Electronic states of florinated amorphous silicon

Abstract

The electronic structures of florinated amorphous silicon (a-Si) and that with hydrogen atom incorporated are studied by means of the first principles orthogonalized linear combination of atomic orbitals (OLCAO) method. The structural models employed are similar to those used in the study of hydrogenated a-Si in which the F atoms appear as SiF, SiF 2, and SiF 3 in an otherwise perfect random tetrahedral network. To elucidate the bonding character, the local density of states (LDOS) of F, H and the Si atom to which they bond, are calculated and resolved into orbital components. The F 2s levels are localized well below the valence band (VB). In the VB region, the LDOS are dominated by F 2p components; in the conduction band (CB) region, F has negligible LDOS. In the case of SiF 3, a gap state exists near the middle of the band gap with predominately Si 3s character. Results for the cases where F atoms are partially substituted by H atoms in SiF 2 and SiF 3 are also presented. It is found that the presence of H atoms has a pacifying effect near the gap region due to the effective bonding between Si 3s, Si 3p, and H 1s wave functions.