify the material properties ZnO by plasma hydrogenation for various technological applications.


K10.41
Electrical and Optical Properties of Ga2O3-ZnO Films Deposited by Room-temperature Sputtering.Han-Chang Pan1, Chien-Ying Su1, Chien-Nan Hsiao1, Si-Pin Lin2 and Chuan-Sheng Chiou2; 1Vacuum Technology Division, Instrument Technology Research Center, Hsinchu, Taiwan; 2Department of Mechanical Engineering, Yuan Ze University, Taoyuan, Taiwan.

Gallium doped zine oxide (GZO) thin films are prepared by radio frequency sputtering on glass using a co-sputtering technique varying with sputtering power of Ga2O3 target as the Ga doping source. The structural, electrical and optical properties of the GZO films were investigated in terms of the deposition conditions such as the GZO film composition, film thickness, partial oxygen pressure and film thickness. The optical and the electrical properties of GZO films were investigated by spectrometer, Hall effect measurement, X-ray diffractometery (XRD), atomic force microscopy (AFM), conducting atomic force microscopy (CAFM) and X-ray absorption near-edge spectroscopy (XANES). Relationship among the optical, electrical properties and microstructures of transparent gallium doped zinc oxide (GZO) films on the glass using a co-sputtering technique with a varying sputtering power of Ga2O3 target as the Ga doping source was discussed. The experimental results showed the deposited GZO films at room temperature were polycrystalline with a hexagonal wurtzite structures and preferential orientation along (0002) plane. The SEM micrographs exhibited the grain size increases as function of the Ga content. The average optical transmittance of a 200 nm-thick film in the visible region (400 ~ 700 nm) is about 80%. The optical band gap of GZO films was calculated to be about of 3.4 eV. GZO films with a 5 wt.% Ga content show low resistivity of 2.96×10-3 Ω-cm , carrier concentration of 2×1020 /cm3 and mobility of 9.18 cm2/V-s, respectively. Chemical change of different Ga doping level in zinc oxide films was investigated using XANES. Intensities of the peaks appearing at the same energy of ZnO in XANES spectra were decreased with the Ga doping level.


K10.42
Abstract Withdrawn


K10.43
Transport Studies of Transition Metal Ion Doped ZnO: Bulk and Thin Films. Shubra Singh and Mahidanna Ramachandra Rao; Physics, Indian Institute of Technology, Chennai, TamilNadu, India.

Dilute magnetic semiconductors (DMS) offer the possibility of studying magnetic phenomena with a simple band structure, magneto-optical and transport properties. The existence of DMS behaviour based on Mn doped p-type ZnO and V, Ti, Fe, Co or Ni doped n-type ZnO has been predicted in theory [1]. Besides this, n-type ZnO doped with Fe and Co has been found to exhibit ferromagnetism[2]. In this paper we focus on electrical and optical properties of bulk and thin film form of Fe and Co doped ZnO samples. All doped and undoped samples were prepared using standard solid state reaction route and thin films were grown on quartz substrates by pulsed laser deposition (PLD) technique. Magnetic moment of bulk Zn0.99Fe0.01O and Zn0.99Co0.01O at room temperature (in a field of 1T) was found to be 2.3 emu/g and 2.01 emu/g respectively. Diffuse reflectance spectra (DRS) of doped ZnO gave rise to bands from crystal-field transitions of ions in tetrahedral coordination. Co doped samples exhibited maxima at 652, 612 and 568 nm corresponding to 4A 2 to 4TI(P) absorption bands and 425 nm transition corresponding to Co (III) species [3]. For Fe doped samples the energy transition 6A1→ 4E corresponding to the UV band at 375 nm is a strong indication for a tetrahedral coordination of Fe species. Absorption band at 450nm can also be attributed to Fe3+ in tetrahedral co-ordination. From the R-T plots we observe a decrease in resistivity for bulk and thin films of Fe doped samples as compared to the undoped sample, but in case of Co doped samples the decrease was observed only in case of thin films. Possible reason of such a behaviour is presented in this work based on our recent work [4]. Transmittance plot for undoped thin films show the first excitonic absorption maximum of ZnO at 357 nm (corresponding to 3.48eV) and other transitions which corroborate the data obtained from DRS for bulk samples. The results of electrical as well as optical studies will be discussed in detail based on the d-d transition band model proposed in previous works [1, 4] References [1] K. Sato and H. Katayama-Yoshida, Semicond. Sci. Tech. 17, 367 (2002). [2] M. Venkatesan, C. B. Fitzgerald, J. G. Lunney, and J. M. D. Coey, Phys. Rev. Lett. 93,177206 (2004). [3] S.Ezhilvalavan, T.R.N. Kutty, Journal of Materials Science Materials in electronics 7(1996) 137-148 [4] Shubra Singh, N. Rama and M. S. Ramachandra Rao, Appl. Phys. Lett., 88, 222111 (2006)


