Characteristics of hydrogenated amorphous silicon-germanium alloys

Data from the literature are summarized for hydrogenated amorphous silicon-germanium (a-SiGe:H and a-SiGe:H:F) alloys, showing the relation between germanium content, hydrogen content, deposition methods, feed gas mixture, and other deposition parameters, and properties such as the optical bandgap, dark conductivity and photoconductivity, photosensitivity, Urbach parameter, and spin density. Examining RF glow discharge with both a diode and a triode geometry, DC proximity glow discharge, and photo-CVD (chemical vapor deposition), using various hydrogen-diluted and undiluted gas mixtures, the author observes that hydrogen (or inert gas) dilution is essential for obtaining high photosensitivity in SiGe alloys (in contrast to a-Si:H). Hydrogen dilution results in about an order-of-magnitude higher photosensitivity in a-SiGe:H than do undiluted gas mixtures. Aside from hydrogen dilution, neither the gas mixture nor the deposition method (various glow discharges or photo-CVD) appears to affect the photosensitivity of SiGe alloy films with optical bandgaps of approximately 1.5 eV.