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.

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The analysis of modulated photocurrents is reviewed, and a novel experimental technique for measurements of modulated photocurrents over a wide frequency range is applied to undoped a-Si:H in sandwich contact configuration. By analysis of the phase response the energetic as well as spatial (versus distance to the top contact) distribution of gap states above midgap is obtained.Metastable changes in the gap-state distribution by light soaking (Staebler-Wronski e8'ect) and by depletion bias annealing are studied as a function of illumination time, illumination temperature, annealing time, annealing temperature, and applied bias during annealing. The main experimental results are as follows: Undoped a-Si:H exhibits a peak in the distribution of gap states at about 0.6 eV below the conduction-band edge E, and in the depletion region a peak of shallow states at 0.4 eV below E,. Upon light soaking the deep peak increases according to a power law and the shallow one is quenched. The original distribution is restored by annealing above 420 K. At lower degradation temperatures, creation and quenching rates are enhanced but the established changes are less stable against annealing. Both peaks show exponentiaI tails towards midgap with slopes that depend on annealing and degradation temperatures. Activation energies for annealing of deep states show a broad variation between 0.9 and 1.3 eV and are strongly correlated with the energetic position of the defect states in the gap. Annealing with depletion bias produces a metastable increase of both defect peaks above midgap. Based on the thermodynamical considerations and on theoretical calculations of the dangling-bond correlation energies, a model of the defect structure is discussed that is able to account for the presented'results as well as various other experimental observations concerning metastable changes and the energetic position of defects in amorphous silicon.

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