Electronic anisotropy of the GaAs(001) surface studied by energy loss spectroscopy

High-Resolution Electron-Energy-Loss Spectroscopy (HREELS) has been applied to investigate the anisotropy of the GaAs(001)-c(4 x 4) and beta2(2 x 4) reconstructions. 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 construct relative difference intensity spectra which can be directly compared with the differential reflectivity spectra of the RAS spectroscopy. A one-to-one correspondence is found between experimental and calculated electronic transitions up to about 3 eV. The surface anisotropy given by EELS is about two orders of magnitude higher than that measured optically. The contributions to the anisotropy originate entirely from a few atomic layers beneath the surface. In the beta2 phase we find direct evidence of transitions involving the dimers of the top atomic layer which are well separated by those involving bulk states modified by the surface.