Nb Junctions Research Articles

Controllable Manipulation of Semifluxon States in Phase-Shifted Josephson Junctions.

The utilization of Josephson vortices as information carriers in superconducting digital electronics is hindered by the lack of reliable displacement and localization mechanisms. In this Letter, we experimentally investigate planar Nb junctions with an intrinsic phase shift and nonreciprocity induced by trapped Abrikosov vortices. We demonstrate that the entrance of a single Josephson vortex into such junctions triggers the switching between metastable ±π semifluxon states. We showcase controllable manipulation between these states using short current pulses and achieve a nondestructive readout by a nearby junction. Our observations pave the way toward ultrafast and energy-efficient digital Josephson electronics.

Optimization of Nb/Al-AlO x /Nb Josephson junctions through wafer-scale anodic oxidation: a systematic characterization and performance analysis

The Nb/Al-AlO x /Nb (SIS) Josephson junction is a crucial component in many types of superconducting devices. However, it can be easily damaged during the plasma fabrication processes. Anodic oxidation is an effective method for protecting SIS junctions by oxidizing Nb and Al from the junction profile. We used a custom wafer-scale anodic oxidation system and a neutral electrolyte to study the oxidation process of Nb films and Nb-Al(-AlO x )-Nb junctions. The oxidation process was thoroughly characterized by considering factors such as morphology and electrical properties. Anodization spectroscopy revealed varying oxidation sections from the top Nb layer to the bottom layer, extending across the Al-AlO x interlayer. This indicates that a 4 nm Al layer is sufficient to cover the surface of the bottom Nb film. High-resolution transmission electron microscopy and energy dispersive spectroscopy revealed that the Nb2O5 layer produced from the oxidation of the bottom Nb layer penetrated the Al2O3 layer and migrated to the top surface as the oxidation voltage increased. The top Nb layer of the SIS junction was also subjected to oxidation, despite the presence of a protective photoresist. Following the anodic oxidation process, the entire wafer surface was coated with an insulating Nb2O5 film. This film provided protection for the SIS junctions during the subsequent microfabrication process. The fabricated junction array, consisting of 128 junctions, demonstrated uniform electrical properties benefiting from the anodic oxidation process. This systematic analysis will further the research and practical applications of SIS junctions.

Comparison of Double Relaxation Oscillation SQUIDs and DC-SQUIDs of Large Stewart-McCumber Parameter

Increasing the flux-to-voltage transfer coefficient of the SQUID output voltage is necessary to release the constraint on the preamplifier input noise. As one of the approaches to increase the transfer coefficient, we have been using a double relaxation oscillation SQUID (DROS) having a transfer coefficient of about 10 times larger than those of DC-SQUIDs. Traditionally, DC-SQUIDs were operated in the non-hysteretic condition with the Stewart-McCumber parameter (β <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) less than 1. In this study, we fabricated hysteretic DC-SQUIDs with the β <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ranging from 2.5 and compared the transfer coefficient and the system noise of DC-SQUIDs and DROSs. Both DROS and DC-SQUID have the same structure and design parameters for the SQUID loop inductance, resonance-damping circuits in the SQUID loop, and input coil. All the SQUIDs were fabricated in the same wafer using the Nb/AlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> /Nb junction process. The transfer coefficients of DROSs were about 1 mV/Φ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> , regardless of SQUID parameters. For DC-SQUIDs, the transfer coefficients were sensitive to the β <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> and the operating margin was narrower than that of DROS. The white noise of both SQUIDs in the flux-locked loop mode was around 1.5 μΦ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> /√Hz at 100 Hz, measured with a preamplifier of input voltage noise of 1 nV/√Hz. DC-SQUIDs had mostly a narrower operating margin against the bias current, and the modulation voltage was smaller than DROS. In a near-optimized operating condition, both DROS and hysteretic DC-SQUID showed comparable noise performance.

Nb/a-Si/Nb-junction Josephson arbitrary waveform synthesizers for quantum information.

