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 degreesC does not remove the oxide layer but simply changes the oxide composition from a Nb2O5-dominated layer to a WO-dominated one through oxygen diffusion. The latter is stable in ultrahigh vacuum and metallic in nature. For oxygen diffusion into the bulk at 250 degreesC, 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. (C) 2002 Elsevier Science B.V. All rights reserved.