Detection of surface states anisotropies at GaAs(001)(2x4) decapped surfaces

The surface or bulk origin of the optical anisotropies detected by reflectance anisotropy spectroscopy (RAS) at GaAs(001)(2 x 4) surfaces has been extensively investigated in the last years and a quite general agreement has been reached that the dominating character would be bulk-like. Nevertheless, a very recent paper has again issued the presence of surface states contributions in optical anisotropies, revealing a structure at 2.5 eV due to surface states, in addition to the well known features around 2.9 eV and 4.5 eV related to the bulk critical points El and Eo. We have carried out a new experiment to prove this conclusion by following the changes in the optical anisotropy of a GaAs(001)(2 x 4) surface in the range 2.0-5.0 eV induced by Ag/Sb-codeposition. The interface Ag/GaAs(001) is known to be not reactive. Due to its surfactant effect, codeposition of Sb leads to a nearly epitaxial growth of the Ag overlayer. We show that at the early stages of deposition (nominally at 0.25 monolayer) an evident modification of the RAS spectrum is detected at 2.5 eV, well below the photon energy (2.9 eV) where bulk-like anisotropies appear. We relate this modification to the disappearance of surface states characteristic of the (2 x 4) reconstruction, in excellent agreement with previous conclusions and experiments.