Ultrafast carrier dynamics in wide gap hydrogenated amorphous silicon

We report on picosecond and femtosecond pump and probe measurements of the dynamics of photoexcited carriers in wide gap a-Si :H prepared by microwave electron-cyclotron resonance plasma-enhanced chemical-vapour-deposition. W e interpret the picosecond dynamics of transient absorption under strong pi cosecond excitation in terms of a bimolecular recombination process with th e rate constant B approximate to 5 x 10(-10) cm(3) s(-1), followed by a slo wer nanosecond decay. In the femtosecond measurements, we observed an initi al decay with effective time constant approximate to 20 ps. We have not fou nd any change in the picosecond dynamics when tuning the excitation wavelen gth through photoluminescence (PL) excitation spectrum profile. The ultrafa st dynamics do not differ in the samples with PL efficiency differing in mo re than one order of magnitude, and they agree well with those in standard a-Si :H. (C) 1999 Elsevier Science B.V. All rights reserved.