Symposium Organizers
John D. Perkins National Renewable Energy Laboratory
Thomas O. Mason Northwestern University
John F. Wager Oregon State University
Yuzo Shigesato Aoyama Gakuin University
B1: TCO Fundamental Electronic Structure
Session Chairs
Thomas Mason
Julia Medvedeva
Monday PM, December 01, 2008
Room 203 (Hynes)
9:30 AM - **B1.1
Basic Physics of the Coexistence of Transparency and Conductivity in Oxides and their Design Principles.
Alex Zunger 1
1 , National Renewable Energy Laboratory, Golden , Colorado, United States
Show Abstract10:00 AM - B1.2
Control of the Optical and Electronic Structure Properties of Multi-component Transparent Conducting Oxides from First-principles Calculations.
Aron Walsh 1 , Juarez L. F. Da Silva 1 , Yanfa Yan 1 , Su-Huai Wei 1
1 , National Renewable Energy Laboratory, Golden , Colorado, United States
Show AbstractMulti-component oxides formed from post-transition metal cations (e.g. Zn, In, Ga, Al) represent the state-of-art in n-type transparent conducting oxides (TCOs), exhibiting simultaneously high electron concentrations, carrier mobility, optical transmission and chemical stability. In recent work we have demonstrated that the properties of the binary and quaternary oxides are more complex than previously assumed: (i) the inequivalence of the fundamental and optical band gaps of indium oxide, arising from parity forbidden band edge transitions [1], (ii) the formation of an inversion boundary domain in the quaternary oxides to satisfy the electronic octet rule [2,3]. In addition to reviewing these results, we will present new findings on the electronic structure, including optical transitions and band level offsets, of the ordered crystalline superlattice and disordered amorphous phases of InMO3(ZnO)n (M = Al, Ga, In) obtained through the application of first-principles band structure theory. The origins of enhanced electronic properties, understanding of current experimental trends, and new avenues for tuning the optoelectronic properties will be explored.
Research supported by the U.S. Department of Energy under Contract No. DE-AC36-99GO10337.
[1] A. Walsh, J. L. F. Da Silva, S.-H. Wei et al., Physical Review Letters 100, 167402 (2008).
[2] Y. Yan, J. L. F. Da Silva, S.-H. Wei and M. M. Al-Jassim, Applied Physics Letters 90, 261904 (2007).
[3] J. L. F. Da Silva, Y. Yan and S.-H. Wei, Physical Review Letters, In Press (2008).
10:15 AM - B1.3
Oxygen Vacancy Levels in Conducting Oxides SnO2 and ZnO.
S. Clark 2 , John Robertson 1
2 Physics, Durham Unversity, Durham United Kingdom, 1 Engineering Dept, Cambridge University, Cambridge United Kingdom
Show AbstractSnO2 and ZnO are two important transparent conducting oxides, which are used in transparent ocnducting films and thin film transistors. Shallow donors for these systems, but the nature of the intrinsic defects is still contentious. This is largely because the oxides have a band gap which needs substantial correction from the LDA value to give the experimental value, and this makes theory predictions difficult. The energy and energy levels of oxygen vacancy has been calculated for these oxides, using an LDA method, screened exchange, which does not require a band gap correction. The vacancy in SnO2 is found to be slightly deep, 0.4 eV below the conduction band edge. In ZnO the vacancy ism uch deeper, near midgap.
10:30 AM - **B1.4
Quantum Computational Approach to Transparent Conductors and Semiconductors for Optoelelectronics.
Arthur Freeman 1 , Jung-Hwan Song 1 , Giancarlo Trimarchi 1 , Linhui Ye 2
1 Physics Department, Northwestern University, Evanston , Illinois, United States, 2 College of Chemistry and Molecular Engineering, Peking University , Beijing China
Show Abstract11:30 AM - **B1.5
Tuning Transparent Conducting Oxides for Opto-electronic Functionality.
Tobin Marks 1
1 , Northwestern U., Evanston, Illinois, United States
Show AbstractTransparent conducting oxides (TCOs) are unique materials that, as thin films, find many current applications of great importance in display, photovoltaic, energy conservation, and lighting applications. However, it is not clear that present generation TCO materials (e.g., tin-doped indium oxide, ITO) will meet the stringent performance and cost requirements for next-generation opto-electronic technologies. This lecture focuses on the scientific aspects of designing, growing, characterizing, understanding, manipulating, and implementing next-generation transparent conducting oxide thin films. This includes implementation in: 1) transparent electronics, 2) organic photovoltaics, 3) nanoscale patterning of TCOs.
12:00 PM - **B1.6
Surface Properties of Polycrystalline Transparent Conducting Oxides.
Andreas Klein 1
1 Materials Science, Darmstadt University of Technology, Darmstadt Germany
Show AbstractProperties of transparent conducting oxide surfaces have been investigated using photoelectron spectroscopy (XPS, UPS) and conductivity relaxation experiments. The TCO films are prepared by magnetron sputtering. Surface analysis is performed in integrated systems, allowing for vacuum transfer between different preparation and analysis chambers. With XPS and UPS, it is possible to assess chemical as well as electronic properties of surfaces and interfaces. The electronic properties include the work function, the Fermi level position with respect to the band edges and barrier heights at interfaces between TCOs and other materials. Results will be presented for magnetron sputtered films of doped and undoped zinc oxide, indium oxide, and tin dioxide. The variation of surface properties with deposition parameters is described, highlighting the influences affecting the work function and the importance of non-equilibrium conditions for the carrier concentration of magnetron sputtered films. In addition, post deposition changes induced by heating in different oxygen partial pressures and air as well as by deposition of ultrathin oxide layers will be addressed.
12:30 PM - B1.7
The Surface Electronic Structure of In2O3 and Sn-doped In2O3.
David Payne 1 , Anne Bourlange 1 , Philip King 2 , Timothy Veal 2 , Chris McConville 2 , Russell Egdell 1
1 Inorganic Chemistry Laboratory, University of Oxford, Oxford United Kingdom, 2 Department of Physics, University of Warwick, Coventry United Kingdom
Show AbstractTransparent conducting oxides (TCOs) such as In2O3, SnO2 and ZnO find widespread application as contacts for photovoltaic devices, liquid crystal displays, and light emitting diodes. Despite the obvious importance of these materials a complete understanding of their electronic structure is yet to be achieved. For example it is only recently that it has been shown that a weak absorption onset in In2O3 at 2.67 eV arises from direct forbidden transitions and that the often quoted value of 3.75 eV for the bulk bandgap is over 1 eV too high [1,2]. Here X-ray photoemission spectroscopy (XPS), infrared (IR) reflectivity and Hall effect measurements have been performed on single crystalline undoped In2O3 and Sn-doped In2O3 grown by O plasma assisted molecular beam epitaxy on Y-stabilised ZrO2 substrates. Single field Hall effect measurements reveal an electron density and mobility of 7.5 x 1018 cm-3 and 32 cm2V-1s-1 for nominally undoped In2O3 and 4.2 x 1020 cm-3 and 27 cm2V-1s-1 for Sn-doped In2O3. These results are consistent with IR reflectivity measurements which give carrier concentrations of 7.4 x 1018 cm-3 and 4.0 x 1020 cm-3 for nominally undoped In2O3 and Sn-doped In2O3 respectively. However valence band photoemission measurements show that the valence band maximum (VBM) to surface Fermi separation is 2.94 eV for the undoped sample and 3.06 eV for the doped samples. For a direct band gap of 2.67 eV, this implies that the Fermi level is pinned above the conduction band minimum (CBM) at the surface. Assuming an effective electron mass of 0.35m0, bulk Fermi levels are calculated as 0.02 eV and 0.54 eV for the nominally undoped and doped samples respectively. These results are incompatible with the measured conduction band photoemission spectra if it is assumed that XPS data probes the bulk electronic structure of the material. Space charge calculations suggest there must be pronounced electron accumulation at the surface of the undoped material, probably arising from surface states close to the charge neutrality level. Increasing the bulk Fermi level to above the charge neutrality level results in a slight depletion of electrons at the surface. This allows the charge neutrality level to be located between 2.94 eV and 3.06 eV above the VBM, and consequently about 0.4 eV above the conduction band minimum. These results combine to show that single-crystalline In2O3 exhibits electron accumulation at its surface, in contrast to the majority of other semiconductors. It can be explained in terms of the conduction band minimum in In2O3 lying below the charge neutrality level, due to the particularly low conduction band energy at the Γ point. These results also explain the propensity for n-type conductivity in In2O3 and the ease of n-type doping in this material.[1] A. Walsh et al., Phys. Rev. Lett. 100, 167402 (2008).[2] A. Bourlange et al., Appl. Phys. Lett. 92, 092117 (2008).
12:45 PM - B1.8
Correlation Between Bulk Defect Chemistry and Surface Electronic Properties of Zinc- and Tin- Co-Doped Indium Oxide.
Steven Harvey 1 , Thomas Mason 1 , Andreas Klein 2 , Christoph Koerber 2
1 Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States, 2 Materials Science and Engineering, Darmstadt University of Technology, Darmstadt, Hessen, Germany
Show AbstractThe Zn- and Sn-codoped bixbyite solid solution, (In(2-2x)SnxZnxO3), hereafter referred to as ZITO, should be self-compensated, i.e., there should be a balance of Zn-acceptors and Sn-donors. Yet the ZITO materials are persistent n-type transparent conducting oxides (TCOs). In the present work, bulk phase equilibrium and electrical property studies (conductivity, thermopower) showed that there is an inherent cation off-stoichiometry favoring Sn-donors over Zn-acceptors. The bixbyite phase field lies to the Sn-excess side of nominal ([Zn]=[Sn]) stoichiometry. It was also shown that the equilibrium defect chemistry of ZITO is dominated by the same neutral Frank-Koestlin (F-K) cluster, (2SnIn●OI")x, as previously reported for indium-tin oxide (ITO). This was demonstrated by equilibrium in situ variable-pO2 conductivity and thermopower measurements on the terminal composition of the ZITO solid solution, In1.2Sn0.40Zn0.40O3, at 750 oC. The characteristic F-K pO2-1/8 dependence was observed. The surface electronic properties of the zinc- and tin- co-doped In2O3 were studied by X-ray and UV photoelectron spectroscopy. Thin films of In1.8Sn0.10Zn0.10O3 (ZITO10) and In1.4Sn0.30Zn0.30O3 (ZITO30) were deposited via magnetron sputtering with varying oxygen contents in the sputtering environment. The films were measured, and then subjected to in situ oxidation and reduction and subsequent analysis without removing the specimens from the UHV system. Reversible changes in the surface Fermi level positions and core level binding energies of 300-600 meV were observed upon oxidation and reduction. These changes were consistent with the presence of the F-K defect cluster affecting the carrier content in ZITO, i.e., reduction lead to higher Fermi level positions, and oxidation to lower Fermi level positions.
B2: Unconventional Transparent Conductors
Session Chairs
Andreas Klein
Tobin Marks
Monday PM, December 01, 2008
Room 203 (Hynes)
2:30 PM - **B2.1
Conventional TCO and Beyond: Band Engineering Approach.
Julia Medvedeva 1
1 Physics, Missouri University of Science & Technology, Rolla, Missouri, United States
Show Abstract3:00 PM - **B2.2
Growth of Transparent Conducting Nb-doped Anatase TiO2 Thin Films on Glass using Seed Layers.
Naoomi Yamada 1 , Taro Hitosugi 1 2 , Shoichiro Nakao 1 3 , Junpei Kasai 1 , Yasushi Hirose 1 3 , Ngoc Lam Huong Hoang 3 , Toshihiro Shimada 1 3 , Tetsuya Hasegawa 1 3
1 , Kanagawa Academy of Science and Technology (KAST), Kawasaki Japan, 2 Advanced Institute for Materials Research (WPI-AIMR), Tohoku University, Sendai Japan, 3 Department of Chemistry, University of Tokyo, Tokyo Japan
Show Abstract Nb-doped anatase TiO2 (TNO) is a wide-gap, n-type degenerate semiconductor, exhibiting low resistivity (ρ) of the order of 10-4 Ω cm and high visible transparency (T>75%). Hence, TNO is promising as a transparent conducting oxide (TCO) that has potential to substitute Sn-doped In2O3, which, in recent years, suffers from indium-shortage problem. In order to establish TNO as a practical TCO material, it is highly desirable to develop sputter-based fabrication process on glass substrate. Recently, we have shown that conductive TNO films can be obtained when they are prepared in strongly reducing conditions. That is, introduction of oxygen deficiencies during film deposition is a key to achieving highly conductive TNO. There are two different routes for fabricating TNO polycrystalline films on glass using sputtering. One is to crystallize amorphous films, deposited at room temperature, by annealing under reducing atmosphere. The other is direct synthesis of polycrystalline films from vapor phase onto heated (Ts=200∼400oC) glass. In the former route, it is easier to obtain strongly reduced anatase phase on glass, resulting in low ρ value of 6∼8×10-4 Ω cm. Meanwhile, direct deposition of reduced anatase is difficult, because under reducing conditions, i.e., low O2/Ar ratio and/or high Ts, highly resistive rutile phase is preferentially formed. Here, we report on our attempts to fabricate conductive TNO films directly on glass by using seed layers, which prevents the formation of rutile. TNO films were deposited on glass and LaAlO3 (100) (LAO), for comparison, at substrate temperature Ts=400 oC and O2/Ar=0.3% under total pressure of 1.0 Pa. A Ti0.94Nb0.06O2-δ sintered pellet was used as a target. TNO on LAO was in epitaxial anatase phase with ρ=3.6×10-4 Ω cm. In contrast, TNO on glass was found to be in polycrystalline rutile phase of ρ=1.7×100 Ω cm. These imply that strongly reduced anatase phase is stabilized on LAO by epitaxial interaction between film and substrate even under reducing conditions. Polycrystalline anatase TNO can be grown on glass by increasing O2/Ar up to 1% and lowering Ts down to 250oC. However, the obtained films are less conductive with ρ∼50 Ω cm, since incorporation of oxygen deficiency is insufficient. We overcame this difficulty by using the resistive anatase film, described above, as an anatase template (seed layer). We deposited 170-nm-thick TNO films on the template under a strongly reducing condition of O2/Ar=0.05% and Ts=400oC. The resultant polycrystalline film was confirmed to be of single phase anatase TNO and showed metallic transport behavior with ρ=1.3×10-3 Ω cm at room temperature. As a consequence, use of resistive anatase seed layers is quite effective in depositing conductive TNO polycrystalline films on glass.
3:30 PM - B2.3
Large Electron Mass Anisotropy in Anatase Ti1-xNbxO2 Transparent Conductor.
Yasushi Hirose 1 2 , Naoomi Yamada 2 , Shoichiro Nakao 2 , Taro Hitosugi 2 3 , Toshihiro Shimada 1 2 , Tetsuya Hasegawa 1 2
1 Department of Chemistry, School of Science, University of Tokyo, Tokyo Japan, 2 nano-structured magneto-optical device project, KAST, Kawasaki Japan, 3 WPI-AIMR, Tohoku University, Sendai Japan
Show AbstractTransparent conducting oxide (TCO) has attracted much attention as a key material indispensable in opto-electronic devices. Recently, we found that Nb-doped anatase TiO2 (TNO) shows excellent conductivity (ρ < 1 x 10-3 Ω cm) and transparency (T >75 %) for visible light in both epitaxial and polycrystalline film form. In addition, TNO possesses additional unique features, such as high chemical stability and large refractive index.The conduction band of TNO is mainly composed of Ti 3d-orbitals in contrast to other conventional TCOs, such as In2-xSnxO3, ZnO and SnO2, with s-nature conduction band, leading us to anticipation that electrical transport in TNO is highly anisotropic. If this is the case, for further improving conductivity of TNO polycrystalline films, it is needed to control their crystallographic orientation. Transport properties of TNO have been mainly studied using epitaxial c-axis oriented films, so far, and TNO films with the other orientation have not been subjected to such measurements. Therefore, no anisotropy data is available for carrier transport of TNO. In this study, anisotropy in electron mass (m*) of TNO was determined from optical measurements for (012)-oriented TNO epitaxial films, in which the c-axis of the anatase structure is tilted from surface normal.Ti1-xNbxO2 films with x = 0 - 0.06 were grown on LaAlO3 (110) substrates by using pulsed laser deposition (PLD) technique. X-ray diffraction and cross-sectional transmission electron microscope (TEM) measurements confirmed epitaxial growth of (012)-oriented TNO films without any impurity phase. Polarized infrared (PIR) spectra of the films were measured by an FT-IR spectrometer with a grid polarizer. The incident angle of IR light was 0o, and the polarization was set parallel or perpendicular to the [100] direction of the TNO films.The observed PIR spectra of TNO films showed free-carrier absorption in IR region, and absorption wavelength was strongly dependent on the polarization condition, suggesting large anisotropy in m*. We analyzed the spectra using Drude model to derive m* values. As a result, we found that m* along the [100] direction is ~0.6 m0, which is almost the same as that of (001)-oriented TNO films. In contrast, m* along [001] was 3-4 times larger than m*[100]. These results indicate that conductivity of polycrystalline TNO films can be further improved by orienting the c-axis of each grain parallel to film surfaces.
3:45 PM - B2.4
Low-temperature Fabrication of Transparent Conductive Polycrystalline Nb-doped TiO2 Films by Sputtering.
Ngoc Lam Huong Hoang 1 , Naoomi Yamada 2 , Taro Hitosugi 2 3 , Junpei Kasai 2 , Shoichiro Nakao 2 , Toshihiro Shimada 1 2 , Tetsuya Hasegawa 1 2
1 Department of Chemistry, The School of Science, The University of Tokyo, Tokyo Japan, 2 , Kanagawa Academy of Science and Technology, Kawasaki, Kanagawa, Japan, 3 Advanced Institute for Materials Research, Tohoku University, Sendai Japan
Show Abstract Recently Nb-doped anatase TiO2 (Ti1-xNbxO2; TNO) in both epitaxial and polycrystalline was found to exhibit low resistivity ρ in the order of 10-4 Ω cm and high transmittance in the visible region, which suggested that TNO has sufficient potential of being a next-generation transparent conductive oxide (TCO). In previous studies on TNO, polycrystalline TNO films with low ρ were obtained only by annealing amorphous films at temperatures exceeding 500 oC. However, such high process temperatures substantially limit the applications of TNO. This study presents a novel low temperature process for preparing highly conductive TNO films on conventional substrates, such as glass and plastics. Polycrystalline TNO films were crystallized from amorphous films deposited on unheated non-alkali glass substrates in vacuum (3×10-3 Pa) or in pure H2 atmosphere. Sputtering technique was used to fabricate these amorphous films onto the substrates for the advantages of low-cost and uniform coating on large-area substrates and it has been recognized as a standard technique to prepare TCO films. Experimental data showed that the crystallization temperature (Tcrys) was suppressed to 250 oC with increasing oxygen partial pressure during the deposition, i.e. with decreasing oxygen deficiencies in amorphous films. Meanwhile, the ρ value after annealing tends to decrease by incorporating more oxygen deficiencies into amorphous films. Therefore, there must be a compromise between decreasing Tcrys and pursuing low ρ. To overcome this difficulty, this study proposes the use of an amorphous film with a double-layered structure, composing of oxygen-rich bottom layer with Tcrys = 300 oC and oxygen-deficient top layer with Tcrys = 350 oC. The oxygen-rich bottom layer with low Tcrys was expected to act as a nucleation center, from which crystallization of the top oxygen-deficient layer with higher Tcrys was initiated. Indeed, the double-layered amorphous film was found to undergo crystallization at around 300 oC, which was identical to Tcrys of the bottom layer. This proves that crystallization process propagates from the bottom layer to the top one at 300 oC. Notably, the ρ values after annealing the above-mentioned double-layered films are essentially independent of annealing atmosphere, proving that incorporation of hydrogen into TNO is not related to low ρ of the crystallized TNO films. The optimum annealing temperature is in a range from 300 to 400 oC, and the lowest ρ of 7.0×10-4 Ω cm was obtained for the film annealed at 400 oC. With the double-layer process proposed here, the highly-conductive TNO film on polyimide foil was successfully fabricated with low ρ = 1.9×10-3 Ω cm obtained after annealing at 300 oC.
4:30 PM - **B2.5
Chalogenide-based p-Type Wide-gap Semiconductors for Optoelectronics.
Janet Tate 1 , Andriy Zakutayev 1 , Robert Kykyneshi 1 , Paul Newhouse 2 , David McIntyre 1 , Guenter Schneider 1 , Peter Hersh 2 , Douglas Keszler 2
1 Physics, Oregon State University, Corvallis, Oregon, United States, 2 Chemistry, Oregon State University, Corvallis, Oregon, United States
Show AbstractThin-film chalcogenide-based wide-gap p-type semiconductors BaCuChF (Ch = chalcogenide S, Se, Te and solid solutions) and Cu3TaCh4 (Ch = S, Se) exhibit a wide range of properties that make them potential candidates for transparent optoelectronic applications. BaCuSF is very transparent with a relatively low carrier density (estimated <1017 cm-3 ), while nominally undoped BaCuTeF is slightly colored with a carrier concentration in excess of 1020 cm-3. Thin film solid solutions across the entire range of x in BaCu(S1-xSex)F and y in BaCu(Se1-yTey)F can be made, allowing for carrier density tuning. Excitonic absorption is observed for all x and almost all y with very sharp features at low temperature that persist to room temperature. Excitonic emission at room temperature is strongest in BaCuSeF. In contrast to the anisotropic BaCuChF, Cu3TaCh4 is cubic, which is unusual, and desirable for isotropic transport characteristics. Solid solutions allow tunabilty of this family's optoelectronic properties.
5:00 PM - B2.6
Synthesis of LaCuOCh (Ch=S, Se) Single Crystals by Flux Method.
Kazushige Ueda 1 2 , Yutaka Nakachi 1 , Hidenori Hiramatsu 2 , Koichi Kajihara 2 4 , Masahiro Hirano 2 3 , Hideo Hosono 2 3
1 Department of Materials Science, Kyushu Institute of Technology, Kitakyushu Japan, 2 , JST SORST in Tokyo Institute Technology, Yokohama Japan, 4 , Tokyo Metropolitan University, Hachioji Japan, 3 , Tokyo Institute Technology, Yokohama Japan
Show AbstractThe layered oxychalcogenides, LaCuOCh (Ch=S, Se), are known as wide gap (2.8-3.1eV) transparent p-type semiconductors that show band gap emission at room temperature. The crystal structure of LaCuOCh was composed of LaO oxide layer and CuCh sulfide layer stacked alternately along the c-axis. This particular layered structure is considered to bring about the unique electrical and optical properties. The electrical and optical properties of LaCuOCh have been examined using powder samples or thin film samples so far. However, analysis of the properties on single crystals has not been carried out because single crystals, that are large enough to use in several conventional measurements, have not been obtained yet. In previous study, preparation of LaCuOS single crystals was reported by chemical transport reaction using the starting material of LaCuOS powder and the vapor of iodine. However, the size of the largest crystal was as small as 100×100×5 μm3. In this study, LaCuOCh single crystals were grown by flux method using NaCl+KCl(1:1) as flux. The crystals were grown in double alumina and evacuated silica glass tube using the starting materials of La2O2S and Cu2S, as well as the flux. After the crystal growth under several conditions, mm-sized transparent colorless laminated crystals were obtained and it was found that the starting materials of La2O2S and Cu2S were essential to obtain large crystals in the flux method. The optimized crystal growth conditions are growth temperature of 850 oC, growth time of 72h and cooling rate of 10 oC /h and the size of the largest LaCuOS crystal obtained was 3.0×2.8×0.049 mm3. The crystals showed transmission as high as 60 % for LaCuOS and 40% for LaCuOSe and the intrinsic absorption edge was observed at 400 nm for LaCuOS and at 440 nm for LaCuOSe. In addition, p-type electrical conductivities of 7.1×10-4 Scm-1 for LaCuOS and 5.3×10-2 Scm-1 for LaCuOSe were observed at room temperature.
5:15 PM - B2.7
Transport, Structure, and Optical Properties of Delafossite CuSc1-xMgxO2 Wide Bandgap Semiconducting Thin Films and Heterostructures.
Patrick Sadik 1 , Fernando Lugo 1 , Hyun-Sik Kim 1 , David Norton 1
1 Materials Science & Engineering, University of Florida, Gainesville, Florida, United States
Show AbstractFor transparent thin-film electronics, there is a generic need to develop p-type transparent semiconductors for field-effect transistors, pn junctions, and sensors. A candidate class of materials known as delafossites, with formula AIIIMIO2, exhibits optical transparency in the visible, p-type behavior, and relatively high conductivities. This talk will examine the thin-film growth behavior, electrical, optical, and structural properties of a specific delafossite member, namely CuScO2. One of the motivations for developing p-type delafossites is the possibility of forming pn heterostructures with n-type ZnO. Many of the p-type delafossites exhibit p-type behavior and low resistivity, but tend to form the spinel phase with ZnO-based heterostructures. CuScO2 is interesting in its favorable lattice match to ZnO and absence of competing spinel phases frequently observed in the growth of other delafossites thin films. The lattice match between CuScO2 (a~3.22 Å) and ZnO (a=3.25 Å) makes this an attractive heterostructure. In this talk, we report the growth and properties of Cu(Sc,Mg)O2 thin films, Cu(Sc,Mg)O2/ CuCrO2, and ZnO/Cu(Sc,Mg)O2/ CuCrO2 heterostructures using pulsed laser deposition. C-axis oriented Cu(Sc,Mg)O2 p-type films are realized with resistivity on the order of 80 ohm-cm. We will discuss the use of buffer layers, namely CuCrO2, for nucleating c-axis delafossite Cu(Sc,Mg)O2 thin films on sapphire. Specific issues that are addressed in this study include achieving highly oriented epitaxial films, realizing high p-type carrier concentrations, and avoiding the formation of competing secondary phases in heterojunctions with ZnO. Evidence for a direct bandgap will be presented as well. Optical characterization included room temperature and low temperature photoluminescence. The transport and structural properties of ZnO/Cu(Sc,Mg)O2 heterostructures will also be discussed.This work was supported through grants from the National Science Foundation, the Department of Energy, and the UF Center for NanoBio Sensors. The researchers would also like to acknowledge the Major Analytical Instrumentation Center and the University of Florida.
5:30 PM - B2.8
The Electronic Structure of Pure and Defective SrCu2O2 Studied by DFT, DFT + U, High Resolution X-ray Photoemission and Electron Paramagnetic Resonance Spectroscopy.
Graeme Watson 1 , Kate Godinhok 1 , Aron Walsh 1 , Carey John 1 , Benjamim Morgan 1 , David Scanlon 1 , David Payne 2 , Jeffrey Harmer 2 , Russel Egdell 2
1 School of Chemistry, Trinity College Dublin, Dublin Ireland, 2 Dept. of Chemistry, Oxford University, Oxford United Kingdom
Show AbstractExploitation of transparent conducting oxides (TCOs) in electronic and optoelectronic devices will only become feasible if suitable wide gap oxides that can be p-doped become available. Currently Delafossite type ternary oxides (CuMO2; M = In, Al, Sc etc.) are of great interest[1], but the low energy indirect band gaps these materials possess might hinder their usefulness in optoelectronic devices[2]. SrCu2O2, however, possesses a direct bandgap of ~3.3 eV[3] and UV-emitting heterojunction involving p-type SrCu2O2 and n-type ZnO was recently fabricated[4]. Therefore the electronic structure and the nature of the hole charge carriers in SrCu2O2 is of major interest.[5]
The electronic structure of stoichiometric, Cu-deficient and K-doped SrCu2O2 are investigated using GGA[6] and GGA + U in conjunction with high resolution X-ray photoemission spectroscopy (XPS) and electron paramagnetic resonance spectroscopy (EPR). Acceptor levels above the valence band are noted for Cu-deficient and K-doped SrCu2O2 from the GGA + U characterization, consistent with the experimental data. These results are discussed in relation to the p-type conduction mechanism.
References:
1)A. N. Banerjee, K. K. Chattopadhyay, Progress in Crystal Growth and Characterisation of Materials, 50, 52 (2005)
2)X. Nie, S. H. Wei, S. B. Zhang, Physics Review Letters, 88, 066405 (2002)
3)A. Kudo, H, Yanagi, H. Hosono, H. Kawazoe, Applied Physics Letters, 73, 220, (1998)
4)H. Ohta, M. Orita, M. Hirano, H. Hosono, Journal of Applied Physics, 89, 5720 (2001)
5)C. C. B. Lynch, R. G. Egdell, D. S. L. Law, Chemical Physics Letters, 401, 223 (2004)
6)K. G. Godinho, G. W. Watson, A. Walsh, A. J. H. Green, D. J. Payne, J. Harmer, R. G. Egdell, Journal of Materials Chemistry, 18, 2798 (2008)
5:45 PM - B2.9
DC Reactive Sputtering, Annealing, and Characterization of CuAlO2 Thin Films.
Blake Stevens 1 , Cathleen Hoel 2 , Kenneth Poeppelmeier 2 , Scott Barnett 1
1 Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States, 2 Department of Chemistry, Northwestern University, Evanston, Illinois, United States
Show AbstractB3: Poster Session I
Session Chairs
Thomas Mason
Yuzo Shigesato
Tuesday AM, December 02, 2008
Exhibition Hall D (Hynes)
9:00 PM - B3.1
Transparent EMI of Conductive Zn1-xAlxO Films Prepared by DC-reactive Magnetron Co-sputtering Method.
Shi-Yuan Tong 1 , Min Da Yang 1 , Mean Jue Tung 1
1 Electromagnetic Material and Device Lab. , Industrial Technology Research Laboratories, Hsinchu Taiwan
Show AbstractHighly transparent conductive oxide (TCO) films which hexagonal ZnO lattice substituted by Al atoms were extensively used as conduction electrode for solar cell and flat panel display. Besides, multifunctional ZnO films were very suitable for surface acoustic-wave (SAW) device, diluted magnetic semiconductor (DMS) and CO gas sensor due to its unique intrinsic piezoelectric and semiconducting characteristics[1-3]. The purpose of Al doping is to contribute more free electrons and carrier concentration due to ionic radius of Al3+ is 0.53A which is similar to that of Zn2+, partial Zn sites were substituted by Al atom. In case of excess concentration of Al doping, more Al atoms were forced to locate interstitial Zn-O site in the formation of scattering center, resulting in resistivity increase. Recently, more efforts were taken on electrical and optical properties, but less work was done in the research of electromagnetic shielding (EMI). Furthermore ZnO-Al thin films were candidate for transparent EMI materials for EM absorber and noise filter. In this work, Zn1-xAlxO (x=0~4.83%) films were prepared by co-titled DC-reactive magnetron sputtering with atmosphere of mixed Ar/O2 ratio. The orientation of crystal growth strongly depends on different Ar/O2 ratio. Strong c-axis (002) orientation was observed at Ar/O2 (100/0.5) ratio. Surface resistivity were performed by standard four points and transmittance were measured in the visible region (400~700nm). However less effort was focused on transparent EMI study of conductive Zn1-xAlxO films at high frequency ranged from 40MHz to 20GHz. At x=4.83%, the result shows low resistivity of 0.1Ω-cm and transmittance were higher than 80% in the visible light as shown in fig.1, indicating these properties were strongly dependence on crystal structure and doping concentration. For EMI analysis as shown in fig.2, S21 attenuation increase as increasing frequency up to 20GHz. At x=2.73%, S21 value of -4dB was observed. Transparent EMI property is a very interesting phenomenon because of no other magnetic ions in the lattice, supposing the possible reason may originates dielectric loss or conduction loss. This EMI results has great opportunity to make ZnO-Al film advanced transparent EMI materials.
