Surface versus bulk contributions from reflectance anisotropy and electron energy loss spectra of the GaAs(001)-c(4x4) surface

We report on the results of a combined experimental study of the optical anisotropy of GaAs(001)-c(4x4) surfaces grown by molecular beam epitaxy and examined in situ by reflectance anisotropy spectroscopy (RAS) and high resolution electron energy loss spectroscopy (HREELS). A correspondence is found between the spectral features detected with RAS and HREELS. The results clearly show that electronic states localized at the surface strongly contribute to RAS and HREELS signals below the E-1 bulk critical point (similar to3 eV), while above E-1 the contribution of bulk states (modified by the surface) becomes largely predominant in RAS. Progressive oxidation of the clean surface by molecular oxygen modifies the RAS features providing additional experimental evidence of their origin.