Fabrication Of Junctions Research Articles

High performance rapid single-flux-quantum bit-slice arithmetic logic unit

Two optimization technologies, namely, bypass and carry-control optimization, were demonstrated for enhancing the performance of a bit-slice Arithmetic Logic Unit (ALU) in 2n-bit Rapid Single-Flux-Quantum (RSFQ) microprocessors. These technologies can not only shorten the calculation time but also solve data hazards. Among them, the proposed bypass technology is applicable to any 2n-bit ALU, whether it is bit-serial, bit-slice or bit-parallel. The high performance bit-slice ALU was implemented using the 6 kA/cm2 Nb/AlOx/Nb junction fabrication process from Superconducting Electronics Facility of Shanghai Institute of Microsystem and Information Technology. It consists of 1693 Josephson junctions with an area of 2.46 × 0.81 mm2. All ALU operations of the MIPS32 instruction set are implemented, including two extended instructions, i.e., addition with carry (ADDC) and subtraction with borrow (SUBB). All the ALU operations were successfully obtained in SFQ testing based on OCTOPUX and the measured DC bias current margin can reach 86% - 104%. The ALU achieves a 100% utilization rate, regardless of carry/borrow read-after-write correlations between instructions.

A novel phosphorus diffusion process for front-side P–N junction fabrication in PERC solar cells

P–N junction technology underlies photovoltaic conversion in passive emitter and rear cell (PERC) solar cells. Although the front-side phosphorus diffusion method for creating P-type PERC cells is well researched, avenues for innovation persist. We introduce a P–N junction fabrication technique for PERC solar cells via precisely controlling the surface doping concentration and junction depth. Through pressure modulation and carefully selected annealing duration at a defined temperature, the technique yields a P–N junction with a surface doping concentration of 2.37 × 1020 cm−3 and a junction depth of 0.29 μm on the front surface of PERC cells. Compared to conventional diffusion processes, our approach results in an average increase of 1.3 mV in open-circuit voltage, 20 mA in short-circuit current, and an efficiency gain of 0.05 %. The maximum efficiency achieved is 23.68 %, representing an improvement of 0.16 %.

Spin Coupling in the Initial Stages of the Zinc‐Blende MnN Growth on the CrN (111) Surface

AbstractSpin‐polarized first‐principles calculations are carried out to study the structural, electronic, and magnetic properties of the MnN growth on the CrN (111) surface. Different magnetic configurations between Mn‐deposited atoms and the substrate are considered. The H3 site is the most favorable site for the adoption and incorporation cases and the Mn spins switch from paramagnetic to antiferromagnetic arrangement. The minimum energy pathway for the Mn diffusion is calculated, showing magnetic modifications in the substrate. A structural transition from tetragonal rock‐salt to zinc‐blende (ZB) structure as the number of MnN deposited layers increases is noticed. Thermodynamic analysis demonstrates that the growth of MnN on CrN is feasible. The density of states for an MnN layer with an extra nitrogen layer shows a half‐metallic behavior and the remaining structures exhibit a metallic character. The ZB‐MnN structures have an antiferromagnetic behavior with a metallic character. Magnetic anisotropy energies reveal a switching of easy magnetization axis, from in‐plane to out‐of‐plane with the formation of MnN on the surface. These results suggest that the CrN/ZB‐MnN system may be employed in antiferromagnetic spintronics applications as the fabrication of perpendicular magnetic tunnel junctions.

Fabrication and properties of lateral Josephson junctions with a RuO2 weak link

Ruthenium dioxide (RuO2) is a metallic rutile oxide with a number of interesting properties. For a long time, it was considered to be a highly conductive normal metal and a Pauli paramagnet. Recently, it was found that the material is antiferromagnetic, with small magnetic moments of the order of 0.05 Bohr magneton and an ordering temperature above 300 K. The presence of magnetic moments should have clear consequences when trying to induce superconductivity in RuO2. We used a selective area chemical vapor deposition method to grow nanostrips of RuO2 on TiO2 substrates. On these nanostrips, superconducting contacts were made of MoGe, and a weak link was fabricated with a Focused Ion Beam. We find that the device behaves as a Josephson junction, including a Fraunhofer-like response to a magnetic field, for distances between the contacts below 70 nm. We estimate the induced singlet coherence length ξ to be about 12 nm, which seems a reasonable number when small magnetic moments are present.

