Overdamped NbN junctions with Nb/AlO/sub x//Nb multilayered barriers

This paper describes electrical characteristics of NbN junctions with Nb/Al/sub 2/O/sub 3/-Al/(Nb) multilayered barriers. The junctions show overdamped behavior at 10 K because of Nb working as a normal layer, which enables us to use them as basic elements in single flux quantum (SFQ) circuits. The junctions, with critical current densities J/sub c/'s up to 3.4 kA/cm/sup 2/ and characteristic voltages V/sub c/'s up to 0.20 mV, were obtained by changing the film thicknesses of lower and upper Nb layers and the Al oxidation conditions when using a conventional batch-type sputtering system. The run-to-run variations of J/sub c/ and V/sub c/ were the maximum-to-minimum spreads of /spl plusmn/32% and /spl plusmn/4.3%, respectively. On the other hand, a V/sub c/ of 0.70 mV and a J/sub c/ of 13 kA/cm/sup 2/ were obtained for the junction fabricated by the ultrahigh-vacuum sputtering system. These results show that NbN/Nb/AlO/sub x//Nb/NbN junctions have high potential and can be applicable to high performance 10 K SFQ circuits.