Transient-photomodulation-spectroscopy studies of carrier thermalization and recombination in α-Si:H

The transient response of mid-gap absorption in a-Si:H to pulsed optical excitation is studied as a function of time (300 ns to 30 ms) and sample temperature (10-220 K) with use of probe wavelengths ranging from 0.75 to 5.5 fim. A numerical inversion process applied to the data gives the distribution of excess carriers in both energy and time. Analytical and numerical results describing the thermalization of carriers in an exponential bandtail have been obtained which agree well with the data. A detailed recombination model which includes both carrier release from the demarcation energy and direct tunneling to recombination centers is discussed in association with the experimental results along with a comparative analysis of spectrally resolved transient-photoluminescence decays.