Electronic Properties of Improved Amorphous Silicon-Germanium Alloys Deposited by a Low Temperature Hot Wire Chemical Va
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A7.2.1
Electronic Properties of Improved Amorphous Silicon-Germanium Alloys Deposited by a Low Temperature Hot Wire Chemical Vapor Deposition Process Shouvik Datta 1, J. David Cohen 1, Yueqin Xu 2, and A. H. Mahan 2 1 2
Department of Physics, University of Oregon, Eugene, OR 97403, USA. National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO 80401,USA.
ABSTRACT We report novel material properties of a series of a-Si,Ge:H alloys grown by hot-wire chemical vapor deposition under low filament temperature (~1800°C) and low substrate temperature (~200-300°C). These alloys exhibit significantly improved electronic properties including low defect densities and sharp band tails (Urbach energies ≤ 45meV even for Ge fractions as high as 47at.%). On the other hand, comparisons of the transient photocapacitance and transient photocurrent spectra do not indicate very efficient hole collection in these materials. We found two distinct regimes of light-induced degradation in the alloy sample with 29at.% Ge fraction, possibly corresponding to the light induced increase of Ge and Si dangling bonds, respectively.
INTRODUCTION Amorphous silicon-germanium alloys (a-Si,Ge:H) [1] are a very important component for the production of the highest efficiency amorphous silicon based multi-junction & tandem solar cells. Unfortunately, optimized a-Si,Ge:H alloys using the RF Plasma Enhanced Chemical vapor deposition (PECVD) process require deposition at very low rates, typically less than 1Å/s. This translates into relatively high manufacturing costs. Efforts to increase the PECVD deposition rates without degrading the electronic properties have not yet been realized for these alloys. The hot wire chemical vapor deposition (HWCVD) method [2] has been shown to greatly increase the deposition rate (~16 Å/s as in [2]) of pure amorphous silicon without significantly degrading its electronic properties. However, several previous attempts to apply this method to the a-Si,Ge:H alloys have resulted in materials with poor electronic properties including, in particular, large deep defect densities and broad band tails. Recently, however, a variation of this technique using lower filament temperatures has shown considerable promise since it appears to lead to improved microstructure of the hot-wire a-Si,Ge:H alloys as revealed by small angle x-ray scattering (SAXS) studies [3,4]. In this paper we report studies of the electronic properties of a-Si,Ge:H alloys produced by this low temperature HWCVD process. We employed a variety of junction capacitance methods to characterize their electronic properties. These included transient photocapacitance (TPC) spectroscopy [5] to determine the sub-band-gap optical spectra, the transient junction photocurrent (TPI) method to address the issue of minority carrier collection and the drive-level capacitance profiling (DLCP) method [6,7] to deduce the deep defect densities. We also report results of light-induced degradation of one HWCVD a-Si,Ge:H alloy with a 29at.% Ge fraction.
A7.2.2
Table I. HWC
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