Vertical Junction Research Articles

Array Synthesis Horn Antenna with an Extended Horn and a Stepped Corrugated Structure for High-Power Microwave Applications

This study proposes an array synthesis horn antenna with an extended horn and a stepped corrugated structure for a high-power microwave system. The horn antenna is designed by joining four pyramidal horn antennas and an extended horn to obtain a high gain. To improve the beam pattern in the H-plane, the length of the vertical junction of the pyramidal horns is controlled. Two-stepped and partitioned corrugated structures are attached to both horizontal edges of the aperture for a good front-to-back ratio. The designed 2 × 2 array synthesis horn antenna has a gain of 19.7 dBi and front-to-back ratio of 39.6 dB in the measurement.

Scale-Up of Room-Temperature Constructive Quantum Interference from Single Molecules to Self-Assembled Molecular-Electronic Films.

The realization of self-assembled molecular-electronic films, whose room-temperature transport properties are controlled by quantum interference (QI), is an essential step in the scale-up of QI effects from single molecules to parallel arrays of molecules. Recently, the effect of destructive QI (DQI) on the electrical conductance of self-assembled monolayers (SAMs) has been investigated. Here, through a combined experimental and theoretical investigation, we demonstrate chemical control of different forms of constructive QI (CQI) in cross-plane transport through SAMs and assess its influence on cross-plane thermoelectricity in SAMs. It is known that the electrical conductance of single molecules can be controlled in a deterministic manner, by chemically varying their connectivity to external electrodes. Here, by employing synthetic methodologies to vary the connectivity of terminal anchor groups around aromatic anthracene cores, and by forming SAMs of the resulting molecules, we clearly demonstrate that this signature of CQI can be translated into SAM-on-gold molecular films. We show that the conductance of vertical molecular junctions formed from anthracene-based molecules with two different connectivities differ by a factor of approximately 16, in agreement with theoretical predictions for their conductance ratio based on CQI effects within the core. We also demonstrate that for molecules with thioether anchor groups, the Seebeck coefficient of such films is connectivity dependent and with an appropriate choice of connectivity can be boosted by ∼50%. This demonstration of QI and its influence on thermoelectricity in SAMs represents a critical step toward functional ultra-thin-film devices for future thermoelectric and molecular-scale electronics applications.

Redox Control of Charge Transport in Vertical Ferrocene Molecular Tunnel Junctions

Summary Controlling charge transport through molecular tunnel junctions is of crucial importance for exploring basic physical and chemical mechanisms at the molecular level and realizing the applications of molecular devices. Here, through a combined experimental and theoretical investigation, we demonstrate redox control of cross-plane charge transport in a vertical gold/self-assembled monolayer (SAM)/graphene tunnel junction composed of a ferrocene-based SAM. When an oxidant/reductant or electrochemical control is applied to the outside surface of the neutral single-layer graphene top electrode, reversible redox reactions of ferrocene groups take place with charges crossing the graphene layer. This leads to counter anions on the outer surface of graphene, which balance the charges of ferrocene cations in the oxidized state. Correspondingly, the junctions switch between a high-conductance, neutral state with asymmetrical characteristics and a low-conductance, oxidized state with symmetrical characteristics, yielding a large on/off ratio (>100).

Vertical bipolar junction transistor triggered silicon‐controlled rectifier for nanoscale ESD engineering

In this Letter, a novel vertical bipolar junction transistor (BJT) triggered silicon-controlled rectifier (VBTSCR) is proposed for advanced nanoscale electrostatic discharge (ESD) protection applications. With the help of a vertical NPN transistor in floating base configuration, the new silicon-controlled rectifier (SCR) structure achieves lower trigger voltage and better clamping capacity than the prior enhanced modified lateral SCR (EMLSCR) under the same layout footprint. The ESD characteristics are measured using the transmission line pulsing tester. Compared with that of EMLSCR, the trigger voltage of VBTSCR drops by 1 V, and the effective ESD robustness of VBTSCR has been doubled significantly. Based on these advantages, the proposed VBTSCR can provide ESD protection for the advanced 1.8/2.5 V input/output circuits more efficiently.

