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Niobium oxide-barrier tunnel junction

We describe the fabrication and electrical characteristics of niobium oxide-barrier tunnel junctions having counterelectrodes of either Pb alloys or Nb. The conditions necessary to obtain good tunnel barriers are discussed in detail. These include RF plasma etching of the Nb film before oxidation, the oxidation step itself and, in the case of Nb counterelectrodes, exposure of the oxide to an RF plasma of N2containing traces of hydrocarbon. The single-particle current of Nb/Pb junctions at 4.2 K is close to the value calculated from BCS theory in the voltage range0 < V < 2mV. With Nb counterelectrodes the subgap current is higher, but in the best examples it is comparable to Nb/Pb at voltages below 1 mV. Finally, it is shown that the junctions exhibit outstanding stability during storage, annealing, and thermal cycling between room temperature and 4.2 K.