Reduction of gap state density in a-SiGe : H alloys

The reduction of the gap states in a-SiGe:H alloys down to the level in a-Si:H was realized by the low-pressure microwave plasma chemical vapour deposition (CVD) which utilizes hydrogen plasma stream. It was found that the densities of defect states and tail states in the alloy vary with the flow rate of hydrogen introduced into the deposition chamber, and a-SiGe:H alloy with low defect density was attained with an optimized flow rate of hydrogen when the flow rate of source gas (SiH4 and GeH4) was reduced. The Urbach energy also decreased when the flow rate of hydrogen was optimized, and a-SiGe:H alloy with an Urbach energy of 48 meV was obtained. It was also found that the photoconductive property of a-SiGe:H alloys is not sensitive to the density of gap states.