9:00 PM - B3.10
Characterization of MoO3-x Films Deposited by Reactive Sputtering.
Watanabe Hiroki 1 , Oka Nobuto 1 , Satou Yasushi 1 , Ito Norihiro 2 , Tsuji Hiroya 2 , Shigesato Yuzo 1
1 Graduate School of Sience and Engineering , Aoyama Gakuin University, Sagamihara, Kanagawa Japan, 2 , Matsushita Electric Works, Ltds. Advanced Technologies Development Laboratory, Kadoma, Osaka Japan
Show AbstractMolybdenum trioxide (MoO3) films have been expected as a material that accelerate the hole injection from the anode to the organic layer in organic light-emitting diode (OLED) devices, where the electron injection mechanisms into the organic layer have been discussed actively. I could be considered that the hole injection ability should depend on the stoichiometry 3-x of MoO3-x [1]. Thus, characterization of the various properties of MoOx films deposited under various conditions and clarify the injection mechanisms should be necessary to improve the ability as the injection layer. OLED devices have been fabricated by the evaporation and the spin coating methods. In order to consider the industrial applications, we paid attention to the sputter deposition method which should have large advantages to deposit uniformly in the large area. Especially, the amount of O in the films could be controlled in the wide range for the reactive sputtering using a metal target by controlling the O2 gas flow ratios [O2 / (Ar+O2)] during the depositions.In this study the MoO3-x films were deposited on unheated fused silica glass or silicon single crystal substrates by an rf magnetron sputtering using a Mo metal target in the mixture of Ar and O2 gases. Sputtering power was fixed at 200 W. The total gas pressure was kept constant at 1.0Pa when the O2 gas flow ratios were varied from 0 to 100 %. The MoO3-x films were analyzed by x-ray diffraction (XRD) to identify the crystal phases and crystallinity, atomic force microscope (AFM) to analyze the surface morphology. ATR-FTIR and Raman spectroscopy was used to analyze the interface between the MoO3-x and organic layers (α-NPD). Electronic state of the film surfaces was analyzed by X-ray photoelectron spectroscopy (XPS) and photoelectron spectrometer in air (PESA), where the photoemission characteristics were investigated in detail. The chemical shift of the XPS Mo3d peaks revealed that the valence electron number of Mo was approximately six for all the films. Nevertheless, the intensities of photoemission observed by PESA increased systematically with the increase in x, indicating the generation of localized defect levels in the band gap between Fermi level and the valence band.A part of this work belongs to "High-efficiency lighting based on the organic light-emitting mechanism” supported by New Energy and Industrial Technology Development Organization (NEDO).[1] G. Xie, Y. Meng, F. Wu, C Tao, D Zhang, M Liu, Q Xue, W Chen, and Y Zhao, Appl. Phys. Lett. 92, 093305 (2008)
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High Rate Deposition of Al-doped ZnO (AZO) by Reactive Sputtering (3); Hollow Cathode Gas Flow Sputtering.
Hiroshi Takeda 1 , Yoshinori Iwabuchi 2 , Masato Yoshikawa 2 , Nobuto Oka 1 , Yasushi Sato 1 , Yuzo Shigesato 1
1 Graduate School of Science and Engineering, Aoyama Gakuin University, Sagamihara, Kanagawa Japan, 2 Chemical & Industrial Products Technology Division, Bridgestone Co., Kodaira, Tokyo Japan
Show Abstract9:00 PM - B3.12
In-situ Analyses on Magnetron Sputtering Processes to Deposit ITO and IZO Films.
Norihiro Ito 1 2 , Nobuto Oka 2 , Yasushi Sato 2 , Yuzo Shigesato 2
1 Advanced Technologies Development Laboratory, Matsushita Electric Works, Ltd., Osaka Japan, 2 Graduate School of Science and Engineering, Aoyama Gakuin University, Kanagawa Japan
Show Abstract Magnetron sputtering should be one of the most promising techniques for uniform coatings in large area with high packing density and strong adhesion. In this process, structure and various properties of the deposited films are heavily affected by the kinds of sputtered fragments, flux or kinetic energies of bombarding ions on the growing film surfaces [1-3]. In this study, the sputtering processes to deposit Sn doped In2O3 (ITO) films and indium zinc oxide (IZO) films using the corresponding oxide targets were analyzed in detail by the in-situ analyses of the energy and species of ion fragments. A Quadrupole mass spectrometer (PPM 422, Pfeiffer Vacuum) combined with an energy analyzer were used for the in-situ analyses. Beside the expected negative oxygen ions (O-, O2-), the various other high-energy negative ions (InO-, InO2-, ZnO-, ZnO2-) was observed, corresponding to ions formed at the target surface and accelerated towards the substrate over the sheath potential. As the total pressure was increased the high-energy negative ions diminished due to gas-phase scattering. For the positive ions the many kinds of sputtered fragments (In+, In2+, InO+, In2O+, InO2+ etc.) were observed, which were produced possibly by electron-impact ionizations of atoms/molecules in gas phase. A similar shape of energy corresponded to the subtraction between the plasma and the floating potentials was also observed for Ar+ ions. Based on these results, the effects of the energetic particle bombardments on ITO and IZO films are discussed in detail. A part of this work belongs to"High-efficiency lighting based on the organic light-emitting mechanism”supported by New Energy and Industrial Technology Development Organization (NEDO).[1] P. K. Song, Y. Shigesato, I. Yasui, C. W. Ow-Yang, and D. C. Paine, Jpn. J. Appl. Phys. , Vol.37, (1998) 1870.[2] P. K. Song, Y. Shigesato, et al., Jpn. J. Appl. Phys. Vol.38, (1999) 2921. [3] T. Sasabayashi, N. Ito, M. Kon, P. K. Song, K. Utsumi, A. Kajio and Y. Shigesato, Thin Solid Films, 445 (2003) 219.
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Blue Luminescence in Ferromagnetic Zn1-xCuxO Thin Films.
Kousik Samanta 1 , Pijush Bhattacharya 2 , Ram Katiyar 1 , K. Rao 3
1 Physics, University of Puerto Rico, San Juan, Puerto Rico, United States, 2 Physics, Fisk University, Nashville , Tennessee, United States, 3 Material Science, Royal Institute of Technology, Stockholm Sweden
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, optical, and magnetic properties of PLD grown nanocrystalline thin films of Zn1-xCuxO (x = 0, 1, 3, and 5%) on Al2O3 substrate. The first order Raman spectrum at room temperature of the Zn-faced (0002) polycrystalline Zn1-xCuxO thin films shows two characteristic optical modes of wurtzite ZnO at 98.5 and 439 cm-1 corresponding to the E2low and E2high modes. The crystalline grain sizes of 1, 3, and 5% Cu doped ZnO thin films were calculated by phonon confinement model as 12.33, 9.39, and 8.38 nm respectively. The PL spectrum of the ZnO thin film at 77 K is shows the excitonic emissions at 3.369 and 3.354 eV were observed corresponding to the free exciton (FX) and donor bound exciton (D0X) respectively. The most intense peak at 3.309 eV is attributed as donor acceptor pair (DAP) recombination. For the DAP emission, a new peak emerges at the higher energy side with increase in temperature. This feature is typically for DAP transition, and the new peak at higher energy side is caused by band-to-impurity transition (eA0). In the case of Cu doped ZnO thin films, the near band edge transitions consists of three peaks in which two are similar to undoped ZnO excitonic transitions and an additional peak appears at 3.353 eV belongs to the neutral acceptor bound exciton (A0X). This prominent A0X emission in the PL spectrum of Cu doped ZnO thin film suggests the incorporation of acceptors due to Cu doping. The intensity of donor acceptor pair (DAP) transition at 3.309 eV in Cu doped ZnO sample decreased abruptly compared to pure ZnO thin film. The decrease of DAP intensity may attributed to the additional acceptor levels which favors more A0X recombination. The strong blue emission at 2.97 eV was observed in the PL spectrum of Cu doped ZnO thin films. This blue emission might come from the intrinsic defects in ZnO and/or Cu impurities. The ferromagnetic properties of Cu doped ZnO thin films were carried out using SQUID magnetometer measurement. We have obtained room temperature ferromagnetism with saturation magnetization Ms = 0.48 μB/Cu in 1% Cu doped ZnO thin film. The temperature dependence of magnetization at 100 Oe confirms the ferromagnetic nature and transition temperature above 300 K in our Zn1-xCuxO thin films.
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Growth of Conducting Zinc Oxide Films by SMI-MOCVD System.
Shangzhu Sun 1 , Gary Tompa 1 , Gary Provost 1 , Roy Roy Moonsammya 1 , Zhe Chuan Feng 2
1 , Structured Materials Industries, Inc., Piscataway, New Jersey, United States, 2 Department of Electrical Engineering, Graduate Institute of Electro-Optical Engineering of National Taiwan University, Taipei Taiwan
Show Abstract9:00 PM - B3.15
Influence of ZnO/LT-ZnO Multi-buffer-layer Structures on the Characteristics of ZnO Films Prepared ALD using DEZn and N2O.
Dong-Yuan Lyu 1 , Tai-Yuan Lin 1 , Kuang-Pi Liu 2 , Wei-Hse Chi 2 , Ting-Hsuan Chen 3 , Hsun-Feng Hsu 3 , Song-Ping Szu 2 , Jyh-Rong Gong 2
1 Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung City Taiwan, 2 Department of Physics, National Chung Hsing University, Taichung City Taiwan, 3 Department of Materials Science and Engineering, National Chung Hsing University, Taichung City Taiwan
Show AbstractZnO thin films were grown on (0001) sapphire substrates using certain ZnO/low-temperature(LT)-ZnO multi-buffer-layer structures by atomic layer deposition (ALD). Based on experimental results, it was found that the use of certain ZnO/LT-ZnO multi-buffer-layer structures helped to accommodate the large lattice mismatch between (0001) ZnO film and (0001) sapphire substrate so that completely coalesced ZnO films could be achieved.ZnO nanowires and nano-rods were commonly observed because ZnO tended to form prefer-orientation along [0001]ZnO. Recently, coalesced ZnO films have been reported to achieve on ZnO substrates by employing certain high temperature (HT)-ZnO/LT-ZnO multi-buffer layer structures. In this work, we conducted ALD growth of highly mismstched ZnO films on the (0001) sapphire substrates using multi-buffer layer structures. It was found that ALD allowed to deposit coalesced ZnO films on (0001) sapphire substrates using ZnO/LT-ZnO multi-buffer-layer structures.ZnO films, 0.4 μm in thickness, were deposited at 500°C on (0001) sapphire substrates by ALD using diethyl-zinc (DEZn) and nitrous oxide (N2O) with ZnO/LT-ZnO multi-buffer-layer structures in an atmospheric pressure quartz reactor. ZnO and LT-ZnO constituent layers were deposited at 600°C and 200~400°C, respectively. As-grown ZnO films were characterized by field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), x-ray diffractometry (XRD), and photoluminescence(PL) spectroscopy to explore their morphological, structural and optical characteristics.SEM observations show that a completely coalesced ZnO film can be achieved using a 3-pair 18.7nm-thick/15.6nm-thick multi-buffer-layer structure with ZnO and LT-ZnO constituent layers being deposited at 600°C and 300°C. For ZnO films having a 3-pair ZnO(600°C)/LT-ZnO(200°C) or a 3-pair ZnO(600°C)/LT-ZnO(400°C) multi-buffer-layer, the SEM image shows [0001]ZnO prefer-oriented granular ZnO structure without coalescence. θ-to-2θ XRD measurements indicate that the use of 3-pair ZnO(600°C)/LT-ZnO(300°C) multi-buffer-layer helps to accommodate the large lattice mismatch between (0001) ZnO film and (0001) sapphire substrate. Both RT PL and AFM studies confirmed that the ZnO film with a 3-pair ZnO(600°C)/LT-ZnO(300°C) multi-buffer-layer exhibited stronger PL intensity and less surface roughness than those observed in other ZnO samples. It is believed that the above-mentioned ZnO/LT-ZnO multi-buffer-layer structure is beneficial to reduce the mismatch between ZnO and sapphire so that coalesced ZnO film can be achieved.We report ALD deposition of coalesced ZnO films on (0001) sapphire substrates using ZnO/LT-ZnO multi-buffer-layer structures. It was found that completely coalesced ZnO films could be achieved with improved morphological and optical properties.This work was supported in part by the National Science Council of Taiwan, Republic of China, under contract no. NSC942212-M-005-014.
9:00 PM - B3.18
Effect of Thickness on Surface Plasmon Resonance Reflectivity in Nickel Alloy Films.
Maarij Syed 1 , Azad Siahmakoun 1
1 Physics & Optical Engineering, Rose-Hulman Institute of Technology, Terre Haute, Indiana, United States
Show AbstractWe have performed surface plasmon resonance (SPR) experiments in the Kretchmann configuration on prisms coated with metal films. The experiment is performed at a number of wavelengths that include the most commonly available laboratory wavelengths (e.g., HeNe laser) and 1320 nm/1550 nm wavelengths that are important for optoelectronic applications. The metal films of 20nm/50nm thickness are grown by magnetron sputtering and are binary alloy films of Nickel and Chromium, and Nickel and Titanium (Nichrome & NiTi). In addition, for comparison we will also present results for pure metal films (Nickel, Chromium, and Titanium). The aim of this study is two-fold. Our results would show SPR behavior as a sensitive function of film composition and film thickness. We will present results that identify a range of alloy composition that enhances SPR and other interesting features in the reflectance signal from these films. Our results will also show two interesting thickness regimes where the reflectance is dominated by different processes that take place at the interface between the metal and the dielectric (fused silica prism in our study). Our measurements reveal a contrast reversal in plasmonic signal (only observed for p-polarization) as we change the alloy thickness from 50nm to 20 nm. Aided by these observations, we hope to better understand the optical properties of the surface plasmons that are excited in these films and also learn about the effective dielectric constant of the metal alloy films. To this end, we present results obtained from spectroscopic ellipsometry that inform our discussion of the dielectric function of these metal films. We will discuss a theoretical model that accounts reasonably well for the features see in the reflectance spectrum of these films. The role played by spectroscopic ellipsometry in providing information for this theoretical model will be highlighted.We will show the effects of growth conditions and the resulting surface morphology on the SPR signal. This in turn would be helpful in understanding the surface details of the individual metal films. We will also present on how SPR results can be better understood by analyzing the SPR data alongside data obtained on these films from x-ray analysis (for composition and structure information), and AFM analysis (for surface topography). Finally, we will comment on the prospects of technological applications of these alloy films.
9:00 PM - B3.19
Preparation of Copper Oxide Thin Films by ALD using Cu(dmamb)2 and Water.
Sun Sook Lee 1 , Byung Kook Lee 1 , Taek-Mo Chung 1 , Young Kuk Lee 1 , Chang Gyoun Kim 1 , Ki-Seok An 1
1 , Korea Research Institute of Chemical technology, Daejeon Korea (the Republic of)
Show AbstractCopper oxide(CuO) is one of binary p-type conducting oxides. Amorphous copper oxide films have been deposited on Si and Pt/Ti/SiO2/Si substrates by atomic layer deposition (ALD) using a liquid precursor Cu(dmamb)2[dmamb=1-dimethylamino-2-methyl-2-butanolate]. The self-limiting ALD process by alternative surface reactions of Cu(dmamb)2 and water was confirmed by thickness measurements of the grown copper oxide films measured as functions of the Cu(dmamb)2 supply times and numbers of Cu(dmamb)2-H2O cycles. The ALD temperature window for this precursor was found to be between 90 and 140 oC. The growth rate of the copper oxide films was 0.56 Å / cycle in optimal reaction condition. The X-ray diffraction patterns showed no distinct peaks for copper oxide, indicating that the films at the films deposited at this temperature were amorphous. The AES analysis evidently shows that the copper oxide film obtained is stoichiometric and contains any appreciable amounts of carbon impurities. Also, the resistance switching of Pt/CuOx/Pt capacitor structure was investigated by the current-voltage (I-V) measurement.
9:00 PM - B3.2
Comparison of ITO by Capacitive RF Magnetron Sputtering and Facing Target DC Sputtering as an Anode on the Organic Light Emitting Diode.
Jun Tak Kim 1 , Chul Yoon 1 , Hee Taek Shin 2 , Sang Ho Kim 1
1 , Korea University of Technology and Education, Cheonan Korea (the Republic of), 2 , BOWON light metal, Cheonan Korea (the Republic of)
Show Abstract9:00 PM - B3.20
Ultraviolet Photoconductive detector using Cerium Fluoride Thin Film Grown by Pulsed Laser Deposition.
Kazuyoshi Tabata 1 , Tomohito Tamakoshi 1 , Yohei Masukawa 1 , Yuki Kuwana 1 , Yo Ichikawa 1 , Shingo Ono 1 , Kentaro Fukuda 2 , Toshihisa Suyama 2 , Takayuki Yanagida 3 , Akira Yoshikawa 3 , Fumio Saito 3
1 , Nagoya Institute of Technology, Nagoya, Aichi Japan, 2 , Tokuyama Corporation, Tokyo Japan, 3 , Tohoku University, Sendai, Miyagi Japan
Show AbstractRecently, there has been interest in photoconductive detectors made with wide gap materials for solar insensitive ultraviolet (UV) detectors. This is an attractive scheme because of its simplicity, reliability, and efficiency. The operating wavelength of these devices is basically limited by the band gap of the materials and is controlled by the composition of complex materials selected. There has been continuous effort in such research about wide gap materials involving ZnO, AlN and diamond. Some of the other most prominent candidates are fluorides. The main advantages of fluorides are their relatively wide band gaps. Additionally, fluorides offer a wide selection of complex materials with wide band gaps and identical crystal structures, and it is possible to select the band structure, band gap and lattice constant. In this work, we present the growth of cerium fluoride (CeF3) films on quarts glass substrates by pulsed laser deposition (PLD) and evaluation of CeF3 thin films as UV photoconductive detectors.CeF3 thin films were deposited on quarts glass substrates by PLD using third harmonics of Nd:YAG laser (355 nm). The repetition rate of laser was 10 Hz. The laser spot size focused on the CeF3 target was 1-mm diameter with a laser fluence of 100 J/cm2. The deposition was carried out in a base pressure of 5 x 10-5 Pa. The temperature of substrate was controlled from R.T. to 870 K. The CeF3 thin film grown at 670 K shows highest transparency in visible region. For the evaluation of these thin films as photoconductive detectors, a pair of interdigitated aluminum electrodes were fabricated onto the CeF3 thin films. The gap between the electrodes was 0.4 mm. The bias voltage was applied to the electrodes up to 1 kV. Mercury lamp was irradiated to the patterned area of interdigitated electrodes. The center wavelength of the radiation source was 370 nm. As a result, we observed that the photocurrent of CeF3 thin films grown at 670-K was approximately 1 order higher than dark current and that the output current linearly increased without saturation up to 25-kV/cm electric field. Additionally, this sample shows the response below 400-nm wavelength. In conclusion, we have successfully fabricated CeF3 thin films on quarts glass substrates by PLD. Furthermore, CeF3 thin films irradiated with UV light below 400 nm shows photoconductivity. By mixing with other fluorides, the band gap and sensitive area will be controlled. These observed characteristics open up the possibility of using fluorides as UV photoconductive detectors.
9:00 PM - B3.21
Layer-by-layer Self-assembly of Unilamellar Nanosheet Crystallites of Ruthenium Oxides.
Katsutoshi Fukuda 1 , Hisato Kato 2 , Wataru Sugimoto 1 2 , Yoshio Takasu 2
1 Nano-FIC, Shinshu University, Ueda, Nagano, Japan, 2 Textile Science and Technology, Shinshu University, Ueda, Nagano, Japan
Show AbstractSelf-organization and self-assembly processes using various nano-scaled fragments including organic and inorganic materials have attracted much attention in the area of nanotechnology because they have latent ability to construct sophisticated nanostructures with unique performances and dimensions. Recently, we have demonstrated that unilamellar nanosheet crystallites of ruthenium oxides can be derived by total exfoliation of a layered potassium ruthenate. The obtained nanosheet, which is regarded as a family of RuO2 and hydrated RuO2 well known for their high electronic or protonic conductivity and catalytic activity, is characterized by a thickness in the range of molecular dimension and a lateral size in bulk dimension. Such a very high two-dimensional anisotropy may have some advantages to construct advanced architectures in the layering process and some enhanced physical-chemical properties may be expected for resulting monolayer or multilayer assemblies in comparison with the film being composed of ruthenic acid nanoparticles. Here, we report a successful film growth via an electrostatic layer-by-layer self-assembly using the ruthenate nanosheets and organic polymers as a counter part, presumably yielding a promising candidate for chemically and electrochemically stable transparent conductor.
9:00 PM - B3.22
Influence of High Energy Negative Ion Bombardments on Structure and Electrical Properties of Al Doped ZnO (AZO) Films Deposited by dc Magnetron Sputtering.
Daisuke Watanabe 1 , Norihiro Ito 1 , Nobuto Oka 1 , Yasushi Sato 1 , Yuzo Shigesato 1
1 Graduate School of Science and Engineering, Aoyama Gakuin University, Kanagawa Japan
Show AbstractThe deposition method mostly used for AZO films is magnetron sputtering using a ceramic oxide target because it is very well suited for large area uniform coatings with high packing density and strong adhesion. However, the degradation of electrical properties has been observed at the positions opposite to the erosion track on the target. The degradation of the crystallinity for ZnO films is considered to be caused by the bombardment of high energy particles such as energetic Ar atoms (high energy neutrals) or negative oxygen ions. On the other hand, Minami et al. suggested that the increase in resitivity of AZO films was caused by the excess oxidation at the position opposite to the erosion track [1]. In this study, we analyzed the flux and energy distributions of high energy negative ions during the dc magnetron sputtering using an AZO target and discussed the influence of high energy negative ion bombardments on the structure and electrical properties of the films. High energy negative ions were analyzed using a quadrupole mass spectrometer combined with an electrostatic energy analyzer, which was positioned at the substrate position opposite to the AZO (Al2O3: 2.0 wt%) target. The sputtering power during the analyses was maintained at 50 W. The O2 flow ratio [O2 / (Ar+O2)] were controlled from 0 to 5 %. For the analysis of the flux of the negative ions at the different substrate locations, the sputtering target was perpendicularly moved to the quadrupole mass spectrometer. In order to control the cathode voltage, the magnetic field strength was selected as 0.025, 0.06 and 0.1 T. In order to discuss the influence of the bombardments on the film properties, AZO films were deposited on unheated alkali-free glass substrate under the same condition of the fragment analysis. The atomic oxygen negative ion (O-) and molecular oxygen negative ion (O2-) were observed as the high energy negative ions which possessed the energy corresponding to the cathode sheath voltage. The flux of O- was much larger than O2-. The maximum flux of O- was observed at the location opposite to the erosion track on the target. The flux of O- decreased slightly with increasing O2 ratio. These results indicate that high energy negative ions were not formed by electron attachment in the cathode sheath but should be sputtered from the target surface. Depending on the magnetic field strength, the cathode voltage varied from 337 V at 0.1 T to 403 V at 0.025 T. While the peak of O- shifted to lower energies with increasing the magnetic field strength, the flux of O- was hardly changed. The electrical conductivity of AZO films was improved with increasing the magnetic field strength and decreasing plasma impedance during the depositions.[1] T. Minami et al., J. Vac. Sci. Technol. A18, 1584 (2000).
9:00 PM - B3.23
Ink-Jet-Printed Transparent Oxide Semiconductor Thin Film Transistors.
Keun Kyu Song 1 , Dongjo Kim 1 , Chang Young Koo 1 , Youngmin Jeong 1 , Jooho Moon 1
1 Department of Materials Science and Engineering, Yonsei University , Seoul Korea (the Republic of)
Show AbstractSolution-processed thin film transistors possess many advantages such as low-costs and simplicity. Of solution-processing methods, inkjet printing offers an opportunity for direct patterning of various materials. In this work, we have fabricated a transparent oxide semiconductor thin film transistor (TFT) using ink-jet-printed semiconductor and electrode layers. For solution processed transparent oxide semiconductor, an active material of Zn-Sn-O system was synthesized by sol-gel method, from which the channel layer was deposited on the heavily doped n-type silicon wafer with 200-nm thick thermal SiO2 layer as a substrate. After annealing of 500oC, the transparent oxide electrode material (i.e. source and drain) was printed on top of the Zn-Sn-O semiconductor layer. For optimal device performance, we investigated the influence of the fabrication conditions on the electrical characteristics by controlling the sol-gel composition, annealing temperature and atmosphere, and the substrate treatment method. The output and transfer characteristics of the TFT devices were measured in air at room temperature. The absorption and transmission analysis of the transparent oxide TFT were measured at various wavelengths using a UV-vis spectrophotometer.
9:00 PM - B3.24
Synthesis of Size and Morpholgy Controlled Zinc Oxide (ZnO) Nanoparticles.
Ozlem Altintas 1 , Caner Durucan 1
1 Metallurgical and Materials Engineering, Middle East Technical University, Ankara Turkey
Show AbstractControlling particle morphology is a critical issue in synthesis of nanoparticles, as the size and shape at nano scale can greatly influence material properties and different technological applications may require specific morphologies. Among numerous processing possibilities microemulsion techniques are especially attractive as they do not require sophisticated equipment or rigorous experimental conditions, but still providing possibilities in controlling the size and morphology of the oxide particles in a size scale approaching to nanometers. This work reports synthesis of phase pure ZnO nanoparticles in different geometrical forms by aqueous methods. One specific objective of the study is to define the morphological stability regions for a selected microemulsion system where sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and zinc salt solutions were employed as the surfactant and aqueous components, respectively. Complete characterization of microemulsion products have been performed by using a variety of microstructural (XRD and FESEM), optical (Uv-Vis, FTIR) and thermal (TGA-DTA) techniques. Morphological variations have been achieved by choosing different formulations in the ternary microemulsion system. The choice of various concentrations in the surfactant:oil:water ternary system resulted in formation of ZnO nanoparticles with distinct morphologies. Sphere-like ZnO nanoparticles with an average diameter of 30 nm were synthesized with AOT:water:oil molar ratio of 8:5:87 and elongated rod-like particles with an approximately aspect ratio of 5:1 were obtained by choosing 8:10:82 AOT:water:oil molar ratio.
9:00 PM - B3.25
Characteristics and Reliability of Ink-Jet Printed Zinc Tin Oxide Semiconductor Transistors.
Youngmin Jeong 1 , Dongjo Kim 1 , Jooho Moon 1
1 Department of Materials Science and Engineering, Yonsei University , Seoul Korea (the Republic of)
Show AbstractWe have synthesized Sn-doped ZnO (ZTO) sol-gel precursor solution for depositing oxide semiconductor layer by either spin-coating or ink-jet printing. Bottom contact-type ITO electrodes were used for the fabrication of the transistors. The ZTO ink had the viscosity of 13.8 mPa-s. The ink was printed using a piezoelectric DOD ink-jet printing device and the nozzle orifice size was 50 μm. The ejected droplet from nozzle has a diameter of about 59 μm and the velocity of 1.5 m/s. The transistor based on spin-coated ZTO exhibits the mobility of 1.14 cm2/Vs and the on/off ratio of 5x106 when annealed at 500oC. By contrast, the electrical performance of the ink-jet printed ZTO transistors significantly depend on the substrate treatment method. Alcohol-treatment allows the transistors to show good electrical performance as compared to the treatment with self-assembled monolayer. The ink-jet printed transistors exhibit the mobility of 0.58 cm2/Vs and the on/off ratio of 107. We fabricated at least 20 ink-jet printed transistors under the identical conditions to test the device reliability. The average electrical parameters for the printed transistors include the mobility of 0.51±0.1 cm2/ Vs, the threshold voltage of 10.0±3.5 V, the on/off ratio of 4.7x106, and sub-threshold swing of 0.95±0.21 V/dec. Furthermore, the influence of bias stress on the printed transistors was investigated.
9:00 PM - B3.26
Nb Doped TiO2 Films with Low Resistivity Deposited by dc Magnetron Sputtering using a TiO2-x Target for Transparent Conductive Electrodes.
Yasushi Sato 1 , Yuta Sanno 1 , Nobuto Oka 1 , Toshihisa Kamiyama 2 , Yuzo Shigesato 1
1 Graduate school of Science and Engineering, Aoyama Gakuin University, 5-10-1 Sagamihara, Kanagawa 229-8558 Japan, 2 , AGC Ceramics Co., Ltd, 5-6-1, Umei, Takasago, Hyogo 676-865 Japan
Show AbstractNb doped anatase TiO2 films have been paid attention as Indium-free transparent conductive oxide films [1]. It has been reported that the films deposited on glass substrate by a reactive magnetron sputtering using a Ti metal target and O2 gas exhibited much higher resistivity than the conventional transparent conductive films, unless they were postannealed at 500 oC under 100% H2 gas of around 1 atm. Such reactive sputtering process is a highly non linear process, where the deposition rate shows the “hysteresis” as a function of reactive gas flow rate. Such behavior originates in the oxidation of the target surface, which results in a drastic decrease in the sputtering yield. In order to deposit polycrystalline anatase TiO2 films, the reactive sputtering process should be in “oxide mode”, where the deposited films are fully oxidized and the carrier density becomes very small. This must be the reason why the postannealing in strongly reductive atmosphere was necessary. Furthermore the deposition rate in the “oxide mode” is very small of 2-5 nm/min and the postannealing under such high pressure of H2 gas should be avoided from a practical point of view. In this study we deposited Nb doped TiO2 films by dc magnetron sputtering using slightly reduced TiO2-x target (AGC Ceramics Co., Japan) [2]. By using this TiO2-x target, very stable dc discharge was maintained because the TiO2-x target of x = 0.02 exhibited a sufficient conductivity of about 3.7 Scm-1 to avoid charge up of the target surface and hence to suppress arcing. Nb doped TiO2 films with various Nb contents were deposited on unheated glass substrates by controlling the number of Nb2O5 pellets on the TiO2-x target. XRD profiles of all the as-deposited films exhibited amorphous structure, whereas all the films were crystallized into polycrystalline anatase structure after post-annealed in vacuum (6×10-4 Pa) at 400 oC for 1 hr. Resistivity of the post-annealed films decreased from 3.8×10-2 to 1.3×10-3 Ωcm with increasing Nb content from 0 to 6.4 at. %. The carrier density of the films with Nb content of 6.4 at.% was 1.3×1021 cm-3, where the doping efficiency was estimated to be 65 %. All the films possessed high transmittance of over 60-70 % in the visible region of light. In the case of using the TiO2-x-Nb2O5-x target (9.5 at.% of Nb), the lowest resistivity of Nb doped TiO2 films was 6.6×10-4 Ωcm after the post-annealing in vacuum at 500 oC for 1 hr, where the Nb content in the films was analyzed to be 7.4 at.% by EPMA. [1] Y. Furubayashi et al., Appl. Phys. Lett. 86 (2005) 252101. [2] Y. Sato, Y. Shigesato, et al., Thin Solid Films 516 (2008) 5758.
