New thickness control process of oxide barrier for Nb-based tunnel junctions

The Nb-based superconductor-insulator-superconductor (SIS) tunnel junctions have been broadly used in many applications. The critical current density (J_C), one of the most important parameters of SIS tunnel junction, is usually controlled by the oxygen exposure (E_O2) of the Al oxidation process. R. E. Miller et al. demonstrated the relation between J_C and oxygen exposure using the SNEP process. However, the value of J_C still varies with Nb/AlO_xAl/Nb deposition system, even run-to-run process. A new AuAl₂/Al composite, instead of pure Al, has been used in the oxidation process. From the J_C-E_O2 relation, we have demonstrated the oxidation rate of AuAl₂ is about 400 times lower than that of Al. Using AuAl₂ layer, two advantages are observed. 1) For low J_C tunnel junctions, the thickness of AlO_X, or J_C, can be controlled easily by inserting AuAl₂ layer as a blocking layer in oxidation process. 2) High quality factor tunnel junctions with J_C>100 kA/cm² are achieved by oxidation of AuAl₂ layer directly.