Universality of transport properties of ultrathin oxide films

We report low-temperature measurements of current-voltage characteris- tics for highly conductive Nb/Al-AlOx-Nb junctions with thicknesses of the Al inter- layer ranging from 40 to 150 nm and ultra-thin barriers formed by di®usive oxidation of the Al surface. In a superconducting state these devices have revealed a strong subgap current leakage. Analyzing Cooper-pair and quasiparticle currents across the devices, we conclude that the strong suppression of the subgap resistance comparing with conventional tunnel junctions is not related to technologically derived pinholes in the barrier but rather has more fundamental grounds. We argue that it originates from a universal bimodal distribution of transparencies across the Al-oxide barrier proposed earlier by Schep and Bauer. We suggest a simple physical explanation of its source in the nanometer-thick oxide ¯lms relating it to strong local barrier-height °uctuations in the nearest to conducting electrodes layers of the insulator which are generated by oxygen vacancies in thin aluminum oxide tunnel barriers formed by thermal oxidation.