9:00 PM - B3.27
Absorption and Photoluminescence from Highly Conductive N-type ZnO.
Xiaocheng Yang 1 , John Neal 2 , Lynn Boatner 2 , Nancy Giles 1
1 Physics, West Virginia University, Morgantown, West Virginia, United States, 2 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractZinc oxide has a room temperature band gap of 3.37 eV. In this material, n-type conductivity can be greatly enhanced by intentionally doping with shallow donor impurities, such as gallium or aluminum. Results are reported for a set of highly-conducting gallium-doped n-type ZnO single crystals grown by the high-pressurized melt technique (Cermet, Inc.). We describe changes in the optical absorption, reflectance, and luminescence due to free carriers. Gallium donors were intentionally introduced during growth to produce samples with carrier concentrations at 300 K varying from 7 x 1016 cm−3 to 2 x 1019 cm−3. The sample with the highest carrier concentration had a conductivity of 200 (Ωcm)−1. Our experimental study includes Hall-effect measurements (85 to 380 K), UV/VIS/IR absorption, photoluminescence (PL) (5 to 300 K), and infrared reflectance. Free carriers produce absorption in the infrared region with a tail extending into the visible at high doping levels, causing the samples to have a blue tint. The free-carrier absorption coefficient (α) in these samples has a wavelength dependence of α ~ λp, where p varies from 3.0 to 3.6 (consistent with ionized impurity scattering). Infrared reflectance spectra are also affected by free carriers, and the reflectance minimum (i.e., the “plasma dip”) can be used to determine carrier concentrations. To establish the effects of free carriers on the band gap energy of highly conducting ZnO:Ga, an analysis of temperature-dependent band-edge PL was carried out. The results are in good agreement with predictions of band gap renormalization in ZnO due to free carriers [1]. At the highest doping level, bright uv emission is still observed even though the samples are degenerately doped at levels exceeding the Mott density. We discuss the origin of the radiative recombination and show at what carrier concentrations the PL changes from excitonic to band-to-band recombination. Based on our results, we can predict optical properties of n-type ZnO with even higher carrier concentrations, as needed for transparent conducting oxide applications. Research supported by the DOE Office of Nonproliferation Research and Engineering in the National Nuclear Security Administration (NNSA), US Department of Energy under contract DE-AC05-00OR22725 with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC. Work done at WVU also supported by NSF Grant Nos. 0508140 and 0804352.[1] N. C. Giles et al., Appl. Phys. Lett. 89, 251906 (2006).
9:00 PM - B3.28
PMP-MOCVD Grown ZnxCd1-xSe cladded ZnyCd1-ySe Quantum Dot Structures Exhibiting Diode Like Characteristics for Electroluminescent Application.
Faquir Jain 1 , Fuad Alamoody 1 , Angel Rodriguez 2 , Ernesto Suarez 1 , Wenli Huang 3 , Fotios Papadimitrakopoulos 1
1 Electrical and Computer Engineering, University of Connecticut, Storrs, Connecticut, United States, 2 , Intel Corp, Rio Rancho, New Mexico, United States, 3 Electrical Engineering, US Military Academy, West Point, New York, United States
Show AbstractThis paper reports the fabrication of devices that exhibit diode like characteristics using pseudomorophic ZnxCd1-xSe (core) quantum dots (QDs) cladded with ZnyCd1-ySe (x < y). The cladded dots were grown using Photoassisted Microwave Plasma Metalorganic Chemical Vapor Phase Deposition (PMP-MOCVD) reactor. The devices were fabricated by growing QDs on ITO coated glass substrates with CsF as the thin barrier layer between the dots and the aluminum cathode. Current-voltage characteristics are presented with and without CsF layers.Earlier we reported growing 3-8 nm CdSe and pseudomorophic ZnxCd1-xSe (core) quantum dots (QDs) cladded with ZnyCd1-ySe (x < y) in a Photoassisted Microwave Plasma Metalorganic Chemical Vapor Phase Deposition (PMP-MOCVD) reactor [1, 2]. Influence of growth parameters including microwave power, ultraviolet intensity, gas phase II/VI [Zn+Cd/Se] molar ratio, temperature of growth, and post-growth processing was investigated. The grown dots are compared [using high-resolution transmission electron microscopy (HR-TEM)] with those reported before [1]. They are also compared with dots prepared by other methods [2, 3]. It has been shown [1] that photoluminescence (PL) peaks and full width at half maximum (FWHM), and X-ray diffraction (XRD) data have been used to calculate dot size. The recently obtained photoluminescence spectrum on our cladded dots deposited on glass substrate shows the exciton peaks. Two different dot sizes were observed.We have also simulated the optical gain of cladded quantum dots including the effect of strain in the cladding for different composition of cladding layer. Simulation is based on excitonic model reported by Jain and Huang [4] with some modification.References: [1] A. Rodriguez, R. Li, P. Yarlagadda, F. Papadimitrakopoulos, W. Huang, J. Ayers, and F. Jain, NSTI, Boston Conference, May 8, 2006.[2] X. Peng, M.C. Schlamp, A.V. Kadavanich and A. P. Alivisatos, J. Am. Chem. Soc., 119, 7019-7019.(1997).[3] B.O. Dabbousi, J. Rodriguez-Viejo, F.V. Mikulec, J.R. Heine, H. Mattoussi, R. Ober, K.F. Jensen, M.G. Bawendi. J. Phys. Chem. B, 101, 9463-9475(1997).[4] F. Jain, W Huang, J. Appl. Phys., 85, 2706-2712 (1999).Acknowledgement: This work is supported by ONR Contracts N00014-02-1-0883 and N00014-06-1-0016, and NSF-Grant ECS 0622068. Discussions with Dr. D. Purdy (ONR) and Dr. R. Khosla (NSF), and technical assistance in processing by S. Vaddiraju are gratefully acknowledged.
9:00 PM - B3.29
Electrical Properties of Polyaniline/indium Tin Oxide Nanocomposites.
Gislayne Herculano 2 1 , Cauê Ribeiro 3 , Rodrigo Bianchi 1
2 , Centro Federal de Educação Tecnológica de Ouro Preto, Ouro Preto, MG, Brazil, 1 Departamento de Física, Universidade Federal de Ouro Preto, Ouro Preto, MG, Brazil, 3 , Embrapa, São Carlos, SP, Brazil
Show AbstractSince the discovery of electroluminescence in polymers, substantial research has been gone into improving the light output, power efficiency, and lifetime of polymer light emitting devices (OLEDs). Typically, OLEDs consists of a transparent hole-injecting anode, an electroluminescent conjugated polymer, and an electron injecting cathode in which their optoelectronic is strongly dependent on the electrical, morphological and optical properties of these materials. The indium-tin oxide - ITO is nowadays their most widely used transparent and electrode, while barium, aluminum and calcium as the metallic one. However, ITO is a non-stoichiomectric material which presents a high work function (4.5 - 5.0 eV) and high electrical conductivity, but also acts as an oxygen and metal ion source, resulting in quenching sites for electroluminescence (EL) devices. In order to minimize this effect, an intermediate hole transport layer (HTL) between the ITO and the photoemissive layer is required. Several studies of utilizing doped polymers as HTLs have been reported on literature. Among these polymers, the polyaniline shows attractive characteristics since it presents the possibility to be processed as ultrathin films, and also appropriated work function for OLEDs applications. In this context, the possibility to prepare transparent and ultrathin films based on ITO nanoparticles and doped polyaniline appears here as a great opportunity to develop efficient materials for applications as HTL of OLEDs, in which thorough study is needed to understand the role of ITO on the electrical properties of PANI/ITO medium. In this work we have investigated the alternating conductivity of ultrathin PANI/ITO films obtained from doped PANI and ITO nanoparticles (~ 8 nm). The ac results are typical of a disordered medium in which the logarithm of the real component, σ’ (ω), exhibits two frequency regions, one plateau at low frequencies (the dc plateau) followed by a region of increasing conductivity, obeying σ’ (ω) α ω^n, where 0 ≤ n ≤ 1, for higher values of frequencies. In order to interpret both the real and the imaginary components of σ* (ω) , we developed a model which considers the doped PANI as a semiconductor matrix, sprinkled with conductive ITO nanoparticles. The conduction through the insulating matrix obeys the random free energy barrier model, while ITO nanoparticles a metallic frequency-independent conductivity is considered. From the fittings is possible to obtain the activation energy value of the maximum energy barrier of the doping mechanism and to estimate the concentration of hopping sites. This work was sponsored by FAPEMIG and CNPq.
9:00 PM - B3.3
Optical and Electrical Properties of Al Doped ZnO Layers Measured by Wide Angle Beam Spectroscopic Ellipsometry.
Csaba Major 1 , Gyorgy Juhasz 1 , Agoston Nemeth 1 , Zoltan Labadi 1 , Peter Petrik 1 , Zoltan Horvath 2 , Miklos Fried 1
1 Research Institute for Technical Physics and Materials Science, Hungarian Academy of Sciences, Budapest Hungary, 2 Department of Solid State Physics, Hungarian Academy of Sciences, Budapest Hungary
Show Abstract9:00 PM - B3.30
Raman Scattering Studies of Anharmonic Contribution to Phonon in p-type CuAlO2 Thin Films.
Manoj Singh 1 , Sandra Dussan 1 , Ganpat Sharma 1 , Ram Katiyar 1
1 Physics, University of Puerto Rico, San Juan, Puerto Rico, San Juan, Puerto Rico, United States
Show AbstractTransparent conducting oxides (TCO) are currently the subject of intensive investigation because of these materials potentially offering a whole range of new applications, including the flat panel display, hetro junctions for solar cells, ultraviolet emitting diodes, transparent diodes and transistors, dilute magnetic semiconductors, and ozone sensing devices. The well known n type materials, such as ZnO, In2O3, are widely used in flat panel display technology because of their high transmission in visible range and good electrical conductivity. However, there are few oxide materials in nature exhibiting p type conductivity and high optical transitions. CuAlO2 is known as a transparent oxide material that exhibits p type conductivity up to 1 S cm-1 at room temperature. It has a wide band gap energy of about 3.5 eV (direct) and 1.8 eV (indirect). It belongs to a family of delafossite structure with rhombohedral symmetry (space group R3m). CuAlO2 thin films were grown on single crystalline sapphire substrates with c-axis orientation by rf sputtering method. The X – ray diffraction data indicates the formation of delafossite structure and tended to be oriented along (001). Temperature dependent Raman spectra of CuAlO2 thin films were measured from 80K to1273K and we observed two optical modes at Eg ( 418 cm-1 ) and A1g (767 cm-1) showing anomalous frequency and line width shifts with temperature, which were interpreted as an experimental evidence of combined effect of lattice expansion and anharmonic phonon – phonon interaction in CuAlO2. At high temperatures, polaronic state and change in effective mass due to lattice expansion also affects the frequency shift and the line width of the observed Raman modes.
9:00 PM - B3.31
High Mobility ITO-ZnO:Al Thin Films Grown by Combinatorial Pulsed Laser Deposition.
Doina Craciun 1 , Gabriel Socol 1 , Magda Nistor 1 , Valentin Craciun 1 2
1 Laser, NILPRP, Bucharest Romania, 2 Materials Science and Engineering, University of Florida, Gainesville, Florida, United States
Show Abstract9:00 PM - B3.32
Effect of Annealing on Rectifying Contacts on ZnO Thin Films Grown using Pulsed Laser Deposition.
A. Bhattacharya 1 , R. Gupta 1 , P. Kahol 1 , K. Ghosh 1
1 Department of Physics, Astronomy, and Materials Science, Missouri State University, Springfield, Missouri, United States
Show AbstractZinc oxide (ZnO) is probably the most explored oxide based semiconductor in this century, for its versatile characteristics in number of domains. ZnO has drawn a strong attention in optoelectronics for being transparent as well as highly conducting in nature, making it suitable to be used for transparent electrodes. Also having wide band gap (3.3 eV), ZnO offers promise to be utilized as UV detectors. ZnO has tasted some success in the domain of spintronics as well. But in-spite of this fame, the thirst of understanding the basic transport phenomena in ZnO is still in search, as the situation is enough challenging to make good contact on ZnO. Based upon the growth technique a metal-ZnO contact can be ohmic or rectifying in nature. The ultimate hurdle therefore lies to make a stable rectifying contact with metal ( Au, Ag, Pt) and ZnO. In this article we therefore aim to make some systematic study towards the metal (Au, Ag) and ZnO contact , and optimization of the growth parameters. It has been found that inherent oxygen vacancies act as source of conducting nature of ZnO. Here we grow ZnO films in ambient oxygen pressure (~ 20 mTorr) at 400 C using pulsed laser deposition technique. The films with (100) preferential growth, are annealed for different time scales in higher oxygen pressure. This makes the carrier concentration of ZnO to reduce from 1020/ cm3 to 1017 / cm3 or order below that. Thickness of the films were limited to 200nm, with surface roughness ranging 10 ~ 15 nm. The ZnO surface was etched with energy pulses 50 mJ / cm2 to reduce some surface defects. The transparency of the films grown in ambient oxygen pressure is nearly 80% . Later, Ag and Au electrodes were grown using same technique on the ZnO film. Detailed observations of I-V characteristics with different annealing time, surface roughness, and XRD data will be presented.
9:00 PM - B3.33
Luminescence of ZnO Thin Films Grown on Glass by Radio-frequency Magnetron Sputtering.
Kai Liu 1 , Michael Shur 1 , Gintautas Tamulaitis 2 , Shinho Cho 3
1 Department of ECE and CIE, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Semiconductor Physics Department and Institute of Materials Science and Applied Research, Vilnius University, Saultekio 9-III, Vilnius, Lithuania, 3 Department of Electronic Materials Engineering, Silla University, Busan Korea (the Republic of)
Show AbstractZnO is promising material for the blue and UV optoelectronics, Photoluminescence (PL) studies allow for comparative characterization of ZnO grown by different techniques. We report on PL steady state and time resolved photoluminescence spectroscopy as well as on light-induced transient grating technique (LITG) studies of ZnO thin films grown on glass by radio-frequency magnetron sputtering with different O2/O2+Ar ratio. The crystallographic structure and surface morphology by X-ray diffraction (XRD) and atomic force microscopy(AFM) showed that the flattest surface and the strongest (002) orientation of the ZnO layers was achieved in the sample grown with the highest O2/O2+Ar ratio (50%). Layers grown with higher O2/O2+Ar also had narrower PL bands. At low temperatures, PL spectra strongly depended on excitation intensity. At low excitation levels, a broad band peaked at 3.10 eV was dominant in the spectrum. The bound exciton recombination was prevailing under high excitation intensity. Free exciton recombination was observed indicating good material quality achieved. Under short pulse excitation at room temperature, a blue shift increased and narrow PL peaks became narrower with increasing the O2 ratio. PL decay kinetics was not significantly dependent on the O2/O2+Ar ratio. The initial PL decay was very fast with the effective decay time below the measurement time resolution (~40 ps). This fast decay followed by a slow decay (with the characteristic time constant ~200 ps confirmed by LITG measurements). The carrier lifetime estimated from the PL slow decay was comparable to the longest carrier lifetime achieved in the ZnO layer grown on sapphire by PLD under different growth temperature [1] and longer than that of the ZnO layer grown on Si by MOCVD [2].[1] S. Cho, S. I. Kim, Y. H. Kim, J. Mickevièius, G. Tamulaitis, M. S. Shur, phys. stat. sol. (a), 203, 3699 (2006) [2] B. Guo, Z. R. Qiu, K. S. Wong, Appl. Phys. Lett. 82, 2290 (2003)
9:00 PM - B3.34
Reducible and Nonreducible Defect Clusters in Tin-Doped Indium Oxide.
Hiroshi Mizuseki 1 , Talgat Inerbaev 1 2 , Ryoji Sahara 1 , Takashi Nakamura 3 , Yoshiyuki Kawazoe 1
1 Institute for Materials Research, Tohoku University, Sendai, Miyagi, Japan, 2 (present) NanoScience Technology Center, University of Central Florida, Orlando, Florida, United States, 3 IMRAM, Tohoku University, Sendai, Miyagi, Japan
Show AbstractTin-doped indium oxide (ITO) is a degenerated semiconductor that is one of the most commonly used transparent conducting oxides. Its optical and electrical properties are exploited in expanding variety of transparent electrode applications, including flat-panel displays, electrochromic windows, organic light-emitting diodes, and solar cells. Although the defect structure of pure and tin-doped In$_2$O$_3$ is being studied over the past three decades it continues to attract the unrelenting interest of researchers. In present density functional theory (DFT) calculations are used to estimate the energy of interstitial oxygen (O$_i$) release from ITO. The defect clusters of different topology and local dopant atoms arrangement around the interstitials are examined. It is found that in contrast to the current concept, the local arrangement of tin around O$_i$ has only minor effect on the defect clusters non-reducibility. The extraction energy of interstitial oxygen from ITO critically increases simultaneously with the charge carrier concentration. This effect is rationalized as the effect of doubly charged oxygen deionization energy change due to ion-ion Coulomb interaction screening by conductive electrons.
9:00 PM - B3.35
Functional Multi-layered Transparent Conducting Oxide Thin Films for Photovoltaic Devices.
Jun Hong Noh 1 , Sangwook Lee 1 , Hyun Soo Han 1 , Chin Moo Cho 1 , Jae-sul An 1 , Hyun Suk Jung 2 , Kyung Hyun Ko 3 , 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 Materials Science and Engineering, Ajou University, Suwon Korea (the Republic of)
Show AbstractNb-doped TiO2 (NTO) layers deposited on conventional Sn-doped In2O3 (ITO) substrates using pulsed laser deposition (PLD) enhanced the optical-to-electrical conversion efficiency of the dye-sensitized solar cells (DSSC) by as much as 17 % compared to that of bare ITO-based DSSCs. The electrical properties and J-V characteristics of the multilayered NTO/ITO films showed that the improved cell performance was due to the facile charge injection from TiO2 nanoparticles to ITO that resulted from their forming an ohmic contact with ITO as well as to the conserved high conductivity of ITO after the oxidizing annealing process.
9:00 PM - B3.36
Transparent Active Matrix Organic Light-Emitting Diode Driven by In2O3 Nanowire Transistor.
Jun Liu 1 , Sanghyun Ju 2 , Jianfeng Li 1 , Chongwu Zhou 3 , David Janes 2 , Tobin Marks 1
1 Department of Chemistry, and the Materials Research Center, Northwestern University, Evanston, Illinois, United States, 2 School of Electrical and Computer Engineering,and Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana, United States, 3 Department of Electrical and Engineering, University of Southern California, Los Angeles, California, United States
Show AbstractFor next-generation display technologies, fully transparent displays are of great interest, with potential applications such as windshield displays, head-mounted displays, and transparent screen monitors. Organic light-emitting diode (OLED) technologies satisfy several key requirements for this application, including potential transparency, low-temperature processing, light weight, mechanical flexibility, and high quantum efficiency. In order to realize transparent or flexible active matrix (AM) OLED displays, it is necessary to develop appropriate thin-film transistor technologies. Nanowire transistors (NWTs) are promising candidates to fulfill this requirement, due to their outstanding electrical characteristics (including high mobilities, associated compact size, and fast switching properties) and high optical transparency compared to conventional thin film transistors. Here we report the first transparent AMOLED display elements, in which the switching and driving circuits are comprised exclusively of NWT electronics fabricated at room temperature via a relatively simple and scalable process. The NWTs use In2O3 nanowires as the active channel materials, a performance-enhancing high-k organic self-assembled nanodielectric (SAND) as the gate insulator, and indium tin oxide (ITO) as the transparent conducting gate, source and drain electrodes. Proof-of-concept green-emitting polymer LEDs (PLEDs) utilizing high-efficiency interfacial charge-blocking and hole transporting materials are integrated with a transparent bottom contact electrode to achieve efficient optical emission through the glass substrate, and an opaque or semi-transparent top contact (cathode), with the latter allowing emission through both top and bottom sides of the structure. The optical transmittance of the 2 x 2 mm NWT arrays is 72%, and that of the NW-AMOLED display through a thin Al cathode is ~35% in the visible range. A luminescence of 300 cd/m2 is obtained for light emission through the ITO side at 10 V.
9:00 PM - B3.37
P-type Conduction in Amorphous Zn-Co-O Thin Films.
Seonhoo Kim 1
1 , University of Florida, Gainesville, Florida, United States
Show Abstract9:00 PM - B3.38
Metal Oxide-based (IZO and ZnO) TFTs for Flexible Electronics.
Shahrukh Khan 1 , Abbas Jamshidi-Roudbari 1 , Miltiadis Hatalis 1
1 Electrical Engineering, Lehigh University, Bethlehem, Pennsylvania, United States
Show Abstract9:00 PM - B3.4
Electrical and Magnetic Properties of ZnO Single Crystals Grown by Hydrothermal Method.
Young Kuk Lee 1
1 , Korea Research Institute of Chemical Technology, Taejon Korea (the Republic of)
Show AbstractUndoped- and TM-doped(TM= Mn, Co, Fe) ZnO single crystals were grown by hydrothermal method using a Pt-lined high pressure autoclave in basic hydrothermal solution. Maximum size of the as-grown single crystal is up to 60 X 60 X 10mm3, which can be fabricated to 2-inch ZnO crystal substrate. Maximum growth rate were about 0.27mm/day. Addition of some alkaline mineralizers were observed to enhance the growth rate along a-axis, resulting in the as-grown ZnO crystals being bulkier. Some important structural, electrical and magnetic properties of un-doped and TM-doped ZnO single crystals are also discussed.
9:00 PM - B3.5
Thermophysical and Electrical Properties of Polycrystalline ITO and Amorphous In2O3-ZnO Films.
Toru Ashida 1 , Nobuto Oka 1 , Yasushi Sato 1 , Takashi Yagi 2 , Naoyuki Taketoshi 2 , Tetsuya Baba 2 , Yuzo Shigesato 1
1 Graduate School of Science and Engineering, Aoyama Gakuin University, Sagamihara, Kanagawa Japan, 2 National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki Japan
Show AbstractThermal diffusivity of polycrystalline tin-doped indium oxide (ITO) and amorphous indium zinc oxide (IZO) films with a thickness of 200 nm has been measured using nanosecond thermoreflectance system. IZO films sandwiched by molybdenum (Mo) films were prepared on fused silica substrate by dc magnetron sputtering using ITO, IZO and Mo targets. The ITO and IZO layers were deposited under different oxygen or hydrogen flow rate of 0-10% or 0-20%, respectively. Such Mo/(ITO or IZO)/Mo layered structure was fabricated without exposure to the atmosphere between each deposition. The Mo films with thickness of 70 nm are necessary because the wavelengths of pulse laser used in this study are 782 nm and 1064 nm, at which wavelength ITO or IZO films are transparent. To derive the thermal diffusivity of the ITO and IZO films, the thermoreflectance signals were analyzed based upon an analytical solution of the one dimensional heat flow across the three-layered film/substrate system. The resistivity, carrier density and hall mobility of the IZO films ranged from 4.2×10-4 to 22.7 Ω cm, from 2.6×1016 to 4.2×1020 cm-3, from 10 to 51 cm2/Vs, respectively [1]. The interface between the Mo films and the fused silica substrate was irradiated by nanosecond laser pulse. Heat generated by the pump laser pulse diffuses toward the top Mo surface across the three-layered films. Then the temperature changes at the Mo film surface, which was probed by reflectance of another nanosecond laser pulse. The heat diffusion time changed significantly with changing the gas flow rate. The thermal diffusivity of the IZO and polycrystalline ITO films deposited at the unheated substrate, under total gas pressure of 0.7 Pa was 1.2×10-6 m2/s and 2.2×10-6 m2/s, respectively. The thermal diffusivity of the IZO film was approximately, equivalent as that of amorphous ITO films [2]. Based on Wiedemann-Franz law, approximately 40-50% of the thermal conduction was considered to be carried by free electrons.[1] T. Ashida, Y. Shigesato, et al., J. Vac. Sci. Technol. A, Vol. 25, No. 4 (2007) 1178.[2] T. Yagi, Y. Shigesato, et al., J. Vac. Sci. Technol. A, Vol. 23, No. 4 (2005) 1186.
9:00 PM - B3.6
Atomic Layer Deposition of CuAlOx as a Transparent Ternary Oxide.
Sun Sook Lee 1 , Byung Kook Lee 1 , Taek-Mo Chung 1 , Young Kuk Lee 1 , Chang Gyoun Kim 1 , Ki-Seok An 1
1 , Korea Research Institute of Chemical technology, Daejeon Korea (the Republic of)
Show AbstractTransparent conductive oxides (TCOs) are of interest as transparent electrodes in flat panel displays, solar cells, and other applications. N-type TCOs have been already developed; p-type TCOs is not yet to create. CuAlOx is one of ternary p-type conducting oxides.In this study, we report transparent CuAlOx semiconductor films are prepared by the use of the atomic layer deposition (ALD) technique with Cu(dmamb)2, Al(CH3)3 precursors, and H2O. The properties of deposited films in the various conditions were characterized. The chemical composition in the film was estimated by energy-dispersive x-ray spectroscopy (EDX) and x-ray photoelectron spectroscopy (XPS), calibrated with known samples of different compositions. The morphology and film thickness were examined using a field-emission scanning electron microscope (FE-SEM). An x-ray diffraction (XRD) and a transmission electron microscope (TEM) were employed to reveal the structure of the CuAlOx films. The transmittance and absorbance of the film were measured by employing the UV–visible spectrophotometer.
9:00 PM - B3.7
High Rate Deposition of Al-doped ZnO (AZO) by Reactive Sputtering (1); Unipolar Pulsing with Plasma Emission Control.
Kento Hirohata 1 , Yasutaka Nishi 1 , Nobuto Oka 1 , Yasushi Sato 1 , Isao Yamamoto 2 , Yuzo Shigesato 1
1 Graduate School of Science and Engineering, Aoyama Gakuin Unversity, Sagamihara, Kanagawa Japan, 2 , Nissan Motor Co., Atsugi, Kanagawa Japan
Show AbstractRecently, there have been strong industrial demands for a much higher deposition rate for Al-doped ZnO (AZO) film. The reactive sputtering process using metal or alloy targets should have high-potential for the lower cost deposition with a very high deposition rate because the higher sputter power can be applied on the cheaper metallic targets. However, the reactive sputtering process is highly non-linear, where the deposition rate exhibits the hysteresis with respect to the reactive gas flow. Such a behavior originates from the oxidation state of the target surface, resulting that the deposition rate changes markedly with increasing O2 flow. Therefore, a precise control of the oxidation on the target surface during the reactive sputtering process should be required for the high rate deposition of AZO film with the reproducibility. In this study AZO films were deposited on unheated and heated (200 °C) glass substrates by the reactive sputtering with mid-frequency pulsing and the plasma control unit (PCU) with a special feedback system for reactive gas flow. A Zn–Al (Al: 1.5 wt%, 130mm × 400mm in size) alloy target was connected with the mid-frequency (50 kHz) pulse unit which was operated in unipolar mode. In order to reduce the arcings on the alloy target due to the impression of high power density, the duty cycle, i.e. ratio of on-period to cycle duration, was adjusted. The O2 reactive gas flow was precisely controlled using the PCU. The plasma emission of an atomic O* line at 777 nm was led into optical emission detector (containing a photomultiplier and optical filter) to transform the optical intensity into the photovoltage. The PCU adjusted the piezoelectric value to control O2 flow precisely and rapidly until the photovoltage reached the value equal to the set-points for the optical emission intensity (OEI). The set-points could be chosen in the “transition region” between the metallic and the oxide states on the target surface in order to achieve high deposition rate. Total gas pressure and sputtering power were 0.5 Pa and 2-4 kW, respectively. First of all, it was confirmed that each set-point of OEI corresponded to O2 flow ratio, indicating that the oxidation of the target surface was precisely controlled by the feedback system for the entire O2 flow ratios including the unstable transition region. When AZO was deposited on an unheated glass substrate, the lowest resistivity was 2.0×10-3 Ωcm and the deposition rate was 235 nm/min. On the other hand, AZO films deposited on glass substrate heated at 200 °C possessed the lowest resistivity of 5.0×10-4 Ωcm. The deposition rate was 132 nm/min, which was about 8-10 times higher than those of the films deposited by the conventional magnetron sputtering. Both these AZO films possessed optical transmittance of over 80 % in the visible region. These results indicated that the unipolar pulsing with the PCU should be promising techniques for the AZO film’s deposition with the high deposition rate.
9:00 PM - B3.8
High Rate Deposition of Al-doped ZnO (AZO) by Reactive Sputtering (2); Unipolar Pulsing with Impedance Control.
