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 thicknesses which range from a few nm to zero. Low temperature measurements 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 voltage electrical properties only when a continuous interlayer exists, and that for discontinuous layers the low voltage conductance is controlled by Coulomb blockade.