Unusual I/V characteristics of Nb/Al/AlOx/Al/AlOx/Nb SINIS tunnel junctions

Double-barrier Nb/Al-AlOx-Al-AlOx-(Al/)Nb devices with various thickness and purity of the middle Al layer were fabricated and investigated. It is found that the subgap structure that appears in the current voltage characteristics is very sensitive to the parameters of the middle film. In addition to the formerly reported gap-difference structure, we observed its half-voltage "subharmonic" and a novel magnetic-field-sensitive structure that develops at a voltage V approximate to Delta (Nb)/e (Delta (Nb) is the superconducting energy gap of Nb). In general, the devices with a cleaner Al electrode reveal better reproducibility of their characteristics, whereas the devices with "impure" Al (deposited in presence of a small amount of oxygen added to the Ar) tend to have more complicated and difficult to reproduce subgap structure. In addition, an anomalously large Josephson critical current was observed for devices with a small thickness of the middle Al layer.