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To optimize the current-voltage characteristics of Nb/AlOx-Al/Nb Josephson tunnel junctions, a uniform and well defined insulating barrier (AlOx) is required so that no leakage current occurs between the upper and lower Nb electrodes. We investigated the dependence of the surface morphology of dc magnetron-sputtered Nb thin films on deposition parameters using atomic force microscopy (AFM), the cross-sectional microstructure of the Nb/AlOx-Al/Nb trilayers using transmission electron microscopy (TEM), and anodization profiling. The surface roughness of the base Nb layer was found to affect the AlOx-Al layer and thus the quality of Nb/AlOx-Al/Nb trilayer. Diffusion of Al at the Al/Nb interface increases with increasing roughness of the base Nb layer, which increases the minimum Al thickness required to cover base-Nb. The importance of sufficient sample-cooling during the trilayer deposition was also confirmed by the TEM study and anodization profiling.

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.

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