Acoustic phonon propagation and elastic properties of nano-sized carbon films investigated by Brillouin light scattering

Abstract

Carbon films having a structure at the nanometer scale are of paramount interest. Experimental tools for the mechanical characterization of such materials are still an open issue, due to the relevance of mesoscopic scales. Brillouin light scattering by acoustic phonons of sub-micrometric wavelength is among the few techniques sensitive at the appropriate length scale. Elastic properties can be derived from measured acoustic properties, if the film thickness and mass density are independently measured, typically by X-ray reflectivity. Applications to two very different types of films are discussed. Tetrahedral amorphous carbon (ta-C) films of high density and stiffness can be deposited with thickness down to a few nanometers. We show that combining Brillouin scattering and X-ray reflectivity the elastic properties of films can be measured for thicknesses down to 2 nm. We also measure the dependence of stiffness on thickness, finding that the high stiffness of thicker ta-C films is reached for thicknesses of approximately 10 nm. Films deposited by low energy cluster beam deposition are characterized by granularity and porosity at scale lengths ranging from nanometers to micrometers. We show that Brillouin scattering discriminates well among films compact enough to support the propagation of acoustic phonons and films in which such phonons are confined and/or over-damped. When acoustic propagation is supported the elastic properties can be obtained, at wavelengths of hundreds of nanometers. It is thus shown that Brillouin scattering has a peculiar potential for the characterization of films having structures of nanometric size.