Junction Behavior Research Articles

Suppression of Interfacial Diffusion in Mg3Sb2 Thermoelectric Materials through an Mg4.3Sb3Ni/Mg3.2Sb2Y0.05/Mg4.3Sb3Ni-Graded Structure.

The Zintl compound, n-type Mg3Sb2, has been extensively investigated as a promising thermoelectric material. However, performance degradation caused by the loss of Mg element during device preparation and service is a main disadvantage in its utilization in thermoelectric devices. To suppress volatilization, diffusion, or reaction of Mg, we designed a graded concentration junction to control the interfacial elemental diffusion and improve the stability of the thermoelectric joint. We utilized the reaction product at the Ni/Mg3.2Sb2Y0.05 interface, the phase Mg4.3Sb3Ni, as a barrier layer material, and prepared Mg4.3Sb3Ni/Mg3.2Sb2Y0.05/Mg4.3Sb3Ni junctions. The results show that the interface behavior of the thermoelectric junction is optimized by the gradation of elemental concentration, thermal expansion coefficient, and work function. The Mg4.3Sb3Ni/Mg3.2Sb2Y0.05/Mg4.3Sb3Ni single-leg device showed high thermal stability at 673 K for 20 days, the contact resistance was stable at around 10 μΩ cm2, and the shear strength was maintained at about 20 MPa. The conversion efficiency of its single-leg device maintains nearly 90% of the best performance after aging at 673 K for 20 days.

The effect of boron concentration on the electrical, morphological and optical properties of boron-doped nanocrystalline diamond sheets: Tuning the diamond-on-graphene vertical junction

In this paper, the effect of boron doping on the electrical, morphological and structural properties of free-standing nanocrystalline diamond sheets (thickness ~ 1 μm) was investigated. For this purpose, we used diamond films delaminated from a mirror-polished tantalum substrate following a microwave plasma-assisted chemical vapor deposition process, each grown with a different [B]/[C] ratio (up to 20,000 ppm) in the gas phase. The developed boron-doped diamond (BDD) films are a promising semiconducting material for sensing and high-power electronic devices due to band gap engineering and thermal management feasibility. The increased boron concentration in the gas phase induces a decrease in the average grain size, consequently resulting in lower surface roughness. The BDD sheets grown with [B]/[C] of 20,000 ppm reveal the metallic conductivity while the lower doped samples show p-type semiconductor character. The charge transport at room temperature is dominated by the thermally activated nearest-neighbor hopping between boron acceptors through impurity band conduction. At low temperatures (<300 K), the Arrhenius plot shows a non-linear temperature dependence of the logarithmic conductance pointing towards a crossover towards variable range hopping. The activation energy at high temperatures obtained for lowly-doped sheets is smaller than for nanocrystalline diamond bonded to silicon, while for highly-doped material it is similar. Developed sheets were utilized to fabricate two types of diamond-on-graphene heterojunctions, where boron doping is the key factor for tuning the shape of the current-voltage characteristics. The graphene heterojunction with the low boron concentration diamond sheet resembles a Schottky junction behavior, while an almost Ohmic contact response is recorded with the highly doped BDD sheet of metallic conductivity. The free-standing diamond sheets allow for integration with temperature-sensitive interfaces (i.e. 2D materials or polymers) and pave the way towards flexible electronics devices.

Electrical Scanning Probe Microscopy Investigation of Schottky and Metal-Oxide Junctions on Hetero-Epitaxial 3C-SiС on Silicon

This paper presents a macro-and nanoscale electrical investigation of Schottky and metal-oxide junctions with hetero-epitaxial 3C-SiC layers grown on Si. Statistical current-density-voltage (J-V) characterization of Pt/3C-SiC Schottky diodes showed an increase of the reverse leakage current with increasing the devices diameters. Furthermore, C-V and J-V analyses of SiO2/3C-SiC capacitors revealed non-idealities of the thermal oxide, such as a high trapped positive charge (3×1012 cm−2) and a reduced breakdown field (EBD=6.5 MV/cm) compared to ideal SiO2. Nanoscale electrical characterizations by conductive atomic force microscopy (CAFM) and scanning capacitance microscopy (SCM) allowed to shed light on the origin of non-ideal behavior of Schottky and thermal oxide junctions, by correlating the morphological features associated to 3C-SiC crystalline defects with local current transport and carrier density.

