NONEQUILIBRIUM OCCUPANCY OF TAIL STATES AND DEFECTS IN ALPHA-SIH - IMPLICATIONS FOR DEFECT STRUCTURE

A detailed investigation of the electron and hole occupancy of tail st ates in undoped amorphous silicon (a-Si:H) as well as changes in the d angling-bond occupancy as a function of excitation intensity was carri ed out using light-induced electron-spin-resonance (LESR) measurements . For very thick films the band-tail electron and hole densities are n ot 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 densi ty in annealed or as-grown a-Si:H. Light soaking increases mainly the neutral defect density. The dependence of the excess hole density on f ilm thickness and absorption profiles indicates that this effect is a bulk property, which may be masked in thinner films by the comparative ly 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 co nsequence of the asymmetries observed between electron and hole captur e rates and tail-state distributions. The calculations demonstrate tha t the lack of this signature does not imply a defect structure that co ntains predominantly neutral defects.