Electronic structure and localized states in a model amorphous silicon

The electronic structure of a model amorphous silicon ($a$-Si) represented by a supercell of 4096 silicon atoms [B.R. Djordjevic, M.F. Thorpe, and F. Wooten, Phys. Rev. B 52, 5685 (1995)] and of a model hydrogenated amorphous silicon ($a$-Si:H) that we have built from the $a$-Si model are calculated in the tight-binding approximation. The band edges near the gap are characterized by exponential tails of localized states induced mainly by the variations in bond angles. The spatial localization of the states is compared between $a$-Si and $a$-Si:H. Comparison with experiments suggests that the structural models give good descriptions of the amorphous materials.