O2/GaAs(110) interface formation at 20 K: Photon-induced reaction and desorption

High-resolution synchrotron-radiation photoemission studies of molecular O2 condensed on GaAs(110) at 20 K show that oxidation is a consequence of photon irradiation. Core-level results for 2 L O2 [1 langmuir (L)==10-6 Torr sec] demonstrate that the topmost layer of As atoms is initially involved in a sequential, two-step reaction to produce As1+- and As3+-like oxides. These reactions are mediated by secondary electron capture by O2 which then dissociates to form surface oxides. As5+-like bonding configurations are formed when additional O2 is condensed on the surface and exposed to photon irradiation. O2-GaAs interface reactions slow as transport through the thickening oxides is impeded, and photon-induced desorption of oxygen becomes significant. Studies of Fermi-level movement into the gap as a function of O2 exposure suggest that oxidation at 20 K produces acceptorlike states. Fermi-level evolution for n-type GaAs is strongly dependent on dopant concentration, O2 dose, and light exposure, indicating band flattening for lightly doped samples due to surface photovoltage effects. These effects are not significant for p-type GaAs at 20 K, consistent with the formation of acceptorlike states. Together, these results show a complex dependence of surface chemistry on photon irradiation, but remarkably little dependence of the surface Fermi-level position on the reactions.