The electronic structure of amorphous silicon-carbon alloys

The electronic density of states and the localization behavior of the state s due to disorder of amorphous silicon-carbon alloys (a-Si(1-x)Cx and a-Si( 1-x)Cx:H) have been calculated for carbon concentrations smaller than x = 0 .20. The structures contain no unsaturated bonds and only tetrahedrally coo rdinated carbon atoms. The electronic structure is described by a tight-bin ding Hamiltonian; the localized or extended character of the states is inve stigated via a scaling approach. Alloying a-Si:H with carbon broadens the v alence band and increases the mobility gap. The experimentally observed lin ear relation between the size of the gap and the carbon concentration is we b reproduced. This holds also for the structure of the valence band edge in the vicinity of the Fermi level, which for small carbon concentrations con sists of two parts (first linear and then exponential). Localized states ar e found at all band edges. (C) 1999 Elsevier Science B.V. All rights reserv ed.