Femtosecond transmission studies of a-Si : H, a-SiGe : H and a-SiC : H alloys pumped in the exponential band tails

The results of transient transmission studies utilizing femtosecond laser p ulses on hydrogenated amorphous silicon alloys are presented. In these stud ies, both the pump and probe photon energies are tuned through the exponent ial band tail region. The responses of the different alloys are similar, bu t the technique is able to clearly distinguish between them based on their alloy composition and bandgap. It is shown that the results can be modeled as a free-carrier absorption spike followed by a residual plateau caused by both recombination and intraband heating. This model is adapted to work in the band tail region. (C) 2001 Elsevier Science B.V. All rights reserved.