XPS and AES studies on oxide growth and oxide coatings on niobium

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Abstract
Niobium surfaces show features that have been blamed on the unavoidable oxidation that occurs during the preparation of the Nb. To quantitatively measure the oxidation, XPS and AES profile measurements have been carried out for the typical procedures used in the preparation of Nb surfaces, such as ultrahigh vacuum annealing (UHV), oxipolishing (OP), electropolishing (EP), or handling in air, H2O, and H2O2. For the different surface preparations the oxidation proceeds as follows: At the Nb surface an inhomogeneous Nb2O-NbO layer 1 nm or less thick exists which does not seem to depend much on oxidation technique. Then a dielectric oxide Nb2O5 grows homogeneously in UHV (?2 nm) or in OP(?3 nm), whereas EP(?3 nm) yields rough oxide layers that pick up impurities and charges easily. With time, all oxides grow more or less rapidly to a stable thickness of 6 nm. These results explain the observed behavior of Nb surfaces, especially their responses to the most sensitive measurements, namely, in superconducting rf cavities and tunnel junctions.