Raman spectra of (001) (GaAs)m(AlAs)n, 1 less-than-or-equal-to m, n less-than-or-equal-to 7 superlattices have been measured in backscattering along x' \\ [110BAR], x \\ [010], and z \\ [001] directions with a microprobe. Confined longitudinal (LO) and transverse (TO) optical phonons with q either parallel or normal to the z axis have been studied in all the independent scattering geometries. A strong anisotropy is observed when the direction of the transferred momentum is changed from the z axis to x' or x. The most evident feature is that the Raman spectra in the z(x'x')zBAR geometry axe dominated by LO1 confined phonons, whereas no signal at this energy is present in the y'(x'x')y'BAR geometry. Modes evolving from TO1 and LO(n) (n odd) phonons give rise to structures at intermediate frequencies in the x'(y'y')x'BAR spectrum. In contrast to LO1, the measured TO1 energy remains constant, showing the isotropy of the lower-energy branch of this doubly degenerate mode. These results axe explained by microscopic lattice-dynamical calculations performed in an ab initio scheme, which properly accounts for angular dispersion and mode mixing. We have also found that in the ultimate limit of confinement, when only one principal mode per branch can be considered, the phonon frequencies can be reproduced by the macroscopic dielectric continuum model, provided that the effect of phonon confinement is considered
Using Raman scattering we have measured the longitudinal optic phonon energies of Ga1−xInxAs layers with 0.48≤x≤0.55 grown on InP substrates. In order to interpret these data successfully it was necessary to consider the effect of the strain induced by the lattice mismatch. Existing theory has been extended to consider the case of alloy pseudomorphic layers and is approximated to a linear form. This theory is also applied to Ga1−xInxAs on GaAs and compared with previously published data with 0≤x≤0.2. Close agreement between theory and experimental data is found indicating that the Raman technique, combined with this theory, can be used to measure the alloy composition of pseudomorphic strained layers of diamond or zinc blende structure accurately without the need for detailed experimental calibration. It is suggested that this method could be particularly useful for determining the composition of alloy layers in thin layer pseudomorphic Ga1−xInxAs/InP heterostructures.
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