Junction Arrays Research Articles

Plasmonic Au-decorated hierarchical p-NiO/n-ZnO heterostructure arrays for enhanced photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting is one of most effective ways for converting solar energy into chemical energy. Single-component semiconductor structures as photoanodes always suffer from poor ability to capture photons and serious recombination of electron-hole pairs, which pose inferior photocurrent performance. In order to increase the photocurrent density, here, a hierarchical heterostructure of p-NiO/n-ZnO decorated with Au nanoparticles (NPs) was designed and prepared as photoanodes. Specifically, a n-ZnO nanorods@nanodisks (NRs@NDs) constructure was completely coated with a thin p-NiO layer to construct a miniature p-n junction arrays, and plasmonic Au NPs were attached on the surface of the thin p-NiO layer. Benefiting from such p-n heterostructure, p-NiO/n-ZnO NRs@NDs heterojunction arrays exhibited excellent rectifying behaviour for their I–V and semi-log I–V curves. Moreover, the photoanode of p-NiO/n-ZnO NRs@NDs had a lower emission intensity than that of n-ZnO NRs@NDs, which indicated the effective inhibition of electron-hole pairs recombination. More importantly, the photocurrent density of Au/p-NiO/n-ZnO NRs@NDs was measured to be as high as 1.78 mA cm −2 , outperforming that of pure n-ZnO NRs@NDs (0.61 mA cm −2 ). This method provides a green, and highly efficient photoanode for the PEC water splitting. • A micro p-NiO/n-ZnO NRs@NDs heterostructure decorated with plasmonic Au NPs was prepared. • p-NiO/n-ZnO NRs@NDs heterojunction arrays exhibited excellent rectifying behaviour. • The synergetic effect of inner electric field of p-n junctions and plasmonic Au effectively inhibit charge recombination. • The photocurrent density increased by approximately 2.9-fold compared to that of pure n-ZnO NRs@NDs.

Влияние контактного сопротивления интерфейса YBCO|Au на транспортные и СВЧ-свойства массивов джозефсоновских контактов из высокотемпературных сверхпроводников

The influence of the contact resistance of the interface of the YBCO/Au structure on the transport and microwave properties of arrays of high-temperature bicrystal Josephson junctions embedded in a coplanar transmission line has been studied. The current-voltage characteristics of Josephson structures fabricated using various technologies are studied: in-situ and ex-situ with annealing in an oxygen atmosphere. The results obtained can be used to create a quantum ac voltage generator based on high-temperature Josephson junctions.

Method to create cuprate tunnel junctions with atomically sharp interfaces

We present a method for producing tunnel junctions from cuprate thin films grown by atomic layer-by-layer molecular beam epitaxy (ALL-MBE). Our approach utilizes microfabrication processing and relies on the atomically precise interfaces provided by the ALL-MBE synthesis technique. The method is suitable for a wide range of junction dimensions and materials and allows for more complex designs such as superconducting quantum interference devices and junction arrays. Fully superconducting electrodes ensure that the contact resistances in the devices created by this method remain very low.

Evaluation of Self-Field Effects in Magnetometers Based on Meander-Shaped Arrays of Josephson Junctions or SQUIDs Connected in Series.

Arrays of superconducting quantum interference devices (SQUIDs) are highly sensitive magnetometers that can operate without a flux-locked loop, as opposed to single SQUID magnetometers. They have no source of ambiguity and benefit from a larger bandwidth. They can be used to measure absolute magnetic fields with a dynamic range scaling as the number of SQUIDs they contain. A very common arrangement for a series array of SQUIDs is with meanders as it uses the substrate area efficiently. As for most layouts with long arrays, this layout breaks the symmetry required for the elimination of adverse self-field effects. We investigate the scaling behavior of series arrays of SQUIDs, taking into account the self-field generated by the bias current flowing along the meander. We propose a design for the partial compensation of this self-field. In addition, we provide a comparison with the case of series arrays of long Josephson junctions, using the Fraunhofer pattern for applications in magnetometry. We find that compensation is required for arrays of the larger size and that, depending on the technology, arrays of long Josephson junctions may have better performance than arrays of SQUIDs.

