Surface states at the GaAs(001)2X4 surface

We have clearly evidenced the contribution of true surface states, in the range 1.8-2.5 eV, to the optical anisotropy of the GaAs(001)2X4 surface grown in situ by molecular beam epitaxy. These spectral features are well below the photon energy (2.9 eV) where bulklike anisotropies appear in coincidence with the Et bulk critical point. The surface character is established by studying the evolution of the reflectance anisotropy spectroscopy spectra versus oxygen exposure. The interpretation is strengthened by comparison with high-resolution electron-energy-loss spectra measured on the same surface and by first-principles density-functional theory calculations.