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

Abstract Band tail states are routinely invoked in a wide variety of models of a-Si:H including the 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 there is no theoretical or experimental work to justify these assumptions. In this work we use ab initio calculations to study the structure of these states and confirm that band tail states are delocalized – involving large numbers of atoms as the energy is varied from midgap into the tails. Surprisingly, 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. Our primary models contain over 500 Si atoms with and without hydrogenation. It turns out that a minimal amount of hydrogen affects the topological structure of the band tail states.