Influence of crystal-potential fluctuations on Raman spectra of coupled plasmon–LO-phonon modes in disordered systems

Abstract

The effect of the structural disorder in the Raman scattering of the coupled plasmon--LO phonon modes was studied in doped ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ alloys and in doped GaAs/AlAs superlattices. It was observed that the asymmetry in the Raman lines, caused by this effect, is opposite to that observed for the optical phonons. This fact is explained by the differences in the dispersion curves of the optical phonons and plasmons---a negative dispersion for the phonons and a positive one for the plasmons. The analysis of the Raman line shapes by means of a Gaussian spatial correlation function allowed us to obtain the localization lengths for the LO phonons and for the plasmons in the alloys and in the superlattices; the dispersions of the coupled plasmon--LO-phonon modes were studied and an evidence for a metal-dielectric transition occurring in superlattices when the lowest miniband is completely occupied was found.