It was recalled that band-tail states were often assumed in models of amorphous hydrogenated material, and were expressed in terms of defect-pools and light-induced defects. Such models described the band-tail states as being localized at a single stretched bond. It was pointed out that there was no theoretical or experimental work which justified these assumptions. Here, ab initio calculations were used to support earlier tight-binding calculations which had shown that the band-tail states were very delocalized and involved large numbers of atoms as the energy was varied from mid-gap into the tails. The present results also showed that valence-band-tail states were statistically associated with short bonds, and conduction-band states with long bonds. A modified 512-atom assembly was used to produce a large model, of amorphous hydrogenated material, which comprised realistic band-tails, radial distribution functions and vibrational spectra. A model with no spectral or geometrical defects was created.

Theoretical study of the nature of band-tail states in amorphous Si P.A.Fedders, D.A.Drabold, S.Nakhmanson: Physical Review B, 1998, 58[23], 15624-31