Brillouin light scattering study on the elastic properties of thick sputtered c-BN films

The Brillouin light scattering technique is used to observe thermally excited acoustic phonons in backscattering geometry in a transparent, 1.8 μm thick diode sputtered c-BN film. The Rayleigh mode at the film surface and the quasilongitudinal bulk mode were detected under various angles of light incidence. The angular dependent phase velocity of the bulk wave provides evidence for an elastic anisotropy of the film material. Complete descriptions of the elastic properties are presented, assuming either isotropic or hexagonal film symmetry. The reduction of the film stiffness in comparison to the single crystal and the origin of the elastic anisotropy of c11/c33≈4/5, where c11 and c33 are the respective extensional stiffness constants parallel and perpendicular to the film, is discussed. Both effects are predominantly caused by sp2-bonded material and a structured film growth.