K10.44
Investigating the Instability of ZnO Thin Film Transistors. Richard Barry Cross and Maria Merlyne De Souza; Emerging Technologies Research Centre, De Montfort University, Leicester, United Kingdom.

In this paper, we describe for the first time gate bias and temperature induced device instabilities of inverted staggered ZnO-TFTs. ZnO with a bandgap of 3.3eV has a significant potential for transparent electronics since it offers a mobility at least 10 times higher than that of the a-Si counterpart. However, variation in sub-threshold properties and device instability currently hamper the use of this technology. The device consisted of an inverted staggered structure with a highly-doped silicon wafer which serves as both the substrate and back gate contact, a layer of 150 nm of thermally-grown SiO2 as the gate insulator, a ZnO channel layer deposited at room temperature by RF magnetron sputtering, and thermally evaporated ITO source and drain contacts. Typical as-deposited devices demonstrated On/Off ratios of ~105, and mobilities of ~ 1-2.1 cm2V-1s-1 which are comparable to those reported in the literature. However, the subthreshold slope is of the order of 3-10 V/dec, which indicates the presence of a large number of traps throughout the bandgap. The transistors have a negative threshold voltage of about -20V displaying an n-channel enhancement mode of operation. Upon annealing, at temperatures upto 600 C in air, oxygen or hydrogen, the threshold voltage either shifts to more negative values or a reduction in the on/off current ratio is observed in comparison to as-deposited samples. To investigate device stability, the TFTs were subjected to prolonged periods of gate-bias stressing at different positive and negative stress voltages. All of the stress conditions resulted in appreciable shifts in the threshold voltage and degradation of the on-off ratio as well as saturation currents. However, after a brief period of relaxation lasting about 15 minutes-1 hour, after the removal of the cause of stress, the characteristics return to their original, pre-stressed state without any thermal or bias annealing. Furthermore measurements carried out at elevated temperatures also reveal similar reversible instability and degradation of transistor characteristics with increase of temperature, in agreement with the annealing study. A characterisation of the activation energy (Ec-EF) from temperature dependent measurements shows an increase with gate bias, contrary to that expected in a-Si TFTs. A detailed analysis of the behaviour will be presented at the conference.


K10.45
Abstract Withdrawn


K10.46
Growth and Characterization of Conducting and Transparent ZnO Films Doped with Ga and Al.Sean Cherry1, D. Hunter2, H. Mustafa2, S. L Jones1 and A. K Pradhan2; 1Optical Engineering, Norfolk State University, Norfolk, Virginia; 2Center for Materials Research, Norfolk State University, Norfolk, Virginia.

The transparent semiconducting oxides have created a lot of research interest due to their applications in p-n junction based-short wavelength light emitting diodes, photovoltaics, and architectural window applications. Two important characteristics are a direct wide band gap (3.37 eV) and large exciton binding energy (60 meV), making ZnO a promising material for above applications. The importance of p-type doping in ZnO remains a clue to realize potential applications as UV light emitters, transparent high-power electronics, piezoelectric transducers, chemical and gas sensors. We report here the synthesis of epitaxial Ga- and Al-doped ZnO films by the pulsed-laser deposition technique. The films were grown in the pressure range 10 to 1 mTorr of oxygen and substrate temperature 400-700°C. The growth conditions were optimized. The films show excellent crystalline quality with atomically smooth surface morphology. The electrical resistivity was approximately 1.5×10-4 ohm-cm and transmittance >85%. The temperature dependent resistivity measurements of these highly conducting and transparent films show several transitions. The extensive results will be presented.


