Ion beam induced changes in the oxide composition of PbIn alloy tunnel junction electrodes

A systematic correlation between krypton ion beam cleaning time and critical current density has been observed in Josephson tunnel junctions fabricated from thermally evaporated PbIn alloys which were ion milled prior to room temperature oxidation under water saturated O2. The observed effect is strongly nonlinear and is in a direction such that the critical current density increases with increased ion milling. Auger and x-ray photoemission studies have revealed that the ratio of lead oxide to indium oxide also increases with increased milling time. The lead oxide is observed to be spatially concentrated at the oxide/ambient interface rather than being uniformly distributed. Vacuum annealing of the alloy subsequent to ion milling but preceding the oxidation step systematically reduces both the critical current density and the quantity of lead oxide observed in the films. These results have a direct bearing on the technological problem of obtaining uniformity and day-to-day reproducibility of Josephson critical currents.