Influence of Nb film surface morphology on the sub-gap leakage characteristics of Nb/AlOx-Al/Nb Josephson junctions

In this work, we investigate the influence of the Nb film surface roughness on the microstructure of Nb/ AlOx - Al/ Nb trilayers and the current - voltage ( I - V) characteristics of the trilayer junctions. The surface morphology of the base- Nb film was found to directly affect the Al layer coverage of the base- Nb and the quality of the Al/ Nb interface. Diffusion of Al at the Al/ Nb interface increases with increasing roughness of the base- Nb layer, which results in micro- shorts or pinholes in the insulating barrier layer and increases the sub- gap leakage current of the junctions. We correlate the I - V characteristics of the junctions to their microstructures and show that Nb surface roughness and Nb/ Al interface quality play critical roles in determining the sub- gap leakage characteristics of the trilayer junctions. High- quality Nb/ Al - AlOx/ Nb trilayer junctions with very low sub- gap leakage currents were obtained after optimizing the Nb film surface morphology.