Light-scattering study of the localization of longitudinal acoustic pseudomodes in a buried silica layer

Brillouin light spectroscopy in p-p backscattering geometry is used to study sagittal surface acoustic phonons in silicon on insulator structures formed on a silicon buffer. The experimental spectra show, near the longitudinal threshold of silicon, two peaks whose physical meaning is discussed by comparison with theoretical cross sections. Calculations of Brillouin cross sections were performed, taking into account both the ripple and elastooptic coupling mechanisms. The peaks originate from two pseudomodes: the first is highly localized in the buried ${\mathrm{SiO}}_{2}$ layer and the second in the top silicon layer. The dependence of the pseudomode localization and cross section intensity with the parallel wave vector and with the thickness of the top silicon layer are discussed.