A Combinatorial Approach to TCO Synthesis and Characterization
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A Combinatorial Approach to TCO Synthesis and Characterization John Perkins, Dennis Readey2, Jeff Alleman, Joe del Cueto, Xiaonan Li, Tim Coutts, Renaud Stauber1, Chris Duncan2, David Young, Phil Parilla, Brian Keyes, Lynn Gedvilas, Davor Balzar3, Qi Wang and David Ginley National Renewable Energy Laboratory, Golden, CO, 80401, U.S.A. 1 University of Colorado, Boulder, CO, 80309, U.S.A. 2 Colorado School of Mines, Golden, CO, 80401, U.S.A. 3 National Institute of Standards and Technology, Boulder, CO 80303, U.S.A. ABSTRACT We have developed the deposition, characterization and analysis tools necessary for a combinatorial approach to thin film metal oxides, with a special focus on transparent conducting oxides (TCOs). We are presently depositing compositionally graded libraries using mutli-target sputtering and CVD. The initial collection of characterization tools includes UV/VIS/NIR transmission/reflection, FTIR reflectance, Raman scattering, 4-point conductivity, thickness, xray diffraction and electron microprobe. In addition to allowing for a more complete empirical optimization of TCO properties, we expect to develop an improved basic understanding of TCOs, especially in the area of p-type TCOs. This paper provides an overview of our current combinatorial material science program as applied specifically to TCOs. INTRODUCTION Transparent conducting oxides play a key role in a number of thin film optoelectronic devices including flat panel displays, low-e windows, photovoltaics, electrochromic devices and anti-static coatings [1]. The bulk of these applications rely on the established n-type transparent conducting oxides (TCOs), such as SnO2:F, ZnO:Al and Indium-Tin-Oxide (ITO) [2]. For many of these technologies, improved next-generation devices will require improved or new TCOs [3]. An exciting possibility is the potential for all metal-oxide transparent electronics which would require TCO p-n junctions [4,5]. Figure 1 illustrates the optical transmission spectra for two commercial SnO2 films on glass with a sheet resistances of 100 W/sq. and 5 W/sq. The short wave length cut off at ~ 350 nm is due the band gap of intrinsic SnO2 and the infrared (long wave length) cut off is due to reflection from plasma oscillation excitations of the conducting electrons. Hence as the sheet resistance is reduced from 100 W/sq. to 5 W/sq. by increasing the conducting free electron concentration through doping, the “plasma edge” moves to shorter wavelengths. One of the current dilemmas for TCOs is that while improved carrier concentrations can be obtained, the result is not always increased conductivity. Rather both transparency in the IR and conductivity can suffer due to free electron adsorption and increased defects caused by the doping process. Hence, one objective in current TCO research is finding higher mobility materials which could work at lower doping levels. This is leading to the investigation of new “exotic” n-type materials such as Cd2SnO4 [6,7], Zn2In2O5 [8,9] and In2-2xSnxZnxO3 [10] and the quest for improved simp
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