Applications and non-idealities of submicron Al–AlOx–Nb tunnel junctions

Abstract

We have developed a technique to fabricate sub-micron, Al–AlOx–Nb tunnel junctions using a standard e-beam resist, angle evaporation and double oxidation of the tunneling barrier, resulting in high quality niobium, as determined by the the high measured values of the critical temperature K and the gap meV. The devices show great promise for local nanoscale thermometry in the temperature range 1–7.5 K. Electrical characterization of the junctions was performed at sub-Kelvin temperatures both with and without an external magnetic field, which was used to suppress superconductivity in Al and thus bring the junction into a normal-metal-insulator-superconductor configuration. We observed excess sub-gap current, which could not be explained by the standard tunneling theory. Evidence points towards materials science issues of the barrier or Nb/AlOx interface as the culprit.