Symposium Organizers
David P. Norton University of Florida
Chennupati Jagadish Australian National University
Irina Buyanova Linkping University
Gyu-Chul Yi Pohang University of Science and Technology (POSTECH)
L1: ZnO Nanowires for Applications
Session Chairs
Monday PM, November 26, 2007
Constitution A (Sheraton)
9:30 AM - **L1.1
ZnO Nanopiezotronics.
Zhong Wang 1
1 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractDeveloping novel technologies for wireless nanodevices and nanosystems are of critical importance for in-situ, real-time and implantable biosensing, biomedical monitoring and biodetection. 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. We have demonstrated an innovative approach for converting nano-scale mechanical energy into electric energy by piezoelectric zinc oxide nanowire (NW) arrays [1-3]. We have recently developed DC nanogenerator driven by ultrasonic wave [4], which is a gigantic step towards application in practice.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. Based on this principle, piezoelectric-field effect transistor [5], piezoelectric gated diode [6], sensors and resonators have been fabricated, which are the fundamental components of nano-piezotronics. Piezotronics is a field of using piezoelectric-semiconducting coupled property for fabricating novel and unique electronic devices and components [7].[1] Z.L. Wang and J.H. Song “Piezoelectric Nanogenerators Based on Zinc Oxide Nanowire Arrays”, Science, 312 (2006) 242-246.[2] P.X. Gao, J.H. Song, J. Liu and Z.L. Wang “Nanowire Nanogenerators on Plastic Substrates as Flexible Power Source”, Adv. Mater., 19 (2007) 67-72.[3] J.H. Song, J. Zhou, Z.L. Wang ”Piezoelectric and semiconducting dual-property coupled power generating process of a single ZnO belt/wire – a technology for harvesting electricity from the environment”, Nano Letters, 6 (2006) 1656-1662.[4] X.D. Wang, J.H. Song J. Liu, and Z.L. Wang “Direct current nanogenerator driven by ultrasonic wave”, Science, 316 (2007) 102-105.[5] X.D. Wang, J. Zhou, J.H. Song J. Liu, N.S. Xu and Z.L. Wang “Piezoelectric-Field Effect Transistor and Nano-Force-Sensor Based on a Single ZnO Nanowire”, Nano Letters, 6 (2006) 2768-2772.[6] J. H. He, C.H. Hsin, L.J. Chen, Z.L. Wang ”Piezoelectric Gated Diode of a Single ZnO Nanowire”, Adv. Mater., 19 (2007) 781.[7] Z.L. Wang “Nano-piezotronics”, Adv. Mater., 19 (2007) 889.
10:00 AM - L1.2
Gas Sensor Array made of ZnO Nanorods.
Junghwan Huh 1 , Hyeyoung Kim 1 2 , Kangho Lee 1 , Jonghyurk Park 2 , Gyutae Kim 1
1 School of Electrical Engineering, Korea University, Seoul Korea (the Republic of), 2 , Electronics and Telecommunications Research Institute, Daejeon Korea (the Republic of)
Show AbstractThe array of ZnO nanorod networks were fabricated by the sol-gel process, which was selectively grown on the patterned Si3N4 substrates. The exposure of the active region of electrodes to the sols following the lithographic process could form the ZnO nanorods at the pre-defined positions. After the annealing process under the nitrogen environment at the temperature of T=300°C, a considerable enhancement of the conductance was observed. Under the hydrogen environments, the electrical conductance of the ZnO nanorod device increased with the response time of about 360sec, fitting to the reductive process by the hydrogen ions. The validity of the passivation was approved by coating with the 5% PVA solution on the array of ZnO nanorods. The easy process for the array of nanowire devices enables a large scale and a low cost fabrication with the longer life time.
10:30 AM - L1.4
Electrical Properties of ZnO Nanowire Field Effect Transistors on Bended Flexible Substrate.
Soon-Shin Kwon 1 , Woong-Ki Hong 1 , Jongsun Maeng 1 , Tae-Wook Kim 1 , Gunho Jo 1 , Takhee Lee 1
1 , GIST, Gwangju Korea (the Republic of)
Show AbstractOne-dimensional nanostructures such as nanowires and nanobelts have been extensively studied as build blocks for nanoscale electronic devices. Recently, ZnO nanowires have attracted considerable interest in wide applications for optoelectronics, piezoelectric devices, chemical sensors, and solar cells due to high transparency, piezoelectricity, and direct wide bandgap (3.37 eV) semiconductor with large exciton binding energy. (60meV) [1]. In addition, the electronic devices on plastic substrate have attracted extensive attention owing to many attractive properties including biocompatibility, flexibility, light weight, shock resistance, softness and transparency [2]. The combination of these advantages can give a wide range of applications in displays, sensors, medical devices, and other systems [3]. In this presentation, we report on the electrical properties of ZnO nanowire field effect transistors (FETs) on a flexible substrate as a function of bending configuration of the substrate. We fabricated ZnO nanowires FETs on a flexible PET substrate with poly(4-vinylphenol) (PVP) and cross-linked PVP as a gate dielectric layer. The transistor characteristics such as drain current-drain voltage characteristics and drain current-gate voltage characteristics of the ZnO nanowire FETs on the flexible substrate will be explained under different bending configuration of the flexible substrate. [1] Z. L. Wang et al., Adv. Funct. Mater. 14, 943 (2004).[2] R. H James et al., Nature Mater. 6, 379 (2007).[3] B.J. David et at., Nature Nanotechnology 2, 378 (2007).
10:45 AM - L1.5
ZnO Nanorod Arrays as Efficient Antireflection Coatings in Solar Cells.
Yun-Ju Lee 1 , Douglas Ruby 1 , David Peters 1 , Bonnie McKenzie 1 , Julia Hsu 1
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractAntireflection coatings (ARCs) play an important role in improving the performance of solar cells utilizing high refractive index materials such as Si and GaAs. Current ARCs typically consist of layer(s) of dielectric films or etched textured surfaces to generate a refractive index gradient and decrease reflectance. Here, we investigate solution-grown ZnO nanorod arrays on Si as ARCs. By varying the synthetic chemistry, we grew nanorods with different nanoscale morphology, which leads to significant changes the macroscopic reflectance response. The effect of parameters such as nanorod dimensions and amount of tapering at the nanorod tips on the reflectance spectra are examined. Compared to traditional single layer ARCs, ZnO nanorod arrays exhibited a much more wavelength independent suppression of reflectance from 400 to 1200 nm. By reducing the average tip diameter of a tapered ZnO nanorod array to 10 nm, a weighted global front surface reflectance of 6.6% was achieved, superior to commercially used single-layer SiN ARC. The experimental data are compared to results from rigorous coupled wave analysis to determine the parameters most critical to ARC performance and to suggest possible future direction for optimizing ARC characteristics through nanorod array synthesis.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.
11:30 AM - L1.6
Nanostructured Degeneratively Doped ZnO as a Solution-processable Transparent Conductor for Solar Cells.
Alberto Salleo 1 , Sam Rosenthal 1 , Ludwig Goris 1
1 Materials Science and Engineering, Stanford University, Stanford, California, United States
Show AbstractCurrently, the material most widely used as transparent electrode in thin-film solar cells is Sn-doped In2O3 (ITO). The natural abundance of In however is insufficient for high-volume production of low-cost solar cells. Moreover, ITO made by vacuum deposition at temperatures compatible with transparent flexible substrates, as needed for high-throughput roll-to-roll manufacturing, has poor crystalline quality and high sheet resistance (Rs>50 Ω/sq.). Zn-based oxides are extremely attractive replacement materials for ITO due to the great natural abundance and low toxicity of Zn. ZnO can be made conductive by doping with heteroatoms, like Al and Ga. The performance of these films approaches that of state-of-the-art ITO. Currently however, the only methods available to deposit high-quality degeneratively-doped ZnO films involve costly vacuum processing techniques. Therefore, the same throughput and temperature limitations as those described for vacuum processing of ITO apply to ZnO. Moreover, uniform Al doping of ZnO by sputtering or CVD is challenging because of the high reactivity of Al with residual O2. We opted to explore colloidal chemistry as a cheap and simple alternative to manufacture ZnO nanostructures and thin film, transparent electrodes. ZnO is known to form nanostructures of varied shapes. For instance, ZnO nanowires have been grown by many methods in aqueous solvents. Doping of these wires however is problematic because the dopant atoms typically form stable inert aquo-complexes. In order to solve this problem, we synthesized ZnO nanowires in a non-aqueous solvent. A dispersion of these highly doped nanowires is effectively a solution-processable precursor to a transparent conducting film. We successfully synthesized ZnO nanowires in liquid phase by refluxing a solution of Zn acetate in tri-octylamine at 365°C under nitrogen for 15 minutes to 2 hours. SEM characterization confirmed the formation of nanowires with a width typically smaller than 50 nm. TEM characterization confirmed by X-ray diffraction indicated that the nanowires are crystalline. By introducing Al acetate in the reaction vessel, Al-doped nanowires were successfully synthesized as confirmed by TEM and EDS. We cast films from nanowire solutions. Electrical testing showed that films made with pure ZnO are insulating while those made with Al-doped ZnO are conductive thus providing a convincing proof-of-principle that introducing Al salts in the reaction bath produces indeed doped nanowires. Thermal annealing at moderate temperatures (T~200°C) increased the conductivity by almost 5 orders of magnitude.The process described here decouples the materials synthesis from successive device fabrication, which constitutes a great advantage from the standpoint of materials and process integration. Indeed, ZnO nanowires with the most suitable properties can be obtained regardless of the thermal budget limitations imposed by successive device fabrication steps or substrates.
11:45 AM - L1.7
Optical Properties of Surface-Related States in ZnO Nanowires and ZnO-Al2O3 Nanowire Core-Shell Structures.
Lars Wischmeier 1 , Tobias Voss 1 , Dong Sik Kim 2 , Margit Zacharias 2
1 Institute of Solid State Physics, University of Bremen, Bremen Germany, 2 , Max Planck Institute of Microstructure Physics, Halle Germany
Show AbstractZnO nanowires have recently attracted much attention due to their high potential as nanoscale optoelectronic devices. Although the properties of these nanostructures are mainly governed by the well-known ZnO bulk properties, the influence of the surface increases with decreasing wire diameter [1]. For nanowire-based applications a detailed knowledge of the microscopic processes in this surface layer is mandatory.We performed micro-photoluminescence investigations on individual ZnO nanowires with diameters ranging from 34 nm to 520 nm. Here the nanowires are grown by vapour transport method using Au nanoparticles as catalyst [2]. The photoluminescence of the as-grown nanowires shows pronounced deep-level and near-band-edge emission. After keeping the samples in air for a few days, the deep-level emission is drastically reduced. All wires exposed to air show near-band-edge emission that is comparable to our previous results [1,3]. In all cases, surface-related excitonic emission strongly contributes to the near-band-edge emission in addition to the emission caused by the recombination of excitons bound to neutral donors and free excitons. The observation of the surface-related excitonic emission from wires grown with different conditions underlines that this emission is a general phenomenon for ZnO nanowires.To analyze possible interconnections between the reduction of deep-level emission and the occurrence of surface-related excitonic emission for nanowires exposed to air, we measured the photoluminescence in regular time intervals starting directly after the growth to trace the changes of the optical properties. We compare the results to those obtained with a second sample that was grown under the same conditions but additionally covered with a thin Al2O3 layer directly after the growth process. The results show the effect of the passivation of the ZnO surface layer and the influence of the shell on the optical properties of the ZnO core.
[1] L. Wischmeier et al., Appl. Phys. A 84, 111 (2006)[2] D. S. Kim et al., Small 3, 76 (2007)[3] L. Wischmeier et al., Phys. Rev. B 74, 195333 (2006)
12:00 PM - L1.8
Fabrication of Vertically Aligned Single-Nanometer-Diameter ZnO Nanorods Using MOVPE with a Two-Step Temperature Growth Method.
Kokoro Kitamura 1 , Takashi Yatsui 2 , Motoichi Ohtsu 1 2 , Gyu-Chul Yi 3
1 Department of Electronic Engineering, The University of Tokyo, Tokyo Japan, 2 Solution-Oriented Research for Science and Technology, Japan Science and Technology, Tokyo Japan, 3 Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang Korea (the Republic of)
Show AbstractZinc oxide (ZnO) nanostructures have attracted much attention for use in optical and electrical devices because of the large exciton binding energy and large oscillator strength of ZnO. ZnO also has great potential as a material for room temperature nanoscale photonic devices. A nanophotonic switch using one-dimensional ZnO nanorod double-quantum-well structures has been reported. Additional functional devices can be realized using the additional radial quantum confinement effect of ZnO nanorods. Although ultrafine (less than 10 nm in diameter) ZnO nanorods exhibiting the quantum confinement effect have been reported, they grow in random directions. In this study, we used a two-step temperature growth method to fabricate vertically well-aligned ultrafine ZnO nanorods.The ZnO nanorods were grown on sapphire (0001) substrate using a catalyst-free metalorganic vapor phase epitaxy (MOVPE) system. Diethylzinc (DEZn) and oxygen were used as the reactants and argon as the carrier gas. The pressure inside the reaction chamber was maintained at 5 Torr. Our method consists of low-temperature growth for the vertical alignment of ZnO nanoneedles and high-temperature growth for the ultrafine nanorod structure. In the first step, vertically well-aligned ZnO nanoneedles measuring 1 μm in length and 100 nm in diameter were grown at 450 °C for 35 min. Before the second step, the substrate temperature was raised to 750 °C without DEZn gas flow. In the second step, we supplied DEZn gas with oxygen for 10 min at 750 °C. Consequently, ultrafine ZnO nanorods structures were grown from the tips of the preformed nanoneedles. Furthermore, the ultrafine ZnO nanorods were aligned in the direction of the preformed vertically well-aligned nanoneedles; that is, the ultrafine ZnO nanorods were vertical to the substrate. Scanning electron micrographic images revealed that the minimum diameter of the ZnO was less than 10 nm and the length was 500 nm with a uniform diameter along the axis. Since the diameter of the nanorods is decreased with increasing temperature, the successful fabrication of well-aligned ultrafine ZnO nanorods described here is a promising step toward designing three-dimensionally controlled nanophotonic devices.
12:15 PM - L1.9
Correlating the Optical and Electrical Properties of Electrodeposited ZnO Nanowires.
Chegnui Bekeny 1 , Tobias Voss 1 , Juergen Gutowski 1 , Ramon Tena-Zaera 2 , Jamil Elias 2 , Claude Levy-Clement 2 , Ivan Mora-Sero 3 , Juan Bisquert 3
1 Institute of Solid State Physics, University of Bremen, P.O. Box 330440, 28344 Bremen Germany, 2 , Institut de Chimie et Matériaux de Paris-Est (ICMPE), CNRS, UMR 7182 Bat. F, 2-8 rue Henri Dunant, 94320 Thiais France, 3 Departament de Física, Universitat Jaume I, Av. de Vicent Sos Baynat s/n, 12071 Castelló de la Plan Spain
Show Abstract12:30 PM - L1.10
Inkjet Printing of ZnO Nanorods for Low Cost Field Effect Transistors.
Cyril Martini 1 , Guillaume Poize 1 , Sebastien Sanaur 2 , Michael Barret 2 , Philippe Collot 2 , Frederic Fages 1 , Joerg Ackermann 1
1 Laboratoire des Matériaux Moléculaires et des Biomatériaux, GCOM2 UMR CNRS 6114, , Université de la Méditerranée, Faculté des Sciences de Luminy, Case 901, 163, avenue de Luminy,, Marseille France, 2 Center for Microelectronics of Provence “Georges Charpak” (CMP-GC, Department of Packaging and Flexible Substrates (PS2), Laboratoires Morandat – impasse de la Plaine, Gardanne France
Show AbstractZnO nanorods represent promising nanomaterials for future low cost flexible electronics such as RFID tags or solar cells. Recently high mobility field effect transistors, hybrid bulk heterojunction solar cells, hydrogen storage and light emitters based on ZnO nanorods have been reported. Therefore combining these materials with low cost deposition techniques such as inkjet printing could overcome the limite of organic semiconducting materials usually used in flexible electronics.In this communication we present our results on ink jet printing of ZnO nanords and show prelimimary resutls on printed ZnO field effect transistors. The effect of surface functionalization of the ZnO nanorods as well as thermal annealing treatment on the charge carrier transport properties of the resulting ZnO transistors will be discussed
12:45 PM - L1.11
Electrospun InGaZnO4 Nanofiber Based Field-effect Transistors.
Seung-Hoon Choi 1 , Jae-Min Hong 1 , Il-Doo Kim 1
1 , Korea Institute of Science and Technology, Seoul Korea (the Republic of)
Show AbstractTransparent ZnO, GaZnO (GZO), and InGaZnO4 (IGZO) films have been used as an active channel in exhibiting n-type semiconductor characteristics with high optical transmittance in the visible spectrum. In particular, the ZnO based thin-film field-effect transistors (FETs) are of great interest due to their high field-effect mobility and good compatibility with plastic substrates. In addition, the pursuit of new materials and/or structures with novel functionalities has led, over the past several years, to the use of wide band gap semi-conducting oxides comprising one-dimensional (1D) nanostructures. Efforts have been focused on achieving increased surface-to-volume ratios and reduced cross sections, offering more effective charge transport. In this regard, we report on the fabrication of low-voltage operating FETs using IGZO nanofiber as an active channel. ZnO/PVA, GZO/PVA, and IGZO/PVA composite nanofibers utilizing sol-gel precursors(i.e., InCl3, GaCl3, Zn acetate / Poly(vinyl alcohol) were directly electrospun onto the patterned gate insulators, i.e., Mg doped Ba0.6Sr0.4TiO3, and calcined at 450-550°C for 1 hr. The morphology, crystal structure and element analysis of ZnO, GZO, and IGZO nanofbiers were investigated by FE-SEM, XRD, EPMA and TEM. These resulted in polycryatalline ZnO, GZO nanofibers and amorphous-like IGZO nanofibers composed of 200-600 nm diameter cores. Electrical transport of each nanofibers were measured by semiconductor device analyzer (Agilent B1500A). We observed that the IGZO nanofibers exhibited much enhanced conductivity as compared to those ZnO and GZO of nanofibers. The semiconducting behavior of ZnO, GaZnO, and InGaZnO4 nanofibers and FETs characteristics are discussed in detail in an accompanying presentation.
L2: ZnO Nanowire Synthesis and Properties
Session Chairs
Monday PM, November 26, 2007
Constitution A (Sheraton)
2:30 PM - L2.1
Second-harmonic Generation and Thermal Effects in ZnO Nanorod Arrays.
Iryna Kudyk 1 , Lars Wischmeier 1 , Bianca Postels 2 , Andrey Bakin 2 , Andreas Waag 2 , Tobias Voss 1
1 Institute of Solid State Physics, University of Bremen, Bremen Germany, 2 Institute of Semiconductor Technology, Braunschweig University of Technology, Braunschweig Germany
Show AbstractDensely packed arrays of hexagonal ZnO nanorods are currently considered as basic platforms for the realization of complex optoelectronic devices. In addition to their excellent crystalline structure and preferentially c-axis orientation, these nanorods possess a large surface-to-volume ratio which allows to efficiently functionalize their surface.We present investigations of the non-linear optical properties of ZnO nanorod ensembles fabricated by an aqueous-chemical growth technique. For the excitation, we used a tunable femtosecond oscillator providing 50-fs pulses with 12 nJ in a spectral range between 700 nm and 900 nm. The signal was detected spectrally resolved in both reflection and transmission geometry at room temperature. The emission from the nanorods consists of second-harmonic generation (SHG), near-band-edge photoluminescence (PL), and deep-level emission (DLE). The PL and DLE are generated by multi-photon excitation and absorption of SHG photons. We find that the ratio of SHG, PL, and DLE contributions strongly depends on the photon energy of the femtosecond pulses. We analyze the wavelength and excitation-power dependence of the non-linear emission for different angles between the incident laser pulses and the c-axis of the nanowires. From the results we determine the non-linear optical (NLO) coefficients of the ZnO nanorod ensemble. We present corresponding results for samples with different nanorod lengths and compare the NLO coefficients of the nanorod arrays to those of ZnO bulk crystals. Due to the wurtzite structure of the single-crystal nanorods the obtained values for the second-order NLO coefficients χ(2)311 and χ(2)333 are of opposite sign. For both the nanorod arrays and the bulk crystals we find |χ(2)311/ χ(2)333| = 0.10 pointing to comparable crystalline qualities of the samples. The peak spectral position of near-band-edge PL is shifted to a range of 400 – 415 nm due to local heating of the sample by the femtosecond excitation pulses. This thermal effect is a result of the reduced thermal conductivity of the nanorod ensemble compared to conventional bulk crystals. The shift of the PL allows us to determine the local temperature in the nanorods under sub-bandgap femtosecond-pulse excitation. We present simulations of the heat transport in ZnO nanorods and investigate the correlation between nanorod size and heat transport.
2:45 PM - L2.2
Heteroepitaxial Orientations of ZnO Nanorods on Ag.
Jerrold Floro 1 , Joseph Michael 2 , Luke Brewer 2 , Julia Hsu 2
1 Materials Science and Engineering, University of Virginia, Charlottesville, Virginia, United States, 2 , Sandia National Labs, Albuquerque, New Mexico, United States
Show AbstractFine-grained polycrystalline silver is an effective nucleation layer for growth of spatially well-aligned ZnO nanorod arrays from aqueous solution. In order to explore heteroepitaxy in the ZnO/Ag system, wurtzite ZnO nanorods were grown onto coarse-grain bulk polycrystalline Ag substrates, providing a means to efficiently investigate the range of preferred heteroepitaxial orientations. Scanning electron microscopy shows that ZnO nanorods grow only on select grains, which are found to have 〈111〉 fiber-texture using electron backscatter detection analysis. The nanorods, which are faceted and grow perpendicularly to the {111} surface, exhibit a tightly-defined epitaxial orientation of (0001) ZnO ‖ {111} Ag and 〈11-20〉 ZnO || 〈1-10〉 Ag. This “hex-on-hex” orientation has a nominal 11% lattice misfit, but is characterized by a near-coincidence site lattice with reasonable coincidence site density and very small CSL misfit strain. We will discuss the use of CSL concepts in understanding heteroepitaxy in highly dissimilar materials systems. This work was supported by the Sandia LDRD program and the DOE Office of Basic Energy Sciences. 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-AC04-94-AL-85000.
3:00 PM - L2.3
Characterization of Vertical Arrays of ZnO Nanorods by AFM.
Christian Teichert 1 , Yue Hou 1 , Andrey Andreev 1 , Gerhard Brauer 2 , Kai Tam 3 , Aleksandra Djurisic 3
1 Institute of Physics, University of Leoben, Austria, Leoben Austria, 2 Institut f. Ionenstrahlphysik und Materialforschung, Forschungszentrum Rossendorf, Dresden Germany, 3 Department of Physics, University of Hong Kong, Hong Kong China
Show AbstractOne-dimensional nanostructures, such as nanorods or nanotubes, exhibit technological potential for many device applications, like efficient low-cost ZnO nanorod-polymer solar cells [1]. However, achieving control over the growth of such nanostructures leading to proper dimensional confinement (like nanorod diameter, length, density and orientation) is still a challenging task. On the other hand, atomic-force microscopy (AFM) is well known as a valuable tool for nanometer scale characterization of different types of nanostructures [2]. The capabilities of AFM techniques are demonstrated for the characterization of vertical arrays of various ZnO nanorods. In detail, the topography of the ZnO nanorods, grown on Si and ITO substrates, was examined. It was found that tapping mode AFM is an appropriate tool to reveal the morphological features of vertical ZnO nanorods on the nanoscale, i.e. 3D rod shape, lateral size, average height, and rod height uniformity [3]. Moreover, by cross-sectional measurements it was established that intermediate {10-13} facets appear between the top (0001) and side {10-10} facets. Further, polymer coated ZnO nanowires have been investigated with respect to the possibility of measuring their elcetric properties by Conductive Atomic Force Microscopy. [1] E. Greene, et al., Nano Lett. 5 (2005) 1231. [2] C. Teichert, Phys. Rep. 365 (2002) 335. [3] G. Brauer, W. Anwand, D. Grambole, W. Skorupa, Y. Hou, A. Andreev, C. Teichert, K. H. Tam, A. B. Djurisic, Nanotechnology 18 (2007) 195301.
3:15 PM - L2.4
Direct Formation of ZnO Nanowire Arrays by Exposure of a Zn Compound to Extreme Ultraviolet Interference Patterns.
Vaida Auzelyte 1 , Harun Solak 1 , Gerald Egeland 2
1 Laboratory for Micro and Nanotechnology, Paul Scherrer Institut, Villigen Switzerland, 2 Spallation Neutron Source Division, Paul Scherrer Institut, Villigen Switzerland
Show AbstractRegular arrays of ZnO nanowires are of high interest for their potential use in new devices such as transparent transistors or light emitting diodes. ZnO nanostructures can be fabricated via synthesis or lithographic lift-off techniques. In general the synthesis route does not lead to the creation of regular, addressable structures and the lift-off method faces limitations in resolution. Here, we demonstrate a technique for the fabrication of large arrays of dense and regular ZnO nanowires by exposure of a Zinc Naphthenate film with extreme ultraviolet interference lithography (EUV-IL). Zinc Naphthenate works as a negative resist when it is exposed to energetic radiation such as electron beam or EUV light, i.e. it becomes insoluble in certain solvents [1]. We deposited this compound onto Si wafers with spin-coating and exposed the resulting films using EUV-IL at a wavelength of 13.5 nm. We measured the EUV lithographic sensitivity and contrast of Zinc Naphthenate to be 500 mJ/cm2 and 3.45, respectively. High-quality arrays of regular and parallel lines with periods from 100 nm down to 32 nm were achieved. The highest resolution lines were only 12 nm wide. They exhibited very low line edge roughness and had a height-to-width aspect ratio of 2:1. The cross sections showed vertical side walls and complete clearing of the spaces between the lines. Patterned Zinc Naphthenate lines were annealed to form poly-crystalline ZnO nanowires. The lines shrank by about 30%, and the structures became transparent. The nanowire arrays were characterized using SEM, TEM, and XRD. Creation of large area ZnO nanowire arrays through EUV-IL exposure may enable use of these structures in research and industrial applications. Use of EUV light in the exposure brings certain benefits including high-resolution and high exposure dose sensitivity due to the exceptionally efficient absorption of EUV radiation by Zn.1. S. M. Saifullah, K. R. V. Subramanian, D.-J. Kang, D. Anderson, W. T. S. Huck , G. A. C. Jones, M. E. Welland, Adv.Mater. 17 (2003) 1757
4:00 PM - L2.5
New Insights in how to Control Size Dispersion and Surface Roughness of Solution-phase Grown ZnO Nanorods.
Cyril Martini 1 , Aurore Said 2 , Wladimir Marine 2 , Suzanne Giorgio 2 , Frederic Fages 1 , Joerg Ackermann 1
1 Laboratoire des Matériaux Moléculaires et des Biomatériaux, GCOM2 UMR CNRS 6114, , Université de la Méditerranée, Faculté des Sciences de Luminy, Case 901, 163, avenue de Luminy, Marseille France, 2 Centre de Recherche en Matière condensée et Nanosciences UPR CNRS 7251, Campus de Luminy, 163 Avenue de Luminy,, Marseille France
Show AbstractThe synthesis of semiconductor nanorods and –wires has attracted much interest due to their unique electronic and optical properties during the last decade. Among these semiconductors Zinc oxide are of particular interest because of its low cost solution synthesis, environmental and biomedical compatibility. Furthermore high mobility field effect transistors, hybrid bulk heterojunction solar cells, hydrogen storage and light emitters based on ZnO nanorods have been reported which stimulate the research for further improved ZnO nanomaterials. Amongst the different solution-phase route for the synthesis of ZnO nanorods, the alkaline hydrolysis of zinc salts in alcoholic or aqueous media has been the most widely applied. These synthesis methods lead to highly crystalline nanorods of high yields, but are usually accompagnied by a high degree of polydispersity in length and diameter. Although the control of size distribution is very crucial for improved material performance, so far only small attention has been paid to study the underlying processes leading to the statistical distribution and the question in how far it can be controlled. We present in this communication a detailed investigation of the influence of the base on the growth of ZnO nanorods synthesized by alkaline hydrolysis of zinc salts in methanol. The synthesis of the ZnO nanorods are performed in a two step process (Ref 1) in which first spherical nanoparticles are formed by the reaction of Zinc acetate with different bases (KOH, NaOH, LiOH and CsOH) followed by a condensation step which induces the nanorod growth. Histograms representing length and diameter distribution at different stage of the ZnO nanorod growth obtained from TEM analysis are used to study the nanorod formation in detail. In general, our results reveal that high quality nanorods can be synthesized with all four alkalis, and surface roughness as well as length and diameter of the nanorods are strongly effected by the nature of the alkali. From studies of the early stage of nanorod formation, we find that the growth mode is alternated by the base. In the presence of KOH, mainly oriented attachement along the C-axis is observed, while a isotropic oriented attachement leads first to the formation of large nanoparticles followed by an anisotropic Oswald in the case of NaOH. This change in the growth process leads to ZnO nanorods of better crystallinity and narrower size distribution with atomically flat surfaces in the case of NaOH.
4:15 PM - L2.6
Rapid, Microwave Synthesis of Aligned Zinc Oxide Nanowires.
Husnu Unalan 1 , Pritesh Hiralal 1 , Gehan Amaratunga 1
1 Engineering Department, University of Cambridge, Cambridge United Kingdom
Show AbstractSolution growth approach for zinc oxide (ZnO) nanowires is highly appealing because of the low growth temperature and possibility for large area synthesis. Reported reaction times for ZnO nanowire synthesis, however, are too long which span from several hours to days. In this work, we have rapidly synthesized ZnO nanowires on various substrates (such as PET, silicon and ITO) using a commercially available microwave oven. The average growth rate of our samples is determined to be larger than 1 µm/h depending on the microwave power. We have analyzed the as-grown ZnO nanowires with high resolution transmission electron microscope (TEM), field-emission scanning electron microscope (SEM), and photoluminescence and absorption spectroscopy. Electrical properties of individual nanowires were obtained by lithographical patterning in a field effect transistor configuration. SEM showed the nanowire diameters of 20-50 nm and lengths up to several microns and TEM analysis revealed single crystalline lattice of the nanowires. We will present a detailed analysis of the growth properties of ZnO nanowires as functions of growth time and microwave power. In brief, our work demonstrates the possibility of a fast synthesis route using microwave heating for nanomaterials synthesis.
4:30 PM - L2.7
Selective Area Growth of ZnO Nanowires on Diamond-like Carbon Film and Applications for UV Emission and Detection.
R. Vispute 1 , Sheng-Yu Young 1 , Brian Tran 1 , Arun Luykx 1 , T. Venketesan 1
1 CSR, University of Maryland, College Park , Maryland, United States
Show AbstractNew generation of electronics needs multifunctional materials, devices, and sensors integrated in a single platform via nanotechnology. To realize faster, compact and efficient electronics, various nanostructures could be integrated using selective area growth approach. Here we report selective area growth of ZnO nanowires using diamond-like-carbon (DLC) nucleation layers. DLC is of interest due to its hardness, insulating properties, high thermal conductivity, and high chemical inertness. Zinc oxide (ZnO) was chosen for nanowire growth because of its direct optical band-gap (3.37 eV) and its large exciton binding energy (60 meV) which boasts near UV emission, tunable electrical conductivity, piezoelectricity, and chemical sensing characteristics; these can all be used for photonics, optoelectronics, and chemical and bio-sensors. The nanowires were grown using vapor phase deposition, with 99.999% pure zinc and argon and oxygen as the carrier gasses. The deposition took place at around 650-700°C. The DLC layer was deposited using a pulsed laser deposition system at room temperature with a graphite target. The grown nanowires were characterized using a scanning electron microscopy, photoluminescence, electroluminescence, and x-ray photoelectron spectroscopy. Our results indicate that the nanowires grow selectively on the DLC surface. Interestingly, nanowires do not grow on regions of the substrate where no DLC was deposited. The average diameter size ranged from about 15-40 nm, depending on the growth temperature. An emission of UV light at 386 nm (with a considerably lower green band emission) was clearly observed. Our results clearly indicate selective and high density nucleation ZnO on DLC. Dependence of thickness of DLC on nucleation, selectivity, orientation of nanostructures will be discussed. Results on material compatibility with DLC and silicon that can yield nanostructures with minimum inter-diffusion of chemical constituents of nanostructure across interfaces and dependence of the surface morphology and thickness of the DLC on the nucleation of ZnO nanowires will be presented. Applications such as UV emitter and UV detector based on the ZnO nanostructures fabricated using our approach will be highlighted. Generic comments will be made on the possibility of fabrication of nanostructure with tunable band gaps for tunable UV applications.
4:45 PM - L2.8
Strong UV Emission from Electrodeposited ZnO Nanorods on Non-epitaxial Au coated Si(100).
Soo-Jin Chua 1 2 , Miao Wang 2 , Carl Thompson 3
1 , Institute of Materials Research and Engineering, 3 Research Link, Singapore Singapore, 2 , Siingapore-MIT Alliance, E4-04-10, 4 Engineering Drive 3, Singapore Singapore, 3 Department of Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States
Show AbstractVarious approaches have been developed to synthesize ZnO nanostructures. Among them, electrodeposition possesses the advantages of low temperature, low cost, high throughput and high deposition rate. Considerable work has been done on electrodepostion of ZnO on epitaxial Au and a heteroepitaxial electrodepostion of (0001)-oriented ZnO has been reported on single-crystalline Au (111) [1,2]. However, there are very few reports regarding the growth of ZnO on non-epitaxial Au. In our work, we demonstrated that good quality of polycrystalline ZnO nanorods with mainly preferred (0001) orientation and with comparable photoluminescence property could be grown on non-epitaxial Au. A template-free potentiostatic cathodic electrochemical deposition method was employed to grow one-dimensional ZnO facet nanorods in zinc nitrate aqueous solution. The experiment was conducted under low temperature from 60 DegC to 80 DegC. The concentration of zinc nitrite was 0.005 M and the potential were ranging from –1 to -1.4 V (with current density varied from 100 to 1000 uA/cm2). Facet ZnO nanorods were grown on both epitaxial Au (111) and non-epitaxial Au coated Si (100) substrates. Under more positive deposited potential with current density within 100 to 300 uA/cm2, the as-grown ZnO nanorods deposited on these two substrates both show a strong narrow UV peak (FWHM 270 meV) at around 370 nm wavelength and a weak wide peak at visible light wavelength in room temperature photoluminescence spectrum. It is impressive that the ZnO nanorods grown on non-epitaxial Au possess a comparable intensity ratio between UV light and visible light peak, which are considered as evidence of the high quality of (0001) oriented ZnO. In our result, the highest intensity ratio of as-electrodeposited sample is 15, which is higher than the former reported value of 1.3 from ZnO grown by electrodepositon, 1.0 by MOCVD and 0.18 by MBE from other groups [2]. Under more negative deposition potential with increased current density, more defects will be introduced, resulting in a decrease of the UV peak intensity and an increase of the visible light peak intensity. Two peaks within visible light range: 580 nm (yellow) and 680 nm (red) were detected. The result is different from the normal green light emission reported from other deep level emission of electrodeposited ZnO. The origin of these emissions are being investigated and results of post-annealing of electrodeposited ZnO nanorods will be reported as well. [1] R. Liu, A. A. Vertegel, E. W. Bohannan, T. A. Sorenson, J. A. Switzer, Chem. Mater. 2001, 13, 508[2] M. Izaki, S. Watase, H. Takahashi, Adv. Mater. 2003, 15, 2000
5:00 PM - L2.9
Synthesis and Mechanical Properties of Zinc oxide Nanowires.
Baomei Wen 1 , John J. Boland 1
1 , Trinity College Dublin, Dublin Ireland
Show AbstractNanowires have attracted great attention because of potential application in the area of nanoscale interconnects and the active components of nanodevices. Nanowires also provide a good system to investigate the dependence of electrical and mechanical properties on dimensionality and size reduction. ZnO is a wide band gap (3.37 eV) semiconductor with a large exciton binding energy (60 meV). ZnO nanowires are of interest in nanodevice applications such as light-emitting diodes, field-effect transistors, gas sensors, and solar cells. Here, we report a mild sovolthermal route to fabricate ZnO nanowires using a thin nucleation layer of nanostructural ZnO deposited by room temperature magnetron reactive sputtering. Small nanowires with typical diameters of 20 nm have been successfully synthesized. By adjusting reaction parameters, diameters in the range of 100–300 nm nanowires have been also obtained. These nanowires exhibit excellent single crystalline structure with [0001] growth direction. Mechanical properties of these nanowires were measured using a AFM tip-induced lateral bending technique developed recently by our group, which in contrast to earlier methods, allows the full spectrum of mechanical properties to be measured. During nanowire manipulation both the normal and lateral force (F) signals are recorded as a function of displacement (d). The results demonstrate that in contrast to previous reports [1], Young’s modulus E of ZnO nanowires is essentially diameter-independent and comparable to that of bulk ZnO. The average value of Yang’s modulus E is 114 ± 25 GPa in this work. The yield strength σy will also be discussed.References[1] C. Q. Chen, Y. Shi, Y. S. Zhang, J. Zhu, and Y. J. Yan, Phys. Rev. Lett. 2006, 96, 075505.
5:15 PM - L2.10
Growth and Luminescence of Doped ZnO Nanoplate and Nanowire Arrays.
Javier Piqueras 1 , Ana Urbieta 1 , Yanicet Ortega 1 , Paloma Fernandez 1
1 Dpt. Fisica de Materiales, Universidad Complutense de Madrid, Madrid Spain
Show AbstractElongated ZnO nanostructures, as wires, rods or belts have been grown by different methods in the last years and their properties have been reported. Contrary to the case of one-dimensional nanostructures, plate shaped ZnO nanostructures have been less investigated. In the present work, Sn and Mg doped ZnO nano- and microstructures, including stacks of nanoplates, have been grown by thermal treating of compacted mixtures of ZnO-SnO2, ZnS-SnO2 and ZnO-MgO powders under argon flow.The starting materials were commercial powders of ZnO, ZnS, SnO2, and MgO. ZnO-SnO2 powders containing 1, 5 and 10 wt % of SnO2, ZnS-SnO2 powders with 1 and 5 % wt of SnO2 and ZnO-MgO powders with 1, 5 and 20 wt % of MgO, were prepared by milling the corresponding mixtures in a centrifugal ball mill. The final powder was compacted under a compressive load to form disk shaped samples. The samples were annealed under argon flow and were then characterized by XRD and the SE, cathodoluminescence (CL) and EDS modes of SEM. The annealing lead to the growth of elongated structures in the sample surface as previously found in undoped ZnO powder (1) (2). In the case of the ZnO-SnO2 sample containing 1 wt % of SnO2 rods with, cross-sectional dimensions from hundreds of nanometers to few microns, formed by stacked nanoplates, are observed. In some rods the stacks present two well defined orientations. The plates have specific orientations which correrspond to the slip systems in ZnO. The presence of Sn in ZnO influences the internal stress resulting in slip structures. Cathodoluminescence measurements show that the near band edge emission in the undoped powder appears at 3.26 eV, while in the doped samples the band is peaked at about 3.33 eV which is explained by the increase of band gap due to Sn doping. When ZnS-SnO2 mixture is used as precursor, nanowire arrays with 6-fold symmetry are formed in a first stage of growth. In a more advanced stage of growth arrays of stacks of nanoplates with 6-fold symmetry are formed. The formation of the 6-fold structures depends on the presence of Sn and on the defect structure.In the samples prepared from ZnO-MgO mixtures containing 5 or 20 wt. % of MgO, rods formed by oriented stacks of nanoplates have been observed. Contrary to the case of the samples containing Sn, the nanoplates have not different orientations, but appear normally perpendicular to the growth axis . In addition needles grow along the growth axis. Local EDS measurements show that the at. % of Mg in the needles and in the plates is in the range 5- 10 %. The CL spectra in the nanoplate stacks, correspond to that of ZnO, while in the surface background of the disk, luminescence features associated to Mg are observed.1) J.Grym, P.Fernandez and J.Piqueras, Nanotechnology 16, 931 (2005)2) L.Khomenkova, P.Fernandez and J.Piqueras, Crystal Growth and Design, 7, 836 (2007)
5:30 PM - L2.11
ZnO/TiO2 Nanohybrid Structures Synthesized by Site-Specific Deposition.
Cheng Chun 1 2 , Wang Ning 1 2
1 the Institute of Nano Science and Technology, the Hong Kong University of Science and Technology, HongKong China, 2 Department of Physics, the Hong Kong University of Science and Technology, HongKong China
Show AbstractSemiconductor nanostructured composites are of interest in many technological applications, such as biolabels, photochemical solar cells, photo catalysis, and sensors. Considerable effort has been devoted to combining semiconductor nanoparticles with suitable materials to synergize the properties of both components, which has led to many promising applications such as the enhancement of photocatalytic performance through the deposition of metal or metal islands on ZnO and TiO2 nanoparticles [1]. Compared to pure ZnO and TiO2, coupled ZnO/TiO2 polycrystals display a greater photocatalytic activity [2]. In our previous works, we have systematically investigated the formation mechanisms of ZnO [3] and TiO2 [4] nanostructures synthesized by our unique methods. Currently, we developed a simple hydrothermal method for the preparation of ZnO/TiO2 nanohybrid structures through the site-specific deposition of TiO2 on ZnO nanorods. Each ZnO nanorod is assembled with one TiO2 cap at one end of the rod. We have found that the polarity of the ZnO (0001) surface plays an important role in the formation of ZnO/TiO2 nanohybrid structures. Using high-resolution transmission electron microscopy, we observed that the amorphous TiO2 particles that are connected to ZnO nanorods are transformed to nanocrystals of the anatase and rutile phases, which have a particular relationship with the orientation of ZnO nanorods and good interface structures. This work provides a rational approach to the assembly of complex nanohybrids using the intrinsic properties of ZnO nanocrystals that allows the size, aspect ratio, and TiO2 cap structures to be fully controlled in the fabrication process and by subsequent annealing treatments [5].[1] C. Pacholski, A. Kornowski, H. Weller, Angew. Chem. Int. Ed. 2004, 43, 4774.[2] V. Sukharev, R. Kershaw, Photochem. Photobiol. A: Chem. 1996, 98, 165.[3] B.D Yao, Y. F. Chan, N. Wang, Appl. Phys. Lett. 2002, 81, 757.[4] B.D. Yao, Y.F. Chan, X.Y. Zhang, W.F. Zhang, Z.Y. Yang and N. Wang, Appl. Phys. Lett. 2003, 82, 281.[5]C. Cheng, K. F. Yu, Y. Cai, and K. K Fung N. Wang, Submitted to Angew. Chem. Int. Ed.
