Dopant and defect states in a-Si:H

The changes in the density-of-states distribution upon doping of hydrogenated amorphous silicon (a-Si:H) are discussed. A previous doping model is extended to explain the presence of occupied band-tail states, and their predicted doping dependence is shown to be consistent with experimental observations. The energy levels of dangling bonds are discussed, and we describe a complete set of optical and thermal emission data which place the upper dangling bond level at around 0·8 eV below the conduction band. The results also indicate that the defect has little atomic relaxation. It is argued that the defect-dopant pair does not have a negative correlation energy, contrary to a recent suggestion.