Tunnel junctions fabricated from coherent NbN/MgO/NbN and NbN/Al2O3/NbN structures

Trilayer structures consisting of two layers of epitaxial, single-crystal NbN and an intervening 2-nm-thick layer of epitaxial MgO or Al2O3, have been formed in four or two, respectively, crystallographic orientations. The NbN films were deposited by dc magnetron sputtering and oxide films were grown by dc sputtering or evaporation. Single-crystal NbN films did not have significantly higher superconducting transition temperatures or significantly lower normal-state resistivities than films grown by polycrystalline epitaxy. However, the use of single-crystal epitaxy to deposit tunnel barriers provided new experimental parameters for control of thickness uniformity that affect the tendency of an epitaxial layer to grow as a 2-D or 3-D film. One of these experimental parameters, increasing the barrier deposition temperature, was shown to decrease the junction subgap conductance and to increase the effective barrier height for epitaxial barriers, but was detrimental to polycrystalline junctions.