Co-doped BaFe2As2 Josephson junction fabricated with a focused helium ion beam

Josephson junction plays a key role not only in studying the basic physics of unconventional iron-based superconductors but also in realizing practical application of thin-film based devices, therefore the preparation of high-quality iron pnictide Josephson junctions is of great importance. In this work, we have successfully fabricated Josephson junctions from Co-doped BaFe2As2 thin films using a direct junction fabrication technique which utilizes high energy focused helium ion beam (FHIB). The electrical transport properties were investigated for junctions fabricated with various He+ irradiation doses. The junctions show sharp superconducting transition around 24 K with a narrow transition width of 2.5 K, and a dose correlated foot-structure resistance which corresponds to the effective tuning of junction properties by He+ irradiation. Significant J c suppression by more than two orders of magnitude can be achieved by increasing the He+ irradiation dose, which is advantageous for the realization of low noise ion pnictide thin film devices. Clear Shapiro steps are observed under 10 GHz microwave irradiation. The above results demonstrate the successful fabrication of high quality and controllable Co-doped BaFe2As2 Josephson junction with high reproducibility using the FHIB technique, laying the foundation for future investigating the mechanism of iron-based superconductors, and also the further implementation in various superconducting electronic devices.

Superconducting Qubit Based on Twisted Cuprate Van der Waals Heterostructures.

Van-der-Waals assembly enables the fabrication of novel Josephson junctions featuring an atomically sharp interface between two exfoliated and relatively twisted Bi_{2}Sr_{2}CaCu_{2}O_{8+x} (Bi2212) flakes. In a range of twist angles around 45°, the junction provides a regime where the interlayer two-Cooper pair tunneling dominates the current-phase relation. Here we propose employing this novel junction to realize a capacitively shunted qubit that we call flowermon. The d-wave nature of the order parameter endows the flowermon with inherent protection against charge-noise-induced relaxation and quasiparticle-induced dissipation. This inherently protected qubit paves the way to a new class of high-coherence hybrid superconducting quantum devices based on unconventional superconductors.

Fabrication of multi-functional molecular tunnelling junctions by click chemistry.

Continuous advancement in molecular electronics demands increasing functionality and diversity of integrated molecular junctions; however, single-functional molecular junctions fail to meet these requirements. In this article, we propose the use of a widely applicable and efficient click reaction on the surface to modify self-assembled monolayers (SAMs) to achieve multifunctional molecular tunnelling junctions, current rectification and memristance, on a single chip. This approach has allowed us to meet the growing demand for versatility and functionality in molecular electronic devices.

Methods for the analysis, interpretation, and prediction of single-molecule junction conductance behaviour

This article provides an overview of measurement methods and interpretations of data in the field of molecular electronics, together with a summary of predictive models that assist in establishing robust structure–property relationships.

The impact of boron doping in the tunneling magnetoresistance of Heusler alloy Co2FeAl

Heusler alloy-based magnetic tunnel junctions have the potential to provide high spin polarization, small damping, and fast switching. In this study, junctions with a ferromagnetic electrode of Co2FeAl were fabricated via room-temperature sputtering on Si/SiO2 substrates. The effect of boron doping on Co2FeAl magnetic tunnel junctions was investigated for different boron concentrations. The surface roughness determined by atomic force microscope, and the analysis of x-ray diffraction measurement on the Co2FeAl thin film reveals critical information about the interface. The Co2FeAl layer was deposited on the bottom and on the top of the insulating MgO layer as two different sample structures to compare the impact of the boron doping on different layers through tunneling magnetoresistance measurements. The doping of boron in Co2FeAl had a large positive impact on the structural and magneto-transport properties of the junctions, with reduced interfacial roughness and substantial improvement in tunneling magnetoresistance. In samples annealed at low temperature, a two-level magnetoresistance was also observed. This is believed to be related to the memristive effect of the tunnel barrier. The findings of this study have practical uses for the design and fabrication of magnetic tunnel junctions with improved magneto-transport properties.

