All refractory NbN/MgO/NbN tunnel junctions

Abstract

We report the fabrication of all-refractory superconductor-insulator-superconductor tunnel junctions of the form NbN/MgO/NbN. The MgO insulating barrier was deposited by e-beam evaporation. High quality junctions were fabricated with sum gaps of 5.2 meV, and a small subgap leakage parameter (V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</inf> =30 mV, measured at 3mV). These devices are for eventual use as quasiparticle mixer elements in millimeter/submiltimeter wave heterodyne receivers. Fabrication techniques and current-voltage characteristics are discussed. We also propose a new growth mode for MgO films on NbN.