Experimental evidence of the transformation of the Rayleigh surface phonon in CaF2/GaAs(111) heterostructures of the accelerating type

The spectrum of surface acoustic phonons in films of CaF2 on GaAs(111) has been investigated by means of Brillouin light scattering. We report the first experimental observation of the disappearance of the Rayleigh phonon with increasing film thickness in these structures of the fast-on-slow or accelerating type. For propagation along at the disappearance of the Rayleigh surface phonon, a quasi-resonance in the continuum, whose spectral position tends to that of the Rayleigh phonon of CaF2(111) for larger value of film thickness, is detected in the spectrum. Because of this behaviour we identify this quasi-resonance as the pre-Rayleigh mode of the film. When propagation is along (110) the role of the pre-Rayleigh mode of the film is played by the pseudosurface acoustic mode. The theoretical discussion is performed by comparing the Brillouin spectra with the surface projected phonon density of states for shear vertical phonons numerically calculated for CaF2/GaAs(111) heterostructures.