NbN/Nb/AlOx/Nb/NbN junctions fabricated using an ultra-highvacuum dc-magnetron sputtering system

Abstract

This paper presents the electrical characteristics of NbN/Nb/AlOx/(Nb)/NbN junctionsfabricated using an ultra-high vacuum (UHV) dc-magnetron sputtering system. TheNbN/Nb/AlOx/(Nb)/NbN junctions show overdamped characteristics at or over 10 K, although theyshow small- or medium-hysteretic characteristics at 4.2 K. The UHV sputtering system with aload-lock chamber enables us to prepare high-quality NbN films with high reproducibility and verythin Al2O3 films with high homogeneity. We have obtained the overdamped junctions with thecritical current density Jc up to 10 kA cm-2 and the characteristic voltage Vc up to 0.50 mV over10 K. When Al oxidation conditions were changed, Jc and Vc were obtained in the range of 0.63-10 kA cm-2and 0.22-0.44 mV, respectively. These Jc and Vc values were much larger than those, typicalvalues of 1.1 kA cm-2 and 0.20 mV, of junctions fabricated using a conventional batch-type sputteringsystem. When the upper-Nb film thickness was changed, Jc and the degree of hysteresis, Ir/Ic, wereobtained in the range of 0.31-0.64 kA cm-2 and 0.87-1.0, respectively. The spread of Ic at 10 K was astandard deviation, 1σ, of 6.3% for a series array of 1000 junctions and 4.4% for 100 junctions.