High-quality Junctions Research Articles

Anomalous superconductivity in twisted MoTe2 nanojunctions.

Introducing superconductivity in topological materials can lead to innovative electronic phases and device functionalities. Here, we present a unique strategy for quantum engineering of superconducting junctions in moiré materials through direct, on-chip, and fully encapsulated 2D crystal growth. We achieve robust and designable superconductivity in Pd-metalized twisted bilayer molybdenum ditelluride (MoTe2) and observe anomalous superconducting effects in high-quality junctions across ~20 moiré cells. Unexpectedly, the junction develops enhanced, instead of weakened, superconducting behaviors, exhibiting fluctuations to a higher critical magnetic field compared to its adjacent Pd7MoTe2 superconductor. In addition, the critical current further exhibits a notable V-shaped minimum at zero magnetic field. These features are unexpected in conventional Josephson junctions and absent in junctions of natural bilayer MoTe2 created using the same approach. We discuss implications of these observations, including the possible formation of mixed even- and odd-parity superconductivity at the moiré junctions. Our results also demonstrate a pathway to engineer and investigate superconductivity in fractional Chern insulators.

Point-contact spectroscopy of Leggett modes in superconducting compounds with unconventional pairing

Proximity-coupled nanostructures of conventional superconductors (SCs) and half-metallic ferromagnets (hmFs) are promising candidates as materials with unconventional superconductivity. The interrelated superposition of spin singlet-triplet and frequency even-odd superconducting condensates characterizes the superconducting state in such heterostructures. In a multi-band SC, the collective modes associated with the excitations of the relative phase between superconducting bands without perturbation of the Cooper pairs symmetry (Leggett modes) are allowed. In this report, we present the results of experimental investigations via the point-contact transport measurements of the Leggett-like collective excitations in the superconducting state of the nanocomposite of s-wave two-band superconductor MgB2 and half-metallic ferromagnet (La,Sr)MnO3. Two types of point contacts (PCs) have been used: the nanocomposite-nonmagnetic metal PCs and the nanocomposite–hmF PCs. The conductance equidistant peaks against the background of the gap structure were observed in both types of high-quality point junctions. Their distinctive feature was their period: two times shorter for the nanocomposite–hmF contacts compared to the nonmagnetic metal PCs. We attribute these spin-selective conductance periodic peaks to the relative phase Leggett’s excitations between “parents” MgB2 even-frequency singlet condensates and proximity-induced triplet superconducting condensates. The data obtained on the hmF PCs also demonstrate the features that may indicate a dynamic coupling between even-frequency condensates and odd-frequency gapless superconducting condensates.

Anodization-free fabrication process for high-quality cross-type Josephson tunnel junctions based on a Nb/Al-AlO x /Nb trilayer

Josephson tunnel junctions form the basis for various superconductor electronic devices. For this reason, enormous efforts are routinely taken to establish and later on maintain a scalable and reproducible wafer-scale manufacturing process for high-quality Josephson junctions. Here, we present an anodization-free fabrication process for Nb/Al-AlO x /Nb cross-type Josephson junctions that requires only a small number of process steps and that is in general intrinsically compatible with wafer-scale fabrication. We show that the fabricated junctions are of very high quality and, compared to other junction types, exhibit not only a significantly reduced capacitance but also an almost rectangular critical current density profile. Our process hence enables the usage of low capacitance Josephson junctions for superconductor electronic devices such as ultra-low noise dc-superconducting quantum interference devices (SQUIDs), microwave SQUID multiplexers based on non-hysteretic rf-SQUIDs and RFSQ circuits.