5:45 PM - L2.12
Investigation of Copper Incorporation into ZnO Nanowires.
Susie Eustis 1 , Douglas Meier 1 , Babak Nikoobakht 1
1 Surface and Microanalysis Science Division, National Institute of Standards and Technology, Gaithersburg, Maryland, United States
Show AbstractThe applications of zinc oxide (ZnO) nanowires (NWs) in devices are promising due to the optical, mechanical and electrical properties of these one-dimensional structures, but current uses are limited by the ability to produce high quality nanowires at desired locations. Copper is an attractive catalyst for generating zinc oxide nanowires due to the long length and high density of ZnO NWs produced. However, defects due to impurities, oxygen deficiencies, and structural defects lead to decreased optical and electrical transport. Photoluminescence (PL) microscopy found that ZnO NWs grown by high temperature evaporation on a bulk copper substrate display the expected band gap emission at 380nm. A larger visible emission is also observed in the PL spectrum around 520nm due to defect states. High-resolution transmission electron microscopy (HR-TEM) shows the ZnO nanowires are single crystalline with hexagonal structure. Auger electron spectroscopy (AES) reveals that copper atoms are present all along the length of the NW. AES also confirmed that the surface of NWs is oxygen deficient. The metallic tip shows a polycrystalline structure in HR-TEM images and a mixture of zinc, copper and oxygen atoms are detected in AES. The results and implications to uses of ZnO NWs will be discussed.
Symposium Organizers
David P. Norton University of Florida
Chennupati Jagadish Australian National University
Irina Buyanova Linkping University
Gyu-Chul Yi Pohang University of Science and Technology (POSTECH)
L3: ZnO Doping and Defects
Session Chairs
Irina Buyanova
Bengt Svensson
Tuesday AM, November 27, 2007
Constitution A (Sheraton)
9:30 AM - **L3.1
Impurity and Point-Defect Issues in ZnO.
David Look 1 2
1 Semiconductor Research Center, Wright State University, Dayton, Ohio, United States, 2 Materials and Manufactuing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Ohio, United States
Show AbstractHere we report on recent progress in our understanding of impurities and point defects in ZnO, including those associated with surfaces. One of the most common impurities is H, but the role of H in affecting electrical properties seems to be under continuous revision. There is little doubt that H produces a 45-meV donor, as evidenced by the I4 donor-bound-exciton PL line, and indeed Hall-effect measurements find a donor of similar energy. However, the Hall and PL donors have differences, such as in their annealing properties. Also, the possible structures of H-related donors are in question, because recent theory suggests that interstitial H is unstable at temperatures much above 300 K. Furthermore, forming-gas (5% H2 in N2) annealing of hydrothermal ZnO at 600 °C produces large increases in both concentration and mobility, and evidence suggests that passivation of acceptors (e.g., H+ + LiZn- → H-LiZn) and/or depassivation of donors (e.g., H+ + OI-AlZn → H-OI + AlI +) may be as important as the direct creation of donors. Note that passivation of acceptors by H may also be useful for the successful production of p-type ZnO, as is the case in p-type GaN. For example, the formation of neutral H-LiZn- complexes during growth may suppress the formation of compensating lattice donors (e.g., VO) and allow a subsequent anneal to break up the H-LiZn- complexes, drive out the H, and leave isolated and uncompensated LiZn. Besides H, other donor-type co-dopants, such as Al, Ga, and In, in conjunction with acceptor dopants such as N, have enabled p-type material to be realized.With regard to point defects, there is now evidence that Zn-interstitial donor complexes may be important in some cases. Also, the O vacancy VO continues as perhaps the most studied point defect in ZnO. Recent work has correlated a VO-related ODMR signal with the 2.45-eV PL band and the 0.55-eV DLTS trap.Conductive n-type surfaces are present in almost all as-grown ZnO samples. Fortunately, the donor concentrations near the surface can now be determined with a fair degree of accuracy by low-temperature Hall-effect measurements; however, the thicknesses of these conductive layers still elude easy measurement. It’s likely that H is either directly or indirectly involved in producing the surface donors, since forming-gas anneals usually increase the net donor concentration,Finally, the presence of H in highly conductive ZnO nanorods has been confirmed by nuclear magnetic resonance. A certain portion of this H is quite stable, surviving at 500 °C in air.Other recent results regarding impurities and point defects in ZnO will be included in the discussion.
10:00 AM - L3.2
Improvements in the Conductivity of n-type ZnO Through Codoping with Al and In.
Steven Kirby 1 , R. van Dover 1
1 Material Science and Engineering, Cornell University, Ithaca, New York, United States
Show AbstractZnO has been identified as a viable candidate to replace expensive Sn-doped In2O3 due to its high transparency and good conductivity. ZnO has been successfully doped with many elements including Al and In to achieve resistivities near or below 10-4 Ω-cm. One of the limiting factors in improving conductivity beyond this is the limited solubililty of the dopants in ZnO. We proposed that codoping with Al and In will increase the overall dopant solubility, since Al3+ and In3+ are smaller and larger than Zn2+ respectively. This will increase the carrier concentration with less degradation of mobility due to formation of secondary phases. We have studied the effect of codoping ZnO with Al and In using a combinatorial technique. Cosputtering from three elemental targets in the presence of oxygen allows for a range of ZnO:(Al, In) compositions to be deposited on a single substrate. Deposition temperatures range from room temperature to 350°C. High temperature annealing is performed in air to eliminate the effect of oxygen vacancies as an uncontrolled variable. The material properties are determined using XRD, electron microprobe, four-point resistivity probe, Hall effect, and reflection and transmission spectroscopy at optical wavelength. These films are found to be highly c-axis oriented. Measurements demonstrate an improvement in the maximum conductivity attributed to codoping: conductivities in the range of 104 S/cm are achieved along with greater than 85% transmission in visible light.
10:15 AM - L3.3
Optical Properties of ZnO Including Fresh Dislocations Induced by Plastic Deformation.
Yutaka Ohno 1 , Haruhiko Koizumi 1 , Toshinori Taishi 1 , Ichiro Yonenaga 1 , Katsushi Fujii 1 2 , Hiroki Goto 1 2 , Takafumi Yao 1 2
1 Institute for Materials Research, Tohoku University, Sendai Japan, 2 Center for Interdisciplinary Research, Tohoku University, Sendai Japan
Show AbstractDefects inducing long-range strain fields, as dislocations, are of considerable importance in semiconductor electronics, since they are frequently introduced during crystal growth, device fabrication, device operation, etc. and affect the electronic and optical properties of semiconductor devices. Recently, ZnO is expected for UV light-emitting devices and is supposed that dislocations are more easily introduced than GaN and SiC. However, the nature of optical properties of dislocations in ZnO has not been fully elucidated. This paper reports photoluminescence study of the optical properties of ZnO in which dislocations are freshly induced by the plastic deformation.Wurtzite ZnO bulk single-crystals, purchased from Goodwill (Russia), were compressively deformed at elevated temperatures 650 ~ 850 degrees C. In photoluminescence studies at 11 K, a deformed ZnO shows an intense light emission with photon energy around 3 eV, while no emission of the light was observed in non-deformed ZnO. The higher the light intensity was, the lower the deformation temperature was. These results suggest that the light emission should be originated from induced fresh dislocations.
10:30 AM - L3.4
Degradation Mechanism and Reversible Recovery of Electrical Properties in Al-doped ZnO Thin Films.
Jun Hong Noh 1 , Hyun Suk Jung 2 , Jung Kun Lee 3 , Jin Young Kim 1 , Chin Moo Cho 1 , Jae Sul An 1 , Kug Sun Hong 1
1 Materials Science and Engineering, Seoul National University, Seoul Korea (the Republic of), 2 Advanced Materials Engineering, Kookmin University, Seoul Korea (the Republic of), 3 Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania, United States
Show AbstractAluminum-doped zinc oxide (AZO) films have been extensively explored as a transparent conducting oxide (TCO) to alternate ITO film. However, the AZO films shows the most abrupt increase in resistivity after air annealing among the known TCOs such as ITO, FTO, and SnO2:Sb, etc. This is known as a thermal degradation issue of AZO films. Therefore, studies about a mechanism of the thermal degradation and methods to prevent the degradation or to recover the degraded film are need.We prepared undoped ZnO and aluminum (1.6 at%)-doped ZnO (AZO) films epitaxially grown on sapphire (0001) substrates using pulsed laser deposition (PLD). To exclude grain boundary effect on conductivity for AZO film, we chose the epitaxial films. As-deposited AZO films shows a low resistivity of 8.01×10-4 Ωcm with carrier concentration (n) of 1.72 ×1020 cm-3 and Hall mobility (μ) of 45.4 cm2/V-S. However, after annealed at 450 oC in ambient air atmosphere, the electrical resistivity of AZO film increased to 1.97×10-1 Ωcm, which was traced to the decrease in the carrier concentration to 2.13 × 1018 cm-3. Subsequent annealing of air-annealed AZO films in H2 was found to recover the electrical conductivity of the AZO films. We calculated concentrations of neutral and ionized scattering center (NN, NI) using Dingle’s model for a degenerate electron gas. The result shows that the total concentration of scattering centers (NI+NN) in air-annealed AZO film is much larger than that in as-deposited AZO film, while as-deposited and reducing-environment treated AZO films have almost a same amount of scattering centers. Neutral or negatively charged scattering centers are newly produced during the air-annealing. In photoluminescence study, it is notable that strong orange emission (1.9 eV) is only showed in air-annealed AZO films. The overall consensus of previous studies is that single negatively charged interstitial oxygen plays an important role in emitting the orange light in ZnO. Al doping drastically suppressed the formation of intrinsic defects such as oxygen vacancies or zinc interstitials which can promote the formation of oxygne interstitial. The single negatively charged interstitial oxygen act as acceptors in ZnO and provide one hole per one interstitial. Therefore, the formation of the oxygen interstitial during air-annealing reduces the electron concentration and increases density of scattering center. Our study suggests that the degraded electrical properties of transparent conducting ZnO films can be recovered by removal of the oxygen interstitial through subsequent annealing at reducing atmosphere or the degradation is prevented by using the less sensitive donors to form the oxygen interstitial because oxygen interstitial is a critical material-parameter for the degradation of electrical conducting properties of Al-doped ZnO films.
10:45 AM - L3.5
Visible Luminescence Related to Defects in ZnO.
Michael Reshchikov 1 , Hadis Morkoc 2 1 , Bill Nemeth 3 , Jeff Nause 3 , Junqing Xie 4 , Brian Hertog 4 , Andrei Osinsky 4
1 Physics Department, Virginia Commonwealth University, Richmond, Virginia, United States, 2 Electrical Eng. Dept., Virginia Commonwealth University, Richmond, Virginia, United States, 3 , Cermet, Inc., Atlanta, Georgia, United States, 4 , SVT Associates, Inc., Eden Prairie, Minnesota, United States
Show AbstractZnO is promising material for optoelectronics owing to its direct wide band gap and large exciton binding energy. A typical low-temperature photoluminescence (PL) spectrum of undoped ZnO contains sharp excitonic lines in the UV range and one or more broad bands in the visible part of the spectrum. While the excitonic spectrum of ZnO is well studied, available data on defect-related broad bands in the visible part of the PL spectrum are highly controversial. In particular, the bands having maxima in the range from 2.1 to 2.5 eV (yellow and green luminescence bands) in n-type ZnO are commonly attributed to oxygen vacancy. However, the oxygen vacancy is a deep donor which should not play a significant role in PL of n-type ZnO. Numerous reports can be found on red, orange, yellow, green, and blue broad bands, mostly studied at room temperature. However, comprehensive studies of defect-related PL bands in ZnO are rare. In this presentation, we will briefly review the luminescence bands associated with defects in ZnO, categorize them, and focus on those for which most knowledge has been garnered. We studied in detail several of the PL bands in undoped, Li-, Ga-, and N-doped high-quality ZnO bulk crystals and thin films grown by different techniques. By studying PL in a wide range of excitation power densities, sample temperature, and decay time after a laser pulse, we were able to classify and analyze in detail more than 10 broad bands with unique luminescence properties. Among these bands, only the Cu-related green band with a characteristic fine structure was well-studied and reliably identified in the past. Two other PL bands in undoped ZnO, namely the orange luminescence (OL) band peaking at 1.96 eV at 10 K and the yellow luminescence (YL) band at 2.19 eV, will be discussed in more detail owing to their unusual properties. The OL band, distinguished by its enormous blue shift with temperature and fluorescence lasting for hours after termination of excitation at 10 K, is attributed to transitions from shallow donors to a deep acceptor having an activation energy of 0.56±0.11 eV. The YL band quenches with an activation energy of 85 meV, yet the related acceptor level is located at about 0.5 eV above the valence band. We suggest that the defect responsible for the YL band is either isolated zinc vacancy or a Zn-vacancy-based complex.
11:30 AM - **L3.6
Kinetics of Capture, Relaxation and Recombination in ZnO-based Nano-structures: nm-spatially- and ps-time-resolved Cathodoluminescence Spectroscopy.
Juergen Christen 1 , Frank Bertram 1
1 Natural Science, Magdeburg University, Magdeburg Germany
Show AbstractThe 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). A high quality 8 µm thick ZnO epi-layer grown by MOVPE on a ZnO/GaN/sapphire template is investigated. At T = 4 K the CL 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. While no spectral shift with time is observed for all lines a distinct change in intensity ratio of I8 / I9 as compared to I1 / I2 is found in time-delayed (td-) spectra: I8 and I9 exhibit a perfectly time-constant intensity ratio. While I1 and I2 are less intense in the in-pulse-spectrum they catch up with I8 / I9 with ongoing delay time eventually dominating the td-spectra after 1.9 ns. XA and the excited state I6/8* disappear extremely fast after the in-pulse spectrum having already completely vanished in the td-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 τ(I8) = 310 ps, τ(I9) = 270 ps, respectively. The same is found for XA , however, exhibiting a very short life time of τ (XA)= 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 (τinitial = 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 thus feeding the neutral impurity bound exciton channels. Consequently, the decays of I8 and I9 are delayed with respect to the initial fast I1 / I2 decay. This capture kinetics is also observed in the CL onset.Alloying ZnO with MgO (Eg = 7.8 eV) further expands the optical properties into the UV and far UV - however further complicating the kinetics. A nano-scale characterization of MgZnO/ZnO/MgZnO quantum wells is presented and the impact of potential fluctuations in the MgZnO-barriers on the kinetics of carrier transfer and capture into the quantum well is investigated in spatially(nm)-time(ps)-resolved CL experiments.
12:00 PM - L3.7
Effect of Growth Polarity on the Formation of Zn Vacancies in Heteroepitaxial MOCVD-ZnO.
Filip Tuomisto 1 , Asier Zubiaga 1 , Jesus Zuniga-Perez 2 , Vicente Munoz-San Jose 2
1 Laboratory of Physics, Helsinki University of Technology, TKK Espoo Finland, 2 Departamento de Fisica Aplicada i Electromagnetisme, Universitat de València, Valencia Spain
Show AbstractThe growth polarity has been shown to have a dramatic effect on the in-grown point defects in GaN [1, 2], a material similar to ZnO in the sense of structure and properties defined by the spontaneous polarization [3, 4]. In c-plane GaN, the incorporation of impurities and formation of native defects are significantly enhanced in growth in the N polar (as opposed to Ga polar) direction and their distributions of impurities and native defects is highly nonuniform as a function of the distance from the GaN/sapphire interface. On the other hand, in non-polar a-plane GaN the defect distributions are flat and the concentrations are similar to N polar GaN.We have applied positron annihilation spectroscopy to study the effect of growth polarity on the vacancy defects in ZnO grown by metal-organic vapor phase deposition (MOCVD) on sapphire. Both c-plane and a-plane ZnO layers were measured, and Zn vacancies were identified as the dominant defects detected by positrons. The results are qualitatively similar to those of earlier experiments in GaN. The Zn vacancy concentration decreases in c-plane ZnO by almost one order of magnitude (from high 1017 cm-3 to low 1017 cm-3) when the layer thickness is increased from 0.5 to 2 μm [5]. Interestingly. in a-plane ZnO the Zn vacancy concentration is constant at a level of about 2 × 1017 cm-3 in all the samples with thicknesses varying from 0.6 to 2.4 μm.[1] F. Tuomisto, K. Saarinen, B. Lucznik, I. Grzegory, H. Teisseyre, T. Suski, S. Porowski, P. R. Hageman, and J. Likonen, Applied Physics Letters 86, 031915 (2005).[2] F. Tuomisto, T. Paskova, R. Kröger, Figge, D. Hommel, B. Monemar, and R. Kersting, Defect distribution in a-plane GaN on Al2O3, Applied Physics Letters 90, 121915 (2007).[3] F. Bernardini, V. Fiorentini, and D. Vanderbilt, Phys. Rev. B 56, R10024 (1997).[4] C. Morhain, T. Bretagnon, P. Lefebvre, X. Tang, P. Valvin, T. Guillet, B. Gil, T. Taliercio, M. Teisseire-Doninelli, B. Vinter, and C. Deparis, Phys. Rev. B 72, 241305(R) (2005).[5] A. Zubiaga, F. Tuomisto, F. Plazaola, K. Saarinen, J.A. Garcia, J. F. Rommeluere, J. Zuñiga-Pérez, and V. Muñoz-San José, Applied Physics Letters 86, 042103 (2005).
12:15 PM - L3.8
Exposure of ZnO(0001)-Zn to H2 at Different Temperatures.
Johannes vandenHeuvel 1 , Erdny Batyrev 1
1 Chemistry, Universiteit van Amsterdam, Amsterdam Netherlands
Show AbstractCu-ZnO based catalysts are very active and selective in methanol synthesis, however, the reaction mechanism and nature of active sites are still under debate. It is known that ZnO plays an important role which ranges from hydrogen supplier by spillover to anti-sintering agent, and even as active site. Reduction at 750 K strongly enhances the activity of the ZnO promoted Cu catalyst. Little information is available on the surface structure of the activated catalyst after reduction at high temperature. This study is devoted to the characterization of the ZnO phase after exposure to H2 at different temperatures.Since in polycrystalline powder catalysts it was shown that only the polar Zn-terminated ZnO face is active in methanol synthesis, a sputter/annealed ZnO (0001)-Zn single crystal was studied in a UHV system with a base pressure of 2x10-10 mbar. The preparation chamber was equipped with a high pressure reaction cell and a mass spectrometer for TDS, the analysis chamber enabled XPS, STM and STS. STM reveals that the clean ZnO (0001)-Zn single crystal contains substantial structural defects, i.e. triangular islands and pits with mono-atomic step height; the edges of these defects are terminated by oxygen. This structure completely disappears after exposure to 10-5 mbar H2 at T > 500 K to yield a rough surface. Some reduction of the surface is obvious from STS, which shows that the initial semiconducting character has become more metallic. Furthermore, XPS revealed that stable OH groups were formed at the surface up to 700 K. After exposure to 1 bar H2 at 700 K significant amounts of H2 dissolved in the ZnO matrix, as apparent from TDS spectra. Desorption of metallic Zn confirmed that the ZnO subsurface layer was significantly modified by the exposure to H2 at 700-800 K. These findings support the idea of ZnO being not only a hydrogen reservoir, but also a catalyst for methanol synthesis.Additional SANS on polycrystalline hydrogen (deuterium) saturated Cu-ZnO was performed at LLB, Saclay, France. Upon cooling to 4 K, H2 (D2) fills the nanopores of the ZnO matrix. In the temperature range of 90-300 K, approximately one-third of the hydrogen participates in the fast diffusion (~8×10–9 m2/s), while the other two-thirds are immobilized. At 20 K, the mobile fraction decreases to ~10%. Analysis of the results demonstrates that the energy barriers retaining hydrogen in defect regions are relatively low. This offers interesting perspectives for the development of hydrogen storage technologies.
12:30 PM - L3.9
Influence of Hydrogen on the Electrical Transport in Zinc Oxide.
N. Nickel 1 , L. Scheller 1 , M. Gluba 1 , M. Weizman 1
1 , Hahn-Meitner-Institut Berlin, Berlin Germany
Show Abstract12:45 PM - L3.10
Hydrogen Migration in Single Crystalline ZnO.
Klaus Magnus Johansen 1 , Jens Christensen 1 , Edouard Monakhov 1 , Andrej Kuznetsov 1 , Bengt Svensson 1
1 Department of Physics/Centre of Material Science and Nanotechnology, University of Oslo, Oslo Norway
Show AbstractHydrogen has been proposed as one of the contributors to the native n-type doping in as-grown ZnO and can also be used as an active (intentional) n-type dopant. In this work we have employed Secondary Ion Mass Spectrometry (SIMS) to study deuterium diffusion profiles in single crystalline Zinc Oxide (ZnO). Because of an improved detection limit by more than 2 orders of magnitude in the SIMS measurement relative to that of hydrogen, the chemically equal isotope of deuterium is used to study the diffusion. The samples used are hydrothermally grown, high-resistive (10 kΩ cm) monocrystalline ZnO. They have been implanted with deuterium at an energy of 1.4 MeV to a dose of 1×1015 cm-2 through a 15 μm Al-film. The implantation result in a peak concentration of approximate 5×1018 2H/cm3 at a depth of 2.2 μm. Diffusion profiles have been studied after 30 minutes isochronal heat treatments from 100°C up to 400°C in steps of 50°C. The observed redistribution can be explained by employing a diffusion model which includes trapping of 2H by Li-impurities. We have used this model to extract the diffusion activation energy for 2H in ZnO.
L4: ZnO Crystal Growth and Properties
Session Chairs
Tuesday PM, November 27, 2007
Constitution A (Sheraton)
2:30 PM - **L4.1
Hydrothermally Grown Single-crystalline Zinc Oxide; Characterization and Modification.
Bengt Svensson 1 , Thomas Moe-Borseth 1 , Klaus Magnus Johansen 1 , Ramon Schifano 1 , Ulrike Grossner 1 , Jens Christensen 1 , Peter Klason 2 , Qing Zhao 2 , Magnus Willander 2 , Filip Tuomisto 3 , Wolfgnag Skorupa 4 , Eduoard Monakhov 1 , Andrej Kuznetsov 1
1 Physics, University of Oslo, Oslo Norway, 2 Physics, University of Gothenburg, Gothenburg Sweden, 3 Physics, Helsinki University of Technology, Helsinki Finland, 4 , Forschungszentrum Rossendorf, Dresden Germany
Show AbstractIn this overview four different topics will be discussed and they are briefly illustrated below, (i) – (iv). The zinc oxide (ZnO) samples investigated are bulk ones grown by hydrothermal (HT) techniques. (i) Photoluminescence spectroscopy has been employed to characterize samples heat-treated in inert Zn-rich and O-rich atmospheres, and a striking correlation is observed between the ambient and the position of the so-called deep band emission (DBE), commonly detected in ZnO. Treatment in O-rich ambient yields a DBE peak at 2.35±0.05 eV while the corresponding peak position after treatment in Zn-rich ambient is shifted to 2.53±0.05 eV. Consequently, it is tempting to assign the former band to zinc-vacancy (VZn) related defects and the latter to oxygen-vacancy related defects. (ii) Lithium (Li) occurs typically with concentrations in the range of 1017 cm-3 in the HT-samples and is anticipated to compensate both n- and p-type doping because of its amphoteric nature. However, by a combination of ion implantation and flash lamp annealing (duration 20 ms) in the 900-1400 C range VZn-clusters are formed which trap and deactivate Li. As a result, low-resistivity layers occur where the electrical properties are determined by other impurities and defects. Outdiffusion of Li is observed after heat treatment above ~1200 C with a duration of the order of 1 h. (iii) The evolution of implanted hydrogen (H) in high-resistivity samples during post-implant annealing has been investigated by secondary ion mass spectrometry (SIMS) and scanning nanoprobe microscopy (SPM) measurements. Varying the H-dose the n-type conductivity can be tuned over at least five orders of magnitude and the n-type doping persists after annealing up to temperatures in excess of 600 C. Further, a detailed comparison between the SIMS and SPM depth profiles reveals that a substantial fraction of the hydrogen is not electrically active and provides experimental evidence of more than one atomic configuration of H in ZnO. (iv) Finally, the issue of realizing Schottky diodes with a high barrier height and long term stability is addressed.
3:00 PM - L4.2
Electronic Structure of the Oxygen Vacancy in ZnO Including a Self-consistent Band-gap Corrected Approach.
Tula Paudel 1 , Walter Lambrecht 1
1 Department of Physics, Case Western Reserve University, Cleveland, Ohio, United States
Show AbstractThe electronic structure of the oxygen vacancy in ZnO has been controversial. Two models have been proposed in recent literature: one in which the 0/++ transition level lies in the lower and one in which it lies in the upper half of the band gap. Both can be reconciled with experimental data on the 0/+ level extracted from electron paramagnetic resonance. The reason for this controversy lies in the underestimate of the band gap by the local density approximation (LDA) and the different a-posteriori approaches used to correct for it. Here we propose an extension of the LSDA+U approach by adding a U shift for the Zn-s as well as the Zn-d levels, implemented in a full-potential linearized muffin-tin orbital method. While the main reason for the underestimate of the gap by LDA is due to long-range Coulomb effects, the resulting upward shift can be mimicked by adding a shift to the Zn-s level. LSDA+U achieves this by adding a potential V_s=U_s(½ -n_s). For the density matrix n_s≈0, this leads to an upward shift by U_s/2. The total energy functional is consistently updated. We also study the size dependence on the supercell. We found no clear evidence for a 1/L dependence as suggested by Makov and Payne and instead extrapolate the results of three different cell sizes using a 1/Volume linear fit from supercells ranging from 72 to 192 atoms. We find that the system is a negative U system but with slightly smaller |U|=0.7 than found in previous work. The 0/++ transition level is found at 2.16 eV above the valence band maximum, clearly supporting the defect level position is in the upper half of the gap. In contrast to previous approaches, ours is not based on an a-posteriori correction. The full band gap correction is included consistently within the total energy formalism.
3:15 PM - L4.3
Origin of the Near-Band-Edge Emission in ZnCdO Alloys.
Irina Buyanova 1 , J. Bergman 1 , G. Pozina 1 , W. Chen 1 , S. Rawal 2 , D. Norton 2 , S. Pearton 2 , A. Osinsky 3 , J. Dong 3
1 Physics, Chemistry and Biology, Linkoping University, Linkoping Sweden, 2 Department of Materials Science and Engineering, University of Florida, Gainesville, Florida, United States, 3 , SVT Associates, Eden Prairie, Minnesota, United States
Show AbstractThe desire for blue and ultraviolet optoelectronic devices has prompted extensive research efforts into wide-bandgap group III-nitride semiconductors as well as ZnO and related alloys. ZnO-based alloys, such as ZnCdO, add tunability in the alloy bandgap that can be utilized for bandgap engineering. For example, by choosing an appropriate Cd content the bandgap energy of the alloy can be tuned as desired within the blue-yellow spectral range. Therefore, ZnCdO can be used as an active material in modern Zn(Mg)O/ZnCdO heterostructures and superlattices, which are among key elements of ZnO-based light emitters and detectors. In spite of that, many fundamental optical properties and associated recombination processes in ZnCdO are still not fully understood. For example, the origin of the near band edge emission in ZnCdO alloys remains largely unknown. The purpose of the present study is to gain a better understanding of the dominant mechanisms for the radiative recombination near the band edge in ZnCdO alloys with high Cd compositions by employing temperature dependent cw- and time resolved photoluminescence (PL) spectroscopies combined with absorption measurements. The near-band-edge emission is attributed to recombination of excitons localized within band tail states likely caused by non-uniformity in Cd distribution. Energy transfer between the tail states is argued to occur via tunneling of localized excitons. The transfer is shown to be facilitated by increasing Cd content, due to a reduction of the exciton binding energy and, therefore, an increase of the exciton Bohr radius in the alloys with a high Cd content.
4:00 PM - L4.4
Microwave Growth of ZnO Bulk Crystals.
Jiping Cheng 1 , Yunjin Zhang 1 , Ruyan Guo 1
1 , Pennsylvania State University, University Park, Pennsylvania, United States
Show AbstractA microwave self-encapsulated melting growth technique is developed for fabrication of ZnO single crystals, which features a contamination-free, high rate and good quality crystal growth process. The microwave grown ZnO crystals sized in millimeters came out in light brown color due to high oxygen vacancies, and turned to colorless and highly transparent crystals after annealing in air. The X-ray diffraction, Laue back reflection, Hall Effect and photoluminescence spectrum measurements were carried out for characterization of microwave grown ZnO single crystals.
4:15 PM - L4.5
Optically Induced Changes to the Surface Conduction in Hydrothermal ZnO Studied by Photo-Hall-effect.
Bruce Claflin 1 2 , David Look 1 2
1 Materials and Manufacturing Directorate, AFRL/MLPS, WPAFB, Ohio, United States, 2 Semiconductor Research Center, Wright State University, Dayton, Ohio, United States
Show AbstractHigh quality, bulk, ZnO crystals grown by Tokyo Denpa using the hydrothermal process typically exhibit a room temperature carrier concentration in the low 1014 cm-3 range and low temperature conduction dominated by a degenerate surface layer with a sheet concentration of 2x1012 cm-2 and low mobility. In the sample discussed here, bulk conduction is controlled by two donor levels at 52 and 400 meV with concentrations of 1.2x1016 cm-3 and 1.5x1016 cm-3 respectively. Temperature-dependent photo-Hall-effect measurements up to 400 K, in vacuum, using blue/UV light, show an increase in the surface sheet carrier density to more than 1x1013 cm-2 at low to intermediate temperatures while the two bulk donors continue to dominate the high temperature behavior. Long-lived persistent photoconductivity (PPC) is observed when the sample is returned to the dark. When the PPC is allowed to fully relax, there is surprisingly no longer any surface conduction at low temperature, while the two bulk donors remain unaffected. In this state, the 52 meV bulk donor level is observed to control the conduction over five orders of magnitude, down to a concentration of 1.0x109 cm-3. This corresponds to an upper limit for the surface sheet carrier density of 5.5x107 cm-2. This is the lowest surface concentration we have ever observed in any ZnO sample and demonstrates that exposure to blue/UV light, in vacuum, at moderate temperatures is very effective at cleaning the surface.
4:30 PM - L4.6
MEMSA Analysis of Transport Properties in Bulk ZnO (and Evidence of Conduction Channels).
Celine Tavares 1 , Johan Rothman 1 , Ivan-Christophe Robin 1
1 , Leti-MINATEC, CEA-Grenoble, DOPT, Grenoble France
Show AbstractThe characterization of transport properties in Zn0 is known to be challenging , particularly due to surface [1] or interface [2] conduction channels, which puts severe limitations on the interpretation of Hall Effect measurements. In this communication, we report the first study of transport properties of n-type ZnO bulk material grown by the hydrothermal method using Hall mobility spectrum analysis estimated through the algorithm known as full Maximum Entropy Mobility Spectrum Analysis, f-MEMSA [3]. f-MEMSA is a complete version of the simplified MEMSA, which was first proposed by Kiatgamolchai [4]. Using the concept of mobility spectrum analysis, proposed by Beck and Anderson in 1987 [5], the distribution of carrier type and mobility can be estimated from magnetic field dependant Hall measurements. The transport properties of bulk Zn0 will be discussed in terms of Mobility spectra estimated from Hall data measured for applied magnetic fields µ0H in the range 0T-9T and for temperature between 50K and 400K.In the measured ZnO bulk material, Hall data analysis at fixed fields yields low mobility, µH < 100 cm2/Vs, n-type conduction and a temperature dependence which gives a low ionisation energy, Ea= 20 meV. The f-MEMSA analysis shows that this observation correlates with the presence of two different types of carriers, which is consistent with the existence of a low mobility conduction channel at the surface: µS(50K) ~ 100cm2/Vs and a high mobility bulk conduction µB(50K) ~ 1000cm2/Vs, in agreement with the bulk values reported by Look[6]. Hence, f-MEMSA analysis gives a strong indication that surface conduction is an issue in bulk ZnO and allows for a correct estimation of the transport properties in the bulk of the material. In addition, X-ray diffraction measurements showing the existence of a surface layer with a lattice parameter different from that of bulk ZnO give an indication as to the origin of the surface conduction channel.[1] D. C. Look, H. L. Mosbacker, Y. M. Strzhemechny, L. J. Brillson, Superlat. and Microstruct. 38, 406 (2005)[2] H. Tampo, A. Yamada, P. Fons, H. Shibata, M. Matsubara, K. Iwata, S. Niki, APL 84, 4412(2004)[3] J. Rothman, J. Meilhan,G. Perrais, J-P. Belle, O. Gravand, J. Electron Mater. 35, 1174 (2006)[4] S. Kiatgamolchai, M. Myronov, O.A. Mironov, V. G. Kanter, E. H. C. Parker, T. E. Whall, Phy. Rev. E 66, 36705 (2002) [5] W. A. Beck, J. R. Anderson, J. Appl. Phys. 62, 541 (1987)[6] D. C. Look, D. C. Reynolds, J. R. Sizelove, R. L. Jones, C. W. Litton, G. Cantwell, W. C. Harsch, Solid State Communications 105, 399 (1998)
4:45 PM - L4.7
Role of Neutral Impurity Scattering in the Analysis of Hall Data from ZnO and Other II-VI Materials.
Xiaocheng Yang 1 , Chunchuan Xu 1 , Nancy Giles 1
1 Department of Physics, West Virginia University, Morgantown, West Virginia, United States
Show AbstractZinc oxide is a wide band-gap semiconductor with bright UV emission. To determine donor and acceptor concentrations affecting electrical properties in n-type ZnO crystals, the relaxation time approximation (RTA) has been used to analyze mobility and carrier concentration data measured from 80 to 400 K. Five scattering mechanisms are included: polar-optical-phonon, piezoelectric potential, deformation potential, ionized impurity, and neutral impurity (NI) scattering. The NI scattering is often ignored but plays an important role in limiting the total mobility. By including NI scattering, the experimental deformation potential E1 = 3.8 eV for ZnO can be used, rather than treating E1 as a fitting parameter. At 300 K, “pure” ZnO has an electron mobility of about 210 cm2/Vs and an intrinsic Hall mobility of about 230 cm2/Vs. Analysis of Hall data from n-type ZnO crystals grown by high pressure melt, hydrothermal and chemical-vapor-transport techniques is presented. The RTA approach with the five scattering mechanisms was also used to analyze Hall data from other wide-bandgap II-VI semiconductors. Experimental E1 values were used and intrinsic Hall mobilities at room temperature are predicted to be 210, 380 and 600 cm2/Vs for ZnS, CdS, and CdSe, respectively. Temperature dependences of the Hall r factors were calculated for each case and used to obtain corrected electron mobility and carrier concentrations. Acceptor concentrations, donor concentrations, and donor activation energies were determined in each case. The temperature dependence of the intrinsic hole mobility in ZnO has also been predicted, and the 300-K value is about 50 cm2/Vs. The effect of acceptor concentration on Hall mobility in p-type ZnO has been determined and compared to reported work on ZnO:As [1,2]. This work was supported by NSF Grant No. DMR-0508140 and Oak Ridge National Laboratory.[1] D. C. Look, Semicond. Sci. Technol. 20 S55-61 (2005). [2] Y. R. Ryu, T. S. Lee and H. W. White, Appl. Phys. Lett. 83 87-89 (2003).
5:00 PM - L4.8
Separation of Surface and Bulk Conduction in Hydrothermally grown ZnO Using Variable Magnetic Field Hall Effect Measurements.
Craig Swartz 1 2 , Steven Durbin 1 2 , Martin Allen 1 2 , Thomas Myers 3
1 Electrical and Computer Engineering, University of Canterbury, Christchurch, Canterbury, New Zealand, 2 , The MacDiarmid Institute, Christchurch New Zealand, 3 Physics and Astronomy, West Virginia University, Morgantown, West Virginia, United States
Show AbstractA major advantage of ZnO over other wide bandgap semiconductors is the availability of bulk, single crystal growth of high quality material. In particular, the hydrothermal growth of ZnO can be used to achieve carrier concentrations 2 – 3 orders of magnitude lower than other bulk ZnO, likely due to compensation from unintentionally introduced Li and Na acceptor impurities. The high resistivity of the bulk material makes the measurement of its electrical properties particularly susceptible to complications such as persistent photoconductivity, increased temperature sensitivity, and surface conduction effects. The surface conductivity of ZnO is known to depend strongly on the ambient atmosphere and can increase significantly under vacuum conditions. After being placed in a vacuum, atmosphere-induced surface effects can persist for several hours at room temperature, and even after an equilibrium condition is reached, the surface conductivity can in some situations dominate electrical measurements. The separation of surface conducting layers from the bulk conductivity of a given sample can be performed by the use of variable magnetic field Hall effect measurements. Here we examine temperature dependent (80 – 300 K), variable magnetic field (up to 12 T) Hall effect measurements on ZnO single crystal wafers from Tokyo Denpa Co. Ltd. (Japan) grown using a hydrothermal technique. Multiple carrier fitting was used at each temperature to remove surface conduction effects and extract the mobility and carrier concentration data for the bulk carriers only. A significantly higher bulk carrier mobility was found than is apparent from standard single field Hall effect measurements. The temperature dependence of the mobility was then fitted to a theoretical model accounting for the various scattering mechanisms believed to be prevalent in ZnO, polar optical phonon, acoustic phonon, and ionized impurity scattering. This resulted in an order of magnitude lower ionized impurity concentration. These results also indicate that the use of single field, temperature dependent Hall effect measurements to determine donor concentration and activation energies may be problematic in resistive hydrothermallly grown material unless the bulk conduction is isolated from surface conduction effects.
5:15 PM - L4.9
Stabilization Mechanisms of Polar ZnO Surfaces Revisited.
Mao-Hua Du 1 2 3 , Shengbai Zhang 3 , Steven Erwin 2 , John Northrup 4
1 , Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 , Center for Computational Materials Science, Naval Research Laboratory, Washington, District of Columbia, United States, 3 , National Renewable Energy Laboratory, Golden, Colorado, United States, 4 , Palo Alto Research Center, Palo Alto, California, United States
Show AbstractIdentifying the mechanisms responsible for the morphology of surfaces is crucial to gaining better control of materials grown both by epitaxial and colloidal methods, for which surface structure can strongly affect crystal quality and even impurity incorporation. The polar (0001) surfaces of ZnO exhibit a variety of different morphologies. The mechanisms underlying this diversity have not been definitively identified. Many efforts to understand the polar surfaces of ZnO have focused on electrostatic considerations. For example, the formation of triangular islands and pits observed on ZnO(0001)-Zn has been argued to originate from the requirements that the macroscopic electrostatic field arising from the polar wurtzite structure be electrostatically compensated through surface reconstruction. Here we evaluate the role of several possible surface stabilization mechanisms. We show that electrostatics does not play a significant role. Instead, we propose that surface morphology is determined by a competition between two other mechanisms. The first is the electron counting rule, which leads to semiconducting surfaces. The second arises from the large cohesive energy of ionic crystals such as ZnO, which tends to preserve the surface stoichiometry at its bulk value, leading to metallic surfaces. Our calculations show a crossover between semiconducting and metallic surfaces as the O chemical potential is varied. On the (000-1)-O surface, semiconducting surfaces with 1/4 monolayer (ML) oxygen vacancies are stable at low O chemical potential (O-poor conditions), and the unreconstructed metallic surface is stable at high O chemical potential (O-rich conditions). Analogously, on the (0001)-Zn surface, semiconducting surfaces with 1/4 ML O adatoms are stable at high O chemical potential, while metallic surfaces are stable at low O chemical potential. Near the crossover, triangular islands and pits of varying sizes may form on both surfaces. Experimentally, ZnO surfaces are usually prepared in ultrahigh vacuum with only residual O2 pressure, corresponding to O-poor conditions. Under O-poor conditions, our calculations predict the formation of triangular islands/pits with varying sizes on the (0001)-Zn surface, consistent with experimental observations. On the (000-1)-O surface our prediction of 1/4 ML O vacancy must await STM images with atomic resolution. To experimantally test our model at O-rich conditions, oxygen-plasma treatment may offer a route. Indeed, a recent STM study of the (000-1)-O surface after O-plasma treatment at 800 °C shows the first atomically resolved image of the unreconstructed surface, consistent with our calculations.
5:30 PM - L4.10
Defects and Diffusion of Fe3+ in ZnO.
Herbert Schmid 1 , Oliver Koester-Scherger 1 , Markus Waelle 2 , Detlef Guenther 2 , Werner Mader 1
1 Chemistry, University of Bonn, Bonn Germany, 2 Chemistry, ETH Zürich, Zürich Switzerland
Show Abstract5:45 PM - L4.11
Structural, Electrical and Optical Properties of Epitaxial Zn1-xGaxO Films Grown on Sapphire (0001) Substrate.
Ashutosh Tiwari 1 , Michael Snure 1
1 Materials Science & Engineering, university of utah, Saltlake city, Utah, United States
Show AbstractHere we report the structural, electrical and optical properties of epitaxial Zn1-xGaxO films (x=0-0.05) grown on single crystal Sapphire (0001) substrate. Thin film processing was performed using pulsed laser deposition technique. Structural and elemental analysis was performed using high resolution X-ray diffraction (θ-2θ and Φ scan) and energy dispersive X-ray spectroscopy (EDX). Temperature dependent electrical resistivity and thermoelectric power measurements were performed over the temperature range of 77-300 K and 296-373 K, respectively. Hall effect and optical transmission measurements were preformed at room temperature. All these studies showed that the structural, electrical as well as the optical characteristics of Zn1-xGaxO films depend very sensitively on the Ga contents. As the Ga doping concentration is increased, initially an increase in carrier concentration and optical bandgap is observed (till x=0.04), which is followed by a decrease at higher concentrations. These features were attributed to the combined effect of band filling (Burstein Moss effect), electronic correlation and epitaxial strain present in the system. Above parameters also affected the electrical properties of the films quite significantly. Zn1-xGaxO films with 1% of Ga doping (x=0.01) showed metal-like electrical resistivity. However, for higher doping levels, enhanced scattering potential, arising from randomly distributed impurity atoms, resulted in the Anderson localization of electronic states.
