Hydrogen and defects in first-principles molecular-dynamics-modeleda-Si:H

We use ab initio pseudopotential local-density-approximation methods to create and study supercells of a-Si:H containing from 66 to 71 atoms. These supercells contain 0, 1, and, 2 well-localized defects. These geometrical defects are each associated with a unique localized state in the gap. The 0- and 1-defect supercells are particularly interesting because the defect banding present in all other theoretical supercells is missing. We study the properties of H in passivating dangling bonds, in bond-centered positions, in tetrahedral positions, and, in one case, as part of a floating-bond defect. Among other properties, we investigate band gaps, binding energies, relaxation effects, localization of defects, NMR linewidths and line shapes, and phonon spectra associated with the samples.