Resistance switching in large-area vertical junctions of the molecular spin crossover complex [Fe(HB(tz)3)2]: ON/OFF ratios and device stability

Multilayer crossbar junctions composed of ITO/[Fe(HB(1,2,4-triazol-1-yl)3)2]/M (with M = Al or Ca) were fabricated and investigated for their resistance switching properties. Current–voltage–temperature maps revealed ON/OFF resistance ratios as high as 400, with the ON and OFF states defined, respectively, as the low-resistance, low spin state and the high-resistance, high spin state of the spin crossover layer. Similar results were obtained with Al and Ca cathodes indicating that the charge transport in the insulating spin crossover film is at the origin of the resistance switching instead of electron injection at the electrodes. The reproducibility and stability of the device properties were also studied.

Graphene/ZnO Nanowire/p-GaN Vertical Junction for a High-Performance Nanoscale Light Source.

We report on a high-brightness ultraviolet (UV) nanoscale light source. The light emission diodes are constructed with graphene/ZnO nanowire/p-GaN vertical junctions, which exhibit strong UV electroluminescence (EL) emissions centered at a wavelength of 397 nm at one end of the ZnO nanowire. Compared to the horizontal heterojunction, the vertical junction based on the ZnO nanowire increases the interface area of the heterojunction along with a high-quality interface, thus making the device robust under a large excitation current. In this structure, transparent flexible graphene is used as the top electrode, which can effectively improve performance by increasing the carrier injection area. Moreover, by analyzing the relationship between the integrated light intensity and applied bias, a superlinear dependency with a slope of 3.99 is observed, which means high electrical-to-optical conversion efficiency. Three electron–hole irradiation recombination processes are distinguished according to the EL emission spectra.

Gate field effects on the topological insulator BiSbTeSe2 interface

Interfaces between two topological insulators are of fundamental interest in condensed matter physics. Inspired by experimental efforts, we study interfacial processes between two slabs of BiSbTeSe2 (BSTS) via first principles calculations. Topological surface states are absent for the BSTS interface in its equilibrium separation, but our calculations show that they appear if the inter-slab distance is greater than 6 Å. More importantly, we find that topological interface states can be preserved by inserting two or more layers of hexagonal boron nitride between the two BSTS slabs. In experiments, the electric current tunneling through the interface is insensitive to back gate voltage when the bias voltage is small. Using a first-principles based method that allows us to simulate the gate field, we show that at low bias, the extra charge induced by a gate voltage resides on the surface that is closest to the gate electrode, leaving the interface almost undoped. This provides clues to understand the origin of the observed insensitivity of transport properties to back voltage at low bias. Our study resolves a few questions raised in experiment, which does not yet offer a clear correlation between microscopic physics and transport data. We provide a road map for the design of vertical tunneling junctions involving the interface between two topological insulators.

High Responsivity and Photovoltaic Effect Based on Vertical Transport in Multilayer α‐In2Se3

Herein, device demonstration based on vertical transport in multilayer α‐In2Se3 is reported. Photodetectors realized using a metal/α‐In2Se3/indium tin oxide (ITO) vertical junction exhibit clear signature of the band edge in spectral responsivity. The wavelength at 680 nm corresponding to an ultrahigh responsivity of 1000 A W−1 and a detectivity of >1013 cm Hz0.5 W−1 at a bias of 0.5 V. The variation of responsivity and detectivity with optical power density is studied, and a transient response of 20 ms is obtained for the devices (instrument limitation). In addition, an asymmetric barrier height arising out of ITO and Au contacts to a vertical α‐In2Se3 junction resulted in a photovoltaic effect with VOC ≈0.1 V and ISC ≈0.4 μA under an illumination of 520 nm.

