Phonon wind on excitons in silicon

We observed experimentally the strong effect of nonequilibrium phonons on the intensity of the luminescence of free and bound excitons, created by optical illumination of a Si surface immersed in liquid helium (T=1.8 K). The polarity of the phonon-induced luminescence signal strongly depends on the perfection of the surface. Two factors were included in the theoretical explanation of this effect: (1) the phonon-induced dissociation of bound excitons, and (2) the drag of free excitons by phonon flux (phonon-wind effect) which shifts the exciton cloud to the surface, where exciton recombination is strong. This effect is used to study spectral features of phonon propagation in Si and a-Si:H films. (orig.)