Niobium films oxidized to thicknesses less than 30 Å by using different oxidation techniques (e.g. thermal, dc glow discharge, and rf glow discharge) were studied by XPS. Chemical shifts in the binding energies of Nb 3d (3/2) and Nb 3d (5/2) levels were used to identify the various oxides of Nb. Niobium films oxidized by thermal or dc glow discharge oxidation showed an outermost layer of Nb2O5. A pure Nb signal could be seen when the oxide films were thin. Suboxides could not be seen directly in XPS data. The composition of the oxide films produced by an rf glow discharge oxidation was found to depend on the rf peak to peak voltage and under certain conditions the XPS spectra clearly showed the existence of NbO. A discrepancy was observed between the oxide thicknesses estimated from the XPS data and the relative values of resistances of Nb–Nb oxide–Pb tunnel junctions fabricated by using these oxide films as barriers. The results are briefly discussed in terms of the difference in the oxidation processes and their importance in junction fabrication.

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