Influence of Filament and Substrate Temperatures on Structural and Optoelectronic Properties of Narrow Gap a-SiGe:H Alloys Deposited by Hot-Wire CVD

We have found that narrow-bandgap - 1.25 < Tauc Gap < 1.50 eV - amorphous silicon germanium (a-SiGe:H) alloys grown by hot-wire chemical vapor deposition (hot-wire CVD) can be improved by lowering both substrate and filament temperatures. We systematically study films deposited using a one-tungsten filament, decreasing filament temperature (T_f) from our standard temperature of 2150° down to 1750°C, and fixing all other deposition parameters. By decreasing T_f at the fixed substrate temperature (T_s) of 180°C, the Ge-H bonding increases, whereas the Si-H₂ bonding is eliminated. Films with higher Ge-H bonding and less Si-H₂ have improved photoconductivity. For the series of films deposited using the same germane gas fraction at 35%, the energy where the optical absorption is 1 × 10⁴ (E04) drops from 1.54 to 1.41 eV with decreasing T_f. This is mainly due to the combination of an increasing Ge solid fraction (x) in the film, and an improved homogeneity and compactness due to significant reduction of microvoids, which was confirmed by small angle X-ray scattering (SAXS). We also studied a series of films grown by decreasing the T_s from our previous standard temperature of 350°C down to 125°C, fixing all other deposition parameters including T_f at 1800°C. By decreasing T_s, both the total hydrogen content (C_H) and the Ge-H bonding increased, but the Si-H₂ bonding is not measurable in the T_s range of 180°-300°C. The E04 increases from 1.40 to 1.51 eV as T_s decreased from 350° to 125°C, mainly due to the increased total hydrogen content (C_H). At the same time, the photo-to-dark conductivity ratio increases almost three orders of magnitude over this range of T_s.