Transient Photoconductivity of a-Si:H at Low Temperatures Induced by Bandgap Light

Abstract

The rise and decay of the photoconductivity σ_p of intrinsic and strongly p-type hydrogenated amorphous silicon (a-Si:H) samples was studied as a function of photocarrier generation rate G between 4.2K and 300K. In intrinsic samples the temperature regime T<60K of energy-loss hop** is clearly distinguishable from T>80K where thermal re-excitation of photocarriers is possible. For the p-type sample the temperature dividing the two regimes is near 170K. In intrinsic samples the rise of σ_p is faster when residual photocarriers exist from previous light exposures than when the samples are in the electronic dark equilibrium. This indicates that geminate recombination decreases with increasing photocarrier concentration. In the p-type sample we observe an up to 20 percent overshoot in the rise of σ_p before σ_p settles down to its steady state value. This overshoot increases with G. We interpret the overshoot as an interplay of two recombination channels with different time scales.