Experimental Investigation on Failure Mechanisms of HDPE Welded Geocell Junctions under Different Clamping Distances

Geocells are three-dimensional honeycomb-reinforced geotechnical materials composed of strips and junctions. Its junctions can support and transmit forces in several directions. The performance of geocells has a considerable impact on engineering applications. However, the testing program of geocell junctions still lacks standardization, and limited research has been undertaken regarding the failure mechanisms of junctions when subjected to various stress types. In this paper, four test procedures for HDPE welded geocell junctions were performed, including weld tensile, shear, peeling, and splitting strength tests. The influence of tests under different clamping distances (10.5 mm, 25 mm, 50 mm, and 100 mm) was analyzed, and the stress–strain behavior, peak elongation, and peak and residual strength of junctions under various force states were analyzed in detail. Finally, considering the strength and deformation, the slope laying method of geocells was proposed. The results show that the tensile strength and shear strength decrease with the clamping distance, whereas the peeling strength and splitting strength remain essentially unchanged. Under a 100 mm clamping distance, the tensile strength and shear strength are decreased by 4.51% and 14.08%. Geocells spreading vertically along the surface on a subgrade slope are thought to be more reliable, improving the geocell’s service life in slope protection. The test results can be used to improve a standardized geocell junction testing procedure as well as to guide, evaluate, and enhance the quality and application dependability of geocells.

Flux focused series arrays of long Josephson junctions for high-dynamic range magnetic field sensing

Series arrays of closely spaced, planar long Josephson junctions were demonstrated to be transducers of magnetic flux featuring high-dynamic range, wide-bandwidth, and the capability to operate at cryogenic nitrogen temperatures. By tuning and scaling the geometry of these devices, it is possible to improve their sensitivity to an applied magnetic field and to generate higher voltage responses. Moreover, these devices feature linear voltage responses allowing for the potential of unlocked operation. Herein, we study the flux focusing effect in series arrays of planar Josephson junctions, which are well-suited to fabrication in thin films of the high-transition temperature superconductor YBa2Cu3O7−δ via helium focused ion beam irradiation. We present efforts to characterize the array geometry and properties for magnetic field sensing, with investigations of single Josephson junction behavior and demonstrations of small and large series arrays of Josephson junctions. Furthermore, two-tone spectroscopy is performed to quantify the practical linearity of the voltage response. In this work, a series array of 2640 long Josephson junctions is demonstrated, achieving a sensitivity of 1.7 mV/μT and a linear response over a region of 10.6 μT resulting in a dynamic range of 117 dB while operating at 40 K.

Josephson effect in superconductor/normal-dot/superconductor junctions driven out of equilibrium by quasiparticle injection

We study theoretically the large variations of the supercurrent through a normal dot that are induced by a small quasiparticle injection current from normal leads connected to the dot. We find that the supercurrent decomposes into a subgap contribution, which depends on the voltages applied to the normal leads, as well as a contribution with opposite sign from energies outside the gap, which is insensitive to the voltages. As the voltages gradually suppress the subgap contribution, a critical voltage exists above which the contribution from energies outside the gap dominates, leading to a sign reversal of the current-phase relation, namely a transition to a so-called $\pi$-junction behavior. We determine the critical voltage and analyze the robustness of the effect with respect to temperature and inelastic relaxation in the dot.

Theoretical insights into the diverse and tunable charge transport behavior of stilbene-based single-molecule junctions

Single-molecule junctions are an elementary class of future electronics devices, and therefore, it is of vital importance for deep understanding the charge transport behavior through molecular building blocks. Here, based on density functional theory (DFT) and non-equilibrium Green’s function (NEGF) method, we systematically investigate the electronic and charge transport properties of single-molecule junctions constructed with a series of stilbene molecules and gold electrodes. Consistent with available experimental results, the calculated conductance and current of para-connected molecular junction are significant larger than those of meta-connected molecular junction, indicating that the charge transport behavior is strongly correlated to the connectivity configurations. Our further frontier orbital analysis demonstrates that this phenomena is originated from the wave function population on the terminating anchor S atoms, which influences the quantum interference properties of single-molecule junctions. Furthermore, we discuss the effect of electrochemical gating on the conductance of single-molecule junctions and find that the conductance are greatly enhanced since the energies of molecular frontier orbitals can be well aligned with the Fermi level. These calculations not only offer a deep understanding of charge transport properties of stilbene-based single-molecule junctions, but also facilitate the design of high-performance field-effect transistors (FETs) in future.

