The electronic nature of the Cl adatom at GaAs(1 1 0) surfaces due to Cl2 dissociative adsorption and CCl4 photodecomposition

Clean, well defined GaAs(1 1 0) surfaces at room temperature, were exposed to carbon tetrachloride vapor. Only when illuminated during the exposure (with 400 mW/cm2 of 532 nm light) could adsorbed species be observed with scanning tunneling microscopy (STM) on flat terraces. The features in the STM images were compared to those due to exposure to Cl2 gas in the dark, and the CCl4 decomposition products were shown to be identical to chlorine adatoms which are bound to surface Ga atoms (A-type). Since the Cl adatoms appear dark at low positive sample bias, and bright at high bias, and from the absence of localized contrast in atomically-resolved surface photovoltage images, we deduce that the Cl adatom interacts with the Ga-induced C3 surface resonance, that it (or the Cl–Ga complex) is not charged, and it does not give rise to any surface state in the GaAs band gap that could pin the Fermi level.