Two-dimensional van der Waals spinterfaces and magnetic-interfaces

Two-dimensional (2D) materials have brought fresh prospects for spintronics, as evidenced by the rapid scientific progress made in this frontier over the past decade. In particular, for charge perpendicular to plane vertical magnetic tunnel junctions, the 2D crystals present exclusive features such as atomic-level thickness control, near-perfect crystallography without dangling bonds, and novel electronic structure-guided interfaces with tunable hybridization and proximity effects, which lead to an entirely new group of spinterfaces. Such crystals also present new ways of integration of atomically thin barriers in magnetic tunnel junctions and an unprecedented means for developing composite barriers with atomic precision. All these new aspects have sparked interest for theoretical and experimental efforts, revealing intriguing spin-dependent transport and spin inversion effects. Here, we discuss some of the distinctive effects observed in ferromagnetic junctions with prominent 2D crystals such as graphene, hexagonal boron nitride, and transition metal dichalcogenides and how spinterface phenomena at such junctions affect the observed magnetoresistance in devices. Finally, we discuss how the recently emerged 2D ferromagnets bring upon an entirely novel category of van der Waals interfaces for efficient spin transmission and dynamic control through exotic heterostructures.

Self-driven all-inorganic perovskite microplatelet vertical Schottky junction photodetectors with a tunable spectral response

An all-inorganic perovskite self-driven vertical Schottky junction photodetector with a tunable spectral response is reported.

An ultrathin two-dimensional vertical ferroelectric tunneling junction based on CuInP2S6 monolayer.

Ferroelectric (FE) materials, especially ABO3 FE perovskite oxides, have been extensively studied for their important applications in memory devices, electronics and sensors. However, the integration of FE perovskite oxides into miniaturized memory and electronic devices has been impeded by the critical thickness limitation, as out-of-plane ferroelectricity in most FE perovskite oxides will disappear when the oxide thin film thickness is below a critical value. On the other side, CuInP2S6 (CIPS) nano-flake, a prototypical two-dimensional (2D) FE material, has recently been demonstrated to display stable out-of-plane ferroelectricity at the atomic layer thickness by experiment, which offers a new candidate for developing FE devices in the 2D nanoscale regime. Herein, after investigation of the structural and ferroelectric properties of 2D CIPS layers, especially the interactions between out-of-plane polarization and the corresponding depolarization field using first-principles calculations, we reveal that out-of-plane ferroelectricity can even persist in the CIPS monolayer, which is only 3.4 Å in thickness. Moreover, in order to explore the potential application of 2D FE CIPS layers as minimized FE devices, we design an ultrathin ferroelectric tunneling junction (FTJ) composed of a graphene/CIPS monolayer/graphene vertical van der Waals (vdW) heterostructure. Our transport simulations based on the non-equilibrium Green's function formalism predict that such an ultrathin FTJ device can still exhibit the typical tunneling electroresistance (TER) effect, where tunneling current strongly depends on the direction of FE polarization. Our work not only elucidates the origin of stable out-of-plane ferroelectricity appearing in 2D CIPS layers, but also demonstrates the practical application of a CIPS based 2D FTJ as a miniaturized, multi-functional and low-power consumption memory device for modern electronics.

Evaluation of residual defects created by plasma exposure of Si substrates using vertical and lateral pn junctions

Defect creation in both the vertical and lateral directions of Si substrates during plasma processing has become a critical problem in the fabrication of three-dimensional structural devices. In this study, the authors present pn junction structures that can be used to evaluate defects in both the vertical and lateral directions of a Si substrate. Samples with these pn junction structures were exposed to fluorocarbon plasma; after plasma exposure, a chemical dry etching process was employed to determine the influence of residual species on damaged layer formation, and capacitance–voltage measurements were conducted to detect the formation of defects in the Si substrate. The results confirmed that defects created by plasma exposure act as carrier trapping sites. Spectroscopic ellipsometry and time-of-flight secondary ion mass spectrometry revealed that the damaged layers were tens of nanometers thick, and cathodoluminescence analysis identified the presence of “latent” defects in the damaged layer even after furnace annealing. Moreover, current–voltage measurements of devices with different pn junction distances revealed that leakage current in both the vertical and lateral directions increased with decreasing pn junction distance. The experimental results of this study demonstrate that plasma-induced damage (PID) creates defects in both the vertical and lateral directions; lateral defects are assumed to be caused by the stochastic straggling of incident ions, which has been predicted by molecular dynamic simulations. The implementation of devices with lateral pn junctions is essential in improving the understanding of PID mechanisms and designing future electronic devices that are sensitive to latent defects.

Alignment tolerant, low voltage, 0.23 V.cm, push-pull silicon photonic switches based on a vertical pn junction.