Current–voltage characteristics of strained, highly underdoped LaSr x CuO4 thin films

The temperature dependence of resistance and of current–voltage characteristics are studied for two underdoped La1.952Sr0.048CuO4 films with different thickness and different degrees of built-in compressive strain. Strain induces an inhomogeneous superconducting (SC) state in the films, with two-step SC transition, consistent with the presence of SC islands coupled by the proximity effect. On further cooling, superfluid density is suppressed and global phase coherence is never reached. Instead, a reentrant increase of resistance appears, reaching several orders of magnitude in the thinner, more strongly strained film, and saturating at lowest temperatures. The saturation is accompanied by Coulomb blockade, which resembles the blockade reported for Josephson junction (JJ) arrays. However, increasing magnetic field quickly diminishes the influence of Coulomb blockade on transport, which is distinct from the behavior observed for JJ arrays. The observation of Coulomb blockade indicates that Cooper pairs survive in the insulating state, suggesting the local character of pairing.

Control of global variables for identical and non-identical Josephson junctions arrays

In the paper a way to solve an LQ-problem for a Josephson junction array with a common LRC-load is proposed. The cases of identical and non-identical Josephson junctions are considered. The solution ensures phase stabilization of Josephson current in every junction. The results are obtained using computer simulation in Jupyter Notebook and MATLAB.

Geometric Superinductance Qubits: Controlling Phase Delocalization across a Single Josephson Junction

There are two elementary superconducting qubit types that derive directly from the quantum harmonic oscillator. In one the inductor is replaced by a nonlinear Josephson junction to realize the widely used charge qubits with a compact phase variable and a discrete charge wavefunction. In the other the junction is added in parallel, which gives rise to an extended phase variable, continuous wavefunctions and a rich energy level structure due to the loop topology. While the corresponding rf-SQUID Hamiltonian was introduced as a quadratic, quasi-1D potential approximation to describe the fluxonium qubit implemented with long Josephson junction arrays, in this work we implement it directly using a linear superinductor formed by a single uninterrupted aluminum wire. We present a large variety of qubits all stemming from the same circuit but with drastically different characteristic energy scales. This includes flux and fluxonium qubits but also the recently introduced quasi-charge qubit with strongly enhanced zero point phase fluctuations and a heavily suppressed flux dispersion. The use of a geometric inductor results in high precision of the inductive and capacitive energy as guaranteed by top-down lithography - a key ingredient for intrinsically protected superconducting qubits. The geometric fluxonium also exhibits a large magnetic dipole, which renders it an interesting new candidate for quantum sensing applications.

Tunable Low Crystallinity Carbon Nanotubes/Silicon Schottky Junction Arrays and Their Potential Application for Gas Sensing.

Highly ordered nanostructure arrays have attracted wide attention due to their wide range of applicability, particularly in fabricating devices containing scalable and controllable junctions. In this work, highly ordered carbon nanotube (CNT) arrays grown directly on Si substrates were fabricated, and their electronic transport properties as a function of wall thickness were explored. The CNTs were synthesized by chemical vapor deposition inside porous alumina membranes, previously fabricated on n-type Si substrates. The morphology of the CNTs, controlled by the synthesis parameters, was characterized by electron microscopies and Raman spectroscopy, revealing that CNTs exhibit low crystallinity (LC). A study of conductance as a function of temperature indicated that the dominant electric transport mechanism is the 3D variable range hopping. The electrical transport explored by I–V curves was approached by an equivalent circuit based on a Schottky diode and resistances related to the morphology of the nanotubes. These junction arrays can be applied in several fields, particularly in this work we explored their performance in gas sensing mode and found a fast and reliable resistive response at room temperature in devices containing LC-CNTs with wall thickness between 0.4 nm and 1.1 nm.

