Tunnel barrier shape for rf-oxidized Nb/Pb-alloy junctions

Tunnel barriers produced by rf oxidation of Nb films were characterized by measuring the tunneling current density as a function of voltage j(V) and the junction specific capacitance Cs. The j(V) curves of high resistance junctions for voltages up to and exceeding the barrier heights showed that the tunnel barriers were approximately trapezoidal. The barrier heights at the base electrode and counterelectrode interfaces, which were both about 0.85 eV for Nb/Pb junctions, were reduced to about 0.57 and 0.44 eV, respectively, for Nb/PbInAu junctions. Deviations of the j(V) curves from those calculated for a trapezoidal barrier included evidence of barrier rounding at the base electrode interface as well as substantial zero-bias anomalies, which suggest the presence of traps in the barrier. The large increase in resistance on thermal annealing observed for Nb/Pb junctions appeared to result primarily from an increase in barrier height.