The electronic structure of amorphous silicon–carbon alloys

Abstract

The electronic density of states and the localization behavior of the states due to disorder of amorphous silicon–carbon alloys (a-Si (1− x) C x and a-Si (1− x) C x :H) have been calculated for carbon concentrations smaller than x=0.20. The structures contain no unsaturated bonds and only tetrahedrally coordinated carbon atoms. The electronic structure is described by a tight-binding Hamiltonian; the localized or extended character of the states is investigated via a scaling approach. Alloying a-Si:H with carbon broadens the valence band and increases the mobility gap. The experimentally observed linear relation between the size of the gap and the carbon concentration is well reproduced. This holds also for the structure of the valence band edge in the vicinity of the Fermi level, which for small carbon concentrations consists of two parts (first linear and then exponential). Localized states are found at all band edges.