Hydrogen-inclusion-induced variation of critical current in Nb–AlOx–Nb Josephson junctions

The critical current density (Jc) of Nb–AlOx–Nb Josephson-junction (JJ) arrays was found to depend on their wiring structure. The Jc values of all JJs wired with a niobium electrode covered with a palladium layer increased by about 20%, while the Jc values of those with electrodes without palladium coverage stayed unchanged (except for that of the two junctions directly connected to the pads of an electrical probe covered with palladium.) To explain this Jc increase, we propose a “hydrogen mechanism,” that is, the hydrogen inclusion into niobium electrodes occurs during fabrication, and its desorption occurs after fabrication. Hydrogen atoms incorporated in the electrodes are thought to influence the mechanical and the electronical properties of niobium, resulting in the deviation of critical current density. Hydrogen desorption analysis and measurements on niobium-film properties verified the occurrence of hydrogen incorporation into the niobium films during the fabrication process for superconducting JJ circuits. The incorporation and desorption processes were confirmed to proceed, even in air, if the niobium film is covered with palladium. As hydrogen diffuses quickly in niobium but stops in aluminum or aluminum oxide, differences in hydrogen concentration can happen within a circuit consisting of electrically connected multiple junctions. This hydrogen concentration difference can explain the observation that two junctions with increased Jc exist in the serial junction array without palladium coverage.