Nb/TaOx/Nb Josephson junctions for a programmable voltage standard

To fabricate SNS Josephson junctions for a programmable voltage standard, nonstoichiometric oxygen-enriched tantalum has been used for the junction barrier. TaOx has been deposited by rf sputtering using the bias voltage to change the composition and the electrical properties of the films. A marked dependence of the resistivity on bias voltage has been observed. Correspondingly, the tantalum-oxide structure varies from amorphous to bcc-like. Nb/TaOx/Nb Josephson junctions have been fabricated with different compositions of the TaOx layer. The best I-V curves have been obtained so far for a bias voltage close to 100 V with a TaOx thickness of 10 nm.