Brillouin spectroscopy of acoustic modes in porous silicon films

A systematic Brillouin study has been carried out on acoustic modes in porous silicon films prepared from p-type Si(100). A surface Rayleigh mode and two bulk acoustic modes, transverse and longitudinal, were observed. Results are presented for the dependence of the phase velocities of these three modes on sample porosity and film thickness. The phonon velocities, especially those of the bulk modes, increase sharply with decreasing film thickness below ∼10 μm. This thickness dependence is attributed to the presence of a transition layer between the porous silicon film and crystalline Si substrate. Its influence on the film bulk modes increases with decreasing thickness. Elastic constants and Young's modulus of porous Si are estimated for different porosities from the phase velocities of the two bulk acoustic modes and are compared with the corresponding data obtained by other techniques.