Band-tail states and the localized-to-extended transition in amorphous diamond

The electronic structure of a large (4096 atom) and realistic model of amorphous diamond is studied. The density of states and the individual eigenstates in the valence-band tail and midgap region are computed with two ``order-N'' spectral electronic-structure methods: the maximum entropy method and the shifted Lanczos method. We observe approximately exponential band tails at both valence- and conduction-band edges. The electronic states are explicitly computed in the vicinity of the gap through the valence-band tail region, and we track their spatial transition from highly local to extended. A simple model leading to exponential band tailing is described. {} 1996 The American Physical Society.