72 nm Pitch Hexagonal MTJ Array on DRAM Platform for High-Density MRAM

Well isolated 72 nm pitch hexagonal honeycomb magnetic tunneling junction (MTJ) array with about 45 nm MgO CD was successfully fabricated on a DRAM platform by using mature DRAM array patterning solution with self-aligned double patterning technology and proposed triple hard mask scheme. The 72 nm pitch MTJ array had an equivalent cell size as small as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.0044~\mu \text{m}$ </tex-math></inline-formula> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , the world’s smallest pitch size and cell size for industrial MTJ array to our knowledge. The hexagonal array geometry offered considerably merit for storage density enhancement and MTJ etch convenience. A reactive ion etch (RIE) and ion beam etch (IBE) combined MTJ etching solution was adopted to optimize pillar isolation and mitigate metallic re-deposition and etch damage. Perpendicular magnetic anisotropy was maintained in MTJ array; however, free layer coercivity was smaller than expected, indicating the existence of etch damage or fringe magnetic field interaction. Potential challenges for the chip integration of high-density hexagonal MTJ array are also discussed.

Toward an Intelligent Synthesis: Monitoring and Intervening in the Catalytic Growth of Carbon Nanotubes.

The bottom-up approach to directly synthesizing low-dimensional materials with outstanding performance has extended the material basis for the next generation integrated circuit industry. All the low-dimensional semiconductors, metals, dielectrics, and their heterojunctions are very promising bricks to build faster and more efficient chips because of their atomically smooth surface and interfaces. The greatest challenge in the synthesis of nanomaterials is how to precisely control the structure, crystalline orientation, defects, dimensions, etc. In past decades, both the methodology and the mechanism of synthesis have been systematically investigated to improve the controllability. However, few studies focused on sensing the synthesis processes in situ and responding to the synthesis immediately. Here, we propose the concept of intelligent synthesis in which the final product can be automatically fine-controlled by a closed loop including in situ monitoring and real-time interventions. As a model system, a high-temperature-tolerant circuit is fabricated on the single-walled carbon nanotube (SWCNT) growth substrate for sensing and responding to the synthesis processes. As a result, either highly pure semiconducting (s-) SWCNT arrays or metallic-semiconducting (m-s) junction arrays with different junction positions is simply synthesized by programming the responding signal. The intelligent synthesis shows much higher efficiency and controllability compared to conventional methods and will lead to the next leap in nanotechnology.

A novel physical unclonable function (PUF) using 16 × 16 pure-HfO x ferroelectric tunnel junction array for security applications

As the computing paradigm has shifted toward edge computing, improving the security of edge devices is attracting significant attention. However, because edge devices have limited resources in terms of power and area, it is difficult to apply a conventional cryptography system to protect them. On the other hand, as a simple security application, a physical unclonable function (PUF) can be implemented without power and area problems because it provides a security key by utilizing process variations without additional external circuits. Ferroelectric tunnel junctions (FTJs) are 2-terminal devices that store information by changing the resistance of a ferroelectric material, where the resistance is determined by the polarization states of the ferroelectric domains. Because polycrystalline ferroelectric materials have a multi-domain nature, domain variation can also be used as a randomness source to induce cell-to-cell variations along with process variations. In this paper, we demonstrate PUF operations of a low-power, small area 16 × 16 hafnium oxide (pure-HfO x )-based FTJ array using certain metrics. It is clear that the proposed array consisting of scaled FTJs has adequate randomness for security applications such that the array-level PUF operations are robust against model-based machine learning attacks.

