Angle resolved XPS studies of oxides at Nb-, NbN-, NbC- and Nb3Sn-surfaces

Nb, NbN, NbC and Nb3Sn are known to be chemically inert with passivating oxides only solvable in HF-acid. Despite Nb2O5as outermost oxide layer, the oxides of Nb compounds show large differences in thicknesses and in electronic properties. To quantify the differences, angle resolved XPS (ARXPS) measurements have been performed. The simultaneous fitting for different angles and preparations of the Nb, Sn, C, N and O XPS lines of the oxides yielded the following stoichiometries and distributions : a) Nb2O5is the outermost oxide layer on all Nb-compounds. In the case of Nb3Sn the SnO2is substituted throughout the Nb lattice ((Nb2O.5)1.5SnO2), whereas for NbN the N substitutes O sites in Nb2O5(Nb2N2-xO3+x, ) adjacent to NbN in air oxidation. b) For Nb and Nb3Sn underneath Nb2O5the NbO matches to the metallic suboxides. c) NbX1-xOx(x<0.5) as metallic suboxide forming lumps. The oxide growth a-c is not planar, instead the oxides serrate the metal surface on a nm scale. The serration is strongest for soft, defective Nb and smallest for the harder compounds NbN and NbC in Parallel to the oxidation rate which is slowest for NbC. This first identification of the oxinitrides of the metallic suboxides and of the reduced serration of NbN (NbC) explains the improved oxide quality and quality of tunnel junctions and rf cavities as compared to defective Nb single cristals. The first identification of NbX1-xOx(x<0.5) compounds serrating the metals explains many deteriorations of superconductivity by oxidation. In granular NbN metallic and dielectric oxides have been identified between the grains.