Thermal stability of Pb-alloy Josephson junction electrode materials. VIII. Effects of Au addition to Pb-Bi counterelectrodes

Abstract

In previous studies, strain-relaxation-induced changes in film microstructure, such as grain rotation, hillock formation, and dislocation slip bands, were observed after repeated thermal cycling between 298 and 4.2 K in 0.4-μm-thick ε-phase Pb-Bi films that are used as the counterelectrode material of experimental Pb-alloy Josephson junction devices. In the present paper, the effects of small additions of Au (1-8 nm thickness) to the Pb-Bi films on the changes in the film microstructure were studied by using x-ray diffraction, and scanning and transmission electron microscopy. It was found that the changes in microstructure observed in the Pb-Bi films were significantly reduced by the Au addition, although the average level of strain relaxation in the Pb-Bi alloy films upon thermal cycling between 298 and 4.2 K was not influenced by the Au addition. Experimental junctions were made using fine-grained Pb-In-Au films prepared at 77 K as the base electrode material, and Pb-Bi or Pb-Bi-Au films prepared at 273 K as the counterelectrode material. The junctions were cycled 200 times between 298 and 4.2 K. The junction failure level was observed to be reduced by a factor of ∼10 times compared to that of control samples by using Pb-Bi-Au films with 1-nm-thick Au as the counterelectrode material. An additional order of magnitude reduction in the junction failure level was obtained by increasing the Au thickness to 8 nm. The results indicate that reducing the changes in microstructure of the counterelectrode material reduces the incidence of junction failure upon repeated cycling between 298 and 4.2 K. However, adding Au to Pb-Bi counterelectrodes also causes an increase in the low-bias tunneling current. This increase in current, which is believed to be due to the presence of nonsuperconductive Pb3Au compounds in the vicinity of the tunnel barrier, would be undesirable for some device applications.