In this paper, we present the design, fabrication and characterization of a carrier depletion silicon-photonic switch based on a highly doped vertical pn junction. The vertical nature of the pn junction enables the device to exhibit a modulation efficiency as high as 0.23 V.cm. Fast switching times of 60ps are achieved in a lumped configuration. Moreover, the process flow is highly tolerant to fabrication deviations allowing a seamless transfer to the 350 nm process node of a commercial complementary-metal-oxide semiconductor (CMOS) foundry. Overall, this work showcases the possibility of fabricating highly efficient carrier depletion-based silicon photonic switches using medium resolution lithography.

In situ and selective area etching of GaN by tertiarybutylchloride (TBCl)

In situ etching (ISE) of gallium nitride (GaN) can enable lateral and vertical junctions through selective area etching (SAE) and regrowth. We report the study of ISE and SAE of GaN using an organometallic precursor, tertiarybutylchloride (TBCl), in a metal-organic chemical vapor deposition reactor. Compared to the conventional etching in hydrogen, the use of TBCl allows in situ etching at a much lower temperature (<850 °C), likely due to a more reactive etchant and a more efficient desorption rate of the etching products. The TBCl etching is near equilibrium and can be significantly changed with the change of the NH3 flow rate. We also report initial results of SAE on SiO2 patterned GaN samples. An important finding is the need to control the desorption of the reaction products in order to achieve smooth surfaces. TBCl etching is crystallographically anisotropic with low etch rates on N-terminated facets. The use of TBCl and possibly other organometallic halogen precursors is expected to enable the design and implementation of III-nitride lateral junction devices that have not been possible.

Junction Temperature Measurement Method for SiC Bipolar Junction Transistor Using Base–Collector Voltage Drop at Low Current

This paper proposes an electrical method for estimation of the vertical junction temperature of silicon carbide bipolar junction transistors (SiC BJTs). This measurement method is based on measurement of the base-collector voltage (VBC) drop at a low current (VBC(low)) during the turn-off process. This voltage shows both good sensitivity and linearity with respect to temperature. The traditional temperature-sensitive electrical parameter VBE(low) (i.e., the base-emitter voltage at a low current) and an infrared camera are used to compare the characteristics of VBC(low). The results show that use of VBC(low) provides more accurate junction temperature and thermal resistance measurement results, which can then be used to extract the vertical junction temperature of the SiC BJT under test.

Interfacial States and Fano–Feshbach Resonance in Graphene–Silicon Vertical Junction

Interfacial quantum states are drawing tremendous attention recently because of their importance in design of low-dimensional quantum heterostructures with desired charge, spin, or topological properties. Although most studies of the interfacial exchange interactions were mainly performed across the interface vertically, the lateral transport nowadays is still a major experimental method to probe these interactions indirectly. In this Letter, we fabricated a graphene and hydrogen passivated silicon interface to study the interfacial exchange processes. For the first time we found and confirmed a novel interfacial quantum state, which is specific to the 2D-3D interface. The vertically propagating electrons from silicon to graphene result in electron oscillation states at the 2D-3D interface. A harmonic oscillator model is used to explain this interfacial state. In addition, the interaction between this interfacial state (discrete energy spectrum) and the lateral band structure of graphene (continuous energy spectrum) results in Fano-Feshbach resonance. Our results show that the conventional description of the interfacial interaction in low-dimensional systems is valid only in considering the lateral band structure and its density-of-states and is incomplete for the ease of vertical transport. Our experimental observation and theoretical explanation provide more insightful understanding of various interfacial effects in low-dimensional materials, such as proximity effect, quantum tunneling, etc. More important, the Fano-Feshbach resonance may be used to realize all solid-state and scalable quantum interferometers.

