Brillouin Scattering by Surface Acoustic Modes for Elastic Characterization of ZnO Films

Brillouin scattering from surface phonons has been used for determining the dispersion curves of guided acoustic modes propagating along piezoelectric ZnO films. Measurements were performed on films of different thicknesses in the range between 20 and 320 nm, deposited by RF magnetron sputtering on Si and SiOz substrates. Brillouin spectra from Rayleigh acoustic modes were taken in the backscattering geometry at different incidence angles between 30 and 70 degrees. The experimental data for the ZnO/Si films fit the expected theoretical dispersion curves fairly well for film thicknesses greater than 150 nm, while they appreciably depart from the same curves for smaller thicknesses. This behavior has been interpreted in terms of a reduction of the effective elastic constants of the film in a layer near the interface, due to the lattice misfit between the film and the substrate. This effect was not observed in the case of ZnO films deposited on fused quartz substrates.