Fabrication of High Quality Nb/AlOx-Al/Nb Josephson Junctions: II-Deposition of Thin Al Layers on Nb Films

The surface coverage of thin Al layers deposited on Nb films with different sputtering parameters was studied. The anodization profile and x-ray photoelectron spectroscopy (XPS) confirmed that the underlying Nb layer is wet by a thin 2.8-nm Al layer, and the oxide formation on the Nb layer is completely suppressed. The excellent surface coverage is due to the affinity of Al for Nb and is quite different from the degraded coverage of Al on Si. We observed that the surface coverage of Al on Nb depends on the film characteristics of the underlying Nb layers. We fabricated Nb/AlOx-Al/Nb junctions having lower Nb layers deposited with different sputtering parameters. We verified that the quality of the junction is closely related to the surface coverage as analyzed by the anodization profiles and XPS.