A triptych photocatalyst based on the Co-Integration of Ag nanoparticles and carbo-benzene dye into a TiO2 thin film

This work proposes a new efficient, long-lasting scalable and low-cost triptych photocatalyst by assembling a semiconductor thin film (planar anatase TiO2), a photosensitive molecule of the carbo-benzene (Cbz) family and plasmonic Ag nanoparticles with exquisite degree of intimacy with the semiconductor. Under simulated sunlight conditions over 48 h, the triptych TiO2/Ag/Cbz photocatalyst allows a hydrogen production rate of 0.18 mmol gphotocatalyst−1 h−1 in conditions of applicative pressure (2.2 bars) and temperature (ambient) suitable for commercial applications. A ternary synergy (~33%) for hydrogen production is clearly evidenced with the triptych material in comparison with the diptych counterpart.The role of each component (TiO2, Ag and Cbz) on the H2 production is investigated systematically by discriminating the light absorption from the different materials and interfaces. We show how to achieve an efficient vertical Schottky junction between Ag nanoparticles and the TiO2 substrate that is demonstrated to be of crucial importance in the water-splitting process.

The heterostructure and electrical properties of Sb2Se3/Bi2Se3 grown by molecular beam epitaxy

We report the growth and characterization of Sb2Se3/Bi2Se3 bilayer films fabricated by molecular beam epitaxy. High quality heterostructures are obtained as evidenced from the X-ray diffraction (XRD), atomic force microscopy and high-resolution transmission electron microscopic (HRTEM) analysis. Interestingly, Sb2Se3 grows as a (120) hexacrystal film in orthorhombic phase on rhombohedral Bi2Se3 (0001) plane, as verified by the out-of-plane and in-plane XRD scans. The cross-sectional TEM studies indicate a sharp interface between Sb2Se3 and Bi2Se3, which is important for the protection of surface states Bi2Se3. The ultraviolet photoelectron spectroscopy indicates that the Fermi level located 0.95 eV above the valence band maximum in Sb2Se3. The insulating nature of Sb2Se3 is confirmed by the nonlinear current-voltage curve via the vertical junction electrical measurement. By four point probe measurements, we confirm the charge transfer effect from Sb2Se3 into Bi2Se3, and such effect can be reduced in the Sb2Se3/(Bi0.7Sb0.3)2Se3 bilayer. This work opens a new avenue for the synthesis of multilayers consisting of topological insulators and ordinary insulator, which is important for harvesting of the multiple surface states for advanced electronic and spintronic applications.

Experimental assessment of a new device for gas–liquid separation

An experimental investigation was conducted with a combined impacting tee junction in order to study its phase-separation capability for air–water two-phase flows. The combined junction consisted of one horizontal and two vertical impacting tee junctions; all equal-sided with internal diameter 13.5-mm. All experiments were conducted at a pressure of 200kPa (abs) and ambient temperature. Two sets of data were generated; one set defines the limiting conditions of inlet gas and liquid superficial velocities (JG1 and JL1) for full separation of phases, and the second set shows the improvement in the separation effectiveness of the combined junction relative to a single vertical junction in the region where full separation cannot be achieved. Results show that, compared to a system of a single vertical impacting tee junction, the combined junction significantly increases the limiting values of JL1 and JG1 at which full separation of phases takes place. In the region where full separation is not possible, the results show that the combined junction is a much more effective separator than the single junction. The present results for a combined impacting tee junction compared favorably against data of combined branching tee junctions with the same number of branches.

Room-temperature-operated fast and reversible vertical-heterostructure-diode gas sensor composed of reduced graphene oxide and AlGaN/GaN

A vertical heterostructure diode (VHD) based on a van der Waals heterojunction between reduced graphene oxide (rGO) and Al0.3Ga0.7N/GaN/sapphire was fabricated for use in the chemical sensing of toxic gases. Target gases interacted with the atomically thin rGO layer, which served as a contact and sensing material; this interaction induced a change in the forward bias current of the VHD through modulation of the effective Schottky barrier height (SBH). The VHD gas sensor showed fast, repeatable, reproducible, recoverable, and stable room-temperature (RT)-operable gas-sensing performance for toxic gases, including nitrogen dioxide, sulfur dioxide, and ammonia. The variations of the SBH, ideality factor and series resistance of the VHD gas sensor upon gas exposure were systematically analyzed by studying the changes in the current transport mechanism through the vertical junction due to the presence of various gases. The analysis revealed that the variation of the SBH upon gas exposure is the primary sensing mechanism of the VHD gas sensor. The VHD device has great promise as the fundamental structure of simple, low-power, low-noise, and RT-operable chemical sensors.