Electronic structure of dangling and floating bonds in amorphous silicon

The electronic structure of amorphous silicon models has been calculated with the tight-binding parameters of Chadi. An energy gap of {~}1.0 eV is obtained for a fully four-coordinated model, whereas gap states occur for models with coordination defects. Dangling-bond gap states are well localized on the three-coordinated site. Floating bonds lead to considerably less localized gap states with wave-function amplitudes on the neighbors of the five-coordinated atom. The hyperfine splittings of these coordination defects are calculated and compared to experiment.