Fabrication and characterization of sub-micron high current density NbN Josephson junctions for large-scale digital circuits

We have developed a fabrication process based on Chemical Mechanical Polishing (CMP) for the sub-micron NbN Josephson junctions on single-crystal MgO substrates. The NbN junctions were fabricated with epitaxial NbN/AlN/NbN trilayers deposited by reactive DC magnetron sputtering. The top electrode of the junction was exposed using CMP to contact the NbN wire without opening the submicron via through the dielectric. It has produced submicron NbN junctions with areas as small as 0.09 μm2. We have fabricated NbN tunnel junctions with a wide range of critical current density Jc. The NbN junctions show excellent Josephson tunneling characteristics, the gap voltage Vg is 5.8 mV and the quality factor Rsg/Rn is 12 for the junctions with a Jc of 10.7 kA/cm2, and Vg is 5.3 mV and Rsg/Rn is 4 for the junctions with a Jc of 150.5 kA/cm2. The 1000-junction arrays were fabricated and their I-V characteristics were measured, critical current uniformity was obtained and the standard deviation is about 5%. The high Jc sub-micron NbN junctions will be helpful in developing large-scale NbN SFQ circuits with high operating frequency.

Thickness-Dependent Charge Transport in Three Dimensional Ru(II)- Tris(phenanthroline)-Based Molecular Assemblies.

We describe here the fabrication of large-area molecular junctions with a configuration of ITO/[Ru(Phen)3]/Al to understand temperature- and thickness-dependent charge transport phenomena. Thanks to the electrochemical technique, thin layers of electroactive ruthenium(II)-tris(phenanthroline) [Ru(Phen)3] with thicknesses of 4-16 nm are covalently grown on sputtering-deposited patterned ITO electrodes. The bias-induced molecular junctions exhibit symmetric current-voltage (j-V) curves, demonstrating highly efficient long-range charge transport and weak attenuation with increased molecular film thickness (β = 0.70 to 0.79 nm-1). Such a lower β value is attributed to the accessibility of Ru(Phen)3 molecular conduction channels to Fermi levels of both the electrodes and a strong electronic coupling at ITO-molecules interfaces. The thinner junctions (d = 3.9 nm) follow charge transport via resonant tunneling, while the thicker junctions (d = 10-16 nm) follow thermally activated (activation energy, Ea ∼ 43 meV) Poole-Frenkel charge conduction, showing a clear "molecular signature" in the nanometric junctions.

The effect of under and over develop time in Josephson junction fabrication process using electron beam lithography

In the 1960s, the physicist Brian Josephson introduced the concept of Josephson tunnel junctions, which involve the tunneling of Cooper pairs across a barrier between superconducting electrodes. The typical methods of fabricating Josephson junction such as Dolan bridge and cross-type technique require specialized equipment for angle deposition. In this work, we present Josephson junction fabrication process using electron beam lithography that eliminates the need for angle deposition. The dose test was performed to optimize the right dose and we also explored various develop time. We observed that for PMMA A9 film with thickness 1.3 μm, the electron dose 280 μC/cm2 and develop time between 1.30 – 2.00 minutes produces clear pattern with sharp features. The effect of under and over develop time is shown such that there are resist residue left on the substrate after lift-off process and the longer develop time created an undercut.

Ultrafast self-powered phototransistor based on Te-WSe2 van der Waals heterojunction

Band engineering through the integration of van der Waals heterostructures composed of 2D materials offers a promising approach to achieving high-performance optoelectronic devices. This strategy allows for the fabrication of ultrathin and uniform PN junctions with well-defined band edges, enabling efficient charge transfer and separation. In this study, we have developed a self-powered and highly sensitive photodetector using a van der Waals heterostructure composed of WSe2 and Te. The Te layer was employed as the channel material for the phototransistor, while the out-of-plane Te/WSe2 PN junction served as the charge transfer layer. The vertical built-in electric field present in the PN junction facilitated the separation of photogenerated carriers. The proposed phototransistor demonstrated exceptional self-powered performance characteristics, including a photoresponsivity of 33.08 mA/W, a specific detectivity of 1.57 × 106 Jones, and a fast response time of 2 μs. These results highlight the effectiveness of the van der Waals heterostructure design in achieving high sensitivity and efficient carrier separation in photodetection applications.

