Experimental investigations and analysis for high-quality Nb/Al-AlOx/ Nb Josephson junctions

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Abstract
We have made a detailed study of the fabrication process of Nb/Al‐AlOx/Nb Josephson junctions with the aim of introducing guiding principles to obtain high‐quality junctions. Junctions were fabricated by a novel process named the self‐aligned contact process. We investigated the deposition conditions of Nb and Al films and optimized them. Oxidation conditions for obtaining target critical current were also investigated. Then composition of the tunneling barrier material and the coverage of the thin Al layer on the base electrode were analyzed by x‐ray photoelectron spectroscopy, Auger electron spectroscopy, etc. We investigated aging and annealing effects on the junctions. We confirmed that the current‐voltage (I‐V) characteristics of the junctions did not change after storage for 450 days at room temperature or when subjected to temperatures of up to 150 °C for shorter periods. Applying the McMillan theory to our experimental results, we explained the characteristics of the junctions. As a result, we have demonstrated that the characteristics are determined by two proximity layers: The first is a double proximity layer composed of an elemental Al layer and Nb/Al alloy layer in the base electrode, and the second is a Nb layer with low transition temperature in the counterelectrode. We have demonstrated that the characteristics are drastically improved by reducing the Nb/Al alloy layer and the Nb layer with low transition temperature and have obtained high‐quality junctions with little leakage current.