A numerical study was made of the electronic properties of various structural models for amorphous Si and hydrogenated amorphous Si. Beginning with an ideal random network, dangling bonds, floating bonds, double bonds, micro-voids, hydrogenated dangling bonds and hydrogenated floating bonds were introduced. The concentrations of these defects could be varied independently, and the amount of disorder which was introduced into the system was therefore strictly controllable. Two continuous random networks, a vacancy model and a bond-switching model, were investigated. The Keating and Stillinger-Weber potentials were used for relaxation of the structures. The electronic structure was described by using a tight-binding Hamiltonian, and the character of the eigenstates was investigated via a scaling approach. The vacancy model indicated a band gap for small defect concentrations, but this filled up with increasing disorder. A similar behavior was found in the case of the other models. In general, the defects introduced states into the gap region of amorphous material, while dangling bonds led to the largest density of states in the gap region; for a given defect concentration. This model was unique. For small system sizes, an impurity band resulted which markedly changed its character, for systems with more than 4000 atoms, to a nearly uniform density of states; as observed experimentally. In the case of amorphous hydrogenated material, the dangling and floating bonds were removed and a mobility gap resulted whose width was in good agreement with experiment. An experimentally observed tailing of the band into the gap region (first linear, then exponential) was well described only for an amorphous hydrogenated material model which was derived from the vacancy model and for system sizes above 4000 atoms. The Wooten-Winer-Weaire model did not reproduce this tail behavior. Localized states were found at all band edges, but states at the bottom of the conduction band were more strongly localized than those at the top of the valence band.

Defects in a-Si and a-Si:H S.Knief, W.Von Niessen, T.Koslowski: Physical Review B, 1998, 58[8], 4459-72