In this work, we report the electrical properties of NbN internally shunted Josephson junctions with NbN x barriers. Cross-sectional scanning transmission electron microscopy analysis shows that all layers have the same cubic structure; NbN/NbN x /NbN trilayers were epitaxially grown on MgO substrates. The resistivity of the NbN x films could be varied in the range of 1–10 4 mΩ cm by controlling both the N 2 partial pressure and the deposition time during reactive sputtering. The temperature dependence of the critical current density ( J c ) and characteristic voltage ( I c R n ) of the junctions with different barrier resistivities were measured for various barrier thicknesses. For the 10 nm-thick NbN x layer with resistivities of 73.44, 385.72, and 711 mΩ cm, the coherence length of the barrier was determined to be 5.55 ± 0.07, 4.88 ± 0.06, and 1.40 ± 0.13 nm, respectively, corresponding to carrier diffusion rates of 2.741 ± 0.004, 1.211 ± 0.002, and 0.008 ± 0.001 cm 2 s −1 , respectively. Thus, the reduction in barrier resistivity leads to a larger coherence length and a faster diffusion rate, which will further increase the J c and I c R n of the junction. By adjusting the barrier resistivity and thickness, the J c of the junction can be easily tuned over more than four orders of magnitude, and an I c R n value of 0.97 ± 0.07 mV was obtained at 10 K. The results indicate that the all-NbN self-shunt junction is a promising candidate in high-speed and high-temperature applications.

In this work, we present a detailed study 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 all 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 shows excellent Josephson properties with characteristic voltage V c of 3.54 mV, which is about four times the 0.88 mV of the single junction. The temperature dependence of critical current density J c and V c of the stacked junction with N = 1, 2, 4 were measured, all of which can be fitted with dirty-limit theory. Stacked junctions with larger V c or more stacked layers can be achieved by optimizing electrode and barrier thickness, barrier resistivity, and thermal relaxation rate, etc.

We investigate the current–voltage characteristics and the interface properties of epitaxial NbN/TaN/NbN Josephson junctions in this study. The crystal structure and interface properties of the NbN/TaN/NbN junctions are determined using x-ray diffraction and cross-sectional scanning transmission electron microscopy, and the epitaxial growth of the NbN/TaN/NbN trilayer exhibited a smooth and clear interface. The characteristic voltage can be easily tuned over more than one or two orders of magnitude by precisely and independently controlling the barrier thickness and resistivity. For the barrier resistivity of 79 mΩ cm, the junctions show excellent Josephson properties with a characteristic voltage of 2.04 ± 0.08 mV at 4.2 K, corresponding to a large normal metal coherence length of 2.51 ± 0.03 nm and a fast diffusion rate of 24.9 ± 0.4 mm2/s. A high junction quality was also confirmed by the large characteristic voltage of 0.88 ± 0.03 mV at 10.0 K, making the intrinsically shunted NbN junctions promising for use in higher speed and temperature-superconducting devices and circuits.