Urbach tail and gap states in hydrogenated a-SiC and a-SiGe alloys

By combining photothermal deflection spectroscopy (PDS) and transmission measurements, the localized-state distribution inside the mobility gap has been optically investigated in amorphous silicon based alloys. For both silicon-carbon and silicon-germanium alloys, a rigid shift of the seudo-gap is observed with increasing deviation from pure a-Si:H. The effect is accompanied by a significant broadening of the Urbach edge. We explain the latter in terms of potential fluctuations due to compositional disorder. For Ge-rich a-SiGe:H, the large density of defect states in the upper half of the gap makes this analysis less quantitative. In the carbon-based alloy, compositional disorder does not bring about additional defect related states throughout the range of compositions explored. This indicates that a-SiC:H, is a promising material for optoelectronic applications.