Combined HREELS, Raman and X-ray diffraction study of short-period GaAs-AlAs superlattices

We have investigated short-period GaAs-AlAs superlattices with layer thickness down to a few monolayers (ML) by combining HREELS, Raman spectroscopy and X-ray diffraction. The superlattices were grown on semi-insulating GaAs(100) substrates by molecular beam epitaxy (MBE). The experimental results are analyzed by the dielectric theory of HREELS. The transverse-optical (TO) phonon frequencies and the damping constants of GaAs and AlAs of individual samples, required for the theoretical calculations, were determined by Raman-scattering measurements. For the sample with a layer thickness of 18 ML an excellent agreement was obtained between the theory and experiment, However, the theory fails to reproduce the experimental intensity ratio between the GaAs-like and AlAs-like loss peaks when the layer thickness becomes shorter (6 and 12 ML). From X-ray diffraction measurements we found that the layer thickness fluctuation due to interface steps becomes large relative to the period for the superlattices with very thin layers. The thickness fluctuation significantly affects the HREEL spectra of the superlattices with layer thickness less than a few ML.