Recombination at dangling bonds and band tails: Temperature dependence of photoconductivity in hydrogenated amorphous silicon

The theory of recombination at dangling bonds and band tails is developed. The statistics for correlated defects are included and the Shockley-Read formalism is extended to asymmetric exponential distributions of tail states. Parametrical representations of the photoconductivity and the charge densities are obtained. A comparison of the results explains the thermal quenching of the photoconductivity experimentally observed. At low temperatures, the photoconductivity is determined by the balance between the two band- tail charge densities. At higher temperatures, one band-tail density becomes negligible compared with the dangling-bond charge density and the photoconductivity is controlled by the latter. Finally, the variations in photoconductivity with the density-of-states parameters are discussed. The agreement between the model and experimental results on undoped hydrogenated amorphous silicon is excellent for temperature above 150K. The divergence observed below 150K can be interpreted with capture cross-sections of band-tail states far from the middle of the gap that are dependent on their energy location.