Fabrication and characterization of NbN/(TaN/NbN) _N stacked Josephson junctions

Abstract

In this work, we present a detailed investigation of the electrical properties of stacked NbN/(TaN/NbN) N Josephson junctions. Cross-sectional scanning transmission electron microscopy analysis of the 5-stacked junction shows that the multilayer interface is very flat, each barrier has the same thickness, and the sidewalls of the junctions are nearly perpendicular to the substrate. Stacked junctions of different sizes and stacking numbers have only one transition in their current–voltage curves. This indicates that the critical currents of the junctions in the stacked junctions are almost the same, showing the stability and repeatability of the multilayer fabrication and etching process. At 4.2 K, the 4-stacked junction exhibits excellent Josephson properties with characteristic voltage V c of 3.54 mV, which is approximately four times the 0.88 mV of the single junction. The temperature dependence of critical current density J c and Vc of the N-stacked junction with N = 1, 2, and 4 were measured, all of which can be fitted with the dirty-limit theory. Stacked junctions with larger Vc or more stacked layers can be achieved by optimizing the electrode and barrier thickness, barrier resistivity, thermal relaxation rate, etc.