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/sub C/), one of the most important parameters of SIS tunnel junction, is usually controlled by the oxygen exposure (E/sub O2/) of the Al oxidation process. R. E. Miller et al. demonstrated the relation between J/sub C/ and oxygen exposure using the SNEP process. However, the value of J/sub C/ still varies with Nb/AlO/sub x/Al/Nb deposition system, even run-to-run process. A new AuAl/sub 2//Al composite, instead of pure Al, has been used in the oxidation process. From the J/sub C/-E/sub O2/ relation, we have demonstrated the oxidation rate of AuAl/sub 2/ is about 400 times lower than that of Al. Using AuAl/sub 2/ layer, two advantages are observed. 1) For low J/sub C/ tunnel junctions, the thickness of AlO/sub X/, or J/sub C/, can be controlled easily by inserting AuAl/sub 2/ layer as a blocking layer in oxidation process. 2) High quality factor tunnel junctions with J/sub C/>100 kA/cm/sup 2/ are achieved by oxidation of AuAl/sub 2/ layer directly.