The role of surface roughness in the fabrication of stacked Nb/Al–AlOx/Nb tunnel junctions

The surface roughness of sputtered Nb films was determined with high precision using x‐ray specular reflectivity measurements in the 10 keV range. The roughness of Nb films increased from 0.9 nm for a 70‐nm‐thick film to 1.8 nm for a 210‐nm‐thick film. The roughness of the Nb surface strongly influences the tunnel barrier formation and the electrical properties of that barrier. For stacked tunnel junctions each thermal Al oxidation has to be adjusted as a function of the underlying Nb film thickness. Alternatively, one can use an Al or Al/AlOx underlayer in order to obtain stacks with small spread in critical currents.