Nb Josephson tunnel junctions with thin layers of Al near the barrier

Josephson tunnel junctions are finding numerous applications as SQUIDS, far infrared detectors, mixers, and in digital logic circuits. For most of these applications the ability of the devices to withstand the repeated thermal cycling from room to liquid helium temperature is required, as well as stability of the device characteristics after prolonged storage at room temperature. Niobium, with its high tensile strength and hardness was for a long time considered an attractive metallization for Josephson junctions. The two major types of junctions based on Nb are Nb-oxidePb (or Pb alloy) junction and the all-Nb junction: Nb-oxide-Nb. While both types were fou excellent cycling and storage stability clearly the durable all-Nb junction is more desirable. However it was generally fo~nd(l9~ y3) that it is much harder to make all-Nb junctions with acceptable I-V characteristics than to make junctions with soft Pb or Pb alloy counterelectrodes. This was blamed on the high reactivity of Nb which tends to reduce art of the oxide formed on the base electrode(l7. With thg coherence length of Nb being rather short ('L100A at 4.2K) (4), this reaction with the oxide layer contaminates the all-important near surface layer of the Nb counterelectrode, thus leading to inferior quality tunneling characteristics. In addition to these difficulties, Nb oxide has high dielectric constant (€2 30) (5) which leads to increased switching times compared to Pb-In alloy junctions(6). Hence, these disadvantages of Nb based junctions to some extent offset the advantages described above. Certain steps were taken to improve upon this