Properties of glow-discharge deposited amorphous germanium and silicon

TLDR: In this article, it is suggested that the de-localisation of states in the valence and conduction bands as the short range order increases is due to a hopping process in an impurity band.

Abstract: Films of silicon and germanium are deposited on glass using the radio-frequency glow-discharge decomposition of silane and germane gases respectively. When grown on a substrate at room temperature the films are amorphous, with a short range order of about 20 A. The resistivities of these films, as deposited, are typically 108Ω cm for silicon and 7 × 103 Ω cm for germanium, measured at 294°K. Thermal activation energies for conduction decrease continuously below the deposition temperature, and at low temperatures germanium follows the relation log ’ = A/T 1 4 , where A is a constant. This would seem to indicate that a hopping process in an impurity band is responsible for conduction at low temperatures. Photoconductivity has been observed in silicon but not in germanium. The threshold energy for this effect decreases with increasing deposition or annealing temperatures. This is also true of the high temperature thermal activation energy. It is suggested that this is due to the de-localisation of states in the valence and conduction bands as the short range order increases. The optical absorption coefficients of germanium and silicon have an exponential dependence on photon energy and the considerable absorption below the fundamental absorption edge of the crystalline form may indicate the presence of localised states in the band gap.