Degradation of superconducting tunnel junction characteristics with increasing barrier transparency

Measurements are reported of the single-particle characteristics of Nb-Al oxide-Nb superconducting tunnel junctions. Subgap leakage, subharmonic gap structure, and excess current are observed to increase systematically with increasing Josephson critical current density above a few kA/cm2. These phenomena are attributed to multiple Andreev reflections in high transmittance regions of the barrier. Such effects may significantly impact the applications of high critical current density junctions in advanced circuits.