Distributed polarization-doped GaN p–n diodes with near-unity ideality factor and avalanche breakdown voltage of 1.25 kV

Polarization-induced (Pi) distributed or bulk doping in GaN, with a zero dopant ionization energy, can reduce temperature or frequency dispersions in impurity-doped p–n junctions caused by the deep-acceptor-nature of Mg, thus offering GaN power devices promising prospects. Before comprehensively assessing the benefits of Pi-doping, ideal junction behaviors and high-voltage capabilities should be confirmed. In this work, we demonstrate near-ideal forward and reverse I–V characteristics in Pi-doped GaN power p–n diodes, which incorporates linearly graded, coherently strained AlGaN layers. Hall measurements show a net increase in the hole concentration of 8.9 × 1016 cm−3 in the p-layer as a result of the polarization charge. In the Pi-doped n-layer, a record-low electron concentration of 2.5 × 1016 cm−3 is realized due to the gradual grading of Al0-0.72GaN over 1 μm. The Pi-doped p–n diodes have an ideality factor as low as 1.1 and a 0.10 V higher turn-on voltage than the impurity-doped p–n diodes due to the increase in the bandgap at the junction edge. A differential specific on-resistance of 0.1 mΩ cm2 is extracted from the Pi-doped p–n diodes, similar with the impurity-doped counterpart. The Pi-doped diodes show an avalanche breakdown voltage of ∼1.25 kV, indicating a high reverse blocking capability even without an ideal edge-termination. This work confirms that distributed Pi-doping can be incorporated in high-voltage GaN power devices to increase hole concentrations while maintaining excellent junction properties.

Graph-theoretical exploration of the relation between conductivity and connectivity in heteroatom-containing single-molecule junctions.

In this study, we employ the Sachs graph theory to formulate the conduction properties of a single-molecular junction consisting of a molecule in which one carbon atom of an alternant hydrocarbon is replaced with a heteroatom. The derived formula includes odd and even powers of the adjacency matrix, unlike the graph of the parental structure. These powers correspond to odd- and even-length walks. Furthermore, because the heteroatom is represented as a self-loop of unit length in the graph, an odd number of passes of the self-loop will change the parity of the length of the walk. To confirm the aforementioned effects of heteroatoms on conduction in an actual sample, the conduction behavior of meta-connected molecular junctions consisting of a heterocyclic six-membered ring, whose conductive properties have already been experimentally determined, was analyzed based on the enumerated number of walks.

Analytical Modeling of Short-Channel TMD TFET Considering Effect of Fringing Field and 2-D Junctions Depletion Regions

An analytical drain current model is developed for a short-channel, dual-gate, monolayer 2-D transition metal dichalcogenide (TMD), lateral tunnel field-effect transistor (TFET) by considering 2-D <inline-formula> <tex-math notation="LaTeX">${p}^{+}-{n}$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">${n}-{n}^{+}$ </tex-math></inline-formula> junction at source&#x2013;channel and channel&#x2013;drain junction, respectively. This work includes the effect of both front- and back-gate dielectric fringing field effects and 2-D source and drain depletion charges. A unified piecewise surface potential model is developed by capturing the fringing field effects in the channel and 2-D junction behavior at the source&#x2013;channel and channel&#x2013;drain junction by ensuring continuity of electric field at the junction regions. A tangent-line approximation method is employed for accurate numerical evaluation of the tunneling current. The proposed model is validated with simulation results obtained using nonequilibrium Green&#x2019;s function (NEGF)-based simulator for different 2-D layered materials and a close agreement is observed. The veracity of the proposed model is tested with data in reported studies for different bias conditions and excellent matching is observed. The applicability of the model is demonstrated for any 2-D layered material-based TFET from ultrashort channel to a channel length of 50 nm. The proposed model will be extremely useful for further exploration of 2-D TMD material TFET-based very large-scale integration (VLSI) circuits.

