Spatially resolved and energy-resolved defect kinetics ina-Si:H: A comprehensive study by phase-shift analysis of modulated photocurrents

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 effect) 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 $E_c$ and in the depletion region a peak of shallow states at 0.4 eV below $E_c$. 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 exponential 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.