I–V characteristics of the GdBa2Cu3O7-x/YBa2Cu3O7-ẟ double-layer micro-structures prepared by self-sensitive chemical modification method

Herein, a novel method of photoresist-free micropatterning, i.e., self-sensitive chemical modification (SSCM) photolithography, is developed to prepare micron-sized superconductor fine-pattern arrays and cross-shaped GdBa2Cu3O7-x/YBa2Cu3O7-ẟ heterojunctions. The X-ray diffraction and R-T analysis revealed that the SSCM photolithography did not influence the structural and superconducting electronic properties of the GdBa2Cu3O7-x/YBa2Cu3O7-ẟ heterojunctions. Therefore, the proposed method is well suitable for the microfabrication of multilayer superconductor films and is expected to widen the application horizon of high-quality superconductor junction arrays. Furthermore, the electrical test results of cross-shaped structure demonstrated that the I–V curves of GdBa2Cu3O7-x/YBa2Cu3O7-ẟ heterojunctions are similar to that of DC Josephson effect. The transmission electron microscopy revealed the presence of an intermediate transition layer (~1.2 nm) at the GdBa2Cu3O7-x/YBa2Cu3O7-ẟ interface, which may hamper the transmission of Cooper pairs. These results provide novel insights into the preparation of Josephson junctions and other junction devices.

Phase interference for probing topological fractional charge in a BiSbTeSe2-based Josephson junction array

Fractional charges can be induced by magnetic fluxes at the interface between a topological insulator (TI) and a type-II superconductor due to axion electrodynamics. In a Josephson junction array with a hole in the middle, these electronic states can have phase interference in an applied magnetic field with period, in addition to the 2π interference of the Cooper pairs. Here, we test an experimental configuration for probing the fractional charge and report the observation of phase interference effect in superconducting arrays with a hole in the middle in both Au- and TI-based devices. Our numerical simulations based on resistive shunted capacitive junction model are in good agreement with the experimental results. However, no clear sign of an axion charge-related interference effect was observed. We will discuss possible reasons and perspectives for future experiments.

Coherent Radiation From Active Josephson Antennas

We performed spectral measurements of two low-temperature discrete Josephson junction (JJ) arrays on a Fourier-transform spectrometer. The arrays contain up to 9000 niobium based junctions located in the area 5 × 5 mm and composed of different designs. The spectrum of each array demonstrates a narrow peak in the frequency range near 140 GHz with the line width 160-170 MHz corresponding to the resolution limit of the spectrometer. It implies the coherent subterahertz radiation from niobium JJ arrays that is necessary for most applications where the JJ array based on similar technology would be implemented. The radiation coherence follows from numerical simulation of Josephson antennas with the self-made algorithm combining the Finite-Difference Time Domain Method with the Resistively and Capacitively Shunted Junction (RCSJ) model. Exploring the spectrum of Beverage-like Josephson antenna simulated by this algorithm we revealed the pure monochromatic emission of all junctions.

An 8–10-GHz Low-Loss Image-Reject HTS Mixer Based on Cascaded Josephson Junctions

A 7.8-9.8-GHz low-loss high- T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> superconducting (HTS) image-reject Josephson junction mixer is presented in this letter. Image rejection is achieved using low-loss image rejection filters and a pair of parallel dual-mode resonators. Three cascaded Josephson junctions are monolithically integrated with impedance matching networks and bandpass/low-pass filters on a single chip for low loss and compactness. The implementation of series junction array increased the overall junction equivalent impedance, so that a better impedance matching is achieved. Measurement results of the novel HTS mixer showed a low conversion loss at 5.5 dB and an image-reject ratio over 30 dB within the desired frequency. The proposed cascaded junction configuration provides a new approach for impedance matching in HTS mixer designs, and the demonstrated mixer performance makes it an ideal candidate for X-band radar systems.