Yasutaka Nishi 1 , Kento Hirohata 1 , Nobto Oka 1 , Yasushi Sato 1 , Isao Yamamoto 2 , Yuzo Shigesato 1
1 Graduate School of Science and Engineering, Aoyama Gakuin University, Sagamihara, Kanagawa Japan, 2 , Nissan Motor Co., Atsugi, Knagawa Japan
Show AbstractTransparent conductive oxide (TCO) is a highly degenerated wide band-gap semiconductor with low electrical resistivity and high transparency in the visible regions. Al doped ZnO (AZO) films should be one of the most potential candidates as indium-based TCO films such as ITO or IZO films. Most of the AZO films have been deposited by dc or rf magnetron sputtering using a ceramic oxide target because of its advantages of stable depositions with high reproducibility. However, to achieve high deposition rate is difficult in the case of using the ceramic target because of its small sputtering yield. Reactive sputtering using a Zn-Al alloy target should be one of the most promising techniques for high rate depositions of TCO films because much higher sputter power can be applied on the targets. On the other hand, the electrical properties of AZO films deposited by reactive sputtering should be strongly affected by O2 flow ratio. Therefore, in order to obtain the high-quality AZO films by reactive sputtering with high deposition rate, the sputtering conditions should be precisely controlled. In this study, AZO films were deposited on glass substrates unheated or heated at 200 °C by reactive sputtering with a special feedback system of discharge impedance combined with mid-frequency (mf) pulsing. A planar Zn-Al alloy target (Al: 1.5wt.%, 130mm×400mm in size) was connected with the UBS-C2 which was operated in the unipolar pulse mode. The mf pulse possesses the approximate shape of a square wave with frequency of 50 kHz and duty cycle of 80%, which make it possible to impress very high power density without arcing at the cathode. The reactive O2 gas flow was precisely controlled by the discharge impedance feedback system where the oxidation of target surface can be monitored by the variation in the cathode voltage caused by the variation in secondary electron emission coefficient of the target surface. The set points of cathode voltage could be chosen in the “transition region” between the metallic and the reactive (oxide) sputter mode in order to achieve high deposition rate. Total gas pressure and sputtering power were 0.5 Pa and 2000-4000W, respectively. It was observed that each set point of the cathode voltage corresponds to O2 flow ratio, indicating that the oxidation of the target surface was precisely controlled by the feedback system for the entire O2 flow ratios including “the transition region”. The deposition rates of the AZO films with the lowest resistivity of 1.3×10-3 Ωcm were around 250 nm/min in the case of the deposition on the unheated glass. In the case of the depositions on the heated substrate at 200 °C, the deposition rate was around 160 nm/min, where the lowest resistivity of the AZO film were 3.9×10-4 Ωcm. Transmittances in visible region for these films were more than 85%.
9:00 PM - B3.9
Properties of IGZO Films Deposited by Dc Magnetron Sputtering with H2O Introduction.
Takafumi Aoi 1 , Daisuke Watanabe 1 , Nobuto Oka 1 , Yasushi Sato 1 , Ryo Hayashi 2 , Hideya Kumomi 2 , Yuzo Shigesato 1
1 Graduate School of Science and Engineering, Aoyama Gakuin University, Sagamihara, Kanagawa Japan, 2 Canon Research Center, Canon Inc., Tokyo Japan
Show AbstractRecently, flexible thin film transistors (TFTs) have been studied powerfully. The thin film layers for the flexible TFTs including semiconductor layers should be formed at a temperature which is lower than the softening temperature of a flexible polymer substrate. In addition, in order to achieve high stability of TFT characteristics, carrier density of semiconductor layer should be controlled precisely. It was reported that amorphous indium-gallium-zinc-oxide (IGZO) was deposited by magnetron sputtering at room temperature, where the carrier density could be controlled to be less than 1016 cm-3. Therefore, this material attracts much attention as the candidate for the semiconductor layer for TFTs. In fact, it is reported that the high performance TFTs was already demonstrated using amorphous IGZO films as the semiconductor layers deposited on plastic substrates by the sputtering method. [1] In this study, we introduced H2O into the magnetron sputter deposition process and analyzed how the H2O partial pressure during the deposition affects the electrical properties of the IGZO films. IGZO films were deposited on unheated fused silica glass substrates by dc magnetron sputtering using an InGaZnO4 target. Sputtering power was kept at 50W. Total gas pressure of the mixture of Ar and H2O gases was maintained at 0.5Pa during the deposition. H2O partial pressure was varied quantitatively using a precise needle valve with monitoring partial pressure of H2O using quadrupole mass spectrometer (QMS). Incorporation of H in the films was investigated by depth profiles of the hydrogen content, which was analyzed in detail by a secondary ion mass spectroscopy (SIMS). Electrical properties of the films was analyzed by the four-point probe method and hall effect measurement. As a result, it is demonstrated that electrical conductivity of IGZO films can be controlled precisely by controlling the H2O partial pressure during the depositions. It should be noted that, the carrier density of the IGZO films was largely reduced at H2O partial pressure above 1.0×10-2 Pa, indicating the possibility for controlling device characteristics. We fabricated TFTs using the IGZO as channels, which were deposited by dc magnetron sputtering with H2O introduction and evaluate the TFT characteristics. The TFT showed the sufficient operations with on-to-off current ratios larger than 105.[1] H.Yabuta, M.Sano, K.Abe, T.Aiba, T.Den, H.Kumomi K.Nomura, T.Kamiya, H.Hosono, Appl.Phys.Lett., 89, 112123.
Symposium Organizers
John D. Perkins National Renewable Energy Laboratory
Thomas O. Mason Northwestern University
John F. Wager Oregon State University
Yuzo Shigesato Aoyama Gakuin University
B4: Amorphous Oxide Semiconductors I
Session Chairs
Hideya Kumomi
David Paine
Tuesday AM, December 02, 2008
Room 203 (Hynes)
9:30 AM - **B4.1
Indium Free High Mobility Amorphous Oxide Based Thin Film Transistors.
Elvira Fortunato 1 , P. Barquinha 1 , L. Pereira 1 , G. Goncalves 1 , R. Martins 1
1 , CENIMAT/I3N, Caparica Portugal
Show AbstractAmorphous oxide semiconductors are nowadays playing an important scientific and technological role for the next generation of optoelectronic devices. The recent applications using these new semiconductor materials, besides covering both p and n-type semiconductor properties present a high electronic mobility being the structure amorphous. This specific characteristic contrast to conventional amorphous semiconductors like a-Si:H, where carrier transport is limited by the disorder of the amorphous phase, ie band tail limited. In this paper we will present some of the recent achievements already obtained with special attention to thin film transistors.In this paper we will focus on high mobility bottom gate thin film transistors (TFTs) with an amorphous gallium tin zinc oxide (a-GSZO) channel layer have been produced by rf magnetron cosputtering using a gallium zinc oxide (GZO) and tin (Sn) targets. The effect of post annealing temperatures (200 °C, 250 °C and 300 °C) was evaluated and compared with two series of TFTs produced at room temperature (S1) and 150 °C (S2) during the channel deposition. From the results it was observed that the effect of post annealing is crucial for both series of TFTs either for stability as well as for improving the electrical characteristics. The a-GSZO TFTs (W/L = 50 µm/50 µm) operate in the enhancement mode (n-type), present a high saturation mobility of 24.6 cm2/Vs, a subthreshold gate swing voltage of 0.38 V/decade, a turn-on voltage of -0.5 V, a threshold voltage of 4.6 V and an ION/IOFF ratio of 8×107, satisfying all the requirements to be used as active-matrix backplane.
10:00 AM - B4.2
First-principles Calculation for Effect of Impurities on Electronic States of Amorphous In-Ga-Zn-O.
Hideyuki Omura 1 , Tatsuya Iwasaki 2 , Hideya Kumomi 2 , Kenji Nomura 3 , Toshio Kamiya 3 4 , Masahiro Hirano 3 5 , Hideo Hosono 3 4 5
1 Analysis Technology Development Center, Canon INC., Tokyo Japan, 2 Materials Technology Development Center, Canon INC., Tokyo Japan, 3 ERATO-SORST, JST, in Frontier Research Center, Tokyo Institute of Technology, Yokohama Japan, 4 Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama Japan, 5 Frontier Research Center, Tokyo Institute of Technology, Yokohama Japan
Show Abstract10:15 AM - B4.3
Favorable Elements for an Indium-based Amorphous-oxide TFT Channel: Study of In-X-O (X=B, Mg, Al, Si, Ti, Zn, Ga, Ge, Mo, Sn) Systems.
Amita Goyal 1 , Tatsuya Iwasaki 1 , Naho Itagaki 1 , Tohru Den 1 , Hideya Kumomi 1
1 , Canon Inc., Tokyo Japan
Show Abstract Amorphous oxide semiconductors (AOSs) have acquired more importance for applications in thin film transistors (TFTs) owing to their advantages of high performance along with large mobility and low temperature fabrication. Though TFTs using channel materials of In-Ga-Zn-O, Zn-Sn-O, In-Ga-O, In-Zn-O etc. have been reported so far, the compositional study was limited in a narrow region of the periodic table. In the present paper, we present results of wide-ranging elemental survey for In based AOSs. Many kinds of In-X-O semiconductor systems were studied through TFT fabrication, where X is one of B, Mg, Al, Si, Ti, Zn, Ga, Ge, Mo and Sn. For each In-X-O system, the cation compositional dependence was systematically studied by using the combinatorial techniques [1]. The TFT characteristics were compared to each other for the optimal compositional ratio of X/(In+X). The TFTs have a device structure of bottom-gate top-contact type with Au/Ti source and drain electrodes and gate-insulator of thermally oxidized silicon. The channel layers were formed at room temperature by radio frequency magnetron sputtering method followed by post-annealing treatment at 300°C. For each In-X-O system, there exists an appropriate range of compositional ratio for which the oxide system is transparent, semi-conductive and amorphous. Successful TFT operation is observed for the optimized semi-conductive channels except for X = Mo and Sn. Good TFT performance in terms of mobility μFE, on-off current ratio, threshold voltage, and subthreshold swing can be obtained in systems of In-Ge-O (μFE ~ 6cm2/V-s, on/off ~ 1010), In-Zn-O (μFE ~ 26cm2/V-s, on/off ~ 1010), In-Si-O (μFE ~ 3 cm2/V-s, on/off ~ 109). Especially, In-Ge-O TFTs show relatively wide compositional margin and good temporal stability. On the other hand, In-Ti-O TFTs, including transition metal element in AOS, show poor performances. Binary multi-cation oxides like In-Ge-O, which are beneficial in terms of production cost and impurity management, are promising candidates for future TFT applications. Empirical guidelines for elemental choice in AOS will be proposed through these TFT studies. [1] T.Iwasaki et. al., Appl. Phys. Letters 90, 242114 (2007)
10:30 AM - B4.4
Local and Defect Structures of Zn-In-Sn-O (ZITO) TCO Crystalline and Amorphous Thin Films by EXAFS
Diana Proffit 1 2 , D. Bruce Buchholz 1 2 , Robert P. H. Chang 1 2 , Michael Bedzyk 1 2 , Thomas Mason 1 2 , Qing Ma 3
1 Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States, 2 Materials Research Science & Engineering Center, Northwestern University, Evanston, Illinois, United States, 3 DND-CAT, Northwestern Synchrotron Radiation Center at Advanced Photon Source, Argonne, Illinois, United States
Show AbstractZn-In-Sn-O materials (ZITO) are promising alternatives to In-Sn-O (ITO) for transparent electrode and transparent semiconductor applications. The local structures of crystalline and amorphous Zn-In-Sn-O (c-ZITO, a-ZITO) thin films grown by pulsed laser deposition were determined by x-ray absorption spectroscopy (XAFS). Measurements around the Zn-, In-, and Sn-K absorption edges were taken at the Advanced Photon Source at Argonne National Laboratory, and the first nearest-neighbor and second nearest-neighbor distances and coordination numbers were obtained. The short Sn-O bond distances and relatively large coordination numbers around Sn provide evidence for the Frank-Köstlin cluster, (2SnIn●Oi″)x, which was first reported for ITO. Two distinct Zn-O first nearest-neighbor distances indicate a more complicated local structure associated with the substitution of Zn for In. These results are discussed with reference to the underlying defect structure(s) and physical properties of ZITO vs. ITO
10:45 AM - B4.5
Conductivity and Transparency in Amorphous In-Zn-O Transparent Conductors.
John Perkins 1 , Joseph Berry 1 , Maikel van Hest 1 , Thomas Gennett 3 , Andrew Cavendor 2 , Andrew Leenheer 2 , Ann Deml 2 , Reuben Collins 2 , Ryan O'Hayre 2 , David Ginley 1
1 , National Renewable Energy Lab., Golden, Colorado, United States, 3 , Rochester Institute of Technology, Rochester, New York, United States, 2 , Colorado School of Mines, Golden, Colorado, United States
Show Abstract11:30 AM - **B4.6
Recent Advances in Oxide Semiconductor-based TFTs: Tail State in TAOS and High Performance P-channel TFT.
Hideo Hosono 1
1 , Tokyo Institute of Technology, Yokohama Japan
Show Abstract12:00 PM - B4.7
Combinatorial Study of ZTO and ZIO Transparent Thin-Film Transistors.
Matthew McDowell 1 , Ian Hill 1
1 Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia, Canada
Show AbstractThin-film transistors having channels composed of Zinc Tin Oxide (ZTO) and Zinc Indium Oxide (ZIO) have been fabricated using a combinatorial RF sputtering technique. This technique yielded hundreds of devices per sample, each with a unique channel stoichiometry. These ranged from x=0 to x=1 for both ZTO of the form (ZnO)x(SnO2)1-x and ZIO of the form (ZnO)x(In2O3)1-x. Film stoichiometry was measured directly using WDS mode electron microprobe, and film structure was examined using glancing-angle XRD. Films were grown on heavily n-doped silicon wafers (acting as the gate contact) capped with a 1000Å SiO2 dielectric. Aluminium source and drain contacts were formed via vacuum vapor deposition. The large number of devices combined with a broad range of stoichometries allowed for fine-grained characterization of transistor performance as a function of thin-film composition.It had been predicted[1,2] that ZTO mobility would be optimized for x=0.50 or x=0.67, due to the likely formation of crystalline phases at the 1:1[3] and 2:1[4] ratios. Surprisingly, our results contradict this prediction, showing two peaks in mobility of ~12cm2/Vs, one at x=0.25 and another at x=0.80, with a trough in mobility (~3 cm2/Vs) around x=0.50, for films annealed in oxygen at 600°C. On/off ratios were as high as 108, but were highly sensitive to stoichiometry, falling off quickly for x<0.33. Threshold voltages were also strongly influenced by stoichiometry, and became significantly more negative for low ZnO content. Additionally, the ZTO devices exhibited a strong sensitivity to light exposure, with on/off ratios dropping by as much as 105 under room lighting.ZIO devices were found to have a maximum channel mobility of over 30 cm2/Vs at a stoichometry corresponding to x = 0.67 when films were annealed under oxygen at 300°C, which was consistent with predictions[5]. The best sub-threshold slopes were as low as 0.3 V/dec at a stoichiometry of x = 0.75. Oxygen partial pressure during sputtering was found to have a significant effect on device performance, and devices are currently being fabricated to work without need of a high temperature annealing step.[1] H. Q. Chiang, J. F. Wager, R. L. Hoffman, J. Jeong, and D. A. Keszler, Appl. Phys. Lett. 86, 013503 2005.[2] W. B. Jackson, G. S. Herman, R. L. Hoffman, C. Taussig, S. Braymen, F. Jeffrey, and J. Hauschildt, J. Non-Cryst. Solids 352, 1753 2006.[3] D. Kovachera and K. Petrov, Solid State Ionics 109, 327 1998.[4] D. L. Young, H. Moutinho, A. T. Yan, and T. J. Coutts, J. Appl. Phys. 92, 310 2002.[5] N. L. Dehu, E. S. Kettenring, D. Hong, H. Q. Chiang,J. F. Wager, R. L. Homan, C.-H. Park, and D. A. Kes-zler, Applied Physics Letters 97, 064505 (2005).
12:15 PM - B4.8
Interface Properties of ZTO and IGZO Metal-Insulator-Semiconductor Devices.
Peter Erslev 1 , David Hong 2 , John Wager 2 , J. David Cohen 1
1 Department of Physics, University of Oregon, Eugene, Oregon, United States, 2 Department of Electrical Engineering and Computer Science, Oregon State University, Corvallis, Oregon, United States
Show AbstractAmorphous oxide semiconductors (AOS) are an exciting new class of materials which are making a large impact in the fields of transparent and oxide electronics. We have used various junction capacitance methods to examine the interfacial and bulk defect properties of zinc tin oxide (ZTO) and indium gallium zinc oxide (IGZO) incorporated into metal-insulator-semiconductor (MIS) capacitors. Investigating such devices, which are much simpler to understand than thin film transistors, allows us to focus more directly on the defect electronic structure of the insulator-semiconductor interface, as well as on the AOS bulk defects. The samples studied include several series of ZTO samples in which the insulator thickness and deposition temperature were varied to distinguish the response of interface states from that of the AOS bulk defects. In addition, we have compared the properties of IGZO devices deposited on thermal SiO2 and on solution-deposited aluminum phosphate insulators of different thicknesses. Such studies address the nature of interface defects that arise for different types of insulating layers. The measurements we used to characterize these sample devices included drive level capacitance profiling (DLCP), which gives a spatially sensitive free carrier and defect density profile, and transient photocapacitance spectroscopy (TPC), a sub-band-gap optical absorption-like measurement from which one can obtain information about the Urbach edge and optically active deep defects which reside well below the conduction band mobility edge. Admittance spectroscopy under variable applied DC bias has shown that as the as the post-deposition anneal temperature of ZTO is increased the concentration of defects at the insulator-semiconductor junction is drastically decreased. Transient photocapacitance spectra for both ZTO and IGZO clearly revealed exponential band-tails with Urbach energies consistently between 100 meV and 150 meV for both materials. The TPC spectra also show a defect band located near the center of the band gap (1.7 eV) in nearly all of the devices examined. However, the magnitude of this defect band was found to vary significantly between different samples. DLCP shows free carrier densities for ZTO near 5x1014 cm-3 and 1-2x1015 cm-3 for IGZO.
12:30 PM - B4.9
Resistive Memory Using Surface-oxidized C12A7:e− Single Crystal.
Yutaka Adachi 1 , Sung-Wng Kim 1 , Toshio Kamiya 1 2 , Hideo Hosono 1 2 3
1 Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama Japan, 2 ERATO-SORST, Japan Science and Technology Agency, Yokohama Japan, 3 Frontier Research Center, Tokyo Institute of Technology, Yokohama Japan
Show AbstractRecently, ReRAM (resistive random access memory) has attracted much attention due to the possibility for non-volatile memory with high integration and low power consumption. ReRAM utilizes a reversible resistance switching induced by applying DC voltage in a device structure of electrode / oxide (or organic) / electrode. However, there are several issues to be solved for a wide application of ReRAM. Because many ReRAMs utilize changes of a phase or an electronic structure induced by a pre-breakdown phenomenon, the long-term stability is controversy. Moreover, the origin of the resistance switching effects is unclear yet because it is induced by chaotic pre-breakdown effects. To clear these problems, we propose a new material as an active layer of ReRAM.
In this study, we utilized 12CaO7Al2O3 (C12A7:O2−) as an active layer of ReRAM due to its unique structural features. C12A7:O2− consists of twelve subnanometer-sized cages and free oxygen ions occupied in two of the twelve cages. Because the free oxygen ions are bound weakly in the cages, the free oxygen ions are much mobile than those of conventional oxides. Therefore, C12A7:O2− shows fast oxygen ion conduction. In addition, although C12A7:O2− is a typical insulator with a bandgap ~7 eV, the electrical property is converted to a metallic state by appropriate chemical reduction treatments. In this process, C12A7 are doped electrons by removing the free oxygen ions in order to keep charge neutrality. Applying the DC voltage, we can expect the electron doping to C12A7:O2− by the oxygen ion conduction, i.e. migration of the free oxygen ions. In this situation, C12A7:O2− can exhibit a reversible resistance switching effect by controlling the polarity of DC voltage.
We employed a Pt / C12A7:O2− / C12A7:e− / Pt stacking structure, in which the free oxygen ions and mobile electrons can be exchanged between the C12A7:O2- and the C12A7:e− layers. The oxide layer was formed by a low temperature surface oxidation of a C12A7:e− single crystal. The thickness of the C12A7:O2− layer was measured using spectroscopic ellipsometry.
We found that this device structure exhibited a bistable resistance switching effect and operated as a ReRAM. We also confirmed that this switching effect was reproduced by 50 cycle tests by keeping an on/off resistance ratio of ~ 102.
12:45 PM - B4.10
Optimization of Oxide Semiconductors for Low Temperature and High Performance TFTs.
P. Barquinha 1 , L. Pereira 1 , G. Goncalves 1 , R. Martins 1 , E. Fortunato 1
1 Materials Science Department, CENIMAT/13N, New University of Lisbon and CEMOP/UNINOVA, Caparica Portugal
Show AbstractOxide semiconductors are nowadays one of the most appealing research topics, having direct application on the next generation of electronic devices, where low temperature and cheap processing, as well as high performance, are persuaded. In this work is shown that proper adjustment of (post)deposition conditions of the semiconductor, in this case Gallium-Indium-Zinc oxide (GIZO), is critical to obtain high performance devices. The processing parameters studied for the room temperature sputtered GIZO were oxygen content in the sputtering chamber, rf sputtering power and annealing temperature. Little to none oxygen content in the sputtering chamber was found to be crucial to enhance device performance, being possible to obtain good electrical properties even on non-annealed devices. On the contrary, high oxygen content and r.f. power led, respectively, to unstable/poor performing and depletion mode TFTs before annealing, and mainly for these “non-ideal” processing conditions annealing proved to be highly effective to improve device performance and stability but also to decrease the performance discrepancy among devices processed under different oxygen and r.f. power conditions. For the best TFTs, annealed at temperatures as low as 150°C, properties like field effect mobility of 46 cm,2/Vs, subthreshold swing of 0.26 V/dec, threshold voltage of 0.70 V and On/Off ratio 108-109 were obtained, which are among the best properties ever reported for GIZO TFTs. As will be shown, these results are a consequence not only of the optimized processing conditions but also of the usage of a proper GIZO target composition.
B5: Amorphous Oxide Semiconductors II
Session Chairs
Elvira Fortunato
Yuzo Shigesato
Tuesday PM, December 02, 2008
Room 203 (Hynes)
2:30 PM - **B5.1
Gate Dielectrics for Amorphous IZO-based Transistors.
David Paine 1 , Zach Beiley 1 , Rod Beresford 1
1 Division of Engineering, Brown University, Providence, Rhode Island, United States
Show Abstract3:00 PM - **B5.2
Materials and Device Structures for Amorphous Oxide Semiconductor TFTs.
Hideya Kumomi 1
1 , Canon Inc, Tokyo Japan
Show Abstract3:30 PM - B5.3
Optical and Electrical Characteristics of Amorphous InGaZnO after Thermal Annealing.
Satoshi Taniguchi 1 , Norihiko Yamaguchi 1 , Takao Miyajima 1 , Masao Ikeda 1
1 Photonic Device Section, Advanced Materials Laboratories, Sony Corporation, Kanagawa Japan
Show AbstractWe measured photoluminescence (PL) spectra from amorphous In-Ga-Zn-O (a-IGZO) layer before and after thermal annealing for the first time. In addition to a weak near band-edge emission which was clearly observed around 400nm, an intense broad deep emission around 700nm was detected at liquid nitrogen temperature for a 700nm-thick a-IGZO layer deposited on the glass substrate by Faced Target Sputtering method at room temperature. The PL intensities from each emission were strongly dependent on the thermal annealing temperature ranging from 150 degrees centigrade to 350 degrees centigrade. Thermal annealing at 350 degrees centigrade led to strong PL intensities in both near band-edge emission and deep emission. These characteristics of the a-IGZO after thermal annealing are suggesting defect/impurity incorporations and annihilations by the process. The depth profile of carrier concentration in a-IGZO was estimated by Hall-effect measurement using step etched sample. Volume carrier concentration evaluated from the relationship between sheet carrier concentration and residual layer thickness exhibited the linear dependence revealing the uniform carrier concentration in depth. From Hall-effect measurement using thermally annealed samples, there appeared two different behaviors with increasing annealing temperature and time. One was an increase of carrier concentration, which appeared for short annealing times (a few minutes) and the other was an opposite behavior at higher annealing temperature than 250 degrees centigrade. The activation energy estimated from the Arrhenius plot with the increase of carrier concentration after thermal annealing for a few minutes was extremely low with 0.1eV. Hall measurements of step etched samples after thermal annealing at 250 degrees centigrade showed the existence of two regions whose carrier concentration was different in depth direction. The carrier concentration at the near surface region (-100nm) was found to be much higher, using non-uniform layer analysis of Hall-effect measurement, while the carrier concentration of the pre-annealing sample was constant at middle of 10^17cm-3. This result indicates the possibility of several origins of carriers after in thermal annealing and this thermal instability might cause inferior characteristics of Thin-Film Transistors. From the results of Secondary Ion-microprobe Mass Spectrometry, some atom species were clarified to be incorporated in the a-IGZO layer after thermal annealing. The possible mechanism of the annealing effect and the way to avoid this effect will be discussed in detail.
3:45 PM - B5.4
Stability and Temperature Dependence of Indium Gallium Zinc Oxide-based Thin-film Transistors.
Ken Hoshino 1 , David Hong 1 , Hai Chiang 1 , John Wager 1
1 School of Electrical Engineering and Computer Science, Oregon State University, Corvallis, Oregon, United States
Show AbstractAmorphous oxide semiconductors (AOS) are of growing interest in the context of thin-film transistor (TFT) channel layers for transparent electronics and, more broadly, oxide electronics applications. The most popular AOS material-of-choice is currently indium gallium zinc oxide (IGZO), as a consequence of its high performance at low temperature processability. Hence, IGZO TFTs show much promise for various types of commercial applications. Many of these applications, however, require TFT devices to be stable under prolonged voltage stressing and operate over a wide range of operating temperature. The objective of this presentation is to provide an overview of long-term (i.e., 10^5 s) device stability and temperature dependence properties of IGZO-based TFTs processed by sputter deposition and subjected to a post-deposition channel anneal at 200, 250, or 300 °C. IGZO TFT stability is assessed via constant-voltage bias-stress testing for a stress duration of 10^5 s. Typically, drain current transfer curves (i.e., log(ID)-VGS) are found to rigidly shift to positive voltages as a function of stress time. Thus, aging is primarily characterized by a positive shift in the turn-on voltage, VON. When the initial (pre-stressed) VON of a TFT is zero, such a device exhibits negligible aging. When present, transfer curve hysteresis is invariably clockwise. Bias-stress induced aging is reversible; devices are found to slowly recover by themselves if left unbiased in the dark. Taken together, these observations are consistent with an instability mechanism involving electron trapping within the channel or at the channel-insulator interface. Significantly, TFTs processed at higher temperatures exhibit improved stability.The temperature dependence of IGZO TFTs over a range –50 to +50 °C is dominated by a monotonic decrease in VON with increasing temperature, except when the initial VON is equal to zero, in which case VON is found to be temperature-independent. For TFTs in which VON is temperature-dependent, the channel electron mobility is observed to increase slightly with increasing temperature.
4:30 PM - B5.5
GIZO as Active Material for Transparent Flexible TFTs.
Anna Vila 1 , Pedro Barquinha 2 , Luis Pereira 2 , Gonçalo Goncalves 2 , Joaquim Font 1 , Antonis Olziersky 1 , Rodrigo Martins 2 , Juan Morante 1 , Elvira Fortunato 2
1 Electronics, University of Barcelona, Barcelona Spain, 2 CEMOP-CENIMAT, New University of Lisbon, Lisbon Portugal
Show Abstract4:45 PM - B5.6
The Effects of Dry-etching and Passivation for a-GIZO Thin Film Transistors.
Sang-Wook Kim 1 , Jaechul Park 1 , Chang Jung Kim 1 , Sunil Kim 1 , Inhun Song 1 , Sung-Hoon Lee 1 , Huaxiang Yin 1 , Jaecheol Lee 2 , Eunha Lee 2 , Youngsoo Park 1
1 Semiconductor Device Laboratory, Samsung Advanced Institute of Technology, Yongin-Si Korea (the Republic of), 2 Analytical Engineering Center, Samsung Advanced Institute of Technology, Yongin-Si Korea (the Republic of)
Show AbstractRecently, ZnO-based thin-film transistors (TFTs) have been attracting much attention due to their potential application in active matrix organic light emitting diodes (AMOLED). Among them, amorphous-gallium indium zinc oxide(a-GIZO) TFTs are one of the promising candidates owing to high mobility (over 10cm2/Vs), high on-off current ratio (Ion/off), stable electrical properties against current stress, and low temperature process. However, since oxide semiconductors are generally vulnerable to various circumstances (such as plasma, the bombardment of energetic particles, and humidity), the electrical properties of a-GIZO TFTs have been easily degraded by dry-etching and passivation processes. In this work we have analyzed the effects of dry-etching and passivation using rutherford backscattering spectroscopy (RBS), time of flight secondary ion mass spectrometry(ToF-SIMS), and x-ray photoelectron spectroscopy(XPS). And we fabricated a-GIZO TFTs using conventional a-Si:H TFTs processes and equipments. The a-GIZO thin film transistors showed excellent electrical properties such as a mobility of 36 cm2/Vs, a Ion/off ratio of 7, and a subthreshold swing of 0.25V/decade.
5:00 PM - B5.7
High Mobility and High Stability Amorphous In-Ga-Zn-O Transparent Thin Film Transistors.
Hsing-Hung Hsieh 1 2 , Cheng-Han Wu 1 2 , Chang-Ken Chen 1 2 , Chao-Shun Yang 1 2 , Chih-Wei Chien 1 2 , Chung-Chih Wu 1 2
1 Graduate Institute of Electronics Engineering, National Taiwan University , Taipei, Taiwan, Taiwan, 2 Department of Electrical Engineering, National Taiwan University, Taipei, Taiwan, Taiwan
Show AbstractOxide semiconductors composed of heavy-metal cations with specific electronic configurations have been aggressively pursued in recent years due to their several merits, such as large carrier mobilities, transparent in visible range, good uniformity, and low processing temperature etc. Although oxide-based thin film transistors (TFTs) with high mobilities by using various gate dielectrics have been reported, their stability was rarely studied. In this paper, we report the amorphous In-Ga-Zn-O (a-IGZO) transparent TFTs fabricated by using all lithographic and etching processes. The completed devices show rather high field-effect mobilities and good stability.The a-IGZO transparent TFTs in this work adopt the top-gate staggered structure. Firstly, ITO was patterned on glass substrate to serve as the source/drain electrodes. Then the a-IGZO active layer was deposited by sputtering without intentional heating. Next, SiNx was deposited by plasma enhanced chemical vapor deposition (PECVD) to serve as the etching mask for a-IGZO wet etching. Different dielectric materials and different growth conditions of a-IGZO were also investigated. After patterning the SiNx etching mask and the a-IGZO active layer, SiNx was deposited by PECVD again to complete the gate insulator stack. Subsequently, ITO was deposited by sputtering and patterned as the gate electrode, and SiNx insulator was etched to expose the source/drain contact pads. Finally, a post annealing was performed to improve devices performances.The a-IGZO transparent TFTs operated in n-type enhancement mode and showed well-behaved saturation. For the devices with optimized conditions, a high mobility (μ) of ~45 cm2/Vs and a threshold voltage (Vt) of ~2 V are extracted from the saturation regime. Also, the subthreshold slope and the on/off current ratio are estimated to be ~0.4 V/decade and >10^7, respectively. The device structure and processing sequence have significant influences on the performances of the oxide-based TFTs. In this work, the SiNx was deposited right after IGZO deposition, thus effectively passivated the IGZO active layer from damages or contaminations and kept good channel/dielectric interface properties. The stability of the a-IGZO TFTs was also studied by applying constant-current stressing. With a normalized stressing current density of 0.2 μA/μm (vs. channel width) for hours, the a-IGZO TFTs showed only slight threshold voltage shift of 0.3 V. These results show that a-IGZO TFTs are much more stable than a-Si:H TFTs now widely used.