L5: Poster Session
Session Chairs
Irina Buyanova
Chennupati Jagadish
David Norton
Gyu-Chul Yi
Wednesday AM, November 28, 2007
Exhibition Hall D (Hynes)
9:00 PM - L5.1
Excitonic Transport in ZnO - Investigated by Temperature, Time, and Spatially Resolved Cathodoluminescence.
Barbara Bastek 1 , Juergen Christen 1 , Frank Bertram 1 , Martin Noltemeyer 1 , Heiko Frenzel 2 , Holger Hochmuth 2 , Michael Lorenz 2 , Matthias Brandt 2 , Marius Grundmann 2
1 Institute of Experimental Physics, University of Magdeburg, Magdeburg Germany, 2 Institut für Experimentelle Physik II, Universität Leipzig, Leipzig Germany
Show AbstractIn this work we investigate the microscopic excitonic transport in ZnO bulk material (substrate) and its temperature dependence on an nm-scale by highly spatially and ps time resolved cathodoluminescence (CL) spectroscopy. The characteristic T-dependence gives information about the underling scattering mechanism. Covering the ZnO surface with 156 nm thick rectangular Ti-masks allows the excitation by electrons with a primary energy of 7 keV (zBethe =356 nm) through this masks while completely absorbing the emerging excitonic luminescence of the subjacent ZnO. The Ti-masks where deposited by electron beam evaporation and structured by optical lithography. They have an edge length of 520 µm x 460 µm and exhibit a lateral distance of 210 µm and 160 µm, respectively. The structure allows the realization of CL-linescans perpendicular to the mask’s edge on the one hand and the undisturbed time resolved transient measurements on the other hand for the identical sample structure on the unmasked ZnO layer. Only the fraction of generated free excitons which are able to reach the uncovered ZnO surface area by lateral transport within their lifetime contribute to the CL intensity. All luminescence of the excitons generated below the Ti-masks is blocked. A fit of the cw CL-linescans of the free exciton luminescence to the one-dimensional diffusion equation yields the excitonic diffusion length λFX. The perfectly exponential time-decay of the free exciton is recorded at an undisturbed position > 100 µm away from any Ti-mask on the identical ZnO sample and yields τFX. By variation of the temperature during the experiment in the range from 5 K to 300 K the temperature dependence of the diffusion length λFX(T) as well as τFX(T)are determined. The decline of the diffusion length λFX for T < 30 K visualizes directly the exceeding effective capture of the free exciton and their binding to neutral donors. At T = 30 K the diffusion length λFX reaches its maximum of 22 µm. With further rising T (30 K < T < 300 K) λFX decreases monotonously. In contrast, the “integral diffusion length”, which means the spatial decay of the cumulative CL intensity integrated over all recombination channels including donor-bound excitons (D0,X) increases monotonously with decreasing temperatures, reaching 29 µm at 5 K. From the measured λFX(T) and τFX(T)we calculate the mobility of the free exciton µFX(T) in ZnO. Astonishingly, the mobility of the free exciton µFX monotonously increases by more than three orders of magnitude with decreasing T from a value of 0.55 cm2/Vs at 300 K to 1.48*103 cm2/Vs at 5 K. This is a clear hint for the reduction or screening of ionised impurity scattering for the excitons. We will also present our microscopic results achieved on ZnO layers PLD (pulsed laser deposition) grown on sapphire as well as on ZnO substrates and will correlate these microscopic results with macroscopic Hall-effect measurements and (unipolar) mobility data from FET structures.
9:00 PM - L5.10
Transport Measurements on Artificially Structured ZnO Epitaxial Layers.
M. Piechotka 1 , M. Elm 1 , S. Lautenschlaeger 1 , T. Henning 1 , Bruno Meyer 1 , P. Klar 1
1 Institute of Experimental Physics, Justus Liebig University, Giessen Germany
Show AbstractZnO layers of about 1000 nm thickness were grown homoepitaxially on ZnO substrates by chemical vapor deposition. The layers are n-type with electron concentrations of about 1017 cm-3. Stripes of the as grown samples were artificially structured by photolithography. Each pattern consists of two contact pads with a regular array of wires of the same width. The wire thickness varies throughout the series of patterns from 4 μm to 100 μm whereas the total cross section area of the wires is kept constant. The patterns were transferred either by wet-chemical etching or by ion-beam etching. The resistance of the wire patterns was measured in the temperature range of 2 K to 300 K. The analysis of the resistivity as a function of wire width yields information about the depth of the surface layer damaged during the etching process.
9:00 PM - L5.11
Synthesis, Magnetism, and Exchange Bias of Cu- and Mn-Doped ZnO Nanocrystals.
Xuefeng Wang 1 , Jianbin Xu 1
1 Department of Electronic Engineering, The Chinese University of Hong Kong, Hong Kong China
Show AbstractIn this presentation, we present a reaction-limited aggregation-based growth of 2 at.% Cu- and 5 at.% Mn-doped ZnO nanocrystals and their above-room-temperature ferromagnetism. Interestingly, they both display exchange biasing phenomena at low temperatures after field cooling, revealing the complexity of magnetic phases therein. (i) For Cu-doped ZnO, a spin-split acceptor impurity band model is proposed to explain the high-temperature ferromagnetism in the Cu-poor cores, while in the Cu-rich surface layers, the shallow donorlike defects generated during aggregation-based growth can not only compensate a few holes but also readily occupy the empty Cu 3d states to destroy CuO4 tetrahedra where magnetic interactions are highly restricted, thus resulting in the short-range antiferromagnetic clusters [1]. This exchange-coupling structure [Zn1–xCuxO (core)/Zn1–yCuyO (shell); x < y] is thought to be the origin of exchange bias. (ii) For Mn-doped ZnO, a Curie temperature is found at ~43 K and a blocking effect occurs at ~40 K, which is characteristic of ZnzMn3–zO4 secondary phase [2]. The room-temperature hysteresis loop is ascribed to the locally p-type (Zn, Mn)O in terms of the spin-split acceptor impurity band model, while the exchange biasing effect is attributed to the exchange interaction between ferrimagnetic ZnzMn3–zO4 [or ferromagnetic (Zn, Mn)O] and antiferromagnetic (or spin-glass-like) phase in (Zn, Mn)O.[1] X. F. Wang, J. B. Xu, W. Y. Cheung, J. An, and N. Ke, Appl. Phys. Lett. 90, 212502 (2007).[2] R. K. Zheng, H. Liu, X. X. Zhang, V. A. L. Roy, and A. B. Djurisic, Appl. Phys. Lett. 85, 2589 (2004).
9:00 PM - L5.12
Visible Green Luminescence from ZnO Nanobrushes and Nanocombs Prepared by a Direct Atmosphere Evaporation Method using Zn Metal.
Shubra Singh 1 2 , Mamidanna Rao 1 2
1 Physics, Indian Institute of Technology, Chennai, TamilNadu, India, 2 Materials Science Recearch Centre, Indian Insitute of Technology Madras, Chennai India
Show Abstract9:00 PM - L5.13
Order and Disorder in Covalent Semiconductors and Ionic Oxides.
Rodrigo Martins 1 2 , Luis Pereira 1 2 , Pedro Barquinha 2 1 , Elvira Fortunato 2 1
1 CEMOP, Uninova, Caparica Portugal, 2 CENIMAT I3N, FCTUNL, Caparica Portugal
Show AbstractThis paper aims to discuss the effect of order and disorder on the electrical performances of covalent silicon semiconductors and ionic oxide semiconductors namely the Zinc oxide alloys used as active channel layer in thin film transistors or p-n junctions like solar cells. The effect of disorder on covalent semiconductors affects directly their electrical transport properties due to the asymmetric behavior of sp states while in ionic oxide semiconductors it is found that this effect is small, because angular disorder has no effect on spherical symmetry of s states. To this, we have to add that the carriers’ mobility in both systems is quite different, being also affected by electron-phonon interactions (weak in silicon and strong in ionic oxides leading to formation of polarons). Besides, the impurity doping effect and the presence of vacancies in disordered silicon and in ionic oxides has different behaviours, which will influence the thin film properties and so, the performances of the devices produced.
9:00 PM - L5.14
Preparation and Characterization of Ga-Doped ZnO Nano powder by Sol-Gel Process.
Kuo-Chuang Chiu 1 , Ren-Der Jean 1
1 , Industrial Technology Research Institute, Chutung, Hsinchu Taiwan
Show AbstractGa-doped ZnO powder was prepared by the low temperature Soft Chemical route Process. X-ray diffraction patterns indicate a pure phase is formed by 600°C without impure phase. TEM observations show an average 80nm particle size of the powder after annealing at 700°C by using H2 as a carrier gas. Electrical properties of Ga-doped ZnO film resistivity was 6.2 × 10-4 Ωcm which deposited on glass substrates by Atmospheric Plasma annealing method at room temperature. This paper reports the preparation of the fine grains size powder and low resistivity high mobility film.
9:00 PM - L5.15
Electrophoretic Deposition of ZnO Quantum Dots, from Substrate Coverage to Local Deposition.
Petra Lommens 1 , Dries Van Thourhout 2 , Zeger Hens 1
1 Department of Inorganic and Physical Chemistry, Ghent University, Ghent Belgium, 2 Department of Information Technology, Ghent University, Ghent Belgium
Show AbstractTo use colloidal quantum dots in devices like sensor, LEDs or transistors, controllable deposition processes are needed. A possible technique to fabricate films or micropatterns of quantum dots is electrophoretic deposition (EPD). It is based on the idea that charged particles in suspension will be driven to and deposit on a surface when an electric field perpendicular to the substrate is applied. We have prepared layers of ZnO quantum dots with tunable thickness by EPD. The ZnO quantum dots, 2 to 3 nm in diameter, are prepared using a well-established colloidal synthesis route. After washing and resuspending the quantum dots in an appropriate solvent, thin layers of ZnO can be deposited on bare silicon or gold substrates. This is done by applying voltages from 20 to 60V over a distance of 1-3 cm between a Cu-electrode and a substrate, submerged in the quantum dot suspension. By varying this concentration from 5 to 30 µM and deposition time and voltage, deposits covering the entire substrate and ranging from monolayer coverage to 250 nm can be made. Besides homogeneous substrate coverage, we showthat EPD can also be used to deposit quantum dots on silicon substrates covered with a patterned, developed photoresist. Using the appropriate combination of resist and solvent, it is possible to remove the resist after the EPD process, without damaging the ZnO deposit. In this way, small micropatterns, with dimensions in the range of 5x5 µm and a thickness, varying between 10 and 300 nm, can be fabricated.In summary, this work shows that EPD is a versitale technique for depositing charged particles, with the advantage of short deposition times, low voltages and the possibility to deposit on bare or lithographically patterned substrates at low cost.
9:00 PM - L5.16
Deposition of Piezoelectric ZnO Films by RF Magnetron Sputtering and its Application to SAW Devices.
Seol Hee Choi 1 , Chan Hyoung Kang 1
1 Department of Advanced Materials Engineering, Korea Polytechnic University, Kyonggi-Do Korea (the Republic of)
Show AbstractHighly c-axis oriented, dense, and fine-grained polycrystalline ZnO films with smooth surface and high resistivity were deposited on 4 inch silicon wafers by employing ZnO targets in a radio-frequency magnetron sputtering system. By changing applied rf power, substrate temperature and O2/Ar gas ratio, the optimum process parameters were found to be 150 W, 200oC and 30/70, respectively. Applying the ZnO films deposited under these optimum conditions, surface acoustic wave (SAW) devices of ZnO/IDT/SiO2/Si structure were fabricated by conventional photolithography and etching processes. The interdigital transducers (IDT), made of aluminium deposited by DC magnetron sputter, were patterned with 2.5/2.5 μm finger width/spacing. Another type of SAW filter of IDT/ZnO/diamond/Si structure was fabricated. High-quality nanocrystalline diamond (NCD) films were deposited on 4 inch silicon wafer by direct current (DC) plasma assisted chemical vapor deposition method using H2-CH4 mixture as precursor gas. On the top of the diamond films, ZnO films were deposited under the optimum conditions. Then aluminium IDTs were fabricated on the ZnO/diamond/Si layered films. The characteristics of the fabricated SAW devices were evaluated in terms of center frequency, insertion loss and wave propagation velocity.
9:00 PM - L5.17
Tailoring of Optical and Field Emission Properties of Aligned ZnO Naorods.
O. Srivastava 1 , Jai Singh 1 , R. Tiwari 1
1 physics, bhu, Varanasi, u.p., India
Show Abstract9:00 PM - L5.18
Proposal of High Temperature MBE Growth of ZnO Films with Using Ionized Zn Flux.
Koji Omichi 1 , Yoshikazu Kaifuchi 1 , Munehisa Fujimaki 1 , Talai Jin 2 , Akihiko Yoshikawa 2 3 4
1 Optics and Electronics Laboratory, Fujikura.Ltd, Sakura, Chiba, Japan, 2 Graduate School of Electrical and Electronics Engineering, Chiba University, Chiba, Chiba, Japan, 3 Venture Business Laboratory, Chiba University, Chiba, Chiba, Japan, 4 InN-project as a CREST program of JST, Chiba University, Chiba, Chiba, Japan
Show Abstract ZnO is one of the most promising materials for ultraviolet light emitting devices. Although a lot of papers have been reported for achieving p-type ZnO epilayers, these methods are not reproducibly/widely followed by other groups. One of the key points for successful p-type doping is to improve the quality of ZnO epilayers, e.g., higher crystalline structural quality with remarkably low threading dislocation density and much more reduced concentration of residual impurities, etc. Higher growth temperatures are favorable for improving the quality of ZnO epilayers due to the effects of enhanced surface migration of adsorbed Zn species and reduced residual impurities as well. However, the growth rate of ZnO epilayers drastically decreases with increasing growth temperature, in particular in MBE. In this paper, we propose a novel method to achieve higher growth temperatures in MBE up to 900 °C with using ionized Zn (Zn+) flux leading to higher Zn-sticking coefficient. Effects of using Zn+ flux on the crystalline quality are also investigated. ZnO epilayers were grown on a sapphire (0001) substrate using a conventional low temperature ZnO buffer layer grown at 400 °C. A high power laser diode module was used for heating the substrate to perform high temperature crystal growth. A novel structure Zn ionizer with extraction grid and energy-acceleration functions was designed to realize effective Zn-ionization and higher Zn-sticking coefficient at high growth temperatures, and it was set between the Zn cell and substrate. ZnO epilayers were grown at 650-900 °C under two different conditions with and without Zn-ionization, and the results in both cases were comparatively studied. First we have confirmed that the growth rate was increased with using Zn+ flux. In particular, this was remarkable at the growth temperature of 900 °C; the growth rate was 2 times higher when using Zn+ flux. As for their properties in 190 nm-thick ZnO epilayers grown at 850 °C using Zn+ flux, the rms surface roughness was about 20 nm in 10 μm x 10 μm area, and the electron concentration and Hall mobility were ~3x1016 cm3 and ~190 cm2/Vs, respectively. Thus it was confirmed that the growth of ZnO epilayers with using Zn+ flux is an effective method to achieve higher growth rate and higher crystalline quality at high growth temperature in MBE.
9:00 PM - L5.19
Morphology Dependence of Lasing Characteristic in ZnO Nanostructures.
SangHyun Lee 1 , Takenari Goto 1 , Hiroshi Miyazaki 2 , Meoung-Whan Cho 3 , Takafumi Yao 1 3
1 Center for Interdisciplinary Research, Tohoku University, Sendai Japan, 2 Department of Applied Physics, Tohoku University, Sendai Japan, 3 Institute for Materials Research, Tohoku University, Sendai Japan
Show AbstractAn interesting topic of recent ZnO research is the relationships between its own properties and morphologies including dimensionality, size, and shape. It is revealing through the analysis of fundamental properties and realization of device using various shaped nanostructures. The optical properties of ZnO nanostructures provide hope applying it to a UV optoelectronic nano-device material instead of GaN due to more easy growth technology of high quality ZnO nano-crystal and a large excition binding energy of 60 meV. Here, we present the optical properties of various ZnO nanostructures with different size and morphologies; rod, needle, and multi-tapered structures. The temperature and excitation intensity dependence of photoluminescence (PL) in ZnO nanostructures was accomplished. These nanostructures showed strong and sharp excitonic emissions comparable to bulk ZnO at 10 K. Emission spectra and images using micro PL measurement system proved different emission distributions within single nanoneedle and multi-tapered. The strong emission was observed in the 100 ~ 400 nm diameter region, while the emission was significantly decreased at the part by decreasing the diameter. The results suggest that the generated emission within crystal can’t progress into narrow structure. No stimulated emission from ZnO nanorods and needle could be also understood in terms of high optical loss and transmission of light wave in nanorods due to smaller diameter than wavelength. We observed interestingly the stimulated and laser emission from only tapered structures from 5 µm to 30 nm. The laser emission was observed near the tip of tapered structure.We also suggest the possible mechanism and optical cavity for laser emission in a multi-tapered ZnO structure at room temperature. A prediction of optical behavior within single nanoneeldle and tapered structure using 3D-FDTD simulation is discussed.
9:00 PM - L5.2
Effect of ZnO Buffer Layer on ZnO Nanorods Growth by MOCVD.
Changa Ha Kwak 1 , Sang Wook Han 2 , Sun Hong Park 1 , Soo Young Seo 1 , Yong Byung Lee 1 , Seon Hyo Kim 1
1 Department of Materials Science and Engineering, Postech, Pohang Korea (the Republic of), 2 , Chonbuk University, Jeonju Korea (the Republic of)
Show Abstract9:00 PM - L5.20
Tunable N-type Conductivity and Transport Properties of Ga-doped ZnO Nanowire Arrays.
Guodong Yuan 1 , Wenjun Zhang 1 , Jiansheng Jie 1 , Xia Fan 1 , Chunsing Lee 1 , Shuittong Lee 1
1 Department of Physics and Materials Science, Center of Super-Diamond and Advanced Films, Hongkong China
Show AbstractIn this work, we demonstrate a controlled growth and doping process of well-aligned ZnO nanowire (NW) arrays by introducing Ga2O3 as a dopant source. First, we show the crystallographic orientation of NWs is dictated by dopant content, while all nanowires possess high crystal quality. We then systematically study the electrical and transport properties of Ga-doped ZnO nanowires via field-effect transistors (FETs) fabricated from single nanowires. FET measurements show, with high reproducibility, that increasing Ga2O3 in the source material from 0 to 1% can realize tunable and controllable n-type conductivity, with resistivity decreasing from 0.25 Ohm cm to 2.2×10-3 Ohm cm. All NWs show sharp near-band edge PL with negligible defect-related emissions suggesting good crystal quality. The emission peak for the undoped ZnO nanowires is centered at 374 nm, which is red-shifted by 1.8 and 15 nm respectively in the lightly and heavily Ga-doped ZnO nanowires. The capability of tuning n-type conduction and the understanding of the transport properties of ZnO NWs are important for the development of ZnO NW-based electronic and photoelectronic devices.
9:00 PM - L5.22
Optical Characterization of Acceptors Implanted into ZnO.
Joachim Dürr 1 , Daniel Stichtenoth 1 , Sven Mueller 1 , Lars Wischmeier 2 , C. Bekeney 2 , Tobias Voss 2 , Carsten Ronning 1
1 II. Institute of Physics, University of Goettingen, Goettingen Germany, 2 Institute for Solid State Physics, University of Bremen, Bremen Germany
Show Abstract9:00 PM - L5.24
Shape-controlled Solution-synthesis of Single Crystalline ZnTe 1D Nanocrystals.
Jun Zhang 1 , Amar Kumbhar 2 , Jiye Fang 1
1 Department of Chemistry, State University of New York at Binghamton, Binghamton, New York, United States, 2 Electron Microscope Facility, Clemson University, Anderson, South Carolina, United States
Show AbstractOne dimensional nano-structured ZnTe have attracted more interests because of their potential applications of optoelectronic devices in the blue-green region of the spectrum. Our study demonstrates shape-controlled synthesis of high-quality ZnTe nanobelts and nanorods in high-temperature organic solution. Zinc acetate was dissolved in benzyl ether under argon protection in the presence of oleic acid. Solution of metallic tellurium in trioctylphosphine (TOP) was introduced into this zinc solution along with superhydride (lithium triethyl borohydride), forming a stock solution as reaction precursor. The reactions were carried out by multiple injections of such stock solution into pre-heated benzyl ether. The products were isolated by centrifugation and were re-dispersed in hexane, resulting one-dimensional (1D) ZnTe nanocrystals (NCs). The size and morphology were observed using transmission electron microscopic technique (TEM), whereas the phase structure was analyzed using powder x-ray diffraction method (XRD). Photoluminescence (PL) spectra of these 1D ZnTe NCs were also explored. Investigation reveals that these 1D ZnTe NCs are monodispersed and single crystalline.
9:00 PM - L5.25
Electrochemical Fabrication of Zn1-xMgxO Films from an Aqueous Solution Containing Magnesium Nitrate and Zinc Sulfate.
Hiroki Ishizaki 1 2 , Hideaki Maeda 1 2
1 Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology, Tosu, Saga, Japan, 2 Crest, Japan Science and Technology Agency, Tosu, Saga, Japan
Show AbstractRecently, oxide films with wide band gap energy were paid much attention for many applications such as optic devices, electric luminescence devices and transparent conductive oxide of solar cells. Many authors reported the preparation of oxide film with the band gap energy, such as ZnO:Ga, Zn1-xMgxO, and Zn1-xCdxO. In particular, Zn1-xMgxO films presented interesting electrical and optical properties, which found wide applications in the fields of optoelectronic and transparent conductive oxide. Because the band gap energy of Zn1-xMgxO films was easily controlled by Mg/(Mg+Zn) atomic ratio. Zn1-xMgxO films were prepared commonly by the physical vapor deposition such as RF-magnetron sputtering, molecular beam epitaxy, metal organic chemical vapor deposition and pulsed laser deposition. On the other hand, the electrochemical preparation of oxide films presented the several advantages over these techniques; (1) the thickness and morphology of film could be controlled by electrochemical parameters, (2) the relatively uniform films could be obtained on the substrates with complex shape, (3) the films could be grown on substrates with a melting point below 373K such as polymer, (4) the technique was considered to be less hazardous and thus more environmentally friendly and (5) the equipment was less expensive. In this work, we investigated the evaluation and the electrochemical preparation of Zn1-xMgxO films on the conductive glass substrates from a 0.1mol/L magnesium nitrate and 0.001mol/L zinc sulfate at the cathodic potential ranging of -0.8V and -0.95V. The Mg/(Zn+Mg) atomic ratio, the structural and optical properties of these Zn1-xMgxO films were investigated using inductively coupled plasma spectroscopy (ICP), X-ray diffraction, scanning electron microscope (SEM) and the UV-VIS-NIR scanning spectrophotometer. Zn1-xMgxO films with the wurtzite structure were electrochemically grown on the NESA glass substrates from a 0.1 mol/L magnesium nitrate aqueous solution containing 0.001mol/L zinc sulfate, regardless of the cathodic potential. And the lattice length of the c-axes increased with the decreasing the cathodic potential. The bandgap energy and Mg/(Zn+Mg) atomic ratio of Zn1-xMgxO film decreased with a decrease in the cathodic potential. Thus, the present results indicate that the band gap energy and the lattice parameters depend on the cathodic potential. In the conclusion, the optical, structural properties and Mg/(Zn+Mg) atomic ratio of Zn1-xMgxO films can be controlled by the cathodic potential.
9:00 PM - L5.26
Tunneling Spectroscopy of Magnetic Impurities (=Co,Mn,Fe) in ZnO.
Takashi Tamura 1 , Changman Kim 1 , Yasushi Oikawa 1 , Hajime Ozaki 1
1 , Waseda university, Tokyo Japan
Show AbstractRecently, ZnO has attracted interest in the spintronics as a candidate for a diluted magnetic semiconductor (DMS) which shows the room temperature ferromagnetism and has been investigated by many workers. However there is a lot of controversy on the origin of the magnetic properties and it is important to know fundamental characteristics of magnetic impurities in ZnO to elucidate the origin of the magnetism.In this study we firstly observed the density of states (DOS) of M-doped ZnO (M=Co,Mn,Fe) by tunneling spectroscopy which is one of the useful methods for analyzing the DOS directly with high resolution. M-doped ZnO single crystals were prepared by chemical vapor transport (CVT) method using ZnCl2 or ZnCl2 plus C as transport agents. The composition x of magnetic impurity M in MxZn1-xO was about 0.07 for Co and about 0.10 for Mn and Fe.A swelling was observed in the dI/dV vs. V tunneling characteristics around 2.8 eV- 3.2eV below the Fermi level and just above the valence band edge of ZnO which is about 3.2eV below the Fermi level. This swelling in DOS is considered to indicate the 3d states of Co ions which is in agreement with the result of a theoretical calculation. [1]In the conference the tunneling spectroscopies will be reported also for Mn and Fe doped ZnO. [1] M.Toyoda, H.Akai, K.Sato, H.Katayama- Yoshida :Physica B, Vol:376-377 (2006), 647
9:00 PM - L5.27
Structural and Optical Properties of High Quality Homoepitaxial ZnO Layer.
Frank Bertram 1 , Alexander Franke 1 , Juergen Christen 1 , Thomas Hempel 1 , Soeren Heinze 1 , Armin Dadgar 1 , Alois Krost 1
1 , Otto-von-Guericke-University Magdeburg, Magdeburg Germany
Show AbstractIn recent years, ZnO and related alloys are attracting attention as promising candidates for the use of light emitters and detectors. Although ZnO single crystals were available for a long time epitaxially ZnO are usually grown on foreign substrates. Despite of the advantages of homoepitaxy, i.e., perfect matching of lattice constants and thermal expansion coefficients and the expected lower dislocation densities, only a few reports on optical properties of homoepitaxially grown ZnO layers exist. In this work, we report on a comprehensive characterization of homoepitaxially grown ZnO films by means of scanning electron microscopy (FE-SEM), photoluminescence (PL) and spatially and spectrally resolved cathodoluminescence spectroscopy (CL). The nominal 500 nm thick epitaxial ZnO layer is grown by MOVPE on commercially available (0001)-oriented ZnO substrate. The ZnO layer is directly deposited on the thermally pretreated ZnO substrate without any buffer layer. The ZnO layer has a flat, mirror-like surface and exhibits no features in plan view SEM. In cross- sectional FE-SEM images, the substrate merges smoothly into the ZnO layer: no contrast is visible at the ZnO-substrate-to-ZnO-epilayer homo-interface! This visualizes the structural perfect quality of the ZnO/ZnO homointerface which is further evidenced by TEM investigations. The low temperature (4 K) PL spectrum of the substrate and of the homoepitaxial layer is dominated by the neutral impurity bound exciton line I8 (E = 3.360 eV). The free exciton line XA (E = 3.377 eV) and the ionized impurity bound excitons I1 (E = 3.371 eV) as well as a deeply bound exciton at E = 3.342 eV are also observable. An additional new luminescence line in the spectral region between I9 and I10 (E = 3.355 eV) is exclusively visible in the ZnO layer.Intense and very homogeneous CL intensity with a narrow spatial distribution of emission wavelength is obtained in plan view CL microscopy. The dominating I8 exhibits a standard deviation of σ = 0.4 meV. We further focus on the spectral evolution as a function of depth to the interface of the ZnO/ZnO homostructure. Cross-sectional CL spectrum linescan, i.e., sets of CL spectra recorded while scanning the e-beam in the growth direction along the freshly cleaved cross section were recorded. The ZnO substrate and the subsequent following ZnO layer can be clearly resolved: The CL of the ZnO substrate is completely governed by a strong I8 emission while slight variations of intensity and spectral position indicate the limited structural quality of the commercial substrate. In contrast, the CL of the ZnO epilayer is characterized by an additional line in the spectral region between I9 and I10 as has been recorded in plan view PL / CL. No shift or fluctuation of spectral position or intensity of these excitonic lines is observed across the ZnO/ZnO homointerface and along the layer thickness indicating the high optical quality of the ZnO epilayer.
9:00 PM - L5.28
Solution-deposited ZnO-organic diodes with high current density and high frequency rectification under ambient conditions
Bhola Pal 1
1 Department of Material Science and Engineering, Johns Hopkins University, Baltimore, Maryland, United States
Show Abstract9:00 PM - L5.29
Aqueous Chemical Synthesis of Small Diameter ZnO Nanorods with Superb Photocatalytic Activity.
Yao-An Chung 1 , Yu-Cheng Chang 1 , Ming-Yen Lu 1 , Lih-Juann Chen 1
1 Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu Taiwan
Show AbstractZnO nanorods have been prepared from aqueous solution containing Zn(OH)42- ions, manganese acetate and sodium dodecyl sulfate (SDS) at room temperature. Zn(OH)42- ions were produced with zinc acetate and sodium hydroxide. The as prepared products were characterized by SEM, EDS, TEM, SAED, HRTEM, XRD, BET and CL. The SEM and TEM images showed that the nanorods are 300 – 400 nm in length and 10 – 20 nm in diameters. The EDS, XRD, SAED, and HRTEM confirmed the product is single crystal ZnO nanorods with the growth direction along [001]. The CL spectrum shows a strong and broad emission at 580 nm. The surface area of sample is 80 m2/g determined by BET. Superb photocatalytic activity of ZnO nanorods toward NO gas was detected.
9:00 PM - L5.3
Properties of Zinc Oxide Doped Indium, Magnesium and Aluminum Oxide Films used on Flexible Substrates.
Shih Hsiu Hsiao 1 , Yoshikazu Tanaka 1 , Ide-Ektessabi Ari 2
1 Mechanical Engineering and Science, Kyoto University, Kyoto Japan, 2 International Innovation Center, Kyoto University, Kyoto Japan
Show AbstractZinc oxide was considered to be substitute for Indium Tin oxide (ITO) as the transparent conductive oxide (TCO) films used in display industry according to Indium is a rare and expensive metal. For the flexible display, the polymeric materials used as the flexible substrates are more bendable and lighter weight compared to the glass substrates used in flat panel display. However, the thermal sensitivity of polymer affects the properties of TCO layers when it was deposited at room temperature. In this study, we doped the different ratio of Indium oxide, Magnesium oxide and Aluminum oxide into Zinc oxide to improve the properties of Zinc oxide used as the TCO films deposited by the sputtering method. The Polyethylene Terephthalate (PET) was used as the flexible substrate without thermal process and the glass was used as the comparison substrate with annealing. The light transmittance and the surface resistivity were measured. The chemical composition and the crystal structure were analyzed by using Rutherford Backscattering Spectrometry and X-ray Diffraction measurement.
9:00 PM - L5.4
Effect of Fabrication Variables on the Performance of Zinc Oxide Metal-Semiconductor-Metal Photodetectors.
Tingfang Yen 1 , Michael DiNezza 1 , Sung Jin Kim 1 , Wayne Anderson 1
1 EE, SUNY-Buffalo, Amherst, New York, United States
Show AbstractThe performance of ZnO metal-semiconductor-metal (MSM) photodetectors can be significantly influenced by modifications in the fabrication process. This paper identifies the effects of some of these modifications. Zinc oxide (ZnO) thin films were deposited onto silicon substrates by radio frequency magnetron sputtering using argon and oxygen ratios of 15 mT/ 5 mT or 5 mT/ 15 mT. Substrates were heated to 400 C during ZnO deposition and later annealed by conventional furnace, rapid thermal anneal or laser anneal. Atomic force microscopy revealed surface features for the different annealing methods. Stoichiometry of the films was verified by electron photoelectron spectroscopy analysis and photoluminescence spectroscopy (PL) was used to reveal the optical bandgap. The PL analysis revealed that laser annealing at 200 mW/cm^2 increased the 370 nm peak from 500 to 1300 a.u. and reduced the defect peak from 250 to 50 a.u.. ZnO films behaved as n-type upon deposition but as p-type after annealing in nitrogen. Values of Hall effect mobility ranged from 100-200 cm^2/V-s. MSM photodetectors were fabricated using an interdigitated metal pattern with metal and spacing dimensions ranging from 25 microns to 2 microns. Performance levels were compared for different Schottky metals such as Cr, Al, Pd, and Yb. Values of current responsivity ranged from about 200 A/W to above 800 A/W depending upon fabrication conditions. Performance was also modified using ozone treatment of the ZnO or the introduction of about 2 nm of silicon oxide between the ZnO and silicon substrate.
9:00 PM - L5.6
Effects of Substrate and Post-Gas-Annealing Temperature on Electric Properties of Al and Ti doped-ZnO Films.
Shinji Takayama 1
1 Systems and Control Engineering, Hosei University, Tokyo Japan
Show AbstractZinc oxide films are now receiving high attention as alternative materials to widely used Indium-tin-oxide (ITO) films, which have high conductivity and optical transparency in visible range. In this report, we systematically investigated the effect of substrate temperature and post-annealing in various gas atmospheres on properties of TM (TM=Al, Ti, Zr, Nb, Y) doped ZnO films. Samples were prepared in pure Ar gas atmosphere using RF magnetron sputtering method. Among these adding elements, Al and Ti doped films showed a lower resistivity than other adding elements after annealing in hydrogen gas (reducing) atmosphere. So, more extensive investigations were further performed on Al and Ti-doped ZnO films as functions of substrate temperatures and post-annealing temperatures in various gas atmospheres (O2, N2 and O2). On post-annealing in H2 gas, Al and Ti doped ZnO samples prepared at room temperature showed the lowest values of 5x10-4Ωcm and 9x10-4Ωcm, respectively,after annealing at 573K. They also show about 90% high transparency in visible range. On the other hand, Internal stresses of as-deposited Al or Ti doped ZnO films showed the compressive stress of about 300MPa and did not change significantly even after 673K anneal. In the case of the effect of a substrate temperature (Ts) during sputtering, Al and Ti doped ZnO films prepared at Ts=573K showed 2.0 and 17 x 10-4Ωcm, respectively. Particularly, Al-doped ZnO films showed nearly the same values as that of ITO films. They also showed a high transparency, about 90% in visible range. However,the internal stress of all Ti or Al doped ZnO samples showed nearly constant values of about 300MPa at substrate temperatures in the range from room temperature to 673K.From these results, we obtained Al-ZnO films with low resistivity and high transparency comparable to those of ITO films. However, compared with ITO films, the processing temperature and internal stress of Al-doped ZnO films are still high and hence need to be improved further in future work.
9:00 PM - L5.8
Annealing Effect on the Structure of RF Sputtered ZnO Thin Films.
Xu Lu 1 , Rui Yang 1 , Zhuo Xu 1 , Wei Ren 1 , Zhong yang Cheng 2 , Shao kang Li 1
1 Electronic science, Electronic Martial Research Lab, Xi'an, Shannxi, China, 2 Mechanical Engineering, Materials Research and Education Center, Auburn, Alabama, United States
Show Abstract9:00 PM - L5.9
Structural and Physical Ccharacterisation of Zinc Oxide Thin Films Prepared from Zinc Acetate via the Sol-gel Method.
Lee Koh 1 2 , Shane O'Brien 1 , Mehmet Copuroglu 1 2 , Gabriel Crean 1 2
1 Tyndall National Institute, University College Cork, Cork Ireland, 2 Department of Microelectronic Engineering, University College Cork, Cork Ireland
Show Abstract
Symposium Organizers
David P. Norton University of Florida
Chennupati Jagadish Australian National University
Irina Buyanova Linkping University
Gyu-Chul Yi Pohang University of Science and Technology (POSTECH)
L6: Magnetically Doped ZnO
Session Chairs
Wednesday AM, November 28, 2007
Constitution A (Sheraton)
9:30 AM - **L6.1
ZnO for Spintronics: Some Critical Issues.
Weimin Chen 1 , Irina Buyanova 1 , A. Murayama 2 , Y. Oka 2 , D. Norton 3 , S. Pearton 3 , A. Osinsky 4 , J. Dong 4
1 , Linkoping University, Linkoping Sweden, 2 Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai Japan, 3 Department of Materials Science and Engineering, University of Florida, Gainesville, Florida, United States, 4 , SVT Associates, Eden Prairie, Minnesota, United States
Show AbstractDue to their attractive physical and chemical properties, ZnO and related materials have emerged to be one of the most promising material systems for potential applications in optoelectronics operating within the UV-blue spectral range and also in chemical and bio sensing. The demonstration of room temperature ferromagnetism in its magnetic counterpart, i.e. diluted magnetic semiconductors based on ZnO, has also shown the potential of this material system for future applications in spintronics. Progress in this research topic has unfortunately been hindered by rather poor understanding of the spin-dependent phenomena in the materials, apart from the difficulties in material preparation and control. In this talk, we shall review our recent work on spin injection and spin detection in ZnO-based materials by spin-polarized cw and time-resolved magneto-optical spectroscopy. We shall address some critical issues that are important for future applications in spintronics, such as spin loss in spin detectors and the fundamental limits in the efficiency of spin detection by using the ZnO-based hetero- and quantum structures. Detailed physical mechanisms leading to spin depolarization and strategy for improvement will be discussed.
10:00 AM - L6.2
Growth and Properties of ZnxMn1-xO Thin Films Grown by Plasma-assisted MBE on (0001) Sapphire Substrates.
Christiane Deparis 1 , Christian Morhain 1 , Jesus Zuniga-Perez 1 , Pascal Sati 2 , Jean-Michel Chauveau 1 3 , Marguerite Laugt 1 , Philippe Vennegues 1 , Borge Vinter 1 3 , Anatoli Stepanov 2
1 , Centre de Recherche sur l'Hétéro-Epitaxie et ses applications CRHEA-CNRS, Valbonne Sophia Antipolis France, 2 L2MP - Université Paul Cézanne, Faculté de St-Jérôme, Marseille France, 3 Physics Dept, University of Nice-Sophia-Antipolis, Nice France
Show AbstractIn the search for a high Tc ferromagnetic semiconductor, p-type doped ZnxMn1-xO appears as an attractive diluted magnetic semiconductor (DMS) since it has been predicted that this material would be a ferromagnet at room temperature. Contradictory results regarding the magnetic properties of ZnMnO have been published so far, showing the importance of sample quality and the need to avoid any Mn-related secondary phases which could mask the intrinsic magnetic properties. MBE appears therefore an excellent technique to deposit ZnO-based DMS and to investigate their growth and properties, especially since it offers the possibility of monitoring the growth.In this communication, we shall report on the growth of ZnMnO DMS by plasma assisted MBE on (0001) sapphire substrates. The Mn content was varied from x=0.001 to x=0.16. The role of the growth conditions on the structural and optical properties of ZnMnO samples as well as the effect of Mn incorporation on the growth kinetics were investigated in detail.It was found that Mn concentrations as low as ~1% could induce dramatic changes in the RHEED diagrams and growth rate of ZnMnO as observed by in situ monitoring of the growth by reflection high energy electron diffraction as well as optical reflectivity. Indeed, a key feature is that Mn incorporation results in a clear (3x3) RHEED pattern. While 3x reconstructions are often observed after the growth of ZnO epilayers when cooling down the samples, here the reconstruction is present during the growth. This change in the surface stoichiometry is clearly related to Mn and manifests itself also on the growth rate which was measured to increase by >50% in the investigated Mn composition range. Mn incorporation has also been found to strongly modify the surface morphology of the samples (AFM) which present very smooth terraces with randomly distributed dips with hexagonal symmetry. The sample microstructure and the origin of the dips are under investigation by means of TEM. Finally, the influence of Mn incorporation on the change in electronic properties has been investigated (PL) as well as the local surrounding of the Mn ions (EPR), and high resolution X ray diffraction measurements show that both lattice parameters linearly increase with Mn content, although the a([Mn]) and c([Mn]) dependence is different.
10:15 AM - L6.3
Ferromagnertic Properties Controlled by Carrier Concentrations in Co:ZnO Thin Films on ZnO Single Crystals.
Hitoshi Tabata 1 , Hiromasa Saeki 2 , Hiroaki Matsui 2
1 School of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo, Japan, 2 ISIR-Sanken, Osaka University, Ibaraki, Osaka, Japan
Show AbstractWe reported ferromagnetic properties of ZnO:Co thin films formed on sapphire substrates.[1] Ferromagneticities of them are confirmed by SQUID, XMCD and anomalous hall effect measurements.[2, 3] Here, we have described a new growth technique realizing the simultaneous control of growth mode and ferromagnetic ordering in Co-doped ZnO layers on ZnO single crystals.[4] Remarkable changes in growth occurred as a result of doping with Co ions. Exposure to O2 plasma to generate an oxygen-rich atmosphere was indispensable for the fabrication of undoped ZnO layers given stabilization of the negatively charged growing surface in a twodimensional(2D) mode. In contrast, the 2D mode of Zn0.94Co0.06O layers was adequately retained in the absence of a plasma source, and was attributable to the covalence provided by the 3d character of Co ions compared with the 4s character of Zn ions. Spontaneous magnetization of Zn0.94Co0.06O layers was closely correlated with the number of free electrons, which was controlled by the activation energy of donor levels. This was also confirmed for O-polar Zn0.94Co0.06O layers. Ferromagnetic _FM_ ordering was observed at ne values close to the Mott transition and led to hopping conduction between shallow donor bands. However, levels of p_O2_ below 10−6 mbar that yielded FM ordering generated a pitted surface and led to crystalline deterioration. Given our understanding of the formation mechanism of pits, we applied a periodic oxygen pressure-modulated epitaxy that resulted in the coherent growth of Co-doped ZnO layers with a 2D mode. This specific growth was effective in producing pit-free surface uniformities over large areas that maintained FM ordering.1) Appl.Phys.Lett., 79 (2001) 988. 2) J.Phys.:Condens. Matter, 16 (2004) S5533.3) Phys. Rev. B, 72 (2005) 201201(R) 4) Phys. Rev. B. 75 (2007) 014438
11:00 AM - **L6.4
Magnetic Dopants and Charge Carriers in Colloidal ZnO Nanocrystals and Epitaxial ZnO Thin Films.
