The effect of the structural disorder in the Raman scattering of the coupled plasmon-LO phonon modes was studied in doped Al x Ga 1Ϫx 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.
The plasmon-longitudinal optical phonon vibrational modes in heavily doped GaAs/AlAs superlattices were analyzed both with Fourier transform infrared and Raman spectroscopies. The modes polarized in the direction normal to the layers were observed in the infrared spectra, while the Raman spectra revealed those modes polarized along the layers. The experimental results allowed us to derive the dispersion relations of the electrons in both directions and these relations were found to be in good agreement with the miniband dispersions calculated in the envelope function approximation when both Γ and X conduction electron states were taken into account. No evidence of the contribution of the two-dimensional electron states to the vertical transport was found.
Raman scattering was applied to a study of collective plasmon-LO phonon excitations polarized normal to the layers in strongly coupled GaAs/AlAs and ␦-doped GaAs superlattices. Thus, the corresponding ͑vertical͒ motion of electrons and their localization were studied and compared in both types of superlattices. The localization of the collective excitations caused by the localization of the electrons at the top of the miniband was found in the GaAs/AlAs superlattices. Much stronger localization effects were observed in the ␦-doped structures than in the GaAs/AlAs superlattices. Our results reveal evidence of the formation of a miniband structure in the ␦-doped structures even in the case of complete localization of electrons inside one period.
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