Surface photovoltage spectroscopy investigations of the electronic surface states on clean and oxygen-exposed polar GaAs(100) and GaAs(111) surfaces

Surface photovoltage spectroscopy (SPS) combined with Auger electron spectroscopy (AES) control have been used in the investigation of the electronic surface states on atomically clean, polar n-type GaAs(100) and GaAs(111) surfaces prepared by means of the ion bombardment annealing (IBA) procedure and subsequently exposed to oxygen. For both of the clean surfaces the shape of the surface photovoltage spectrum corresponds to the onset of the band-to-band semiconductor transitions. After oxygen adsorption the amplitude of the surface photovoltage signal for photon energy hν > E_g increases significantly, especially for the GaAs(111) surface. Moreover, for photon energy hν < E_g an additional wide shoulder in the surface photovoltage appears for both of the investigated GaAs surfaces. It becomes more prominent with increasing oxygen exposures and extends down to photon energies of 1.26 and 1.22 eV, respectively and was attributed to oxygen-induced electronic surface states localized below the middle of the band gap at the surface.