Vv. Aleksandrov et al., EXPERIMENTAL-EVIDENCE OF THE TRANSFORMATION OF THE RAYLEIGH SURFACE PHONON IN CAF2 GAAS(111) HETEROSTRUCTURES OF THE ACCELERATING TYPE/, Journal of physics. Condensed matter, 6(10), 1994, pp. 1947-1954
The spectrum of surface acoustic phonons in films of CaF2 on GaAs(111)
has been investigated by means of Brillouin light scattering. We repo
rt the first experimental observation of the disappearance of the Rayl
eigh phonon with increasing film thickness in these structures of the
fast-on-slow or accelerating type. For propagation along (121) at the
disappearance of the Rayleigh surface phonon, a quasi-resonance in the
continuum, whose spectral position tends to that of the Rayleigh phon
on 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 pseudos
urface acoustic mode. The theoretical discussion is performed by compa
ring the Brillouin spectra with the surface projected phonon density o
f states for shear vertical phonons numerically calculated for CaF2/Ga
As(111) heterostructures.