In this paper, the enhancement of the Al(111) oxidation rate by simultaneous oxygen exposures and electron irradiation is investigated at 90 K using X-ray photoelectron spectroscopy (XPS) and work-function (WF) measurements. Electron-beam energies of ∼0–100 eV and current densities up to 80 μ${\mathrm{A}/\mathrm{c}\mathrm{m}}^2$ are used. No effect of electron irradiation is found at the initial stages of oxidation. The effect (enhanced oxidation rate) is only observed at higher ${\mathrm{O}}_2$ exposures (above 100 L), when clusters of aluminum oxide are formed on the surface. The existence of a molecular ${\mathrm{O}}_2$ precursor state associated with these clusters is proposed, based on both the surface temperature and beam-energy dependence of the oxidation rate. In addition, charge trapped on the oxide grains sets up an electrostatic field (detected by XPS and WF measurements) stimulating ion diffusion and further oxide film growth.

• Received 26 December 2000

DOI:https://doi.org/10.1103/PhysRevB.65.195409