New technique for electron-tunneling junction fabrication and its application to tantalum and niobium

A new technique for the fabrication of electron-tunneling junctions is presented. Ion etching is used to provide a hole in an insulating layer to define the junction area and to produce a clean surface of the underlying superconductor, as indicated by Auger electron spectroscopy, prior to the barrier formation and the evaporation of the counterelectrode. Knee structure in the current-voltage characteristics of $\mathrm{Ta}-{\mathrm{Ta}}_x{\mathrm{O}}_y-\mathrm{Pb}$ and $\mathrm{Nb}-{\mathrm{Nb}}_x{\mathrm{O}}_y-\mathrm{Pb}$ tunneling junctions above the voltage corresponding to the sum of the energy gaps is shown to be due to a proximity effect between a thin ion implanted normal layer and the underlying superconductor, and can be explained quantitatively using McMillan's theory of the proximity effect. Lowering the energy of the etching ions is shown to reduce the thickness of this normal layer and the associated proximity-induced structure in the tunneling characteristics, so that good values for ${\Delta }_{\mathrm{Ta}}$ and ${\Delta }_{\mathrm{Nb}}$ are obtained. It is proposed that the excess current observed below the voltage corresponding to the sum of the energy gaps is due to tunneling through normal regions of Nb or Ta in parallel with tunneling through the superconducting regions.