Evidence of Josephson Coupling in a Few-Layer Black Phosphorus Planar Josephson Junction.

Setting up strong Josephson coupling in van der Waals materials in close proximity to superconductors offers several opportunities both to inspect fundamental physics and to develop cryogenic quantum technologies. Here we show evidence of Josephson coupling in a planar few-layer black phosphorus junction. The planar geometry allows us to probe the junction behavior by means of external gates, at different carrier concentrations. Clear signatures of Josephson coupling are demonstrated by measuring supercurrent flow through the junction at milli-Kelvin temperatures. Manifestation of a Fraunhofer pattern with a transverse magnetic field is also reported, confirming the Josephson coupling. These findings represent evidence of proximity Josephson coupling in a planar junction based on a van der Waals material beyond graphene and will expedite further studies, exploiting the peculiar properties of exfoliated black phosphorus thin flakes.

Mechanical Stress Modulation of Resistance in MoS2 Junctions.

Strain engineering is a powerful strategy to control the physical properties of material-enabling devices with enhanced functionality and improved performance. Here, we investigate a modulation of the transport behavior of the two-dimensional MoS2 junctions under the mechanical stress induced by a tip of an atomic force microscope (AFM). We show that the junction resistance can be reversibly tuned by up to 4 orders of magnitude by altering a tip-induced force. Analysis of the stress-induced evolution of the I-V characteristics indicates a combined effect of the tip-induced strain and strain gradient on the energy barrier height and profile. In addition, we show that the tip-generated flexoelectric effect leads to significant enhancement of the photovoltaic effect in the MoS2 junctions. A combination of the optical and mechanical stimuli facilitates reversible photomechanical tuning of resistance of the narrow-band 2D semiconductors and development of devices with an enhanced photovoltaic response.

Approximate MRAM: High-performance and Power-efficient Computing with MRAM Chips for Error-tolerant Applications

Approximate computing (AC) leverages the inherent error resilience and is used in many big-data applications from various domains such as multimedia, computer vision, signal processing, and machine learning to improve systems performance and power consumption. Like many other approximate circuits and algorithms, the memory subsystem can also be used to enhance performance and save power significantly. This paper proposes an efficient and effective systematic methodology to construct an approximate non-volatile magneto-resistive RAM (MRAM) framework using consumer-off-the-shelf (COTS) MRAM chips. In the proposed scheme, an extensive experimental characterization of memory errors is performed by manipulating the write latency of MRAM chips which exploits the inherent (intrinsic/extrinsic process variation) stochastic switching behavior of magnetic tunnel junctions (MTJs). The experimental results and error-resilient image application reveal that the proposed AC framework provides a significant performance improvement and demonstrates a maximum reduction in MRAM write current of ~66% on average with negligible or no loss in output quality.

Джозефсоновские туннельные переходы с интегральным СИН-шунтированием

This work is devoted to the study of tunnel Josephson superconductor-insulator-superconductor (SIS) junctions with a new type of shunting based on the usage of an additional superconductor-insulator-normal metal (NIS) junction located around the SIS junction. Numerical calculations of the parameters of such shunted junctions were carried out and modeling of their IVC (current-voltage characteristics) was performed. The designed samples were manufactured, their parameters were studied. To investigate the behavior of junctions under the influence of high-frequency signals in the sub-THz range, their IVCs were measured.

Josephson tunnel junctions with integral SIN shunting.

This work is devoted to the study of tunnel Josephson superconductor-insulator-superconductor (SIS) junctions with a new type of shunting based on the usage of an additional superconductor-insulator-normal metal (NIS) junction located around the SIS junction. Numerical calculations of the parameters of such shunted junctions were carried out and modeling of their IVC (current-voltage characteristics) was performed. The designed samples were manufactured, their parameters were studied. To investigate the behavior of junctions under the influence of high-frequency signals in the sub-THz range, their IVCs were measured. Keywords: Superconducting devices, superconductor-insulator-superconductor tunnel junction, Josephson effect, shunting of Josephson junctions.

