Dependence of oxide thickness on O2 pressure and experimental determination of Gibbs free energies of ultrathin oxide films in vacuum

Abstract

Ultrathin oxide films grown in a vacuum are important in many industrial processes, with the values of Gibbs free energies of the gas and films determining the oxide type that grows on a surface, and its thickness. A high-stability quartz-crystal microbalance is used to provide quantitative experimental data on these free energies, for the model case of oxidation of a lead film on a gold substrate. The surface oxide, PbO, forms a single molecular layer at 10−6Torr of O2, but thickens abruptly to two layers at 1×10−4Torr, and becomes even thicker by 0.33Torr. This is explained using film free energies.