We present electrical characterization data of sputtered Nb/a-Si/Nb Josephson junctions (JJs) for high-speed and high-density superconducting circuits. Junctions were studied with critical current densities (Jc) ranging from 0.01 to 3 mA/μm2 at 4 K. For junctions deposited at room temperature and processed to a maximum temperature of 150 °C, the dependence of Jc on barrier thickness d is exponential, Jc∝exp (−d/d0), with d0 constant over the entire range of Jc values studied. Junctions were annealed at temperatures up to 300 °C to study changes in their electrical properties and possible compatibility with high temperature fabrication processes. Current–voltage characteristics, critical current uniformity, critical current modulation with in-plane magnetic field, and sub-gap resistance behavior of these junctions were measured at 4 K and demonstrate that the junction properties do not degrade with annealing. These data indicate that Nb/a-Si/Nb JJs are a potential candidate for higher speed and higher density superconducting circuits.
We present electrical characterization data of sputtered Nb/a-Si/Nb Josephson junctions (JJs) for high-speed and high-density superconducting circuits. Junctions were studied with critical current densities (Jc) ranging from 0.01 to 3 mA/μm2 at 4 K. For junctions deposited at room temperature and processed to a maximum temperature of 150 °C, the dependence of Jc on barrier thickness d is exponential, Jc∝exp (−d/d0), with d0 constant over the entire range of Jc values studied. Junctions were annealed at temperatures up to 300 °C to study changes in their electrical properties and possible compatibility with high temperature fabrication processes. Current–voltage characteristics, critical current uniformity, critical current modulation with in-plane magnetic field, and sub-gap resistance behavior of these junctions were measured at 4 K and demonstrate that the junction properties do not degrade with annealing. These data indicate that Nb/a-Si/Nb JJs are a potential candidate for higher speed and higher density superconducting circuits.
We investigated current-voltage characteristics of unshunted and externally shunted Josephson junctions (JJs) with high critical current densities Jcin order to extract their basic parameters and statistical characteristics for JJ modeling in superconducting integrated circuits as well as to assess their potential for future technology nodes. Nb/AlOx-Al/Nb junctions with diameters from 0.5 to 6 μm were fabricated using a fully planarized process with Mo or MoNxthin-film shunt resistors with sheet resistance Rsq= 2 Ω/sq and Rsq= 6 Ω/sq, respectively. We used our current standard MIT Lincoln Laboratory process node SFQ5ee to fabricate JJs with Jc= 0.1 mA/μm2and our new process node SFQ5hs (where “hs” stands for high speed) to make JJs with Jc= 0.2 mA/μm2and higher current densities up to about 1 mA/μm2. Using LRC resonance features on the I-V characteristics of shunted JJs, we extract the inductance associated with Mo shunt resistors of 1.4 pH/sq. The main part of this inductance, about 1.1 pH/sq, is the inductance of the 40-nm Mo resistor film, while the geometrical inductance of superconducting Nb wiring contributes the rest. We attribute this large inductance to “kinetic” inductance arising from the complex conductivity of a thin normal-metal film in an electromagnetic field with angular frequency ω, σ(ω) = σ0/(1 + iωτ), where σ0is the static conductivity and r the electron scattering time. Using a resonance in a large-area unshunted high-Jcjunction excited by a resistively coupled small-area shunted JJ, we extract the Josephson plasma frequency and specific capacitance of high-Jcjunctions in 0.1-1 mA/μm2Jcrange. We also present data on Jctargeting and JJ critical current spreads. We discuss the potential of using 0.2-mA/μm2JJs in very large scale integration single flux quantum circuits and 0.5-mA/μm2JJs in high-density integrated circuits without shunt resistors.
We have fabricated NbTiN/Al-AlNX/NbTiN Josephson tunnel junctions on thermally oxidized Si substrates for their applications to single-flux-quantum circuits operating around 10 K. NbTiN films with a critical temperature Tcof 14.2 K and a root-mean-square value of their surface roughness of 0.45 nm were used as junction electrodes. For a junction barrier, radical-nitrided AlNXwas used to realize high uniformity in the critical current Icof the junctions. We obtained NbTiN junctions with a gap voltage Vg of 4.3 mV and an IcRn. product of 2.6 mV at 4.2 K, where R. is the junction resistance. The critical current density Jc, at 4.2 K was obtained in the range between 0.2 and 6.2 kA/cm2by changing a nitridation time for the AlNXbarrier formation. A series array of 200 junctions with an area of 10 μm × 10 μm exhibited a small I', spread, and the standard deviation 1-σ in Icuniformity was less than 1.6% in the temperature range between 4 K and 10 K.