K10.47
The Growth and Characterization of Ga-doped Mg0.15Zn0.85O Thin Film by Pulsed Laser Deposition.Chunming Jin1, Wei Wei2,1, Vikram Bhosle2, Andy Doraiswamy1, Roger Narayan1 and Jagdish Narayan2; 11Joint Department of Biomedical Engineering, University of North Carolina and North Carolina State University, Chapel Hill, North Carolina; 2Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina.

Transparent oxide semiconductors are being considered for use in several advanced devices, including conductive contacts, solar cells, laser diodes, ultraviolet lasers, and thin-film transistors. For example, gallium- and aluminum- doped zinc oxide thin films have attracted significant interest for use in optoelectronic applications. These materials exhibit large bandgap values and optical transparency from 1100 nm to 370 nm. By incorporating magnesium into zinc oxide, magnesium zinc oxide alloys may be obtained that retain a hexagonal structure and exhibit large bandgap values. The bandgap for magnesium zinc oxide alloys can be tuned between 3.4 eV (value for unalloyed zinc oxide) and 4.42 eV. Magnesium zinc oxide alloys may provide an even larger range of optical transparency (1100 nm-280 nm) than conventional gallium- or aluminum- doped zinc oxide thin films. We have deposited gallium-doped Mg0.15Zn0.85O thin films using pulsed laser deposition on amorphous fused silica and sapphire (0001) substrates. Film microstructure was examined using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The films demonstrated textured structures with the c-axis perpendicular to the substrate surface on the fused silica substrates and epitaxial structures on the sapphire (0001) substrates. The optical properties of the films were examined with transmission, absorption, and photo luminescence measurements. Electrical conductivity was investigated using the four-point probe method at several temperatures between 30 K and 300 K. The films exhibited optical transparency from 1100 nm to 330 nm. Excitonic absorption peaks were not observed in the gallium-doped magnesium zinc oxide films due to the effects of the gallium doping. The conductivity values for gallium-doped magnesium zinc oxide thin films were comparable to those observed for gallium-doped zinc oxide thin films. The effect of the growth temperature on the optical and electrical properties of the films was also investigated. These results suggest that gallium-doped MgxZn1-xO thin films may provide novel functionalities to optoelectronic devices.


K10.48
Abstract Withdrawn

K10.49
ZnO Nanofibers Doped with Ga, In and Er Fabricated with Electrospinning Technique.Aurangzeb Khan1, Saima Naz Khan1, Wojciech Jadwisienczak2 and Martin E Kordesch1; 1Physics& Astronomy and CMSS Program, Ohio University, Athens, Ohio; 2School of Electrical Engineering and Computer Science, Ohio University, Athens, Ohio.

ZnO nanofibers doped with Ga, In and Er metals have been fabricated by electrospinning technique. The fibers are then characterized by x-ray diffractometery (XRD), scanning and transmission electron microscopy (SEM & TEM) and photoluminescence (PL). PL is performed on the samples in the temperature range of 10-300 K. PL spectra exhibit near band edge (NBE) ultra violet (UV) emission (381 nm) and blue-green band emission (510 nm) in both undoped and doped samples. A red-shift in the band edge peak is observed with increasing temperature for the undoped ZnO nanofibers. It is also observed that the In and Ga incorporation in ZnO crystal have brought more defects and show stronger blue-green emission peak relative to the band edge emission peak. PL spectra of the Er doped ZnO have shows peaks at 527 nm, 559 nm, 604 nm, 622 nm, which are coming from active Er. The electrospinning mechanism used for the fabrication of both doped and undoped ZnO nanofibers was found to be productive, simple and easy to implements.


K10.50
Doped ZnO Nanoclusters: Ferromagnetism and UV Photoluminescence at Room TemperatureYou Qiang, Jiji Antony, Amit Sharma, Muhammad Faheem, Daniel Meyer and Michael Campanell; Physics, University of Idaho, Moscow, Idaho.

