CROSS-SECTIONAL TRANSMISSION ELECTRON-MICROSCOPY STUDY FOR NB/ALOX-AL/NB, NB/ZROX-ZR/NB, AND NB/HFOX-HF/NB JOSEPHSON-JUNCTIONS

We studied the interfaces of Nb Josephson junctions with an overlayer structure using cross-sectional transmission electron microscopy. We compared Nb/AlOx-Al/Nb, Nb/ZrOx-Zr/Nb, and Nb/HfOx-Hf/Nb junctions. The interface between the Zr layer and Nb base electrode, and that between the Hf layer and Nb were smoother than that between the Al layer and Nb. The roughness of the interface between Al and Nb is caused not by the roughness of the original Nb base electrode but grain boundary diffusion following Al deposition. Annealing increased the roughness of the interface between Al and Nb, but had very little effects on the interfaces between Zr and Nb, or Hf and Nb. We attribute these effects to the differences in the melting points of the various overlayers.