5:15 PM - B5.8
Low Voltage High Performance Amorphous InGaZnO Based Thin Film Transistors.
Jungbae Kim 1 , William Potscavage 1 , Xiaohong Zhang 1 , Vaibhav Vaidya 2 , Denise Wilson 2 , Bernard Kippelen 1
1 Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 Department of Electrical Engineering, University of Washington, Seattle, Washington, United States
Show AbstractLow-voltage high-performance enhancement-mode n-channel thin-film transistors (TFTs) were fabricated using amorphous indium gallium zinc oxide (a-IGZO) and Al2O3 as the channel layer and gate dielectric material, respectively. The a-IGZO TFT (W/L=200 µm /5 µm) using a 50 nm-thick Al2O3 gate dielectric by atomic layer deposition shows a large saturation mobility of ~10 cm2/Vs, an excellent sub-threshold slope of 0.06 V/decade, a low threshold voltage of 0.8 V, and an on-off current ratio of above 107 at 3 V. This TFT shows a width-normalized series contact resistance < 200 Ω-cm as a function of channel length in the linear region. Minimizing contact resistance plays an important role in achieving high performance short channel a-IGZO TFTs below 10 µm.
5:30 PM - B5.9
Improved Electroluminescent Polymer Light-emitting Diodes by using Anodes Based on Amorphous Indium-zinc Oxide.
Goncalo Goncalves 1 , Gabriel Bernardo 2 , Pedro Barquinha 1 , Quirina Ferreira 2 , Luis Pereira 1 4 , Graça Brotas 2 , Jorge Morgado 2 3 , Rodrigo Martins 1 4 , Elvira Fortunato 1
1 , Materials Science Department, CENIMAT/I3N, Faculty of Sciences and Technology, Universidade Nova de Lisboa, Caparica Portugal, 2 , Instituto de Telecomunicações, Instituto Superior Técnico, Lisboa Portugal, 4 , CEMOP-UNINOVA, Caparica Portugal, 3 , Department of Chemical and Biological Engineering, Instituto Superior Técnico, Lisboa Portugal
Show AbstractTransparent conducting oxides (TCOs) are key components of various optoelectronics devices, namely photovoltaic systems (PVs), light-emitting diodes (LEDs) and displays. Although indium-tin oxide (ITO) (Sn-doped In2O3) has been so far the main TCO of choice in these fields, the optimisation of its electrical and optical properties can only be obtained at substrate temperatures above 300 C, which makes it difficult for use on flexible plastic substrates.In the last few years, there has been an extensive search for new TCOs as potential alternatives to ITO. In particular, new materials have been investigated as potential anodes in organic and flexible LEDs.Amorphous ZnO-doped In2O3 (IZO) films have recently emerged as promising anode materials for organic LEDs due to their low deposition temperature, high work function, low resistivity, high transparency in the visible region and low surface roughness. Unlike amorphous ITO films, amorphous IZO anodes with optimised electrical and optical properties can be easily prepared by low-temperature sputtering. For all these reasons, amorphous IZO anode films are currently considered among the best candidates for high-quality transparent conducting anodes in flexible displays.The authors report on the performance of polymer-based light-emitting diodes, LEDs, using amorphous IZO films produced by rf magnetron sputtering at room temperature, as anode. LEDs with poly[(2-methoxy-5-(2’-ethyl-hexyloxy)-1,4-phenylene vinylene] as electroluminescent layer and aluminium cathodes, show higher efficiency with this IZO anode (0.015 cd/A) than with indium-thin oxide (ITO) (0.010 cd/A). In spite of the higher resistivity of this IZO electrode (ρ= 3.44x10-4 Ω.cm), compared with ITO (ρ=2.48 x 10-4 Ω.cm), the fact that it can be processed at lower temperatures and allows an efficiency increase of polymer LEDs make this material a good candidate for display and other optoelectronic devices applications.
B6: Poster Session II
Session Chairs
Wednesday AM, December 03, 2008
Exhibition Hall D (Hynes)
9:00 PM - B6.1
MOCVD Growth of GaN-based Materials on ZnO and Si Substrates.
William Fenwick 1 , Hongbo Yu 1 , Shen-Jie Wang 1 , Nola Li 1 , Andrew Melton 1 , Jeff Nause 3 , Ian Ferguson 1 2
1 Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States, 3 , Cermet, Inc., Atlanta, Georgia, United States, 2 Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractWe explore metal organic chemical vapor deposition (MOCVD) of GaN-based materials on ZnO (0002) and Si (111) substrates in this work. This work is being done because of the promise that both ZnO and Si hold as substrate materials for MOCVD growth of GaN thin films, though most commercial GaN MOCVD processes deposit on either sapphire or SiC. ZnO is a promising substrate for growth of low defect density GaN because of its small lattice and thermal mismatch with GaN, and MOCVD growth of GaN on Si would allow integration of GaN-based optoelectronics with Si-based electronics. However, there are issues to overcome with both materials that are not present with GaN on sapphire or SiC. The major issues with ZnO are hydrogen etching when using NH3 at high temperatures and interdiffusion at the interface. The major issue with Si is strain in the epilayer, causing cracking.GaN and InxGa1-xN thin films have been deposited on both Zn-face and O-face c-plane ZnO substrates by MOCVD using a highly modified vertical injection MOCVD system. Structural properties were investigated using X-ray diffraction (XRD) and Raman spectroscopy, and optical properties were investigated using room temperature (RT) photoluminescence (PL) and low temperature PL. XRD shows single phase InxGa1-xN with x ≤ 0.27, and PL measurements show optically active GaN and InxGa1-xN epilayers on ZnO. It has been found that the use of a low temperature GaN buffer layer (~540°C) on ZnO does not increase crystal quality significantly, and instead a higher temperature layer (~740°C) leads to better surface morphology. Effects of substrate polarity on film polarity and morphology are also studied. Further work will be done to investigate the GaN/ZnO (or InxGa1-xN/ZnO) interface and study the possible formation of a second phase at the interface and the effects that this interfacial layer has on subsequent film properties. InxGa1-xN and AlxGa1-xN superlattices have also been studied to prevent zinc and oxygen diffusion from the substrate, and the effects of well and barrier doping on diffusion in the superlattices are investigated. InxGa1-xN/GaN multiple quantum wells have also been grown and their optical and structural properties studied for use as the active region in GaN-based green emitters on ZnO.GaN epilayers have also been grown on Si (111) substrates, and their optical, electrical, and structural properties investigated. The use of AlN and AlxGa1-xN buffer layers on Si has been studied to decrease strain in subsequent GaN epilayers on Si. The GaN-based materials on both ZnO and Si will be compared to material grown on sapphire in order to determine the usefulness of these materials in similar optoelectronic applications.
9:00 PM - B6.10
Transport Processes in Doped ZnO Nanoparticles under Different Atmospheres.
Sonja Hartner 1 , Moazzam Ali 2 , Markus Winterer 2 3 , Hartmut Wiggers 1 3
1 Institute of Combustion and Gasdynamics, University of Duisburg-Essen, Duisburg, NRW, Germany, 2 Nanoparticle Process Technology, Universitiy of Duisburg-Essen, Duisburg, NRW, Germany, 3 CeNIDE, Center for Nanointegration Duisburg-Essen, Duisburg, NRW, Germany
Show AbstractZinc oxide (ZnO) is a II-VI wide and direct band gap semiconductor and has shown promising results as an inexpensive alternative for transparent conductive materials which are used for flat panel displays, solar cells, varistors and sensors. Aluminum-doped ZnO (AZO) is discussed as an alternative material to substitute Fluorine doped Tin Oxide (FTO) and the more expensive Indium Tin Oxide (ITO). Especially printing technologies based on nanoparticles containing inks and pastes are a promising way to cheap and large area functional structures. The electrical properties of ZnO powders consisting of nanoparticles in the size regime between 6 to 16 nm were investigated by means of impedance spectroscopy. Aluminum-doped as well as undoped ZnO was produced by gas phase synthesis. For the electrical characterization the as-prepared powders were pressed into pellets with a green density of around 80%. Impedance spectroscopy was performed under hydrogen and synthetic air at temperatures ranging from 323K to 673K. The ZnO nanoparticles show different transport properties with changing atmosphere. The impedance spectra under synthetic air exhibit typical semi-circles which are well known for semiconducting materials, under hydrogen atmosphere a high conductivity and ohmic behavior is shown. The difference between the conductivity under hydrogen atmosphere and under synthetic air of the undoped nanocrystal constitutes six orders of magnitude. The measurements under hydrogen as well as under synthetic air show a switching transport process between ptc and ntc conduction behavior with respect to the doping level. Exposing the samples to hydrogen achieves a high concentration of free electrons, caused by oxygen vacancies and shows ptc behavior like a metal. Doping with aluminum as an n-type dopant also increases the charge carrier concentration, leading to high conductivity, which can be found in the experiments under synthetic air. As a result, highly doped ZnO also shows ptc behavior. It has been found, that a doping level of about 7.74 atom % shows a maximum conductivity.
9:00 PM - B6.12
Nanoscale Growth Control of ITO Thin Films on the Nanoimprinted Glass Substrates.
Yasuyuki Akita 1 , Makoto Hosaka 1 , Yuki Sugimoto 1 , Yushi Kato 1 , Yusaburo Ono 1 , Mamoru Yoshimoto 1
1 Department of Innovative and Engineered Materials, Tokyo Institute of Technology, Yokohama, Kanagawa, Japan
Show AbstractIndium tin oxide (ITO) is a highly degenerated n-type semiconductor, with a wide-bandgap (3.3 eV to 4.3 eV). ITO thin films have been widely used as transparent conductive electrodes for flat-panel displays, liquid crystal displays, solar cells, and organic light emitting devices, because the low resistivity (~10 -4 Ωcm) and high transmittance (~90 %) in the visible region. There are several deposition techniques for ITO thin films on the glass and plastic substrates, that is, sputtering, chemical vapor deposition (CVD), and pulsed laser deposition (PLD). PLD used in this work has an advantage that the composition of the films is quite close to that of the target.Recently, we investigated nano-scale surface modifications of borosilicate glass plates by applying a thermal nanoimprinting technique, in which self-organized nanopattern molds of oxides (NiO, α-Al2O3) were used [1,2]. Using these nanoimprinted glass substrates for ITO thin film deposition has possible merits to reduce the resistivity due to high crystallinity via homogenization of crystal nucleation sites on the regular nanopattern (height of about 10nm, stripe distance of about 100nm) as well as to reduce the grain boundaries due to enlargement of the crystal grain sizes. Here we report the fabrication and characterization of ITO thin films on the nanoimprinted glass substrates. We have grown ITO thin films on the nanoimprinted glass and commercial glass substrates by PLD. KrF excimer laser beam (wavelength of 248 nm, duration time of 20 ns) was irradiated onto the sintered ceramics target. Sintered ceramics target of ITO (5 wt%-Sn doped) was used. Crystal grain sizes of ITO thin film fabricated on the nanoimprinted glass at 400 °C were about 6 times larger than those of ITO thin film fabricated on the commercial glass. Amorphous ITO films deposited at room temperature were annealed for 3 h in ultrahigh vacuum (UHV) at 400 °C. Amorphous ITO thin films surface deposited on the nanoimprinted glass at R.T. reflected nanopattern of glass surface even at 500 nm in thickness. By annealing the amorphous ITO thin film on the nanoimprinted glass, we could obtain the crystalline orientation different from that of the ITO thin film fabricated on the commercial glass plate at 400 °C. Anisotropic electrical property of these films was also characterized. [1] Akiba et al. Appl. Surf. Sci. 253 (2007) 4512 [2] Akita et al. Ipn. J. Appl. Phys. 46 (2007) L342
9:00 PM - B6.13
Transparent Conducting Al-doped ZnO Thin Films Prepared by Laser Induced High Current Pulsed Arc at Low Deposition Temperature.
Jin-Bao Wu 1 , Chao-Ying Chen 1 , Jia-Jen Chang 1 , Mei-Yi Li 2 , Ming-Sheng Leu 1 , Ai-Kang Li 1
1 , Materials Research Laboratories, Industrial Technology Research Institute, Hsinchu Taiwan, 2 , National Nano Device Laboratories, Hsinchu Taiwan
Show AbstractHighly transparent conductive Al-doped ZnO (AZO) thin films were deposited at 100°C by laser induced high current pulsed arc (LIHCPA) with Al-Zn alloy target. A pulsed current more than 1 kA was generated on the Al-Zn target in order to make high ion energy and fully ionized plasma. The film properties were correlated with the growth conditions, including O2 flow rate, pulsed arc current applying to the target and Al doping content (Al: 2-5 wt%). The characteristics of the microstructure of the films such as surface morphology, crystallinity and chemical compositions were studied using scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The electrical properties and transmittance of the AZO films were investigated by the Hall measurement and UV/VIS spectrometer respectively. From experimental XRD results, it shows that the AZO films have preferred c-axis orientation along the (002) plane. The minimum resistivity is as low as 4.2×10-4 Ω-cm with the carrier concentration of 7.3×1020 cm-3 and Hall mobility of 20 cm2 V-1 s-1 . On the other hand, the average transmittance of the films in the visible range (400-700 nm) is above 84 %. The band gap calculations as a function of O2 flow rate and Al doping concentration are also discussed.
9:00 PM - B6.14
Organic Photovoltaic Cells Using Metal Electrodes and Dielectric Coatings as Indium Tin Oxide Replacement.
Brendan O'Connor 1 , Chelsea Haughn 2 , Kwang-Hyup An 1 , Kevin Pipe 1 , Max Shtein 2
1 Mechanical Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractIndium tin oxide (ITO) is used ubiquitously as the transparent anode in organic photovoltaic (PV) cells. However, several considerations including high cost and sub-optimal mechanical properties motivate the search for an alternative transparent contact. Here, we investigate the possibility of using a semi-transparent metal electrode (e.g. silver) along with optical coatings as a low-cost, high performance alternative to ITO. Continuous thin metal films can be deposited at low cost and with low surface roughness, low sheet resistance, and low contact resistance. While the far-field optical transmittance is typically lower for continuous thin metal films than ITO, decreasing with metal layer thickness, we show that the high reflectivity allows for a stronger optical microcavity that can be tuned to overcome the transmittance losses.In this work, we demonstrate experimentally and theoretically that OPV cells utilizing thin (e.g. 9 nm of Ag) metal electrodes can match the performance of ITO-based cells. Furthermore, our calculations show that adding optical coatings to the metal-organic-metal OPV cells can result in larger relative gains in performance than when the coatings are introduced into ITO-based devices. For example, a 12 nm Ag electrode is found to have a sheet resistance < 5 Ohm/square and, with optimized coatings, results in a cell that exhibits a power conversion efficiency equivalent to an ITO-based device. Optical coatings potentially permit using a thicker metal film, which helps reduce sheet resistance and improves film uniformity, resistance to corrosion, and device reliability. The ability to use thin metal films as the transparent electrode provides a simple, cost-effective replacement for ITO, and enables novel device structures (e.g. OPV cells deposited on opaque substrates, fiber-based cells, etc.).
9:00 PM - B6.15
Impact of Surface Morphology and Polarity on ZnO Optical Emission.
Leonard Brillson 1 2 , Y. Dong 1 , D. Doutt 2 , Y. Kramer 2 , D. Tayim 3 , C. Zgrabik 2 , Z. Fang 4 , D. Look 4 , G. Cantwell 5 , J. Zhang 5 , J. Song 5 , H. Mosbacker 2 1
1 Department of Electrical & Computer Engineering, The Ohio State University, Columbus, Ohio, United States, 2 Department of Physics, The Ohio State University, Columbus, Ohio, United States, 3 , Columbus School for Girls, Columbus, Ohio, United States, 4 Semiconductor Research Center, Wright State University, Dayton, Ohio, United States, 5 , ZN Technology, Inc., La Brea, California, United States
Show AbstractAdvances in ZnO growth and processing are enabling optoelectronic applications, yet control of ZnO’s optical emission properties at the nanoscale remain to be explored. We have used nanoscale depth-resolved cathodoluminescence spectroscopy (DRCLS), atomic force microscopy (AFM), and Kelvin probe force microscopy (KPFM) to measure the spatial distribution of native point defects within the outer few hundred nanometers of the surface for leading ZnO growth and surface finishing techniques and identified how surface morphology and polarity impacts the efficiency of near band edge (NBE) optical emission. These studies reveal a wide variation in defect concentration and depth dependence. ZnO crystals grown by melt, hydrothermal, and vapor-phase transport methods display independent, orders-of-magnitude variation in 2.1, 2.5, and 3.0 eV native point defect optical transitions at their free surfaces and as a function of nanometer depth. Melt- and vapor-phase-grown ZnO exhibit uniformly distributed defect emission from the surface to the bulk, whereas hydrothermal ZnO from different sources displays up to hundred-fold increases in defect density from <5 to ~1000 nm below the surface. These results highlight the value of near-surface, depth-resolved probes in assessing crystal quality. AFM scans of surface morphology reveal large variations in surface roughness, asperities, and extended features related to the growth method, subsequent polishing and etching. UHV-clean Zn-polar surfaces exhibit significantly higher NBE emission and lower defects than O-polar surfaces of the same crystals along with 0.2 vs. 0.4 nm rms roughness, respectively. KPFM maps of surface and subsurface electric potential acquired simultaneously exhibit systematic correlations that depend on the distribution of native point defects measured by DRCLS. When DRCLS defect emissions are low, surface roughness is low and morphology matches its respective KPFM potential map. When DRCLS emissions vary with depth, surface morphology and potential maps can be strikingly different. Indeed, electric potential can vary by hundreds of meV across micron square areas, emphasizing the impact of sub-surface defects on electronic properties. Detailed depth correlations show that chemomechanical polishing can reduce outer layer roughness and native defects while sub-surface defect and potential features remain. Furthermore, pronounced surface asperities with large KPFM changes can be associated with native point defects. The relative strength of near band edge to deep level defect emissions exhibits a dramatic threshold dependence on surface roughness: surface optical emission efficiency increases over ten-fold as roughness decreases to unit cell dimensions, highlighting the role of surface polishing and etching for high efficiency emitters. Furthermore, surface and near-surface states due to morphology and polarity provide a guide to understanding and controlling and optical emission.
9:00 PM - B6.16
High Quality ZnO Thin Films for TCOs and Transistors by MOCVD.
Bruce Willner 1 , Shangzhu Sun 1 , Gary Tompa 1
1 , Structured Materials Industries, Inc., Piscataway, New Jersey, United States
Show AbstractThe wide bandgap semiconductor Zinc Oxide (ZnO) and its alloys are used as transparent contact layers in applications such as solar cells and LEDs. ZnO-based transistors have recently gained significant interest because of their potential for use in displays and as low-noise, high-voltage, high-power devices. Wide bandgap ZnO, and related materials offer the potential for high power and performance transistors at low cost, with the proper deposition techniques. Metal-organic chemical vapor deposition (MOCVD) is an optimal scalable production approach to manufacturing ZnO and its alloys. We have developed high speed rotation susceptor style reactors scaled from deposition planes of 3” to 16” in diameter. As we report herein, we have shown that MOCVD can produce high quality films of desired composition uniformly over large areas on a variety of substrates. In particular, we report on deposition parameters for compositionally uniform pin-hole free smooth morphology insulating and doped films of varying composition as well as multi-layer dielectric-ZnO structures and the characteristics of the films produced using Hall, optical, XRD, XRF, and other techniques. Further, we report on ZnO thin film device performance parameters.
9:00 PM - B6.17
Defect Induced Bandgap Narrowing of TiO2 by Thermal Heating.
Hui Pan 1 , Baohua Gu 1 , Wei Wang 1 , Xiaofeng Qiu 1 , Wengguan Zhu 1 , M. Parans Paranthaman 1 , Gyula Eres 1 , Zhenyu Zhang 1
1 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractAs a photocatalyst, titanium dioxide, TiO2, has received much attention since the discovery of photoelectrochemical splitting of water into hydrogen and oxygen on n-TiO2 electrodes. However, the high band gap of TiO2 (3.0 eV) makes it highly inefficient in the conversion of solar energy. Narrowing its bandgap has thus been recognized as one of the major avenues for increasing the conversion efficiency, but to date no major breakthrough has been achieved despite numerous attempts by doping TiO2 with various metallic or non-metallic elements or by fabricating composite materials. In this study, we reported a simple method to narrow the band gap of TiO2 without doping. TiO2 was synthesized by directly heating the Ti metal foil in air at varying temperatures and holding times. The as-grown TiO2 was found to exhibit various spectral absorbance characteristics due to surface defects produced in the thermal process at different heating conditions. An extra absorbance band was observed in the visible spectral region, which shifted from 450 to 750 nm with an increase in heating temperature from 400 to 600 oC. The full width at half maximum (FWHM) appeared broad, suggesting that the absorption was induced by defects. This extra absorbance band disappeared with increasing the heating temperature to 700oC or above, and Raman spectra showed that all TiO2 samples were in rutile structures. The intensity ratio between Eg and A1g modes increased and the FWHM of the two modes decreased with an increase of the heating temperature, suggesting an increased crystallinity and reduced defects. The as-grown TiO2 was further investigated for its photocleavage of water with visible light, and preliminary results indicated its higher efficiency due to the band gap narrowing.
9:00 PM - B6.19
Comparison on Optimized Optical Transmission and Electrical Resistivity between Indium Tin Oxide and Gallium Doped Zinc Oxide.
Wei-Lun Hsu 1 , Fan-Shuen Meng 1 , Cheng-Tao Lin 1 , Kuang-Chung Liu 1 , Chee-Wee Liu 1 , JianJang Huang 1 , Gong-Ru Lin 1
1 Graduate Institute of Photonics and Optoelectronics, National Taiwan University , Taipei Taiwan
Show Abstract9:00 PM - B6.2
Amorphous Copper-Zinc-Oxide Thin Films by RF Sputtering.
Christopher Gorrie 1 , John Perkins 2 , David Ginley 2
1 Materials Science and Engineering, University of Florida, Gainesville, Florida, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractAmorphous copper-zinc-oxide (a-CZO) is of interest as a p-type semiconductor for flexible transparent thin film transistors. An absorption edge at ~ 850 nm makes it a candidate for use in PV applications as well, and the ability to deposit this material at low temperatures allows for its potential use with low-cost polymeric substrates. Recently, RF sputtering has been used to deposit a-CZO films using ceramic ZnO and CuO targets in an off-axis combinatorial configuration. The metals ratio in the resulting films spans a range from 4% to 84% Cu-to-Zn, with an amorphous region at approximately 45-50% Cu-to-Zn. These depositions were performed at ambient temperature, in 30 mtorr of argon, with a power density of 4.8 W/cm2 on the ZnO target and 1.9 W/ cm2 on the CuO target. At deposition temperatures of 100°C and higher, the CZO films were crystalline for all compositions deposited. However, in-situ XRD measurements made while annealing in air have shown that once the a-CZO film has been deposited at ambient temperature it remains stable up to 250°C. In addition, it was found that these depositions were very sensitive to deposition pressure and to power density on the sputter targets. Amorphous films were not obtained when using lower pressures or higher powers. Films deposited in 100% argon were too resistive to measure. However, films grown with 25% oxygen in argon resulted in a conductivity of 4.8 x 10-3 S/cm. Currently, an attempt is being made to optimize the sputtering atmosphere to yield films with higher conductivity. The results of these ongoing studies will be presented.
9:00 PM - B6.20
Solution Deposition of Indium-Zinc Oxide Films by Ultrasonic Spray Pyrolysis.
Robert Pasquarelli 1 , Maikel van Hest 2 , Alexander Miedaner 2 , Calvin Curtis 2 , John Perkins 2 , Joseph Berry 2 , Jennifer Leisch 2 , David Ginley 2 , Ryan O'Hayre 1
1 Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, Colorado, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractTransparent conducting and semiconducting thin films play a critical role in many current and emerging opto-electronic devices. Their unique combination of high transparency in the visible region of the spectrum and tunable electronic conductivity is ideal for applications in transparent thin-film transistors (TTFTs) and as the collection layer for solar cells. Amorphous indium zinc oxide (IZO) is a promising material system for these applications due to the potential combination of both high performance and a low deposition temperature. Currently, these thin film materials are primarily deposited by vacuum-based physical vapor deposition techniques such as sputtering and pulsed laser deposition. This study investigates atmospheric-pressure solution deposition routes as an alternative to these traditional high-vacuum techniques. Solution processing is attractive due to its ease and potential to lower device manufacturing costs. Here we report on IZO films prepared by ultrasonic spray pyrolysis from solutions of indium and zinc metal-organics and salts, principally formates and acetates. The potential of a novel indium-zinc formate precursor is demonstrated, along with variations of film properties as a function of substrate deposition temperatures and annealing conditions. The structural, optical, and electronic properties of the deposited films will be reported.
9:00 PM - B6.21
Properties of RF Sputtered Transparent Conducting ZnO:F Films.
Stanley Potoczny 1 , Adam Bowen 1 , Jia Li 1 , Kartik Sivaramakrishnan 2 , Terry Alford 2 , Jay Lewis 3 , Eric Forsythe 4 , Shanthi Iyer 1
1 Electrical and Computer Engineering, North Carolina Agricultural and Technical State University, Greensboro, North Carolina, United States, 2 Flexible Display Center at ASU and School of Materials, Arizona State University, Tempe, Arizona, United States, 3 Center for Materials and Electronic Technologies, RTI International Inc., RTP, North Carolina, United States, 4 , Army Research Laboratory, Adelphi, Maryland, United States
Show AbstractDoped ZnO films are presently being investigated as an alternative to indium tin oxide (ITO) films, due to the high cost and scarcity of indium, for applications in displays, solar cells as well as other opto-electronic devices. Polymers are being investigated to replace glass substrates to move towards a future of flexible displays. Among the polymers polyethylene napthalate (PEN) has been found to offer better thermal and dimensional properties than most polymers. In this work we present a systematic and detailed investigation of F doped ZnO films deposited on both glass and PEN substrates by RF magnetron sputtering and the effect of post growth annealing in different ambients. Thin films of ZnO:F have been deposited onto glass and PEN substrates in an Ar plasma. The various properties of these films have been analyzed using a variety of techniques including atomic force microscopy, x-ray diffraction (XRD), Hall measurements, transmission and reflectance measurements, x-ray photoelectron spectroscopy (XPS), and mechanical bend testing. Post deposition annealing treatments include annealing in high vacuum, nitrogen, 5% H2/Ar as well as argon at 150°C. XRD indicated (002) preferred orientation in all the films. The ZnO:F samples on glass had an absorption edge at 3.32 eV with an average transmission of 83% in the visible region. The resistivity of the annealed samples varied from 10-2 to10-3 ohm-cm depending on the ambient. Further, the large reduction in resistivity on annealing has been found to be primarily caused by the change in the carrier concentration. The highest carrier concentration in the range of 1020 cm-3 has been obtained on the films annealed in either high vacuum or argon environments. The variation in resistivity observed in different annealing ambients will be discussed in terms of differences in the oxygen vacancies and F in the substitutional sites. Mechanical testing on the films grown on PEN substrates were performed using a collapsing radius setup. The mechanical failure of the layers has been tested as a function of bend cycles of the polymer substrate for both close to and far away from the critical radius. The differences in the mechanical behavior between these and our earlier work on ITO films will also be reported. This work is supported by The Army Research Office (Contract No. W911NF-04-2-0051)
9:00 PM - B6.22
Decreasing Sheet Resistance of ITO Thin Films using Current Source.
Ehsan Davoodi 1 , Ebrahim Asl Soleimani 1
1 Electronics & Computer, Tehran University, Tehran Iran (the Islamic Republic of)
Show Abstract9:00 PM - B6.23
Transparent Metal Oxide-based Colloidal Quantum Dot LEDs.
Vanessa Wood 1 , Matthew Panzer 1 , Jean-Michel Caruge 2 , Jonathan Halpert 2 , Moungi Bawendi 2 , Vladimir Bulovic 1
1 Department of Electrical Engineering and Computer Science, MIT, Cambridge, Massachusetts, United States, 2 Department of Chemistry, MIT, Cambridge, Massachusetts, United States
Show Abstract9:00 PM - B6.24
Indium Zinc Oxide Based Transistors for Thermal Stability and Electric Properties Measurements.
Charles Sievers 1 , Joseph Berry 2 , Thomas Gennett 2 , Charles Rogers 1 , John Perkins 2 , David Ginley 2
1 Physics, Unversity of Colorado, Boulder, Colorado, United States, 2 , National Renewable Energy Lab, Golden, Colorado, United States
Show AbstractAmorphous Indium zinc oxide (a-IZO) is a promising component for transparent devices. However, recently it has been shown that more resistive a-IZO can undergo an irreversible increase in the conductivity from ~1e-4 S/cm to ~1e3 S/cm when annealed at 170 degrees for a few minutes. To better understand the thermal instability in the electrical properties of a-IZO, we have designed a Hall Bar/Transistor pattern. We will fabricate devices using standard photolithography, aqueous HCl etching, Ti/Au ohmic contacts, and a variety of gate structures. The Hall Bar pattern will allow us to measure both the mobility and carrier concentration while the transistor will allow us to measure mobility separately from initial carrier density for the more insulating a-IZO films. We will report the electrical properties for both as-grown and annealed semi-conducting materials and the characterization of these transistors to better understand how a-IZO can be used in transparent devices.
9:00 PM - B6.25
Fusion of Transparent Semiconductors and Microstructured Optical Fibers via High-Pressure Microfluidic Chemical Deposition.
