Transitioning from tunnel to weak-coupling NbN/TaN/NbN junctions with tunable TaN barriers

TaN films were employed as the barrier layer, and by adjusting the nitrogen content f N2 during the deposition of the TaN barrier layer, underdamped and overdamped Josephson junctions were fabricated using NbN/TaN/NbN trilayers. As f N2 was varied from 60% to 100%, the optical band gap of the TaN film exhibited a non-monotonic trend, peaking at f N2 of 80%, where the NbN/TaN/NbN trilayer achieved its best epitaxial quality. For f N2 ranging from 60% to 90%, the NbN/TaN/NbN junction exhibited typical tunnel junction characteristics, also with peak performance at f N2 = 80%: the energy gap voltage V g was 5.5 mV, the characteristic voltage I c R n reached 4.5 mV, and the critical current density J c could be finely tuned between 40 kA cm −2 to 0.6 kA cm −2 by adjusting the barrier thickness from 2.3 nm to 3.3 nm. When f N2 increased to 100%, the counter NbN electrode evolved into a polycrystalline structure. Concurrently, the resistance of the NbN/TaN/NbN junction dropped sharply, accompanied by the vanishing of V g in the I – V curve and a marked surge in J c , signaling a shift from superconductor–insulator–superconductor to superconductor–normal metal–superconductor junction. At f N2 of 80%, the average barrier height ϕ was roughly 0.04 eV, substantially lower than that of AlN. This lower ϕ suggests that the same J c can be maintained even with a thicker TaN barrier, thereby improving the process stability and reproducibility of NbN junctions at high J c .