PRECISE MEASUREMENTS OF THE GAP ENERGY OF FOLDED ACOUSTIC PHONONS IN SI/SI1-XGEX SUPERLATTICES

We report high-resolution Raman measurements of the gap widths of folded longitudinal-acoustic phonons in Si/Si1-xGex superlattices. The first four gap widths at the minizone boundary were studied systematically. It was found that to fit the measured gap widths by Rytov theory one has to use a reduced sound velocity and density from that of bulk Si for the Si layers and a higher sound velocity and density of the alloy layers than the value deduced by linear interpolation. In the high-resolution measurements fine structure of the phonon peaks was observed together with a linewidth narrowing when approaching the minizone boundary. The variation of the scattering intensities recorded at different wavelengths and for several orders of folding provides an opportunity for a precise test of light scattering theory near the minizone boundary.