Abstract
GaAs (110) surfaces of $p$- and $n$-type crystals cleaved in ultrahigh vacuum are investigated by ellipsometry, surface photovoltage (SPV) spectroscopy and low-energy-electron loss spectroscopy (ELS). Changes of the ellipsometric angle $\ensuremath{\delta}\ensuremath{\Delta}$ which are induced by a Franz-Keldysh effect in the space-charge layer due to adsorbed oxygen indicate that oxygen adsorption changes the band bending of the clean surface at dosages (\ensuremath{\sim} 1 langmuir, 1 L = ${10}^{\ensuremath{-}6}$ Torr sec) much lower than those which produce measurable Auger-electron signals. In SPV spectroscopy on a clean perfectly cleaved (110) surface empty or occupied surface states can not be detected in the forbidden band whereas oxygen adsorption and/or crystallographic defects produce such states. On crystallographic irregularities oxygen also induces a new set of extrinsic surface states close to the valence or the conduction-band edge. The ELS data in combination with SPV results support the interpretation of the $\mathrm{Ga}(3d)\ensuremath{-}\mathrm{G}\mathrm{a}$ (surface state) transition in terms of a surface exciton. ELS furthermore suggests a contribution of Ga surface atoms to the chemisorption bond of oxygen. The results from ellipsometry and SPV spectroscopy can be understood by means of a model in which two discrete sets of surface states near midgap are induced by adsorbed oxygen.
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