Niobium tunnel junctions with multi-layered electrodes

The current-voltage characteristics of the niobium - aluminum oxide - niobi um tunnel junctions have been studied systematically and are compared with numerical simulations based on the microscopic theory of the proximity effe ct. The thickness of the base niobium layer is varied from 35 to 500 mn whi le the thickness of the aluminum layer is kept constant (about 9 mm). In a separate series of experiments the aluminum thickness is varied from 2 to 3 0 mn for two fixed thickness of the base electrode: 50 and 200 mn. The appr opriate conditions for a full suppression of the so called "knee" structure at the gap voltage in the current-voltage characteristic are experimentall y determined and theoretically interpreted in the framework of the microsco pic theory. The influence of the additional aluminum layer in a composite b ase electrode on the properties of the tunnel junction have been studied in dependence on the aluminum thickness and distance of this layer from the b arrier. The obtained results demonstrate that the current-voltage character istics of tunnel junction can be engineering by an appropriate layer thickn ess of compound base electrode.