Effects of isotropic overlayer on the focusing of surface phonons

Based on the geometrical acoustic approximation, we study theoretically the phonon focusing at the (100) face of silicon with a thin oxide overlayer. The oxide layer is assumed to be isotropic elastically and the focusing factors of both the Rayleigh-surface and pseudosurface phonons are calculated for several values of k (wave number) x h (thickness of the oxide layer). As kh increases we find that the strong focusing region of Rayleigh-surface phonons in between two caustic directions becomes narrow and moves toward the [110] direction. Also the directions around the [110] axis, where the pseudosurface phonons are allowed to exist, are diminished and eventually their spatial intensity distribution disappears.