Critical current uniformity and stability of Nb/Al-oxide-Nb Josephson junctions

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Abstract
The critical current stability of Nb Josephson junctions using thin Al oxide layers as the tunneling barrier has been investigated with respect to repeated thermal cycling, aging, and annealing at elevated temperatures as a function of time. The Nb electrodes and in situ thermally oxidized Al barriers were deposited by dc magnetron sputtering prior to the lithographic formation of the individual junctions. The arrays of junctions were formed by standard photolithography techniques, reactive ion etching, and anodization. A population of 400 20-μm-square junctions were thermally cycled from room temperature to 6 K 4880 times with no failures due to shorts and no change in the average critical currents. These same unpassivated junctions were annealed at elevated temperatures for various periods of time until the critical currents had decreased by 50% at temperatures near 275 °C.