The Effects of Coordination and Local Disorder on Impurity States in Hydrogenated Amorphous Silicon

The self-consistent pseudopotential method has been used to calculate the electronic structure of a periodic model of hydrogenated amorphous silicon containing, independently, P and B impurities. We have investigated both impurities in threefold- and fourfold-coordinated sites. Both types of sites studied also include the realistic distortion inherent in disordered systems. In fourfold coordination, both P and B introduce gap states, the energy and spatial extent of which depend upon the local geometrical distortion. In threefold coordination, the lone pair state of P is found below the valence band edge, while the B non-bonding state is found above the conduction band edge. The effects of local geometrical distortion on both of these states is also discussed. Comparisons with other calculations and experimental results are made.