SURFACE BRILLOUIN-SCATTERING IN SEMICONDUCTOR FIBONACCI MULTILAYERS

Brillouin spectra from surface (Rayleigh) acoustic waves in a-Si:H/a-SiNx:H (x0.94) Fibonacci multilayers have been measured with a tandem multipass Fabry-Pérot interferometer. To make a detailed analysis for the observed surface (Rayleigh) waves, following Merlin et al. [Phys. Rev. Lett. 55, 1678 (1985)] and Dharma-wardana et al. [Phys. Rev. Lett. 58, 1761 (1987)], we have formulated the propagation of surface acoustic waves parallel to the surface of the quasiperiodic multilayers using the Fourier-transform method in the long-wavelength regime. We found that the obtained Brillouin spectra can be accounted for by this new derivation of the Fourier transform of the surface acoustic waves, but a rather fascinating feature for the phonon frequency is that it is slightly modified by quasiperiodic indices (n,m) or p. Such a modulation is attributed to the existence of a small but nonvanishing acoustic impedance. Our theoretical approach gives the correct order of amplitude modulation for surface (Rayleigh) waves within an error of <1%. We thereby obtain an overall understanding of the elastic properties of the Fibonacci multilayers. © 1990 The American Physical Society.