Coordination defects in amorphous silicon and hydrogenated amorphous silicon: a characterization from first-principles calculations

We study by means of the first-principles pseudopotential method the coordi nation defects in amorphous silicon and hydrogenated amorphous silicon, als o in their formation and their evolution upon hydrogen interaction. An accu rate analysis of the valence charge distribution and of the 'electron local ization function' allows us to resolve possible ambiguities in the bonding configuration, and in particular to identify clearly threefold (T-3) and fi vefold (T-5) coordinated defects. We found that electronic states in the ga p can be associated with both kinds of defect and that in both cases the in teraction with hydrogen can reduce the density of states in the gap.