Photoconductivity in hydrogenated amorphous silicon. II. Relaxation of trapped charge carriers

Dual-beam modulated photoconductivity (DBMP) experiments allow measurement of three relaxation times that characterize the trapping-detrapping and the recombination processes participating in the steady-state photoconductivity of hydrogenated amorphous silicon. The shortest times t 1 and t 2 associated with electron thermal emission from doubly occupied dangling bonds and from band-tail states respectively are shown to be related to the response time τR of the initial decay of photoconductivity measured in a transient experiment. To discover the relationship existing betweent 1, t 2 and τR, we first analyse the temperature dependence of the steady-state mobility μ ss = μ n τR, /τr where μ n and τn are the free electron mobility and lifetime. We reach the conclusion that μ ssis the weighted average of the mobilities characterizing the electron traffic between D− centres and the conducting states on the one hand, and that between tail states and the conduction band on the other hand. The model allows us to calculate the response time in terms of DBMP quantities only, namely t 1,t 2 and the relative contributions a 1 γ 1 and a 2 γ 2 of the detrapping processes. Fair agreement with the measured values is found in the whole temperature range. Finally the magnitude and variations with temperature of the third relaxation time associated with thermally assisted diffusion of holes trapped in the valence-band tail, are discussed in terms of existing models.