Transparent conducting oxides of doped ZnO are of great interest to current research due to its wide variety of applications in spintronic materials. We prepared Ti, V, Co or Ni-doped ZnO nanoclusters using a third generation nanocluster source that utilize a combination of magnetron sputtering and gas-aggregation technique. TEM images show that the nanoclusters are monodispersive with a nanocrystalline size < 10 nm. XRD patterns are identical to the bulk ZnO wurtzite structure. XPS detected the dopant elements in clusters and showed Ti in +4 oxidation state, V in +4 and +5, Co in +2, and Ni in +2 and +3. These analyses indicate that dopant elements do not exist as independent aggregates but are incorporated into the ZnO structure. All the doped ZnO nanoclusters are ferromagnetic above room temperature. Magnetic moments of Ni and V-doped ZnO (1.5mB or 3.5mB per dopant atom) are much larger than Ti or Co doped ZnO clusters (0.2mB or 0.6mB per dopant atom) at 300K. Double exchange interactions due to the mixed valance states can be the reason that Ni or V-doped ZnO clusters a better magnetic moment than the Ti or Co-doped clusters. UV-photoluminescence is observed at pure and low dopant concentration ZnO nanoclusters. Both magnetic and UV optical properties of doped ZnO nanoclusters are dopant concentration dependent. Supported by NSF-EPSCoR, DOE-EPSCoR and Battelle-PNNL Reference: 1. J. Antony, X.B. Chen, J. Morrison, L. Bergman, Y. Qiang, D.E. McCready, and M. Engelhard, “ZnO nanoclusters: Synthesis and Photoluminescence”, Appl. Phys. Letters, 87, 241917 (2005). 2. J. Antony, S. Pendyala, A. Sharma, X.B. Chen, J. Morrison, L. Bergman, Y. Qiang, “Room temperature ferromagnetic and UV optical properties of Co-doped ZnO nanocluster films”, J. of Appl. Phys. 97, 10D307 (2005). May 16, 2005 3. J. Antony, D.E. McCready, M. Engelhard and Y. Qiang, “Anomalous double-exchange enhanced ferromagnetism in transition metal-doped ZnO nanoclusters above room temperature”, Phys Rew. Letters, submitted, 2006. 4. Jiji Antony, Sweta Pendyala, David E McCready, Mark H Engelhard, Daniel Meyer, Amit Sharma and You Qiang, “Ferromagnetism in Ti-Doped ZnO Nanoclusters above Room Temperature”, accepted by IEEE Transactions on Magnetics, 2006.


K10.51
Structural, Optical and Magnetic Properties of Co and Fe doped ZnO Thin Films Growth by Radio-frequency Magnetron SputteringLuis Manuel Angelats, Maharaj S. Tomar, Oscar Perales-Perez, Ricardo E. Melgarejo, Hector Jiménez and Ricardo Martinez; Physic, University of Puerto Rico, Mayaguez, Puerto Rico.

Co-doped and CoFe-doped ZnO films were growth onto quartz fused from ceramic targets using the magnetron sputtering technique under the following sputtering conditions: rf power 125 W, argon working pressure 8.5 x 10 -3 Torr and substrate temperature 300°C. The X-ray diffraction patterns of Zn0.90Co0.10O and Zn0.85[Co0.50Fe0.50]0.15O films showed only (002) peak indicating the strong preferred orientation along these planes. Raman spectra for thin films did not any significant additional modes for Co and Fe. Zn0.90Co0.10O film showed transmittance above 70% with three absorption peaks attributed to d-d transitions of tetrahedrally coordinated Co2+. Transmittance optic of Zn0.85[Co0.50Fe0.50]0.15O film was less than Zn0.90Co0.10O film in the visible range. The band gap values for Zn0.90Co0.10O and Zn0.85[Co0.50Fe0.50]0.15O films were 2.95 and 2.70 eV respectively, which are slightly less than ZnO films found in this work. The Zn0.90Co0.10O film showed a relatively large positive magnetoresistance (MR) at the high magnetic field in the temperature range from 7 to 50K, which reached 11.9% a 7K for the magnetoresistance. The lowest MR was found at 100K. From M-H curve of Zn0.90Co0.10O and Zn0.85[Co0.50Fe0.50]0.15O films measured at room temperature shown a coercive field of 30 and 40 Oe respectively.


K10.53
Electrical and Optical Properties of Al-doped ZnO Thin Films Prepared by Magnetron Co-sputtering Tin Yan Kwok1, Ning Ke1, Wing Yiu Cheung1 and S. P. Wong1,2; 1Dept of Electronic Engineering, Chinese University of Hong Kong, Shatin, Hong Kong; 2Materials Science and Technology Research Centre, Chinese University of Hong Kong, Shatin, Hong Kong.