Laser shock-enabled optical–thermal–mechanical coupled welding method for silver nanowires

Silver nanowires (AgNWs) are recognized as highly promising materials for flexible and transparent electrode applications. However, existing material-processing methods fail to achieve uniform and reliable AgNWs junctions. In this study, we propose a new method using the laser shock effect combined with the laser heating effect, for creating AgNW junctions within thin films. We explored the welding mechanism of AgNWs through optic-thermal welding, laser shock-enabled mechanical welding, and laser-shock-enabled optical-thermal-mechanical (LS-OTM) experiments, as well as numerical simulations, and the results demonstrate that the innovative mechanism of the LS-OTM process lies in its utilization of laser shock to adjust the gap between the nanowire junctions, which in turn achieves a fine control of the thermal effect of the heating laser localised surface plasmon resonance, and the atomic diffusion in the solid state at intermediate temperature under the action of the impact force is the mechanism of the formation of high-quality junctions. We prepared flexible transparent conductive films and studied their transmittance, conductivity, and thermal properties, the results show that the flexible transparent conductive films prepared by LS-OTM welding method have excellent transmittance, conductivity, and thermal properties, this verifies the feasibility and effectiveness of this processing strategy. The LS-OTM method is a viable solution for manufacturing transparent, conductive films from AgNWs for emerging applications such as flexible heated films, flexible displays, and wearable medical devices.

NbN-based half-flux-quantum element for integration with superconducting qubits

Half-flux-quantum (HFQ) logic is a superconductor logic family comprising conventional Josephson junctions (0-JJs) and π Josephson junctions (π-JJs). The energy scale of HFQ logic can be flexibly reduced by phase shifts owing to π-JJs; thus, HFQ circuits are a promising building block for qubit interface circuits. In this study, we demonstrate an NbN-based HFQ circuit element compatible with NbN-based superconducting qubits for all-NbN monolithic integration of qubits and HFQ circuits. The use of NbN is beneficial for both qubits and HFQ circuits owing to its high-quality junctions and high kinetic inductance. First, we developed a prototype of an NbN-based 0-JJ/π-JJ hybrid fabrication process for designing HFQ circuits. We evaluated 0-JJs and π-JJs fabricated through the hybrid process, which demonstrated that the 0-JJs had a small deviation with regard to critical current density and the π-JJs had a sufficiently high critical current density to work as π phase shifters. Furthermore, we fabricated an HFQ superconducting quantum interference device, which is one of the most fundamental elements in HFQ circuits, and observed clear HFQ-period modulation in the magnetic flux dependence of the maximum current at 4.2 K.

Surface heterojunction based on n-type low-dimensional perovskite film for highly efficient perovskite tandem solar cells.

Enhancing the quality of junctions is crucial for optimizing carrier extraction and suppressing recombination in semiconductor devices. In recent years, metal halide perovskite has emerged as the most promising next-generation material for optoelectronic devices. However, the construction of high-quality perovskite junctions, as well as characterization and understanding of their carrier polarity and density, remains a challenge. In this study, using combined electrical and spectroscopic characterization techniques, we investigate the doping characteristics of perovskite films by remote molecules, which is corroborated by our theoretical simulations indicating Schottky defects consisting of double ions as effective charge dopants. Through a post-treatment process involving a combination of biammonium and monoammonium molecules, we create a surface layer of n-type low-dimensional perovskite. This surface layer forms a heterojunction with the underlying 3D perovskite film, resulting in a favorable doping profile that enhances carrier extraction. The fabricated device exhibits an outstanding open-circuit voltage (VOC) up to 1.34V and achieves a certified efficiency of 19.31% for single-junction wide-bandgap (1.77eV) perovskite solar cells, together with significantly enhanced operational stability, thanks to the improved separation of carriers. Furthermore, we demonstrate the potential of this wide-bandgap device by achieving a certified efficiency of 27.04% and a VOC of 2.12V in a perovskite/perovskite tandem solar cell configuration.

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.

