Barrier-Thickness Dependence of the dc Quantum Interference Effect in Thin-Film Lead Josephson Junctions

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Abstract
The barrier‐layer thickness of cross‐type square Pb–PbO–Pb thin‐film Josephson junctions has been varied systematically and reproducibly by employing a mask‐substrate changer and a low‐pressure‐glow‐discharge oxidation technique. A series of junctions has been fabricated in which the ratio of film width w to the Josephson penetration depth λJ is varied from λJ≫w to λJ≪w while the width of both films is kept constant at 0.3 mm. The magnetic‐field dependence of the maximum zero‐voltage current in these junctions has been measured as a function of the barrier‐layer thickness. For non‐negligible self‐fields of the tunneling currents, the field dependence follows a Fresnel diffraction pattern rather than a Fraunhofer diffraction pattern. For very thin barrier layers, the maximum zero‐voltage current is eventually limited by the critical current of the Pb films. Relatively thick barrier layers, on the other hand, tend to be shorted at the edges, as can be seen from the magnetic‐field dependence. The stability of Pb–PbO–Pb Josephson junctions is compared with that of Nb–NbOx–Pb Josephson junctions.