Surface acoustic modes in InSe and GaSe lamellar films

Brillouin light scattering has been used for studying surface acoustic mode propagation in InSe and GaSe films epitaxially grown on hydrogen-terminated non-reconstructed Si(111) surface. These films are characterized by strong covalent intra-layer bonds and weak van-der-Waals interlayer interaction. A large anisotropy of the elastic properties is therefore expected. Both Rayleigh and Sezawa guided acoustic modes have been revealed, and their velocity dispersion measured as a function of the ratio between the film thickness and the acoustic wavelength. The data obtained enabled us to determine four of the five elastic constants of the films through a non-linear best fit procedure. In addition, measurement of shear horizontal (Love) modes enabled us to determine the fifth elastic constant C₆₆. The obtained sets of elastic constants show a strong anisotropy with c₃₃ and c₄₄ much lower than c₁₁ and c₆₆, respectively.