Performance Evaluation of Junctionless Cylindrical Gate-All-Around FET for Low Power Applications

The advent of device miniaturization techniques and the evolution of very deep submicron technology have led to the increased prominence of short channel effects (SCEs) in conventional transistors (CTs). Now, in the era of nanoengineering and nano-wires, current research is centered around a novel device known as the Junctionless Field Effect Transistor (JLFET), which incorporates gate-all-around engineering applications. Given the challenges associated with scaling transistor sizes, such as creating high-quality junctions and changing doping concentrations (~1019 cm–3) over a 10 nm distance, JLFET emerges as a promising alternative to CTs. Notably, JLFET lacks junctions and doping concentration gradients. In this study, the authors have conducted a comprehensive analysis and performance evaluation of JLFET and CTs, specifically in the context of low-power applications. Various performance parameters of JLFET, including SS, DIBL, transconductance, output conductance, and Ion/Ioff, have been assessed. The findings indicate that JLFET exhibits reduced susceptibility to SCEs compared to CTs and demonstrates exceptional current driving capability.

In situ construction of PtSe2/Ge Schottky junction array with interface passivation for broadband infrared photodetection and imaging

AbstractInfrared (IR) detection is vital for various military and civilian applications. Recent research has highlighted the potential of two‐dimensional (2D) topological semimetals in IR detection due to their distinctive advantages, including van der Waals (vdW) stacking, gapless electronic structure, and Van Hove singularities in the electronic density of states. However, challenges such as large‐scale patterning, poor photoresponsivity, and high dark current of photodetectors based on 2D topological semimetals significantly impede their wider applications in low‐energy photon sensing. Here, we demonstrate the in situ fabrication of PtSe2/Ge Schottky junction by directly depositing 2D PtSe2 films with a vertical layer structure on a Ge substrate with an ultrathin AlOx layer. Due to high quality junction, the photodetector features a broadband response of up to 4.6 μm, along with a high specific detectivity of ~1012 Jones, and operates with remarkable stability in ambient conditions as well. Moreover, the highly integrated device arrays based on PtSe2/AlOx/Ge Schottky junction showcases excellent Mid‐IR (MIR) imaging capability at room temperature. These findings highlight the promising prospects of 2D topological semimetals for uncooled IR photodetection and imaging applications.image

Polarization-sensitive UV photodetector based on ReSe2/GaN mixed-dimensional heterojunction.

Polarization-sensitive photodetectors in the ultraviolet (UV) region have been favored for their great meaning in the field of military and civilian. UV photodetectors based on GaN have aroused much attention due to high photocurrent and high sensitivity. However, the dependence on external power sources and the limited sensitivity to polarized UV light significantly impede the practical application of these photodetectors in UV-polarized photodetection. Herein, a polarization-sensitive UV photodetector based on ReSe2/GaN mixed-dimensional van der Waals (vdWs) heterojunction is proposed. Owing to the high-quality junction and type-II band alignment, the responsivity and specific detectivity reach values of 870 mA/W and 6.8 × 1011 Jones, under 325 nm illumination, respectively. Furthermore, thanks to the strong in-plane anisotropy of ReSe2, the device is highly sensitive to polarized UV light with a photocurrent anisotropic ratio up to 6.67. The findings are expected to bring new opportunities for the development of highly sensitive, high-speed and energy-efficient polarization-sensitive photodetectors.

Flexo-Pyrophotronic Effect Modulated Giant Near Infrared Photoresponse from VO2 -Based Heterojunction for Optical Communication.

The flexoelectric phenomenon, which occurs when materials undergo mechanical deformation and cause strain gradients and a related spontaneous electric polarization field, can result in wide variety of energy- and cost-saving mechano-opto-electronics, such as night vision, communication, and security. However, accurate sensing of weak intensities under self-powered conditions with stable photocurrent and rapid temporal response remains essential despite the challenges related to having suitable band alignment and high junction quality. Taking use of the flexoelectric phenomena, it is shown that a centrosymmetric VO2 -based heterojunction exhibits a self-powered (i.e., 0V), infrared (λ = 940nm) photoresponse. Specifically, the device shows giant current modulation (103 %), good responsivity of >2.4mA W-1 , reasonable specific detectivity of ≈1010 Jones, and a fast response speed of 0.5ms, even at the nanoscale modulation. Through manipulation of the applied inhomogeneous force, the sensitivity of the infrared response is enhanced (> 640%). Ultrafast night optical communication like Morse code distress (SOS) signal sensing and high-performing obstacle sensors with potential impact alarms are created as proof-of-concept applications. These findings validate the potential of emerging mechanoelectrical coupling for a wide variety of novel applications, including mechanoptical switches, photovoltaics, sensors, and autonomous vehicles, which require tunable optoelectronic performance.

