Abstract
Highly photoconducting (ημτ) films of a-SiGe: H with very low defect densities have been produced by a plasma-enhanced chemical vapour deposition technique under low flow rate (2·5 sccm) of source gases (silane and germane) and hydrogen dilution. The defect states have been characterized by photothermal deflection spectroscopy, the constant photocurrent method dual-beam photoconductivity and electron spin resonance. A low deposition rate is necessary to obtain high-quality alloy materials with comparatively high band gaps, i.e., 1·7eV>E g> 1·5 eV, as manifested by a small disorder (lower Urbach parameter E 0) and defect density, N D (mainly reduced Ge dangling-bond density), whereas additional hydrogen dilution of the source gases is necessary to reduce N D in the low band-gap region. Hydrogen dilution improves homogeneity throughout the band-gap region with a corresponding decrease of E 0 values which has no correlation with the reduction of N D. Studies of the temperature dependence of the photoconductivity and infrared quenching reveal that the large ημτ values in the optimized materials are not due to sensitization effects, but because of extremely low dangling-bond densities. This is supported by light-induced electron spin resonance studies where a significant increase in the number of carriers in the band tails has been observed in the optimized materials. The experimental results on the deposition rate and hydrogen dilution dependence of electronic and structural properties are consistent with a bulk thermal equilibration model.