Nonequilibrium occupancy of tail states and defects ina-Si:H: Implications for defect structure

A detailed investigation of the electron and hole occupancy of tail states in undoped amorphous silicon (a-Si:H) as well as changes in the dangling-bond occupancy as a function of excitation intensity was carried out using light-induced electron-spin-resonance (LESR) measurements. For very thick films the band-tail electron and hole densities are not proportional. Over a wide range of excitation conditions the excess hole density is constant, suggesting the presence of charged defects with a density that is 5--10 times larger than the neutral defect density in annealed or as-grown a-Si:H. Light soaking increases mainly the neutral defect density. The dependence of the excess hole density on film thickness and absorption profiles indicates that this effect is a bulk property, which may be masked in thinner films by the comparatively high interface defect density. Model calculations of nonequilibrium occupation statistics confirm the experimental results. For a defect distribution that includes charged defects, the calculations suggest a very small positive LESR signature of the dangling bond, in spite of the high density of charged defects in the material, as a necessary consequence of the asymmetries observed between electron and hole capture rates and tail-state distributions. The calculations demonstrate that the lack of this signature does not imply a defect structure that contains predominantly neutral defects.