2D semimetal with ultrahigh work function for sub-0.1 V threshold voltage operation of metal-semiconductor field-effect transistors

Metal-semiconductor field-effect transistors (MESFETs) offer the advantages of efficient gate control and low power consumption due to the large junction capacitance. However, the strong Fermi-level pinning caused by the metal-induced gap states makes it a great challenge to build a high-quality Schottky junction with a large energy barrier and low leakage current. Moreover, the upper limit of the work function of conventional metals is around 5 eV, which prevents the formation of a wide depletion region and the reduction of power consumption. In this work, 2D semimetals with high work function have been used as the gate to fabricate 2D van der Waals MESFETs. Compared with PtSe2 and PdSe2, TiS2 with ultrahigh work function of 6.6 eV forms the largest Schottky barrier height with MoS2. And TiS2/MoS2 MESFETs display a sub-0.1 V threshold voltage owing to the large built-in potential and exhibit a small subthreshold swing of 73 mV/dec. The tuning of the channel carrier density from the back gate can further optimize the device performance and provide a strategy to build AND logic gate in a dual-gated transistor. Our work successfully demonstrates the use of high-work-function 2D semimetal to realize high-performance MESFET with low power consumption.

Novel Mg- and Ga-doped ZnO/Li-Doped Graphene Oxide Transparent Electrode/Electron-Transporting Layer Combinations for High-Performance Thin-Film Solar Cells.

Herein, a novel combination of Mg- and Ga-co-doped ZnO (MGZO)/Li-doped graphene oxide (LGO) transparent electrode (TE)/electron-transporting layer (ETL) has been applied for the first time in Cu2 ZnSn(S,Se)4 (CZTSSe) thin-film solar cells (TFSCs). MGZO has a wide optical spectrum with high transmittance compared to that with conventional Al-doped ZnO (AZO), enabling additional photon harvesting, and has a low electrical resistance that increases electron collection rate. These excellent optoelectronic properties significantly improved the short-circuit current density and fill factor of the TFSCs. Additionally, the solution-processable alternative LGO ETL prevented plasma-induced damage to chemical bath deposited cadmium sulfide (CdS) buffer, thereby enabling the maintenance of high-quality junctions using a thin CdS buffer layer (≈30nm). Interfacial engineering with LGO improved the Voc of the CZTSSe TFSCs from 466 to 502mV. Furthermore, the tunable work function obtained through Li doping generated a more favorable band offset in CdS/LGO/MGZO interfaces, thereby, improving the electron collection. The MGZO/LGO TE/ETL combination achieved a power conversion efficiency of 10.67%, which is considerably higher than that of conventional AZO/intrinsic ZnO (8.33%).

Si:HgTe Colloidal Quantum Dots Heterojunction-Based Infrared Photodiode

Integrated circuits and optoelectronics are currently dominated by silicon technology. However, silicon’s response wavelength is typically less than 1,100 nm, limiting the application of silicon in machine vision, autonomous vehicles, and night vision. For infrared photodetectors, HgTe colloidal quantum dots (CQDs) are promising materials. Because of the adjustable bandgap, it responds over a wide spectral range. However, the construction of a high-quality junction between Si and HgTe CQDs continues to be difficult, thus restricting the scope of its application. In this article, we describe the synthesis, characterization, and correlation of HgTe CQDs with reaction temperature and nanocrystal size. We then fabricated HgTe-CQDs/silicon infrared photodiodes and discussed how the silicon resistivity affected their performance. We found that the devices prepared from 9.1 nm HgTe quantum dots synthesized at 80°C and a silicon substrate with a resistivity of 20–50 Ω·cm has optimal performance parameters. This results in a responsivity of 0.2 mA/W for 1,550 nm incident light at room temperature. These results provide a direction for future silicon-compatible HgTe quantum dot infrared optoelectronics.

