Nonequilibrium carrier dynamics in ultrathin Si-on-glass films

Abstract

We present a femtosecond pump-probe spectroscopy approach for characterization of amorphous and microcrystalline silicon films grown on glass substrates. Such films are presently being considered as absorbers in tandem-type, Si-based photovoltaic cells. Our experiments consisted of time-resolved, two-colour femtosecond optical measurements, performed in the transmission mode in a wide range of delay times. Depending on the sample growth process, collected normalized transmissivity change (ΔT/T) waveforms exhibited a bi-exponential relaxation dynamics with the characteristic times varying from picoseconds to nanoseconds. Experimental data were interpreted using a three-rate-equation models, and the relaxation was identified as, depending on the Si film type, being dominated by either carrier trapping or electron-phonon cooling and followed by electron-hole recombination. An excellent fit between the model and the ΔT/T transients was obtained and a correlation between the Si film growth process, its hydrogen content, and the associated trap concentration was demonstrated.