Oxidation of the surface ofA15Nb3Ge

An oxide layer grown on the $A15\mathrm{}$ ${\mathrm{Nb}}_3$Ge surface after exposure to pure oxygen is found to be mainly ${\mathrm{Nb}}_2$ ${\mathrm{O}}_5$ from the in situ observation of x-ray photoelectron spectroscopy. The thickness of the ${\mathrm{Nb}}_2$ ${\mathrm{O}}_5$ layer varies with the oxidation time by following the logarithmic-growth law. The oxide layer serves as a protective stable film with a thickness of around 1.5 nm. Beneath the oxide layer an Nb-deficient layer occurs in a thickness of 2 nm due to the preferential diffusion of Nb atoms. This is caused by the contact potential difference between the ${\mathrm{Nb}}_3$Ge and adsorbed oxygen layers and the ionization potential difference between Nb and Ge atoms.