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

High-resolution synchrotron-radiation photoemission studies of molecular ${\mathrm{O}}_2$ condensed on GaAs(110) at 20 K show that oxidation is a consequence of photon irradiation. Core-level results for 2 L ${\mathrm{O}}_2$ [1 langmuir (L)==${10}^{\mathrm{-}6}$ Torr sec] demonstrate that the topmost layer of As atoms is initially involved in a sequential, two-step reaction to produce ${\mathrm{As}}^{1+}$- and ${\mathrm{As}}^{3+}$-like oxides. These reactions are mediated by secondary electron capture by ${\mathrm{O}}_2$ which then dissociates to form surface oxides. ${\mathrm{As}}^{5+}$-like bonding configurations are formed when additional ${\mathrm{O}}_2$ is condensed on the surface and exposed to photon irradiation. ${\mathrm{O}}_2$-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 ${\mathrm{O}}_2$ exposure suggest that oxidation at 20 K produces acceptorlike states. Fermi-level evolution for n-type GaAs is strongly dependent on dopant concentration, ${\mathrm{O}}_2$ 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.