Angle-resolved X-ray photoelectron spectroscopy study of the oxides on Nb surfaces for superconducting r.f. cavity applications

Angle-resolved X-ray photoelectron spectroscopy is used to study the oxidation of chemically etched, polycrystalline Nb surfaces and thermal effects on the oxide layer. Annealing the sample at 250 °C does not remove the oxide layer but simply changes the oxide composition from a Nb2O5-dominated layer to a Nb2O-dominated one through oxygen diffusion. The latter is stable in ultrahigh vacuum and metallic in nature. For oxygen diffusion into the bulk at 250 °C, estimations are made of the composition of the oxygen-enriched metal layer and the density change of this layer due to interstitial oxygen. Room temperature re-oxidation of the annealed surface follows similar kinetics as the oxidation of as-etched surfaces. Nb2O5 is simply re-generated in the existing oxide layer by inward oxygen diffusion starting out from the oxide surface. It is demonstrated that the electric field-assisted, Cabrera–Mott mechanism operates in the initial Nb surface oxidation. Its effectiveness diminishes quickly as the insulating oxide layer grows over 3 nm. The existing electric field is evidenced by the shifts in the binding energy of Nb2O5 towards that of Nb metal, indicating a potential drop across the oxide.