Fabrication and dc characteristics of small-area tantalum and niobium superconducting tunnel junctions

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Abstract
We discuss the fabrication and dc electrical characteristics of small‐area (1–6 μm2) superconducting tunnel junctions with Ta or Nb base electrodes and Pb or Pb0.9Bi0.1 counterelectrodes. These junctions have very small subgap leakage currents, a ‘‘sharp’’ current rise at the sum‐gap voltage, and show strong quantum effects when used as microwave mixers. The use of a low‐energy (∼150 eV) ion cleaning process and a novel step‐defined fabrication process that eliminates photoresist processing after base electrode deposition are discussed. Tunnel barriers formed by dc glow discharge oxidation were the most successful. Tunnel barrier formation by thermal oxidation and ion‐beam oxidation is also discussed. An oxidized Ta overlayer (∼7 nm thick) was found to improve the characteristics of Nb‐based junctions. The electrical characteristics of junctions with different electrode and barrier materials are presented and discussed in terms of the physical mechanisms that lead to excess subgap current and to a width of the current rise at the sum‐gap voltage.