While indium tin oxide has been widely used as transparent electrodes for many optoelectronic devices, to match the optical properties of various semiconductor materials systems in order to achieve better device performance, there are increasing interest to develop new materials with various optical properties for transparent electrode applications. In this work, we performed a systematic study of the electrical and optical properties of transparent conductive Al-doped ZnO (AZO) thin films prepared by magnetron co-sputtering with an RF source for ZnO and a DC source for Al. The Al doping composition was varied by adjusting the power of the two sources and was determined by Rutherford backscattering spectrometry. The thickness of the films was determined by an alpha-step surface profiler. The electrical properties were studied by resistivity and Hall effect measurements from 40K to 300K using the van der Pauw method. The optical properties were studied by optical transmittance spectroscopy and spectroscopic ellipsometry. The results showed that for samples with an Al atomic percentage falling between 3 to 4.5 %, they were optically transparent in the visible range with sufficiently low resistivity values suitable for transparent electrode applications. The thickness of these films ranged from about 150 nm to 300 nm. The transmittance values were observed to be larger than 80% in the wavelength range from 400 to 800 nm and the resistivity values varied from about 6x10-3 to 8x10-3 ohm-cm. The band gap energy was observed to increase slightly with increasing Al composition from 3.36 eV to 3.54 eV for the Al atomic percentage range from 2 to 4.5%. The refractive index n and extinction coefficient k of these films in the wavelength range from 400 nm to 700 nm were also determined by fitting of the spectroscopic ellipsometry spectra. A general decreasing trend of n values with increasing Al composition was observed. This work is supported in part by a direct grant for research from the Faculty of Engineering of CUHK.


K10.54
Abstract Withdrawn

K10.55
Lasing Characteristics of Nano-structured Zinc OxideShou-Yi Kuo1, Wei-Chun Chen1 and Fang-I Lai2; 1Instrument Technology Research Center, National Applied Research Laboratories, Hsinchu, Taiwan; 2Department of Electronic Engineering, Ching Yun University,, Taoyuan, Taiwan.

Highy-quality nano-structured ZnO samples, including thin films and nanorods, have been synthesized by simple chemical solution and thermal evaporation methods without any catalysts. The samples were characterized by x-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), temperature-dependent photoluminescence (PL) spectra measurements. XRD patterns illustrated that there were no second phases in these ZnO samples, and the TEM results indicated that the well-aligned ZnO nanorods are single crystalline with a hexagonal structure and grow along the [001] direction. Room-temperature PL spectra of ZnO thin films showed a strong UV near-band-edge (NBE) emission located at about 390 nm and a green defect-related (G) emissions, where the intensity ratio of NBE-to-G emission varies with the annealing temperatures. Meanwhile, the ZnO nanorods only revealed strong UV emission. The ZnO samples exhibited free exciton and very sharp exciton emissions at low temperatures. Particularly, room-temperature UV random lasing characteristic of ZnO films and nanorods has been observed as well. It is shown that these nano-structured ZnO samples can exhibit random laser action depending on the growth condition. The threshold intensity for the lasing is comparable to earlier reported data. These results indicate that nano-structured ZnO samples prepared by both simple techniques may be a promising material for further photonic devices. Possible lasing mechanism is discussed and further investigation to clarify the mechanism between the nano-structured ZnO samples is still underway.


K10.56
Abstract Withdrawn

K10.57
Modelling the Preferred Shape and Orientation of ZnO Nanowires and Nanobelts.Amanda S Barnard1 and Yanan Xiao2; 1Department of Materials, University of Oxford, Oxford, United Kingdom; 2Advanced Photon Source, Argonne, Illinois.

At the nanoscale, zinc oxide (ZnO) forms in many different shapes, the dimensions of which are often closely related to specific properties. Although much attention is focussed on finding methods for controlling the morphology, explanations of phenomena such as the change in crystallographic orientation of the principle axis of one-dimensional (1-D) nanostructures (with size) are still largely empirical. We have used a shape-dependant thermodynamic model to identify the relationship between size, shape and orientation of ZnO in 1-D. Using this method we are able to approximate the stability regions for nanowires or nanobelts, and show that growth in the ±[1-210] direction is a decisive factor in determining the orientation of nanobelts once formed.
 

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