Dual-beam-modulated photoconductivity in hydrogenated amorphous silicon response-time and drift-mobility measurements

Abstract

Dual-beam-modulated photoconductivity in a-Si:H is studied at room temperature. The complex modulation signal is analysed in terms of three independent relaxing components. It is shown that the fast-relaxing term is due to electron detrapping from states in the conduction-band tail and that its relaxation time is the response time τ_R of the photoconductivity. The product μτ is measured by steady-state photoconductivity, and we proceed by investigating the dependence of the ratio μτ/τ_R = μ/(1 +n _t/n) on the photogeneration rate f where n _t and n are the trapped and free electron densities respectively. The f dependence of the ratio n _t/n is discussed. A model based on the existence of an exponential distribution of tail states allows us to show that n _t/n∝ f ¹⁻²γ (σ_p∝fγ), in agreement with the experimental data. This implies that the response time of the photoconductivity follows a τ_R∝ fγ dependence. The link between μτ/τ_R and the drift mobility μ_D measured by time of flight is discussed. It is found that μ/τ_R is, at the low photogeneration rates studied in this paper, lower than μ_D and increases with f The reason is that, whereas time of flight probes shallowly trapped electrons, and measures a ratio (n ^t/n)*, secondary steady-state photoconductivity probes deeply trapped electrons and measures a ratio xn _t/n>(n _t/n)*.