Surface Brillouin scattering study of the surface excitations in amorphous silicon layers produced by ion bombardment

Thin amorphous silicon layers on crystalline silicon substrates have been produced by argon-ion bombardment of (001) silicon Surfaces. Thermally induced surface excitations characteristic of this example of a soft-on-hard system have been investigated by surface Brillouin scattering (SBS) as a function of scattering-angle and amorphous-layer thickness. At large scattering angles or for sufficiently large layer thickness, a second peak is present in the SBS spectrum near the low-energy threshold for the continuum of bulk excitations of the system. The measured spectra are analyzed on the basis of surface elastodynamic Green's functions, which successfully simulate their detailed appearance and identify the second peak as either a Sezawa wave (true surface wave) or a pseudo-Sezawa wave (attenuated surface wave) depending on the scattering parameters. The attributes of the pseudo-Sezawa wave are described; these include its asymmetrical Line shape and variation in intensity with k(parallel to)d (the product of the surface excitation wave vector and the layer thickness), and its emergence as the Sezawa wave from the low-energy side of the Lamb shoulder at a critical value of k(parallel to)d. Furthermore, the behavior of a pronounced minimum in the Lamb shoulder near the longitudinal wave threshold observed in the experiments is reported and is found to be in good agreement with the calculated spectra. The elastic constants of the amorphous silicon layer are determined from the velocity dispersion of the Rayleigh surface acoustic wave and the minimum in the Lamb shoulder. [S0163-1829(98)04039-9].