The electronic structure of amorphous diamond-like C was considered. The sp2 sites formed small, mainly chain-like clusters, which controlled the band-gap. Detailed analysis showed that all π states within the σ-σ* gap were localized, so that the mobility gap greatly exceeded the optical gap; as reflected by the photoluminescence excitation spectrum. Dangling-bond states were predicted to possess some s-orbital character. This lowered their energy level in the gap, and accounted for the p-type conduction of tetrahedral amorphous C. The defect density was high in hydrogenated amorphous C and was found to be quite well described by the weak-bond to dangling-bond conversion model. It was concluded that paramagnetic defects were the predominant recombination center.

J.Robertson: Philosophical Magazine B, 1997, 76[3], 335-50