Transient decay from the steady-state in microcrystalline silicon

We measured the transient photocurrent decay from the steady state in microcrystalline silicon from plasma-enhanced chemical vapor deposition and hot-wire chemical vapour deposition. Samples exhibiting a range of photoconductive properties in terms of both the majority and minority carrier mobility-lifetime products or sub-gap absorption coefficients were studied. Measurements were made over a wide range of steady-state photogeneration rates for which we detail the variation with generation rate of the decay time. Samples with a short steady-state photocarrier lifetime show a long decay time. We relate the slow decay process to the much larger density of traps in the band gap in the poor-quality samples. Trapped carriers are released, undergo emission and trapping processes and eventually recombine in these samples, on a much longer time-scale than in the higher-quality samples so that the decay time cannot be taken as a fingerprint for photo-electronic quality. Analytical and numerical modeling indicate bimolecular recombination behavior during the decay. Results are in agreement with free-carrier interaction with exponentially distributed band-tail states.