Surface and guided acoustic phonons in porous silicon

Brillouin spectroscopy of porous Si (pi-Si) formed on p-type (100) Si has r evealed two acoustic-phonon peaks associated with the surface and film mode s of the thick porous slab. They exhibit relatively large peak widths that are attributed to fractons. Longitudinal wave guiding is enhanced in high p orosity pi-Si due to its very low sound velocities compared with Si. Unlike opaque semiconductors where ripple scattering is dominant, the elasto-opti c contribution to the scattering mechanism is found to be significant in pi -Si. Both phonon velocities show appreciable negative dispersion due to a p orosity gradient.