Optimization on Temperatures of Filament and Substrate for High-Quality Narrow Gap a-Si1-xGex:H Alloys Grown by Hot-Wire CVD

We improve narrow-bandgap (1.2 < ETauc< 1.3 eV) amorphous silicon germanium (a-Si1-xGex:H) alloys grown by hot-wire chemical vapor deposition (HWCVD) by lowering both substrate and filament temperatures. We grew two series of films using a tungsten filament. First we systematically varied the filament temperature (Tf) from our standard temperature of 2150 deg C down to 1750 deg C, while fixing alldeposition parameters. Secondly we systematically varied the substrate temperature (Ts) from our previous optimized temperature of 350 deg C down to 125 deg C, while fixing all other deposition parameters including Tf = 1800 deg C. Films with the best properties are grown with Tf < 1880 deg C and Ts between 200-250 deg C. Improvement of the material properties are characterized by improvements in reduced microvoid density, and good photoresponse (for a given ETauc). There are about 15% more Ge-H bonds-passivating Ge-dangling bonds-relative to our previous work. The films are more compact due to microvoid reduction as measured by small-angle X-ray scattering (SAXS). We also fabricated solar cells with these optimized materials and obtained~3.58%-efficient devices without doing bandgapprofiling yet. Due to the high optical absorption of these a-Si1-xGex:H (~1.25 eV bandgap) alloys, we need an i-layer that is only~1200 ..ANG.. thick to obtain a Jsc of~20 mA/cm2. Additionally, we increased the GeH4 gas utilization relative to SiH4 from previous work, which was about 1:1 (GeH4 in gas to Ge in film). Under the current conditions, a 35% GeH4 gas fraction produces an a-Si1-xGex:H filmwith x = 0.7.