Niobium tunnel junctions with multi-layered electrodes

The current-voltage characteristics of the niobium - aluminum oxide niobium tunnel junctions have been studied systematically and are compared with numerical simulations based on the microscopic theory of the proximity effect. The thickness of the base niobium layer is varied from 35 to 500 mm while the thickness of the aluminum layer is kept constant (about 9 nm). In a separate series of experiments the aluminum thickness is varied from 2 to 30 nm for two fixed thicknesses of the base electrode: 50 and 200 nm. The appropriate conditions for a full suppression of the so called "knee" structure at the gap voltage in the current-voltage characteristic are experimentally determined and theoretically interpreted in the framework of the microscopic theory. The influence of the additional aluminum layer in a composite base electrode on the properties of the tunnel junction have been studied in dependence on the aluminum thickness and distance of this layer from the barrier. The obtained results demonstrate that the current-voltage characteristics of tunnel junction can be engineering by an appropriate layer thickness of compound base electrode.