Depletion Region Research Articles

2D WSe2/MoS2 p‐i‐n Vertical Heterojunction Photodetectors by Selective Plasma Doping

AbstractCompared to traditional pn junctions, photodetectors based on 2D materials with a p‐i‐n structure offer enhanced photoresponsivity by broadening the depletion region and improving response speed by reducing junction capacitance. However, due to the lack of a mature, controllable doping process, p‐i‐n heterostructure photodetectors based on 2D materials are rarely reported. A 2D WSe2/MoS2 p‐i‐n vertical heterojunction photodetector created through plasma selective doping is presented. This device not only retains the wide junction region of the vertical heterojunction, but in coordination with the intrinsic layer (i layer), the width of the depletion region is broadened, increasing the photoactive area, Additionally, a strong built‐in electric field is formed internally, greatly accelerating the rapid separation and transport of photogenerated carriers. The p‐i‐n vertical heterojunction photodetector exhibits a high responsivity and an ultra‐fast response time (τr = 7.3 µs, τf = 5.49 µs), achieving nearly a 100 fold improvement over the pristine WSe2/MoS2 heterojunction. Under self‐driven conditions, the device achieves a maximum responsivity of 0.32 A W−1 and a detectivity of 3.41 × 109 Jones at 637 nm. Additionally, stable detection in the near‐infrared (NIR) is realized due to the interband transitions. These findings are expected to advance the development and application of photoelectric detection technologies.

Pitting Behavior of a Super Ferritic Stainless Steel: Role of Primary and Secondary Precipitates after Heat Treatment

Abstract This study investigates the influence of sigma phase in 446 stainless steel on pitting process in a Br- environment after heat treatment, as well as its relationship with (Ti, Nb) primary precipitates. We find that the chemical dissolution of (Ti, Nb) primary precipitates takes top priority, while the formation of stable pitting tends to occur around sigma phase at the depletion region with size wider than at least 1.5μm. Furthermore, the mesh-like sigma phase along grain boundaries limits the lateral growth of pitting on the surface, while increasing the tendency for vertical growth, eventually forming a mesh-like pitting morphology.

Near-Surface Concentration Profile of Sheared Semidilute Polymer Solutions.

Controlling the structure of polymer solutions near a solid surface is crucial for many industrial processes as it significantly impacts solution flow and influences slip at the interface. To date, only a few techniques have been developed to experimentally investigate this type of interface at the nanometric scale of solid/liquid interactions. In this study, we probe the interface between a smooth sapphire surface and a semidilute polystyrene solution, using neutron reflectivity. A special setup for flow measurements under shear has been designed and optimized. Our results show that, at rest, polymer chains are globally depleted from the solid surface. Contrary to common assumptions, some polystyrene chains do adsorb onto the wall. Under flow conditions, we experimentally demonstrate that the depletion layer remains stable, a finding that has been hypothesized but is only vaguely confirmed in the literature.

Characterization and optimization of high-efficiency crystalline silicon solar cells: Impact of recombination in the space charge region and trap-assisted Auger exciton recombination

Characterization and optimization of high-efficiency crystalline silicon solar cells: Impact of recombination in the space charge region and trap-assisted Auger exciton recombination

Operando spin observation elucidating performance-improvement mechanisms during operation of Ruddlesden–Popper Sn-based perovskite solar cells

Sn-based perovskite solar cells (PSCs) have attracted attention because of their low environmental impact. Unfortunately, the readily occurring oxidation of Sn2+ inhibits further improvement of their efficiency and stability. Ruddlesden–Popper (RP) Sn-based perovskites are considered promising candidates as absorbers that improve the performance and stability of Sn-based PSCs. However, microscopic understanding of performance-enhancing mechanisms remains insufficient. For this study, electron spin resonance (ESR) spectroscopy measurements were taken of RP Sn-based PSCs with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole-transport layers and (BA0.5PEA0.5)2FA3Sn4I13 perovskite layers to clarify the space-charge region formation mechanism at the PEDOT:PSS/(BA0.5PEA0.5)2FA3Sn4I13 interface. These results indicated electron-barrier formation in the (BA0.5PEA0.5)2FA3Sn4I13 layer near the PEDOT:PSS layer. Moreover, the electron barrier was found to be enhanced during device operation. The enhanced interface band bending reduces interface recombination and thereby improves the device's performance. These findings might provide important progress in practical applications of PSCs and might advance the realization of a carbon-neutral society.

Quantification of mobile charge carrier yield and transport lengths in ultrathin film light-trapping ZnFe2O4 photoanodes.