Neil Baril 1 2 , John Badding 1 , Mahesh Krishnamurthi 1 , Justin Sparks 1 , Rongrui He 1 , Pier Sazio 3 , Venkatraman Gopalan 2
1 Chemistry, The Pennsylvania State University, University Park, Pennsylvania, United States, 2 Materials Science and Engineering, Penn State, University Park, Pennsylvania, United States, 3 Optoelectronics Research Centre, University of Southampton, Southampton United Kingdom
Show AbstractThe introduction of a crystalline semiconductor within the capillaries of a microstructured optical fiber (MOF) presents tremendous potential for the development of in-fiber optoelectronic devices. We have developed a high-pressure microfluidic process that allows us to adapt traditional chemical vapor deposition chemistries to incorporate materials within the capillaries of MOFs. Pressures up to 35MPa are used to force a precursor/carrier gas mixture through the capillaries enabling deposition within microscale capillaries over meters in length. The materials can be organized within the MOFs for in-fiber applications, or the MOF can be used as a template for the formation of highly uniform extreme aspect ratio tubes and wires. Our efforts in the deposition of silicon carbide within the microscale capillaries of MOFs from a single source precursor will be presented. Crystalline semiconducting materials such as SiC are of particular interest to us owing to their ability to generate light. The introduction of SiC into the capillaries presents tremendous potential for the development of in-fiber optoelectronic devices with potential applications including light generation, modulation, and amplification.
9:00 PM - B6.26
Study of Current Stability and Fluctuations of Field Emitted Electrons from ZnO Nanostructure.
Kishore Uppireddi 1 , Boqian Yang 1 , Peterxian Feng 1 , Gerardo Morell 1
1 Dept of Physics, University of Puerto Rico, San Juan, Puerto Rico, United States
Show Abstract9:00 PM - B6.27
M3+-doped ZnO (M = Al, Ga) Powder : Effect of Annealing (Temperature/atmosphere) on Near-infrared Absorption and Electrical Conductivity.
Helene Serier 1 , Manuel Gaudon 1
1 , ICMCB, Pessac France
Show AbstractZinc oxide is a wide band-gap semiconductor (3.5 eV) which can exhibit n-type conductivity when it is doped by efficient shallow donors such as aluminium, gallium, indium, fluorine… In this investigation, XRD analyses along with RMN have been performed in order to determine the solubility limits of aluminium and gallium. These solubility limits have been assessed for powder synthesized via a soft chemistry route : Pechini process, and for several annealing. Under air, the Al solubility limit is very low (under 0.5 mol.%) compared to the Ga solubility limit (around 1.5 mol.%), the results being explained by the small ionic radius of aluminium which induces a too large compression of the cationic site in the ZnO lattice compared to gallium.Al-doped ZnO (AZO) and Ga-doped ZnO (GZO) materials exhibit optoelectronic properties mainly useful for transparent electrode application or thermal insulator in smart windows. These properties are the consequence of a wide band-gap along with a high dispersive conduction band which enables a high mobility of carriers. The electronic conduction involves a plasma effect and consequently IR reflectivity. In this work, optical properties, assessed on powder by diffuse reflection, have been correlated to electrical measurements, carried out on pellets. Thus, powder getting a large conductivity exhibits also a good IR absorption.Finally, IR absorption depends firstly on the dopant rate, related to carrier concentration. However, this investigation presents also the relevance of the cations/anions ratio in the lattice. In fact, the use of high temperature and/or reductive atmosphere enables to improve substantially IR absorption either by changing solubility limit or by inducing defects (interstitial cations or anionic vacancies) in the lattice.
9:00 PM - B6.28
Electrical and Optical Properties of GaN and ZnO Studied by Surface Photovoltage and Photoluminescence Techniques.
Michael Foussekis 1 , Alison Baski 1 , Michael Reshchikov 1
1 Physics Department, Virginia Commonwealth University, Richmond, Virginia, United States
Show AbstractAlthough significant progress has been made in the development of wide-band-gap semiconductors for optoelectronics, the detrimental effect of the surface on the electrical and optical properties of materials and devices based on these semiconductors is often underestimated. In particular, a depletion region caused by upward bend bending near the surface can affect device properties. The height of the near-surface barrier and the width of the depletion region are ambient dependent. We have studied the effect of temperature and different ambients (air, oxygen, nitrogen, and vacuum) on the electrical and optical properties of GaN and ZnO in an original set-up based on a high-vacuum Kelvin probe system combined with an optical cryostat. The value of band bending at the surface and its change due to illumination (photovoltage) were studied with the Kelvin probe. Complementary information about the depletion region and carrier recombination paths was obtained from the analysis of photoluminescence (PL) measured using the same experimental conditions. We observed the photovoltage (PV) signal at photon energies between 1.2 and 4.0 eV. In GaN, the PV signal reached its highest value of 0.7 V at photon energies close to the GaN bandgap, indicating that the initial (dark) upward band bending decreased by at least 0.7 eV under UV light with an intensity of about 0.03 W/cm2. Note that in the intensity range from 10-9 to 10-2 W/cm2 the PV signal increased as a logarithm of the light power. Transients of the PV, as indicated by the decrease of the near-surface barrier under continuous illumination and its rise after switching off the light, strongly depended on the intensity and wavelength of the light, the sample temperature, and the ambient. In air ambient and room temperature, the PV signal under near-band-edge illumination increased quickly to a maximum and then gradually decayed while illumination of the sample continued. In contrast, in vacuum at room temperature, the PV signal continuously and slowly (during two hours) increased under UV illumination. To explain these effects we propose that in air the UV light assists adsorption, i.e., photo-induced adsorption, whereas in vacuum it causes the desorption of negatively charged species from the surface. These observations are consistent with photoluminescence transients under continuous illumination with a UV laser: the PL intensity gradually decreases in air, whereas it increases in vacuum.
9:00 PM - B6.29
Epitaxial Growth of Non-polar a-plane ZnO Thin Films on r-plane Sapphire Substrate by Pulsed Laser Deposition.
Punam Pant 1 , J. Narayan 1
1 Materials Science and Engineering, North Carolina state University, Raleigh, North Carolina, United States
Show AbstractAs a wide band gap semiconductor with a high excitonic binding energy ZnO offers the possibility of making highly efficient light emitting devices. Conventionally, most of the devices are based on [0001] heterostructures, since [0001] is the natural growth direction of ZnO on (0001) sapphire which is the most commonly used substrate for light emitting devices. However devices based on [0001] wurtzitic materials have electrostatic fields that spatially separate the electrons and the holes in the active layers and therefore reduce the device efficiency. These fields are due to the inherent polar character of the [0001] growth direction and also due to the piezoelectric effect that affects the strained layers.Epitaxial growth of non-polar ZnO would make the film free of electrostatic fields and bring about a significant change in device performance by not only increasing the device efficiency but also by reducing the power consumption. In our study, we report in detail the growth of (11-20)ZnO(a-plane ZnO) on (1-102)sapphire(r-plane sapphire) under varying conditions of temperature and pressure. A comprehensive characterization of the grown non-polar films will be presented. An investigation of the morphology of the grown films by AFM shows that the films have a smooth surface and are striated in the in-plane direction. Epitaxial relationships between ZnO and sapphire substrate are determined to be (11-20)ZnO//(1-102)sapphire in the growth direction. The FWHM of the 2-theta scans shows a variation with temperature with minimum values~0.28deg for films grown at 700°C. Depending on the growth temperature the resistivity measurements of the films show either a semiconducting behavior or metallic conductivity at room temperature with a metal-semiconductor transition at low temperatures. These results will be supplemented by the interfacial structural characterization by high-resolution TEM. Results of the variation in the optical and electrical properties of the films with varying growth conditions will also be presented and structure-property correlation of the films will be discussed.
9:00 PM - B6.3
Electronic Structure of CuAl1-xCrxO2 (x = 0, 0.5, 1): A Theoretical Study.
David Scanlon 1 , Benjamin Morgan 1 , Aron Walsh 1 , Graeme Watson 1 , Russel Egdell 2
1 School of Chemistry, Trinity College Dublin, Dublin, Leinster, Ireland, 2 Dept. of Chemistry, University of Oxford, Oxford United Kingdom
Show AbstractCuAlO2 crystallizes in the delafossite structure and is the prototype p-type TCO with a band gap of 3.5 eV[1]. CuCrO2 is also a p-type TCO which also crystallizes in the delafossite structure, and Mg doping of CuCrO2 has been shown to yield the highest conductivities of all delafossite structured TCOs[2]. Therefore the elucidation of the electronic structure and the nature of the hole charge carriers involved in the p-type conductivity of CuAlO2 and CuCrO2 is of key importance.
Detailed examinations of the electronic structures have been performed on CuAlO2, CuAl0.5Cr0.5O2 and CuCrO2 using DFT corrected for on-site Coulomb interactions (DFT + U). The introduction of Cr into the system results in the shifting of conduction band minimum away from Γ, (where it appears for CuAlO2 and the appearance of Cr d states at the bottom of the conduction band. Comparison of our calculated experimental density of states with XPS results show that changes to top of the valence band are due to changes in the Cu EDOS as opposed to states introduced by Cr. The electronic structure of the relevant defects involved in the p-type conductivity of these delafossite TCOs will also be discussed.
References:
1)X. Nie, S. H. Wei, S. B. Zhang, Physics Review Letters, 88, 066405 (2002)
2)R. Nagarajan, A. D. Draeseke, A. W. Sleight, J. Tate, Journal of Applied Physics, 89, 8022 (2001)
9:00 PM - B6.30
ZnO Chemical Compatibility with Hydrogen: Its Dependence on ZnO Polarity.
Maria Losurdo 1 , Maria Giangregorio 1 , Graziella Malandrino 2 , Maria Fragalà 2 , Pio Capezzuto 1 , Giovanni Bruno 1
1 PlasmaChemistry, IMIP-CNR, Bari Italy, 2 Chemistry, University of Catania, Catania Italy
Show AbstractZnO, whose optical band gap is 3.4 eV, is a versatile semiconductor material investigated and applied in all its forms ranging from epitaxial layers grown by molecular beam epitaxy and metalorganic chemical vapour deposition (MOCVD) for the blue-violet optoelectronics, to polycrystalline ZnO thin films produced by MOCVD and pulsed laser deposition (PLD) for a large variety of applications including sensors, and transparent conductive layers–TCO, and to a myriad of nanostructures including nanorods, nanobelts, nanorings, synthesized by catalyzed vapour-liquid-solid (VLS) and MOCVD. Two aspects still critical and interesting from both fundamental and technological point of views are the polarity of ZnO and the interaction of ZnO with atomic hydrogen. In a perspective of technological relevance, intriguing basic questions, involving the interplay between ZnO orientation/polarity and kinetics of the interaction with hydrogen remain poorly investigated and will benefit from a better understanding. In particular, it is a debated aspect whether the interaction of ZnO with hydrogen is detrimental (i.e., etching, reduction to metallic Zn, loss of optical transparency) or beneficial, i.e., passivation of defects and grain boundaries, increase of electrical conductivity and of photoluminescence.In this contribution, we report the first real time monitoring of the interaction of various ZnO orientations and polarities with atomic hydrogen. ZnO films with different microstructures and polarities have been deposited by MOCVD and exposed at various temperature in the range RT-400°C to atomic hydrogen produced by a remote H2 plasma. The real time monitoring of the interaction of ZnO with hydrogen has been carried out non-invasively by spectroscopic ellipsometry(UVISEL-Jobin Yvon) in kinetic mode with a time resolution of 1s.The present data develop a robust link between the polarity, the microstructures of the ZnO films and their reactivity and demonstrates that a “face/polarity” of ZnO stable to atomic hydrogen attack exists, i.e., -c O-polar ZnO, making it valuable for all those applications in atomic hydrogen environments. In particular, the following order of stability to atomic hydrogen is established for the various faces of ZnO(000-1)O-polar >>(0001)Zn-polar > polycrystalline > (11-20)non-polar demonstrating that the use of the non-polar a-plane ZnO should be avoided in all those technological applications involving hydrogen and of the Zn-polar ZnO as well becoming a Zn-sponge upon hydrogen exposure. The determination of a polar form of ZnO not etched/reduced by hydrogen represents a great issue, since it allows a technological advancement in all applications involving ZnO as hydrogen sensor and as TCO electrode, e.g. a-Si.H and microc-Si:H solar cells. The use of O-polar ZnO could inhibit subsurface reactions and chemical reduction of ZnO during deposition of a-Si:H andor microc-Si:H from highly hydrogen diluted SiH4-H2 plasmas
9:00 PM - B6.31
Electronic Memory Effects in Diodes Containing Metal Oxide Nanoparticles.
Stefan Meskers 1 , Frank Verbakel 1 , Robert Jakobs 1 , Rene Janssen 1 , Albertus Schenning 1 , Dago DeLeeuw 2
1 Chemistry and Chemical Technology, Technical University Eindhoven, Eindhoven Netherlands, 2 , Philips Reserach Lab, Eindhoven Netherlands
Show Abstract9:00 PM - B6.33
Optimization of Transparent Single-Walled Carbon Nanotube Contacts for Photovoltaic Applications.
Robert Tenent 1 , Teresa Barnes 1 , Jeremy Bergeson 1 , Michael Heben 1 , Jeffrey Blackburn 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractTransparent and electrically conducting films of single-walled carbon nanotubes (SWCNTs) hold a great deal of promise to replace conventional transparent conducting oxides (TCOs) in a variety of opto-electronic applications. We present our work to optimize these films for photovoltaic applications with special emphasis on reproducible, efficient and highly scalable fabrication techniques. We have recently reported on a new method that enables spray coating of highly conductive (Rs = 39 Ohm/square) transparent (%T = 74 at 550nm) SWCNT films1. Here we present data using this method to explore a number of factors that influence the electronic and optical properties of SWCNT networks. These factors include the tube source, purification protocol, ink formulation procedure, deposition parameters and post-deposition processing. We will also present preliminary data on integrating these films into photovoltaic devices.1. Tenent, R.C.; Blackburn, J.L.; Barnes, T.M.; To, B; Gedvilas, L.M.; Heben, M.J. , Ultrasonic Spraying of Highly Conductive Transparent Single-Walled Carbon Nanotube Films, 2008, in preparation.
9:00 PM - B6.34
Semitransparent Organic Photovoltaic Cells and Tandem Cells.
Jung-Yong Lee 1 , Steve Connor 2 , Yi Cui 3 , Peter Peumans 1
1 Electrical Engineering, Stanford University, Stanford, California, United States, 2 Chemistry, Stanford University, Stanford, California, United States, 3 Materials Science and Engineering, Stanford University, Stanford, California, United States
Show AbstractOrganic photovoltaic cells are considered a promising solar cell technology because of a potential for low-cost production, compatibility with flexible substrates, and environmental stability. Due to relatively short diffusion lengths of excitons in organic materials, the thickness of active layer needs to be rather thin (~100nm) to ensure efficient exciton dissociation at the donor-acceptor junction. Therefore, devices without the usual reflective metal cathode layer can have a high optical transmissivity, making such organic photovoltaic cells attractive for applications in semitransparent power-generating coatings, if appropriate transparent conductor materials are provided. Recently, a laminated transparent polymer cathode on top of polymer photovoltaic cells was reported [1], but the device performance was poor due to the very low conductivity of the cathode layer. Sputtered indium-tin-oxide (ITO) transparent cathode layers have been also reported [2,3]. However, the slow and expensive sputtering process used is not appropriate for high throughput roll-to-roll process. Moreover, the sputtering process is known to incur damage to the organic active layer. Here, we demonstrate transparent organic photovoltaic cells laminated with silver nanowire meshes functioning as the cathode layer. As we reported earlier, silver nanowire meshes are as transparent as sputter-coated ITO at the same sheet resistance [4]. The low-cost process and fast processing of such silver nanowire meshes makes them suitable for large area applications and roll-to-roll processing. Under 73mW/cm2 AM1.5G illumination, the transparent cell exhibits a power conversion efficiency (PCE) of 0.63%, an open circuit voltage of Voc=0.44V, a short-circuit current of Jsc=1.91mA/cm2, and a fill factor of FF=0.55. The averaged transmittance over the spectral range 400nm-800nm is 26%. For comparison, for the opaque control device we obtain PCE=1.1%, Voc=0.46V, Jsc=2.91mA/cm2, FF=0.60.The silver nanowire meshes can also be used as an excellent contact between adjacent cells in multijunction structures [5] to achieve higher efficiencies. The ability to extract photocurrent laterally from the multijunction cells would allow one to achieved greater efficiencies since one is no longer limited by the cell producing the lowest photocurrent as is the case for a series-connected multijunction cell [5]. The performance data for multijunction cells will be presented as well.References[1]A. Gadisa, et al., Synth. Met. 156, 1102-1107 (2006).[2]R. F. Bailey-Salzman, B. Rand, and S. R. Forrest, Appl. Phys. Lett. 88, 233502-233503 (2006); [3]G. -M. Ng, et al., Appl. Phys. Lett. 90, 103505-103503 (2007).[4]J. -Y. Lee, S. T. Connor, Y. Cui, and P. Peumans, Nano Lett. 8, 689-692 (2008)., Y. Cui, and P. Peumans, Nano Lett. 8, 689-692 (2008).[5]A. Hadipour, B. de Boer, and P.W.M. Blom, Adv. Funct. Mater., 18 169-181 (2008).
9:00 PM - B6.36
Transparent and Conductive ZnO:Al Powder Prepared by Soft Chemical Route Process and Design of Experiment Technique.
Kuo-Chuang Chiu 1 , Yi-Wen Kao 1 , Ren-Der Jean 1
1 Materials Research Laboratories, Industrial Technology Research Institute, Hsinchu Taiwan
Show AbstractAluminum doped zinc oxide polycrystalline powder (AZO) were prepared by soft-chemical route process. The quantity of aluminum in the sol was varied from 1 to 5 mol%. The structural characteristics studied by X-ray diffractometry were complemented resistivity measurement by AC impedance spectroscopy. Prepared under tartaric acid as chelating agents and sintered at 1400 oC and design of experiment (D.O.E.) method was employed to elucidate the AZO formulation for the powder process. Following the design of experiment method, dominant factors were found for [Al/Zn] mol%, sintering temperature, sintering time, annealing environment was optimized for production. The best conductors were obtained for the AZO powder containing 3 mol% of aluminum.
9:00 PM - B6.38
Growth of Microscale In2O3 Islands on Y-stabilised Zirconia(100) by Molecular Beam Epitaxy.
Anne Bourlange 1 , David Payne 1 , Russell Egdell 1 , John Foord 1 , John Hutchison 2 , Robert Jacobs 1
1 Department of Chemistry, University of Oxford, Oxford United Kingdom, 2 Department of Materials, University of Oxford, Oxford United Kingdom
Show AbstractMismatch between the lattice parameters for a substrate and an epilayer deposited by molecular beam epitaxy (MBE) can promote the development of self-assembled “dot” structures, usually on a nanometre length scale. To date most work in this area has concentrated on quantum dots obtained in the hetero epitaxial growth of III-V semiconductors, typified by InAs on GaAs where there is a 7% lattice mismatch. In the present communication we extend this approach to the growth micrometre sized arrays of In2O3 islands on Y-stabilised ZrO2(100). In2O3 is an n-type oxide amenable to degenerate n-type doping to give a material which combines optical transparency in the visible region with a high conductivity. This leads to important applications in liquid crystal displays, solar cells and electroluminescent display devices. We have very recently shown that continuous epitaxial films of In2O3 can be grown on (100) oriented yttria-stabilized zirconia substrates at a temperature of 650 degree C. In2O3 itself has a low symmetry monoclinic structure at room temperature, but a cubic phase can be stabilized by replacement of Zr(IV) with larger cations such as Ca(II) or Y(III) and with concomitant introduction of compensating oxygen vacancies. The lattice parameter of the face centered cubic fluorite phase increases with Y doping level. For the minimum Y concentration of around 17% required to stabilize the cubic phase the lattice parameter a = 5.1423 Å, whilst for 28% Y-doping a = 5.2100 Å. The body centered bixbyite structure of In2O3 has a = 10.1170 Å and is derived from a 2 x 2 x 2 superstructure of the fluorite structure with ¼ of the anion sites vacant. Thus at 17% Y-doping there is a mismatch of 1.6% between 2a for Y-ZrO2 and a for In2O3. For this reason yttria stabilised zirconia has proved to be on of the most popular substrates for attempted epitaxial growth of In2O3. In the present communication it is shown that MBE growth of In2O3 at high temperatures (typically around 900 degree C) on cubic In2O3 stabilized by 17% Y-doping promotes break-up of the continuous films obtained at lower temperatures into striking arrays of highly oriented truncated square pyramidal micrometer sized epitaxial islands with a narrow size distribution. The edges of the islands are uniformly aligned parallel to <110> directions of the substrate and display a narrow size distribution. This unanticipated growth mode is apparently driven by the small mismatch between the inter-cation separations in the two oxides and a propensity for development of (111) side facets on the islands.We further investigate the influence of Sn doping on the growth mode. It is shown that low levels of Sn doping lead to an increase in the lattice parameter and promotes development of larger island sizes.
9:00 PM - B6.39
Specific Contact Resistance of Metal/SWNT Electrodes and its Impact on Photovoltaic Applictions.
Roderick Jackson 1 , Samuel Graham 1
1 Mechanical Engineering, Georgia Institute of Technology, Detroit, Michigan, United States
Show AbstractThin, transparent films of single wall carbon nanotubes (SWNT) are an appealing candidate as a surrogate for ITO in organic photovoltaic (OPV) devices because of the extraordinary electrical and mechanical properties these 1-D structures possess. Recent progress has demonstrated thin film random network SWNT electrodes deposited onto transparent substrates under ambient conditions with high electrical conductivity, mechanical stability, and doping stability.[1-3] The contribution of SWNT electrodes to electrical power losses in OPV devices has been examined with respect to the sheet resistance of the electrode resulting in increased operational series resistance. However, when SWNT electrodes are utilized in photovoltaic applications, a grid system with low resistivity metallic fingers and busbars deposited on the surface to collect electrical charges will be required. Therefore, the interfacial electrical contact resistance, Rc, between the metal fingers and randomly distributed SWNTs must be known to further understand and mitigate the total resistive components that contribute to power loss in device operation. To date, no report has quantified the expected impact of Rc at the SWNT film and metallic interface on overall power losses.This report will present the contact resistance at the metallic/SWNT film interface and provide insight into the quality of the contact through the evaluation of the specific contact resistance, ρc, defined as the product of contact resistance and interfacial contact area. ρc is the primary figure of merit used when assessing interfacial contact resistance and provides a convenient parameter to compare contacts of various sizes. The transmission line method (TLM) was used to ascertain ρc through the direct determination of Rc, the SWNT film sheet resistance, and characteristic transfer length over which the current transfer from the film to metal takes place.[4] In this report, ρc was determined for both silver and palladium contacts deposited onto SWNT films through e-beam evaporation. These metals form ohmic contact with SWNT films and verification will be presented in this report to validate the utility of the TLM in quantifying specific resistance. Silver contacts on SWNT films have resulted in an average ρc value of 0.05 Ω-cm2 which is higher than approximate ρc values on the order of 1 mΩ-cm2 for silicon solar cells. The expected power loss efficiency in organic photovolatics with a ρc as determined from SWNT films with metallic contacts will be evaluated. Furthermore, the impact on ρc of chemically doping SWNT films to reduce sheet resistance will be shown in this report.References:1.Z. Wu et al., Science 305, 1273 (2004).2.G. Gruner, Journal of Materials Chemistry 16, 3533 (2006).3.R. Jackson, B. Domercq, R. Jain, B. Kippelen, S. Graham, Advanced Functional Materials, (2008). (in press)4.D. K. Schroder, D. L. Meier, IEEE Transactions on Electron Devices ED-31, 637 (1984).
9:00 PM - B6.5
First principles calculations of the electronic structure and optical properties of indium- and antimony-doped SnO2.
Luisa Scolfaro 1 , P. Borges 1 , H. Leite Alves 2 , J. A. Alves 2 , E. da Silva Jr. 3
1 DFMT, Instituto de Fisica, Universidade de Sao Paulo, Sao Paulo, SP, Brazil, 2 Depart. de Ciencias Naturais, Universidade Federal de São Joao del Rei, Sao Joao del Rei, MG, Brazil, 3 , Universidade Federal de Pernambuco, Recife Brazil
Show Abstract9:00 PM - B6.7
Metal Doped Indium Oxide Semiconductors for Thin Film Transistors.
Osamu Shiino 1 , Hidefumi Kotsubo 1
1 Chemical & Industrial Products Technology Division, Bridgestone Corporaion, Tokyo Japan
Show AbstractRecently, the applicability of the oxide semiconductors improved remarkably. Especially, the In-Ga-Zn oxide is expected to become the key material for the flexible or large-area electronic devices as they can be manufactured by the conventional sputtering deposition at ambient temperature.In this study, we have tried the fabrication of thin film transistors (TFTs) using indium oxide based semiconductors for the channel layers to obtain deeper information and predict applicability of these materials. Doping of Sn and W to indium oxide is known to improve electron mobility and to preserve amorphous state, respectively. Hence, we fabricated the TFTs with semi-conductive In2O3:Sn (sc-ITO) and In2O3:W (IWO) channel layers and measured the transport characteristics.Si/SiO2 (300 nm) wafer was used for the gate electrode and gate insulator. Metal doped indium oxide channel layers were deposited by the conventional DC magnetron sputtering without substrate heating. The source and drain electrodes of conductive ITO were also fabricated by DC sputtering. The channel length and channel width were 0.1 mm and 6.4 mm, respectively. The TFT with semi-conductive ITO channel exhibited the field-effect mobility of 12.7 cm2/Vs and on/off ratio of 108. The thermal stability, however, was very poor. It was because the ITO channel varied from semi-conductive to conductive by excessive activation of the doped Sn.In the TFT with IWO channel, on the other hand, the field effect mobility reached 2.5 cm2/Vs and on/off ratio 107, though annealing at 180 °C was required to obtain stable functioning free of charging. The material remained amorphous even after the annealing.
9:00 PM - B6.8
Electronic Structure and Conductivity in Indium Zinc Oxide Transparent Conductors.
Jennifer Leisch 1 , Dennis Nordlund 2 , Robert Pasquarelli 3 , Charles Sievers 4 , Philip Parilla 1 , Joseph Berry 1 , John Perkins 1 , David Ginley 1
1 , NREL, Golden, Colorado, United States, 2 , Stanford Synchrotron Radiation Laboratory, Menlo Park, California, United States, 3 , Colorado School of Mines, Golden, Colorado, United States, 4 , University of Colorado - Boulder, Boulder, Colorado, United States
Show AbstractRecent work has identified a promising new ternary TCO material from ZnO-In2O3 (IZO) that shows high optical transmission. Combinatorial studies of IZO synthesis have examined a full range of In content, and helped to identify three separate regions of In concentration that give rise to varying electronic properties. In the range of 45% < In < 85% these compounds have realized the highest mobilities and carrier concentrations, and are highly stable amorphous materials. An amorphous TCO material has a wide range of possible processing routes, including solution, sol-gel and electrochemical synthesis methods.During sputtering of these materials, it has been found that increasing the oxygen content during film growth results in a highly insulating as-grown state. Initial experiments involving low-temperature heating of insulating amorphous IZO in air (or an atmosphere containing small amounts of oxygen) have shown increases in material conductivity on the order of ~107. The insulating state is highly interesting for transistor applications, making the huge thermal switching of great interest. Initial experiments indicate that this may be some kind of subtle structural transformation, though the detailed origin underlying the conductivity change remains unknown, as the amorphous nature of these materials limits lab-source x-ray structural characterization. Here we explore the local and long-range atomic order, as well as the electronic structure in IZO materials in the amorphous regime, including the structural transition causing the conductivity switching at low temperature. This information is crucial to obtaining control over the optical and electronic properties in this system, and the tailoring of properties to specific applications. Fundamental insight into the relationship between carrier mobility and structure in these amorphous materials is needed and may give insight into other amorphous systems.
9:00 PM - B6.9
Effect of Thermal Annealing on Deep and Near-band Edge Emission from ZnO 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, Virginia Commonwealth University, Richmond, Virginia, United States
Show Abstract
Symposium Organizers
John D. Perkins National Renewable Energy Laboratory
Thomas O. Mason Northwestern University
John F. Wager Oregon State University
Yuzo Shigesato Aoyama Gakuin University
B7: ZnO Materials
Session Chairs
Wednesday AM, December 03, 2008
Room 203 (Hynes)
9:30 AM - **B7.1
Group-III Donors in ZnO: Activation, Compensation, and Diffusion.
David Look 1 2 , Gary Farlow 3 1 , F. Yaqiib 4 , L. Vanamurthy 4 , Mengbing Huang 4
1 Semiconductor Research Center, Wright State University, Dayton, Ohio, United States, 2 Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio, United States, 3 Physics Department, Wright State University, Dayton, Ohio, United States, 4 College of Nanoscience and Engineering, University at Albany, Albany, New York, United States
Show AbstractThe Group-III elements, Al, Ga, and In, are important dopants in the production of transparent, conductive ZnO, because they can generate material with resistivities as low as 10-4 ohm-cm. However, stability problems have been reported in such materials, and in some cases the high conductivity degrades with time and/or temperature. The reasons for this degradation are not fully understood and can involve several different phenomena: (1) deactivation, in which the Group-III donors are no longer substitutional on the Zn site, or become neutralized by close association with acceptors, say LiZn or OI; (2) compensation, in which new acceptors are formed, or neutralized acceptors are reactivated, e.g., (LiZn-- H+)0 → LiZn- + H+, with the H leaving the sample; and (3) diffusion, in which the donors move to a surface or other interface and again may be deactivated. To fully understand these issues it is necessary to follow the time evolution of both donors and acceptors, and this process requires temperature-dependent Hall-effect (T-Hall) measurements. Furthermore, the donor and acceptor concentrations are often inhomogeneous with depth, especially near the surface, so that at least a two-layer Hall analysis is required. We have investigated the dynamics of Group-III donors by annealing bulk crystals and thin films, and then following the changes in near-surface donor properties by T-Hall, low-T photoluminescence, and secondary-ion mass-spectroscopy (SIMS) measurements. In one study, an as-grown crystal was annealed at 900 °C in N2, to remove H, and then 600 °C in forming gas (5% H2in N2) to controllably add H. A strongly conductive surface layer (CSL) was formed, and SIMS measurements showed that Group-III elements, especially Al and Ga, had diffused from the bulk into the surface region. The T-Hall analysis of the near-surface donor concentration agreed well with the SIMS results, showing that the increased surface conduction was due to Group-III elements, and not H. In another study, In was implanted into a single crystal of ZnO to a depth of about 100 nm and a concentration of about 1020 cm-3, and studied by low-T PL and T-Hall measurements. In this case, the initial donor concentration surprisingly turned out to be much higher than the In concentration, but still could be well explained by the strong presence of donor-type point defects, as predicted by TRIM calculations. Moreover, low-T PL measurements showed several new lines in the 3.33 – 3.35-eV region, and these are likely due to point defects generated by the implantation. This assignment is supported by the observation that the new lines mostly disappeared during a 400-°C anneal in N2, as was also found to be the case in previous PL studies of electron-irradiated ZnO.
