Phonon generation and decay in hydrogenated amorphous silicon

We report on investigations of the temporal evolution of nonequilibrium pho non populations in hydrogenated amorphous silicon held at 1.8 K. Two types of a-Si:H were examined, one grown by plasma enhanced chemical vapor deposi tion (PE) and one by hot-wire assisted chemical vapor deposition (HW). Phon ons were created during the relaxation and recombination of: optically exci ted charge carders, and detected by means of pulsed anti-Stokes Raman spect roscopy. In the same setup, pulsed luminescence experiments were performed, under identical experimental conditions. The temporal shape of the Raman s ignals turned out to be determined both by the electronic processes respons ible for the phonon generation and by the aanharmonic decay of the excited phonon population itself. In the PE films we observed a slowly decaying (>> 100ns) contribution to the Raman signal, which was not present in the HW l ayers. We propose a model to explain this slow background as resulting from laser-induced fast nonradiative recombination of mobile with localized cha rged carriers. results of the pulsed luminescence experiments support our m odel. In addition, phonon decay times were observed to be the same in all s amples: decay times of similar to 70 ns were obtained for LA- and TO-like v ibrations, whereas TA-like vibrations decayed faster (<10 ns) than could be resolved with the experimental setup. We propose that the extreme longevit y of the LA and TO phonons is related to the microstructure, of amorphous s ilicon.