Ellipsometric Study of Nb-Al-AlOx Layered Structure for All-Refractory Josephson Junctions

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Abstract
The deposition of an ultra-thin Al overlayer on Nb thin films and its thermal oxidation were observed by in-situ ellipsometry. All-Nb tunnel junctions were fabricated using these Nb-Al bilayers and their tunneling characteristics were compared with the ellipsometric results. The ellipsometric parameters of the bilayers are sensitive to Al diffusion into the Nb grain boundaries, and show that the extent of the diffusion largely depends on the cooling interval between the Nb and Al depositions, that is, the substrate surface temperature. The wide range of parameters and the changes in the junction tunnel characteristics observed are systematically explained assuming a trilayer (the base Nb film, the Al-diffused layer and the residual Al layer) model. The thicknesses of the latter two layers can also be semiquantitatively determined from the calculations based on this model.