Analysis of the interfaces of stacked josephson junctions by atomic force microscopy

The surface properties of vertical stacks of Nb/(Al-AlOx/Nb)(n) Josephson junctions have been investigated by Atomic Force Microscopy (AFM). The results indicate that the roughness of the AlOx layers is nearly constant and independent of the surface features of the underlaying Nb films. The superposition of several Nb and Al-AlOx films has an overall smoothing effect on their surfaces, as indicated by the strong reduction of the rms roughness in a stack of junctions with respect to that of the base Nb film. The height-height correlation function shows that the rms roughness increases on small sampling lengths I (l < 20 mn) as l(alpha) with alpha approximate to 0.85 for both Nb and AlOx. The interface profiles between the Al-AlOx and Nb electrodes have been reconstructed from AFM data, and a reasonable agreement with anodization spectroscopy profiles has been found.