Acoustic barriers and observation of guided elastic waves in GaN-AlN structures by Brillouin scattering

We report on the localization of acoustic waves in GaN-AlN heterolayers deposited on Si that lead to the observation, by Brillouin light scattering, of distinctive guided excitations associated with the hexagonal $(2H)$ GaN epilayers. These elastic modes include the longitudinal guided resonance (LGR) propagating parallel to the film surface and a shear horizontal resonance (SHR) polarized in the plane of the surface. The observation in light scattering of guided excitations arise from the strong elasto-optic properties of the GaN thin films as well as the presence of AlN that acts as a barrier by reducing energy leakage to the substrate. Calculations of the mode density and displacements disclose characteristics of the LGR and SHR and account for their localization and polarization features. Several of the principal elastic constants of the GaN layer are determined from the data that also reveal the bulk acoustic phonons and the Rayleigh surface mode.