| Thermal oxidation of Al(110) in O2 is investigated in temperature-pressure-time phase space using medium energy ion scattering and x-ray photoelectron spectroscopy. Alumina film characterization shows the formation of an ultra-thin (<30 Å) stoichiometric Al2O3 overlayer with a fairly abrupt oxide/metal interface. The roughness onset is found only upon oxidation at a higher pressure or prolonged time for oxidation temperatures above 300°C. The growth rate is best described by the Cabrera-Mott theory of field-assisted oxidation and by a square-root pressure dependence. No charging effects have been observed after oxygen exposure, indicating that the surface ionic oxygen layer exists dynamically only during oxidation. Oxygen isotopic substitution (16O/18O) studies reliably identify oxygen ions as at least one of the principal mobile species responsible for material transport via oxide network defects. Analysis of isotope depth distribution shows a strong dependence of the oxygen incorporation at the oxide/metal interface on oxidation temperature, pressure and time, and no or little effect of these parameters on the concentration of oxygen incorporated at the gas/oxide interface. We argue that these observations support oxygen transport via a vacancy mechanism, with fast oxygen vacancy injection into the oxide at the oxide/metal interface. The rate-limiting stage of oxidation is an oxygen vacancy transport across the oxide.; For Si oxidation, the additional reaction channel for unstable SiO species formation can become important. We investigate the role of this reaction in the overall oxidation process in different parts of the pressure-temperature phase diagram. For the first time, SiO formation and out-diffusion into the gas phase at very low rate (∼10−5 monolayer/s) was detected after formation of a continuous SiO2 overlayer. We also demonstrate the significance of the interfacial SiO 2 layer in the thermal stability of metal oxide dielectric stacks grown on silicon. |
