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

NbN, NbC and Nb are known to be chemically inert with passivating oxides only soluble in HF acid. Despite all having 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 x-ray photoelectron spectroscopy (ARXPS) measurements have been performed yielding the following stoichiometries and distributions: (a) Nb2O5 is the outermost oxide layer on NbN, NbC and Nb, followed by: (b) Nb2Nsub(2-x)Osub(3+x)(x < 1) on NbN. Dielectric Nb oxicarbides were not identified for NbC. (c) Underneath the dielectric metallic oxides NbNsub(1-x)Osub(x) for NbN, NbCsub(1-x)Osub(x) for NbC and NbO and NbOsub(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 correlating with the oxidation rate which is slowest for NbC. The stoichiometry, structure, electronic properties and growth dynamics of the coatings are discussed. (author)