Some theoretical aspects of hydrogen motion ina-Si:H

The mechanism (or mechanisms) of hydrogen transport in a-Si:H is of great intrinsic interest but is also important for the understanding of a number of phenomena such as light induced defects. The standard picture for hydrogen migration is that a hydrogen atom leaves a trap site for a higher energy transport site, migrates among the transport sites, and eventually falls back into a trap site. Presumably the transport sites are bond centered sites and the trap sites are H passifying dangling bonds although other possibilities do exist. We find that the literature includes many calculations for the activation energy for hydrogen diffusion that are at best misleading and often interpreted incorrectly. In this paper we exhibit a number of calculations that apply to particles hopping in complex medium and apply the results 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 picture is likely to be incorrect. However, some preliminary calculations and arguments presented in this paper suggest that the mobile species of hydrogen in a-Si:H may well be molecular.