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

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Abstract
NbN, NbC and Nb are known to be chemically inert with passivating oxides only solvable in HF acid. Despite Nb2O5 as outermost oxide layer, the oxides of Nb compounds show large differences in thickness 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, C, N and O XPS lines of the oxides yielded the following stoichiometries and distributions: Nb2O5 is the outermost oxide layer on NbN, NbC and Nb, followed by: Nb2N2−xO3+x(x1−xOx for NbN, NbC1−xOx for NbC and NbO and NbOx for Nb (x1−xOx compounds extending into the metals explains, e.g., regions of depressed superconductivity, leakage current and pinning. The fluxoid pinning is enhanced by dielectric oxides existing for sputtered NbN between the grains and thus explaining the superior superconducting properties of granular NbN.