Driving a low critical current Josephson junction array with a mode-locked laser

We report proof-of-concept experiments on an optically driven Josephson voltage standard based on a mode-locked laser (MLL), a time-division multiplexer, and a cryogenic ultrafast photodiode driving an overdamped Josephson junction array (JJA). Our optical pulse pattern generator (PPG) concept builds on the capability of MLLs to produce trains of picosecond-wide optical pulses with little amplitude and temporal spread. Our present setup enables multiplication of the original 2.3 GHz pulse repetition frequency by a factor of 8. A commercial photodiode converts the optical pulses into about 25 ps wide electrical pulses in liquid helium several cm from the JJA. Using a custom-made MLL, we can drive a JJA with a low critical current of 360 μA at multiple Shapiro steps. We have performed experiments with pulse pairs whose time interval can be set freely without distorting the shapes of individual pulses. Experimental results are in qualitative agreement with theoretical simulations, and they demonstrate, e.g., crossover in the Shapiro step pattern when the time interval between the pulses is approximately equal to the inverse of the characteristic frequency of the JJA. However, there are quantitative discrepancies, which motivate an improved integration of photodiodes and JJAs to improve both the understanding and fidelity of Josephson Arbitrary Waveform Synthesizers. Considering future quantum technologies in a wider perspective, our optical approach is a potential enabler for fast and energy-efficient pulse drive without an expensive high-bandwidth electrical PPG and without high-bandwidth electrical cables that yield too high thermal conductance between cryogenic and room temperatures.

Improvement of Plug-in Wire for 200-pixel Superconducting Tunnel Junction X-ray Detector Array on Helium Three Cryostat

The design of plug-in wire system is improved for 200-pixel superconducting tunnel junction (STJ) array of X-ray detector on a helium three cryostat. The electrical connections between the STJ array to the room temperature electronics consists of plural wire modules, which have connectors at the both ends. A flexible coaxial cable with electrical conductors of stainless steel (SUS-FFC) is manufactured and attached to make the electrical connection between room temperature to the 3 K stage. After the installation of SUS-FFC, temperature of the cold stage is 302 mK, and the holding time is more than 115 hours. Compared with the previous setup, the temperature of 3He stage decreased 5 mK, and the holding time increased 30 hours.

Achieving high-responsivity near-infrared detection at room temperature by nano-Schottky junction arrays via a black silicon/platinum contact approach

A room temperature sub-bandgap near-infrared ( λ &gt; 1100 nm ) Si photodetector with high responsivity is achieved. The Si photodetector features black Si made by wet etching Si (100), Si/PtSi nano-Schottky junction arrays made from black Si/Pt contacts, and chemical and field-effect passivation of black Si. Responsivities are 147.6, 292.8, and 478.2 mA/W at reverse voltages of − 1.0 , − 1.5 , and − 2.0 V for 1550 nm light, respectively, with corresponding specific detectivities being 9.79 × 10 8 , 1.88 × 10 9 , and 2.97 × 10 9 cm · Hz 1 / 2 / W . This work demonstrates a practical room temperature sub-bandgap near-infrared Si photodetector that can be made in a facile and large-scale manner.

Coulomb Blockade Thermometry Beyond the Universal Regime

The charge localization of single electrons on mesoscopic metallic islands leads to a suppression of the electrical current, known as the Coulomb blockade. When this correction is small, it enables primary electron thermometry, as it was first demonstrated by Pekola et al. (Phys Rev Lett 73:2903, 1994). However, in the low temperature limit, random charge offsets influence the conductance and limit the universal behavior of a single metallic island. In this work, we numerically investigate the conductance of a junction array and demonstrate the extension of the primary regime for large arrays, even when the variations in the device parameters are taken into account. We find that our simulations agree well with measured conductance traces in the submillikelvin electron temperature regime.

In-situ control of on-chip angstrom gaps, atomic switches, and molecular junctions by light irradiation

Pairs of electrodes with nanometer gap, termed as nano-gapped electrodes, are fundamental building blocks for the fabrication of nanometer-sized devices and are essential for the examination of molecular properties and extreme nano-optics. Although modern fabrication techniques make it feasible to fabricate nanometer gaps, it is still a formidable challenge to fabricate adjustable gaps arrays with angstrom precision. Here, we demonstrate that in-situ adjustable nanogaps (arrays) with sub-angstrom precision can be fabricated via laser irradiation on the substrate which supports the electrode pairs. We further demonstrate that atomic-level metal contacts can be switched and the direction of the switching can be selectively controlled by the laser irradiation position. By varying the laser power gradually, the nanogap’s size can be continuously changed, providing a reliable break junction technique to address the properties of single-molecule junctions. The small spatial focus size of the laser beam makes it feasible to realize addressable on-chip molecular junction arrays.