Daniel Gamelin 1
1 Department of Chemistry, University of Washington, Seattle, Washington, United States
Show AbstractCarrier-dopant magnetic exchange interactions in diluted magnetic semiconductors (DMSs) provide the basis for many important magneto-electronic and magneto-optical phenomena, including carrier-mediated ferromagnetism, magnetic polaron nucleation, giant excitonic Zeeman splittings, and proposed spin-based quantum information processing schemes. This talk will describe our group's recent investigations into the use of magnetic and magneto-optical techniques to probe dopant-carrier exchange interactions in ZnO DMS thin films and colloidal quantum dots. Apart from the technological interest in these materials, this research is motivated by the new fundamental insights it provides into the microscopic origins of cooperative magnetization in this class of materials.Related references:Liu, W. K.; Whitaker, K. M.; Kittilstved, K. R.; Gamelin, D. R., Stable Photogenerated Carriers in Magnetic Semiconductor Nanocrystals. J. Am. Chem. Soc. 2006, 128, 3910-3911.Kittilstved, K. R.; Schwartz, D. A.; Tuan, A. C.; Heald, S. M.; Chambers, S. A.; Gamelin, D. R. "Direct Kinetic Correlation of Carriers and Ferromagnetism in Co2+:ZnO." Phys. Rev. Lett., 2006, 97, 037203.Kittilstved, K. R.; Liu, W. K.; Gamelin, D. R. "Electronic Structure Origins of Polarity Dependent High-TC Ferromagnetism in Oxide Diluted Magnetic Semiconductors." Nature Materials, 2006, 5, 291-297.Kittilstved, K. R.; Norberg, N. S.; Gamelin, D. R. "Chemical Manipulation of 300K Ferromagnetism in ZnO Diluted Magnetic Semiconductors." Phys. Rev. Lett., 2005, 94, 149049.
11:30 AM - L6.5
Dopant Incorporation in Co:ZnO Colloidal Quantum Dots.
Petra Lommens 1 , Philippe Smet 2 , Frank Loncke 2 , Henk Vrielinck 2 , Dirk Poelman 2 , Zeger Hens 1
1 Physics and Chemistry of Nanostructures, Ghent University, Ghent Belgium, 2 Department of Solid State Sciences, Ghent University, Ghent Belgium
Show AbstractBeing a well-established technique to alter the electric and optical properties of a bulk semiconductor, doping of semiconductor nanocrystals or quantum dots has been intensively investigated over the last ten years. Nevertheless, controlled dopant incorporation in semiconductor nanocrystals remains an important issue. With colloidal quantum dots for example, dopants often end up adsorbed at the nanocrystals surface rather than incorporated in the nanocrystals core. We have performed a case study on the incorporation of Co2+ ions in ZnO quantum dots. This is an interesting system, not only because of its possible ferromagnetic properties, but also because Co2+ ions can be observed with a variety of spectroscopic techniques. The Co:ZnO colloidal nanocrystals were prepared using a colloidal synthesis route. A general characterization of the quantum dots was performed using XRD, XPS, ICP-MS and TEM, while optical spectroscopy and electron paramagnetic resonance (EPR) spectroscopy were used to investigate the Co2+ ions. We found that three different types of Co2+ ions are present in Co:ZnO nanocrystals. Firstly, optical spectroscopy confirmed that a part of the Co2+ ions substitutionally replace Zn2+ ions in the ZnO lattice. Secondly, using an amine surface treatment that detaches Co2+ ions from the Co:ZnO nanocrystals surface, we could demonstrate that about 50 % of the Co2+ ions entering synthesis end up adsorbed at the nanocrystals surface. This observation is confirmed by growing an undoped ZnO shell around the Co:ZnO core. Shell growth leads to a threefold increase of the substitutional Co2+ absorption, while no extra Co-precursor was added. This shows that shell growth leads to the incorporation of surface adsorbed Co2+ ions in the core/shell nanocrystals. Finally, EPR spectra of as synthesized Co:ZnO powders contain contributions of two different types of Co2+ ions. One can be related to substitutional Co2+. The other contribution could however not be attributed to surface adsorbed ions since neither amine treatment nor shell growth significantly changes the intensity ratio of the two contributions. This means a third type of Co2+ ions must be present in the Co:ZnO nanocrystals. Simulations of our EPR spectra show that this additional type of Co2+ ions have an octahedral coordination. We therefore suggest that these Co2+ ions actually are Co2+ interstitials.
11:45 AM - L6.6
Study of the Role of the Free Carrier Concentration on the Ferromagnetism in Co- and Mn-implanted ZnO.
Zheng Yang 1 , Leelaprasanna Mandalapu 1 , Jianlin Liu 1 , Biasini Maurizio 2 , Ward Beyermann 2
1 Department of Electrical Engineering, University of California at Riverside, Riverside, California, United States, 2 Department of Physics, University of California at Riverside, Riverside, California, United States
Show Abstract Diluted magnetic semiconductor (DMS) materials have attracted a great deal of attention for their potential application in spintronics. ZnO is attracting much attention toward this application because of its above room-temperature ferromagnetism based on theoretical calculations. The carrier-mediated ferromagnetism is one of the most widely accepted theories for DMS, in which the ferromagnetic coupling between the moments from the localized magnetic dopants is mediated by itinerant carriers. However, very few experiments have been done so far to verify this theory. In this presentation, we will show our experimental results on the role of the free carrier concentration on the ferromagnetism in ZnO DMS materials. The ZnO thin films were grown on sapphire substrates by molecular-beam epitaxy. The free electron carrier concentrations of the ZnO films were tuned by precisely-controlled doping Ga into the ZnO. The free electron carrier concentrations from 1.4×10^18 cm^-3 to 1.2×10^20 cm^-3 were achieved. The Co- and Mn-implantations were performed on the as-grown ZnO samples with different free carrier concentrations. All the samples were annealed at 900 degree C for 5 minutes after implantation to activate the implanted ions and recover the crystalline quality. The X-ray diffraction measurements show that the ZnO samples after implantation are still of very high crystalline-quality, and no Co or Mn related phase separation was detected. The secondary ion mass spectroscopy measurements show that very tiny amount of the Co/Mn atoms penetrate the ZnO thin films and reach the sapphire substrate. Even though the amount of the residual implanted ions in the substrate is very small, they have been eliminated by subtraction in the measurements. The ferromagnetic hystereses were observed in both Co- and Mn-implanted ZnO samples at room-temperature. The temperature-dependent magnetization measurements were performed on the Co- and Mn-implanted ZnO samples up to 800 K, showing far-above room-temperature ferromagnetism. The Co-ZnO samples show free carrier dependent saturation magnetizations: the larger carrier concentrations, the larger saturation magnetizations. However, when the carrier concentration reaches near 10^20 cm^-3, the saturation magnetization reaches a maximum value and saturates even if more carriers are added. This result gives an experimental support to the carrier-mediation theory of the ferromagnetism from DMS materials. Unlike Co-ZnO, the Mn-ZnO samples do not show any electron concentration dependent behavior. This is interesting since the existing theory predicts that the Mn-ZnO would be ferromagnetic only in hole-mediated heavily doped p-type ZnO. Possible reasons are discussed.
12:00 PM - **L6.7
Microscopic Properties of ZnCoO and ZnMnO: The Role of the Defects and Hydrogen Impurity in Spin-spin Interactions.
C. Park 1 , D. Chadi 2 , I. Kang 1
1 Department of Physics, Pusan National University, Pusan Korea (the Republic of), 2 , NEC Laboratories America, Princeton, New Jersey, United States
Show AbstractIt is well-known that Zn-interstitial and O-vacancy can be easily formed in ZnO. In addition, ZnO can be easily contaminated by hydrogen impurity. Theoretical calculations have indicated that their formation enthalpies are very small. Therefore it is suggested that these defects and hydrogen can lead to the natural n-type conductivity and also prevent the achievement of p-type doping. Since the concentrations can be easily high in the DMS, it is important to understand the role of defects and hydrogen in the magnetic properties of the diluted magnetic semiconductors. Many experimental data have indicated that the O-vacancy may play an important role in the ferromagnetism of ZnCoO. In this work, we investigated the role of these low-energy defects and hydrogen in the ZnO-based magnetic semiconductors (DMS) such as ZnCoO and ZnMnO, We performed the first-principles calculations using local spin density approximation (LSDA) and LSDA+U to investigate the electronic structures and the formation enthalpies of the defects and impurities. It is shown that the hydrogen can induce the strong ferromagnetic coupling between the Co ions and also between the Mn ions with the formation of a bridge bond composed of Co-3d orbitals. The Monte-Carlo calculation using Heisenberg-type artificial Hamiltonian indicates that the contamination of H in the magnetic semiconductors can induce the high-T ferromagnetism. The roles of O-vacancy and the interstitial defects in the spin-spin interaction will be discussed based on the calculational results.
12:30 PM - L6.8
Effect of Defects and Free Carriers on the Magnetic and Electrical Properties of Cu Doped ZnO Based Diluted Magnetic Semiconductor Thin Films.
Deepayan Chakraborti 1 , John Prater 2 1 , Jagdish Narayan 1
1 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 Materials Science Division, Army Research Office, Research Triangle Park, North Carolina, United States
Show AbstractThe exciting possibility of combining the charge and spin functionality of an electron and to incorporate them into novel spintronic devices like spin FETs, spin LEDs, quantum qbits, etc has led to an extensive search for materials in which semiconducting properties can be integrated with magnetic properties. Recent studies have found transition metal doped ZnO to display room temperature ferromagnetism. pbyBut there still exists a debate over the mechanism responsible for the ferromagnetic ordering in these systems. Earlier reports claimed a free carrier mediated mechanism (RKKY) to be responsible for the ordering of spins in ZnO based DMS. But more recent studies found room temperature ferromagnetism in systems with low carrier concentrations indicating that defects like oxygen vacancies play an important role in stabilizing ferromagnetic ordering. For successful fabrication of devices involving injection of spin polarized electrons it is very important to have a good understanding of the origin of ferromagnetism and also how free carriers interact with the electron spins. In this study we have grown Cu doped and Cu,Al co-doped ZnO thin films epitaxially on (0001) α-Al2O3 by pulsed laser deposition technique and annealed them in oxygen at high temperature (600 0C) to study the role of defects like oxygen vacancies on the ferromagnetic properties of these films. Cu-doped ZnO thin films not only rule out the possibility of ferromagnetism arising from extrinsic sources like magnetic secondary phases or nanoclusters, but have also been found to display very high spin polarization.1 The doping of Cu:ZnO specimens with Al changed the carrier concentration by three orders of magnitude (from 1017 to 1020 cm-3) without altering the ferromagnetic ordering. On the other hand, a reduction in the number of oxygen vacancies brought about by high temperature oxygen annealing had a large detrimental effect on the ferromagnetism reducing the total saturation magnetization by almost an order of magnitude. These results tend to rule out a free carrier mediated mechanism as being responsible for the ferromagnetism in ZnO based DMS systems.These studies point towards a defect mediated mechanism, such as a bound magnetic polaron mediated exchange as being responsible for stabilizing the ferromagnetic ordering in these systems.1 D. Chakraborti, J. Narayan and, J.T. Prater, App. Phys. Lett. 90(6),062504 (2007)
12:45 PM - L6.9
Structural and Luminescence Properties of Zn1-xCuxO Thin Films.
Kousik Samanta 1 , Pijush Bhattacharya 2 , Ram Katiyar 1
1 Physics, University of Puerto Rico, San Juan, Puerto Rico, United States, 2 Physics, Fisk University, Nashville, Tennessee, United States
Show AbstractHigh electronic conductivity, optical transparency, and direct band gap (3.37 eV) energy with stable free excitonic binding energy (60 meV) of ZnO make it the most promising multifunctional semiconducting material for the application in optoelectronics devices operating in blue and ultraviolet (UV) region. Moreover, the 3d transition metal (Co, Ni, Mn, Cu, etc.) doped ZnO can be the most promising ferromagnetic semiconductor at room temperature for spintronic applications. In this work we have carried out the structural and optical properties of PLD grown nanocrystalline thin films of Zn1-xCuxO (x = 0, 1, 3, and 5%) on Al2O3 substrate. The structural analysis of these samples was done by micro-Raman scattering studies using 514.5 nm Ar+ laser source. The Raman spectrum of Zn1-xCuxO thin films shows the characteristic normal modes of wurtzite ZnO at 98.5, 378.7, and 439 cm-1 corresponding to the E2low, A1TO, and E2high respectively, no other secondary phase was detected in our samples. The crystalline grain sizes of 1, 3, and 5% Cu doped ZnO samples were calculated by phonon confinement model as 85.2, 67.6, and 63.6 nm, respectively. The PL spectrum of Zn1-xCuxO thin films at 77 K shows the basic excitonic feature of ZnO with the splitting of free exciton peak and additional transitions at 3.159 eV and 3.09 eV due to the Cu doping. This radiative recombination belongs to the electronic transition between the valance band and the CuZn acceptor at different charge state 3d9 and 3d10 respectively. The splitting of the free exciton corresponding to Γ5 and Γ6 excitons in Zn1-xCuxO samples was due to the induced strain by Cu doping. The decrease of optical band gap from 3.276 eV to 3.263 eV with increase in Cu concentration was due to the p-d inter-valance charge transfer (IVCT) transition from p (O2-) to eg (Cu3+).
L7: P-type Doping
Session Chairs
Wednesday PM, November 28, 2007
Constitution A (Sheraton)
2:30 PM - L7.1
Study of p-type Codoped ZnO Films Grown by Pulsed Laser Deposition.
Heungsoo Kim 1 , Mike Osofsky 1 , Ray Auyeung 1 , Dave Knies 1 , Catalina Cetina 1 , Alberto Pique 1
1 Code 6364, Naval Research Laboratory, Washington, District of Columbia, United States
Show AbstractThe ability to deposit both p- and n-type ZnO thin films of high quality is essential for the development of ZnO based optoelectronic devices such as p-n homojunctions. However, it is very difficult to obtain p-type ZnO due to self-compensation from its native intrinsic defects and the low solubility of most p-type dopants. One approach to improve the solubility of the p-type dopants is to use co-doping deposition methods during film growth. Recently, at NRL, we have shown the ability to deposit thin films of Zr-N codoped p-type ZnO by pulsed laser deposition on sapphire substrates. The carrier type and conduction depend on the N2O deposition pressure and film growth temperature. The best Zr-N codoped ZnO films were grown at 500°C in 5x10-5 Torr of N2O. These films showed p-type conduction behavior with a low resistivity of 0.026 Ω-cm, a carrier concentration of 5.5x1019 cm-3, and a mobility of 4.4 cm2V-1s-1. We have investigated the use of these p-type ZnO films for making p-n heterojunctions on n-type Si substrates (p-ZnO/n-Si) as well as p-n homojunctions on sapphire substrates (p-ZnO/n-ZnO/Al2O3). In this talk, we will discuss our work to date on the growth of Zr-N codoped ZnO thin films by PLD and present electrical and optical characterization results from the various types of films deposited.This work was supported by the Office of Naval Research.
2:45 PM - L7.2
Role of Microstructure and Defects in Epitaxial p-Type ZnO Thin Films.
Xiaoqing Pan 1 , Arnold Allenic 1 , Wei Guo 1 , Yanbin Chen 1 , Yong Che 2 , Zhendong Hu 2 , Bing Liu 2
1 , The University of Michigan, Ann Arbor, Michigan, United States, 2 , IMRA America, Inc., Ann Arbor, Michigan, United States
Show AbstractThe potential of ZnO for short-wavelength optoelectronic devices is enormous and to fully exploit this potential, it is imperative to understand the roles of native and doping-induced defects. In this talk, we will focus on our understanding of the role of crystal defects and processing conditions on the optoelectronic properties of p-type ZnO epitaxial thin films. In this talk, we will show that the interplay between dopants and extended defects such as threading dislocations and stacking faults is of fundamental importance for the achievement of p-type conductivity in ZnO. ZnO thin films doped with antimony (Sb) or phosphorus (P) were grown by pulsed laser ablation. We found that dislocations can, under certain conditions, aid the formation of shallow acceptors. The role of dislocations on the dopant solubility, electrical transport properties and photoluminescence will be addressed. We will also show that depending on the oxygen chemical potential, different defect levels can be introduced in the gap. These defects result in various optical transitions which can provide useful information on the donors and acceptors formed. Finally, issues of homoepitaxy and p-n junction fabrication will be presented.
3:00 PM - L7.3
Chemical Nature of N Incorporated in ZnO During Epitaxial Film Growth.
Christian Pettenkofer 1 , Patrik Hoffmann 1 , Stefan Andres 1
1 SE6, Hahn-Meitner-Institut, Berlin Germany
Show AbstractEpitaxial ZnO films are grown by MOMBE. During film growth the samples are exposed to N2+ions with energies ranging from 5kV to 300V. In XPS the observed binding energies are those of O-N-O, N2, and N bonded to Zn. Exposing a grown film after growth to ions yields an O-N-O as the predominant species followed by N2 N-Zn is observed comparingly strong for N admission during film growth. XPS data taken with synchrotron radiation at varying kinetic energies reveal that only the N-Zn componenet is predominantly present in the bulk of the film. These results are corroborated by NEXAFS data taken at the same samples which show an increased signal for N admission during film growth. The overall doping activity of N is still supressed by a high level of hydrogen incorporated in the film. The presence of N2 species is nicely demonstrated by the vibrational features of the molcule in the NEXAFS spectrum.
3:15 PM - L7.4
P-type ZnO Prepared by Microwave Enhanced Nitrogen Plasma Diffusion.
Paris Y. Liu 1 2 , Jiping Cheng 2 , Ryuan Guo 1 2
1 Department of Electrical Engineering , The Pennsylvania State University, University Park, Pennsylvania, United States, 2 Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania, United States
Show AbstractNitrogen doped regions and predominant p-type ZnO crystals and microtubes are obtained using a microwave enhanced nitrogen plasma diffusion process. The enhanced plasma process appears to activate the nitrogen dopant and to enhance its solubility limit in ZnO crystals. An overall p-type doping is verified by Hall effect measurement conducted on the ZnO samples with plasma treated diffusion surfaces. Such nitrogen doping is found to be stable over time, in contrast to the diminishing p-type effects reported by several other groups. A detailed study on diffusion profiles, chemical binding energy determination, and other related defect mechanisms will be presented.
3:30 PM - L7.5
Electrical and Optical Properties of ZnMgO:P Films grown by Pulsed Laser Deposition.
Hyunsik Kim 1 , Stephen Pearton 1 , David Norton 1 , Fan Ren 2
1 Department of Materials Science and Engineering, University of Florida, Gainesville, Florida, United States, 2 Department of Chemical Engineering, University of Florida, Gainesville, Florida, United States
Show AbstractThe ZnMgO:P films were grown on sapphire c-plane by pulsed laser deposition with different substrate temperatures in the range of 550 ~ 650 oC. The 5 and 10 at % Mg mixed ZnMgO:P targets were used for the comparison. After the growth, rapid thermal annealing was performed at 600 ~ 900 oC under oxygen ambient. The 10 at % Mg mixed ZnMgO:P films showed p-type conductivity, while the 5 at % Mg mixed ZnMgO:P films showed only n-type conductivity at as-deposited state. However, the 5 at % Mg mixed ZnMgO:P film which showed n-type conductivity at as-deposited state converted to p-type after the post thermal annealing. ZnMgO:P films containing 10 at. % Mg showed only visible defect levels without the band edge emission, while those containing 5 at % Mg showed the band edge emission even though the defect level is dominant. The properties of these films will be described. This research was sponsored by the Department of Energy (grant DE-FC26-04NT42271)
3:45 PM - L7.6
Surface Doping of p-type ZnO Nanowires using Molecular Adsorption.
Sung-Hoon Lee 1 , Jongseob Kim 1 , Ki-Ha Hong 1 , Jaikwang Shin 1
1 , Samsung Advanced Institute of Technology, Yongin Korea (the Republic of)
Show AbstractWith a direct wide band gap and a large exciton binding energy, ZnO is considered as one of the best materials for photonic applications. Moreover, ZnO can be easily grown into nanowires or nanoribbons, allowing a rich spectrum of nanoscale engineering. Despite this high potential of ZnO nanowires, their applications have been hindered by the extreme difficulty to obtain p-type doping, mainly due to the deep valence band edge of ZnO. Here we propose a novel method of p-type doping of ZnO nanowires, exploiting the high surface to volume ratio. In our approach, p-type dopants that trap electrons from and leave holes onto semiconductor hosts are added on the surfaces of nanowires, not into the bulk as used in the conventional method. As electron-trapping surface dopants, we consider atoms or molecules having a half-filled highest occupied molecular orbitals (HOMO), since they show high electronegativity. Among the several molecules examined by first-principles atomistic calculations, we find fluorine molecules (F2) would be one of good candidates. Fluorine molecules adsorb strongly on the surfaces of ZnO nanowires and dissociate into two fluorine atoms spontaneously. Each of the adsorbed F atoms attracts an electron from valence band of the ZnO nanowire and thus leaves a hole state in the nanowire. Our calculations show that the resulting F-passivated ZnO nanowire surfaces are stable under ambient gas conditions and normal temperature ranges.
4:30 PM - L7.7
Infrared Lattice Viblations of Nitrogen-doped ZnO Thin Films.
Makoto Hirai 1 , Ashok Kumar 2 1
1 Nanomaterials and Nanomanufacturing Research Center, University of South Florida, Tampa, Florida, United States, 2 Department of Mechanical Engineering, University of South Florida, Tampa, Florida, United States
Show Abstract4:45 PM - L7.8
A Density Functional Theory Study of the Band Gap Tuning by Atomic Doping in ZnO.
Muhammad Huda 1 , Yanfa Yan 1 , Mowafak Al-Jassim 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractZnO is a potential candidate as a photoelectrode for hydrogen generation by breaking water through photo-electro-chemical process. For spontaneous generation of H2 several criteria have to be met for the ZnO to be used as a photoelectrode. These include, among others, the band edges have to be matched with the water-splitting potential, as well as the band gap needs to be around 2 eV to operate in the visible spectrum of solar energy. With this goal in mind, we will present density functional theory (DFT) and DFT+U studies of different atomic doping in ZnO and discuss their electronic properties. In particular we would consider nitrogen atom doping, both interstitial and substitutional, and discuss the relative shifting of the band edges. Several codoping situations and transition metal atom doping will also be presented. In general, band gap reduction was found as a common result due to the formation of impurity bands. In addition to this, shift of the band edges, for example valence band maxima, depend critically on the doping concentration and the type of doping.
5:00 PM - L7.9
New Theoretical Approach for Controlling the p-type Doping in ZnO.
George Gavaza 1 , Zhigen Yu 1 , Ping Wu 1
1 Computational Materials Science And Engineering , Institute for High Performance Computing, Singapore Singapore
Show AbstractSystematic realization of the p-type doped ZnO by any usual experimental technique (sputtering, ion implantation, MOCVD, etc) is problematic as the optimal "window" of experimental parameters is very narrow and difficult to determine empirically. Our first-principles studies reveal that p-type dopants such as P would be substituted at Zn or O site under the different oxygen partial pressure. The desired configuration of P substituted at O site (PO) is the potential acceptor in ZnO. We found that P would substitute at Zn site and O site under oxygen rich and poor conditions, respectively. However, with a poor oxygen growth condition, the dense oxygen vacancy would act as hole “killer” and dilute p-type conduction. Hence, we demonstrated that p-type conduction would be achieved in a narrow oxygen partial pressure window.Within the many particle scattering with rearrangements theory framework, we further calculate the probability of obtaining the required atomistic configuration by a certain physical process and in given experimental conditions. This allows us to theoretically estimate the rate of success of the doping process function of experimental parameters. Thus, we can predict the outcome of an experiment function of its parameters (type of experiment, temperature, concentrations, etc) and determine the optimal “window” for the doping process.
5:15 PM - L7.10
Photoluminescence Characterization of Defects in Nitrogen-Implanted and Thermally Annealed ZnO.
Xuemin Dai 1 , Shijie Xu 1 , Qilin Gu 1 , Francis C. C. Ling 1 , Habil Brauer 2 , Dipl.-Phys. Anwand 2 , Wolfgang Skorupa 2
1 Physics, The University of Hong Kong, Hong Kong, Hong Kong, China, 2 , Institut für Ionenstrahlphysik und Materialforschung, Dresden Germany
Show AbstractIon implantation is an important technology for modifying electrical and even optical properties of semiconductor materials. Recently, ZnO has attracted a great deal of renewed research interest due to its unique optoelectronic properties and new application potential in short-wavelength efficient light-emitting devices including laser diodes. Usually undoped ZnO exhibits N-type conductivity, which indicates the presence of native defects. It still remains a challenging issue to realize efficient P-type doping in ZnO nowadays. We thus attempt to prepare P-type ZnO by means of nitrogen implantation. In this work, we concentrate on photoluminescence characterization of defects in ZnO implanted and post-implantation annealed. ZnO samples employed in nitrogen implantation show N-type electrical conductivity and weak green emission. However, a new broad red luminescence peak was observed in all as-implanted ZnO samples, which implies that somewhat new deeper defects are induced by nitrogen implantation. Thermal treatment at different temperatures was adopted to activate the implanted ions and eliminate the implantation-induced lattice distortion. Interestingly, the defect emissions in the implanted ZnO show a strong dependence on the treatment temperature. As the treatment temperature increases, the red emission is significantly suppressed whereas the green emission greatly enhances. In particular, two sets of fine structures with a fixed energy separation of about 30 meV superimposing on the broad background were observed in the samples annealed at moderate high temperatures. The multimode Brownian oscillator (MBO) model accounting for electron-phonon coupling was employed to determine the energetic positions of defects producing the green and red emissions. Excellent agreement between theory and experiment is achieved over the entire experimental temperature range. Our experimental data favor the electronic level structure of native oxygen vacancy theoretically obtained by Janotti and Van de Walle(1) using the first principles calculation. A physical model is proposed to interpret the observed luminescence behavior of defects in ZnO.(1)Anderson Janotti and C. G. Van de Walle, Appl. Phys. Lett. 87, 122102 (2005).
5:30 PM - L7.11
Structural and Optical Properties of Silver Ion Implanted ZnO Crystals.
Lakshmanan Vanamurthy 1 , Mengbing Huang 1 , Hassaram Bakhru 1
1 College of Nanoscale science and Engineering , University AT Albany , State University of New York, Albany, New York, United States
Show AbstractA recent first-principle calculation predicted that silver could readily replace Zn in ZnO and therefore would be a good candidate for p-type acceptors in ZnO crystals (Appl. Phys. Lett. 89, 181912 (2006)). In this work, we have made an experimental investigation of the structural and optical properties of Ag doped ZnO crystals. We relied on ion implantation to incorporate Ag into ZnO crystals. The Ag ion implantation was done at 400 C using four different energies with a total Ag dose of 4X1E15 atoms/cm2 in order to form a uniformly doped thin layer of 100 nm near the surface. Post-implantation annealing was conducted in the range of 700-1100 C. Rutherford backscattering/channeling techniques were utilized to evaluate the crystal quality and the lattice location of Ag in ZnO. Ion channeling measurements along both [0001] and [1011] directions show that the Ag-implanted ZnO crystal starts to recover around 800 C and following post-implantation annealing at 1000 C, its crystal quality is comparable to that of the as-grown ZnO single crystal. Our results suggest that the majority of implanted Ag does not appear to occupy the Zn substitutional site. Even in the case for the 1000 C anneal, the percent of implanted Ag cannot exceed 30%. Furthermore, the optical properties of Ag-implanted ZnO were studied with photoluminescence measurements at liquid helium and room temperatures. The presence of Ag results in dramatic quenching of photoluminescence lines typically found for as-grown ZnO crystals including the green luminescence band and various exciton-associated transitions. Photoluminescence signals suggested for shallow Ag acceptors in ZnO are not detected. These experimental evidence implies that additional mechanisms, possibly related to interactions between Ag and defects in ZnO, may need to be included in theoretical modeling of Ag doping in ZnO crystals.
5:45 PM - L7.12
Comprehensive Review on Doping Mechanism of Group I, III and V Elements into ZnO Thin Films.
Sang Yeol Lee 1
1 Center for Energy Materials, Korea Institute of Science and Technology, Seoul Korea (the Republic of)
Show AbstractZnO thin films have been fabricated by pulsed laser deposition. Doping mechanism of ZnO thin films has been investigated using group I, III and V elements, such as Ag, Ga, P, and As. Two different doping mechanism has been observed with simple model and complex model. Ag and Ga have been easily doped into ZnO resulting in the change of carrier concentration by simple doping model. As has been observed to be doped into ZnO by complex doping model. Structural, electrical and optical properties have been investigated and well agreed with simple and complex doping model.
L8: Poster Session
Session Chairs
Irina Buyanova
Chennupati Jagadish
David Norton
Gyu-Chul Yi
Thursday AM, November 29, 2007
Exhibition Hall D (Hynes)
9:00 PM - L8.10
Fabrication of ZnO Nanoarrays.
PilHo Huh 1 , Fadong Yan 1 , Lian Li 1 , Jayant Kumar 1 , Ravi Mosurkal 2 , Lynne Samuelson 2 , Myunghwan Kim 3
1 Center for Advanced Materials, Departments of Chemistry and Physics, University of Massachusetts Lowell, Lowell, Massachusetts, United States, 2 , U.S. Army Natick Soldier Center RDEC, Natick, Massachusetts, United States, 3 , Samsung Electronics Semiconductor Divison, Suwon Korea (the Republic of)
Show Abstract9:00 PM - L8.11
Raman Spectroscopy of Different Morphologies of ZnO Nanostructures.
Aurangzeb Khan 1 2 , Wojciech Jadwisienczak 3 , Martin Kordesch 1
1 Physics & Astronomy and CMSS Program, Ohio University, athens, Ohio, United States, 2 Physics, University of Peshawar, Peshawar, NWFP, Pakistan, 3 School of Electrical Engineering and Computer Science, Ohio University, athens, Ohio, United States
Show Abstract9:00 PM - L8.12
Luminescence of Er-doped ZnO Films: Effects of Thermal Annealing and Doping Concentration.
Zhengda Pan 1 , S. Morgan 1 , A. Ueda 1 , R. Aga Jr. 1 , A. Steigerwald 1 , R. Mu 1
1 , Fisk University, Nashville, Tennessee, United States
Show AbstractPhotoluminescence (PL) of Erbium-doped Zinc Oxide films with nano-sized grains was studied. The films were grown on fused silica substrates using e-beam evaporation. The evaporating targets used were sintered pellets of ZnO and Er2O3 mixtures with different Er concentration. The films were subsequently annealed at 700 °C in air for an hour. PL was measured at two excitation wavelengths, 325 and 488 nm. The 325 nm is used for exciting the host semiconductor ZnO and 488 nm is used for directly exciting Er3+ ions in the ZnO host. Strong Er3+ luminescence of 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions was observed from annealed film with 4.5 % Er2O3 concentration using either 325 or 488 nm excitation. With 325 nm excitation, the Er3+ luminescence observed is attributed to energy transfer from the excitons in ZnO host to the Er3+ ions doped. The effective energy transfer from ZnO host to the doped Er3+ ions is an essential property for the realization of actual current-injection opto-electronic devices operating at wavelengths of Er3+ emission, for example, at 1.54 μm for the erbium-doped fiber amplifier (EDFA). Our PL results indicate that thermal annealing played an important role for optically activating the doped Er3+ ions in ZnO nano-crystalline grains of the film. This research is supported by US National Science Foundation NSF-CREST- CA: HRD-0420516, NSF-STC CLiPS - grant no. 0423914, and US Department of Defense (DOD)/ARO contracts: W911NF-05-1-0453, and W911NF-04-1-0400.
9:00 PM - L8.13
Characterizing Solid-State Reactions of ZnO Nano-rods.
Joysurya Basu 1 , Divakar Ramachandran 3 , Jessica Riesterer 1 2 , C. Carter 1
1 Chemical, Materials Science & Biomolecular Engineering, University of Connecticut, Storrs, Connecticut, United States, 3 Metallurgy & Materials Group, Indira Gandhi Centre for Atomic Research, Kalpakkam India, 2 Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractZnO is becoming an increasingly important material and is in the forefront of solid-oxide fuel cell and photovoltaic cell development. ZnO nano-rods can be grown on reconstructed c-plane (0001) sapphire via hydrothermal synthesis. Alignment of the nano-rods with the sapphire substrate is ensured by prior deposition of a c-plane oriented ZnO thin-film by pulsed-laser deposition (PLD). Electron backscatter diffraction (EBSD) confirms orientation relationships observed with the TEM between the nano-rods and the sapphire substrate and has the advantage of requiring less sample preparation. Working with TEM-ready sapphire substrates, specimens suited for a direct comparison of interfacial reaction between the sapphire substrate and the ZnO in nano-rod and thin-film forms have been designed. In situ and ex situ TEM may both be carried out in order to understand the reaction kinetics. In situ studies have shown differences in the effect of heat-treatment on the microstructure in the vicinity of the nanorods as compared to the thin-film regions without the nano-rods.
9:00 PM - L8.14
Zinc Vacancies and Oxygen Vacancies in an Electron-Irradiated ZnO Crystal.
Sean Evans 1 , Nancy Giles 1 , Larry Halliburton 1 , Lawrence Kappers 2
1 Dept. of Physics, West Virginia University, Morgantown, West Virginia, United States, 2 Dept. of Physics, University of Connecticut, Storrs, Connecticut, United States
Show AbstractA bulk n-type zinc oxide crystal grown by the seeded chemical vapor transport method was irradiated near room temperature with 1.5 MeV electrons from a Van de Graaff accelerator. Before the irradiation, the primary defects in the crystal were shallow Group III donors and deep Fe2+ donors. The crystal remained n type after the irradiation, but the concentration of carriers at room temperature decreased from 7 x 1016 to 3 x 1013 cm-3. Electron paramagnetic resonance (EPR) at 30 K was used to monitor zinc vacancies, oxygen vacancies, and Fe3+ ions after the irradiation. Because of long spin-lattice relaxation times, these EPR spectra were greatly enhanced by operating the EPR spectrometer in an out-of-phase mode [1]. After the electron irradiation, but before illumination, Fe3+ ions and nonaxial singly ionized zinc vacancies were observed. The Fe3+ ions were observed at this stage because the electron irradiation significantly lowered the Fermi level (the presence of these Fe3+ ions was critical to our study as they provided deep traps to hold photoexcited electrons when the crystal was subsequently exposed to light at low temperature). Illumination with 325 nm light at low temperature (30 K) destroyed the EPR signal from the Fe3+ ions and simultaneously produced EPR spectra from singly ionized oxygen vacancies, neutral zinc vacancies, and axial zinc vacancies, as electrons were pumped from the lower lying vacancy states to the higher lying Fe3+ state. The 325 nm light also produced two previously unreported EPR spectra from zinc vacancies that had an adjacent hydrogen (in the form of an OH- ion) in axial and basal positions. The observation of forbidden EPR transitions (ΔmS = ±1, ΔmI = ±1) provided direct evidence that a proton was associated with these zinc vacancies. Using a lamp and monochromator, the response at 30 K of the irradiated crystal to illumination wavelengths out to 750 nm was determined. Wavelengths shorter than 580 to 600 nm convert the Fe3+ ions to Fe2+ ions and convert the neutral oxygen vacancies to singly ionized oxygen vacancies. Warming above 130 K in the dark reverses the effect of the illuminations. This observation of a threshold near 2.1 eV for light to produce singly ionized oxygen vacancies places the ground state of the neutral oxygen vacancy less than 1.3 eV above the valence band. For illumination wavelengths shorter than 480 to 500 nm, an additional excitation path emerges. These higher energy photons pumped electrons to the conduction band from isolated singly ionized zinc vacancies, and thus formed neutral zinc vacancies. This threshold near 2.5 eV places the ground state of the singly ionized zinc vacancy approximately 0.85 eV above the valence band. This work was supported by NSF Grant DMR-0508140. One of the authors (SME) acknowledges support from the West Virginia STEM Fellowship Program. [1] J. R. Harbridge et al., Journal of Magnetic Resonance 156, 41 (2002).
9:00 PM - L8.15
The ZnO/Zn3N2 Material System Investigated by Soft x-ray Emission and Absorption Spectroscopy: Impact of Air-Exposure on the Chemical and Electronic Surface Properties.
Marcus Bar 1 , Lothar Weinhardt 1 , Clemens Heske 1 , Kwang-Soon Ahn 2 , Yanfa Yan 2 , Mowafak Al-Jassim 2 , Oliver Fuchs 3 , Monika Blum 3 , Jonathan Denlinger 4
1 Chemistry, University of Nevada, Las Vegas, Las Vegas, Nevada, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States, 3 Exp. Physik II, Universität Würzburg, Würzburg Germany, 4 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show Abstract9:00 PM - L8.16
Microwave-assisted Aqueous Synthesis of c-axis Oriented ZnO Nanorods on ZnO Film Coated Glass Substrates.
Ken-ichi Ogata 1 , Kazuto Koike 1 , Shigehiko Sasa 1 , Masataka Inoue 1 , Mitsuaki Yano 1
1 , Osaka Institute of Technology, Osaka Japan
Show AbstractZnO nanorod structures are of great interest because of their unique properties as well as large surface area that enhance the performance of sensing devices. Toward the realization of high sensitive bioFETs based on ZnO/ZnMgO heterostructure grown by molecular beam epitaxy [1], fabrication of ZnO nanorods on FET structures in order to increase the number of immobilized enzymes is one of the key issues.In this contribution, microwave-assisted aqueous synthesis, which has lots of advantages such as less energy consumptions and rapid heating, was employed for the fabrication of c-axis oriented ZnO nanorods.For the ZnO nanorod growth, teflon bottles with autoclavable caps were filled with an equimolar (1-100mM) aqueous solution of zinc nitrate hexahydrate (Zn(NO3)2 6H2O) and hexamethylenetetramine (C6H12N4). Subsequently, boronsilicate glass substrates with ZnO films coated by means of RF magnetron sputtering at room-temperature were immersed in the bottles and heated at about 95oC for 3h via irradiation of 2.45GHz microwave. ZnO-coated glass substrates were placed with the bottom-side up configuration to prevent from undesirable precipitation. X-ray diffraction measurement of the as-sputtered ZnO films indicated that they were c-axis oriented, but that the average grain size of ZnO was smaller than 50nm. Therefore, in prior to the aqueous synthesis, thermal annealing was conducted at 500oC in air atmosphere for 1h. Top view SEM observation revealed that hexagonal nanorods were formed on the ZnO-coated glass substrates; however, their average diameter and orientation were dependent on the properties of base ZnO films. Namely, on the thermally annealed ZnO films, highly c-axis oriented ZnO nanorods with a diameter of about 100nm were grown, while tilted nanorods with much larger diameter were obtained on the as-sputtered films. Although detailed mechanism of those differences is not clarified yet, properties of underlying ZnO films strongly affect the synthesis of ZnO nanorods on them. Effects of concentration of source materials in the solution will be also discussed.[1] K. Koike, D. Takagi, M. Kawasaki, Y. Nakamura, Y. Hirano, S. Sasa, M. Inoue, M. Yano, The 13th International Conference on II-VI Compounds, 10p-17, Jeju, Korea, Sep. 10-14, 2007.
9:00 PM - L8.18
Comparison of Gas Sensing Performance of Thin Film ZnO Chemiresistors and ZnO-based TFTs.
Yoonsil Jin 1 , George Whitfield 1 , Harry Tuller 1
1 Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States
Show AbstractMetal oxide semiconductors such as SnO2, ZnO, and WO3 have been extensively studied as thin or thick film chemoresistive gas sensors owing to their high sensitivity, stability and low cost. The large band gaps (>3eV) of these metal oxides allows them to operate at elevated temperature (200-500degree), a key advantage for solid state gas sensors, given the need to accelerate gas chemisorption kinetics. Recently, there has been growing interest in ZnO-based thin film transistors due to their transparency, relatively high mobility and low processing temperatures, e.g.[ ]. In this study, we report on the relative response of thin film ZnO chemiresistors and ZnO-based thin film transistors to a variety of gases including NOx, CO, O2 under controlled temperature to examine the possible advantage of the FET structure for gas sensing. Bottom gate ZnO TFTs, with variable ZnO channel thickness, are prepared to aid in the analysis of the combined effects of chemisorption and electrical bias on the device I-V characteristics.
9:00 PM - L8.2
UV-Enhanced Photo Response and Quantum Efficiency of PLD Grown n-ZnO/p-Si Photodiode.
E. Senthil Kumar 1 2 , Mamidanna Rao 1 2
1 Department of Physics, Indian Institute of Technology, Chennai, Tamil Nadu, India, 2 Materials Science Research Centre, Indian institute of Technology Madras, Chennai, Tamil Nadu, India
Show Abstract9:00 PM - L8.20
An NO2 Gas Sensor of Sol-gel Synthesized Zinc Oxide Nano-rods.
Hong Yeol Lee 1 , Seong Eon Moon 1 , Eun Kyong Kim 1 , So Jeong Park 1 , Jun Hyuk Kwak 1 , Kang Ho Park 1 , Jong Dae Kim 1
1 , Electronics and Telecommunications Research Institute (ETRI), Daejeon Korea (the Republic of)
Show AbstractWe present a NO2 gas sensor have been fabricated using a novel zinc oxide nano-rods synthesized by the sol-gel process on an inter-digitated electrodes and their electrical transport properties and gas sensor properties have been measured. EDAX, XRD, and TEM were used in order to do constituent analysis and the structure of being grown up of the ZnO NRs. Moreover, the I-V characteristic was measured with varying the measuring temperature to acquire the temperature dependant device resistance. The reactivity experiment about the NO2 gas was performed so that it could confirm whether an application was possible as the gas sensor or not. The result of the I-V measurement revealed that good ohmic contact was formed between electrodes and NRs, and that resistance decreased with increases in measurement temperature In the case of the reactivity experiment about NO2 gas, the sensitivity of a sensor linearly increased according to the concentration of NO2 gas.
9:00 PM - L8.21
ZnO Nanowire-based Logic Circuits: NOT, NAND, and NOR Logic Gates.