We demonstrate Josephson arbitrary waveform synthesizers (JAWS) with increased operating temperature range for temperatures below 4 K. These JAWS synthesizers were fabricated with externally-shunted Nb/a-Si/Nb junctions whose critical current exhibits improved temperature stability compared to the self-shunted Nb/Nb0.15Si0.85/Nb junctions typically used. Vertical stud resistors made of 230 nm of PdAu were developed to provide the milliohm shunt resistance required for junction overdamping while maintaining a small footprint suitable for high-density series arrays embedded in a coplanar waveguide. We evaluated the performance of these resistors from 3.8 K down to 20 mK. We designed, fabricated and tested a JAWS circuit with 4650 externally shunted Nb/a-Si/Nb JJs with a critical current density () of 0.12 and critical current () of 3 mA. This circuit was designed to be mounted to the 3 K stage of a dilution refrigerator and used to control and calibrate a qubit mounted at the 10 mK stage. To increase the circuit density of the JAWS circuits we made arrays of two-junction vertical stacks. Current-voltage () curves of this JAWS circuit with stacked junctions under microwave excitation show Shapiro steps with quantum-locking ranges similar to those of JAWS circuits used for qubit control.

Modeling the Effects of Niobium Surface Roughness on Electrical Conductivity of Nb/Al-AlO$_{x}$/Nb Josephson Junctions

Technology Computer Aided Design (TCAD) is being developed for superconductor electronics (SCE). The AlO <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{x}$</tex-math></inline-formula> tunnel barriers used in SCE have thicknesses on the order of the surface roughness of sputtered niobium. Surface roughness is not included in a standard CMOS TCAD process simulator and is among the unique modules needed for SCE. An empirical model for surface roughness is developed for TCAD process simulators. This model is merged with the existing sputtering module and coupled with chemical mechanical polishing and aluminum oxidation to generate process-simulated structures of the Nb/Al-AlO <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{x}$</tex-math></inline-formula> /Nb junction stack. Electrical simulations of these junctions are used to extract room temperature conductance. The inherent statistical variation of the proposed surface roughness model lends itself to large scale parallel simulation of thousands of junctions. Statistical simulations are performed and statistical analysis is given.

Aspects of long range spin–triplet correlations in superconductor/ferromagnet hetero-structures

The notion of competing ferromagnetic (F) and superconducting (S) orders in F/S hybrid structures was transformed by the first realization of ferromagnetic Josephson π-junctions and the almost simultaneous prediction of a possibility of spin–triplet correlations in such structures, almost two decades back. Such hybrid structures in various configurations are now studied as rich sources of emergent states and new effects. Unlike the spin singlet Cooper pairs, the spin triplet Cooper pairs are much less affected by the exchange field of a ferromagnet and, therefore, immediately finds interest in the field of spintronics. Theoretically, it has been shown that the basic protocol for spin–singlet to spin–triplet supercurrent conversion is the presence of magnetic non-collinearity at the superconductor–ferromagnet interface. Therefore, almost all experiments in this direction have utilized transport measurements on F/S systems with artificial magnetic non-collinearity formed by combination of several ferromagnetic layers next to the superconducting layer. Here we highlight two aspects of studying these heterostructures. Firstly we show that natural magnetic inhomogeneities, found in domain walls of ferromagnets, can also be used to achieve singlet–triplet conversion, instead of artificial magnetic inhomoheneities. This possibility was explored via transport measurements in nano-scale planar Nb–Ni–Nb junctions and nano-SQUIDs, where a domain wall was pinned at the Josephson junction barrier. By this method we were able to show Josephson coupling across about 70 nm of strong ferromagnetic planar barrier. Secondly we show that spin–triplet correlations at the F/S interface are robust enough to be probed by the diamagnetic screening currents at the interface. This was probed by studying the change in sperconducting transition temperature of Nb/Co/Py/Nb multilayers in presence of small in-plane magnetic field. The Co/Py combination, which is a soft-hard type magnetic exchange spring, worked as magnetic inhomogeneity for triplet generation at the interface of the superconducting Nb. These observations may promote new experiments in the field of superconducting-spintronics.

Optimization of fabrication processes for Nb, NbN, NbTiN films and high-quality tunnel junctions for terahertz receiving circuits.

This paper describes the optimization of the existing technology for the fabrication of superconducting films and high-quality tunnel junctions on a magnetron sputtering facility. To expand the frequency range to 1.1 THz and to obtain the limiting parameters of superconducting elements, the regimes of production of Nb, NbN, NbTiN films were optimized. These films are used to fabricate superconductor-insulator-superconductor Nb/Al--AlN/NbN tunnel junctions. Al and NbTiN films are required to create receiving elements at frequencies above 700 GHz (the cutoff frequency for niobium); such structures are being developed for the radio astronomy array receiver located in the Atacama Pathfinder Experiment telescope. Keywords: superconductivity, tunnel junctions, magnetron sputtering, thin films.