To increase the integration scale of superconductor electronics, we are developing a new node, titled SFQ7ee, of the fabrication process at MIT Lincoln Laboratory. In comparison to the existing SFQ5ee node, we increased the number of fully planarized superconducting layers to ten and utilized NbN and NbN/Nb kinetic inductors to increase the possible inductor number density above a hundred million inductors per cm2. Increasing the Josephson junction number density to the same level requires implementing self-shunted high-JcJosephson junctions. We investigated the properties of Nb/NbNx/Nb trilayer junctions as a potential replacement of high-JcNb/Al-AlOx/Nb junctions, where NbNxis a disordered, overstoichiometric, nonsuperconducting nitride deposited by reactive sputtering. Dependences of theIcRnproduct and Josephson critical current density, Jc on the NbNx barrier thickness and temperature were studied in the thickness range from 5 nm to 20 nm. The fabricated junctions can be well described by the microscopic theory of SNS junctions, assuming no suppression of the energy gap in Nb electrodes near the NbNxinterfaces and a Cooper pair decay length in the NbNx barrier, ξnTcof about 2.3 nm. Current-voltage characteristics of the junctions are well described by the resistively and capacitively shunted junction (RCSJ) model without excess current. In the studied rangeJc2, the Nb/NbNx/Nb junctions have lower values of the specific resistanceRnA, lower IcRn products, and a stronger dependence of theIcRnon temperature than the self-shunted or critically damped externally shunted Nb/Al-AlOx/Nb junctions with the same critical current density; A is the junction area,Icthe junction critical current, and Rnthe effective shunting resistance.
We report on the fabrication and millikelvin char acterization of Nb/Al–AlOx/Nb Josephson junctions directly de posited on X-cut LiNbO3 substrates. Motivated by the strong electro-optic and piezoelectric properties of LiNbO3, such devices are of interest for future hybrid superconducting–electro-optic architectures. The junctions were fabricated using standard processes involving DC magnetron sputtering followed by controlled thermal oxidation of the aluminum layer to form an AlOx tunnel barrier with reproducible junction areas. Insulation of the junctions was achieved by anodization and SiOx deposition. Electrical measurements performed at 30 mK reveal high-quality Josephson tunnel junction behavior with a critical current density ofJc= 70 A/cm2and a characteristic voltage reaching values up toVm= 286 mV. Differential conductance measurements exhibit pronounced quasiparticle peaks corresponding to a sum gap energy of 2Δ = (2.73 ± 0.05) meV, confirming the good quality of the Nb electrodes. Magnetic-field-dependent measure ments yield a Josephson penetration depth of λJ= 45 μm, confirming operation in the small-junction regime, with deviations from the ideal Josephson interference pattern attributed to non-uniform current distribution across the junction area. Compared to our previous characterization at 4.2 K, this work demonstrates reliable Josephson junction operation on LiNbO3 at dilution-refrigerator temperatures relevant for quantum circuits, supporting the suitability of lithium niobate as a promising substrate for future hybrid superconducting–electro-optic device implementations.
Abstract 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 1–104 mΩ cm by controlling both the N2 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 cm2 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.
Abstract Superconducting circuits based on Josephson junctions have the potential to achieve high speed and ultra-low power consumption, but their integration is limited by the low controllability of Nb-based tunnel junction and the existence of shunt resistors. In this work, we report the fabrication of superconductor/normal metal/superconductor (SNS) Josephson junctions with high reproducibility on oxidized Si substrates. The junctions based on NbN/TaN/NbN trilayers measured at 4.2 K show excellent Josephson properties with a wide range of critical current ( I c ) from 227 to 2000 μ A. The variations of critical current density ( J c ) and characteristic voltage ( V c ) are respectively less than 7.1% and 7.5% in the 2 inch region. The standard deviation of I c is calculated to be less than 1.7% for Josephson arrays with the number of junctions up to 10 000 occupying an area of 0.34 mm 2 . The results provide a guarantee for the applications of NbN-SNS Josephson junctions in superconducting large-scale integrated circuits.