Kihyun Keem 1 , Jeongmin Kang 1 , Dong-Young Jeong 1 , Changjoon Yoon 1 , Donghyuk Yeom 1 , Sangsig Kim 1 2
1 Electrical Engineering, Korea University, Seoul Korea (the Republic of), 2 , Institute for Nano scicence, Seoul Korea (the Republic of)
Show AbstractZnO nanowires have been particularly interested in the application of the nanodevices due to their high crystalline quality and simple synthetic process. These oxide nanowires have been utilized as the channels of nanodevices including field-effect transistors (FETs) and logic gates. In this study, inverter logic circuits (or NOT logic gates) were built with two top-gate ZnO nanowire-based FETs with Al2O3 gate layers in series on a chip; each of the top-gate FETs was fabricated by a conventional Si processing, and its electrical characteristics were evaluated by the general electrical measurements. Their output to input voltage characteristics exhibited the inverting operation. The logic swing of their voltage transfer characteristics was 98%, and their transition width was 4.35 V. Furthermore, NAND and NOR logic circuits were built by fabricating three top-gate FETs on a chip, and their operation characteristics corresponded to the truth tables.
9:00 PM - L8.24
Coating Characteristics of Aligned ZnO Nanorods with ZnS:Mn Layers by Chemical Vapor Deposition Cooperated with Laser Ablation of Mn.
Takashi Hirate 1 , Kenta Kakio 1 , Tomomasa Satoh 1
1 Department of Electronics and Informatics Frontiers, Faculty of Engineering, Yokohama Japan
Show AbstractZnO 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 aligned ZnO nanorods by a low-pressure thermal chemical vapor deposition (CVD) method cooperated with laser ablation of Mn pellet. In this paper, we report on coating characteristics of the surface of aligned ZnO nanorods with ZnS:Mn layers by CVD method, intending to develop a new electroluminescent device including ZnO nanorods.The fabrication method of ZnO nanorods 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 Mn pellet 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 pressure is 26.6 Pa, the temperature is 550 C and O2 mass flow rate is 0.88 SCCM. Aligned ZnO nanorods with 100 nm diameter and 1700 nm height are grown for 10 min growth-time. After the growth of ZnO nanorods, only the laser ablation of Mn is performed for 5 min without O2. We call this process as intermediate laser ablation process. Finally, ZnS:Mn are grown for 10 min by changing the precursor to H2S. The pressure is from 67 Pa to 133 Pa, and the mass flow rate of H2S is from 2.0 to 4.0 SCCM.It is found that the intermediate laser ablation of Mn has a drastic effect on growth of ZnS:Mn on the surface of ZnO nanorods. When the intermediate laser ablation is not performed, the ZnS:Mn grows mainly along the axis-direction of ZnO nanorod with little growth along the lateral direction, i.e. the length of nanorods becomes longer by the length of ZnS:Mn with little growth of the diameter. However, when the intermediate laser ablation is performed, the growth along the lateral direction becomes dominant, while the growth rate along the axis-direction is not so changed. For example, when the mass flow rate of H2S is 2 SCCM and pressure is 67 Pa, the total length of nanorod is 3600 nm and diameter is 100 nm when the intermediate laser ablation is not performed. On the other hand, when the intermediate laser ablation is performed the total length of nanorod is 3000 nm and diameter is 540 nm. It is concluded that the intermediate laser ablation has a drastic effect to enhance the growth rate of ZnS:Mn for the lateral direction of ZnO nanorods. We estimate that the Mn species that are ablated from a Mn pellet and reaches the surface of ZnO nanorods change any quality of the surface of ZnO nanorods. We are now studying of finding the optimum growth condition of ZnS:Mn for electroluminescent devices.
9:00 PM - L8.25
ZnO-based Electroluminescent Devices Using Molecular Precursor Solutions.
Kaori Yoshioka 1 , Shinichi Egawa 1 , Toshiaki Kobayashi 1 , Yoshihiro Mashiyama 1 , Hirohisa Nomura 2 , Mitsunobu Sato 2 , Tohru Honda 1
1 Electronic Engineering, Kogakuin University, Tokyo Japan, 2 Coordination Engineering Labortory, Kogakuin University, Tokyo Japan
Show Abstract Zinc oxide (ZnO) is one of the candidates for light-emitting devices operating in the UV spectral region [1]. Its spin-coating enables the fabrication of low-cost and large-scale devices. In our previous work, ZnO films fabricated by Sol-Gel processing were investigated [2]. Here, the possibility of ZnO films fabricated by spin-coating technique was confirmed. Although the near-band-edge (NBE) emission was dominated, the deep-level emission band (peak wavelength: 650nm) was observed. For the reduction of the deep level emission band, precise control of process condition will be required. In this paper, the fabrication of ZnO films using molecular precursor solu-tions is reported. The molecular precursor method has an advantage for doping other metal ions and control of the crystal orientation [3]. Furthermore, ZnO-based electroluminescent devices (ELDs) are also reported. UV light emission was observed from the devices. A solution was prepared by the reaction of Zn nitrilotriacetic acid (Zn-nta) complex. It was ob-tained from the reacted aqueous solution of nitrilotriacetic acid with zinc acetate, with butylamine in ethanol. Also, prepared by the reaction of Ga ethylenediaminetetraacetic acid (Ga-edta) complex, then, which doped Zn-nta with zinc acetate, with butylamine in ethanol. The solutions include Ga impuriti-es in this study. ZnO:Ga clear solution was spin-coated on substrate. It is annealed in O2 atmosphere. ZnO-based ELDs were fabricated using the molecular precursor solutions. A double-insulator struc-ture was adapted for the ELDs. The ELDs were operated using a pulsed voltage. The applied voltage was created using a rectangular shape pulse and a transformer. A purplish white emission including UV-light emission was observed form the device during the operation at RT. Using the optimized conditions, the ZnO-based ELDs were fabricated. A purplish white emission including UV-light emission was observed from the device under the pulsed operation. This means that the ZnO film fabricated by the molecular precursor method have a possibility for the fabrication of cost-effective ELDs.[1] D. C. Look, Mater. Sci. Eng. B. 80, 383 (2001).[2] K. Yoshioka, S. Egawa, T. Kobayashi, T. Baba, K. Sugimoto, M. Arai, H. Nomura, M. Sato and T. Honda, physica status solidi (c) 4, No.1, 162-165 (2007). [3] H. Nomura, C. S. Mochizuki, I. Takano, T. Honda and M. Sato, 13TH International SPACC-CSJ symposium, P05.
9:00 PM - L8.26
Synthesis and Characterization of Sisal-like ZnO Nanostructures via Low-temperature Simple Solution Process.
Rizwan Wahab 1 , Sg Ansari 1 , Young soon Kim 1 , Song Min Woo 1 , Hyung Kee Seo 1 , Hyung-Shik Shin 1
1 School of Chemical Engineering , Chonbuk National University, Jeonju, Chollabukto, Korea (the Republic of)
Show Abstract9:00 PM - L8.27
Low Resistivity Gallium-doped Zinc Oxide Films Deposited by Low Temperature Radio-frequency Magnetron Sputtering.
Jiun-Yi Tseng 1 , Cheng-Yi Wang 2 , Wang-Chieh Yu 1 , Chih-Wei Hsu 1 , Chao-Jen Ho 1 , Yung-Fu Hsu 2 , Sea-Fue Wang 2 , Tzer-Shen Lin 1
1 Material and Chemical Research Laboratories, Industrial Technology Research Institute, Chutung, Hsinchu, Taiwan, 2 Institute of Materials Science and Engineering, National Taipei University of Technology, Taipei Taiwan
Show Abstract9:00 PM - L8.28
Nonlinear I-V Characteristics of ZnO Films.
Jung-Wook Lim 1 , Jun Kwan Kim 1 , Sun Jin Yun 1 , Hyun Tak Kim 1
1 Tera-electronic device team, ETRI, Deajeon Korea (the Republic of)
Show Abstract9:00 PM - L8.29
ZnO Thin Films Fabricated by Plasma Enhanced Atomic Layer Deposition.
Doo Lee 1 , Sae Kwon 1 , Young Park 1 , Bo Chung 1 , Hee Kim 1 , Jun Hyun 1 , Yongmin Kim 1 , Seung Noh 1
1 Applied Physics, Dankook University, Seoul Korea (the Republic of)
Show Abstract9:00 PM - L8.3
Indium Zinc Oxide Transparent Thin Film Transistors with MgO Gate Oxide.
W. Lim 1 , H. Kim 1 , Y. Wang 1 , D. Norton 1 , Stephen Pearton 1 , F. Ren 2 , I. Kravchenko 3 , J. Zavada 4
1 Materials Science, University of Florida, Gainesville, Florida, United States, 2 Chemical Engineering, University of Florida, Gainesville, Florida, United States, 3 Physics, University of Florida, Gainesville, Florida, United States, 4 Electronics Division, US Army Research Office, Research Triangle Park, North Carolina, United States
Show Abstract Transparent depletion-mode amorphous indium zinc oxide (IZO) channel thin film transistors with 1 μm gate length and 100 μm gate width were fabricated on glass substrates from layers deposited at room temperature using rf magnetron sputtering. The n-type carrier concentration in the channel was 2x1016 cm-3. The gate oxide was 90 nm of MgO, deposited by pulsed laser deposition at room temperature. The threshold voltage of the transistors was -1.6V with a gate-voltage swing of 1.55 V/decade and the drain current on-to-off ratio was >103.The maximum field effect mobility in the channel was ~0.3 cm2.V-1.s-1, lower than the Hall mobility of ~6.5 cm2.V-1.s-1 in the same layers, suggesting the presence of scattering due to trapped charges at the MgO-IZO interface. The addition of the gate and source/drain contacts caused a relatively small decrease in transmittance of the structure at visible wavelengths.
9:00 PM - L8.30
Direct Fabrication of ZnO Whiskers Bridging Between Micron-gap Electrodes in Aqueous Solution for Highly Gas Sensing.
Hattori Reiko 1 , Imamoto Hiroshi 1 , Kametani Keisuke 2 , Fujita Shizuo 2
1 Advanced Device Lab., OMRON Co.Ltd, Kizugawa Japan, 2 International Innovation Center, Kyoto University, Kyoto Japan
Show AbstractZinc oxide (ZnO) has been conventionally used in the field of gas sensors such as H2 and CO. High sensitive gas sensors have been increasingly expected for safety as monitoring devices of living environment and chemical processes. In recent years, there are also many reports about the fabrication and characteristics of ZnO gas sensors composed of nanostructures as a sensing area owing to their high surface-to-volume ratio. But, it is actually difficult to control the nanostructures placing at the desired positions. We propose an approach for artificially fabricating the ZnO whisker gas sensor between micron-gap electrodes using electrolytic deposition in order to realize the high sensitivity for gases. This method can be safe and easy to handle because of adopting an aqueous solution. The factors effecting the formation of bridging the ZnO whiskers were studied by the observation with a scanning electron microscope equipped with energy dispersive X-ray spectroscopy and then characteristics of H2 gas response as well. Hydrogen has been focused on much attention for fuel cell application, but the high sensitive sensing is essential necessary against the slight leakage of the flammable gas.Micron-gap electrodes (Au/Cr), used both for the sensor device and the plating electrodes, were fabricated by conventional photolithography. In order to optimize the reagent for the electrodepotion, several reagents were examined such as zinc nitrite, zinc acetate, and so on. The concentration of reagent solution was varied from 0.005 to 0. 5 mol/L. The AC bias voltage was applied. The values of voltage and frequency were varied from 1 to 15V and from 100 Hz to 100 kHz, respectively. An additional oxidation process of the deposits was continuously conducted using dry or wet process, if desired. From the analyses of morphology and composition, metal zinc is preferentially electrodeposited under the condition of higher ion concentration of Zn2+, lower frequency, no solvent additive and higher voltage. After oxidation, a typical sample with resistance level in megaohm range was found to have no significant improvement in gas sensitivity. However, under the condition of lower ion concentration, higher frequency, oxidant addition, lower voltage, ZnO is directly deposited without the successive oxidation. This sample with resistance level in gigaohm range is found to respond well for H2 gas of 100 ppm even at room temperature.We propose the simple but convenient method of ZnO fabrication selectively and artificially between two micron-gap electrodes using the electrolytic deposition in aqueous solution. The growth condition with oxidant addition makes it possible that one ZnO whisker bridging between two electrodes is directly electrodeposited. The electrodes for deposition can be diverted to the contact electrodes for sensors in this process. These advantages such as the technological simplicity can facilitate the realization of high sensitive gas sensor for H2 gas.
9:00 PM - L8.31
Optical Quenching of NiO/Ni coated ZnO Nanowires.
Young Park 1 , Yong Shin 1 , Doo Lee 1 , Hee Kim 1 , Seung Noh 1 , Jun Hyun 1 , Yongmin Kim 1 , Ki An 2
1 Applied Physics, Dankook University, Seoul Korea (the Republic of), 2 Chemical Technology, Korea Research, Seoul Korea (the Republic of)
Show Abstract9:00 PM - L8.32
The Charge Sheet Density in Non-polar ZnO Films at the ZnO-Sapphire Interface Determined by Generalized Ellipsometry.
Ch. Sturm 1 , T. Chavdarov 1 , R. Schmidt-Grund 1 , B. Rheinlaender 1 , C. Bundesmann 3 , H. Hochmuth 1 , M. Lorenz 1 , M. Schubert 2 , M. Grundmann 1
1 Institut für Experimentelle Physik II, Semiconductor Physics, Universtät Leipzig, Leipzig Germany, 3 , Leibniz-Institut für Oberflächenmodifizierung e.V., Leipzig Germany, 2 Department of Electrical Engineering, University of Nebraska, Lincoln, Nebraska, United States
Show AbstractZnO is a wide band gap semiconductor and promising material for ultraviolet optoelectronic devices, due to its properties such as the exciton binding energy (Eexb ~ 60 meV) and band gap energy (Eg ~ 3.4 eV). Of special interests are non-polar films since they avoid electric fields at the interfaces, which will influence the performance of the devices. The infrared dielectric function of non-polar a-plane ZnO was determined by C. Bundesmann et al. [1], however, a detailed study of the interface with the substrate was not given. In this work we will report about the investigation of the charge sheet density at the ZnO-sapphire-interface in non-polar a-plane ZnO films.
Two series of a-plane ZnO films were deposited by Pulsed Laser Deposition on r-plane sapphire substrates at constant growth temperature TG=730°C. The first series contains samples with different film thicknesses in the range (30 – 610) nm, whereas, the samples of the second series have nearly the same film thickness of d ~ 240 nm. The samples of the second series were annealed after deposition at TG up to 105 min. All samples were investigated by generalized infrared spectroscopic ellipsometry (gIRSE) in the spectral range (370 – 1300) cm-1.
The dielectric function of the ZnO films was obtained by a line shape analysis using a model dielectric function, which consists of contributions of lattice vibrations and free charge carriers. H. v. Wenckstern [2] reported on diffusion of aluminium (Al) atoms from the sapphire substrates into the ZnO film, which act as donors in the ZnO film. They change the electrical and the infrared-optical properties of the ZnO film near the ZnO-sapphire interface. Hence, an inhomogenous distribution of the free charge carriers was assumed with concentration (n) at the ZnO-sapphire interface larger than in the rest of the ZnO film, which is below the gIRSE detection limit of n ~ 5 × 1016 cm-3. Due to the small thickness (d) of the highly conductive part of the ZnO film, only the product σ=n×d could be determined, representing a charge sheet density. For the first series, σ increases with increasing film thickness, which is related to a longer growth time. Hence this behaviour can be interpreted as an effect of time, e.g., the diffusion of aluminium from sapphire into ZnO. However, σ of the second series shows an opposite behaviour compared to the first series: σ decreases with increasing annealing time. Responsible for the decrease of σ can be the diffusion of Al atoms from the highly conductive part into the main part of the ZnO film, which could not be detected, or the formation of a zinc aluminate layer. The latter one was observed by several groups [3, 4].
1. C. Bundesmann et al., Thin Solid Films 455, 041301 (2004).
2. H. v. Wenckstern et al., Advances in Solid State Physics 45, 263 (2005).
3. Gorla et al., J. Appl. Phys. 87, 3736 (2000).
4. Fan et al., Nature Materials 5, 627 (2006).
9:00 PM - L8.33
Gas-Phase Synthesis and Characterization of ZnO Nanoparticles from a Microwave Flow Reactor.
Klemens Hitzbleck 1 , Matthias Offer 2 , Hartmut Wiggers 1 , Christof Schulz 1
1 Institute for Combustion and Gasdynamics, University Duisburg-Essen, Duisburg Germany, 2 Institute of Physics, University Duisburg-Essen, Duisburg Germany
Show AbstractNano-sized ZnO is a very promising candidate for future nano-technological applications such as varistors, UV-emitting devices, TCOs, transistors or gas sensors. A lot of the unique optical and electrical properties of ZnO nanoparticles originate from the high surface-to-volume ratio which varies with the size. Therefore the synthesis of ZnO particles with tuneable size plays an important role for scientific research. In this study the formation and size dependent properties of ZnO nanoparticles in dependency of different reaction parameters were investigated.The particles were synthesized in a microwave flow reactor by decomposition of Diethylzinc in an argon-oxygen plasma. The precursor material was diluted in argon using a bubbler system and supplied to the reactor via a central nozzle. Additional argon and oxygen were injected trough a surrounding ring nozzle. For argon / oxygen ratios smaller 5:1 a small, discrete plasma torch was formed at the tip of the central nozzle. For higher ratios a diffuse, voluminous plasma downstream the reactor tube was observed. The best results with respect to reaction rate and particle formation were achieved at a ratio of about 3:1. For lower ratios an increasing percentage of the precursor material was lost due to CVD at the reactor walls. Hardly any particles could be sampled under the conditions of the diffuse plasma.A particle laden molecular beam extracted from the reactor was deflected in a particle mass spectrometer to receive the size distribution of the synthesized particles. Depending on the synthesis parameters, the mean particle size was found to be in the range of 2 to 7 nm. Furthermore, the molecular beam device was used for the deposition of size-selected particles for further analysis. X-Ray diffraction of the product reveals the hexagonal crystalline structure of ZnO. The crystallite size, calculated from Scherrer’s equation, was found to match the particle size determined before. The crystalline structure turned out not to depend on the argon / oxygen ratio in the discrete plasma. PL-spectroscopy of the as-prepared nanoparticles exhibits strong photoluminescence in the visible spectrum. The spectra show two broad emission bands, a near bandgap excitonic luminescence in the range of 375 to 440 nm, and a broad defect-luminescence in the range of 500 to 750 nm. They indicate a dependency of the emission band intensity on the argon / oxygen ratio during synthesis, which is under investigation.
9:00 PM - L8.34
Characteristics of Zinc Oxide and Gallium Doped Zinc Oxide Thin Film Transistors Fabricated at Room Temperature by Radio Frequency Magnetron Sputtering.
Hoonha Jeon 1 , Ved Prakash Verma 2 , Sookhyun Hwang 1 , Kyoungseok Noh 1 , Sooyeon Lee 1 , Chiyoung Park 1 , Joonhee Moon 1 , Jaekyu Kim 1 , Dohyun Kim 1 , Wonbong Choi 2 , Minhyon Jeon 1
1 Department of Nano Systems Engineering, Inje University, Gimhae Korea (the Republic of), 2 Department of Mechanical and Materials Engineering, Florida International university, Maimi, Florida, United States
Show Abstract9:00 PM - L8.4
Indium Zinc Oxide Thin Films Deposited by Sputtering at Room Temperature.
W. Lim 1 , Y. Wang 1 , F. Ren 2 , D. Norton 1 , I. Kravchenko 3 , J. Zavada 4 , Stephen Pearton 1
1 Materials Science, University of Florida, Gainesville, Florida, United States, 2 Chemical Engineering, University of Florida, Gainesville, Florida, United States, 3 Physics, University of Florida, Gainesville, Florida, United States, 4 Electronics Division, US Army Research Office, Research Triangle Park, North Carolina, United States
Show Abstract The deposition of amorphous indium zinc oxide (IZO) thin films on glass substrates with n-type carrier concentrations between 1014 and 3x1020 cm-3 by sputtering from single targets near room temperature was investigated as a function of power and process pressure. The resistivity of the films with In/Zn of~ 0.7 could be controlled between 5x10-3-104 Ω.cm by varying the power during deposition. The corresponding electron mobilities were 4-18 cm2.V-1.s-1.The surface root-mean-square roughness was < 1nm under all conditions for film thicknesses of 200 nm. Thin film transistors with 1 µm gate length were fabricated on these IZO layers, showing enhancement mode operation with good pitch-off characteristics, threshold voltage 2.5V and a maximum transconductance of 6 mS/mm. These films look promising for transparent thin film transistor applications.
9:00 PM - L8.6
Room Temperature Deposited Indium Zinc Oxide Thin-Film Transistors.
Y. Wang 1 , F. Ren 2 , W. Lim 1 , D. Norton 1 , Stephen Pearton 1 , I. Kravchenko 3 , J. Zavada 4
1 Materials Science, University of Florida, Gainesville, Florida, United States, 2 Chemical Engineering, University of Florida, Gainesville, Florida, United States, 3 Physics, University of Florida, Gainesville, Florida, United States, 4 Electronics Division, US Army Research Office, Research Triangle Park, North Carolina, United States
Show Abstract Depletion-mode indium zinc oxide (IZO) channel thin film transistors were fabricated on glass substrates from layers deposited at room temperature using rf magnetron sputtering. The threshold voltage was in the range -5.5 to -6.5V depending on gate dielectric (SiO2) thickness and the drain current on-to-off ratio was ~105.The maximum field effect mobility in the channel was ~4.5 cm2.V-1.s-1, lower than the Hall mobility of ~17 cm2.V-1.s-1 in the same layers, suggesting a strong influence of scattering due to trapped charges at the SiO2-IZO interface. The low deposition and processing temperatures may these devices suitable for applications requiring flexible substrates.
9:00 PM - L8.7
Structural and Electrical Properties of Amorphous InGaZnO4 and GaZnO2 Sputtered Thin Films.
Martin Kordesch 1 , Mohammad Ebdah 1
1 Department of Physics and Astronomy, Ohio University, Athens, Ohio, United States
Show AbstractAmorphous InGaZnO4 (a-IGZO) thin films have been recently of great interest due to its flexibility, optical, and transport properties. The incorporation of a-IGZO in device applications such as thin-film transistors (TFTs) has been reported in the literature. In this work, a-IGZO films were successfully fabricated using rf sputtering technique with a sputtering target of polycrystalline In, Ga, and Zn species under the flow of Oxygen and deposited on Si substrate. The structure has been investigated by the x-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive x-rays (EDX) spectroscopy techniques. Multiple crystallographic phases are detected. The ZnO phase has been identified for both non-annealed and 500 C annealed thin films. The existence of Gallium Oxide (Ga2O3) and Indium Oxide (In2O3) phases is under investigation. Characterization of the electrical conductivity, concentration of free carriers, energy gap and optical properties are included within this study. The same study has been made again for amorphous thin films after excluding the Indium due to the formation of Indium islands on the surface of a-IGZO films during the growth, namely for amorphous GaZnO2 (a-GZO) thin films. The corresponding properties and phases have been compared to the findings of other growth conditions of both sputtered films.
9:00 PM - L8.8
Application of ZnO to Passivate the GaN-based Device Structures.
Eliana Kaminska 1 , Anna Piotrowska 1 , Marie-Antoinette di Forte Poisson 2 , Christian Brylinski 2 , Hacene Lahreche 3 , Norbert Kwietniewski 1 , Iwona Pasternak 1 , Piotr Boguslawski 4 , Elzbieta Dynowska 4 , Renata Kruszka 1 , Zuzanna Sidor 1 , Witold Rzodkiewicz 1
1 , Institute of Electron Technology, Warsaw Poland, 2 , Thales-Alcatel III-V Lab., Marcoussis France, 3 , Picogiga, Courtaboeuf France, 4 , Institute of Physics, Polish Academy of Sciences, Warsaw Poland
Show AbstractThe same crystalline structure with a close lattice match, similar band gaps, and thermal stability of ZnO and GaN or other III-nitrides have previously been used in fabrication of ZnO/GaN hybrid substrates and heterojunctions. Here, we propose to farther exploit these features, and use highly resistive ZnO as passivating coating for GaN-based heterostructures. In fact, GaN-based heterostructures are intensively studied owing to their excellent properties for applications in high power/high frequency electronics. While high output power devices have been demonstrated, one of the key unsettled issues is that of large leakage currents in Schottky gates fabricated on GaN and AlGaN surfaces, seriously limiting their performance. It is believed that the dominant leakage mechanism is the tunnelling via donor states (nitrogen vacancy or oxygen impurity) introduced near the semiconductor surface during surface processing of devices [1]. Various surface treatments, such as N2 plasma exposure and/or passivation with dielectric films have been investigated [2]. A novel approach to solve this problem, presented in our work, relies on application of semi-insulating ZnO as passivating lattice-matched coating. AlGaN/GaN structures were grown by MOCVD and MBE. The processing towards fabrication of Schottky barriers and high electron mobility transistors (HEMTs) involved mesa etching, ohmic contacts fabrication and deposition of Ir/Au Schottky gates. Passivating ZnO was deposited by magnetron sputtering. Since nominally undoped ZnO exhibits n-type conductivity we apply additional acceptor doping to achieve semi-insulating material. Finally thin silicon nitride film was deposited to protect ZnO from humidity. Characterisation techniques such as VASE ellipsometry, AFM, current-voltage-temperature, and capacitance-voltage measurements were used to optimise the passivation procedure. Analysis of ZnO/GaN interfaces performed with SIMS and XRD shows their pseudomorphic microstructure.We demonstrate that ZnO-passivation suppresses leakage currents in Schottky diodes by more than one order of magnitude and substantially improves output characteristics of HEMT. Possible mechanisms of surface passivation are discussed. Part of the research was supported by the grant from the European Commission HYPHEN (IST contract n°027455), and by national grants from the Ministry of Science and HigherEducation N50703131/0743 and 72/E-67/SN-033/2006.[1]. J. Kotani et al., J. Vac. Sci. Technol. B 24, 2148 (2006).[2]. P. Kordos et al., Semicond. Sci. Technol. 21, 1592 (2006).
Symposium Organizers
David P. Norton University of Florida
Chennupati Jagadish Australian National University
Irina Buyanova Linkping University
Gyu-Chul Yi Pohang University of Science and Technology (POSTECH)
L9: Epitaxial Growth of ZnO
Session Chairs
Thursday AM, November 29, 2007
Constitution A (Sheraton)
9:30 AM - **L9.1
Growth and Characterisation of ZnO.
Bruno Meyer 1
1 , 1. Physikalisches Institut, Giessen Germany
Show AbstractIn order to realize controlled p-type doping in ZnO it is absolutely necessary to control and understand the role of point and extended defects in the epitaxial films. This starts with the choice of the substrate since for sapphire and GaN on sapphire substrates there is a severe contamination by diffusion of the group III elements in the ZnO films. However, also the choice of a ZnO substrate matters. In the first part of the talk we address to the properties of the ZnO substrates in terms of crystallinity, surface morphology and preparation, polarity etc. Secondly, we report on the homoepitaxial growth and on the structural, electrical and optical properties of the films. In the last part the recombinations caused by excitons bound to donors and/or acceptors will be discussed.
10:00 AM - L9.2
Effect of Growth Conditions on Defect-related Photoluminescence in ZnO Thin Films Grown by Plasma Assisted MBE.
Vitaliy Avrutin 1 , Mikhail Reshchikov 2 , Natalia Izyumskaya 1 , Ryoko Shimada 1 , Hadis Morkoc 1 2
1 Electrical Engineering, Virginia Commonwealth University, Richmond, Virginia, United States, 2 Physics Department, Virginia Commonwealth University, Richmond, Virginia, United States
Show AbstractZnO has attracted a considerable interest owing to its outstanding optical properties. Therefore, it comes as no surprise that a great number of articles have been dedicated to investigation of photoluminescence (PL) in this material. Most studies focus on the excitonic spectral range, whereas much less attention has been paid to defect-related luminescence from ZnO. However, investigation of emission bands (usually classified as yellow-green and red) and their relation with growth conditions can shed some light on the nature of defects responsible for these bands and pave the way to control the defect composition of ZnO that is believed to be a key to successful p-type doping. In this contribution, we report on systematic study of the effect of growth conditions on defect-related PL in ZnO thin films fabricated by plasma-assisted molecular beam epitaxy on a-plane sapphire and GaN/c-sapphire substrates. As confirmed by reflection high-energy electron diffraction, all the samples discussed in this work were grown in two-dimensional mode. The layers were grown at different substrate temperatures and different oxygen-plasma powers. In case of GaN templates on c-plane sapphire, one set of samples was grown with employing low-temperature (LT) buffer layer and another set was fabricated without LT buffer. It was found that the layers grown with the LT buffer show pronounced red PL band in addition to the yellow-green one, in contrast to ZnO films grown without LT buffers, whose defect-related spectra are dominated by the yellow-green emission. PL spectra from layers grown on a-sapphire substrates show only yellow-green line. To study possible contribution of plasma damage caused by high-energy ions into defect formation, plasma power was varied in a wide range. The intensity of excitonic lines was found to increase with reducing plasma power for both types of substrates, whereas the yellow-green emission band red shifts towards the energy position (about 2.2 eV) typical for PL spectra measured from ZnO samples grown using H2O2 oxygen source. This finding indicates that point defects and their complexes induced by plasma irradiation may have a drastic impact on the optical and electronic properties MBE-grown ZnO .
10:15 AM - L9.3
Growth and Doping of ZnO.
Stefan Lautenschlaeger 1 , Joachim Sann 1 , Niklas Volbers 1 , Jan Stehr 1 , Felizitas Eylert 1 , Andreas Laufer 1 , Bruno Meyer 1
1 1st Physics Institute, University of Giessen, Giessen, Hessen, Germany
Show AbstractThe so far not reliably resolved acceptor doping of ZnO is clearly the main obstacle for the successful development of working devices based on ZnO. On the other hand an n- type doping with high carrier densities is required as well.We report on the possibilities of both, acceptor and donor doping of ZnO. To achieve the incorporation of an acceptor we used the group V element arsenic. Donors have been introduced using Aluminium and different group VII elements. We investigated mainly epitaxially grown thin films, some comparison with Ion- implanted samples have been undertaken. All the samples have been analyzed using photoluminescence spectroscopy, secondary ion mass spectrometry, atomic force microscopy and scanning electron microscopy.
10:30 AM - L9.4
ZnO Epitaxy on (111) Si Substrates Using Intervening Epitaxial Lu2O3 Buffer Layers.
Wei Guo 1 , Wei Tian 2 , Arnold Allenic 1 , Yanbin Chen 1 , Carolina Adamo 2 , Darrell Schlom 2 , Xiaoqing Pan 1
1 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Materials Science and Engineering, Pennsylvania State University, University Park, Pennsylvania, United States
Show AbstractZnO epitaxial films are widely explored for optoelectronic applications. Besides the commonly used sapphire substrate, it is significant to grow epitaxial ZnO films on Si substrates for integration of ZnO-based multi-functional devices into Si electronics. However, direct growth of ZnO on Si usually results in polycrystalline or textured films due to the large lattice and thermal expansion mismatches. Here we report on the growth and characterization of high quality epitaxial ZnO films on (111) Si using intervening epitaxial Lu2O3 buffer layers. The epitaxial Lu2O3 thin films were grown by reactive molecular-beam epitaxy and the ZnO films were subsequently grown using pulsed laser deposition. X-ray diffraction shows that the epitaxial orientation relationships are (0001)ZnO||(111)Lu2O3||(111)Si and [11-20]ZnO||[1-10]Lu2O3||[1-10]Si. Our ZnO epilayers have the narrowest (0002) and (10-12) ω-rocking curves ever reported, for a growth on Si, with full width at half-maximum values of 374 arcsec and 565 arcsec. The films are conductive with a mobility of 80 cm2/Vs. Temperature-dependent photoluminescence spectra show sharp transitions, with three free exciton and five bound exciton recombination lines. Cross-sectional and high-resolution transmission electron microscopy studies will be presented. In summary, the epitaxial growth of ZnO on Si represents a significant step toward the integration of ZnO-based optoelectronic devices and complementary metal-oxide-semiconductor devices on Si.
10:45 AM - L9.5
Growth and Characterization of Non-Polar ZnO Epitaxial Films on γ-LiAlO2 Substrates.
Jih-Jen Wu 1 , Wan Hsien Lin 1 , Sian-Jhang Lin 1 , Mitch M.C. Chou 2 , Liuwen Chang 2 , Hsiao-Yi Chung 2 , Teng-Hsing Huang 2 , Chien-Chih Tao 3 , Chun-Wei Chen 3
1 Department of Chemical Engineering, National Cheng Kung University, Tainan Taiwan, 2 Department of Materials Science & Opto-electronic Engineering, National Sun Yat-Sen University, Kaohsiung Taiwan, 3 Department of Materials Science and Engineering, National Taiwan University, Taipei Taiwan
Show AbstractM-plan ZnO epitaxial films have been successfully grown on γ-LiAlO2 substrates using metalorganic chemical vapor deposition. The dependence of growth characteristics on the growth temperature was investigated. The (10-10) nonpolar ZnO films of uniform surface were obtained when the growth temperature was higher than 650oC whereas both m- and c- plane ZnO crystals were formed on the LiAlO2 substrates at lower temperatures. TEM analyses reveal that the m-plane ZnO films were grown on the LiAlO2 substrates by the lattice-match growth mechanism. The micro-PL spectrum of the m-plane ZnO film shows a strong UV emission peaking at around 375 nm. We believed that LiAlO2 single crystal substrate is a potential material to nonpolar ZnO epitaxial growth.
11:30 AM - L9.6
Properties of ZnO Thin Films Grown Homoepitaxially by Pulsed-laser Deposition.
Holger vonWenckstern 1 , H. Schmidt 1 , M. Brandt 1 , C. Czekalla 1 , G. Benndorf 1 , G. Biehne 1 , A. Rahm 1 , H. Hochmuth 1 , M. Lorenz 1 , M. Grundmann 1
1 Institut für Experimentelle Physik II, Semiconductor Physics, Universtät Leipzig, Fakultät für Physik und Geowissenschaften, Leipzig Germany
Show AbstractWe report the structural, morphological, optical, and electrical properties of ZnO thin films grown homoepitaxially by pulsed-laser deposition (PLD). Two-dimensional growth and excellent material properties open the path to homoepitaxial devices. Prior to epitaxy the (000-1) oriented hydrothermal substrates were annealed for two hours in oxygen (850 mbar) at 1000°C. This is indispensable in order to obtain atomically flat or vicinal surfaces (depending on the substrate miscut) perfectly suited for homoepitaxy [1]. A sample series with undoped layers of nominal thickness between 8 nm and 1 µm was used to study the evolution of growth. Already for 8 nm tick films the step edges bend most probably due to pinning by dislocations emerging from the substrates. For thickness exceeding 100 nm, closed loop spirals with step heights of c or c/2 between the loops determine the morphology of the thin films. Nominally undoped 1 µm thick ZnO films were deposited at an oxygen partial pressure of 0.016 mbar at growth temperatures TG ranging from 470°C to 700°C. For temperatures above 600°C smooth surfaces with steps of c/2 are visible. Photoluminescence (PL) measurements (2 K) reveal recombination of free (XA, XB) and bound excitons (I0, I3a, I6, I6a) [1]. The intensity ratio of I6 (3.3608 eV) and I3a (3.365 eV) decreases with decreasing TG. Thermal admittance spectroscopy on an undoped sample grown at 650°C shows three prominent defect levels labelled T1, T2 and T3. The thermal activation energy of T1 is 56(5) meV; it is attributed to AlZn in accordance with the observation of the I6 line in the PL measurements. The concentration of AlZn is only in the 1013 cm-3 range [1]. Due to superposition the levels T2 and T3 cannot be evaluated by Arrhenius analysis, however, their activation energies are estimated in the range from 200 meV to 400 meV.Phosphorous (P-content: 0.01 wt.% - 4 wt.%) doped ZnO thin films were grown by PLD at 650°C. Two-dimensional growth is achieved for the lowest P-content and an oxygen partial pressure of 0.1 mbar. As-grown ZnO:P thin films have I3a and I6 as main recombination features. Their transport properties are comparable to that of commercially available ZnO wafers grown by seeded chemical deposition or pressurized melt growth (µH,max = 800cm2/Vs @ 70 K). The activation of the phosphorus dopants by thermal annealing will be discussed in this contribution.
[1]H. von Wenckstern, H. Schmidt, C. Hanisch, M. Brandt, C. Czekalla, G. Benndorf, G. Biehne, A. Rahm, H. Hochmuth, M. Lorenz, and M. Grundmann, phys.stat.sol. (RRL) 1, No. 4, 129-131 (2007).
11:45 AM - L9.7
New Method of Reducing Threading Dislocation in Epitaxial ZnO Films Grown on C-sapphire Substrates.
Yuekui Sun 1 , David Cherns 1 , Rachel Doherty 2
1 H.H.Wills Laboratory, Physics Department, University of Bristol, Bristol United Kingdom, 2 Chemistry Department, University of Bristol, Bristol United Kingdom
Show AbstractWe present a transmission electron microscopy study of threading dislocations (TDs) in epitaxial ZnO films grown on (0001) sapphire substrates. The films were produced by a two-step method. Firstly, ZnO was deposited by pulsed laser deposition (PLD) on (0001) sapphire substrates at temperature of 600 °C. This gave an epitaxial deposit comprising a continuous buffer ZnO, with thickness about 80 nm and a predominant alignment of (0001)ZnO//(0001)sapphire and [11-20]ZnO//[10-10]sapphire. On the top of buffer layer there was a high density of c-aligned nanorods, which revealed few, if any, TDs, in contrast with the buffer layer where TD density is about 1011/cm2. A further layer of ZnO was then grown under conditions favouring nanorods growth. This was carried out using either chemical vapor deposition in a furnace tube, where ZnO and graphite powders acted as source materials heated to 1000 °C, or hydrothermal deposition, where the PLD pre-deposited samples were put into an aqueous solution of Zn(NO3)2 and HMT for 6 hours at 90 °C. In both cases the nanorods grew laterally and eventually coalesced to form a continuous overgrowth. It was found that the nanorods remained mostly free of dislocations, but that some subgrain boundary dislocations were generated as the nanorods coalesced. Some of these dislocations were subsequently migrated laterally, leading to reactions where many were annihilated. The result was a substantial reduction in the total TD density, which was estimated to be around 109/cm2, two orders of magnitudes down from that in the initial buffer layer. The mechanism by which TDs are reduced during the two-step growth will be illustrated and explained. The wider significance of this method for reducing the density of TDs in epitaxial growth will also be discussed.
12:00 PM - L9.8
A Theoretical Study of Ultra-Thin Films with the Wurtzite and Zinc Blende Structures.
Frederik Claeyssens 1 , Colin Freeman 2 , Neil Allan 1 , John Harding 2
1 School of Chemistry, University of Bristol, Bristol United Kingdom, 2 Department of Engineering Materials, University of Sheffield, Sheffield United Kingdom
Show AbstractThe structure and composition of the substrate often controls the structure of materials grown on them, particularly for ultra-thin films. The interfacial energy between the growing film and the substrate (often determined by matching the structures) dominates the effect of the bulk cohesive energy of the film. Thus the film may initially grow in a phase that is not the thermodynamically stable bulk phase. However, once the film is thick enough, it transforms to the stable phase. An example of this is Stranski-Krastanov growth; where the initial layers of the film are strained to produce epitaxial matching, but the film ultimately grows as islands when the bulk cohesive term dominates. This type of behaviour assumes that there is an easy transition from the initial metastable state to the final stable state at a given film thickness. This need not be the case. If there is no low-energy pathway to the more stable state, the system may become kinetically trapped. This enables crystals to grow in unusual, metastable structures or in unexpected directions.One good example of this behaviour is the growth of polar directions for wurtzite structures on ceramic substrates, e.g. ZnO.[1,2] These produce high-energy surfaces because the macroscopic dipole generated in such a direction must be quenched by massive electronic or ionic reconstruction. Such surfaces are rarely observed for that reason. We show how such directions can initially be favoured for thin enough films and become kinetically trapped beyond a certain film thickness. In this study we demonstrate, using density functional theory calculations, that the stable crystal structure in ultra-thin films of ZnO is a low-energy graphitic-like structure, which initially stabilizes the growth in the (001) direction. This is equivalent to the polar direction in the wurtzite structure and explains why this high-energy face is seen in grown films. Additionally, we confirm this behaviour in other common wurtzite and zinc blende materials, indicating a general mechanism for these crystal structures.[1] C.L Freeman et al., Physical Review Letters 96 (2006), 066102[2] F. Claeyssens et al., Journal of Materials Chemistry, 15 (2005): 139-148
12:15 PM - L9.9
Growth Mechanism and Raman Spectroscopy Analysis of Solution Derived ZnO Films.
Sang Yoon 1 , Dan Liu 1 , Chiwon Kang 1 , Minseo Park 2 , Dong-Joo Kim 1
1 Materials Engineering, Auburn University, Auburn, Alabama, United States, 2 Physics, Auburn University, Auburn, Alabama, United States
Show AbstractChemical solution deposition (CSD) has been employed for the fabrication of piezoelectric ZnO films. For the application to acoustic wave device, the degree of c-axis preferred orientation of ZnO films is a crucial material issue to produce piezoelectric property of the film. In this study, the effect of polymeric sol precursors modified by two different additives, Monoethanolamine (MEA) and Diethanolamine (DEA) has been focused. The phase transformation, texture evolution, and internal stress and defects of solution derived ZnO films were systematically investigated. The XRD results showed that no textured structure was found in ZnO films on amorphous silicon nitride substrates from the solution modified with DEA, while ZnO films from the solution with MEA exhibited strong (002) oriented structure regardless of the substrate types. The results are in good agreement with the results of Raman spectroscopy. The growth of ZnO films from the solution modified with DEA is more dependent on the surface configuration of substrates than that of those with MEA, which indicates different energy barriers for the nucleation and growth of films depending on the solution precursors. The result of Raman spectroscopy also showed the higher internal stress and carrier concentration of ZnO films from DEA-modified solution, comparing with ZnO films with MEA. The internal stress and defect of solution derived ZnO films will be discussed along with the effects of the substrate and the solution additives.
