Oxygen-doped Pb-In-Au films suitable for Josephson tunnel junction base electrodes

Abstract

The effects of oxygen doping during film deposition on film microstructures, surface morphology, strain behavior, and electrical resistivity have been studied by using transmission and scanning electron microscopies, x-ray diffraction, and electrical resistivity measurements for 0.2-μm-thick Pb and Pb-12 wt.% In-4 wt.% Au films prepared at ∼77 K. The average grain size of both types of films was found to decrease with increasing oxygen partial pressure during the film deposition: one order of magnitude smaller grains, compared with those prepared in a high vacuum (≲5×10−8 Torr), were obtained when the oxygen pressure was higher than 1×10−6 Torr. The finest average grain size obtained in the present experiments was ≲15 nm, which is over 10 times smaller than the film thickness. The surfaces of both films were found to be smooth when the films were prepared at oxygen pressures below 1×10−5 Torr. Strain relaxation upon cooling to 4.2 K was observed for oxygen-doped Pb films, but no strain relaxation was observed for Pb-In- Au films. In addition, no hillocks were observed in oxygen-doped Pb-In-Au films cycled 300 times between 300 and 4.2 K. The electrical resistivities of Pb-In-Au films were found to increase with oxygen pressure; an increase of a factor of 10 was observed when the films were prepared at an oxygen pressure of 1×10−5 Torr. The thermal cyclability of large-area Josephson tunnel junctions, in which oxygen-doped Pb-In-Au films were used as the base electrodes, exhibited the lowest cycling-induced failure level ever reported for large-area Pb-alloy Josephson tunnel junctions beyond 400 thermal cycles. The number of thermal cycles to reach 5% cumulative failure increased from 1000 to 5000 cycles by introducing 5×10−7 Torr of oxygen during film preparation at 77 K. However, the σ value (slope of the log normal failure distribution) becomes about three times larger.