Controlling charge and spin transport in an Ising-superconductor Josephson junction

An in-plane magnetic field applied to an Ising superconductor converts spin-singlet Cooper pairs to spin-triplet ones. In this work, we study a Josephson junction formed by two Ising superconductors that are proximitized by ferromagnetic layers. This leads to highly tunable spin-triplet pairing correlations which allow to modulate the charge and spin supercurrents through the in-plane magnetic exchange fields. For a junction with a nonmagnetic barrier, the charge current is switchable by changing the relative alignment of the in-plane exchange fields, and a $\pi$-state can be realized. Furthermore, the charge and spin current-phase relations display a $\phi_0$-junction behavior for a strongly spin-polarized ferromagnetic barrier.

Ion beam etching dependence of spin–orbit torque memory devices with switching current densities reduced by Hf interlayers

We report on the fabrication of nanoscale, three-terminal in-plane spin–orbit torque switching devices with low switching current densities. Critical parameters in the fabrication process, including the ion beam etching angle and time, were optimized to avoid fabrication defects and improve device yield. Measurements of the magnetic field and current-induced switching behavior of the tunnel junctions demonstrate a sensitivity to the nanopillar aspect ratio, which dictates the nanopillars’ anisotropy and thermal stability. Additionally, we show that the current density required for switching can be reduced and the device thermal stability increased by inserting Hf interlayers into the heterostructure. Micromagnetic simulations are generally consistent with the experimentally observed switching behavior, suggesting an increase in the interfacial perpendicular anisotropy at the CoFeB/MgO interface and the reduction in the Dzyaloshinskii–Moriya interaction at the W/CoFeB interface by the Hf interlayers.

Photovoltaic behavior of centimeter-long lateral organic junctions

In this study, the photovoltaic behavior of centimeter-long lateral organic junctions, reaching 1.8 cm, is reported. The organic junctions are formed using organic semiconductor films with high mobilities of holes and electrons. The lateral diffusion lengths of photogenerated electrons and holes are 4.7 and 5.5 mm, respectively. The photovoltaic behavior in the centimeter-long lateral junctions is controlled by the trap-assisted recombination between the electrons and holes.

Experimental investigation on the moment-rotation performance of pultruded FRP web-flange junctions

This paper aims to present an experimental investigation on the behavior of web-flange junctions (WFJs) rotational stiffness of pultruded fiber-reinforced polymer composites (FRP). Channels and I-sections were tested using a simple set-up, which was developed in order to experimentally characterize the junctions in a direct manner. The Digital Image Correlation (DIC) technique was used, allowing overall deflections and relative rotations between web and flange to be monitored. The WFJs' imperfections were analyzed through an optical microscope and correlated with the cracks' formation. Further, damage thresholds are identified using available stress equations for curved composite members and lower bound functions are proposed to simulate the junction stiffness retention. Finally, two Equations are developed in order to analytically predict pultruded junctions' rotational stiffness per unit of width. In general, the theoretical and experimental results agreed fairly well, with a maximum difference of 24% for I-sections and 38% for channels.

Energy-Level Alignment at Interfaces between Transition-Metal Dichalcogenide Monolayers and Metal Electrodes Studied with Kelvin Probe Force Microscopy.

We studied the energy-level alignment at interfaces between various transition-metal dichalcogenide (TMD) monolayers, MoS2, MoSe2, WS2, and WSe2, and metal electrodes with different work functions (WFs). TMDs were deposited on SiO2/silicon wafers by chemical vapor deposition and transferred to Al and Au substrates, with significantly different WFs to identify the metal–semiconductor junction behavior: oxide-terminated Al (natural oxidation) and Au (UV–ozone oxidation) with a WF difference of 0.8 eV. Kelvin probe force microscopy was employed for this study, based on which electronic band diagrams for each case were determined. We observed the Fermi-level pinning for MoS2, while WSe2/metal junctions behaved according to the Schottky–Mott limit. WS2 and MoSe2 exhibited intermediate behavior.