12:30 PM - L9.10
Growth of High Quality (Zn,Mg)O Thick Films on ZnO Sybstrates by Liquie Phase Eptaxy.
Jun Kobayashi 1 2 , Hideyuki Sekiwa 1 2 , Miyuki Miyamoto 1 , Isao Sakaguchi 2 , Naoki Ohashi 2 , Hajime Haneda 3
1 Tokyo Laboratory, Mitsubishi Gas Chemical Co., Inc., Tokyo Japan, 2 Optoelectronics Group, National Institute for Materials Science, Tsukuba, Ibaraki, Japan, 3 Sensor Materials Center, National Institute for Materials Science, Tsukuba, Ibaraki, Jamaica
Show AbstractRecently, wide band gap semiconductors are considered to be very important materials for optoelectronic devices, such as transparent transistors and short wavelength light emitting diodes (LEDs). To utilize ZnO based optoelectronic devices, band engineering, designing and controlling band gap energy and lattice parameters, is one of the most important key technologies. Band engineering is absolutely necessary for fabrication of double hetero structures and multi-quantum-well structure in LED devices. It is well-known that making (Zn,Mg)O alloy (ZMO) enables us to expand band-gap of ZnO, and the double hetero structure such as n-ZMO/i-ZnO/p-ZMO is regarded as the most desirable one to achieve LEDs made from ZnO. From this viewpoint, synthesis of ZMO wafers having very high crystallinity, sufficiently large wafer diameter for industrial applications, and well-controlled optical and electric properties is an indispensable technology for development of ZnO based optoelectronic devices. In the present study, we examined growth of ZMO films on ZnO single crystalline substrate by a liquid phase epitaxy (LPE) technique. Since ZnO single crystal wafers more than two inches diameter have been commercially available, LPE growth of ZMO on ZnO wafers is one of the most appropriate way to produce (Zn,Mg)O layer for device applications. Thus, we studied LPE growth of ZMO layer on ZnO wafers and electric and optical properties of ZMO layers grown by the LPE method. We used single crystalline c(+)-ZnO wafers, 10 × 10 × 0.5 mm3, as the substrates for LPE growth of ZMO layers. The substrate surface was put into the molten precursor including ZnO and MgO, and the ZMO layers were grown on ZnO substrate. The obtained ZMO layers were single crystalline layer. Subsequent compositional analysis and photoluminescence measurements revealed that the highest Mg/(Zn+Mg) ratio in the LPE grown ZMO reached to more than 10% and the near band edge luminescence was found at 354 nm, corresponding to photon energy of 3.5 eV. This Mg concentration is close to the solubility limit of Mg in ZnO lattice under equilibrium condition. We also examined carrier injection into the LPE grown ZMO film by Al-doping. Here, Al-doping was done by adding Al2O3 into the precursor for LPE growth. Since obvious increase of electron concentration corresponding to Al-concentration was observed and temperature dependence of hall coefficient measurement indicated that activation energy of mobile electron in Al-doped ZMO film was several tens meV, Al-doping is found to be an appropriate way to introduce very shallow donor into ZMO films. Thus, we can conclude that growth of ZMO layer on ZnO and controlling of band gap up to 3.5 eV and electron concentration in the ZMO layer are possible by applying LPE technique.
L10: Issues with Optical and Electronic Devices
Session Chairs
Thursday PM, November 29, 2007
Constitution A (Sheraton)
2:30 PM - **L10.1
Electrical Pumped ZnO UV Random Lasers.
S. Yu 1
1 School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue Singapore
Show AbstractUltraviolet semiconductor laser diodes find enormous applications in commercial products (e.g., ultrahigh-density storage in CDs and DVDs), scientific research (e.g. low-cost activated biological or chemical sensors) as well as military applications (e.g., portable on-site detector for natural or human caused epidemics). ZnO has a band-gap of 3.37 eV and a large exciton binding energy of 60 meV at room temperature. Furthermore, ultraviolet lasing has demonstrated from mirrorless ZnO thin films with random media at room and evaluated temperature. Hence, it is believed that ZnO semiconductor is one of the most promising candidates to realize ultraviolet laser diodes. However, the practical application of ZnO random media as the active layer in semiconductor laser diodes is still a major challenge due to the problems of 1) achieving high optical gain under electrical excitation and 2) high scattering loss from the random media. In this presentation, we will discuss on our recent development of the fabrication of high-crystal-quality ZnO films which are suitable for the realization of device-quality ultraviolet light emitting devices. We will also demonstrate the fabrication of hetero-junction light emitting diodes with ultraviolet emission by our filtered cathodic vacuum arc technique. Hence, the approach to realize ultraviolet ZnO random hetero-junction laser diodes will be explained. Furthermore, we will study the possible methods to enhance the internal and external conversion efficiency of the ZnO random media as the active layer. It is believed that our proposed modification of ZnO random media is suitable to realize practical room-temperature operated ultraviolet laser diodes.
3:00 PM - L10.2
Exciton-Polaritons in All-oxide ZnO-based Resonator Structures.
R. Schmidt-Grund 1 , J. Sellmann 1 , C. Sturm 1 , C. Czekalla 1 , B. Rheinlaender 1 , J. Lenzner 1 , H. Hochmuth 1 , G. Zimmermann 1 , M. Lorenz 1 , M. Grundmann 1
1 Institut für Experimentelle Physik II, Semiconductor Physics, Universtät Leipzig, Leipzig Germany
Show AbstractBose-Einstein condensation of exciton-polaritons in semiconductor micro-cavities at room temperature is an attractive subject of current research. By using the optical emission from such a Bose-Einstein condensate, future lasers with a very low threshold can be realised. ZnO-based micro-cavity resonators are promising systems for Bose-Einstein condensation of micro-cavity exciton-polaritons at room temperature and above, owing to the large binding energy (60 meV) and the high oscillator strength of the free excitons in ZnO. For ZnO-based resonators, exciton-polaritons are expected to be stable up to temperatures of about 560 K and huge values of the vacuum Rabi splitting of about 190 meV at room temperature are predicted.
We report on recent results towards Bose-Einstein condensation of exciton-polaritons in ZnO-based all-oxide resonator structures. The resonators consist of a half-wavelength thick ZnO cavity, which acts simultaneously as active medium, and which is embedded between lower and upper Bragg reflectors. High band-gap oxide materials are appropriate candidates for Bragg reflectors operating in the ultraviolet spectral range because of their wide band-gap, i.e. transparency at the ZnO band-gap, and variety in the refractive index. We have studied Bragg reflectors consisting of ZrO2 as high-index material and MgO respective Al2O3 as low-index material.
For optimising the properties (crystal structure, surface respective interface properties, homogeneity of the layer thickness, reproducibility of the materials dielectric functions) of the cavity and Bragg reflector layers with regard to their application in resonators, we have varied the PLD (pulsed laser deposition) growth conditions and the crystallographic orientation of the used substrates [(0001)- and (1-102)-oriented sapphire, and (100) oriented silicon]. The single layers, Bragg reflectors and resonator structures have been structurally and optically characterised by means of X-ray diffraction measurements, atomic force microscopy, secondary electron microscopy, spectroscopic ellipsometry, reflection measurements, and measurements of the photoluminescence.
All-oxide Bragg mirrors with very low surface roughness, Ra=0.46 nm after 10.5 pairs, and very high reflectivity of 99.8% near the the ZnO band egde have been fabricated. We have observed strong coupling of exciton and photon modes in ZnO-based resonators. Here, the photon modes are the first order cavity mode and two Bragg band-edge modes. The exciton-polariton modes have been observed in both, photoluminescence and reflectivity spectra at temperatures between 4 K and 306 K and for various incidence and exit angles of the light at room temperature. We have observed very large energy splitting of the exciton-polariton branches of about 75 meV in maximum, stable at room temperature.
3:15 PM - L10.3
Suppression of the Internal Piezoelectric Field in ZnO/Zn0.7Mg0.3O Quantum Wells by Oxygen Ion Implantation.
Jeffrey Davis 1 , Lap Van Dao 1 , Xiaoming Wen 1 , Peter Hannaford 1 , Victoria Coleman 2 , Hoe Tan 2 , Chennupati Jagadish 2
1 Centre for Atom Optics and Ultrafast Spectroscopy, Swinburne University of Technology, Hawthorn, Victoria, Australia, 2 Department of Electronic Materials Engineering, Research School of Physical Sciences and Engineering, The Australian National University, Canberra, Australian Capital Territory, Australia
Show AbstractThe strong piezoelectric field in ZnO/ZnMgO quantum wells means that the quantum-confined Stark effect can counteract quantum confinement effects in larger quantum wells and can lead to transition energies up to 500 meV below the ZnO exciton bandgap. Implantation with low energy oxygen ions and subsequent rapid thermal annealing at 800°C induces intermixing of Zn and Mg atoms. Large blue-shifts are observed as a result, and can be explained by a combination of three mechanisms: the increased Mg content in the QW region changing the inherent band gap; the change in the confinement potential due to the change in the shape and depth of the well; and as is reported here, the change in the internal electric field reducing the quantum-confined Stark shift.By studying the photoluminescence and time-resolved photoluminescence from ZnO/ZnMgO quantum wells of different width and with varying ion implantation doses we observe that even with the weakest implantation dose (5×1014 cm-2) the internal electric field is substantially reduced. In the 4 nm quantum well, where quantum-confined Stark effects dominate over confinement effects, the depth of the QW and the Mg content in the middle of the well is largely unchanged by the weak implantation dose. It might therefore be expected that the blue-shift observed is smaller than for the narrower wells. However, the interface between the barrier and the QW is smoothed which leads to a major reduction in the strain and hence the piezoelectric field. The ensuing suppression of the quantum-confined Stark shift leads to a blue-shift in the optical transition, which is greater than that observed in narrower wells where quantum confinement effects dominate. Higher implantation doses lead to further blue shifts of similar magnitude largely independent of well width and are largely due to changes in the band gap as a result of the changed Mg concentration. Similar behaviour is also observed for exciton life× for the weakest implantation dose the lifetime decreases from 35 ns to 10.8 ns, whilst for higher doses further reductions in the lifetime are small, down to 9.2 ns at 1×1016 cm-2, and are due to the decreased confinement. This suggests that decreases in the exciton lifetimes are mostly due to the reduction of the piezoelectric field, caused by the intermixing, and the associated increased overlap of electron and hole wave functions.Hence, by carefully controlling the implantation dose it becomes possible to suppress the internal piezoelectric field in ZnO/ZnMgO quantum wells without significantly altering the QW structure. This will be of great advantage to applications where the internal field provides a barrier to greater optical gain such as ZnO-based QW lasing, and is also of significance for GaN based structures where the strong piezoelectric field may also be suppressed by intermixing.
3:30 PM - L10.4
Quantum Hall Effect in ZnO.
Akira Ohtomo 1 , Atsushi Tsukazaki 1 , Tomohiro Kita 2 3 , Yuzo Ohno 3 , Hideo Ohno 2 3 , Masashi Kawasaki 1 4
1 Institute for Materials Research, Tohoku University, Sendai Japan, 2 ERATO Semiconductor Spintronics Project, JST, Tokyo Japan, 3 Laboratory for Nanoelectronics and Spintronics, Research Institute of Electrical Communication, Tohoku University, Sendai Japan, 4 CREST, JST, Toky Japan
Show AbstractZnO exhibits high electron mobility at room temperature, especially the highest among oxide semiconductors. This leads to some interests for extracting its potential to such applications as high electron mobility transistor. Intrinsic electronic properties superior to bulk single crystals have been obtained in ZnO/MgxZn1-xO heterostructures grown by pulsed-laser deposition [1]. Recently, we were able to observe the quantum Hall effect in a high-mobility (~5000 cm2/(Vs) at 1 K) two-dimensional electron gas (2DEG) in the polar (0001) interface [2]. The density of 2DEG was controlled in a range of 0.7 ~ 3.7x1012 cm-2 by tuning polarization mismatch between the ZnO and MgxZn1-xO layers, i.e., the Mg content in the barriers, epitaxial strain and the growth polarity. From the temperature dependence of the amplitude of Shubnikov-de Haas oscillations (SdHOs), the effective mass of the 2DEG was derived as 0.32 ± 0.03m0. We found anomalous behavior in the 1/B period of the SdHOs suggesting possibility of a spin-splitting effect arising from the asymmetric triangular well and a strong internal electric field. Furthermore we applied gate electric field or optical excitation by using an ultraviolet laser to the heterostructures. This resulted in a significant influence on the conductivity demonstrating insulator-to-metal transition in its temperature dependence and switching of a quantum state.[1] A. Tsukazaki, A. Ohtomo, M. Kawasaki, Appl. Phys. Lett. 88, 152106 (2006).[2] A. Tsukazaki, A. Ohtomo, T. Kita, Y. Ohno, H. Ohno, M. Kawasaki, Science 315, 1388 (2007).
3:45 PM - L10:IssOptic
BREAK
4:15 PM - **L10.5
Control of Crystal Polarity of ZnO Layers and Applications to Nonlinear Optical Devices.
T. Minegishi 1 , J. Park 1 , Takafumi Yao 1 2
1 Center for Interdisciplinary Research, Tohoku University, Sendai Japan, 2 Institute for Materials Research, Tohoku University, Sendai Japan
Show AbstractSince the exciton binding energy of ZnO is as large as 60 meV, exciton survives even at room temperature. Hence, exciton plays an important role in optical processes in ZnO both at weak and high excitation. Photoluminescence spectra of both ZnO layers and bulk crystal with reasonable crystalline quality are dominated by free exciton emission, which cannot be realized in other semiconducors. Hence, optically-pumped exciton lasing becomes possible even at higher temperatures than room temperature. Exciton is characterized by enhanced oscillator strength and nonlinear optical effects of materials become enhanced as the oscillator strength of the associated optical processes is increased. Therefore, nonlinear optical effects of ZnO will be greatly strengthened, if exciton resonance is included in the optical processes. This paper will introduce our recent work towards this end.We have established the selective growth technique of Zn-polar and O-polar ZnO layers on sapphire substrate using MgO buffer. The crystal polarity of ZnO layers are successfully controlled by varying the thickness of MgO buffer. Very recently, we have found that ZnO layers grown on CrN/sapphire show Zn polar, while those grown on Cr oxides/sapphire result in O-polar.By combining this technique with photolithography technique, we have fabricated periodically-polarity inverted (PPT) structures with periodicity ranging from 60 μm to 0.5 μm. Such PPI structures enable quasi-phase matching in nonlinear optical effects to achieve effective frequency conversion of the incident laser light. We have succeeded for the first time in the generation of the second harmonics at 402.5 nm of the incident laser light at 805 nm. Detailed analysis shows that quasi-phase matching is achieved.It is possible to fabricate periodically corrugated structures from such PPI structures. Such periodically corrugated structures offer either distributed feed-back cavity or distributed Bragg reflector mirror. We have achieved optically-pumped distributed-feed-back lasing of periodic corrugated ZnO structures for the first time.
4:45 PM - L10.6
Mechanisms in the Formation of High Quality Schottky Contacts to n-type ZnO.
Martin Allen 1 , Craig Swartz 1 , Martin Henseler 2 , Roger Reeves 2 , James Metson 4 , Holger von Wenckstern 3 , Marius Grundmann 3 , S.A. Hatfield 5 , P.H. Jefferson 5 , P.D.C. King 5 , Timothy Veal 5 , Chris McConville 5 , Masakazu Kobayashi 6 , Steve Durbin 1
1 Department of Electrical and Computer Engineering, University of Canterbury, Christchurch New Zealand, 2 Department of Physics and Astronomy, University of Canterbury, Christchurch New Zealand, 4 Department of Chemistry, University of Auckland, Auckland New Zealand, 3 Institut für Experimentelle Physik II, Universität Leipzig, Leipzig Germany, 5 Department of Physics, University of Warwick, Coventry United Kingdom, 6 Faculty of Science and Engineering , Waseda University, Tokyo Japan
Show AbstractCompared to GaN, its main rival as a wide bandgap semiconductor in the UV spectrum, ZnO has a significantly higher ionicity and spontaneous polarization along the c-axis. The former causes the Zn-polar (0001) and O-polar (000bar1) surfaces to be electrostatically unstable requiring some form of a) surface reconstruction, b) internal charge transfer, and/or c) external adsorbates. The latter leads to the presence of bound surface polarization charges which also require internal and/or external compensation. From the perspective of Schottky diode fabrication, a detailed understanding of these phenomena is important for the control of contact properties and device characteristics. We have investigated the influence of surface polarity on the morphology and electrical properties of melt-grown (medium resistivity ~ 0.3 Ωcm) and hydrothermally grown (high resistivity ~ 300 Ωcm) bulk ZnO single crystals. Atomic force microscopy showed the presence of monolayer-height triangular shaped terracing and vacancies on the Zn-polar face of hydrothermal ZnO with no such features observed on the O-polar face. The electrical properties of high resistivity ZnO, investigated through four point probe measurements and variable temperature Schottky diode characterisation, were found to be significantly different when measured in air and under vacuum conditions. The surface conductivity was found to increase significantly when measured in vacuum, indicating the presence of a surface conduction channel which is activated by the removal of adsorbates—most likely oxygen and water vapour—from the ZnO surface. This well-known characteristic of ZnO has important consequences for contacts, which are usually fabricated under vacuum but operated in air. Furthermore, valence band x-ray photoelectron spectroscopy (XPS) revealed the presence of a significant electron accumulation layer on both the Zn-polar and O-polar faces in vacuum, which is not present in air as indicated by scanning Kelvin probe microscopy. Variable emission angle XPS measurements revealed there is a significant coverage of hydroxyl bonds on the Zn-polar and O-polar surfaces under ultra-high vacuum pumping. In light of this, it is surprising that very high quality Au, Pt and silver oxide Schottky contacts, with ideality factors close to the image force controlled limit, were fabricated on the high resistivity material without the need for any special surface treatments. While high quality silver oxide diodes, with barrier heights in excess of 1.0 eV, were routinely achieved on the Zn-polar, O-polar and non-polar faces of both melt and hydrothermal ZnO, those on the Zn-polar face had consistently higher barriers. The barrier height difference between diodes on the Zn-polar and O-polar faces was 200 meV for hydrothermal ZnO and 50 meV for melt ZnO. The larger polarity effect for the high resistivity material is consistent with a model of spontaneous polarization induced surface band bending.
5:00 PM - L10.7
Polymer Schottky Contact on Polar ZnO Surfaces.
Ryosuke Gunji 1 , Masaki Nakano 1 , Atsushi Tsukazaki 1 , Akira Ohtomo 1 , Tomoteru Fukumura 1 , Masashi Kawasaki 1 2
1 Institute for Materials Research, Tohoku University, Sendai Japan, 2 CREST, JST, Tokyo Japan
Show Abstract Schottky contacts have been made on ZnO surface using rare metals such as Au, Pt, and Ag. However, the junction properties are often found to vary depending on the surface polarity and methods used for metal deposition and surface treatments. We have been studying Schottky junctions with a conducting polymer, poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), which is formed by simple spin-coating technique. Good junction properties were reproducibly obtained for various n-type oxide semiconductors including the +c face of ZnO [1]. In case of the -c face of ZnO, however, coating PEDOT:PSS solution resulted in interface reaction due to its acid nature and the higher reactivity of the -c face than +c face. Such reaction has to be avoided since the surface of ZnO thin films is typically the -c face. Here we report on realization of high quality Schottky junction composed of PEDOT:PSS and the -c face of ZnO by using silane-coupling agent. ZnO single crystals were immersed into the diluted solution of the agent, and then PEDOT:PSS was spin-coated. The interface reaction was inhibited by the thin molecular layer, which was formed spontaneously by silane-coupling reaction on the ZnO surface. Au contact electrode was deposited on PEDOT:PSS by thermal evaporation. Circular mesa structures and ring-shaped Ohmic electrodes were formed by ion milling and electron-beam evaporation with photolithography, respectively. Schottky junction properties were examined by current-voltage (I-V) and capacitance-voltage (C-V) characteristics at room temperature. The I-V characteristic was well in accordance with thermionic emission model and extracted ideality factor was nearly unity. From the C-V profile, donor concentration of the ZnO crystal was evaluated to be ~1016 cm-3. Thus, this simple wet process can be used for electrical evaluation of both +c and -c faces of ZnO.[1] M. Nakano et al., Extended Abstracts (The 54th Spring Meeting 2007) The Japan Society of Applied Physics and Related Societies, 29p-ZA-14
5:15 PM - L10.8
Electrical Characterization of N+-implanted n-type ZnO Single Crystal: ZnO p-n Homojunction and Deep Level Defects.
Qilin Gu 1 , Xuemin Dai 1 , Chi-Chung Ling 1 , Shijie Xu 1 , Liwu Lu 1 , Gerhard Brauer 2 , Wolfgang Anwand 2 , Wolfgang Skorupa 2
1 Physics, The University of Hong Kong, Hong Kong China, 2 Institut für Ionenstrahlphysik und Materialforschung, Forschungszentrum Rossendorf, Dresden Germany
Show AbstractUnintentionally doped n-type ZnO single crystals were implanted by nitrogen ions with different fluences of 1013, 1014 and 1015 cm-2 respectively under accelerating voltage of 150 keV. The oxygen face was chosen for the implantation at a substrate temperature of 300°C . Electrical characterization techniques including current-voltage (I-V), capacitance-voltage (C-V), Deep Level Transient Spectroscopy (DLTS) and double-correlation DLTS (DDLTS) were used for investigating all the as-implanted samples and control sample through Au/n-ZnO Schottky diodes. DLTS results showed that N+ implantation with the fluence of 1013 cm-2 introduced one prominent electron trap EN1 at 0.95eV below conduction band with trap concentration of 4×1015 cm-3 and capture cross section of 6×10-12 cm2 which was not observed in non-implanted control sample. This implantation induced deep level was also observed in as-implanted ZnO samples with fluences of 1014 and 1015 cm-2. ZnO p-n homojunction was successfully fabricated due to the formation of p-type layer after 650°C post-implantation annealing in air for 30 minutes. DLTS measurements showed that the deep states in as-implanted samples still persisted after 650°C annealing. However, it was found that EN1 finally disappeared after 750°C thermal annealing through DLTS spectra. On the other hand, another electron trap with activation energy of 0.174 eV and capture cross section of 4.7×10-17 cm2 was detected in 750°C-annealed p-n junction. Detailed properties of implantation induced defects were discussed based on plentiful DLTS spectra. Moreover, a broad red luminescence peak centered at 620nm was observed in low-temperature photoluminescence spectra of all the as-implanted samples, whereas absent in the non-implanted control sample. It was also found that the red luminescence peak existed in 650°C-annealed sample. As for 750°C-annealed sample, there was no obvious red emission peak and a strong green luminescence peak dominated the PL spectrum. Using multimode Brownian oscillator (MBO) model, the induced broadband had a zero phonon wavelength coincided with the energy scheme of the transition between the detected 0.95eV deep level and the valance band. Combined with first-principle calculation results, this EN1 deep level can be assigned to be oxygen vacancy (VO) which was predicted to be located ~1 eV below conduction band.
5:30 PM - L10.9
Influence of Monochromatic Illumination on Chemisorption Processes in ZnO.
George Whitfield 1 , Harry Tuller 1
1 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractZnO, as an n-type semiconducting metal oxide (SMO), exhibits a strong chemoresistive sensitivity to a variety of oxidizing and reducing gases. [1] While SMO based sensors typically need to be heated to temperatures in excess of 300°C to obtain reversible signal response, previous studies have demonstrated sensor operation at room temperature upon illumination with visible and UV light. [2, 3] In the present work, the effect of light on ZnO-based gas sensor performance is investigated, with particular attention toward the effect of varying wavelength. ZnO photoconductance spectra are measured in environments of controlled temperature and atmosphere, and spectral characteristics are correlated to sensor performance during monochromatic illumination. These experiments are used to gain insight into the density of electronic states at the ZnO surface and their relation to chemisorption processes active during gas sensor operation. [1]M. Suchea et. al., Thin Solid Films 515, 551-554 (2006)[2]K. Anothainart et. al., Sensors and Actuators B 93, 580-584 (2003).[3]M. Law et. al., Angew. Chem. Int. Edit. 41, 2405-2408 (2002)
5:45 PM - L10.10
Bilayered (MoOx/ ZnGaO / substrate) Nano-composite Structure Transparent Conducting Oxide with Improved Work Function.
Titas Dutta 1 , Vikram Bhosle 1 , Jagdish Narayan 1
1 , North Carolina State University, Raleigh, North Carolina, United States
Show AbstractResearch on transparent conducting oxides (TCOs) alternative to indium tin oxide (ITO) has attracted lot of attention due to the serious concern related to the cost and chemical stability of indium oxide. ZnO doped with group III elements is a promising candidate because of its superior stability in hydrogen environment, benign nature and relatively inexpensive supply. Ga doped ZnO films with electrical and optical properties comparable to ITO can be grown on sapphire as well as inexpensive glass substrates. However, ZnO based TCO films suffer from low work function (4.1 eV, compared to that of 4.8 eV for ITO), which is a critical parameter for device applications. We report here the growth of a bilayered structure consisting of thin MoOx layer on Zn0.95Ga0.05O film for transparent electrode applications. The thin MoOx buffer layer provides a higher work function, and superior surface and diffusion barrier characteristics without affecting the overall transmittance and sheet resistance. The higher work function of the MoOx (~ 5.0 eV) is envisaged to improve the transport of the carriers across the heterojunction in the device, thus, resulting in the increase in device efficiency. The films (MoOx, ZnGaO) and the heterostructures (MoOx / ZnGaO) have been deposited using pulsed laser deposition (PLD). To understand the growth characteristics of the thin MoOx layer, films (few monolayers) were grown on glass, sapphire and ZnGaO templates at different temperatures and oxygen partial pressure. The characteristics of the MoOx film and the heterostructure have been investigated in detail using X-ray diffraction, TEM, X-ray photoelectron spectroscopy (XPS), and electrical and optical property measurements. The thickness of MoOx layer was determined to be ~1.5 nm ± 0.5 nm which corresponds to 4 unit cells. The thickness of the layer was carefully controlled by controlling the repetition rate and the number of pulses. One of the most important features of this work is the 2-D growth of the MoOx film on the various templates as verified by XPS. The 2-D growth of the film leads to a uniform coverage and a smooth surface, which are expected to be conducive to improve the device performance. In the MoOx film molybdenum exists in Mo4+ , Mo5+ and Mo6+ oxidation states in a monoclinic phase. The MoOx films exhibited transmittance of more than 80% across the visible region and a resistivity of the order of 10-3 Ω-cm. However, the overall transmittance and the resistivity are dictated by the thicker layer of ZnGaO beneath the MoOx. Thus, the sheet resistance and the transmittance of the composite structure are close to that of ITO, albeit with improved work function and diffusion barrier properties. Preliminary results of the performance of test devices based on these novel TCO layers will be presented and the suitability of these layers for transparent electrode application will also be discussed.
L11: Poster Session
Session Chairs
Irina Buyanova
Chennupati Jagadish
David Norton
Gyu-Chul Yi
Friday AM, November 30, 2007
Exhibition Hall D (Hynes)
9:00 PM - L11.1
Electrical and Electro-optical Investigations on the Polarization Coupling in Epitaxial Ferroelectric BTO/ZnO Heterostructures.
Matthias Brandt 1 , Holger Hochmuth 1 , Michael Lorenz 1 , Marius Grundmann 1 , Mathias Schubert 2 , Tino Hoffman 2 , Rao Voora 2 , Nurdin Ashkenov 1 3 , Jürgen Schubert 4
1 Fakultät für Physik und Geowissenschaften, Institut für Experimentelle Physik II, Abteilung Halbleiterphysik, Universität Leipzig, Leipzig Germany, 2 Department of Electrical Engineering and Center for Materials Research and Analysis, University of Nebraska-Lincoln, and Nebraska Center for Materials and Nanoscience , Lincoln, Nebraska, United States, 3 , Present Address: Opteg GmbH, Leipzig Germany, 4 Forschungszentrum Jülich GmbH, IBN 1-IT, D-52425 Jülich Germany
Show AbstractFerroelectrics are a material class of increasing importance in electronics. A large number of electrical and electro-optical applications are known, including ferroelectric switches, capacitors, non volatile memory elements, optical switches and thin film transistors. ZnO is an emerging candidate for UV optoelectronic devices, especially due to its high exciton binding energy. While the polarization in ferroelectric materials is switchable by external electric fields, wurtzite ZnO exhibits a permanent spontaneous polarization. However, publications concerning the effects of polarization coupling in BTO/ZnO structures are sparse.Previously, polarization coupling effects have been observed by us in experiments on Pt/BTO/ZnO/Pt structures on Si[1]. Asymmetric current-voltage (I-V), capacitance-voltage (C-V) and field dependent polarization P(E) hysteresis loops were demonstrated. A change of more than 30% in capacitance could be detected depending on the direction of the bias sweep.Lately, we have investigated the structural, morphological, electrical and electro-optical properties of structurally improved BTO/ZnO heterojunctions grown on SrTiO3 (STO) substrates. BTO films of 800nm thickness have been grown on lattice matched STO substrates by pulsed laser deposition (PLD). Two mechanisms for the fabrication of back contacts have been employed, STO:Nb substrates and 70nm thick conductive SrRuO3 between the substrate and the BTO layer. Further, ZnO top electrodes of 400nm thickness and 2 mm diameter were grown by PLD. Ohmic front contacts have been realized by DC sputtered Au pads with 1 mm diameter. Pt contacts have been DC sputtered instead of the ZnO for comparison.Superior structural properties were observed in comparison to the polycrystalline structures obtained on Si substrates. A very good (001) and (0002) alignment has been observed for the BTO and ZnO layers respectively. The leakage current could be reduced by 4 orders of magnitude in comparison to the previous structures. The influence of persistent photocurrents has been investigated. Depending on the time the sample was kept in darkness, the leakage currents could be reduced by another order of magnitude. This allowed the direct observation of the ferroelectric switching currents of the BTO, which could be used to determine the coercive field in BTO and the spontaneous polarization in ZnO. A detailed model analysis of the electrical switching behaviour will be given. [1] N. Ashkenov, M. Schubert, E. Twerdowski, B. N. Mbenkum, H. Hochmuth, M. Lorenz, H. v. Wenckstern, W. Grill, and M. Grundmann, Thin Solid Films 486, 153-157 (2005)
9:00 PM - L11.10
Optical SHG for ZnO Films with Different Morphology Stimulated by UV-laser Thermotreatment.
Kazimierz Plucinski 1 , V. Kapustianyk 2 , I. Kityk 3
1 Electronics, Military University of Technology, Warsaw Poland, 2 Scientific-Technical and Educational Centre of Low Temperature Studies, , Univ. of Lviv, Lviv Ukraine, 3 Institute of Physics, J. Dlugosz University , Czestochowa Poland
Show AbstractWe have discovered substantial influence of the ZnO grain sizes on the output second harmonic generation stimulated by nanosecond pulses of 371 nm nitrogen laser during simultaneous superposition of the electrostatic electric field with electric strength about 2 kV/cm. To explore an influence of film morphology on the second order optical susceptibility we have explored the films prepared by electron sputtering with average grain sizes about 1000 nm and films synthesized by rf-magnetron sputtering on the two different substrates – glass and amorphous quartz with average grain sizes about 137 nm and 29 nm, respectively. Comparing the UV-induced optical second harmonic generation for the ZnO films with different grain sizes we came to a conclusion that the samples with nanorods are characterised by substantially larger second order susceptibility (up to 5.7 pm/V at 1064 nm fundamental wavelength) compared to those for the films with smaller grain sizes (1.5 pm/V). This may be due to fact that UV-illumination deals only effectively with simultaneous electric filed treatment enhancing the second order optical susceptibility. A correlation between the temperature of local laser thermo-heating and the output optical second harmonic generation may indicate on principal role of the local thermal expansion in the observed output second harmonic generation.
9:00 PM - L11.11
Ultraviolet Luminescence in Epitaxial Phosphorus-Doped ZnO Thin Films.
Xiaoqing Pan 1 , Arnold Allenic 1 , Wei Guo 1 , Yanbin Chen 1 , Yong Che 2 , Zhendong Hu 2 , Bing Liu 2
1 , The University of Michigan, Ann Arbor, Michigan, United States, 2 , IMRA America, Inc., Ann Arbor, Michigan, United States
Show AbstractWhether ZnO can compete with GaN for optoelectronic applications will depend on our ability to fabricate robust p-type material. We will show that while good p-type conductivity can be observed in P-doped ZnO, the near band edge (NBE) luminescence is strongly degraded by the high density of doping-induced defects. In a more general effort to improve on the NBE luminescence of P-doped ZnO and reduce the minority carrier concentration, we have used oxygen plasma-assisted pulsed laser deposition to grow P-doped ZnO films epitaxially on both (0001) sapphire and (0001) ZnO substrates. The oxygen plasma provides oxygen-rich growth conditions. We find that a new ultraviolet luminescence band at 3.1162 eV (at 12.5 K) appears in ZnO films doped with phosphorus. The transition was studied by excitation-intensity and temperature-dependent photoluminescence measurements, and assigned to a donor-acceptor (DA) transition. The behavior and characteristics of the DA band will be presented and the activation energy of the acceptor estimated as a function of the phosphorus concentration in the film. The microscopic origin of the phosphorus acceptor will be discussed in the light of our transmission electron microscopy studies.
9:00 PM - L11.12
Colloidal Synthesis of Optically Active ZnO/ZnS Core/Shell Nanocrystals.
Krishnaprasad Sankar 1 , Brian Akins 1 , Tosifa Memon 1 , Shin Bowers 1 2 , Gennady Smolyakov 1 , Marek Osinski 1
1 Center for High Technology Materials, University of New Mexico, Albuquerque, New Mexico, United States, 2 Present address: Division of Engineering, Brown University, Providence, Rhode Island, United States
Show AbstractRecent advances in colloidal nanomaterials have produced a new class of fluorescent labels by conjugating monodisperse semiconductor nanocrystals with biospecies (antibodies, enzymes, oligonucleotides, etc.). These biofunctionalized nanocrystals provide important advantages over conventional organic dyes: their emission wavelength can be continuously tuned to achieve a range of colors by changing the particle size, and a single light source can simultaneously excite all the different-sized nanocrystals, therefore allowing for simultaneous imaging of different colors. Additionally, semiconductor nanocrystals enable long-time, in-situ cell imaging due to 100-fold greater stability against photobleaching. Moreover, the characteristic optical property of “blinking” provides readily distinguished emission from individual nanocrystals compared to aggregates, or multimeric assemblies. Importantly, by defining the behavior of individual protein assemblies, it is now possible to resolve a detailed picture of cellular trafficking and molecular processing events that have been previously unappreciated, but are critical to developing new insight into biological processes, including receptor trafficking or vesicle recycling.However, colloidal nanocrystals reported to date are not optimized for in vivo studies. In addition the requirements associated with the quality of in vivo imaging data, such as minimal nonspecific deposition and retention of luminescent properties over a sufficiently long time, the most fundamental is: nanocrystals must not be harmful to the investigated cells. This fundamental requirement has so far received relatively little attention, perhaps due to the pioneering character of all nanocrystal imaging studies at this stage. A survey of the materials used to date as components of colloidal quantum dots resembles more a catalog of poisons and carcinogens than desirable non-invasive tools.ZnO nanocrystals possess a number of properties making them very attractive for optical applications, such as wide bandgap, large exciton binding energy, radiation hardness, and strong surface-defect-related green emission. As such, they are of interest as potential high-speed nanophosphors for conversion of UV light into visible emission in UV LEDs and laser diodes. Gas and chemical sensors, electro- and photoluminescent devices, solar energy conversion, and transparent UV-protection films can be named as potential applications of ZnO nanocrystals. Being a non-toxic substance, ZnO is a very attractive material for biomedical applications. When doped with various other materials, the wide bandgap of ZnO could be used to get a wide spectral range of narrow emission lines.In this paper, we report on colloidal synthesis of ZnO/ZnS core/shell nanocrystals and their characterization by TEM, EDS, and steady state UV-VIS optical absorption and photoluminescence spectroscopies.
9:00 PM - L11.13
Sputter Deposition of ZnO Layers at High Substrate Temperatures.
Angelika Polity 1 , Sebastian Eisermann 1 , Joachim Sann 1 , Swen Graubner 1 , Stefan Lautenschlaeger 1 , Niklas Volbers 1 , Andre Krtschil 2 , Alois Krost 2 , Bruno Meyer 1
1 1st Physics Institute, University of Giessen, Giessen, Hessen, Germany, 2 Institute for Experimental Physics, Otto-von-Guericke-University, Magdeburg Germany
Show AbstractPure ZnO and nitrogen doped ZnO thin films have been prepared on sapphire, gallium nitride films, and zinc oxide substrates at substrate temperatures up to 750 °C by radio-frequency (RF) sputtering. A pure ZnO ceramic target was used in pure argon, in a mixture of argon and oxygen or in a mixture of argon, oxygen and nitrogen. By optimizing the sputter parameters, such as sputtering power, temperature of the substrate or Ar/O2/N2 sputtering gas ratios, high quality films were obtained. The thin film crystallinity and surface morphology has been investigated with X-ray diffraction (XRD), atomic force (AFM) and scanning electron microscopy (SEM). Optical properties have been examined by measuring optical transmission and photoluminescence (PL) spectra. Hall and Scanning Capacitance Microscopy (SCM) measurements were carried out to check electric properties. Secondary Ion Mass Spectrometry (SIMS) measurements have been performed to determine the distribution of nitrogen in the doped layers.To conclude, the ZnO film crystallinity greatly increases with increasing substrate temperatures and the layer surfaces become smoother. High quality ZnO layers could be deposited on epi-ready zinc oxide substrates which was proven by PL measurements. Nitrogen was successfully incorporated into the layers, but homogeneous p-type conductivity has not been achieved, p-type domains could be observed at the grain boundaries.
9:00 PM - L11.14
Transient Gain Spectroscopy of ZnO Epilayers After Multiphoton Pumping.
Christoph Lange 2 , Sangam Chatterjee 2 , Stefan Lautenschlaeger 1 , Bruno Meyer 1
2 Faculty of Physics and Material Sciences Center, Philipps-University , Marburg Germany, 1 1. Physics Institute, Justus-Liebig-University, Giessen Germany
Show AbstractZnO layers of about 100 nm thickness were grown heteroepitaxially on three different substrates, i.e. GaN, sapphire, and sapphire/MgO in the same growth run by chemical vapor deposition. Transient gain measurements using transmission pump-probe spectroscopy at room temperature are performed on all three ZnO epilayers. Results are obtained for direct, one photon pumping and multiphoton pumping at different wavelength. The differences between the results on the three samples as well as between the two types of measurements are presented and discussed.
9:00 PM - L11.15
Defects in Bulk ZnO Studied by Time-resolved Photoluminescence.
Michael Reshchikov 1
1 Physics Department, Virginia Commonwealth University, Richmond, Virginia, United States
Show AbstractIn spite of considerable attention to defect-related broad photoluminescence (PL) bands in ZnO, the origin of the deep-level defects in this semiconductor remains uncertain and transitions causing these bands are not well identified. The analysis of defect-related PL in ZnO is complicated by the fact that PL bands from different defects have similar shapes and positions or essentially overlap. Time-resolved PL enables one to resolve PL bands related to different defects and determine types of optical transitions. Unintentionally doped bulk ZnO samples were grown by hydrothermal method in Tokyo Denpa Co. Ltd. (Japan) and MTI Corporation. At low temperatures the PL spectrum contained a very broad band with the peak position observed between 2.0 and 2.4 eV, depending on excitation intensity. Evolution of the PL spectrum after a pulse excitation revealed that the broad band is composed of an orange (OL) and green (GL) luminescence bands having maxima at 1.96 and 2.4 eV, respectively. The GL band dominated at times up to 1 ms and then disappeared. The OL band decayed as approximately t-1 over a wide time interval, and its spectrum could be recorded even 24 hours after the excitation source was switched off. The slow nonexponential decay of the OL band is attributed to transitions from shallow donors to a deep acceptor (donor-acceptor pair transitions). The ionization energy of the acceptor has been determined as 0.5-0.6 eV from the quenching of the OL band at temperatures above 230 K. Large Stokes shift and nearly Gaussian shape of the OL band are explained in a model accounting for strong electron-phonon coupling.
9:00 PM - L11.16
Magneto Properties of Nano-crystalline Co-doped ZnO Thin Films.
Christoph Knies 1 , Swen Graubner 1 , Jan Stehr 1 , Matthias Elm 1 , Detlev Hofmann 1 , Peter Klar 1 , Nikolai Romanov 2 , Bruno Meyer 1
1 1st Physics Institute, Justus-Liebig-University, Giessen Germany, 2 , A. F. Ioffe Institute, St. Petersburg Russian Federation
Show AbstractCo doped ZnO is an interesting material for room temperature spintronic applications. Nano-crystalline ZnO samples doped with Co were synthesized by a wet-chemical synthesis using dip coating for the film formation. Samples with Co contents up to 30% can be synthesized. The as grown films are of a low conductivity. The carrier concentration of the films can be increased by several orders of magnitude up to about 1019 cm-3 by controlled annealing in Zn vapor. The magnetic properties of the samples were investigated by temperature and magnetic-field dependent circular dichroism (MCD) measurements as well as magneto-transport measurements. Our results indicate a transition from paramagnetic to macroscopic ferromagnetic behavior which is caused by the annealing procedure. Currently, we verify whether the ferromagnetic like properties of ZnCoO are mediated by carriers or by a non-random Co distribution within the sample. The somewhat similar synthesis approach of K. R. Kittilstved et al. [Appl. Phys. Lett. 89, 062510 (2006)] yielded comparable results.
9:00 PM - L11.17
A Study of High Quality Al-doped ZnO Thin Films Grown at Low Temperature by Pulsed Laser Deposition.
Ram Gupta 1 , K. Ghosh 1 , S. Mishra 2 , P. Kahol 1
1 Physics, Astronomy and Materials Science, Missouri State University, Springfield, Missouri, United States, 2 physics, the university of memphis, memphis, Tennessee, United States
Show Abstract9:00 PM - L11.18
Thermoelectric Measurements on Artifically Structured ZnO Epitaxial Layers.
