Dependence of the Electronic Properties of Hot-Wire CVD Amorphous Silicon-Germanium Alloys on Oxygen Impurity Levels
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0989-A04-03
Dependence of the Electronic Properties of Hot-Wire CVD Amorphous SiliconGermanium Alloys on Oxygen Impurity Levels Shouvik Datta1, J. David Cohen1, Yueqin Xu2, A. H. Mahan2, and Howard M. Branz2 1 Department of Physics, University of Oregon, 1371 E 13th Avenue, Eugene, OR, 97403 2 National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO, 80401 ABSTRACT We report the effects of intentionally introducing up to ~ 5×1020/cm3 oxygen impurities into hydrogenated amorphous silicon-germanium alloys (of roughly 30at.% Ge) grown by the hot-wire chemical vapor deposition (HWCVD) method. Deep defect densities determined by drive-level capacitance profiling (DLCP) indicated a modest increase with increasing oxygen content (up to a factor of 3 at the highest oxygen level). Transient photocapacitance (TPC) spectra indicated a clear spectral signature for an optical transition between the valence band and an additional defect level which is attributed to oxygen impurities. The oxygen impurity related defect transition has an optical threshold around 1.4eV above the valence band and also results in a negative contribution to the TPC signal. This initially led us to believe that the bandtail for the higher oxygen samples was much narrower than it actually is. Surprisingly, this additional oxygen related defect level appears to have only a very minor effect upon the estimated minority carrier collection fraction. The effects of light-induced degradation upon some of these oxygen contaminated samples were also examined. We infer the existence of a significant thermal barrier to explain the observed spectral signatures of this oxygen impurity defect. INTRODUCTION Electronic properties of a-Si,Ge:H alloys deposited by the hot-wire chemical vapor deposition (HWCVD) method have now improved [1,2] to a level comparable to the best glow discharge (PECVD) a-Si,Ge:H alloy films. As was reported earlier [3-6], these were obtained by replacing the usual tungsten filament with tantalum and using a filament temperature of ~1800°C instead of ~ 2000°C during the HWCVD growth process. We recently reported studies [5] in which we carried out a systematic incorporation of oxygen impurities for a series of a-Si,Ge:H alloy samples. Here we observed that the band tail actually appeared to become narrower as the oxygen content of a-Si,Ge:H films was increased. This seemed to suggest that the oxygen impurities actually improved the quality of these alloy films! However, a more detailed study was needed to understand the root cause of such oxygen induced electronic changes. Subsequently, these alloys were characterized using a variety of junction capacitance based techniques including transient photocapacitance (TPC) [7,8], transient photocurrent (TPI) spectroscopy, and drive-level capacitance profiling (DLCP) [9,10]. After careful analysis of these results, we now have a viable alternative explanation for the oxygen induced changes in aSi,Ge:H alloys. Specifically, we believe our experimental results provide a clear signature of t
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