Gap states in diamond-like amorphous carbon

The electronic structure of amorphous diamond-like carbon is reviewed. The sp(2) sites form small, mainly chain-like clusters which control the bandgap. Detailed analysis shows that all pi states within the sigma-sigma* gap are all localized; so the mobility gap greatly exceeds the optical gap. This accounts for the photoluminescence excitation spectrum. Dangling-bond states are calculated to possess some s orbital character, which lowers their energy level in the gap and accounts for the p-type conduction of tetrahedral amorphous carbon. The defect density is high in hydrogenated amorphous carbon and is found to be fairly well described by the weak-bond-dangling-bond conversion model. The photoluminescence mechanism is described and it is argued that the paramagnetic defects are still the most likely dominant recombination centre.