Gert Homm 1 , Joerg Teubert 1 , Stefan Lautenschlaeger 1 , Thorsten Henning 1 , Peter Klar 1 , Bruno Meyer 1
1 1st Physics Institute, Justus-Liebig-University, Giessen Germany
Show AbstractZnO layers of about 1000 nm thickness were grown homoepitaxially on ZnO substrates by chemical vapor deposition. The layers are n-type with electron concentrations of about 1017 cm-3. Stripes of the as grown samples were artifically structured by photolithography. The patterns consist of regular arrays of holes with different spacings and hole diameters. The patterns were transferred either by wet-chemical etching or by ion-beam etching. The Seebeck coefficient is measured in the temperature range of 80 K to 300 K. The influence of the artificial structuring on the thermodynamic figure of merit is discussed.
9:00 PM - L11.19
Visible Luminescence of Rare Earth Ions Doped Amorphous Zinc Oxide Thin Films Grown by Sputtering Technique.
Wojciech Jadwisienczak 1 , Ajay Vemuru 1 , Aurangzeb Khan 2 , Martin Kordesch 2
1 School of EECS, Ohio University, Athens, Ohio, United States, 2 Department of Physics and Astronomy, Ohio University, Athens, Ohio, United States
Show AbstractWe report on the luminescence of rare earth (RE) (Sm, Tb, Er, Tm) ions doped ZnO films grown by the radio frequency magnetron sputtering technique. Samples were insitu doped with selected RE ions without any intentional co-dopants and deposited on silicon or quartz substrates at low temperature. The as-grown samples morphology was amorphous as was confirmed by the X-ray analysis. RE-doped a-ZnO films were investigated by optical transmission in the ultraviolet-visible spectral range, Raman scattering, photoluminescence (PL) and cathodoluminescence (CL) spectroscopes at 11 K and 300 K. In general, CL spectra of as-grown RE-doped a-ZnO films show characteristic emission lines due to 4f-shell transitions of RE3+ ions without any host background. Optical excitation of a-ZnO doped with a single RE species, using above the bandgap excitation resulted in strong host emission overlapped with RE3+ emission bands. PL of a-ZnO co-doped with Sm, Er and Tm produces strong white light emission easily detected by naked eye. Using collected data we discuss the possibility of sensitization of RE ions using nano-crystals embedded in a-ZnO matrix for white light emitting phosphor applications.
9:00 PM - L11.2
Metal-Catalyst-Free and Surface-Roughness-Assisted Selective Area Growth of Zinc Oxide Nanowires by Thermal Evaporation.
Shu-Te Ho 1 , Hsin-Yu Lin 1 , Heh-Nan Lin 1
1 Materials Science and Engineering, National Tsing Hua University, Hsinchu Taiwan
Show AbstractWe report an effective growth method which can approach selective area growth of zinc oxide nanowires without the employment of metal catalyst. This can exclude effectively metal catalyst as a residual contamination from the Schottky junction or optical resonator. The PMMA film of thickness 50 nm spin-coated on two types of substrates, Si (100) and α-sapphire, was used as a resist layer for fabrication of the selective area patterns, microsquare and nanogroove arrays, by using e-beam lithography and atomic force microscopy nanomechining, respectively. The Si and sapphire substrates were immersed for 15 s in a solution of potassium hydroxide (KOH, 1M) and sodium hydroxide (NaOH, 1M), respectively, and then these arrays were selectively etched. Root-mean-square roughness measurements inside the etched region by using contact-AFM shown a little difference compared with roughness values of the bare substrates. The growth of ZnO nanowires was performed in furnace system with gas flow of mixed O2 and Ar in which the temperatures of substrate zone were set as 620 °C for Si and 580 °C for sapphire. However, the surface-roughness-assisted growth mechanism of ZnO nanowires has been opaque to our knowledge and it could be due to the increase of vacancy sites on etched surface within Langmuir adsorption as assisted heteronucleation of vapor or liquid zinc on substrate surface.
9:00 PM - L11.20
Observation of Room Temperature Ferromagnetism in Ga:ZnO – A Transition Metal Free Transparent Ferromagnetic Conductor.
Vikram Bhosle 1 , Jagdish Narayan 1
1 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States
Show AbstractWe report the observation of room temperature ferromagnetism (RTFM) in group III element (e.g. Ga, Al) doped ZnO. The major emphasis is placed on the defect characterization through detailed structural, chemical, electrical and magnetic property measurements in order to understand the origin of RTFM observed in this system. The ZnO films with varying Ga concentration (1-7 at%) were grown on (0001) sapphire using pulsed laser deposition (PLD). The detailed structural characterization of the films was performed using X-ray diffraction and transmission electron microscopy (TEM). The chemical concentration and purity of the films was verified by using x-ray photoelectron spectroscopy (XPS). The magnetic properties of the films were measured with a vibrating sample magnetometer (VSM) from Quantum Design. The magnetic behavior observed in ZnO is attributed to the concentration of oxygen vacancies which act as F-centers. The electrons trapped in these F-centers tend to get easily polarized under the influence of the magnetic field thus leading to the magnetization of the film. It is also surmised that the degenerate electrons present in the films further aid in the magnetization process by facilitating the transfer of moment from one polarized center (F-center) to another. The role of oxygen vacancies was established unequivocally from the experiments performed as a function of oxygen partial pressure during film deposition and the annealing experiments in air (~ 650 oC). It was observed that the magnetization could be systematically varied by varying the oxygen vacancy concentration in the film. The dependence of magnetic properties with vacancy concentration in ZnGaO films has been complemented with electrical resistivity measurements and Hall measurements. However, it was observed that the presence of Ga is also necessary to instigate the magnetic behavior in this system and has been related to the donor characteristics of the dopant and its complex interaction with the oxygen vacancies. This is the first time ferromagnetic behavior has been observed in Ga:ZnO system. These results help us to understand the cause of ferromagnetism in ZnO doped with elements having fully filled d-orbitals. The situation might be completely different in case of transition metal doped ZnO, however, these results suggest a need to take a closer look into the role of vacancies in those samples as well. The most important implication of this work is the use of ZnGaO films for the realization of transparent spintronics in addition to the large pool of applications for this very interesting system.
9:00 PM - L11.21
Multiphonon Raman Scattering and Optical Properties of ZnMnO Ternary Alloy.
Sandra Dussan 1 , Pijush Bhattacharya 2 , Kousik Samanta 1 , Ram Katiyar 1
1 Physics, University of Puerto Rico, San Juan, Puerto Rico, United States, 2 Physics, Fisk University, Nashville, Tennessee, United States
Show AbstractThe 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 3d transition metal (Mn, Co, Ni, Cu, etc.) doped II-VI compound semiconductor, ZnO is the most promising dilute magnetic semiconductor (DMS) for the spintronic applications. In this work we have carried out the lattice dynamical and optical bandgap studies of Mn substituted ZnO by micro-Raman scattering and optical transmission spectra. Apart from the first order Raman modes of ZnO and Zn1-xMnxO (x = 0, 0.01, 0.03, 0.05, and 0.10) at 99.9, 381, 438.5, 573, and 584 cm-1 corresponding to E2low, A1TO, E2high, A1LO, and E1LO, respectively, we observed five addition optical modes at 327 (I1), 332 (I2), 482 (I3), 525 (I4), and 680 (I5) cm-1, where I1 and I5 is present only in 10% Mn substituted ZnO sample. The modes I2, I3, and I4 are assigned as multi-phonon scattering considering the two phonon processes in the disorder lattice due to Mn doping and these are identified as (E2high – E2low), (A1TO + E2low), and (E2high + E2low) respectively. To identify the additional modes, I1 and I5, we have carried out Raman measurements on ZnMn2O4 target and compare the spectra with 10% Mn doped ZnO; and we found that these two additional modes originated from the precipitation of spinal secondary phase ZnMn2O4 in 10% Mn doped ZnO. The crystalline grain sizes of 1, 3, 5, and 10% Mn substituted ZnO samples were calculated by phonon confinement model as 31.8, 18.3, 15.9, and 14.1 nm, respectively. The optical band gap was found to increase from 3.27 eV to 3.41 eV with Mn concentration and additional broad sub-bandgap absorption was found around 3 eV with higher Mn content.
9:00 PM - L11.22
Blue Luminescence in ZnO Films Grown of by Peroxide MBE.
Vitaliy Avrutin 1 , Mikhail Reshchikov 2 , Natalia Izyumskaya 1 , Ryoko Shimada 1 , Hadis Morkoc 1 2
1 Electrical Engineering, Virginia Commonwealth University, Richmond, Virginia, United States, 2 Physics Department, Virginia Commonwealth University, Richmond, Virginia, United States
Show AbstractInterest in ZnO is due to its potential applications to blue-ultraviolet light emission. Because of precise control over the deposition parameters, molecular beam epitaxy (MBE) is widely employed for ZnO thin films. However, relatively low temperature of the growth by MBE may result in formation of a variety of native defects and complexes. Photoluminescence (PL) is a well suited for studying point defects. Low-temperature PL spectrum of undoped ZnO contains numerous excitonic sharp lines and a defect-related broad band in the yellow/green part of the spectrum. In addition, occasionally a blue band peaking at about 3.1 eV is observed [1]. We observed such a broad band in a set of ZnO films grown by MBE using H2O2 oxygen source on a-plane sapphire. This band was observed for a wide range of oxygen-to-zinc ratio. In contrast, however, no blue band was observed when an RF plasma source was used for oxygen in the same chamber under similar conditions. The PL spectrum in the peroxide grown samples contained a sharp and intense line at 3.360 eV, assigned to an exciton bound to a shallow donor, and phonon replicas as well as a blue-UV band at 2.85 -3.15 eV and a yellow band at 2.13-2.30 eV in different samples under varying excitation conditions. The well-defined blue band had a Gaussian shape with a full width at half maximum of 0.22-0.30 eV. The salient features of this band are (i) strong shift to higher energies (up to 150 meV) with the band narrowing (from 0.30 to 0.22 eV) when the excitation power density varied from 10-5 to 100 W/cm2 ;(ii) a strong red shift (~ 150 meV) with increasing temperature from 10 to 180 K. The blue-UV band intensity decreased by a factor of 103 with increasing temperature from 10 to 250 K, disappearing at room temperature. The yellow band blue-shifted by ~ 150 meV both with increasing excitation intensity at 10 K and temperature from 10 to 300 K at a low excitation intensity. These effects can be explained with band-to-band and band-to-acceptor transitions in the presence of potential fluctuations rather than donor-acceptor pair transitions involving unknown shallow acceptors. The potential fluctuations might be due to the presence of hydroxyl groups when H2O2 is used.[1] B. Kumar et al., Appl. Phys. Lett. 89, 141901 (2006).
9:00 PM - L11.23
MOCVD Growth of GaN and InGaN on ZnO as a Sacrificial Substrate.
Nola Li 1 , Shen-Jie Wang 1 , Eun-Hyun Park 1 , Zhe Chuan Feng 1 2 , Adriana Valencia 3 , Jeff Nause 3 , Ian Ferguson 1 3 4
1 School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 Graduate Institute of Electro-Optical Engineering and Department of Electrical Engineering, National Taiwan University, Taipei Taiwan, 3 , CERMET Inc, Atlanta, Georgia, United States, 4 School of Material Science Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show Abstract ZnO has been considered as a substrate for epitaxial growth of III nitrides due to its close lattice and stacking order match. There have been some reports of the growth of GaN based materials on ZnO substrates by Molecular Beam Epitaxy (MBE). Here, we will report the In0.18Ga0.82N epitaxial layer on lattice-matched ZnO substrates by metal-organic chemical vapor deposition (MOCVD). In addition, the variation of In incorporation in the InGaN films were controlled well on ZnO substrates under different growth temperatures. The high-resolution X-ray diffraction has confirmed the epitaxial growth of InGaN film on ZnO substrates. In this talk, the optical and structural characterization of InGaN epilayer on ZnO substrates were performed by using photoluminescence (PL), optical transmission, and atomic force microscopy (AFM). However, it is well known that there is high reactivity between ZnO and GaN during the high temperature growth, such as with MOCVD. Secondary ion mass spectrometry (SIMS) has revealed Zn and O diffusion occurring at the interface. The mechanisms of epitaxial growth and inter-diffusion between InGaN and ZnO substrates will be discussed. The use of ZnO will also act as a sacrificial substrate that will lead to a thin nitride film leading to higher external quantum efficiency devices, simple low cost fabrication, and high product yield with high brightness.
9:00 PM - L11.24
Erbium Environment in ZnO:Er Polycrystalline Fibers Produced by Electrospinning.
Danilo Mustafa 1 , Ji Wu 2 , Jeffery Coffer 2 , Leandro Tessler 1
1 Instituto de Física "Gleb Wataghin", Unicamp, Campinas, SP, Brazil, 2 Deparment of Chemistry, Texas Christian University, Fort Worth, Texas, United States
Show AbstractErbium-doped polycrystalline ZnO fibers were prepared by electrospinning. Organo-metallic Zn and Er (0.5, 1.0 and 2.0 % Er/Zn ratio) precursors dissolved in methanol, DI water and acetic acid were forced through a syringe needle. An electric field of 20 kV over 16 cm was used to promote the growth of fibers from the needle to a rotating drum. The resulting material consisted of entangled fibers with 0.5 to 1 μm typical diameter. The as-prepared samples present faint Er3+ luminescence at 1.5 μm when excited by the 488 nm line of an Ar+ laser. This luminescence intensity increases up to 25 times when the samples are annealed for 30 min at 500 °C in air.The effect of annealing over the Er environment was determined by EXAFS measurements at the Er L3 edge. In all as-prepared samples Er is coordinated to 8 ± 1 oxygen atoms at up to 0.236 ± 0.001 nm interatomic separation. After annealing the coordination is reduced to 5.9 ± 0.6 and the atomic separation becomes 0.230 ± 0.001 nm. This is comparable to Er2O3 where the coordination is 6 and the first neighbor distance is 0.226 ± 0.002 nm. The overcoordination in the as-prepared samples is consistent with the Er L3 edge at 8361.7 ± 0.5 eV compared to 8360.2 ± 0.5 eV in the annealed samples and 8359.9 ± 0.5 eV in Er2O3.In conclusion, the annealing of electrospun ZnO:Er fibers makes the Er environment more similar to that of Er2O3 increasing the luminescence efficiency.
9:00 PM - L11.25
Diblock Copolymer Reverse Micelles as Nanoscale Reactors for Zinc Oxide Nanoparticles.
Cleva Ow-Yang 1 , Osman el-Atwani 1 , Taner Aytun 1 , Omer Faruk Mutaf 1
1 Materials Science & Engineering, Sabanci University, Istanbul Turkey
Show AbstractTheir potential for blue-light source applications underscores the technological attraction of ZnO nanoparticles. However, the degree of blue-shift in their absorption edge scales with particle size, so the narrow bandwidth requirement places a strict demand for monodispersity in the nanoparticle population size distribution. Thus in the last decade, the use of diblock copolymer micelles in the synthesis of different metallic and semiconductor nanoparticles has become increasingly common, in part because the yield is reasonably monodisperse. In the study that we are presenting, zinc oxide nanoparticles were synthesized inside polystyrene-b-poly2vinlpyridine (PS-b-P2VP) micelles in toluene using zinc acetate and tetra methyl ammonium hydroxide (TMAH) as reactants. The UV-visible absorption spectra of the solutions showed a peak with an onset around 340 nm, indicating the presence of ZnO nanoparticles. Comparison with reference solutions also suggests the presence of zinc oxide in the material system. To prove that these did indeed form inside the micelles, the system was extensively studied by phase and topographical imaging using tapping mode atomic force microscopy, by dynamic light scattering, and by high angle annular dark field imaging in a scanning transmission electron microscope, and these results would be presented as well.
9:00 PM - L11.26
UV Assisted Surface Cleaning and Electrical Characterization of Zinc Oxide Nanowire Field Effect Transistor.
Ved Verma 1 , Hoonha Jeon 2 , Sookhyun Hwang 2 , Minhyon Jeon 2 , Wonbong Choi 1
1 Mechanical and Materials Engineering, Florida International University, Miami, Florida, United States, 2 Department of Nano Systems Engineering, Inje University, Gimhae Korea (the Republic of)
Show AbstractElectrical characteristics of zinc oxide nanowire (ZNW) field-effect transistors (FETs) are investigated at high temperature and vacuum conditions. Ultraviolet (UV) light stimulated oxygen desorption from the active channel improves the device performance of ZNW-FETs. Results show that charge transport in single ZNW depends on its surface environmental conditions. UV irradiation assisted with high temperature and vacuum condition, removes absorbed oxygen from the surface of nanowire and the current values increase upto 400 nA from 50 nA at a bias voltage of 2 V. ZNW-FETs fabricated in this study exhibits a mobility of ~ 4.10 cm2/V.s and high on-off ratio of ~ 104.
9:00 PM - L11.27
The Horizontal Growth of ZnO Nanowires on Si and Sapphire Substrates.
Jian Shi 1 , Qingkai Yu 2 , Shin-Shem Pei 2 , Hao Li 1
1 , University of Missouri-Columbia, Columbia, Missouri, United States, 2 , University of Houston, Houston, Texas, United States
Show AbstractStraight and curled horizontally aligned ZnO nanowires (NWs) were achieved using a chemical vapor deposition method on a-plane sapphire, c-plane sapphire, and (111) silicon substrates. Scanning electron microscopy (SEM) reveals that straight ZnO NWs preferred to align along different orientations on different substrates. No wavy structure was observed on the straight aligned ZnO NWs. The orientations of these ZnO NWs were primarily affected by metal catalysts density and the substrates. In addition, curled ZnO NWs, similar to nanorings, were also observed sticking on some of the substrates. These unique ZnO NWs horizontally grown on substrates may provide unique advantages for fabricating network nanodevices that can be integrated directly by the silicon technology.
9:00 PM - L11.28
Cathodoluminescence Study of Carrier Diffusion in ZnO.
Matthew Phillips 1 , Dominique Drouin 2 , Victoria Coleman 3 , Jodie Bradby 4 , Chennupati Jagadish 4
1 Microstructural Analysis Unit, University of Technology, Sydney, Sydney, New South Wales, Australia, 2 , Université de Sherbrooke, Sherbrooke, Quebec, Canada, 3 Department of Materials Chemistry, Uppsala University, Uppsala Sweden, 4 Department of Electronic Materials Engineering, The Australian National University, Canberra, Australian Capital Territory, Australia
Show AbstractScanning cathodoluminescence (CL) imaging of dislocation contrast has been used to investigate carrier diffusion in ZnO. The experiments were conducted on the oxygen face of high quality c-axis n-type (~ 1 x 10-17 cm-3) ZnO single crystals purchased from Cermet, Inc. Non-radiative dislocations were introduced around pits created using a 4.3 um spherical indenter with maximum loads of 100 and 200 mN. An injected carrier volume approaching a point source was achieved by using an ultra low excitation voltage (1 – 3 kV) with a < 2 nm electron probe diameter. Under these conditions the electron probe can be considered to be purely diffusive and consequently carrier behavior can be investigated at high spatial resolution using scanning CL microscopy. Carrier diffusion was modeled under the injection conditions using normalized depth and radial total electron energy loss profiles calculated using Monte Carlo simulations. The recombination length (L) was estimated by measuring the CL contrast, C(r), around isolated dislocations at normal incidence to the ZnO surface. CL intensity line profiles measured across these dislocations were analyzed using C(r) ~ exp(-r/L), where r is the distance of the electron probe from the dislocation core. From these measurements the carrier diffusion length in ZnO was estimated to be around 100 nm.
9:00 PM - L11.29
Microstructural Study of Zinc Oxide Nanocrystals and their Photoelectric Application.
Jun Guo 1 , Yong Hu 2 , Ting Mei 2 , Tim White 1
1 Material Science and Engineering, Nanyang Technological University, Singapore Singapore, 2 Electrical and Electronic Engineering, Nanyang Technologoical University, Singapore Singapore
Show Abstract In recent years, chemically synthesized colloidal nanocrystals were found to exhibit many attractive advantages in physical properties (e.g. size-tunable optoelectronic characteristics), versatile solution process ability, and low fabrication cost. Nanostructure materials like nanocrystals with dimensions down to nanometer scale, such as quantum dots (QDs), are fundamentally preferred gain material for fabricating low dimensional devices, benefiting from quantum effects. Colloidal synthesis is an economical and efficient approach providing nanocrystal materials with tunable size to meet emission wavelength requirements. In the field of photonics, colloidal quantum dots based materials have the opportunity for exploitation in diverse applications such as biolabeling, photovoltaics, electroluminescence, etc. In this work, the synthesis of ZnO, In2O3 and Ga2O3 (1-3 nm) semiconductor nanocrystal materials were investigated and their photoelectric properties were measured. X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray photoelectron spectrometry (XPS), and photoluminescence (PL) spectroscopy were used to characterize the nanocrystals obtained. The PL shows a strong blue-shift because of small dimension effect, and other results show that oxide nanocrystals were fabricated successfully in certain condition and the morphology of the samples varied with synthesis condition. The relations between property and structure were analyzed for further improvement of the process. A light-emitting device was fabricated based on the experimental results. Through this work, an economical, efficient technological platform based on colloidal synthesis will be developed for the fabrication of oxide semiconductor nanocrystal gain materials, and UV light emitting devices.
9:00 PM - L11.3
Homoepitaxial ZnO-Layers Grown in 2D-Growth Mode by MOVPE.
Soren Heinze 1 , A. Krtschil 1 , J. Blaesing 1 , A. Dadgar 1 , T. Hempel 1 , J. Christen 1 , A. Krost 1
1 Institute of Experimental Physics, Otto-von-Guericke-University, Magdeburg Germany
Show AbstractThe II-VI semiconductor ZnO offers a tremendous potential for optoelectronic devices in the blue and near ultraviolet spectral regime due to its wide direct band gap of 3.4 eV and the large excitonic binding energy of 60 meV. In contrast to GaN large and inexpensive ZnO substrates are available and therefore homoepitaxial growth of ZnO-layers should be possible. In contrast to heteroepitaxial growth, homoepitaxial grown layers are expected to have a significantly lower number of structural defects due to advantages as perfect lattice and thermal expansion match. However, most of the work in recent years has been performed on heteroepitaxial growth of ZnO layers. For the purpose of homoepitaxial growth the substrates were annealed in a rapid thermal-annealer for 15 minutes at 1100°C in a box with ZnO powder and oxygen atmosphere. Annealing under these conditions leads to atomically flat surfaces on a large scale area.On such pretreated substrates we have grown high quality ZnO layers by MOVPE. A systematic variation of growth conditions like VI/II-ratio, growth temperature and reactor pressure finally resultes in atomically flat surfaces grown under a 2D-growth mode. Here, we present our latest results on the morphological and structural properties of these layers.
9:00 PM - L11.30
Synthesis of Assembled ZnO Structures by Precipitation Method in Aqueous Media.
Selene Sepulveda-Guzman 1 , Elder de la Rosa 1 2 , B. Reeja-Jayan 1 3 , Miguel Ramirez 1 , Domingo Ferrer 1 , Xiaoxia Gao 4 , Miguel Jose-Yacaman 1 4
1 Chemical Engineering, The University of Texas, Austin, Texas, United States, 2 , Centro de Investigación en Optica, Leon , Guanajuato, Mexico, 3 Electrical and Computer Engineering, The University of Texas, Austin, Texas, United States, 4 Texas Materials Institute, The University of Texas, Austin, Texas, United States
Show AbstractThe design of structures by the assembly of nanoparticles have received great attention during the last years due to their different properties from those of the single nanoparticles. In addition, the process of assembly allows the study of several growth parameters, such as nucleation, that are difficult to elucidate in single particles. A wide range of methods have been developed to assembly ZnO nanoparticles in complex structures such as spheres, flower-like and web-like agglomerates. However, the synthesis of these structures usually requires two or more steps in order to obtain the assembly. In this work we present a one-step synthetic method to obtain assembled ZnO structures by precipitation in aqueous media. The morphology of the ZnO assemblies was studied by Scanning electron microscopy (SEM). Snowflake-like and flower-like morphologies can be obtained by changing the temperature. Possible mechanism that explain the change in the morphology by the reaction temperature will be discussed. The crystalline structure of the samples was analyzed by X-ray diffraction and HRTEM. STEM analysis was done to study the morphology and the mass distribution of the elements of the assembly. The optical properties of the resulting structures were determinated by photoluminescence (PL). The results indicate that the ZnO structures present UV and visible emissions upon excitation at 350 nm. According to the XPS analysis, the ZnO assemblies obtained at lower temperature present excess of Zn of 4% that generates the yellow emission band.
9:00 PM - L11.31
Electrical and Optical Studies on Bandgap Modified MgxZn1-xO Transparent Thin Films.
Wei Wei 1 , Chunming Jin 1 , Jagdish Narayan 1 , Roger Narayan 2
1 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 Joint Department of Biomedical Engineering, University of North Carolina at Chapel Hill and North Carolina State University, Chapel Hill, North Carolina, United States
Show Abstract9:00 PM - L11.32
Top-gate ZnO-based TFTs and Circuits by RF Sputtering.
Shahrukh Khan 1 , Abbas Jamshidi-Roudbari 1 , Miltiadis Hatalis 1
1 Electrical Engineering, Lehigh University, Bethlehem, Pennsylvania, United States
Show Abstract9:00 PM - L11.35
Structural and Field Emission Properties of Selectively-Grown ZnO Nanorod Arrays by Solution Method.
Ahsan Qurashi 1 , Yoon Hahn 1 , Sang Kim 1 , Jin Kim 1
1 School of Semiconductor and Chemical Engineering , Chonbuk National University, Jeonju, jeoklabukto, Korea (the Republic of)
Show AbstractWe have demonstrated a novel and scalable solution method for the growth of high-quality and uniform ZnO nanorod arrays. The growth location and shape of nanorods were defined by lithography precisely. Growth of bunch-like nanorod arrays with excellent long-range order has been observed when the pattern size was kept as 2x2 µm2, and 500x500 nm2 respectively. The successful synthesis of periodic individual, or two nanorods was achieved by decreasing pattern size to sub 100x100 nm2. The diameter and length of nanorods were found to be 70~80 and 800 ~ 1,000 nm, respectively. Structural analysis revealed that the as-grown nanorods are single crystalline in nature and grown along c-axis direction. The photoluminescence studies of nanorods showed a sharp ultra violet emission at a wavelength of ~381 nm and a broad deep-level visible emission at ~580 nm. Field emission current density (J) versus macroscopic field (E) measurements showed that the emission current density of ZnO nanorods is high as 0.052 mA/cm2 at the field of 3.2 V/micrometer and the turn-on field, a field where the emission current density can be districted from the background noise, is about 2.85 V/micrometer m.
9:00 PM - L11.36
Enhanced Mode of ZnO Field Effect Nanobiosensor.
Sang Yeol Lee 1 , Kyung Ah Jeon 2 , Minsoo Sohn 3 , Kyunghwa Yoo 3 , A Mi Choi 4 , Hyo Il Jung 4
1 Center for Energy Materials, Korea Institute of Science and Technology, Seoul Korea (the Republic of), 2 2Department of Electrical and Electronic Engineering, Yonsei University, Seoul Korea (the Republic of), 3 Department of Physics, Yonsei University, Seoul Korea (the Republic of), 4 4School of Mechanical Engineering, Yonsei University, Seoul Korea (the Republic of)
Show Abstract9:00 PM - L11.37
Valence Band Structure of ZnO and MgxZn1-xO.
R. Schmidt-Grund 1 , C. Sturm 1 , M. Schubert 2 , N. Ashkenov 1 , B. Rheinlaender 1 , D. Faltermeier 3 , H. Hochmuth 1 , A. Rahm 1 , J. Blaesig 4 , C. Bundesmann 5 , J. Zuniga-Perez 6 , T. Chavdarov 1 , M. Lorenz 1 , M. Grundmann 1
1 Institut für Experimentelle Physik II, Semiconductor Physics, Universtät Leipzig, Leipzig Germany, 2 Department of Electrical Engineering, University of Nebraska, Lincoln, Nebraska, United States, 3 Physikalisches Institut, Universität Stuttgart, Stuttgart Germany, 4 FNW/IEP/AHE, Otto-von-Guericke Universität Magdeburg, Magdeburg Germany, 5 , Leibniz-Institut für Oberflächenmodifizierung e.V., Leipzig Germany, 6 , CRHEA-CNRS, Valbonne France
Show AbstractIn recent years, much attention has been devoted to study the optical properties of ZnO. Beyond these, the band-gap near band-to-band transitions and the related excitonic contributions are of specially interest because their relevance for optoelectronic applications. For wurtzite-structure ZnO and MgZnO, due to the spin-orbit and crystal-field interaction, the topmost valence band is split into three subbands with different symmetries. The band-to-band transitions involving these bands and the lowest conduction band have different selection rules for coupling with light polarised parallel or perpendicular to the wurtzite-structure optical axis. So far, the order of the Γ-point valence bands (Γ7-Γ9-Γ7 or Γ9-Γ7-Γ7) is still a question under debate.
We present an attempt to clear up the valence band order of ZnO and MgxZn1-xO thin films grown using pulsed laser deposition on sapphire substrates. The analysis is based on the sign and the value of the spin-orbit and crystal-field splitting energies. We have varied these parameters upon strain-variation, variation of the unit-cell volume, and by cation replacement (Zn with Mg). The found dependencies have been compared with theoretical predictions. The strain variation was provoked by utilising the temperature dependence of the lattice mismatch between the films and the substrates, by using different oriented sapphire substrates (c-, r-, and m-plane), and by variation of the thin film thickness. A variation of the unit-cell volume was induced by temperature variation as well as by changing the Mg content in the mixed crystals.
The spin-orbit and crystal-field splitting energies have been calculated applying the quasi-cubic model for the three band-gap near band-to-band transition energies. The latter have been obtained by means of model analysis of spectroscopic ellipsometry data for temperatures between 5 K and 800 K. The structural properties of the thin films have been investigated using X-ray diffraction measurements (XRD), even in dependence on the temperature. From the XRD data, the lattice constants, the unit-cell volume, and the strain-tensor elements have been calculated. Using k×p perturbation theory, the effect of the strain on the spin-orbit and crystal-field splitting energies was taken into consideration.
From these investigations we have found very strong hints for the valence band order of ZnO thin films to be Γ7-Γ9-Γ7 at room temperature.
9:00 PM - L11.38
Solution-phase Synthesis of ZnO Nanoparticles: Nucleation Kinetics.
Geonel Rodriguez-Gattorno 1 , Gerko Oskam 1
1 Applied Physics, CINVESTAV - IPN, Merida, Yucatan, Mexico
Show AbstractSolution processing has been shown to be a suitable method to prepare ZnO nanoparticles allowing a large degree of control over the particle size and size distribution. ZnO nanoparticles are generally prepared from a non-aqueous solvent such as alkanols by reacting a dissolved Zn-salt with a source of hydroxide such as NaOH or even H2O. The synthesis can be divided in four steps: precursor formation, nucleation, growth, and aging (aggregation and coarsening). The relative rates of the different processes determine the properties of the final product. Extensive work has been published on the coarsening kinetics of ZnO nanoparticles, but very few papers deal with the kinetics of the nucleation process. In this paper, we present results on the nucleation, growth and aging kinetics for the synthesis of ZnO nanoparticles from the system ZnCl2, NaOH, and H2O in ethanol, as a function of reactant concentration. The reaction kinetics are found to be a strong function of their respective concentrations. For the syntesis of ZnO from 1 mM ZnCl2 and 1 - 2 mM NaOH, the nucleation of ZnO is very slow at relatively low water concentration (20 - 50 mM), while the nucleation and growth rates increase with higher water concentration. The coarsening kinetics are found to be essentially independent of water concentration. At a fixed, low water concentration, a time period is observed where only nucleation of ZnO nanoparticles occurs, and the growth and coarsening rates are negligible. In this regime, the nucleation rate was found to be proportional to the ZnCl2 concentration (maintaining the ratio [ZnCl2]:[NaOH:[H2O] a constant). At sufficiently high water concentrations (> 100 mM), the nucleation and growth kinetics are very fast, and mainly coarsening is observed. In this case, it is found that the ZnO formation reaction is only completed if the [ZnCl2]:[NaOH] = 1:2; it can be concluded that at lower concentrations of NaOH, water cannot react with the additional ZnCl2 to form ZnO. The results are discussed in terms of two mechanisms, involving the kinetics of precursor formation and the dependence of the nucleation rate on the supersaturation.
9:00 PM - L11.4
Transparent Thin-film Transistors with ZnO Active Layer Fabricated by RF Magnetron Sputtering Method.
Woo-Seok Cheong 1 , Chi-Sun Hwang 1 , Sang-Hee Ko Park 1 , Jaeheon Shin 1 , Chun-Won Byun 1 , Doo-Hee Cho 1 , Minki Ryu 1 , Shinhyuk Yang 1 , Sung Min Yoon 1 , Jeong-Ik Lee 1 , Hye Yong Chu 1 , Kyoung Ik Cho 1
1 , Electronics telecommunications Research & Institute, Daejeon Korea (the Republic of)
Show AbstractZinc oxide(ZnO) is very interesting material for practical applications because of its high transparency in the visible light and simultaneously high electrical conductivity. In this study, transparent thin-film transistors(TTFTs) with active channel of zinc oxide were fabricated, where zinc oxide thin films were prepared by RF(radio frequence) magnetron sputtering on glass substrates. ZnO films could be made diffrently under the condition of different working pressures, gas ratios (oxygen/oxygen+argon) and rf plasma powers. For our analysis on device properties, as well as bottom-gated structrues (inverted staggered, inverted coplanar), top-gated structures (staggered, coplanar) were used with changing active deposition conditions. The microstructures of films and devices were investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD), FIB(focused ion beam) scanning electron microscopy (SEM) and atomic force microscopy (AFM). From the analysis of microstructures, ZnO films without oxygen gas were similar to the structure zone model (SZM), while other films (with oxygen gas) were shown a similar morphological evolution with each other. At the condition with high kinetic energy, the Rrms was reachied the lowest value, 0.36nm at 100nm (film thickness). The optical transmittance at 550nm was around 80% or higher for most films. In general, current-voltage (I-V) properties measured through the gate showed that the channel was n-type, and Ion/Ioff ratio was more than 106, where the channel mobility had the values of 0.06 ~2.0 cm-2V-1s-1. The trend of transmittance and XRD was closely related with the device properties. The high peak intensity in XRD resulted in the high off current, while the device with low stressed active films did in the high on current.
9:00 PM - L11.5
Donor Doping of ZnO with Group VII Elements.
Joachim Sann 1 , Niklas Volbers 1 , Stefan Lautenschlaeger 1 , Swen Graubner 1 , Felizitas Eylert 1 , Melanie Pinnisch 1 , Kay Potzger 2 , Bruno Meyer 1
1 1st Physics Institute, University of Giessen, Giessen, Hessen, Germany, 2 Institute of Ion Beam Physics and Materials Research, Forschungszentrum Rossendorf, Dresden Germany
Show AbstractIn order to produce ZnO-based devices not only p-type doping, but also controlled n-type doping is essential. For the group VII elements F, Cl, Br, I substituting O neither their donor levels nor their influence on Photoluminescence spectra have been intensely studied.In this work we doped ZnO single crystals with F, Cl, Br or I by diffusion through the surface and by ion implantation with fluences of about 1014 ions/cm2. The samples have been investigated by low temperature and temperature dependent Photoluminescence, and Hall measurements have been performed in the van-der-Pauw configuration. We will give an assignment between dopant and PL-recombinations and determine the respective donor binding energies. First results on CVD-grown in-situ doped samples will be shown.
9:00 PM - L11.6
Optical Properties of Nanoporous ZnO Structures.
Waitz Thomas 2 , Michael Tiemann 2 , Joachim Sann 1 , Jan Stehr 1 , Peter Klar 1 , Bruno Meyer 1 , Swantje Horst 3 , Sangam Chatterjee 3
2 Institute of Inorganic and Analytical Chemistry, Justus Liebig University, Giessen Germany, 1 1st Physics Institute, University of Giessen, Giessen, Hessen, Germany, 3 Faculty of Physics and Material Sciences Center, Philipps-University , Marburg Germany
Show AbstractWe report the synthesis of nanoporous ZnO, which exhibits a periodically ordered, uniform pore system with crystalline pore walls. The crystalline structure is investigated by X-ray diffraction, transmission electron microscopy, and selected area electron diffraction. The large specific surface area and the uniformity of the pore system are confirmed by nitrogen physisorption. Raman spectroscopy along with low-temperature continuous-wave and time-resolved photoluminescence measurements confirm the high degree of crystallinity and give insight into defects participating in the radiative recombination processes. The band edge and near band edge luminescence are dominant in the entire spectral range from 2 to 3.4 eV, the deep recombinations in the visible spectral range being almost five orders of magnitude lower in intensity. Individual recombinations can be assigned to specific extrinsic impurities, i.e. gallium and hydrogen. The I8 recombination line appears to be significantly broadened compared to bulk samples. The broadening is attributed to different local environments of gallium donors situated at the surface or in the interior of the pore walls. This reflects the enhanced surface-to-volume ratio. The dynamics of the recombination processes are compared with corresponding bulk reference samples.
9:00 PM - L11.7
Gas Sensing Properties of Porous ZnO Nano-Platelet Films.
Amandeep Saluja 2 , Seth Hubbard 1 , Jie Pan 3 , Lei Kerr 3 , Eunjung Cho 4 , Dimitris Pavlidis 4
2 Microelectronics Engineering Department, Rochester Institute of Technology, Rochester, New York, United States, 1 Department of Physics, Rochester Institute of Technology, Rochester, New York, United States, 3 Department of Paper and Chemical Engineering, Miami University, Oxford, Ohio, United States, 4 Department of High Frequency Electronics, Technische Universität Darmstadt, Darmstadt, Darmstadt, Germany
Show AbstractSolid state chemical sensors are gaining popularity and finding extensive use in process control, environmental monitoring and residential safety. ZnO, a semiconducting metal oxide, has attracted great interest over the years due to its sensitivity to a variety of gases. Nanostructured sensing materials, such as nanowires, nanotubes and quantum dots offer an inherently high surface area, thus reducing operating temperatures and increasing sensitivity to low concentrations of analytes. In this work, various ZnO nanostructures were synthesized and tested as chemical sensors. Initial ZnO nanoparticles were prepared by a simple solution chemical process. The as-prepared ZnO thin films were characterized by Secondary Electron Microscopy (SEM) and BET Sorptometer. SEM images demonstrate that the particles have uniform diameter of about 15 nm and are aligned to form plate-like nanostructures. The surface area obtained from BET measurement is 21.5 m2/g with 50% of the pore diameters in the range of 20 to 80 nm.The gas sensor platforms consisted of Pt inter-digitated fingers with a spacing of 10 μm. The sensor platform was dip-coated with ZnO nano-platelets suspended in terpineol to form a uniform film. Sensing was performed in a closed quartz chamber and the flow of gases was regulated using calibrated mass flow controllers. For all experiments, the carrier gas was high purity N2 and dry industrial air was used after exposure to recover the sensors to their initial state.The sensitivity [(Rair-Rgas)/(Rair)] was measured for different concentrations of the analytes and values in excess 80% for 60ppm of H2 gas were observed at room temperature. High sensitivity of the sensor at low temperatures is attributed to both the increased surface area of the porous ZnO nano-platelets and the presence of a Pt catalyst. Measurements at higher temperatures (150 °C) show even higher sensitivities, near 96% for a 20 ppm H2 concentration. Short response and recovery times of about 200 seconds were observed for these ZnO nano-platelets. The sensitivity for CO gas was also measured and found to be about near 90% for 80 ppm at operating temperatures of 200 °C. The sensors depict incomplete recovery of resistance at room temperature for both H2 and CO gases. This effect is possibly due to the traces of elemental Zn in the material, which were not oxidized at the time of recovery. However, this effect was not observed at higher temperatures.Gas sensor design and sensor platforms will be optimized for nano-structured materials and a detailed study on the effect of different operating parameters such as temperature, inter electrode spacing, gas concentration and material properties (ZnO nanorod arrays, plates and wires) on gas sensitivity will be presented. The sensing mechanism of each type of gas will be correlated to the nanomaterials properties. Sensitivity of the ZnO nanostructures to other sensing gases such as NH3 and C2H5OH will be presented.
9:00 PM - L11.8
Annealing Studies on Arsenic Implanted Zinc Oxide.
Niklas Volbers 1 , Stefan Lautenschlaeger 1 , Joachim Sann 1 , Shengqiang Zhou 2 , Kay Potzger 2 , Frank Bertram 3 , Andre Krtschil 3 , Jürgen Christen 3 , Alois Krost 3 , Holger Wenckstern 4 , Marius Grundmann 4 , Bruno Meyer 1
1 1st Physics Institute, University of Giessen, Giessen, Hessen, Germany, 2 Institute of Ion Beam Physics and Materials Research, Forschungszentrum Rossendorf, Dresden Germany, 3 Institute of Experimental Physics, Otto-von-Guericke-University, Magdeburg Germany, 4 Institute for Experimental Physics, University of Leipzig, Leipzig Germany
Show AbstractArsenic is considered a possible acceptor in in ZnO and as such it is a possible dopant to achieve p-type conductivity. Ion implantation provides an accurate and reproducable method to incorporate this dopant into ZnO crystals.In the presented work, high quality ZnO crystals were implanted with 75As ions, using implantation doses of 1016 atoms/cm2. The crystals were then annealed at temperatures of up to 1100 °C. Dynamic secondary ion mass spectrometry shows the evolution of the implanted ions in the films and the corresponding motion of residual impurities. For the electrical characterisation, Hall effect, scanning capacitance microscopy, CV and admittance measurements were performed. In addition, the optical properties were investigated by observing the photoluminescence and cathodoluminescence.
9:00 PM - L11.9
Preparation of a Dominant (002) Orientation ZnO Nano-rod by Electrodeposition and its Optical Properties.