Fabrication Process for Deep Submicron SQUID Circuits with Three Independent Niobium Layers.

We present a fabrication technology for nanoscale superconducting quantum interference devices (SQUIDs) with overdamped superconductor-normal metal-superconductor (SNS) trilayer Nb/HfTi/Nb Josephson junctions. A combination of electron-beam lithography with chemical-mechanical polishing and magnetron sputtering on thermally oxidized Si wafers is used to produce direct current SQUIDs with 100-nm-lateral dimensions for Nb lines and junctions. We extended the process from originally two to three independent Nb layers. This extension offers the possibility to realize superconducting vias to all Nb layers without the HfTi barrier, and hence to increase the density and complexity of circuit structures. We present results on the yield of this process and measurements of SQUID characteristics.

Design and Verification of SFQ Cell Library for Superconducting LSI Digital Circuits

We have developed a cell library for superconducting large scale integrated (LSI) digital circuits based on Single Flux Quantum (SFQ) devices. The circuits were designed for a 6-kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> Nb/AlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> /Nb junction process with 10 mask levels including one ground plane and two Nb wiring layers. The initial design and optimization of the circuit parameters were achieved by means of the optimization functions in a circuit simulator, PSCAN2, to ensure that important circuit parameters, Josephson critical current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> ), bias current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">bias</sub> ), and inductance satisfied minimal margin requirements. Critical margins of I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> and I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">bias</sub> were then further improved manually. To compensate the scattering in the circuit parameters from fabrication by design as much as possible, the critical junction parameters were optimized to further centralize the I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> . The library cells were laid out following our design rules determined by systematic experiments on process control monitors (PCM). Basic cells including Josephson transmission lines, splitters, confluence buffers, D flip-flops, T flip-flops, and XOR and NDRO gates were designed, fabricated, and successfully tested at low frequencies. Wide overlaps of the operating regions for the common bias voltages were confirmed.

Planarized Fabrication Process With Two Layers of SIS Josephson Junctions and Integration of SIS and SFS &lt;italic&gt;π&lt;/italic&gt;-Junctions

We present our new fabrication process for superconductor electronics that integrates two layers of Josephson junctions (JJs) in a fully planarized multilayer process on 200-mm wafers. The two junction layers can be, e.g., conventional superconductor-insulator-superconductor (SIS) Nb/Al/AlOx/Nb junctions with the same or different Josephson critical current densities, Jc. The process also allows integration of high-Jc superconductor-ferromagnet-superconductor (SFS) or SFS'S JJs on the first junction layer with Nb/Al/AlOx/Nb trilayer junctions on the second JJ layer, or vice versa. In the present node, the SFS trilayer, Nb/Ni/Nb is placed below the standard SIS trilayer and separated by one niobium wiring layer. The main purpose of integrating the SFS and SIS junction layers is to provide compact π-phase shifters in logic cells of superconductor digital circuits and random access memories, and thereby increase the integration scale and functional density of superconductor electronics. The current node of the two-junction-layer process has six planarized niobium layers, two layers of resistors, and 350-nm minimum feature size. The target critical current densities for the SIS JJs are 100 μA/μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and 200 μA/μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . We present the salient features of the new process, fabrication details, and characterization results on two layers of JJs integrated into one process, both for the conventional and π-junctions.

Film Stress Influence on Nb/Al-AlO x /Nb Josephson Junctions

Nb/Al-AlOx/Nb Josephson junctions are widely used in superconducting quantum interference devices and rapid single flux quantum (RSFQ) circuits. Very large-scale RSFQ integration often has a strict requirement on wafer-scale junction qualities. In particular, Nb film stress in Nb/Al-AlOx/Nb trilayer based Josephson junctions is a critical factor affecting their qualities. In this work, we develop a multi-step sputtering process to prepare Nb films with different combinations of stress and roughness to investigate the correlation between Nb film stress and junction quality. In particular, this method allows us to prepare Nb films with almost the same roughness but different stress, a key factor to study solely the influence of stress on junction qualities. Our measurements indicate that within a very tight range of film roughness (less than 2 nm), tensile film stress is found to decrease the quality of Nb/Al-AlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> /Nb Josephson junctions and larger film roughness has a positive effect on junction quality. By decreasing the total tensile stress and increasing the film roughness properly, we fabricated Nb/Al-AlOx/Nb Josephson junctions with a high quality factor characterized by R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sg</sub> /R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</sub> (30 on average). The minimum size of junctions was 1 μm.

