Nb/AlOx/Al/AlOx/Nb double-barrier junctions with high critical current densities: influence of barrier asymmetry

In order to be useful for high speed digital circuit applications, double-barrier SINIS or SIS´IS junctions must be nonhysteretic, possess high critical current densities (j_c>1 kA/cm²) and high characteristic voltages V_c∼0.3 mV, where V_c=I_cR_sub and R_sub is a characteristic (subgap) resistance damping the junction in the operating range of voltages. This requires high transparencies of barriers and small interlayer thicknesses. Data are presented on fabrication and Josephson properties of SIS´IS junctions with j_c up to 10 kA/cm² at 4.2 K. It is shown that the asymmetry of double-barrier structure starts playing a major role at high j_c (i.e., at thin, high transparency barriers) as evidenced by the temperature dependences of the critical current, the value of the current deficit in the I-V characteristics, and the appearance of multiple Andreev reflection peaks in differential conductance of the junctions.