Chia-Feng Chang 1 , Tsung-Ying Liu 1 , Chen-Feng Huang 1 , Tsing-Hai Wang 2 , Shi-Ping Teng 2 , Jiann-Ruey Chen 1
1 material science and engineering, national tsing hua university, Hsinchu, Taiwan, 2 Dept. Engineering and System Science, National Tsing Hua University, Hsinchu Taiwan
Show AbstractIn this paper, ZnO nano-rods with a dominant (0 0 2) orientation were obtained by chemical electrodeposition on ITO glass substrates at lower temperature under constant current. Our results showed that the grain size and aspect ratio of the ZnO nano-rods are strongly vulnerable with some parameters such as current density, deposition time and temperature of the reaction bath. Results from XRD suggest that the moiety of (0 0 2) orientation increases when the applied current density decreases. The XPS analysis shows a Zn/O ratio between 0.5~1, which is in good agreement with its stoichiometry. In addition, the ZnO nano-rods are composed mostly of hexagonal columns, which is confirmed by SEM analysis. Furthermore, a band gap of 3.3 eV of ZnO is obtained from photoluminescence spectra.
Symposium Organizers
David P. Norton University of Florida
Chennupati Jagadish Australian National University
Irina Buyanova Linkping University
Gyu-Chul Yi Pohang University of Science and Technology (POSTECH)
L12: ZnO Thin Film Devices
Session Chairs
Friday AM, November 30, 2007
Constitution A (Sheraton)
9:30 AM - L12.1
Advanced Zinc Oxide Based Devices for the Next generation of Thin Film Transistors and Solar Cells.
Rodrigo Martins 1 2 , Leandro Raniero 1 , Pedro Barquinha 2 1 , Luis Pereira 1 2 , Goncalo Goncalves 2 1 , Elvira Fortunato 2 1
1 CEMOP, Uninova, Caparica Portugal, 2 CENIMAT I3N, FCTUNL, Caparica Portugal
Show Abstract9:45 AM - L12.2
ZnO Based Transparent Thin Film Transistor with GZO Electrode and HfO Dielectrics.
Sang Yeol Lee 1
1 Center for Energy Materials, Korea Institute of Science and Technology, Seoul Korea (the Republic of)
Show AbstractTransparent thin film transistor (TTFT) with ZnO channel layer has been fabricated and characterized. As grown n-type ZnO channel layer, HfO2 dielectric layer and Ga doped ZnO transparent electrode have been adopted to implement transparent thin film transistor. Pulsed laser deposition and RF magnetron sputtering system have been used to fabricate thin films. Investigation on ohmic contact property between Ga doped ZnO transparent electrode and ZnO channel layer is presented with the variation of energy bandgap for the application of ZnO semiconducting devices. Electrical and optical properties of TTFT show on-off ratio of higher than 105 and transparency of higher than about 80%, respectively.
10:00 AM - L12.3
The Effects of Various Circumstances for GIZO Thin Film Transistors.
Sun Il Kim 1 , Chang jeong Kim 1 , Jae Chul Park 1 , Ihun Song 1 , Jae Chul Lee 1 , Eun Ha Lee 1 , Young Soo Park 1
1 Semiconductor Device Lab., SAIT, Yongin-si, Gyeonggi-do Korea (the Republic of)
Show AbstractIn this study, we investigated the effects of various circumstances, especially the plasma and humidity, for Ga2O3-In2O3-ZnO (GIZO) TFT. Before the TFT is passivated, the active surface of bottom gated GIZO TFT is exposed to the plasma and wet solution in conventional process. Especially, the source/drain metal etching process delivers the plasma damage on the surface of GIZO active layer. When the GIZO TFT was exposed in SF6, Ar and O2 plasma for metal dry etching, a significant change in Ids-Vgs curve was observed. The off current increased dramatically and the threshold voltage shifted negatively. It is considered because the oxygen vacancy is generated and the carrier concentration increases in surface of active layer by ion bombardment. However, the off current and threshold voltage recovered as time was going. The recovery time is longer in Ar plasma than SF6 or O2 plasma. For the effect of water vapor, the GIZO TFT is left in humidity of 80 %. The off current and threshold voltage is changed and the recovery is observed like the exposure in plasma. It is considered because the absorption of OH- radical generates the carriers in the active surface due to the bond break of Zn, Ga, In and oxygen.
10:15 AM - L12.4
Hysteresis Problem in ZnO Electronic Devices.
Hyunjin Ji 1 , Jaewoo Lee 1 , Hyeyeon Ryu 1 , Daehyun Kim 1 , Gyutae Kim 1 , Honyun Lee 2
1 , Korea University, Seoul Korea (the Republic of), 2 , LG electronics, Seoul Korea (the Republic of)
Show AbstractZinc oxide (ZnO) has drawn much attention because of their interesting semiconducting properties as a transparent device to visible lights with a wide band-gap energy, 3.37eV. In fabricating ZnO TFT (thin film transistor) devices, the easy sputtering process can facilitate the formation of thin film as a TFT on a plastic flexible substrate with a rather high mobility and a high on/off ratio over 105. But the stability problems such as reproducibility or hysteresis became crucial for the practical application together with the recipe of enhancing the mobility. Voltage-current characteristics of ZnO devices showed different behaviors under the floating gate or well-defined gate voltages as shown in Fig 1. When measuring the current, the voltages were swept from zero to a positive maximum then to a negative maximum and returned to zero. The current level for the sweeping-up stages was smaller than the case for the sweeping-down stages. The hysteresis in the voltage-current characteristics was significant, attributed to the existence of traps in the gate side and the channel side close to an insulator-semiconductor interface. In the case of the floating gate configuration, the charges in the insulator side traps were maintained so that the polarization of the dielectric (gate insulator) is determined by the initial charge density (electron) of the gate insulator sides. The dielectric polarization can influence on the effective gate voltage owing to the additional dipole field. The gate effect increased as the increase of drain voltage (Vds) can induce the migration of the initial charges from the semiconductor. Based on the assumption of constant charges the gate bias will change, resulting in the positive curvature of Vds characteristics different from the conventional FETs(field effect transistors). The changes of the gate effects can be verified by measuring the variation of the capacitance between the gate and the source with the different drain voltages. The voltage-current characteristics as sweeping the gate voltage showed the opposite direction in the hysteresis different from the output characteristics, which can be explained by the variation of the threshold voltage as Vgs increase. The hysteresis was also found in individual ZnO nanowire FETs. Voltage-current characteristics in a suspended configuration of a ZnO nanowire FET can verify the effect of gate dielectric material to the hysteresis. Pulsed bias was observed to decrease the hysteresis curves, which can be explained by the reduced charging in the traps. The specific conduction mechanism will be discussed together with the experimental proofs.
10:30 AM - L12.5
Solution Processed Doped ZnO-based Thin Film Transistors.
Sunho Jeong 1 , Sul Lee 1 , Youngmin Jeong 1 , Dongjo Kim 1 , Jooho Moon 1
1 Department of Materials Science and Engineering, Yonsei University , Seoul Korea (the Republic of)
Show AbstractChannel layers of thin-film transistors (TFT) have been generally fabricated by vacuum deposition methods. However, recent years have seem a growing interest in realizing TFTs based on solution-processable semiconducting materials for the applications in which low-cost and low-temperature manufacturing is demanded. ZnO-based semiconducting oxides receive a recent spotlight as a new channel material alternative to conventional Si-based materials and organic semiconducting materials. We have developed solution-processble Sn doped ZnO-based semiconductor materials by sol-gel chemical reactions. The precursor solution was spin-cast to form a uniform crack-free dense channel layer of the thickness ~ 100 nm. Stable sol-gel precursor solutions were obtained by controlling the composition of solvents and stabilizers, and their influences on electrical performance of the transistors were demonstrated by measuring electrical parameters such as off-current, on-current, mobility, and threshold voltage. Microstructure and crystallization behavior of the doped ZnO films were investigated by SEM, XRD, and TG/DTA. For improving an electrical performance of the transistors, in addition, Sn doping concentrations varied from 5 to 40 mol% and XPS surface analysis was performed to understand the role of Sn doping. The mobility values of the device heat-treated at 350 - 500 °C are 0.1 - 0.5 cm2/Vs and on/off current ratio is 103 - 105. Furthermore, this sol-gel derived semiconducting oxide precursors are easily adopted for direct-writing method such as ink-jet printing. We have also demonstrated the TFTs based on the ink-jet printed doped ZnO.
10:45 AM - L12: ZnO Thin
BREAK
11:15 AM - L12.6
ZnO Thin Film, Device, and Circuit Fabrication Using Low Temperature CVD Processes.
Devin Mourey 1 3 , Jie Sun 2 3 , Dalong Zhao 2 3 , Shelby Nelson 4 , David Levy 4 , Diane Freeman 4 , Thomas Jackson 2 3
1 Materials Science and Engineering, Penn State University, State College, Pennsylvania, United States, 3 , Center for Thin Film Devices and Materials Research Institute, State College, Pennsylvania, United States, 2 Electrical Engineering, Penn State University, State College, Pennsylvania, United States, 4 , Eastman Kodak Company, Rochester, New York, United States
Show AbstractZnO and similar metal oxides have recently received a high level of interest as a result of their wide bandgap, high transparency, and good electrical transport. [1] We report transparent, boron-doped, ZnO films on glass substrates by plasma enhanced chemical vapor deposition (PECVD) at 200oC. A ~350nm boron-doped ZnO film showed a minimum resistivity of 4 x 10–4 Ω●cm with transmission >85% over the entire visible spectrum. Diethyl zinc (DEZ) [Zn(C2H5)2], carbon dioxide (CO2), and triethylboron (TEB) precursors are used to form the heavily doped zinc oxide film. The maximum free electron concentration, determined by Hall Effect measurement, was 1021/cm3 with a corresponding mobility of 13.5 cm2/V●s. SEM, AFM, XRD, and photoluminescence were done to understand film morphology and crystalline orientation and to correlate materials properties to electrical transport in these films. A comparison was done between doped and intrinsic ZnO films, and relationships between film structure and electrical transport were observed. In addition, undoped ZnO deposited by PECVD was compared to an atmospheric pressure chemical vapor deposition process at the same substrate temperature of 200oC. The two deposition techniques studied have differing crystalline texture and grain structure. Thin film transistors (TFTs) were fabricated and evaluated from both the PECVD and the atmospheric CVD processes. Top contact TFTs were formed on glass with sputtered indium tin oxide or Cr patterned to form the gates, a CVD-based aluminum oxide dielectric, and aluminum source and drain contacts formed by lift off. The zinc oxide and aluminum oxide layers were isolated by a wet etching process. TFTs fabricated using either PECVD or atmospheric CVD had field effect mobility >10 cm2/V●s. The atmospheric CVD process devices were highly uniform with threshold voltage <7 V, and subthreshold slope <0.5 V. Despite the potential advantages of high mobility metal oxide TFTs, few reports have investigated the use of zinc oxide or related materials in circuits. Ofuji, et al, recently reported a 5-stage indium gallium zinc oxide ring oscillator with propagation delay of less than 240 ns at 18V.[2] We have fabricated 7-stage ring oscillators using ZnO deposited by PECVD or atmospheric CVD. Circuits using PECVD ZnO showed relatively slow dynamic response compared to the atmospheric CVD process. Circuits using atmospheric process CVD ZnO had propagation delays <100 ns/stage and oscillation frequencies >1MHz at a supply voltage of 32 V and represent the fastest ZnO circuits reported to date.[1] Ozgur, U. et al. Journal of Applied Physics 98, 041031 (2005).[2] Masato Ofuji, Katsumi Abe, et al., IEEE EDL, Vol. 28, No.4, April 2007
11:30 AM - L12.7
Properties of ZnO/ZnS Mixed Thin Films Grown by Atomic Layer Deposition Method for the TTFT Applications.
Sunyeol Jeon 1 , Seokhwan Bang 1 , Seungjun Lee 1 , Semyung Kwon 1 , Wooho Jeong 1 , Inhoe Kim 1 , Hyeongtag Jeon 1 , Ho Jung Chang 2 , Hyung-ho Park 3
1 Division of Materials Science and Engineering, Hanyang University, Seoul Korea (the Republic of), 2 , Dankook University, Cheonan Korea (the Republic of), 3 , Yonsei University, Seoul Korea (the Republic of)
Show AbstractCompound thin films can exhibit a widely tunable range of physical and electrical properties by varying the ratio of the composition. Atomic layer deposition (ALD) technique is one of methods to grow various compound films because of its basic characteristics of sequential and self-limiting surface reactions. In this study, ZnO/ZnS mixed thin films were grown with a traveling wave type ALD system in order to fabricate the active channel layer of the transparent thin-film transistors (TTFT) for the next generation display device. The ZnO thin films grown by ALD with H2O are too conductive to act as channel layers. In contrast to the ZnO ALD films, ZnS ALD films exhibited the semi-insulating properties due to their low carrier densities. For this reason, we grew the thin films with ZnO/ZnS mixed and/or lamellar structures from alternate reactions of ZnO and ZnS growths. The precursor used was Zn(C2H5)2, and reactant gases for ZnO and ZnS were H2O and H2S, respectively. The deposition rates of the ZnO and ZnS films were approximately 1.9 and 2.1 Å/cycle, respectively. The composition ratio of thin films was controlled by the number of ZnO and ZnS cycles. The chemical, physical, and electrical properties of these thin films were evaluated to be used as channel layers for a TTFT structure. We analyzed chemical bond states of the thin films with X-ray photoelectron spectroscopy (XPS). Auger electron spectroscopy (AES) was used to analyze the chemical composition and impurity contents. The crystallinity of the thin films was characterized by X-ray diffraction (XRD). The interface morphology and thicknesses of these thin films were determined by using high-resolution transmission electron microscopy (HRTEM). We examined the electrical characteristics by the hall effect measurement, capacitance-voltage (C-V) measurement, and current-voltage (I-V) analysis with a capacitor structure, and the proper carrier density ranges of 1012 ~ 1015 cm-3 depending on the compositional variations were observed.
11:45 AM - L12.8
Stability of Transparent Zinc Tin Oxide Transistors Under Irradiation with Visible Light.
Patrick Goerrn 1 , Marcus Lehnhardt 1 , Thomas Riedl 1 , Wolfgang Kowalsky 1
1 Institut für Hochfrequenztechnik, TU Braunschweig, Braunschweig Germany
Show AbstractAmorphous thin film transistors (TFTs) are widely used as switching devices for active matrix displays due to the possibility of large scale integration on glass substrates. To date, the material of choice has been amorphous hydrogenated silicon (a-Si:H). On the other hand, amorphous oxide semiconductors containing post-transition-metal cations like zinc tin oxide (ZTO) exhibit field-effect mobilities one order of magnitude larger.Moreover, the stability of zinc tin oxide TFTs against bias stress can be superior compared to amorphous silicon TFTs. After 1000 hours of bias stress with V_GS=V_DS=10V and I_DS=188uA, which is 2 orders of magnitude higher than the current needed for a pixel, we see a shift of the threshold voltage as low as 320 mV. This long term stability plays a pivotal role for analog applications like active matrix organic light emitting diode (OLED)displays, because a change of the characteristic of the TFTs concomitantly leads to a modification of the individual pixel brightness.In addition, wide band gap oxide semiconductors like ZTO are highly transparent in the visible part of the spectrum. Thus ZTO TFTs allow for the realization of applications beyond reach of a-Si:H e.g. active-matrix, bottom-emitting OLED displays with high filling factors or even entirely transparent active-matrix displays. Of course, these applications require a low photosensitivity of the transparent TFTs. For oxide TFTs very little is known about the influence of visible light irradiation. Owing to the wide band gap (larger than 3 eV), fundamental absorption of visible light can be neglected. Nevertheless, due to the amorphous structure a significant contribution of defect states to the absorption is expected. The variation of the characteristics of transparent zinc tin oxide thin film transistors under illumination of visible light has been studied using light sources with different wavelength and intensity. Upon illumination a decrease of threshold voltage, field effect mobility and an increase of the off current were observed. All these effects have been found to be reversible. The temporal dependence of the change of these device parameters has been researched. Typical time scales are on the order of tens of hours. The effect of composition and processing temperature on the device photosensitivity has been investigated. At a typical display brightness (below 1000 cd/m2) the shift of the threshold voltage was found to be the dominating effect and is typically smaller than 2000 mV, even for illumination with blue light (470 nm). The results of our investigations allow an estimate of the applicability of ZTO TFTs as OLED drivers in transparent displays concerning shelf-life and stability against bias stress, illumination and temperature.
12:15 PM - L12.10
Pulsed Laser Deposition of ZnOxTe1-x Thin Films and Application to p+-Si/ZnOTe/n-ZnO Heterojunction Diodes.
Weiming Wang 1 , Willie Bowen 1 , Shihchun Lin 1 , Sarah Spanninga 1 , Jamie Phillips 1
1 , The University of Michigan, Ann arbor, Michigan, United States
Show AbstractWide bandgap semiconductors based on ZnO and related II-VI oxide alloys are attractive for several device applications including light emitters and detectors operating in the visible/ultraviolet spectral region, and transparent electronics. Research and development efforts on alloys related to ZnO have predominantly focused on the mixed cation materials CdZnO, MgZnO, and BeZnO. Mixed anion alloys related to ZnO may also provide the ability to tune semiconductor bandgap energy in the visible and ultraviolet spectral region, but are much less understood. Alloys of ZnTeO may be of particular interest due the bandstructure of ZnTe and intrinsic p-type properties. ZnTe has a direct bandgap at 2.29eV, corresponding to the green optical wavelength, and predicted large bandgap bowing analogous to the GaAsN alloy. Furthermore, ZnTe has shown the ability for controllable p-type doping by nitrogen with hole concentrations of up to 1020 cm-3. In this work, we report on the deposition of ZnOTe thin films by pulsed laser deposition. Thin films were deposited on sapphire, GaAs substrates and Si substrates using a pulsed excimer laser, ZnTe target, and varying ambients of oxygen, nitrogen, and high vacuum. X-ray diffraction measurements indicate the preferential formation of zinc-blend thin films similar to ZnTe, with higher oxygen pressures resulting in degraded crystal structure rather than the formation of a wurtzite structure similar to ZnO. The optical bandgap energy shows a strong dependence on oxygen partial pressure, ranging from the ZnTe bandgap of 2.3eV for deposition under high vacuum to greater than 3.1eV for deposition at 100mTorr oxygen partial pressure. XPS measurements indicate a preferential formation of TeOx for high oxygen partial pressure. The variable bandgap energy of the ZnTeO thin films is attractive for a variety of optoelectronic devices, including solar cells. These thin films were applied to n-ZnO/ZnOxTe1-x/p+-Si heterojunctions, and demonstrate strong diode rectifying behavior and optical response in the visible spectral region. The electrical and optical characteristics of these heterojunction diodes will be presented.
12:30 PM - L12.11
High-current-density CuOx/InZnOx Thin Film Diodes for Application to Cross-point Resistive Memory.
Bo Soo Kang 1 , Myoung-Jae Lee 1 , Genrikh Stefanovich 1 , Wen Xu Xianyu 1 , Ki Hwan Kim 1 , Seung Eon Ahn 1 , Chang Bum Lee 1 , Youngsoo Park 1 , Bae Ho Park 2 1
1 Semiconductor Device Lab., Samsung Advanced Institute of Technology, Yongin-si, Gyeonggi-do, Korea (the Republic of), 2 Department of Physics, Konkuk University, Seoul Korea (the Republic of)
Show AbstractRecently, there has been a growing need for high performance diode as resistive memory devices emerge with cross-point one diode one resistor (1D1R) structure. The 1D1R structure is a promising building block for 3 dimensional stack memory devices. A diode is a key switch element in cross-point structure which prevents misreading through unexpected circuitous paths. Oxide-based diodes are considered as new candidates for 3D stack structure since it is formidable to use epitaxial silicon-based p-n diodes due to high processing temperatures, significant costs, and formation of silicide on metallic layers. High forward current density, high forward/reverse current ratio, and low processing temperature are important requirements of thin film diodes for application to high density stack memory.We fabricated CuOx/InZnOx thin film heterojunction diode on platinized silicon substrate by RF-sputtering at room temperature. No Schottky contact was formed when Pt and W were used as electrode materials for p-type CuOx and n-type InZnOx respectively. The diode showed good rectifying characteristics at room temperature: a rectifying ratio of >105 at ±1.25 V, a forward current density of >104 A/cm2, an ideality factor less than 2.The diode also exhibited a fast response to pulse signals. Turn-on and turn-off transient responses were stabilized within a few tens of nanoseconds in response to a square pulse signal with amplitude of ±0.85 V and width of 300 ns.
12:45 PM - L12.12
Performance and Stability of ZnO TFTs with SiO2, SiN and AlN Insulators.
Maria De Souza 1 , Richard Cross 1 , Divine Ngwashi 1 , Suhas Jejurikar 2 , K. Adhi 2
1 Emerging Technologies Research Centre, De Montfort University, Leicester United Kingdom, 2 Dept of Physics, University of Pune, Pune, Maharashtra, India
Show AbstractZnO can be deposited at low cost over large areas and at relatively low (or room) temperature, suggesting good compatibility with flexible substrate materials for display applications. It offers the potential of at least 10 times higher mobility in comparison to a:si TFTs. Little is known however, about the extent of instability mechanisms in ZnO TFTs with various insulators.In this work we will describe (A) Instability mechanisms of ZnO TFTs fabricated via RF magnetron sputtering with silicon dioxide and silicon nitride as insulators. Temporary shifts in threshold as a result of gate bias stress are found to arise due to fast interface states at the insulator boundary. In comparison to silicon dioxide, silicon nitride gives a better performance both in terms of stability and transistor parametrics. Under AFM it is found that the growth of ZnO on the two insulators results in different topographies. The shape of the ZnO grains on top of SiN are elongated as opposed to the conical shape demonstrated by the material grown on to SiO2. The overall surface roughness of ZnO/SiO2 is also greater than that of the ZnO/SiN stack. This may be explained by the differences in the lattices at the interface between the ZnO and SiN/SiO2 layers, which contributes to the disparities in the performance of the two sets of devices.(B) Performance and instability mechanisms of ZnO TFTs with AlN as insulator. In comparison to silicon nitride or oxide, the lattice constants (a= 3.1 A and c = 4.98 A) of AlN are more closely matched with ZnO 3.249Å, 5.201Å. The AlN was deposited via pulse laser deposition and the ZnO via RF sputtering. Devices with excellent electrical characteristics were obtained. The performance and stability of these transistors will also be presented at the conference.
L13: ZnO Defects and Surfaces
Session Chairs
Friday PM, November 30, 2007
Constitution A (Sheraton)
2:30 PM - L13.1
Gas Phase Synthesis of Zinc Oxide Nanocrystals and Their Surface Modification Using Small and Large Acidic Ligands.
Sankhanilay Roy Chowdhury 1 3 , Moazzam Ali 1 3 , Daniela Sudfeld 2 3 , Markus Winterer 1 3
1 Faculty of Engineering, University Duisburg-Essen, Duisburg Germany, 3 CeNIDE, University Duisburg-Essen, Duisburg Germany, 2 Faculty of Physics, University Duisburg-Essen, Duisburg Germany
Show AbstractZinc oxide nanocrystals are synthesized using the chemical vapor synthesis (CVS) technique. Diethylzinc is used as zinc precursor. Synthesized zinc oxide nanocrystals are characterized by X-ray diffraction (XRD), nitrogen adsorption and desorption, transmission electron microscopy (TEM) and dynamic laser scattering. As-synthesized zinc oxide has a primary particle size of about 10 nm and an isoelectric point at pH 9.5. The surface of zinc oxide nanocrystals can be modified using short chain organic acids like formic acid and glycine as well as using low molecular weight polyacrylic acid (PAA). The isoelectric point shifts towards acidic pH when glycine is used as surface modifier whereas it shifts towards basic pH with formic acid. The use of PAA generates a very high value of zeta potential over the range of pH studied in these experiments and thereby eliminates the isoelectric point.
2:45 PM - L13.2
Molecular Adhesion Mechanisms on Single Crystalline, Hydroxide Stabilized ZnO(0001) Surfaces.
Markus Valtiner 1 , Guido Grundmeier 2 1
1 Christian Doppler Laboratory for Polymer/Metal Interfaces, Max Planck Institute für Eisenforschung, Düsseldorf Germany, 2 Technical and Macromolecular Chemistry, University of Paderborn, Paderborn Germany
Show AbstractA combined approach of single molecule adhesion studies by means of AFM and of DFT studies, which provides a deeper understanding of the mechanisms of molecular adhesion of organic macromolecules on metal oxides, will be presented. First, the design of suitable model surfaces will be discussed. The main focus is the preparation and characterisation of ZnO surfaces that are comparable with the inevitable idealisations of a theoretical treatment. Therefore, different ambient atmosphere approaches were developed for the preparation of clean, non-reconstructed, single crystalline ZnO(0001) surfaces. Depending on the preparation technique, atomically flat terraces with a width of 100 nm up to several micrometers were observed using an atomic force microscope (AFM). The obtained surface structures were further characterized by means of angle resolved X-ray photoelectron spectroscopy (AR-XPS), time-of-flight secondary ion mass spectroscopy (ToF-SIMS), Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED) measurements. Based on these results it can be shown, that the obtained surfaces are, in contrast to surfaces prepared under UHV condition, stabilised by the adsorption of a monolayer of hydroxides.Secondly, it will be demonstrated that these developed surfaces are suitable to study molecular adhesion in electrolytes as these surfaces are very well defined. Moreover, the surface structure of the used model surfaces turned out to be extremely stable in electrolyte solutions. The stability of the surfaces under the respective ambient conditions will be demonstrated and single molecules desorption events by means of single molecule AFM spectroscopy will be discussed. The main advantage of this setup is, that single molecule desorption events on these specially prepared surfaces are closely comparable with the idealisations of the theoretical modelling. Consequently the theoretical modelling, within the framework of the density functional approach, will be discussed and compared with the obtained experimental results.
3:30 PM - L13.5
Cathodoluminescence Study of Polish Induced Luminescence in Single Crystal ZnO.
Esther Alarcon-Llado 1 2 , Frank Bertram 1 , Juergen Christen 1 , Lluis Artus 2 , Buguo Wang 3 , Michael Callahan 4
1 Institute of Experimental Physics, Otto-von-Guericke-University, Magdeburg Germany, 2 Institut Jaume Almera, CSIC, Barcelona, Catalunya, Spain, 3 , Solid State Scientific Corporation, Hollis, New Hampshire, United States, 4 Optoelectronic Technology Branch, Air Force Research Laboratory, Hanscom AFB, Massachusetts, United States
Show AbstractZnO is a wide band-gap material with growing interest because of its advantageous physical properties. However, no-reproducible p-type doping has been obtained so far. One of the main reasons is the high residual intrinsic defect concentration which is still not completely understood. The dominant defect-related luminescence of undoped ZnO is the well-known green (GL) and the yellow luminescence (YL) band. The origin of these bands in ZnO is still controversial. Copper impurities, oxygen vacancies and zinc interstitial defects are the potential candidates to be responsible for the GL band. On the other hand, the YL luminescence is hardly believed to be originated by Li impurities [1].In this work we investigate the luminescence in bulk, single crystal ZnO by means of spatially resolved cathodoluminescence (CL) spectroscopy. The sample was grown by the hydrothermal method and post-growth rapid thermal annealed at 950oC under continuous O2 flow for 10s. A mechanical polishing process was performed on a surface perpendicular to the c-plane to remove the surface layer of the bulk crystal. SE images of the polished surface show a flat, mirror like surface with some parallel running scratches. CL measurements at T = 5 K were carried out at the polished surface perpendicular to the c-plane of the bulk crystal right after the mechanical polishing process. Without polishing no CL intensity had been observed. The optical spectrum of ZnO bulk material is usually characterized by intense luminescence intensity with rich structure of excitonic lines. Up to 20 emission lines are listed so far [2]. Nevertheless, the sample under study shows only a weak luminescence intensity mainly influenced by the polishing of the surface. The lateral averaged CL spectrum shows dominating near band edge (NBE) luminescence and a broad defect band around 2.2 eV. The NBE is dominated by the bound exciton line I6 (3.360 eV). In addition, the free exciton X (3.375 eV) as well as the bound excitons I0 (3.370 eV), I9 (3.356 eV) and I10 (3.352 eV) are visible.A strong correlation between the CL intensity of the excitonic lines and the morphology is observed. The NBE luminescence strongly quenches at the position of the scratches. No spectral shift is observed while approaching these marks. It is worth noting that even in the regions without any features on the SE image, monochromatic CL images show dark lines running through. These lines mark the position of already removed scratches and cracks from the top surface indicating a long range/distant acting damage of the scratches. In contrast, a complementary behavior is observed for the defect-related luminescence band. This defect band shows its highest normalized intensity at the scratches. This correlation suggests that the mechanical polish process induces a new radiative recombination channel. REFERENCES[1] T.M. Borseth et al., Appl. Phys. Lett. 89, 262112 (2006)[2] B.K. Meyer et al., Phys. Stat. Sol. (b) 241, 231 (2004)
3:45 PM - L13.6
Dynamic of Dislocations in Plastically Deformed ZnO.
Ichiro Yonenaga 1 , Haruhiko Koizumi 1 , Yutaka Ohno 1 , Toshinori Taishi 1
1 Institute for Materials Research, Tohoku University, Sendai Japan
Show AbstractKnowledge on the dislocation characters and dynamical properties is essentially important as a basis for the control of dislocation generation and deformation during crystal growth and device processing. Recently, ZnO oxide semiconductor is expected for UV light-emitting device and is supposed for dislocations to be introduced easier than rival materials as GaN and SiC. However, up to now, far less is known on the dynamic and optical properties of dislocations in ZnO except a limited number of papers on nano-indentation. This paper reports first results of the direct measurement of mechanical strength of ZnO single crystals at elevated temperatures. Specimens prepared from bulk single-crystals h-ZnO, purchased from Goodwill (Russia), were deformed by means of uni-axial compressive deformation at elevated temperatures. The yield stress of ZnO in the temperature range 650 ~ 850 degree C is around 10 ~ 15 MPa, i.e. much easily deformed even at low temperatures in comparison with those of GaN and SiC. From the analysis of the temperature dependence of the yield stress, the activation energy for dislocation motion in ZnO is estimated to be 0.8– 1 eV.
4:30 PM - L13.7
Zn Vacancies and Electrical Isolation of MBE-ZnO by O Irradiation.
Asier Zubiaga 1 , Filip Tuomisto 1 , Victoria Coleman 2 , Chennupati Jagadish 2
1 Laboratory of Physics, Helsinki University of Technology, TKK Espoo Finland, 2 Department of Electronic Materials Engineering, The Australian National University, Canberra, Australian Capital Territory, Australia
Show AbstractIon implantation processes can be of interest for p-type doping or electrical isolation of ZnO based devices but the ion-induced damage should first be better understood. Structural, optical, electrical and magnetic properties in ZnO exposed to irradiation with energetic particles have recently been under study. It has been observed that the electrical properties of ZnO re quite resistant to particle radiation, and the irradiation induced defects anneal at low temperatures. ZnO remains crystalline even after high ion irradiation doses [1].We have studied nominally undoped and Al-doped (concentrations 2×1018, 5×1018 and 1×1019 cm-3) ZnO single-crystal epilayers grown on a-plane sapphire by plasma assisted molecular beam epitaxy. Defect induced isolation was introduced by implantation with 2 MeV O+ ions at room temperature. This incident beam energy was chosen to ensure that both the maximum damage peak and the O ions were placed within the sapphire substrate. Different implantation doses were used ranging from 4×1012 to 1×1017 cm-2. The electrical isolation was measured in situ during the implantation with In-Ga Ohmic contacts. Slow positron Doppler Broadening technique was used to study zinc vacancy related defects introduced during the implantation process. We show that the dominant defects detected by positrons are Zn vacancies that are introduced at a rate of about 2000 cm-1 at low and moderate fluences, and that vacancy clusters are formed at the highest irradiation fluences. The Zn vacancies are a likely cause for the isolation, while the vacancy clusters are possibly related to the loss of resistivity observed at the highest fluences.[1] S. O. Kucheyev, J. S. Williams, C. Jagadish and J. Zou, Phys. Rev. B 67, 94115 (2003).
4:45 PM - L13.8
Micro-Raman and Resonant Raman Scattering in Homoepitaxial- grown ZnO.
Axel Hoffmann 1 , M. Wagner 1 , U. Haboeck 1 , P. Zimmer 1 , J. Sann 2 , S. Lautenschlaeger 1 , B. Meyer 2
1 Inst. f. Fstkoerperphysik, TU Berlin, Berlin Germany, 2 1 Physikalisches Institut, Justus Liebig University, Giessen Germany
Show Abstract5:00 PM - L13.9
Optical Spectroscopy of A-plane ZnO Epilayers and Heterostructures Grown on R-plane Sapphire Substrates by MBE.
Christian Morhain 1 , Bassirou Lo 2 , Monique Teisseire-Doninelli 1 , Jean-Michel Chauveau 1 3 , Borge Vinter 1 3 , Martin Albrecht 4
1 , CRHEA-CNRS, Valbonne Sophia Antipolis France, 2 , Université Cheikh Anta Diop, Dakar Senegal, 3 Physics Dept, University of Nice-Sophia Antipolis, Nice France, 4 , Institut für Kristallzüchtung, Berlin Germany
Show AbstractMBE growth of ZnO on R-plane oriented sapphire substrates results in fully A-plane oriented epilayers. A key advantage of such a non-polar orientation is that it leads to quantum well heterostructures showing no built-in electric field, and hence excitons having large binding energies (no e-h separation by the electric field). With the aim of identifying the specific properties of A-plane ZnO films, these were investigated by detailed optical spectroscopy, carried out by means of reflectivity, temperature-dependent photoluminescence (PL), selective PL as well as cathodoluminescence (CL).The main results are:-(i) The luminescence emission is strongly linearly polarised for free exciton lines (>95%) as well as for defect-related emissions.-(ii) The strain state in the samples is high as testified by the ~ 20 meV shift towards UV of all free excitons as compared to C-plane oriented samples. This property holds even for 1µm thick samples. -(iii) The lowest energy exciton (A) has a greater oscillator strength than the B exciton, unlike c-oriented samples.-(iv) The nature of the main bound exciton emission as well as the chemical origin of the involved defects are determined by SPL.-(v) A new band, not detected in c-oriented layers, is observed in the energy range of the 1st phonon replica of (A). The origin of this line could be unambiguously identified as the recombination of electron-hole pairs decorating basal stacking faults from the comparison of TEM plane views and cathodoluminescence mapping carried out on thin TEM-prepared samples.-(vi) A strain gradient is observed along the growth direction, which is particularly well evidenced on step wet-etched samples. This can result in an energy shift of the band gap by several meV and has to be taken into account for a proper analysis of the optical spectra. Relaxation mechanisms and residual strain are explored using ZnMgO buffer layers with Mg contents up to 40%.
5:15 PM - L13.10
Electrical Characterization of Defects in ZnO Grown by Pulsed-laser Deposition.
Holger vonWenckstern 1 , F. Auret 2 , W. Meyer 2 , P. Janse van Rensburg 2 , G. Biehne 1 , H. Hochmuth 1 , M. Lorenz 1 , M. Grundmann 1
1 Institut für Experimentelle Physik II, Semiconductor Physics, Universtät Leipzig, Leipzig Germany, 2 Physics Department, University of Pretoria, Pretoria South Africa
Show AbstractThe properties of the wide band-gap II-VI semiconductor ZnO make this material interesting for applications in UV optoelectronics. Defects have crucial impact on the performance of devices. Although the defects in bulk-grown ZnO have been studied in some detail [1], much less is known about the defects present in ZnO grown by pulsed-laser deposition (PLD).
We employed deep level transient spectroscopy (DLTS) and thermal admittance spectroscopy (TAS) to investigate the defect states in as-grown ZnO PLD thin films and the impact of annealing (2h, 750°C) in either oxygen, nitrogen and vacuum on the defect spectrum. For that, high quality Pd/ZnO Schottky diodes were evaporated through a shadow mask on the Zn-face of the samples. A n++ ZnO:Al layer is used as ohmic back contact and results in low series resistances between about 50 and 200 Ohm [2].
Four shallow defects with levels at 28 meV, 60 meV, 100 meV and 140 meV below the conduction band minimum Ec, respectively are revealed by TAS and DLTS. The 100 meV defect displayed metastable behavior: annealing under reverse bias at temperatures of above 130 K introduces this defect while annealing under zero bias above 110 K removes it. We have determined, and will discuss, the complete introduction and removal kinetics of this defect. Using high resolution Laplace DLTS we found that close to the commonly observed defect E3 (about 300 meV below Ec) another close lying peak labeled E3’(with a level at 370 meV below Ec) is observed. While the concentration of E3 did not show pronounced changes during annealing, the defect E3’ is strongly diminished for nitrogen or vacuum annealing, respectively.
Irradiating the Schottky diodes with a white LED at 4 K creates a defect with a level at 50 meV below Ec. However, now the 60 meV peak is diminished. This behavior is only observed for diodes prepared on as-grown or oxygen treated samples. Annealing at temperatures above 120 K removes the 50 meV defect and the 60 meV peak is reintroduced.
[1]F. D. Auret, S. A. Goodman, M. J. Legodi, W. E. Meyer, and D. C. Look,Appl. Phys. Lett. 80, 1340 (2002).
[2]H. von Wenckstern, G. Biehne, R. A. Rahman, H. Hochmuth, M. Lorenz, and M. Grundmann, Appl. Phys. Lett. 88, 092102 (2006).
5:30 PM - L13.11
Characterization of Mid-gap Defect Level in ZnO by High Resolution X-ray Photoelectron Spectroscopy Using Synchrotron Radiation.
Naoki Ohashi 1 , Yutaka Adachi 1 , Takeo Ohsawa 2 , Isao Sakaguchi 1 , Haruki Ryoken 1 , Hideki Yoshikawa 2 , Shigenori Ueda 2 , Hajime Haneda 3 , Keisuke Kobayashi 2
1 Optoelectronics Group, National Institute for Materials Science, Tsukuba, Ibaraki, Japan, 2 Beam Line Station, National Institute for Materials Science, Tsukuba, Ibaraki, Japan, 3 Sensor Materials Center, National Institute for Materials Science, Tsukuba Japan
Show AbstractIn the present study, we investigated electronic structure of ZnO films and bulk crystals by X-ray photoelectron spectroscopy (XPS). In particular, we obtained XPS spectra of ZnO using a synchrotron radiation (SR) having high photon energy, 6 keV. Extremely high photon flux utilized by using the SR enables very high resolution and signal/noise ratio, and elongated escape depth of photoelectron by using the high energy incident beam enables us to observe bulk electronic structure. In this paper, we concentrate on detection of mid-gap state in ZnO, which is critical to the charge compensation phenomena. Moreover, we discuss effects of crystalline polarity on formation of the mid gap state from the result of XPS measurement for the single crystalline ZnO with various polarization. Samples were commercially available bulk ZnO single crystals grown by a hydrothermal method or a chemical vapor transport method, ZnO thin films prepared by a pulsed laser deposition method, ZnO thick films prepared by a liquid phase epitaxy method, and ZnO ceramics prepared by an ordinary ceramic synthesis process. Here, the thin and thick film samples includes pure ZnO, (Zn,Mg)O alloy, and Al-doped ZnO, and the film crystals were hoepitaxially grown on ZnO single crystals or heteroepitaxially grown on sapphire or zirconia substrates. Some of these samples were irradiated by ion beam to introduce impurity or to create defects caused by the irradiation damage. The high resolution XPS measurements were performed at the BL15XU beam line constructed in the SPring-8 and the platform for experiments has been collaborated between SPring-8/JAEA (Japan Atomic Energy Agency), SPring-8/JASRI (Japan Synchrotron Radiation Research Institute) and HiSOR (Hiroshima Synchrotron Radiation Center, Hiroshima University). The XPS spectra were taken at room temperature using a Scienta SES-2002 electron energy analyzer with a total energy resolution of 270 meV or higher. The excitation energy was fixed to 6 keV and the inelastic mean free paths of photoelectrons at the kinetic energy of 6 keV range between 5 to 15 nm, which is almost 5 - 10 times larger than those at 1 keV. That enables us to investigate intrinsic electronic structures into the bulk. The pesence of defect/impurity level in mid-gap was clearly detected by the XPS measurement. For example, density of state of the mid-gap state in the Al-doped ZnO films varied with the condition of thermal treatment and the mid-gap state corresponding to the acceptor state was found in ZnO after ion beam irradiation. It is also notable that profile of XPS spectra obtained from (0 0 0 1), (0 0 0 -1), (1 1 -2 0) and (1 0 -1 0) face disagreed to one another, indicating that permanent polarization of ZnO lattice affects to the electronic state of ZnO. In particular, XPS spectra of valence band region were sensitive to the crystalline orientation. The detailed interpretation of the variation of XPS spectra will be discussed at the symposium.
5:45 PM - L13.12
Effect of Ga doping on Luminescence in ZnO.
Michael Reshchikov 1 , Junqing Xie 2 , Brian Hertog 2 , Andrei Osinsky 2
1 Physics Department, Virginia Commonwealth University, Richmond, Virginia, United States, 2 , SVT Associates, Inc., Eden Prairie, Minnesota, United States
Show AbstractIn spite of impressive achievements in growth of high-quality ZnO, point defects in this semiconductor are not well understood. In particular, assignments of the broad bands in the yellow-green part of the photoluminescence (PL) spectrum of undoped and doped ZnO remain highly controversial. In this work we analyze in detail the yellow luminescence (YL) band having a maximum at about 2.2 eV at 10 K in undoped and Ga-doped ZnO films grown on sapphire substrate. The YL band is attributed to transitions from the shallow GaZn donor to a deep acceptor having an energy level about 0.7 eV above the top of the valence band. The activation energy of the YL-related acceptor is close to the 2-/- energy level of the isolated zinc vacancy (VZn) in ZnO (0.8 eV) predicted from the first-principles calculations [1]. It is also likely that the acceptor responsible for the YL band is not an isolated VZn but a complex defect such as VZnGaZn. The intensity of the YL band decreased several orders of magnitude with Ga doping, in agreement with the above assignments. The shift of the YL band maximum with increasing excitation intensity at 10 K is negligible in undoped layers but reaches 0.1 eV in Ga-doped ones. With time delay after a pulse excitation, the YL band shifted to lower energies in Ga-doped ZnO. The noticeable shifts of the YL band with excitation intensity and with time delay are attributed to manifestation of potential fluctuations created by random distribution of impurities in Ga-doped ZnO. [1] A. F. Kohan, G. Ceder, D. Morgan, and C. G. Van de Walle, Phys. Rev. B 61, 15019 (2000).