10:00 AM - B7.2
High Quality ZnO Epitaxy on (111) Si Substrates using Sc2O3 and Gd2O3 Buffer Layers.
Wei Guo 1 , Tassilo Heeg 2 , Christopher Nelson 1 , Darrell Schlom 2 , Xiaoqing Pan 1
1 Materials Sci. & Eng., University of Michigan, Ann Arbor, Michigan, United States, 2 Materials Sci. & Eng., Pennsylvania State University, University Park, Pennsylvania, United States
Show AbstractZnO epitaxial films on sapphire substrates have been widely explored for optoelectronic applications. The potential for integrating multi-functional devices into Si electronics has motivated the growth of epitaxial ZnO films on Si substrates. 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 Sc2O3 and Gd2O3 buffer layers. X-ray diffraction shows that the epitaxial orientation relationships are (0001)ZnO//(111)X2O3//(111)Si and [11-20]ZnO//[-110]X2O3//[1-10]Si (X=Sc or Gd). 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 302 arcsec and 403 arcsec. Cross-sectional transmission electron microscopy studies show atomically sharp interfaces between the ZnO films and the X2O3 buffer layers. The films are conductive with mobility up to 104 cm2/Vs. Temperature-dependent photoluminescence studies show optical properties that are comparable to those of ZnO single crystals. 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.
10:15 AM - B7.3
Influence of Hydrogen on The Electrical Transport in Zinc Oxide.
N. Nickel 1 , L. Scheller 1 , M. Gluba 1 , M. Weizman 1
1 , Helmholtz-Center Berlin (formerly Hahn-Meitner-Institut), Berlin Germany
Show Abstract Research on zinc oxide (ZnO) is driven by a strong desire for blue and ultraviolet light emitting devices. So far, however, the major shortcoming is the lack of reliable p-type doping. Interestingly, the unique properties of hydrogen have been identified as a significant problem that interferes with effective p-type doping. Recently, it was proposed that substitutional hydrogen accommodated at an oxygen site is the cause for the commonly observed n-type conductivity. Theory suggests that this complex is thermally stable up to about 500 °C [1]. In this paper we present an extensive study on the role of hydrogen for electrical transport. For this purpose single crystal ZnO samples with a c-axis orientation were homogeneously hydrogenated using an ampoule hydrogenation. For electrical measurements titanium contacts where evaporated. These contacts showed ohmic behavior in a temperature range from 20 to 300 K. The electrical transport properties of hydrogenated and as-grown ZnO samples were investigated with temperature dependent Hall-effect measurements. Hydrogenation causes an increase of the electron concentration, n, by about 3×10^17 cm^-3 at room temperature. This is accompanied by a decrease of the mobility, µ, from 1100 to about 250 cm^2/Vs; the temperature at which the maximum value of µ occurred increased from 70 to 100 K. This observation confirms that H acts as a shallow donor. The hydrogenated samples are not stable; n and µ decrease and increase with time, respectively. To study the kinetics of this process the time and temperature dependence of the recovery of n and µ were measured. For this purpose the samples were isothermally annealed and subsequently T-dependent Hall-effect measurements were performed. With increasing annealing time the electron concentration decreases and the mobility increases due to a decrease of the concentration of shallow H donors. The temperature dependence of the recovery of n and µ exhibits an activated behavior with an activation energy of 0.9 eV. Interestingly, neither n nor µ approach the original values observed for as-grown ZnO. It is likely that the difference is related to a more stable H-related shallow donor complex such as substitutional H at an oxygen site. [1] Anderson Janotti and Chris G. Van de Walle, Nature Materials 6, 44 (2007)
10:30 AM - **B7.4
Ga:ZnO based Transparent Contacts for Organic Photovoltaics and Light Emitters.
Joseph Berry 1 , Christopher Gorrie 2 , Matthew Reese 1 , Dana Olson 1 , Paul Burrows 3 , David Ginley 1
1 , National Renewable Energy Laboratory, Golden , Colorado, United States, 2 Department of Materials Science and Engineering, University of Florida, Gainesville, Florida, United States, 3 , Reata Research, Kennewick, Washington, United States
Show Abstract11:30 AM - **B7.5
Carrier Transport in Homo- and Heteroepitaxial Zinc Oxide Layers.
Klaus Ellmer 1
1 SE5, Helmholtz-Zentrum für Materialien und Energie, Berlin Germany
Show Abstract12:00 PM - B7.6
Enhancement-mode ZnO Thin-film Transistors Grown by MOCVD.
Jungyol Jo 1 , Junho Yun 1 , Haemi Kim 1
1 Electrical and Computer Engineering, Ajou University, Suwon Korea (the Republic of)
Show AbstractWe developed a new method to realize enhancement-mode zinc oxide (ZnO) thin-film transistors (TFT) by metalorganic chemical vapor deposition (MOCVD). We used growth interruptions during MOCVD to encourage complete oxidation of deposited ZnO film. With this method, turn-off characteristics were significantly improved. Our ZnO TFT showed 1E7 on/off ratio with 18 cm2/Vsec mobility, and +5 V threshold voltage.ZnO has attracted considerable attention because it can be used for transparent electronic circuits and light emitting devices. Among various growth methods, MOCVD has advantages for industry, since it can be applied to large-size substrates more easily. The problem associated with MOCVD is that turn-off characteristics of ZnO TFT is not good compared to those grown by other methods. ZnO grown by MOCVD usually shows oxygen deficiency. We thought that the oxygen deficiency could be improved if we allow sufficient oxidation time by using growth interruptions. Our MOCVD system has a horizontal reactor operating at atmospheric pressure. Diethylzinc (DEZ) and O2 were used as sources. ZnO films were grown on heavily doped n-type Si substrates with a thermal oxide of 110 nm thickness. The Si substrate works as a bottom gate. The growth temperature was 450 degree C, and the thickness of ZnO film was 30 to 50 nm. TFT’s were fabricated by evaporating Al through a shadow mask, and the channel is 15-μm long and 500-μm wide. Current-voltage characteristics were measured by using Keithley 2400 sourcemeters.We studied the effect of growth interruptions by comparing turn-off characteristics of ZnO TFT’s. Sample A was grown with 3 interruptions, and sample B with 5 interruptions. After each 20-sec ZnO growth, DEZ was closed, and O2 and N2 were supplied for 2 min.Sample B showed smaller turn-off current, even though sample B is thicker than sample A. The larger current of sample A indicates that current does not depend on film thickness. We explain that our results are related to oxygen depletion at the front surface [1], which works as n-type doping for the channel. Since our device is a bottom-gate structure, ZnO/SiO2 interface is the conduction channel. In sample A, the front surface is close to the channel, and channel electron density will be higher, resulting in a negative threshold voltage. When the front surface is separated from the channel in sample B, the effect of front-surface doping will be reduced, and positive shift of threshold voltage is observed.After optimizing growth parameters, we obtained high quality ZnO by MOCVD. ZnO TFT grown at 450 degree C showed 1E7 on/off ratio, +5V threshold voltage, and 18 cm2/Vsec mobility. Our results show that MOCVD is a good candidate to grow ZnO for commercial applications.This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD, KRF-2007-313-D00488).[1]. F. Trani, M. Causà, D. Ninno, G. Cantele, and V. Barone, Phys. Rev. B 77, 245310 (2008).
12:15 PM - B7.7
Improved Properties of ZnO by Codoping with Al and In.
Steven Kirby 1 , R. Hennig 1 , R. van Dover 1
1 Materials Science and Engineering, Cornell University, Ithaca, New York, United States
Show AbstractWe employ a combinatorial approach to show that codoping the transparent conductor Al:ZnO with a small concentration of In dramatically improves the electrical properties of ZnO through size compensation of the dopant ions. Cosputtering from three off-axis sputtering sources allows for deposition of ZnO with a range of Al and In doping concentration on a single substrate. We use this technique to obtain a ZnO sample with only 0-7 at.% Al, one with only 0-7 at.% In and one with both Al and In in the same range. The films were deposited with a substrate temperature held at 250°C at a chamber pressure of 30 mTorr of Ar/40 mol.% O2. These process parameters result in undoped ZnO that is insulating; i.e., with few native defects such as oxygen vacancies. Hall measurements and x-ray diffraction have been used to analyze the films. The Al-doped sample was found to have a minimum resistivity of 0.1 Ω-cm at about 1.5 at.% Al, above which the mobility decreases rapidly. The In-doped samples showed a monotonic decrease in resistivity as the doping increased to 7 at.% with the lowest resistivity comparable to that of the best Al doping level. The codoped sample exhibited an order of magnitude improvement in conductivity. For Al contents above 1.5% the carrier concentration was found to increase rapidly when a small amount of In was incorporated. In the codoped samples, a high Al content could be incorporated without decreasing the mobility. This remarkable effect is attributed to the size compensation (Al ions are smaller than Zn, while In ions are larger) inferred by measuring the volume of the unit cell as a function of composition.One of the central issues with the use of Al-doped ZnO is the tendency for the resistivity to increase unacceptably when thin films are processed in air at moderate temperatures largely due to reduction of oxygen vacancies. In order to gain further understanding of the role of dopants, samples were annealed in various atmospheres. Preliminary results show that annealing at 300°C in vacuum (5 uTorr) or in 6 mol.% H2 in Ar improves the conductivity by an order of magnitude, presumably by introducing O vacancies. We tested the robustness of the films to processing by annealing samples in air at 200°C, while monitoring the resistance. The ZnO film doped with 1.5 at.% Al (optimally) showed a resistivity increase of 40% in 1 hour while a codoped sample (1.5 at.% Al and 1.5 at. % In) showed an increase of less than 1%. This shows that codoping can dramatically improve the stability of the electrical properties of ZnO. We conclude that codoping yields improved dopant activation over a larger oxygen activity than doping with a single dopant. These results are being compared to density functional theory calculations. Our combinatorial studies are leading to a deeper understanding of the role of dopants in ZnO and their interaction with each other and native defects.
12:30 PM - **B7.8
LED and 2DEG in ZnO/(MgZn)O Heterostructures.
Masashi Kawasaki 1 2 3
1 WPI Adv. Inst. Mater. Res. (WPI-AIMR), Tohoku University, Sendai, 980-8577, Japan, 2 CMRG, ASI, RIKEN, Wako, 351-0198, Japan, 3 CREST, JST, Tokyo, 102-0075, Japan
Show AbstractAtomically defined heterostructures composed of ZnO and (MgZn)O were grown by pulsed laser deposition (PLD) and plasma-assisted molecular beam epitaxy (MBE). PLD films had been used for making p-ZnO/n-ZnO light emitting diodes (LEDs) [1] but it was difficult to extend the research for making p-(MgZn)O due to impurities in the target. MBE enabled us to make highly pure (MgZn)O and nitrogen doped p-(MgZn)O on +c ZnO substrate. Ultraviolet electroluminescence could be clearly observed for LEDs having p-(MgZn)O and ZnO active layers. This technology also drastically improved the mobility of two-dimensional electron gas accumulated at the ZnO/(MgZn)O interfaces. The highest value of the mobility exceeds 20,000cm2/Vs, giving threefold improvement from that grown by PLD [2]. Zero-resistance in Shubnikov-de Haas oscillation and clear plateau in quantized Hall resistance were observed in MBE grown heterostructures.Part of this work was carried out by the collaboration with A. Tsukazaki, A. Ohtomo, K. Nakahara, S. F. Chichibu.[1] A. Tsukazaki et. al., Nature Materials 4, 42 (2005)[2] A. Tsukazaki et. al., Science 315, 1388 (2007)
B8: ZnO Spintronics and P-type ZnO
Session Chairs
Wednesday PM, December 03, 2008
Room 203 (Hynes)
2:30 PM - **B8.1
Spin-dependent Transport in ZnMnO/ZnO Heterostructures.
Norifumi Fujimura 1 , Keiichiro Masuko 1 , Atsushi Ashida 1 , Takeshi Yoshimura 1
1 Graduate School of Engr., Osaka Prefecture University, Osaka Japan
Show Abstract3:00 PM - B8.2
Magnetic Semiconductors for Negative Index of Refraction in the THz Regime.
Alkim Akyurtlu 1 , Adil-Gerai Kussow 2
1 Electrical and Computer Engineering, University of Massachusetts Lowell, Lowell, Massachusetts, United States, 2 Physics, University of Massachusetts, Lowell, Massachusetts, United States
Show AbstractNegative index metamaterials have received much attention in recent years and homogenous, low-loss, and isotropic designs are very desirable for applications. We have found that homogeneous indium oxide (IO) is a negative refraction index material if doped with Cr. In accordance with calculations, this magnetic semiconductor, or In2-xCrxO3, in its polycrystalline form, should possess a fully isotropic, strongly pronounced negative refraction index, at ~ 10 THz. The effect is due to the coexistence of the spin wave mode with the plasmonic mode, and both modes are activated by the electromagnetic field of the light, with simultaneous permittivity and permeability responses within the frequency band close to the ferromagnetic resonance. Theoretical analysis, as well as ab-initio calculations will be presented, along with preliminary experimental results.
3:15 PM - B8.3
Fabrication of Epitaxial ZnCoO Films Towards a Spin-Photonics.
Hitoshi Tabata 1 , Hiroaki Matsui 1 , Zhiyan Xiao 2
1 School of Engineering, The University of Tokyo, Tokyo Japan, 2 Research Institute of Electronics, Shizuoka University , Shizuoka University, Hamamatsu Japan
Show AbstractDiluted magnetic ZnCoO semiconductors are useful tools for studying basic physical problems and practical applications. The strong exchange interaction between localized Co2+ spins and the extended band states yields the large Zeeman splitting. Thus, ZnCoO is a promising candidate to investigate the correlation between spin and photon towards spin-dependent photonics. Unfortunately, photoluminescence (PL) intensity of ZnCoO decreases remarkably with increasing Co content, which prevents an investigation of spin-dependent PL. In this presentation, we report a recombination process involving a carrier relaxation in ZnCoO using magneto-PL spectroscopy. A PL intensity of band-edge emissions related excitonic transitions decreased remarkably with an increase of Co content up to 0.8%. In contrast, that the relative 3d intra-emissions of Co2+ ions increased despite non-allowed dipole transitions. In this work, we first suppose an “Auger process“ such that recombination energy of electron-hole pairs is transferred to Co 3d levels [Fig. 1(b)]. In this process, the PL intensity of transition-metal ions is strongly dependent on magnetic fields by a spin-splitting on the conduction band (C.B.), originating from the s-d interaction. Figure 1(b) shows dependence of Co 3d emissions on a magnetic field. PL intensity for all samples clearly decreased by around 3 Tesla. We also confirmed a spin-splitting on the C.B based on the s-d interaction from magneto-transport (positive MR) at low temperatures. We suggest that this PL quenching is related to an Auger mechanism attributed to spin selection between band states and localized Co spins, indicating that s,p-d hybridizations exist in ZnCoO for the realization of efficient energy transfer. Phys. Rev. B, 75 (2007) 014438. J. Appl. Phys. 103, (2008) 043504.
3:30 PM - B8.4
Investigation of Tunable Surface Plasmon Resonance (SPR) in Conductive Oxide Thin Films.
Mark Losego 1 , Alina Efremenko 2 , Crissy Rhodes 2 , Marta Cerruti 2 5 , Shereen Elhalawaty 4 , Terry Alford 4 , Dave Aspnes 3 , Stefan Franzen 2 , Jon-Paul Maria 1
1 Materials Science, North Carolina State University, Raleigh, North Carolina, United States, 2 Chemistry, North Carolina State University, Raleigh, North Carolina, United States, 5 , UC Berkeley, Berkeley, California, United States, 4 , Arizone State University, Tempe, Arizona, United States, 3 Physics, North Carolina State University, Raleigh, North Carolina, United States
Show AbstractDevices utilizing surface plasmon resonance (SPR) to sense chemical / biological species or probe molecular interactions at surfaces are well established. Most devices employ the interface between a thin metal film, such as gold or silver, and a dielectric substrate to generate the necessary surface plasmon wave. However, metal films are limited to specific surface chemistries and their opacity restricts multiplexing with complimentary spectroscopic techniques. To expand the capabilities of SPR systems, transparent conducting oxides are investigated as a possible alternative materials set. In this work the SPR characteristics of indium tin oxide (ITO) thin films on glass substrates are investigated. ITO films are prepared by RF magnetron sputtering and crystallized in a controlled atmosphere tube furnace. Film microstructure and composition can be controlled through sputter parameters, particularly sputter pressure. This allows control of electron mobility (35 to 7 cm2/V*s), which strongly impacts the width of the SPR absorption band. Charge carrier density can be modified by controlling the partial pressure of oxygen during post deposition annealing. Controlling the number of charge carriers allows for manipulation of the SPR frequency, which can be shifted by more than 40%. Such control over SPR response is unprecedented in conventional devices using metallic films. We apply the Drude free electron model and the Fresnel equations to explain the observed SPR response. Because the derived model contains no fitting parameters (it uses only measurable values such as carrier concentration and mobility) and generates spectra that match extremely well with experimental data, it is proposed as a valid tool for the prediction and design of new SPR sensors using new material sets.
3:45 PM - B8.5
Stability and Structural Phase Transitions in Mn Doped ZnO.
Laxmikant Saraf 1 , P. Nachimuthu 1 , Zihua Zhu 1 , C. Wang 1 , M. Engelhard 1 , Donald Baer 1
1 Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland , Washington, United States
Show AbstractDoping in ZnO is of great interest due to a variety of thermoelectric, optical, magnetic and dielectric applications associated with this wide band gap semiconductor. In this study, we explore the stability, solubility and structural phase transitions in Mn doped ZnO by comparing results from highly oriented Mn:ZnO films by MOCVD and Mn:ZnO nanoparticle synthesis by sol-gel process. Based on analysis by Micro-XRD and conventional surface and interface analysis techniques like XPS, HRTEM and ToF-SIMS, we attempt to pinpoint the limits of Mn dopant solubility as a function of growth and microstructure. We observe that even though Micro-XRD reveals systematic Mn:ZnO–ZnMnO3– ZnMn2O4 structural phase transition as a function of Mn incorporation, overall Mn variation in EDS chemical mapping as a function of doping may indicate Mn based sub-stoichometric phases with higher x-ray scattering cross section. We correlate the results with earlier observation of suppression of conductivity in oriented Mn:ZnO films grown by MOCVD to study mobility suppression dynamics. These results also will be discussed from a viewpoint of enhanced chemical reactivity in nanoparticles as oppose to oriented films.
4:30 PM - **B8.6
P-Type Conduction and Light-Emitting Diodes Using Phosphorus-Doped ZnO.
David Norton 1 , Hyunsik Kim 1 , Kyeong-Won Kim 1 , Fernando Lugo 1 , Stephen Pearton 1 , Yu-Lin Wang 2 , Fan Ren 2
1 Materials Science and Engr, University of Florida, Gainesville, Florida, United States, 2 Chemical Engr, University of Florida, Gainesville, Florida, United States
Show AbstractA significant challenge in developing ZnO LEDs is the formation of p-type material. In recent studies, we have investigated the growth of p-type ZnO in P-doped ZnO grown by pulsed laser deposition (PLD). As-grown ZnO:P samples showed n-type characteristics, presumably due to the formation of anti-site PZn defects. Rapid thermal annealing yielded a carrier type conversion from n- to p-type for the ZnO:P films grown at ~ 700 C; samples grown at substantially lower or higher temperatures tended to remain n-type even after thermal annealing process. The properties and behavior of the n-to-p conversion is most consistent with the formation of PZn-2VZn as the active acceptor state. Variable magnetic field Hall measurements confirmed the p-type behavior. Phosphorus doping concentrations in the range of 0.5-1.0 at,% were considered, with evidence for P segregation in the higher phosphorus concentrations. The synthesis and properties of ZnO LED heterojunctions employing these P-doped ZnO materials will also be discussed. ZnO-based light emitting diodes were fabricated on c-plane sapphire using ZnO:P/ Zn0.9Mg0.1O/ZnO/ Zn0.9Mg0.1O/ZnO:Ga p-i-n heterostructures. There were no diode characteristics or light emission observed unless the structures were annealed at 350 C in oxygen after fabrication. A decrease in leakage currents in as-fabricated structures was achieved via low temperature oxygen annealing. Annealed p-i-n heterojunction diodes diodes showed band-edge electroluminescence and a broad defect band with a peak at 930nm at room temperature. The electroluminescence spectra shows deep level emission at low bias, but near band edge ultraviolet emission at high voltage bias. Properties of the materials and devices, along with prospects for significant improvement in LED performance will be discussed.
5:00 PM - B8.7
Transparent Polymer Schottky Contact on ZnO and Related Materials.
Masaki Nakano 1 , Atsushi Tsukazaki 1 , Kazunori Ueno 2 , Ryosuke Gunji 1 , Takayuki Makino 2 , Akira Ohtomo 1 , Tomoteru Fukumura 1 , Shunsuke Akasaka 3 , Hiroyuki Yuji 3 , Ken Nakahara 3 , Masashi Kawasaki 1 2 4
1 , Institute for Materials Research, Tohoku University, Sendai Japan, 2 , WPI Advanced Institute for Materials Research, Tohoku University, Sendai Japan, 3 , Advanced Compound Semiconductors R&D Center, ROHM Co., Ltd., Kyoto Japan, 4 , CREST, Japan Science and Technology Agency, Tokyo Jordan
Show AbstractTo date, developments of transparent conducting oxides have been mostly devoted to flat panel display applications including transparent electrodes and thin film transistors. The development of additional applications will be enabled by novel processing techniques and device concepts, which benefit from an overwhelming superiority to current device performance and a decrease in the cost of processing. ZnO is one of the promising materials for realizing light-emitting diodes. Besides its well-known unique optical properties, superior electronic properties have been proven through the observation of quantum Hall effect of two-dimensional electron gas (2DEG) confined at ZnO / MgxZn1-xO interface [1]. In order to develop various optoelectronic devices, we show that a transparent conducting polymer, poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), is of high-quality, low-cost, and easy-to-make Schottky contact on ZnO surface [2]. It is compatible to simple spin-coating process and has low resistivity of about 10-3 Ωcm and an internal transmittance nearly 100% in a wide range of wavelength from 250 to 1000 nm. By using this polymer, we have fabricated a visible-blind photodiode and a metal / semiconductor field-effect transistor (MESFET). PEDOT:PSS aqueous solution was spin-coated on the ZnO surface. Photodiode properties were examined at room temperature by using a monochromatic Xe lamp as a light source. The device showed good rectifying behavior under dark condition with a rectification ratio as high as 1010 at ±2 V and low ideality factor of 1.02. Spectral response of the photodiode shows good UV / visible rejection ratio of about 103 and high quantum efficiency in UV region of about unity, indicating the present junction is of high-quality and defect-free. In addition, field-effect modulation of the 2DEG was successfully demonstrated in MESFET structure, opening a door to realize high-electron mobility transistor applications. [1] A. Tsukazaki et al., Science 315, 1388 (2007)[2] M. Nakano et al., Appl. Phys. Lett. 91, 142113 (2007)
5:15 PM - B8.8
Bilayered (ITO / ZnGa0.05O / Glass) Films for Transparent Electrode Application.
Titas Dutta 1 , Jagdish Narayan 1
1 , North carolina state university, raleigh, North Carolina, United States
Show AbstractZnO doped with group III elements is a promising candidate for transparent conducting oxides (TCOs) 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 indium tin oxide (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 novel bilayered structure consisting of thin ITO layer on Zn0.95Ga0.05O film for transparent electrode applications. Pulsed laser deposition (PLD) technique has been used to deposit the heterostructures (ITO / ZnGa0.05O). To understand the growth characteristics of the thin ITO layer and get the optimized condition for TCO application, few monolayers of ITO were grown on ZnGa0.05O templates at different temperatures and oxygen partial pressure. The characteristics of the ITO 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 the layer was carefully controlled by controlling the repetition rate and the number of pulses. The smooth and uniform surface layer is the key for superior device performance. This is achieved by controlling the growth parameters. It is envisaged that the overall transmittance and the resistivity are dictated by the thicker layer of ZnGa0.05O beneath the ITO layer. Hence, this study of (ITO / ZnGa0.05O) heterostructure is aimed to improve the surface characteristics (work function) without affecting the overall transmittance and sheet resistance. This will enhance the transport of the carriers across the heterojunction in the device, thus, resulting in the increase in device efficiency.
5:30 PM - B8.9
Studies on Electrical Transport in p-ZnO/p-Si Heterojunction.
Sayanee Majumdar 1 , Pallab Banerji 1 , Sourav Chattopadhyay 2
1 Materials Science Centre, Indian Institute of Technology, Kharagpur, West Bengal, India, 2 Department of Physics & Meteorology, Indian Institute of Technology, Kharagpur, West Bengal, India
Show AbstractIn the present work, we report fabrication of p-ZnO/p-Si heterostructures formed by pulse laser deposition. The p-type doping of as-grown ZnO has been done by nitrogen with urea as a source of nitrogen. The pulsed laser deposition of p-ZnO on p-Si was carried out with 248 nm KrF laser at energy 300 mJ. At first a ZnO buffer layer had been deposited at 3500C substrate temperature and vacuum (10^-5 mbar) using 300 shots. Then nitrogen (urea) doped ZnO thin film had been deposited at 6000C substrate temperature at 10^-1 mbar oxygen pressure using 2000 shots at frequency 10 Hz.The urea doped ZnO was examined for its conductivity type. The film deposited was found to be p-type with resistivity 5.0 Ω-cm and carrier concentration 1.3×10^18 cm^-3. The X-ray diffraction clearly showed (002) and (201) diffraction peaks of ZnO and (400) peak of Si.The current density-voltage (J-V) characteristics of a typical p-ZnO/p-Si heterojunction measured at room temperature exhibited a diode-like behavior with turn-on voltage 0.5 V. The ideality factor η was found to be greater than 2 at room temperature. The observed large ideality factors exclude several possible forward current transport mechanisms across the junction barrier, namely thermionic emission, minority carrier injection, and recombination degeneration. The reverse current at room temperature as a function of (Vbi-V)^1/2 in the range of 0.5 V < V < 1.2 V was found to be almost linear, where Vbi, was the built-in potential whose value was measured to be 0.9V by C-V. If the generation current in the Si depletion region dominated under reverse bias, the reverse current should be proportional to the width of the depletion region (which varies as (Vbi-V)^1/2 in the case of abrupt junctions). This was observed experimentally and therefore it was reasonable to assume that the reverse current in our p-ZnO/p-Si junctions was mainly due to the generation current in the Si depletion region.C-V analysis was done at 1MHz as a function of reverse bias at room temperature. The 1/C^2 versus V plot revealed a good linear nature indicating that the abrupt heterojunction theory is indeed applicable to the p-ZnO/p-Si structure. The intercept of 1/C^2–V gave a value of 0.9V which was essentially equal to the diffusion potential within the Si. The slope of the plot gave NA = 9.98 X 10^20 cm^-3 which was consistent with the resistivity of the p-Si used. The depletion width was found to be 0.9 nm. The band diagram of p-ZnO/p-Si was proposed and shown in the Figure below. The electron affinity of Si and ZnO were 4.05eV and 3.7eV respectively which yielded a conduction band offset of 0.35eV. The valanec band offset was found to be 2.6eV. The barrier height calculated from reverse saturation current was 0.74 eV and matched well with others. The heterojunction band diagram is proposed.
5:45 PM - B8.10
NH3 Doping in MOCVD Growth of ZnO Thin Films.
Tahir Zaidi 1 , Andrew Melton 1 , Nola Li 1 , William Fenwick 1 , Hongbo Yu 1 , Ian Ferguson 1
1 Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show Abstract
Symposium Organizers
John D. Perkins National Renewable Energy Laboratory
Thomas O. Mason Northwestern University
John F. Wager Oregon State University
Yuzo Shigesato Aoyama Gakuin University
B9: Nanotube TCs and Atomic Layer Deposition
Session Chairs
Thursday AM, December 04, 2008
Room 203 (Hynes)
9:30 AM - B9.1
Effects of Nanotube Conductivity-Type and Chemical Doping on Transparent Single-Wall Carbon Nanotube Networks.
Teresa Barnes 1 , Jeff Blackburn 1 , Jeremy Bergeson 1 , Rob Tenent 1 , Jao van de Lagemaat 1 , Mike Heben 1 , Tim Coutts 1
1 , National Renewable Energy Lab, Golden, Colorado, United States
Show AbstractSingle-wall carbon nanotube (SWCNT) networks exhibit high electrical conductivity and optical transmittance, enabling their use as transparent contacts in photovoltaic devices. Although SWCNT networks already yield high quality transparent contacts, their optoelectronic performance could be substantially improved by increasing conductivity in the networks. Several factors, including inter-tube/bundle junctions, tube/bundle length, the presence of residual surfactants, doping, and nanotube conductivity (semiconducting or metallic) type are thought to affect conductivity in the networks. We have recently demonstrated the ability to separate SWCNTs by chirality (and therefore conductivity type) and to use these “type-pure” nanotubes to form transparent networks.1 Here, we focus on how changing the abundance ratio of semiconducting and metallic nanotubes and chemical doping affect the temperature dependence of the conductivity. Temperature dependant conductivity data can be combined with optical spectroscopy and temperature programmed desorption data to show that the observed decrease in conductivity with increasing temperatures above 325K is due to dedoping. We also present a model for the temperature dependence for all of the networks below 325 K and for chemically dedoped networks at all temperatures. This modeling and previous work indicates that doped networks comprised primarily of semiconducting nanotubes exhibit the highest conductivity while networks enriched in metallic tubes show the best temperature stability of their conductivity.21.Blackburn, J. L.; Barnes, T. M.; Beard, M. C.; Kim, Y.-H.; Tenent, R. C.; McDonald, T. J.; Coutts, T. J.; To, B.; Heben, M. J., Transparent Conductive Single-walled Carbon Nanotube Networks with Precisely Tunable Ratios of Semiconducting and Metallic Nanotubes. ACS Nano (2008), in press.2.Barnes, T. M.; Blackburn, J. L.; Van de Lagemaat, J.; Heben, M. J.; Coutts, T. J., Temperature effects on resistivity in metallic and semiconducting transparent carbon nanotube networks. ACS Nano (2008), under review.
