Electrical parameters of niobium-based overdamped superconductor-normal metal-insulator-superconductor Josephson junctions for digital applications

Abstract

We have demonstrated superconductor-normal metal-insulator-superconductor Nb/Al–AlOX/Nb Josephson junctions developed with Al layer thicknesses between 30 and 100 nm, resulting in nonhysteretic current-voltage characteristics for critical current densities between 20 and 60 kA/cm2. Specific capacitances ranged from 180 to 480 fF/μm2, depending on temperature and of barrier deposition conditions. It is shown that the high capacitance seen by the junction depends on the Josephson junction barrier but only to a least extent on the parasitic environment. Although the specific capacitance is unusually high, Stewart–McCumber parameters from 1 to 3, at 4.2 K, optimum for digital single-flux-quantum applications were obtained without external shunting.