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. We interpret the picosecond dynamics of transient absorption under strong picosecond excitation in terms of a bimolecular recombination process with the rate constant B≈5×10⁻¹⁰ cm³ s⁻¹, followed by a slower nanosecond decay. In the femtosecond measurements, we observed an initial decay with effective time constant ≈20 ps. We have not found any change in the picosecond dynamics when tuning the excitation wavelength through photoluminescence (PL) excitation spectrum profile. The ultrafast dynamics do not differ in the samples with PL efficiency differing in more than one order of magnitude, and they agree well with those in standard a-Si : H.