Pulsed laser welding of macroscopic 3D graphene materials.

Welding is a key missing manufacturing technique in graphene science. Due to the infusibility and insolubility, reliable welding of macroscopic graphene materials is impossible using current diffusion-bonding methods. This work reports a pulsed laser welding (PLW) strategy allowing for directly and rapidly joining macroscopic 3D porous graphene materials under ambient conditions. Central to the concept is introducing a laser-induced graphene solder converted from a designed unique precursor to promote joining. The solder shows an electrical conductivity of 6700 S m-1 and a mechanical strength of 7.3 MPa, over those of most previously reported porous graphene materials. Additionally, the PLW technique enables the formation of high-quality welded junctions, ensuring the structural integrity of weldments. The welding mechanism is further revealed, and two types of connections exist between solder and base structures, i.e., intermolecular force and covalent bonding. Finally, an array of complex 3D graphene architectures, including lateral heterostructures, Janus structures, and 3D patterned geometries, are fabricated through material joining, highlighting the potential of PLW to be a versatile approach for multi-level assembly and heterogeneous integration. This work brings graphene into the laser welding club and paves the way for the future exploration of the exciting opportunities inherent in material integration and repair.

Composition Dependent Electrical Transport in Si1-x Gex Nanosheets with Monolithic Single-Elementary Al Contacts.

Si1-x Gex is a key material in modern complementary metal-oxide-semiconductor and bipolar devices. However, despite considerable efforts in metal-silicide and -germanide compound material systems, reliability concerns have so far hindered the implementation of metal-Si1-x Gex junctions that are vital for diverse emerging "More than Moore" and quantum computing paradigms. In this respect, the systematic structural and electronic properties of Al-Si1-x Gex heterostructures, obtained from a thermally induced exchange between ultra-thin Si1-x Gex nanosheets and Al layers are reported. Remarkably, no intermetallic phases are found after the exchange process. Instead, abrupt, flat, and void-free junctions of high structural quality can be obtained. Interestingly, ultra-thin interfacial Si layers are formed between the metal and Si1-x Gex segments, explaining the morphologic stability. Integrated into omega-gated Schottky barrier transistors with the channel length being defined by the selective transformation of Si1-x Gex into single-elementary Al leads, a detailed analysis of the transport is conducted. In this respect, a report on a highly versatile platform with Si1-x Gex composition-dependent properties ranging from highly transparent contacts to distinct Schottky barriers is provided. Most notably, the presented abrupt, robust, and reliable metal-Si1-x Gex junctions can open up new device implementations for different types of emerging nanoelectronic, optoelectronic, and quantumdevices.

High-sensitivity silicon: PbS quantum dot heterojunction near-infrared photodetector

The integration of silicon and infrared-sensitive materials to manufacture infrared detectors is very promising. However, the combination of silicon and colloidal quantum dots (CQDs) to form stable and high-quality junctions still has problems. In this work, Si: PbS CQD heterojunction near-infrared photodetector was fabricated. The experimental results show that the PbS CQDs extend the detection band of Si substrate to the near-infrared. What's more, when the bias voltage is 0.05 V at room temperature, the responsivity of the device at 1064 nm and 1310 nm are 0.68 A/W and 0.29 A/W respectively. The specific detection is 7.74 × 1010 Jones for 1064 nm and 3.32 × 1010 Jones for 1310 nm. This research is expected to promote the development of related silicon-based near-infrared photodetectors.

In Situ Fabrication of PdSe2/GaN Schottky Junction for Polarization-Sensitive Ultraviolet Photodetection with High Dichroic Ratio.