4π-Periodic Supercurrent from Surface States in Cd_{3}As_{2} Nanowire-Based Josephson Junctions.

The combination of superconductivity and surface states in Dirac semimetal can produce a 4π-periodic supercurrent in a Josephson junction configuration, which can be revealed by the missing of odd Shapiro steps (especially the n=1 step). However, the suppression of the n=1 step is also anticipated in the high-power oscillatory regime of the ordinary 2π-periodic Josephson effect, which is irrelevant to the 4π-periodic supercurrent. Here, in order to identify the origin of the suppressed n=1 step, we perform the measurements of radio frequency irradiation on Nb-Dirac semimetal Cd_{3}As_{2} nanowire-Nb junctions with continuous power dependence at various frequencies. Besides the n=1 step suppression, we uncover a residual supercurrent of first node at the n=0 step, which provides a direct and predominant signature of the 4π-periodic supercurrent. Furthermore, by tuning the gate voltage, we can modulate the surface and bulk state contribution and the visibility of the n=1 step. Our results provide deep insights to explore the topological superconductivity in Dirac semimetals.

Microscopic Tunneling Model of Nb–AlN–NbN Josephson Flux-Flow Oscillator

Since the very first experimental realization of a Josephson flux-flow oscillator (FFO), its theoretical description has been limited by the phenomenological perturbed sine-Gordon equation (PSGE). While PSGE can qualitatively describe the topological excitations in Josephson junctions that are sine-Gordon solitons or fluxons, it is unable to capture essential physical phenomena of a realistic system such as the coupling between tunnel currents and electromagnetic radiation. Furthermore, PSGE neglects any dependence on energy gaps of superconductors and makes no distinction between symmetric and asymmetric junctions: those made of two identical or two different superconducting materials. It was not until recently when it became possible to calculate properties of FFO by taking into account information about energy gaps of superconductors (Gulevich et al. in Phys Rev B 96:024215, 2017). Such approach is based on the microscopic tunneling theory and has been shown to describe essential features of symmetric Nb–AlO\(_\mathrm{x}\)–Nb junctions. Here, we extend this approach to asymmetric Nb–AlN–NbN junctions and compare the calculated current–voltage characteristics to our experimental results.

Inductance analysis of superconducting quantum interference devices with 3D nano-bridge junctions

Superconducting quantum interference devices (SQUIDs) with 3D nano-bridge junctions can be miniaturized into nano-SQUIDs that are able to sense a few spins in a large magnetic field. Among all device parameters, the inductance is key to the performance of SQUIDs with 3D nano-bridge junctions. Here, we measured the critical-current magnetic flux modulation curves of 12 devices with three design types using a current strip-line directly coupled to the SQUID loop. A best flux modulation depth of 71% was achieved for our 3D Nb SQUID. From the modulation curves, we extracted the inductance values of the current stripe-line in each design and compared them with the corresponding simulation results of InductEX. In this way, London penetration depths of 110 and 420 nm were determined for our Nb (niobium) and NbN (niobium nitride) films, respectively. Furthermore, we showed that inductances of 11 and 119 pH for Nb and NbN 3D nano-bridge junctions, respectively, dominated the total inductance of our SQUID loops which are 23 pH for Nb and 255 pH for NbN. A screening parameter being equal to one suggests optimal critical currents of 89.6 and 8.1 μA for Nb and NbN SQUIDs, respectively. Additionally, intrinsic flux noise of 110 ± 40 nΦ0/(Hz)1/2 is calculated for the Nb SQUIDs with 3D nano-bridge junctions by Langevin simulation.

