  The effect of dc electrical stress and breakdown on Josephson and
quasiparticle tunneling in Nb/Al/AlOx/Nb junctions with ultrathin AlOx barriers
typical for applications in superconductor digital electronics has been
investigated. The junctions' conductance at room temperature and
current-voltage (I-V) characteristics at 4.2 K have been measured after the
consecutive stressing of the tunnel barrier at room temperature. Electrical
stress was applied using current ramps with increasing amplitude ranging from 0
to ~1000 Ic corresponding to voltages across the barrier up to 0.65 V where Ic
is the Josephson critical current. A very soft breakdown has been observed with
polarity-dependent breakdown current (voltage). A dramatic increase in subgap
conductance of the junctions, the appearance of subharmonic current steps, and
gradual increase in both the critical and the excess currents as well as a
decrease in the normal-state resistance have been observed. The observed
changes in superconducting tunneling suggest a model in which a progressively
increasing number of defects and associated additional conduction channels
(superconducting quantum point contacts (SQPCs)) are induced by electric field
in the tunnel barrier. By comparing the I-V characteristics of these conduction
channels with the nonstationary theory of current transport in SQPCs based on
multiple Andreev reflections by Averin and Bardas, the typical transparency D
of the induced SQPCs was estimated as D ~ 0.7. The number of induced SQPCs was
found to grow with voltage across the barrier as sinh(V/V_0) with V_0 = 0.045
V, in good agreement with the proposed model of defect formation by ion
electromigration. The observed polarity dependence of the breakdown current
(voltage) is also consistent with the model.
