COMBINED HREELS, RAMAN AND X-RAY-DIFFRACTION STUDY OF SHORT-PERIOD GAAS-ALAS SUPERLATTICES

We have investigated short-period GaAs-AlAs superlattices with layer t hickness down to a few monolayers (ML) by combining HREELS, Raman spec troscopy and X-ray diffraction. The superlattices were grown on semi-i nsulating GaAs(100) substrates by molecular beam epitaxy (MBE). The ex perimental results are analyzed by the dielectric theory of HREELS. Th e 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 th e sample with a layer thickness of 18 ML an excellent agreement was ob tained 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 1 2 ML). From X-ray diffraction measurements we found that the layer thi ckness fluctuation due to interface steps becomes large relative to th e period for the superlattices with very thin layers. The thickness fl uctuation significantly affects the HREEL spectra of the superlattices with layer thickness less than a few ML.