Sputtered all-niobium Josephson tunnel junctions with barrier oxide protected by gold

When niobium counter-electrode is sputtered onto a tunnel barrier oxide grown on a niobium base electrode, the result is normally a superconductively shorted junction. However, the authors find that a thin (0.3 nm) gold layer deposited on top of the oxide prior to counter-electrode deposition protects the junction oxide and prevents shorting. The authors give a brief summary of junction electrical and mechanical characteristics, and propose a mechanism for the shorting and the protection behaviour which is based on electrochemical enhancement of the Cabrera-Mott oxidation barrier potential. They also observe a large increase in tunnelling current in junctions with niobium top electrodes compared with junctions with lead top electrodes, and propose a tunnelling barrier potential model to account for this.