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Abstract NbN, NbC and Nb are known to be chemically inert with passivating oxides only solvable in HF acid. Despite Nb 2 O 5 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: Nb 2 O 5 is the outermost oxide layer on NbN, NbC and Nb, followed by: Nb 2 N 2− x O 3+ x (x<1) on NbN. Dielectric Nb oxicarbides were not identified for NbC. Underneath the dielectric metallic oxides NbN 1− x O x for NbN, NbC 1− x O x for NbC and NbO and NbO x for Nb (x<0.5) occur. The oxide growth (a)–(c) is not planar; instead, the oxides serrate the metal surface on an nm scale. The serration is strongest for the soft Nb and smallest for the harder compounds NbN and NbC in parallel to the oxidation rate which is slowest for NbC. The improved oxide quality and better quality of tunnel junctions in comparison with Nb is explained by this first identification of oxinitrides and of reduced serration of NbN (NbC). The first identification of Nb(N, C) 1− x O x 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.
Darlinski et al. (Mon,) studied this question.