Plasma-Oxidized Tunnel Barrier of Nb/Nb Josephson Junctions with Copper Interlayer

Niobium/niobium Josephson tunnel junctions were fabricated using electron-beam evaporation of Nb films and rf-plasma oxidation. Niobium reacts easily with oxygen, resulting in a decrease in its transition temperature. In order to suppress this reaction between the tunnel barrier and the Nb counter-electrode, a thin layer of Cu was evaporated on the tunnel oxide before the Nb counter-electrode was deposited. The sub-gap current of Nb/Nb junctions with a Cu interlayer was found to be smaller than without the interlayer. Moreover, it was found that using a clean and stabilized plasma process in forming the tunnel barrier is useful in decreasing the sub-gap current of the Nb/Nb junctions.