Spectra of surface phonon excitations in a CaF2/Si(111) structure deduced from Mandelshtam-Brillouin light-scattering data

The Mandelshtam-Brillouin spectroscopy (MB) was used in an investigation of the spectra of long-wavelength phonon excitations of the surface in CaF 2 /Si (111) structures with the fluoride film ranging in thickness from 0 to 400 nm for different azimuthal directions in the (111) plane. The experimentally determined values of the velocity of sound were compared with calculations carried out using the bulk values of the elastic parameters of the film and substrate materials. The experimental velocities agreed with the calculations with the exception of the case when the film thickness was 100 and 120 nm and the velocity of sound was found to be 5-8% less than the theoretical value, which could be explained by a strong (for films of these thicknesses) influence of the mismatch strain on the elastic properties of the structure