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
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 Show Abstract
1 , National Renewable Energy Laboratory, Golden , Colorado, United States
10:00 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 Show Abstract
1 , National Renewable Energy Laboratory, Golden , Colorado, United States
Multi-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 , (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.
 A. Walsh, J. L. F. Da Silva, S.-H. Wei et al., Physical Review Letters 100, 167402 (2008).
 Y. Yan, J. L. F. Da Silva, S.-H. Wei and M. M. Al-Jassim, Applied Physics Letters 90, 261904 (2007).
 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 Show Abstract
2 Physics, Durham Unversity, Durham United Kingdom, 1 Engineering Dept, Cambridge University, Cambridge United Kingdom
SnO2 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 Show Abstract
1 Physics Department, Northwestern University, Evanston , Illinois, United States, 2 College of Chemistry and Molecular Engineering, Peking University , Beijing China
11:30 AM - **B1.5
Tuning Transparent Conducting Oxides for Opto-electronic Functionality.
Tobin Marks 1 Show Abstract
1 , Northwestern U., Evanston, Illinois, United States
Transparent 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 Show Abstract
1 Materials Science, Darmstadt University of Technology, Darmstadt Germany
Properties 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 Show Abstract
1 Inorganic Chemistry Laboratory, University of Oxford, Oxford United Kingdom, 2 Department of Physics, University of Warwick, Coventry United Kingdom
Transparent 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. A. Walsh et al., Phys. Rev. Lett. 100, 167402 (2008). 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 Show Abstract
1 Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States, 2 Materials Science and Engineering, Darmstadt University of Technology, Darmstadt, Hessen, Germany
The 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
Monday PM, December 01, 2008
Room 203 (Hynes)
2:30 PM - **B2.1
Conventional TCO and Beyond: Band Engineering Approach.
Julia Medvedeva 1 Show Abstract
1 Physics, Missouri University of Science & Technology, Rolla, Missouri, United States
3:00 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 Show Abstract
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
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 Show Abstract
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
Transparent 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  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  direction is ~0.6 m0, which is almost the same as that of (001)-oriented TNO films. In contrast, m* along  was 3-4 times larger than m*. 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 Show Abstract
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
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 Show Abstract
1 Physics, Oregon State University, Corvallis, Oregon, United States, 2 Chemistry, Oregon State University, Corvallis, Oregon, United States
Thin-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 Show Abstract
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
The 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 Show Abstract
1 Materials Science & Engineering, University of Florida, Gainesville, Florida, United States
For 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 Show Abstract
1 School of Chemistry, Trinity College Dublin, Dublin Ireland, 2 Dept. of Chemistry, Oxford University, Oxford United Kingdom
Exploitation 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, but the low energy indirect band gaps these materials possess might hinder their usefulness in optoelectronic devices. SrCu2O2, however, possesses a direct bandgap of ~3.3 eV and UV-emitting heterojunction involving p-type SrCu2O2 and n-type ZnO was recently fabricated. Therefore the electronic structure and the nature of the hole charge carriers in SrCu2O2 is of major interest.
The electronic structure of stoichiometric, Cu-deficient and K-doped SrCu2O2 are investigated using GGA 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.
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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 Show Abstract
1 Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States, 2 Department of Chemistry, Northwestern University, Evanston, Illinois, United States
B3: Poster Session I
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 Show Abstract
1 Electromagnetic Material and Device Lab. , Industrial Technology Research Laboratories, Hsinchu Taiwan
Highly 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 Show Abstract
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
Molybdenum 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 . 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, indic