Electronic structure of amorphous silicon carbide compounds

Abstract

The electronic structure of a-Si_1–x C_x, has been calculated for the whole range of C concentrations (0 ⩽x ⩽1). The atomic structure of the alloy has been modelled by an ideal continuous random network in which some Si-C atomic correlations have been considered. The electronic levels have been obtained from a nearest-neighbour tight-binding Hamiltonian that has been solved within the Bethe lattice and effective-medium approximations. The gap increase that follows C incorporation is correctly reproduced by our calculations up to x values close to 0·7. The employed structural model based on fourfold coordination of both Si and C atomic species loses its validity for higher C contents. Besides average densities of states, other electronic magnitudes of physical interest such as joint densities of states, spectral and optical gaps, valence and conduction band edges, and energy levels of defect states produced by Si and C dangling bonds have been obtained. We have extended the definition of the charge neutrality level introduced by Tejedor and Flores to cover heterojunctions between amorphous semiconductors. With this concept, we have predicted band alignments between alloys having different compositions.