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We report on both optical reflectance anisotropy and electroreflectance measurements carried out in order to determine the physical origin of the impurity-dependent anisotropy observed in the reflectance spectrum of (001) GaAs. We found that such anisotropy is actually a bulk-related, linear electro-optic eFect produced by the electric field present at the semiconductor surface. This electric field is due to the pinning of the Fermi level at this surface because of the presence of surface states. Our results are important for the application of optical reflectance anisotropy measurements to the study of surface processes in zinc-blende semiconductors.The measurements of above-band-gap anisotropies in the reflectance spectrum of cubic semiconductors is ernerging as a tool for the study of surface processes in such materials. ' This is due to the fact that the bulk of cubic semiconductors is nominally isotropic, so that any observed reflectance anisotropy should be related to the lower symmetry of the semiconductor surface. However, bulk-related reflectanee anisotropies could be produced by spatial dispersion effects, 3 or, as shown in this paper, by the breakdown of the cubic symmetry of the semiconductor by a directional perturbation.An unambiguous identification of the origin of the observed anisotropies is then essential in order for the measurement of reflectance anisotropies to be useful in the study of surface processes in cubic semiconductors.Above-band-gap optical anisotropies were first observed by Cardona, Pollack, and Shaklee in the reflectance spec-

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