PHOTOCONDUCTIVITY IN HYDROGENATED AMORPHOUS-SILICON .2. RELAXATION OFTRAPPED CHARGE-CARRIERS

Dual-beam modulated photoconductivity (DBMP) experiments allow measure ment of three relaxation times that characterize the trapping-detrappi ng and the recombination processes participating in the steady-state p hotoconductivity of hydrogenated amorphous silicon. The shortest times t1 and t2 associated with electron thermal emission from doubly occup ied dangling bonds and from band-tail states respectively are shown to be related to the response time tau(R) of the initial decay of photoc onductivity measured in a transient experiment. To discover the relati onship existing between t1, t2 and tau(R), we first analyse the temper ature dependence of the steady-state mobility mu(ss) = mu(n)tau(n)/tau (R), Where mu(n) and tau(n) are the free electron mobility and lifetim e. We reach the conclusion that mu(ss) is 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 an d the conduction band on the other hand. The model allows us to calcul ate the response time in terms of DBMP quantities only, namely t1, t2 and the relative contributions a1gamma1 and a2gamma2 of the detrapping processes. Fair agreement with the measured values is found in the wh ole temperature range. Finally the magnitude and variations with tempe rature 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.