We describe both reflectance anisotropy and electroreflectance measurements carried out to determine the physical origin of the anisotropies observed in the reflectance spectrum of (001) and (110) GaAs. We find an anisotropy component which depends on impurity concentration for both (001) and (110) surfaces [and on conductivity type for (001) GaAs]. This component is actually a bulk-related electro-optic effect produced by the electric field present at the semiconductor surface. This electric field is due to the pinning of the Fermi level at surface states. We find that a linear electro-optic effect is responsible for the impurity-dependent aniso tropies observed in GaAs (001), while a quadratic electro- optic effect is responsible for those observed in GaAs (110). We give an estimate for the linear electro-optic coefficients of GaAs at energies around the E1 and E1 + z transitions.
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