Brillouin spectroscopy was used to study surface acoustic waves on a supported layer of (111)-oriented porous silicon having a thickness of 2.7 μm and a porosity of 30%. The Rayleigh surface wave velocities were found to be significantly lower than corresponding velocities for crystalline silicon. A complete set of elastic constants for the porous layer was determined from the measured directional dependence of the surface wave velocity in the (111) plane. The best-fit constants are C11=56.0±0.7 GPa, C12=6.7±0.3 GPa and C44=37.0±0.8 GPa. The anisotropy factor, η=1.50 indicates that the porous layer is elastically anisotropic.
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