Fabrication of ultrasmall all-Nb tunnel-junction devices with ion beam-oxidized barriers

We report a new type of all-Nb tunnel junction and single electron transistor (SET) fabricated using conventional electron-beam lithography and angle evaporation combined with an ion beam oxidation technique to create a tunnel barrier between the Nb electrodes. Significantly improved reproducibility in fabrication is achieved compared with previous Nb/Al-A1O _x/Nb structures. The basic parameters of the Nb/NbO _x/Nb junctions, namely, the barrier height and width, are extracted from the temperature-dependent current-voltage characteristics. The transport characteristics of various all-Nb superconducting SETs with different tunnel barriers are presented and discussed, Clear gap features are observed, and a corresponding gap energy (Δ _Nb) of up to 1.1 meV is obtained. From the transport characteristics of the Nb-based SET measured at ∼40 mK in the normal state, we estimate the specific junction capacitance to be ∼440 ± 50 fF/μ ². We also discuss how the quality (i.e., T _c) of the Nb stripes is affected by outgassing from the various resists used in the fabrication.