Comparative study of Al(1 1 1) oxidation with O3 and O2

The interaction of O3 and O2 with the Al(1 1 1) surface was investigated over the surface temperature range of 90–600 K using X-ray photoelectron spectroscopy. The oxidation of aluminum at 300 K with ozone gas was found to proceed with an 8.5 times higher initial sticking coefficient compared to oxygen, reaching a 2–2.3 times higher saturation oxide layer thickness of approximately 20 Å. The enhanced oxidation rate observed at lower temperatures indicates that trapping of the molecular ozone species into a shallow precursor well occurs in the course of Al(1 1 1) oxidation. The difference in activation energy for O3 adsorption and desorption to and from this state is (Ed−Ea)=7±2 meV. Subsequent dissociation of the ozone molecules on the surface occurs possibly through the production of chemically active atomic oxygen. O3-induced oxidation occurs by means of a mechanism involving preferential direct formation of Al2O3 clusters. In contrast, for oxidation with molecular oxygen, the formation of a chemisorbed O phase, followed by its slow transformation into oxide clusters, is observed. The higher electron affinity of the ozone molecule compared to oxygen is proposed to also enhance the kinetics of Al oxidation at the later stages of the oxide layer growth (film thickness d⩾2–3 ML), by causing enhanced tunneling of electrons from Al metal to the adsorbed O3. Tunneling electrons create an electrostatic potential across the growing film, stimulating ion-diffusion and, thus, oxide film growth.