Tunneling and surface properties of oxidized metal overlayers on Nb

The present status of Bell Labs tunneling and surface physics on Oxidized Metal Overlayers (OMO) is summarized and placed in perspective with other work in the field of superconducting tunnel junctions. In this paper the emphasis is on features of interest to tunneling applications. The central result of this work consists of the demonstration that using thin OMO's, all-refractory Josephson tunnel junctions with nearly ideal BCS electrical characteristics can be fabricated. These junctions were also shown to possess high thermal cycling and annealing stability. Tunneling and XPS studies of OMO's reveal the reasons for such high performance: complete wetting of the Nb base electrode with a thin reactive metal overlayer, and total suppression of the ''host'' metal oxidation. XPS studies also suggest an interesting grain-boundary diffusion mechanism which may be responsible for the apparent depletion of an overlayer from an excess unoxidized normal metal in structures prepared at temperatures exceeding 1500C