Polarization-sensitive ultraviolet (UV) photodetection is of great technological importance for both civilian and military applications. Two-dimensional (2D) group-10 transition-metal dichalcogenides (TMDs), especially palladium diselenide (PdSe2), are promising candidates for polarized photodetection due to their low-symmetric crystal structure. However, the lack of an efficient heterostructure severely restricts their applications in UV-polarized photodetection. Here, we develop a PdSe2/GaN Schottky junction by in situ van der Waals growth for highly polarization-sensitive UV photodetection. Owing to the high-quality junction, the device exhibits an appealing UV detection performance in terms of a large responsivity of 249.9 mA/W, a high specific detectivity, and a fast response speed. More importantly, thanks to the puckered structure of the PdSe2 layer, the device is highly sensitive to polarized UV light with a large dichroic ratio up to 4.5, which is among the highest for 2D TMD material-based UV polarization-sensitive photodetectors. These findings further enable the demonstration of the outstanding polarized UV imaging capability of the Schottky junction, as well as its utility as an optical receiver for secure UV optical communication. Our work offers a strategy to fabricate the PdSe2-based heterostructure for high-performance polarization-sensitive UV photodetection.

Fabrication of Superconducting Nb–AlN–NbN Tunnel Junctions Using Electron-Beam Lithography

Mixers based on superconductor–insulator–superconductor (SIS) tunnel junctions are the best input devices at frequencies from 0.1 to 1.2 THz. This is explained by both the extremely high nonlinearity of such elements and their extremely low intrinsic noise. Submicron tunnel junctions are necessary to realize the ultimate parameters of SIS receivers, which are used as standard devices on both ground and space radio telescopes around the world. The technology for manufacturing submicron Nb–AlN–NbN tunnel junctions using electron-beam lithography was developed and optimized. This article presents the results on the selection of the exposure dose, development time, and plasma chemical etching parameters to obtain high-quality junctions (the ratio of the resistances below and above the gap Rj/Rn). The use of a negative-resist ma-N 2400 with lower sensitivity and better contrast in comparison with a negative-resist UVN 2300-0.5 improved the reproducibility of the structure fabrication process. Submicron (area from 2.0 to 0.2 µm2) Nb–AlN–NbN tunnel junctions with high current densities and quality parameters Rj/Rn > 15 were fabricated. The spread of parameters of submicron tunnel structures across the substrate and the reproducibility of the cycle-to-cycle process of tunnel structure fabrication were measured.

Analyzing metallurgical interaction during arc surfacing of barrier layers on titanium to prevent the formation of intermetallics in titanium-steel compounds

This paper considers a possibility to obtain high-quality butt junctions of bimetallic sheets from steel clad with a layer of titanium, with the use of barrier layers. The task that was tackled related to preventing the formation of Ti-Fe intermetallic phases (IMPs) between the steel and titanium layer. The barrier layers (height ~0.5 mm) of vanadium and copper alloys were surfaced by arc techniques while minimizing the level of thermal influence on the base metal. To this end, plasma surfacing with a current-driving wire and pulsed MAG surfacing were used. The obtained samples were examined by methods of metallography, X-ray spectral microanalysis, durometric analysis. It has been established that when a layer of vanadium is plated on the surface of titanium, a defect-free structure of variable composition (53.87–65.67) wt % Ti with (33.93–45.54) wt % V is formed without IMPs. The subsequent surfacing of steel on a layer of vanadium leads to the formation of eutectics (hardness up to 5,523 MPa) in the fusion zone, as well as to the evolution of cracks. To prevent the formation of IMPs, a layer of bronze CuBe2 was deposited on the surface of vanadium. The formed layer contributed to the formation of a grid of hot cracks. In the titanium-vanadium-copper transition zones (0.1–0.2 mm wide), a fragile phase was observed. To eliminate this drawback, the bronze CuBe2 was replaced with bronze CuSi3Mn1; a defect-free junction was obtained. When using a barrier layer with CuSi3Mn1, a defect-free junction was obtained (10–30 % Ti; 18–50 % Fe; 5–25 % Cu). The study reported here makes it possible to recommend CuSi3Mn1 as a barrier layer for welding bimetallic sheets "steel-titanium". One of the applications of the research results could be welding of longitudinally welded pipes of main oil and gas pipelines formed from bimetallic sheets of steel clad with titanium.