9:45 AM - B9.2
Influence of Single-Wall Carbon Nanotube Length on the Optical and Conductivity Properties of Thin ‘Buckypaper’ Films.
Daneesh Simien 1 , Jeff Fagan 1 , Jack Douglas 1 , Kalman Migler 1 , Jan Obrzut 1
1 Polymers Division 854, National Institute of Standards and Technology, Gaithersburg, Maryland, United States
Show AbstractThin layers of length-sorted single wall carbon nanotubes (SWCNT), formed through filtration from a dispersing solvent onto a filter substrate (‘buckypaper’), exhibit sharp changes in their optical and conductivity (σ) properties with increasing SWCNT surface coverage. Longer tubes are found to be more transparent and conducting at the same coverage at optical frequencies and the IR absorption increases linearly with SWCNT concentration, regardless of SWCNT length. We show that changes of σ with SWCNT concentration can be quantitatively described by generalized effective medium (GEM) theory. The scaling exponents describing the ‘percolation’ transition from an insulating to conducting state with increasing concentration are consistent with two-dimensional percolation theory, provided that the SWCNTs are reasonably long. The conductivity percolation threshold xc was also found to vary with particle aspect ratio L as, xc ~ 1 / L, a result that is also in agreement with the expectations of conductivity percolation theory. Our results provide a framework for engineering the properties of thin SWCNT layers for the numerous technological applications that are envisioned for buckypaper.
10:00 AM - B9.3
A Nanocomposite Thin Films as Novel Transparent Conductors: a Metalorganic CVD Route Stemming from Thermodynamic Analysis.
Sukanya Dhar 1 , Anil Mane 1 , Shalini Kandoor 1 , M. Sahana 1 , S. Shivashankar 1
1 Materials Res. Centre, Indian Inst. of Science , Bangalore, Karnataka, India
Show Abstract10:15 AM - B9.4
Solution-Processed Graphene Transparent Electrodes for Organic Solar Cells and Light-emitting Diodes.
Junbo Wu 1 , Hector Beccerril 2 , Mukul Agrawal 3 , Zunfeng Liu 4 , Yongsheng Chen 4 , Zhenan Bao 2 , Peter Peumans 3
1 Materials Science and Engineering, Stanford University, Stanford, California, United States, 2 Chemical Engineering, Stanford University, Stanford, California, United States, 3 Electrical Engineering, Stanford University, Stanford, California, United States, 4 , Nankai University, Tianjin China
Show Abstract10:30 AM - B9.5
Enhanced Outcoupling of Organic Light Emitting Devices via Metal Nanowire Mesh Transparent Electrodes.
Jung-Yong Lee 1 , Steve Connor 2 , Xiaoran Tong 3 , Yi Cui 4 , Stephen Forrest 3 , Peter Peumans 1
1 Electrical Engineering, Stanford University, Stanford, California, United States, 2 Chemistry, Stanford University, Stanford, California, United States, 3 Materials Science and Engineering, University of Michigan, Ann Arbor, California, United States, 4 Materials Science and Engineering, Stanford University, Ann Arbor, California, United States
Show AbstractSince the initial demonstration of efficient organic light emitting devices[1] (OLEDs), steady progress in luminous efficiency and device lifetime has further established the potential of OLEDs for next generation displays and solid-state lighting. Despite this marked progress, there is still ample room for improvement in luminous efficiency. In a conventional small molecular weight OLED, approximately 75% of the emitted photons are wasted since they are trapped in the substrate by total internal reflection, or coupled into waveguided and surface plasmon polariton modes [2]. Several schemes have been shown to improve the fraction of photons that are coupled into useful modes. Aperiodic dielectric stacks between the substrate and transparent anode are used to increase the coupling strength to unbound optical modes[2]. Imprinted microlenses increase outcoupling efficiency[3]. Although such schemes bring about improved performance, they require extra costly processing steps.Another issue that cannot be decoupled from the photon outcoupling challenge, is the need for a transparent conductive electrode (TCE) for OLEDs that exhibits none of the disadvantages of the commonly used indium tin oxide (ITO) TCEs. ITO is undesirable because of its cost which is partially attributed to the low deposition rate and cost of elemental Indium. Moreover, for films thick enough to achieve sheet resistances <10Ohm/sq, the optical transparency of ITO is limited to <60%. Finally, ITO films on flexible substrates readily crack when the substrate is bent, leading to device failure. Many efforts are underway to replace ITO[4,5], but none of the proposed schemes have demonstrated the required performance and low cost. We recently demonstrated that solution-processed random meshes of Ag nanowires form transparent and conductive electrodes with a performance that matches or exceeds that of ITO [6]. These Ag nanowire meshes are compatible with flexible substrates. Here, we demonstrate that such Ag nanowire mesh electrodes have additional optical benefits when used in OLEDs that lead to an up to 30% increase in luminous efficiency.[1] Tang, C. W. and Vanslyke, S. A., Appl. Phys. Lett. 51, 913-915 (1987).[2] Agrawal, M., Sun, Y., Forrest, S. R., and Peumans, P., Appl. Phys. Lett. 90, 241112-241113 (2007).[3] Sun, Y. and Forrest, S. R., J. Appl. Phys. 100, 073106 (2006).[4]Aguirre, C. M. et al., Appl. Phys. Lett. 88, 183104-183103 (2006).[5]Rowell, M. W. et al., Appl. Phys. Lett. 88, 233506 (2006).[6] Lee, J. Y., Connor, S. T., Cui, Y., and Peumans, P., Nano Lett. 8, 689-692 (2008).
10:45 AM - B9.6
Electrical Characterization of ZnO Nanorod Thin Film Transistors.
Rebecca Peterson 1 , Baoquan Sun 1 , Henning Sirringhaus 1 , Kiyotaka Mori 2
1 Cavendish Laboratory, University of Cambridge, Cambridge United Kingdom, 2 Cambridge Liaison Office, Panasonic R&D Centre of Europe, Cambridge United Kingdom
Show AbstractZinc oxide is an attractive semiconductor choice for transparent TFTs in active matrix displays because of its large bandgap, high mobility and low toxicity. We have previously described solution deposition of chemically synthesized ZnO nanorods, and have shown several ways to improve TFT device performance to obtain mobilities of ∼1 cm2V-1s-1 [1-3]. Here we explore the electronic structure of these films by temperature-dependent current-voltage characteristics and bias stress stability measurements. We propose that inter-nanorod grain boundaries limit the charge transport through these films, and that the high specific surface area of the nanorod layer limits its stability in air.ZnO nanorods with diameter ∼10nm and length ∼90nm are synthesized using a zinc acetate and KOH reaction at < 100°C as described earlier [1]. The nanorod solution is stabilized by addition of a ligand and diluted, then spin-cast onto a HMDS-treated SiO2 (300nm) / n++ Si substrate. Proper choice of ligand (octylamine) and solution concentration (∼10mg/mL) enables self-assembly of the nanorods into small domains with in-plane alignment, i.e. with the long axis of the rod parallel to the substrate surface [2]. The films are annealed in air at 270°C and aluminum electrodes are evaporated to form bottom-gate, top-contact TFTs with typical W/L=3mm/90μm. The resulting devices have mobilities of about 0.1-0.2 cm2V-1s-1, which can be increased to ∼1.2 cm2V-1s-1 by spin-casting a second chemical precursor layer consisting of zinc acetate dehydrate and aminoethanol in methoxyethanol before film annealing. Such double-layer devices exhibit ON/OFF ratios of 105-106 and VT of ∼-4V [3].The devices operate well in vacuum down to 80K with ION ≥ 1μA. To investigate the electronic structure we assume a mobility edge model with delocalized band-like conduction above the mobility edge and exponential tail-states below characterized by D∼exp(-E/E0) [4]. For the lower-mobility nanorod-only films the sub-bandgap DOS is slightly broader, E0=90meV, compared to double-layer films with E0=81meV. At high gate voltages, the Arrhenius activation energy EA reaches a minimum of 100meV for nanorod-only TFTs and 70meV for double-layer ones. Thus the addition of the second precursor layer to interconnect the nanorods seems to slightly reduce conduction-band tail state disorder and shifts the pinned Fermi level closer to the conduction band, providing higher mobility. However the relatively high E0 and minimum EA values [4] indicate that charge transport in these films is likely limited by the shared feature of inter-nanorod grain boundaries. We will also discuss the bias stress stability of double-layer TFTs in nitrogen and in air.[1] B. Sun and H. Sirringhaus, Nano Lett. 5, 2408 (2005). [2] B. Sun and H. Sirringhaus, J. Am. Chem. Soc. 128, 16231 (2006). [3] B. Sun, et. al., J. Phys. Chem. C 111, 18831 (2007). [4] K. Nomura, et. al., Appl. Phys. Lett. 85, 1993 (2004).
11:30 AM - **B9.7
Oxide Thin-Film Transistors With Gate Dielectrics Grown by Atomic Layer Deposition on Plastic Substrates.
Peter Carcia 1 , Robert McLean 1 , Michael Reilly 1
1 Research & Development, DuPont , Wilmington, Delaware, United States
Show AbstractElectronics on flexible plastic substrates are attractive for portable applications, because plastic is lighter weight and more rugged than glass. However, most plastics are temperature-sensitive. This restricts materials processing to below 200 C in device fabrication. Temperature-sensitive plastics thus preclude use of crystalline silicon and polycrystalline silicon semiconductors, and the lower processing temperature also compromises the performance of devices made with an amorphous silicon (a-Si) semiconductor. In this regard, organic semiconductors, and more recently investigated oxide semiconductors (e.g., ZnO), are emerging as an enablers of flexible electronics, because they can be processed at low temperature with good electronic properties. Oxide semiconductors with mobility >10 cm2/V-s are common, even at low processing temperature.One challenge for both organic and oxide semiconductor technologies on plastic is identifying a gate dielectric material and process that enable low voltage operation, reliability and stability with low device off-state current. Active electronic identification tags, for example, need sub-5 V operation, and organic light emitting diodes require high drive currents at typically less than 10 V. For active-matrix driven liquid-crystal displays, the static off-state current needs are 0.1-1.0 pA.In this presentation we report on high performance oxide thin film transistors (TFTs) with gate dielectrics grown by atomic layer deposition (ALD) on polyethylene napthalate (PEN) and Kapton® polyimide substrates. For ZnO TFTs on PEN, we obtained a saturation mobility of ~ 8 cm2/V-s with Vth=4.7 V and a gate leakage current less than 10 pA up to 10 V with an Al2O3 gate dielectric grown at 150 C by atomic layer deposition on PEN. Results for oxide devices on plastic with ALD HfO2 will also be presented.
12:00 PM - B9.8
Ultraviolet Electroluminescence from n-ZnO/SiO2-ZnO Nanocomposite/p-GaN and n-ZnO/p-GaN Heterojunction Light-Emitting Diodes at Forward and Reverse Bias.
Ying-Tsang Shih 1 , Shing-Chao Chen 1 , Mong-Kai Wu 1 , Miin-Jang Chen 1 , Hon Kuan 2 , Jer-Ren Yang 1 , Makoto Shiojiri 3
1 Materials Science and Engineering, National Taiwan University, Taipei Taiwan, 2 Optoelectronics Engineering, Southern Taiwan University, Tainan Taiwan, 3 , Kyoto Institute of Technology, Kyoto Japan
Show AbstractAtomic layer deposition (ALD) technique was implemented for the first time to grown high-quality ZnO films for the fabrication of n-ZnO/SiO2-ZnO nanocomposite/p-GaN and n-ZnO/p-GaN heterojunction light-emitting diodes(LEDs). The SiO2 layer embedded with ZnO nanodots (SiO2-ZnO nanocomposite) was prepared using spin-on coating of SiO2 nanoparticles together with ALD. The n-type Al-doped ZnO layer was deposited also by ALD. The current vs. voltage (I-V) characteristics of both the n-ZnO/SiO2-ZnO nanocomposite/p-GaN and n-ZnO/p-GaN structure exhibit rectifying, diode-like behavior. The SiO2-ZnO nanocomposite layer in the n-ZnO/SiO2-ZnO nanocomposite/p-GaN structure leads to a larger forward-bias series resistance and a smaller reverse-bias leakage current than the n-ZnO/p-GaN LED. Significant room-temperature ultraviolet (UV) electroluminescence (EL) from ZnO have been observed from both the n-ZnO:Al/SiO2-ZnO nanocomposite/p-GaN:Mg and n-ZnO/p-GaN LEDs at low injection current. The low refractive index and optical scattering of the SiO2-ZnO nanocomposite layer may result in the increase in the light extraction efficiency from the n-ZnO layer. The impact ionization in GaN, ZnO, and GaN:Mg states causes a significant UV emission at the reverse breakdown bias. The negligible defect-related bands in the EL and photoluminescence(PL) spectra and the significant UV emission from ZnO at low injection current reveal that the ALD technique is applicable to deposit high-quality ZnO films for next-generation UV LEDs and lasers.
12:15 PM - B9.9
Growth and Characteristics of ZnO Nanotube Arrays on Si Substrate by Atomic Layer Deposition in Anodic Aluminum Oxide.
Miao Wang 1 , Han Gao 2 , Carl V. Thompson 1 3 , Soo Jin Chua 1 2
1 , Singapore-MIT Alliance, Singapore Singapore, 2 , Institute of Materials Research and Engineering , Singapore Singapore, 3 , Department of Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States
Show Abstract Atomic layer deposition (ALD) is a gas-phase thin film deposition method characterized by sequential surface-saturating reactions combined with alternately dosing the precursors on substrates. Owing to its self-limiting growth mechanism, the ALD technique enables growth of uniform and high quality very thin film over a large area at low temperatures. Anodic aluminum oxide templates are composed of highly uniform, nanometer-scale pores arranged in a closed-packed array. Depositing conformal films inside the nanopores can be achieved through ALD. In this study, ZnO nanotube arrays were successfully deposited in anodic aluminum oxide templates on Si substrates. The zinc precursor diethylzinc (DEZ) and the oxygen precursor (H2O) are pulsed alternately into the deposition chamber using nitrogen as a carrier gas with a flow rate of 200 sccm. Each cycle is separated by nitrogen purges. Typical pulse times for DEZ and H2O feeds were 0.5s and 1.5s, respectively, and the purging time between the reactants was 60s. The substrate temperature was kept at 150°C. The morphology, optical properties and crystallinity of the nanotube arrays were analyzed using scanning electron microscope, photoluminescence, transmission electron microscope and X-ray diffraction. The ZnO nanotubes had outer diameters of 70~80 nm, which is constrained by the diameters of the AAO pores; and inner diameters of 40~50 nm, which can be precisely controlled by the number of precursor pulse/purge cycles. The room temperature PL spectrum of this sample reveals a sharp UV emission from excitons at the wavelength of 370 nm, and a broad visible light emission from intrinsic defects ranging from blue to red was observed. Post annealing is able to remove the defects and enhance the UV emission intensity. By varying the wall thickness, a shift of UV peak position can be detected. The mechanism of this shift, probably due to quantization, is under further investigation.
12:30 PM - B9.10
Fabrication of Ga-doped MgZnO-based Transparent Electrodes by Molecular Precursor Method for GaN-based UV LED.
Tohru Honda 1 , Yoshihiro Mashiyama 1 , Shigetoshi Komiyama 1 , Mitsunobu Sato 2
1 Dept. of Electronic Eng., Kogakuin University, Hachiohji, Tokyo, Japan, 2 Coordination Engineering Laboratory, Kogakuin University, Hachiohji, Tokyo, Japan
Show AbstractMgZnO thin films were fabricated by molecular precursor method (MPM) for the realization of low-cost near-ultraviolet (UV) transparent electrodes for GaN-based UV light-emitting diodes (LEDs). The MgZnO fabrication by MPM requires a common solvent for Zn and the Mg precursor mixture solution. We find that an ammonia solution is one of the common solvents. The base solution was prepared by the reaction of Zn-nitrilotriacetic acid (nta) complex, which was obtained from the reacted NTA aqueous solution with zinc acetate, with butylamine in ammonia aqueous. The dosed solution was prepared by the reaction of Mg-ethylenediamine-N,N,N’,N’-tetraacetic acid (edta) complex. Then, the dosed solution was mixed into the base precursor solution. Here, an amount of the dosed solution (Mg-precursor) was controlled. MgZnO thin film was fabricated on the quartz glass substrate using the MgZnO precursor solution. The Mg composition in the films was estimated using the x-ray photoelectron spectroscopy (XPS). The solid phase composition of Mg in the film is 0.4-0.5 of the liquid phase molar fraction. The X-ray diffraction patterns indicate the films have a hexagonal single phase as the same as ZnO. Ga-doped ZnO and MgZnO films fabricated by MPM using the ammonia solution were also investigated. The possibility of MgZnO films for the application to UV transparent electrodes on GaN-based UV LEDs is discussed. In the case of a GaN emission layer (365 nm), the absorption spectra of the MgZnO films indicate that the amount of Mg of 20 mol% in the MgZnO solution was required for the application to the GaN-based UV LED. Here, excitonic absorption peaks in the MgZnO films affect the limitation of the transparent spectral region. On the other hand, the resistivity of MgZnO films depended on the Mg molar fraction. In the highly Ga-doped ZnO films, c-axis orientation was observed from the XRD and RHEED patterns.
12:45 PM - B9.11
Good Conformability of Indium-tin-oxide Thin Films Prepared by Spray CVD using Inorganic Source.
Hiroshi Funakubo 1 , Takeshi Kondo 2 , Yutaka Sawada 2 , Kensuke Akiyama 3 , Kiguch Takanori 4 , Shigeyuki Seki 5 , Koichi Haga 5 , Minhan Wang 2 , Takayuki Uchida 2
1 , Tokyo Institute of Technology, Yokohama Japan, 2 Center for Hyper Media Research, Tokyo Polytechnic University, Atsugi Japan, 3 , Kanagawa Industrial Technology Center, Ebina Japan, 4 , Tohoku University, Sendai Japan, 5 , Sendai National College of Technology, Sendai Japan
Show AbstractIndium-tin-oxide (ITO; tin-doped In2O3) transparent conducting films are widely used for flat panel displays, transparent heaters, and infrared shielding of buildings and automobiles. Conformal ITO films are favorable for the realization of high density transparent devices. Conformal deposition of patterned substrates is relatively easy with the CVD process, though it is difficult with PVD. Recently, ITO films have been successfully deposited by spray chemical vapor deposition (CVD) using an ethanol solution of indium (III) chloride and tin (II) chloride [1-2]. The advantages of this method are of their simple and inexpensive process operated even under air without vacuum system. In the present study, homogeneous ITO films with good step coverage in view of composition, crystallinity and the resitivtity were deposited on stripe-patterned substrates by original spray CVD using inexpensive inorganic materials as source materials.ITO films were deposited at 200-400oC on stripe-patterned Si substrates by an original spray CVD. The step coverage was almost constant to be about 70% above 280oC corresponding to the mass transport limited region, but drastically improved with decreasing deposition temperature below 250oC in the reaction-limited region. Finally it reached above 85% and 90% for the opening width of the 0.6 μm and 1 μm (correspond to the aspect ratio of 1.7 and 1), respectively when the films were deposited at 200oC. The homogeneous ITO films in view of composition [the Sn/(In+Sn) ratio], crystallinity, and resistivity were deposited on stripe-patterned Si substrates at approximately 200oC.The present results show the possibility of the ITO film deposition with good conformability by spray CVD process.[1] Y. Sawada, C. Kobayashi, S. Seki, H. Funakubo, Thin Solid Films 409 (2002) 46.[2] Y. Sawada, Mater. Sci. Forum 437/438 (2003) 23.
B10: Towards Amorphous Oxide Electronics
Session Chairs
Peter Carcia
John Perkins
Thursday PM, December 04, 2008
Room 203 (Hynes)
2:30 PM - **B10.1
Electrical Characteristics and Practical Properties of Amorphous Indium Zinc Oxide and Related Materials.
Koki Yano 1 , M. Kasami 1 , F. Utsuno 1 , K. Inoue 1 , B. Shinozakia 2 , K. Makiseb 3 , M. Funaki 2
1 , Advanced Technology Research Laboratories, Idemitsu Kosan Co. Ltd, Chiba Japan, 2 Department of Physics, Kyushu University, Fukuoka Japan, 3 , .National Institute for Material Science Ibaraki, Tsukuba Japan
Show Abstract3:00 PM - B10.2
Amorphous Oxide Semiconductor Circuits.
Eric Sundholm 1 , Brian McFarlane 1 , Peter Kurahashi 1 , Rick Presley 1 , Daniel Heineck 1 , John Wager 1
1 School of Electrical Engineering and Computer Science, Oregon State University, Corvallis, Oregon, United States
Show Abstract The majority of research related to amorphous oxide semiconductors (AOS) has been devoted to the development of materials and devices, particularly channel layers and thin-film transistors (TFTs), respectively. In contrast, relatively few reports exist today on the use of AOS for circuits in the context of transparent electronics, or more broadly, oxide electronics. Given the performance and device stability of AOS TFTs compared to that of organic or amorphous silicon TFTs, AOS circuits appear to be attractive for a wide range of low-cost, large-area, flexible, and/or transparent applications. The objective of this presentation is to provide an overview of progress towards the realization of AOS circuits. Examples of digital, analog, and power AOS circuits to be discussed in this presentation include multiplexers / de-multiplexers, oscillators, and rectifiers, respectively. In addition to presenting data related to circuit design, simulation, fabrication, and performance, issues involving material selection and process integration will be addressed.
3:15 PM - B10.3
Fabrication of IGZO Channel Thin Film Transistor with BZN Gate Insulator.
Tomohiro Oiwa 1 , Yohei Kondo 1 , Mitsuru Nakata 1 , Eisuke Tokumitsu 1
1 Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama-City, Kanagawa Japan
Show Abstract3:30 PM - B10.4
Transparent Non-volatile Memory using Platinum Nanocrystals Embedded in an Amorphous IGZO Thin Film Transistor.
Arun Suresh 1 , Steven Novak 1 , Patrick Wellenius 1 , Veena Misra 1 , John Muth 1
1 Electrical and Computer Engineering, North Carolina State University, Raleigh, North Carolina, United States
Show AbstractAmorphous oxide semiconductors (AOS) have attracted considerable attention because of their unique material properties including high field effect mobility, low processing temperatures and transparency. AOS based high performance thin film transistors have been demonstrated for flexible and transparent electronics. Another basic device that can be envisioned for such applications is a memory element. In this work we have developed a transparent non-volatile memory device based on indium gallium zinc oxide (IGZO) thin film transistors. Atomic layer deposition was used to deposit platinum nanocrystals within the transistor dielectric which acts as a floating gate. The memory effect was determined by the hysteresis in the transistor transfer characteristics and is mainly caused by electrons being trapped in the Pt nanocrystals. Large memory operation windows have been observed. Effects of changes in the tunneling oxide and the Pt nanocrystal density on the memory effect have also been explored.
3:45 PM - B10.5
Environmental Stability of Amorphous In-Zn-O Transparent Conductors.
Thomas Gennett 1 3 , Ann Deml 2 3 , Dane Gillaspie 3 , Reuben Collins 3 , John Perkins 3 , David Ginley 3
1 Chemistry, Rochester Institute of Technology, Rochester, New York, United States, 3 , National Renewable Energy Laboratory, Golden, Colorado, United States, 2 Physics, Colorado School of Mines, Golden, Colorado, United States
Show Abstract4:30 PM - **B10.6
Low-Temperature Zinc Indium Oxide Thin-Film Transistors: Progress Toward Roll-to-Roll Manufacturing.
Tim Koch 1 , Randy Hoffman 1 , Timothy Emery 1 , Robert Weber 1 , Bao Yeh 1
1 , Hewlett-Packard Company, Corvallis, Oregon, United States
Show AbstractThursday, 12/4New Presenter *B10.6 @ 3:30 PMLow-Temperature Zinc Indium Oxide Thin-Film Transistors: Progress Toward Roll-to-Roll Manufacturing. Timothy Emery
5:00 PM - B10.7
Roll to Roll Coating, Properties and Device Applications of Transparent Nanotube Thin Films.
Liangbing Hu 1 , Youngbae Park 1 , David Hecht 1 , Mike O'Connell 1 , Corinne Ladous 1 , Ting Huang 1 , George Gruner 1 , Glen Irvin 1 , Paul Drzaic 1
1 , Unidym Inc, Menlo Park, California, United States
Show Abstract Thin films of carbon nanotubes (CNT) are a promising candidate in the development of alternatives to indium-tin oxide thin films in applications requiring transparent, conductive films. While several groups have presented data demonstrating the feasibility of CNT films as transparent conductors over the past few years, most of these systems have suffered from one or more limitations. These limitations have included inferior conductivity, high cost, limited availability, or poor uniformity and quality. This presentation will focus on recent work within Unidym to fabricate high quality, high performance, scalable conductive CNT films on plastic and glass substrates. The quality and performance of the CNT films, coupled with our use of scalable manufacturing processes, indicate that large-scale implementation of CNT transparent conductors is practical.The basic process for fabricating conductive transparent CNT films will be described. CNTs are fabricated using atmospheric pressure chemical vapor deposition, and the materials purified to a level adequate for formation of conductive inks. These inks can be used to fabricate thin films using a variety of high-speed coating methods: here, we demonstrate the use of slot coating methods for high quality films over large areas, on both rigid and flexible substrates. We will review key parameters relevant to the formation of high-performance films, including dispersion, rheology, film fabrication processes, and post treatment such as doping and patterning. Routes for future improvement of the performance of transparent nanotube electrode will be suggested, based on the electronic and transport properties of mesh-type of percolating random network. We will also discuss dry and wet process for patterning carbon nanotube thin films. Various types of devices with incorporated CNT thin films will be described, including touch panel display, liquid crystal display, organic light emitting diodes and solar cells. In particular, device integration issues such as conformal coating and nanotube-metal contact resistance, as well as advantages of using CNT thin films compared to ITO, will be discussed.
5:15 PM - B10.8
Solution-deposited Low Voltage Transparent Zinc Oxide Transistors Using Sodium Beta-Alumina (Superionic Conductor) as Gate Dielectric.
Bhola Pal 1 , Howard Katz 1
1 Material Sc. And Engineering, Johns Hopkins University, Baltimore, Maryland, United States
Show AbstractFor transparent field effect transistor (FET) fabrication, a sol-gel derived n type zinc oxide (ZnO) semiconducting film has been deposited using zinc acetate solution as precursor. A zinc acetate film has been made by dip coating method. A vertically aligned layer of ZnO nanocrystal film has been prepared from this zinc acetate by suitable heat treatment. We used ITO coated glass as substrate for top contact FET where this ZnO film acts as n type semiconductor. For low voltage operation we used sodium beta-alumina (SAB) as dielectric, in which mobile sodium ion (Na+) create high capacitance with a very low leakage current. The capacitance of the dielectric is hundreds of nanofarads/cm2 and the dielectric strength is >5 V. The resulting transparent transistor needs only few volts to produce considerably high drain current of tens of microamps. The maximum field effect mobility of electrons in FETs using these materials and processes is 0.1 cm2/Vs with On/Off ratio of 102 in air.
5:30 PM - B10.9
Indium and Tin Oxide Films by Atmospheric-Pressure Plasma Deposition.
Rob Sailer 1 , Salil Jha 1 , Kyle Johnson 1 , Douglas Schulz 1
1 CNSE, North Dakota State University, Fargo , North Dakota, United States
Show AbstractAtmospheric-pressure plasma (APP) deposition has recently been employed to form thin polymer layers with targeted surface wetting characteristics. In most examples, the reactive portion of the precursor is based upon an olefin functional group that is activated during plasma deposition and fabrics with hydrophobic coatings have been demonstrated. The application of APP deposition to higher value products such as flexible electronics requires the ability to utilize chemistries beyond simple plasma polymerization across double bonds. We have been investigating the utility of metal-organic complexes as precursors to transparent conducting oxides toward roll-to-roll manufacture of photovoltaics, flat panel displays and other electronic applications. Sn(II) and In(III) beta-diketonate complexes have been employed as solid source precursors for atmospheric pressure plasma film deposition with He carrier gas, O2 reactant gas and growth temperatures from ambient to 300 °C. In the initial studies, 20 minute growth cycles typically coated 30 cm2 areas with 30-70 nm thick films as determined by ellipsometry and x-ray reflectivity. The as-deposited films exhibit light transmittance in excess of 90% over the visible spectrum while maintaining resistivities on the order of 10-2 ohm●cm. Improved electrical properties (i.e., ρ < 10-3 ohm●cm) are realized after thermal treatment (T ~ 300 °C) in a controlled gas ambient tube furnace. Recent results from novel precursors and alternative doping strategies will also be presented. This work was funded through the Air Force Research Lab agreement number FA8650-04-1-5045.
5:45 PM - B10.10
Fabrication of Thin Film Transistors based on Sol-Gel Derived Oxide Semiconductor Layers by Ink-Jet Printing Technology.
Dongjo Kim 1 , Chang Young Koo 1 , Keun Kyu Song 1 , Youngmin Jeong 1 , Jooho Moon 1
1 Department of Materials Science and Engineering, Yonsei Univ., Seoul Korea (the Republic of)
Show AbstractWe have fabricated solution processed oxide semiconductor active layer for thin film transistors (TFTs). The oxide semiconductor layers were prepared by ink-jet printing the sol-gel precursor solution based on Ga and In co-doped ZnO (GIZO). Inorganic ZnO-based thin films have drawn significant attention as an active channel layer for TFTs applications alternative to conventional Si-based materials and organic semiconducting materials, due to their wide energy band gap, optical transparency, high mobility, good TFT characteristics and stability. However, in spite of such excellent device performances, the fabrication methods of ZnO related oxide active layer involve high cost vacuum processes such as sputtering and pulsed laser deposition. Herein we introduced the ink-jet printing technology to prepare the active layers of oxide semiconductor. Stable sol-gel precursor solutions were obtained by controlling the composition of Ga, In, Zn precursor as well as 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 thermal behavior of the doped ZnO films were investigated by SEM, XRD, and TG/DTA. Furthermore, we studied the influence of the ink-jet printing conditions such as substrate temperature and surface treatment on the macrostructure of the ink-jet printed active layers and electrical performance. The mobility value of the device with optimized condition was about 0.1-0.3 cm2/Vs and the on/off current ratio was about 106. Our investigations demonstrate the feasibility of the ink-jet printed oxide TFTs toward successful application to cost-effective and mass-producible displays.