Supramolecular Radical Electronics.

Supramolecular radical chemistry is an emerging area bridging supramolecular chemistry and radical chemistry, and the integration of radicals into the supramolecular architecture offers a new dimension for tuning their structures and functions. Although various efforts have been devoted to the fabrication of supramolecular junctions, the charge transport characterization through the supramolecular radicals remained unexplored due to the challenges in creating supramolecular radicals at the single-molecule level. Here, we demonstrate the fabrication and charge transport investigation of a supramolecular radical junction using the electrochemical scanning tunneling microscope-based break junction (EC-STM-BJ) technique. We found that the conductance of a supramolecular radical junction was more than 1 order of magnitude higher than that of a supramolecular junction without a radical and even higher than that of a fully conjugated oligophenylenediamine molecule with a similar length. The combined experimental and theoretical investigations revealed that the radical increased the binding energy and decreased the energy gap in the supramolecular radical junction, which leads to the near-resonant transport through the supramolecular radical. Our work demonstrated that the supramolecular radical can provide not only strong binding but also efficient electrical coupling between building blocks, which provides new insights into supramolecular radical chemistry and new materials with supramolecular radicals.

Single-Molecule Junction Formation in Deep Eutectic Solvents with Highly Effective Gate Coupling.

The environment surrounding a molecular junction affects its charge-transport properties and, therefore, must be chosen with care. In the case of measurements in liquid media, the solvent must provide good solvation, grant junction stability, and, in the case of electrolyte gating experiments, allow efficient electrical coupling to the gate electrodes through control of the electrical double layer. We evaluated in this study the deep eutectic solvent mixture (DES) ethaline, which is a mixture of choline chloride and ethylene glycol (1:2), for single-molecule junction fabrication with break-junction techniques. In ethaline, we were able to (i) measure challenging and poorly soluble molecular wires, exploiting the improved solvation capabilities offered by DESs, and (ii) efficiently apply an electrostatic gate able to modulate the conductance of the junction by approximately an order of magnitude within a ∼1 V potential window. The electrochemical gating results on a Au-VDP-Au junction follow exceptionally well the single-level modeling with strong gate coupling (where VDP is 1,2-di(pyridine-4-yl)ethene). Ethaline is also an ideal solvent for the measurement of very short molecular junctions, as it grants a greatly reduced snapback distance of the metallic electrodes upon point-contact rupture. Our work demonstrates that DESs are viable alternatives to often relatively expensive ionic liquids, offering good versatility for single-molecule electrical measurements.

Visualizing heterogeneous dipole fields by terahertz light coupling in individual nano-junctions

The challenge underlying superconducting quantum computing is to remove materials bottleneck for highly coherent quantum devices. The nonuniformity and complex structural components in the underlying quantum circuits often lead to local electric field concentration, charge scattering, dissipation and ultimately decoherence. Here we visualize interface dipole heterogeneous distribution of individual Al/AlOx/Al junctions employed in transmon qubits by broadband terahertz scanning near-field microscopy that enables the non-destructive and contactless identification of defective boundaries in nano-junctions at an extremely precise nanoscale level. Our THz nano-imaging tool reveals an asymmetry across the junction in electromagnetic wave-junction coupling response that manifests as hot (high intensity) vs cold (low intensity) spots in the spatial electrical field structures and correlates with defected boundaries from the multi-angle deposition processes in Josephson junction fabrication inside qubit devices. The demonstrated local electromagnetic scattering method offers high sensitivity, allowing for reliable device defect detection in the pursuit of improved quantum circuit fabrication for ultimately optimizing coherence times.

