Femtosecond transmission studies of n-type and p-type hydrogenated amorphous silicon pumped in the exponential band tails

The results of transient transmission studies utilizing femtosecond laser pulses on n-type and p-type hydrogenated amorphous silicon are presented. In these studies, both the pump and probe photon energies are tuned through the exponential band tail region. The responses of the different materials are quite different, allowing the technique to clearly distinguish between them. It is shown that while recombination is dominant in intrinsic a-Si:H, trapping becomes important in the doped material. A model of the experimental results gives additional insight into the density of states in the band tail region.