A molecular dynamics study of band tails in a-Si:H

Band tail states are routinely invoked in models of a-Si:H including defect pool models and models of light induced defects. These models describe the band tail states as being localized on a single stretched bond. However to our knowledge, there is no theoretical or experimental work to justify these assumptions. In this work we use ab initio calculations to support earlier tight-binding calculations that show that the band tail states are very delocalized - involving large numbers of atoms as the energy is varied from midgap into the tails. Our work also shows that the valence band tail states are statistically associated with Short bonds (not long bonds) and the conduction-band states with long bonds. We have slightly modified a 512 atom model of a-Si due to Djordjevic et al. to produce a large, defect free model of a-Si:H with realistic band tails.