High density fabrication process for single flux quantum circuits

We implemented, optimized, and fully tested a superconducting Josephson junction fabrication process over multiple runs tailored for integrated digital circuits that are used for control and readout of superconducting qubits operating at millikelvin temperatures. This process was optimized for highly energy efficient rapid single flux quantum (ERSFQ) circuits with critical currents reduced by a factor of ∼10 as compared to those operated at 4.2 K. Specifically, it implemented Josephson junctions with 10 μA unit critical current fabricated with 10 μA/μm2 critical current density. In order to circumvent the substantial size increase in the SFQ circuit inductors, we employed an NbN high kinetic inductance layer with 8.5 pH/sq sheet inductance. Similarly, to maintain the small size of junction resistive shunts, we used a non-superconducting PdAu alloy with 4.0 Ω/sq sheet resistance. For integration with quantum circuits in a multi-chip module, 5 and 10 μm height bump processes were also optimized. To keep the fabrication process in check, we developed and thoroughly tested a comprehensive process control monitor chip set.

Symmetry-broken Josephson junctions and superconducting diodes in magic-angle twisted bilayer graphene

The coexistence of gate-tunable superconducting, magnetic and topological orders in magic-angle twisted bilayer graphene provides opportunities for the creation of hybrid Josephson junctions. Here we report the fabrication of gate-defined symmetry-broken Josephson junctions in magic-angle twisted bilayer graphene, where the weak link is gate-tuned close to the correlated insulator state with a moiré filling factor of υ = −2. We observe a phase-shifted and asymmetric Fraunhofer pattern with a pronounced magnetic hysteresis. Our theoretical calculations of the junction weak link—with valley polarization and orbital magnetization—explain most of these unconventional features. The effects persist up to the critical temperature of 3.5 K, with magnetic hysteresis observed below 800 mK. We show how the combination of magnetization and its current-induced magnetization switching allows us to realise a programmable zero-field superconducting diode. Our results represent a major advance towards the creation of future superconducting quantum electronic devices.

Improving Josephson junction reproducibility for superconducting quantum circuits: junction area fluctuation

Josephson superconducting qubits and parametric amplifiers are prominent examples of superconducting quantum circuits that have shown rapid progress in recent years. As such devices become more complex, the requirements for reproducibility of their electrical properties across a chip are being tightened. Critical current of the Josephson junction Ic is the essential electrical parameter in a chip. So, its variation is to be minimized. According to the Ambegaokar–Baratoff formula, critical current is related to normal-state resistance, which can be measured at room temperature. In this study, we focused on the dominant source of non-uniformity for the Josephson junction critical current–junction area variation. We optimized Josephson junction fabrication process and demonstrated resistance variation of 9.8–4.4% and 4.8–2.3% across 22 × 22 mm2 and 5 × 10 mm2 chip areas, respectively. For a wide range of junction areas from 0.008 to 0.12 μm2, we ensure a small linewidth standard deviation of 4 nm measured over 4500 junctions with linear dimensions from 80 to 680 nm. We found that the dominate source of junction area variation limiting {mathrm{I}}_{mathrm{c}} reproducibility is the imperfection of the evaporation system. The developed fabrication process was tested on superconducting highly coherent transmon qubits (T1 > 100 μs) and a nonlinear asymmetric inductive element parametric amplifier.

Optimization of shadow evaporation and oxidation for reproducible quantum Josephson junction circuits

The most commonly used physical realization of superconducting qubits for quantum circuits is a transmon. There are a number of superconducting quantum circuits applications, where Josephson junction critical current reproducibility over a chip is crucial. Here, we report on a robust chip scale Al/AlOx/Al junctions fabrication method due to comprehensive study of shadow evaporation and oxidation steps. We experimentally demonstrate the evidence of optimal Josephson junction electrodes thickness, deposition rate and deposition angle, which ensure minimal electrode surface and line edge roughness. The influence of oxidation method, pressure and time on critical current reproducibility is determined. With the proposed method we demonstrate Al/AlOx/Al junction fabrication with the critical current variation (sigma /langle {I_{c} } rangle ) less than 3.9% (from 150 × 200 to 150 × 600 nm2 area) and 7.7% (for 100 × 100 nm2 area) over 20 × 20 mm2 chip. Finally, we fabricate separately three 5 × 10 mm2 chips with 18 transmon qubits (near 4.3 GHz frequency) showing less than 1.9% frequency variation between qubits on different chips. The proposed approach and optimization criteria can be utilized for a robust wafer-scale superconducting qubit circuits fabrication.