ELECTRONIC TRANSPORT PROCESSES AND THE TRANSITION FROM DOUBLE TO SINGLE BARRIER TUNNEL-JUNCTIONS

Nb/AlOx/Al/AlOx/Nb devices have been fabricated with Al interlayer thi cknesses which range from a few nm to zero. Low temperature measuremen ts of the current versus voltage characteristics show the coexistence of both a direct tunneling channel via a double thickness barrier, and a two stage process via the interlayer. Despite a considerably higher specific conductance, we show that the latter dominates the low volta ge electrical properties only when a continuous interlayer exists, and that for discontinuous layers the low voltage conductance is controll ed by Coulomb blockade.