Epitaxial Niobium Nitride/Insulator Layered Structures

摘要

The properties of niobium nitride insulator multilayers are reviewed. The NbN layers were polycrystalline or (100), (111), (110) or (135) single crystal films. The insulator was a 3 to 50 A thick epitaxial layer of A1203, MgO, or a metastable pseudo-binary compound, Mg1-xCaxO. The particular composition, x=0.27, was chosen to match the oxide lattice constant to that NbN. Superconductive tunneling measurements were used in conjunction with XPS, LEED, RHEED, and TEM to construct a model of oxide tunnel barrier microstructure and evaluate disorder in the layers of NbN adjacent to oxide interfaces. It was found that the chemistry of the native oxide of NbN can be modified during artificial oxide deposition to form a carboxide that effectively seals thin spots in the artificial oxide. A comparison of tri-layers made with MgO or with lattice-matched MgO-CaO showed little difference in the mode of epitaxial growth or in the superconducting properties of adjacent NbN layers. Niobium nitride films and tunnel junctions grown by single-crystal epitaxy had better superconducting properties than equivalent polycrystalline samples, particulary at temperatures less than 300 C. Keywords: Aluminum oxides, Magnesium oxides, Calcium compounds, Reprints. (MJM)