Micro-Raman scattering in ultrathin-layer superlattices: Evidence of zone-center anisotropy of optical phonons

Raman spectra of (001) (GaAs${)}_{\mathit{m}}$(AlAs${)}_{\mathit{n}}$, 1≤m, n≤7 superlattices have been measured in backscattering along x’∥[1¯10], 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’)z¯ geometry are dominated by ${\mathrm{LO}}_1$ confined phonons, whereas no signal at this energy is present in the y’(x’x’)y¯ ’ geometry. Modes evolving from ${\mathrm{TO}}_1$ and ${\mathrm{LO}}_{\mathit{n}}$ (n odd) phonons give rise to structures at intermediate frequencies in the x’(y’y’)x¯ ’ spectrum. In contrast to ${\mathrm{LO}}_1$, the measured ${\mathrm{TO}}_1$ energy remains constant, showing the isotropy of the lower-energy branch of this doubly degenerate mode. These results are 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.