Oxides grown on the surface of a polycrystalline Nb thin film using rf plasma cleaning and plasma oxidation processes were examined by transmission electron microscopy. The oxides were prepared on the Nb surface using the same rf plasma processes developed for fabricating the tunnel barriers of Nb/Nb oxide/Pb alloy Josephson tunnel junctions. Transmission electron diffraction patterns obtained from the tunnel barrier formed on the Nb surface indicated the presence of a crystalline NbC0.25O0.75 layer between the amorphous Nb2O5 tunnel barrier layer and the Nb base electrode. The thickness of this transition layer was found to be sensitive to the rf plasma cleaning conditions prior to the tunnel barrier formation. The formation of this NbC0.25O0.75 layer of proper thickness is correlated to good junction quality. An epitaxial relationship between the NbC0.25O0.75 layer and the underlying Nb crystal was also observed.