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

The surface properties of vertical stacks of Nb/(Al-AlO/sub x//Nb)/sub n/ Josephson junctions have been investigated by Atomic Force Microscopy (AFM). The results indicate that the roughness of the AlO/sub x/ layers is nearly constant and independent of the surface features of the underlaying Nb films. The superposition of several Nb and Al-AlO/sub x/ 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 l(l<20 mm) as l/sup /spl alpha// with /spl alpha//spl ap/0.85 for both Nb and AlO/sub x/. The interface profiles between the Al-AlO/sub x/ and Nb electrodes have been reconstructed from AFM data, and a reasonable agreement with anodization spectroscopy profiles has been found.