Intrinsically shunted Josephson junctions with high characteristic voltage based on epitaxial NbN/TaN/NbN trilayer

Abstract

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.

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

“…Figures 1(a The thickness of the TaN barrier is set to 13 nm for the 1, 2, and 4 stacked junctions, and 16 nm for the 5 stacked junctions. The resistivity of the 16 nm thick TaN film decreases from 4.34 mΩ cm to 4.11 mΩ cm as the temperature increases from 4.2 K to 10.0 K, showing metallic behavior [12][13][14]. The thicknesses of intermediate and top electrodes are 36 nm and 100 nm, respectively.…”

“…Shoji et al investigated vertical stacking of up to five NbN/TiN/NbN junctions, and obtained uniform critical current distribution and V c of 0.1 mV by optimizing the etching process [11]. The V c of stacked junctions is significantly smaller than 2.04 mV of the reported single junction obtained by adjusting the resistivity or thickness of the barrier layer, and there is still room for optimization [12].…”

Intrinsically shunted NbN/TaN/NbN Josephson junctions on Si substrates for large-scale integrated circuits applications

“…Then, V c , the product of I c and R n , was calculated to be 0.425 mV, corresponding to the maximum operating frequency of around 200 GHz, related by f max = V c /Φ 0 , where Φ 0 is the magnetic flux quantum [21]. Higher V c can be obtained by decreasing the barrier thickness or increasing the nitrogen content of the barrier [16]. Figure 2(b) shows the dependence of I c on the magnetic field parallel to the junctions.…”

“…The resistivities of TaN films directly grown on the SiO 2 /Si substrates were measured by the four-point method using a physical property measurement system, revealing a gradual increase from the center to the edge of the wafer. In the normal metal layer with higher resistivity, the wave function of the superconducting layer decays faster, causing the decrease of J c and V c [16]. Furthermore, the thickness of NbN film is thinner at the wafer edge.…”

“…Simultaneously, the preparation of SNS junction with high quality impose higher requirements on the interface of the trilayer. Hence, nitride materials, such as TaN, NbN and TiN, have been used as the barriers to avoid the formation of compounds across the N-S boundary [16][17][18]. Thus, the study of NbN-SNS Josephson junctions with high uniformity and reproducibility is of great significance for the development of superconducting large-scale integrated circuits.…”