The Effect of Oxygen Contamination on the Electronic Properties of Hot-Wire CVD Amorphous Silicon Germanium Alloys
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0910-A02-05
The Effect of Oxygen Contamination on the Electronic Properties of Hot-Wire CVD Amorphous Silicon Germanium Alloys Shouvik Datta1, J. David Cohen1, Steve L. Golledge2, Yueqin Xu3, A. H. Mahan3, James R. Doyle3, and Howard M. Branz3 1 Physics, University of Oregon, 1371 E 13th Avenue, Eugene, OR, 97403 2 Surface Analytical Facility,CAMCOR, University of Oregon, Eugene, OR, 97403 3 National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO, 80401 ABSTRACT A series of four a-Si,Ge:H alloy samples with Ge fractions near 30 at.% were deposited by hot-wire CVD (HWCVD) using a Ta filament maintained at 1800 oC. During film growth, the level of oxygen contamination was varied from less than 1019 cm-3 to roughly 5 x 1020 cm-3 using a controlled air-leak. The electronic properties of these films were then characterized using transient photocapacitance (TPC) and transient photocurrent (TPI) spectroscopy, as well as the drive-level capacitance profiling (DLCP) techniques. We observed an unexpected systematic improvement of the electronic properties of these HWCVD a-Si,Ge:H with increasing oxygen impurity level, which was reflected by a decrease in the deduced Urbach energies. Comparing these with films co-deposited on stainless-steel versus p+ c-Si substrates, we found significantly better electronic properties in the latter case. Comparisons of the TPC and TPI spectra indicated a very high level of hole collection, consistent with these narrow bandtail distributions. INTRODUCTION Improved electronic properties of a-Si,Ge:H alloys, grown by the hot-wire chemical vapor deposition (HWCVD) method, to a level comparable to the best glow discharge (PECVD) a-Si,Ge:H alloy films have recently been reported [1-5]. This resulted from replacing the usual tungsten filament with tantalum and using a filament temperature of ~1800°C instead of ~ 2000°C. For HWCVD a-Si,Ge:H alloy films deposited in this fashion with Ge fractions in the 15 to 50 at.% range (Tauc gaps were between 1.65 eV to 1.3eV), Urbach energies were found to lie below 45 meV. In addition, for Ge fractions up to 30 at.%, the annealed state deep defect densities were found to lie at or below the mid 1015cm-3 level.[4] These recent HWCVD grown alloys also exhibited roughly an order of magnitude lower structural defect density as measured by small angle X-ray scattering (SAXS) studies for Ge fractions ranging from 20% to as high as 80% compared to all past a-Si,Ge:H thin films grown by either by PECVD or HWCVD. [2,3] Subsequently, a series of such alloy samples exhibited much broader Urbach energies (as high as 60 meV) and significantly higher deep defect densities [5]. Because SIMS analysis indicated that these samples also contained higher levels of oxygen contamination, it seemed likely that this was responsible for the poorer electronic properties. The source of oxygen contamination during the HWCVD growth process has now been identified and eliminated. However, we decided it would be interesting to understand to what extent controlled changes in the
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