NIST 10 V Programmable Josephson Voltage Standard System Using a Low-Capacity Cryocooler

The recent shortage and increasing cost of liquid helium provides motivation for cryogen-free operation of superconducting devices such as NIST programmable Josephson voltage standard (PJVS) systems. However, operation on closed-cycle cryocoolers must not compromise the performance of the PJVS system. New cryogenic packaging and cryostat integration are presented that have been optimized for the NIST 10 V PJVS, demonstrating improved attenuation of coldhead temperature oscillations, and improvement of thermal conductances in the junction-to-coldhead path. When combined with an improved design of 10 V PJVS devices employing Nb/Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> /Nb junctions with increased operating margins, we have operated a NIST PJVS at 10 V with over 1.32 mA current margins on a nominal 200 mW cryocooler using a 3.0 kW water-cooled compressor. The new cryo-package, in conjunction with the improved generation of chips, eliminates the need for liquid cryogens in applications using NIST 10 V PJVS systems.

Electric field and temperature dependence of dielectric permittivity in strontium titanate investigated by a photoemission study on Pt/SrTiO3:Nb junctions

Schottky junctions made from platinum and niobium-doped strontium titanate (SrTiO3:Nb) were investigated by hard X-ray photoemission (HXPES) and through a band bending behavior simulation using a phenomenological model, which assumes a decrease in dielectric constant due to an electric field. Thus, we confirmed that the observed HXPES spectra at relatively high temperatures, e.g., &amp;gt;250 K, were well simulated using this phenomenological model. In contrast, it was inferred that the model was not appropriate for junction behavior at lower temperatures, e.g., &amp;lt;150 K. Therefore, a reconstruction of the phenomenological model is necessary to adequately explain the dielectric properties of SrTiO3.

Current–voltage characteristics of Nb–carbon–Nb junctions

We report on properties of Nb(/Ti)–carbon–(Ti/)Nb junctions fabricated on graphite flakes using e-beam lithography. The devices were characterized at temperatures above 1.8 K where a Josephson current was not observed, but the differential conductivity revealed features below the critical temperature of Nb, and overall metallic conductivity, in spite of a high-junctions resistance. Since the conductivity of graphite along the planes is essentially two-dimensional (2D), we use a theoretical model developed for metal/graphene junctions for interpretation of the results. The model involves two very different graphene “access” lengths. The shorter length characterizes ordinary tunneling between the three-dimensional Nb(/Ti) electrode and 2D graphene, while the second, much longer length, is associated with the Andreev reflections (AR) inside the junction and involves also “reflectionless” AR processes. The relevant transmission factors are small in the first case and much larger in the second, which explains ...

Publisher's Note: “Determination of Schottky barrier profile at Pt/SrTiO3:Nb junction by x-ray photoemission” [Appl. Phys. Lett. 101, 251911 (2012)

Publisher's Note: “Determination of Schottky barrier profile at Pt/SrTiO3:Nb junction by x-ray photoemission” [Appl. Phys. Lett. 101, 251911 (2012)

Determination of Schottky barrier profile at Pt/SrTiO3:Nb junction by x-ray photoemission

The platinum/[niobium-doped strontium titanate] junction (Pt/SrTiO3:Nb) was investigated by x-ray photoemission (XPE) spectroscopy. Aluminum Kα and synchrotron radiation (6 keV) were used to obtain XPE spectra with different probing depths. The broadening and shift of the XPE peaks for SrTiO3:Nb, which resulted from the formation of a potential barrier at the interface, were quantitatively analyzed by fitting simulations. The barrier height was calculated to be 0.7–0.8 regardless of the Nb concentration. Furthermore, the XPE profile of the junction was reproduced when the permittivity of SrTiO3 was assumed to depend on the electric field.

Universality of transport properties of ultrathin oxide films

We report low-temperature measurements of current–voltage characteristics for highly conductive Nb/Al–AlOx–Nb junctions with thicknesses of the Al interlayer ranging from 40 to 150 nm and ultrathin barriers formed by diffusive oxidation of the Al surface. In a superconducting state these devices have revealed a strong subgap current leakage. Analyzing Cooper-pair and quasiparticle currents across the devices, we conclude that the strong suppression of the subgap resistance compared with conventional tunnel junctions is not related to technologically derived pinholes in the barrier but rather has more fundamental grounds. We argue that it originates from a universal bimodal distribution of transparencies across the aluminum oxide barrier proposed earlier by Schep and Bauer (1997 Phys. Rev. Lett. 78 3015). We suggest a simple physical explanation of its source in the nanometer-thick oxide films relating it to strong local barrier-height fluctuations in the nearest to conducting electrode layers of the insulator which are generated by oxygen vacancies in thin aluminum oxide tunnel barriers formed by thermal oxidation.