Anisotropy of the GaAs(001)-beta 2(2x4) surface from high-resolution electron energy loss spectroscopy

High-resolution electron energy loss spectroscopy has been applied to investigate the anisotropy of the GaAs(001)-beta2(2x4) reconstruction. Measurements have been performed on high-quality samples grown in situ by molecular beam epitaxy. The loss intensity is different in the directions parallel and perpendicular to dimers, particularly close to the fundamental gap. We discuss our data in comparison to reflectance anisotropy spectra in the attempt of identifying the source of the anisotropy close to the bulk critical points where surface and bulk like transitions take place. We find direct evidence of transitions involving the dimers of the top atomic layer. The set of data taken on As-decapped surfaces indicates that the intensity of the structures is markedly affected by the As cap removal. The exposure to increasing amounts of molecular oxygen modifies the spectral features in a way consistent with their surface origin.