Brillouin spectroscopy of surface modes in thin-filmSi3N4on GaAs

Abstract

We report here the determination, by surface Brillouin spectroscopy, of the dispersion relation of surface acoustic phonons localized at the free surface and interface of thin film ${\mathrm{Si}}_{3}{\mathrm{N}}_{4}$ on a GaAs substrate. A splitting in the dispersion curve representing velocity versus kh is observed experimentally. With increasing kh, starting from zero the Rayleigh mode of the substrate increases in velocity and then at a cut-off degenerates with the bulk mode continuum, and transforms into a pseudointerfacial wave. This mode propagates with attenuation along the layer-substrate interface, decaying exponentially with depth into the layer and leaking energy into the substrate. In the same range of kh beyond cutoff, a second mode appears that for large kh evolves into the Rayleigh wave of the film. The purpose of this paper is to demonstrate experimentally this splitting of the dispersion relation into two branches. This is the first reported observation, for a stiffening layer on a substrate, of a leaky interfacial wave, a type of mode that is of potential value in the interface characterization of thin supported transparent films.