The mobility edge in amorphous silicon

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Abstract
The calculations of this paper are an attempt to find out whether the range of localized states, ΔE = 0.2 eV, observed in amorphous silicon prepared by glow-discharge decomposition of silane is a property inherent in the silicon, or is a result of the considerable amount of hydrogen incorporated into the material. A simple model is presented which gives a value of ΔE of the observed magnitude for a continuous random network of pure silicon. It is argued that the p-like content of the wavefunctions is imporant for obtaining such a large value of ΔE and that the main source of disorder is the random orientation of the tetrahedra. Calculating the scattering due to this, a mean free path for a conduction-band electron is derived. This is used to fix the amount of disorder in an equivalent Anderson model on the same lattice, from which ΔE may be found using the analysis of Abou-Chacra and Thouless (1974). This is found to give a range of localized states about 0.28 eV wide. Using the same model, the effect of hydrogen is found to be small, a 10% concentration increasing the value of ΔE only by 0.02 eV or less.