Strain relaxation in Pb-alloy Josephson junction electrode materials

An investigation has been carried out of the relationship between strain relaxation in Pb-alloy Josephson junction electrode materials and the failure of Pb-alloy/oxide/Pb-alloy junctions during cycling between 300 and 4.2K. The strain behavior of Pb-12wt% In-4wt%Au alloy films evaporated on oxidized Si substrates has been studied using an x-ray diffraction technique. The thermal expansion coefficient mismatch strain is found to be partially relaxed upon both cooling to 4.2K and rewarming to 300K. The amount of strain relaxation depends strongly on film thickness h, decreasing from 0.5% to 0.2% as h is reduced from 0.5 to 0.1μm. The stability of Pb-alloy junctions during thermal cycling correlates with the observed levels of strain relaxation. A deformation mechanism map has been constructed for Pb films from which the dominant strain relaxation mechanisms can be predicted. From the analysis, two strain relaxation mechanisms are expected: dislocation glide at high strain and grain boundary diffusion creep at low strain near 300K. Dislocation lines and hillocks were observed in films that had been repeatedly cycled, providing supporting evidence for these mechanisms. It is proposed that junction failure occurs when non-uniform deformation in the electrodes causes a high enough local stress to rupture the tunnel barrier, producing a short.