Nonequilibrium phonons in amorphous silicon studied by pulsed Raman spectroscopy

The anti-Stokes Raman scattering of a-Si and a-Si:H is studied at T=2 K for intense (∼1 mJ/${\mathrm{mm}}^2$) pulsed optical excitation. It is found that high-energy phonons (ħω>100 ${\mathrm{cm}}^{\mathrm{-}1}$) emitted as a result of carrier relaxation possess a nonequilibrium energy distribution. A time-resolved study of the Raman signals reveals relaxation time constants for 480-${\mathrm{cm}}^{\mathrm{-}1}$ phonons as long as 10 ns. The effect of phonon localization is suggested as an explanation of the experimental results.