Theoretical study on the nature of band-tail states in amorphous Si

Band-tail states are routinely invoked in models of n-Si:H, including defec t pool models and models of light-induced defects. These models describe th e band-tail states as being localized on a single stretched bond. However, to our knowledge, there is no theoretical or experimental work to justify t hese assumptions. In this work we use ab initio calculations to support ear lier tight-binding calculations that show that the band-tail states are ver y delocalized-involving large numbers of atoms as the energy is varied from midgap into the tails. Our work also shows that valence-band-tail states a re statistically associated with short bonds (not long bonds), and conducti on-band states with long bonds. We have slightly modified a 512-atom model of a-Si due to Djordjevic, Thorpe, and Wooten [Phys. Rev. B 52, 5688 (1995) ] to produce a large model of a-Si:H with realistic band tails, radial dist ribution function, and vibrational spectrum. Above all, we created and used a model with no spectral or geometrical defects. [S0163-1829(98)03148-8].