Some theoretical aspects of hydrogen motion in a-Si : H

The mechanism (or mechanisms) of hydrogen transport in a-Si:H is of great i ntrinsic interest but is also important for the understanding of a number o f phenomena such as light induced defects. The standard picture for hydroge n migration is that a hydrogen atom leaves a trap site for a higher energy transport site, migrates among the transport sites, and eventually falls ba ck into a trap site. Presumably the transport sites are bond centered sites and the trap sites are fl passifying dangling bonds although other possibi lities do exist. We find that the literature includes many calculations for the activation energy for hydrogen diffusion that are at best misleading a nd often interpreted incorrectly. In this paper we exhibit a number of calc ulations that apply to particles hopping in complex medium and apply the re sults to a-Si:H. These calculations, coupled with existing estimates of the energy of various H configurations in a-Si:H, show that the standard pictu re is likely to be incorrect. However, some preliminary calculations and ar guments presented in this paper suggest that the mobile species of hydrogen in a-Si:H may well be molecular.