Experimental investigations and analysis for high-quality Nb /Al-AlO(x)/Nb Josephson junctions

The junction characteristics of Nb/Al-AlO(x)/Nb are examined. The self-aligned contact (SAC) process for fabricating the Nb /Al-AlO(x)/Nb junctions is described. The effects of the deposition conditions, Al layer thickness, aging, and annealing on the junction characteristics are studied. The oxidation conditions required to form an optimized tunneling barrier material, and the coverage of the base electrode by a thin Al layer are analyzed using X-ray photoelectron spectroscopy. The observed junction characteristics are investigated in terms of the McMillan theory (1968) of the proximity effect. It is observed that the characteristics are determined by two proximity layers: a double proximity layer composed of an elemental Al layer and Nb/Al alloy layer in the base electrode and a Nb layer with low transition temperature in the counter electrode. The data reveal that reducing the two layers improves the characteristics of the junction. (I.F.)