Zinc ferrite (ZnFe2O4, ZFO) has gained attention as a candidate material for photoelectrochemical water oxidation. However, champion devices have achieved photocurrents far below that predicted by its bandgap energy. Herein, strong optical interference is employed in compact ultrathin film (8-14 nm) Ti-doped ZFO films deposited on specular back reflectors to boost photoanode performance through enhanced light trapping, resulting in a roughly fourfold improvement in absorption as compared to films deposited on transparent substrates. The spatial charge carrier collection profile and wavelength-dependent photogeneration yield of mobile charge carriers was then extracted via spatial collection efficiency analysis based on optical and external quantum efficiency measurements. We demonstrate that despite the enhanced performance enabled by the light trapping structure, substantial recombination occurs for thin film ZFO photoanodes even within the space charge region of an ultrathin film photoanode. Furthermore, the excitation-wavelength-dependent yield of mobile charge carriers in ZFO is shown to be less than unity across the visible spectrum, ultimately limiting the attainable photocurrent density. These results explain the underperformance of ZFO as a photoanode material and suggest that reduction of the mobile charge carrier yield due to the existence of ligand field states is a dominant loss mechanism for metal-oxides containing Fe metal centers with open d-shell configuration.

Ion concentration polarization causes a nearly pore-length-independent conductance of nanopores.

There has been a great amount of interest in nanopores as the basis for sensors and templates for preparation of biomimetic channels as well as model systems to understand transport properties at the nanoscale. The presence of surface charges on the pore walls has been shown to induce ion selectivity as well as enhance ionic conductance compared to uncharged pores. Here, using three-dimensional continuum modeling, we examine the role of the length of charged nanopores as well as applied voltage for controlling ion selectivity and ionic conductance of single nanopores and small nanopore arrays. First, we present conditions where the ion current and ion selectivity of nanopores with homogeneous surface charges remain unchanged, even if the pore length decreases by a factor of 6. This length-independent conductance is explained through the effect of ion concentration polarization (ICP), which modifies local ionic concentrations, not only at the pore entrances but also in the pore in a voltage-dependent manner. We describe how voltage controls the ion selectivity of nanopores with different lengths and present the conditions when charged nanopores conduct less current than uncharged pores of the same geometrical characteristics. The manuscript provides different measures of the extent of the depletion zone induced by ICP in single pores and nanopore arrays, including systems with ionic diodes. The modeling shown here will help design selective nanopores for a variety of applications where single nanopores and nanopore arrays are used.

The investigation of main electrical parameters, energy dependent profiles of surface states and their lifetimes in the Au/n-Si Schottky diodes with (PVA-Fe3O4) interlayer depend on frequency and voltage

Abstract In this article, the impedance-voltage-frequency (Z-V-f) measurements of the fabricated Au/(PVA-Fe3O4)/n-Si SDs have been performed between 0.1 kHz and 1 MHz, and in the ±3 V range. Main important electronic parameters of the Schottky diode (SD) like diffusion - potential (VD), Fermi - energy (EF), barrier - height (ΦB), depletion layer (WD), and max. electric field (Em) were extracted from the reverse bias 1/C2 - V plots in a wide frequency range. The voltage-reliant variations of the surface states (Nss) have been calculated by using low—high frequency capacitance (CLF-CHF), and parallel conductance or admittance models and compared to each other. The voltage-reliant resistance profile of Ri has also been obtained from the Nicollian & Brews method for all frequencies. All these results indicate that these main electrical parameters are strongly dependent on voltage and frequency due to the existence of Nss, their lifetimes (τ), interfacial organic layer, Rs, interface, and dipole polarizations. But, while Nss is effective, both in depletion and inversion regions, Rs is dominant at the strong-accumulation region at high enough frequency.

Analytical Solutions and Computer Modeling of a Boundary Value Problem for a Nonstationary System of Nernst–Planck–Poisson Equations in a Diffusion Layer

This article proposes various new approximate analytical solutions of the boundary value problem for the non-stationary system of Nernst–Planck–Poisson (NPP) equations in the diffusion layer of an ideally selective ion-exchange membrane at overlimiting current densities. As is known, the diffusion layer in the general case consists of a space charge region and a region of local electroneutrality. The proposed analytical solutions of the boundary value problems for the non-stationary system of Nernst–Planck–Poisson equations are based on the derivation of a new singularly perturbed nonlinear partial differential equation for the potential in the space charge region (SCR). This equation can be reduced to a singularly perturbed inhomogeneous Burgers equation, which, by the Hopf–Cole transformation, is reduced to an inhomogeneous singularly perturbed linear equation of parabolic type. Inside the extended SCR, there is a sufficiently accurate analytical approximation to the solution of the original boundary value problem. The electroneutrality region has a curvilinear boundary with the SCR, and with an unknown boundary condition on it. The article proposes a solution to this problem. The new analytical solution methods developed in the article can be used to study non-stationary boundary value problems of salt ion transfer in membrane systems. The new analytical solution methods developed in the article can be used to study non-stationary boundary value problems of salt ion transport in membrane systems.

High Current Gain MoS2 Bipolar Junction Transistor Based on Metal-Semiconductor Schottky Contacts.

Bipolar junction transistors (BJTs) are crucial components in high-power electronic applications. However, while two-dimensional (2D) semiconductors with exceptional electrical properties have been extensively studied in field-effect transistors, their application in BJTs has received far less attention. In this study, we demonstrate high-gain MoS2 BJTs based on metal-semiconductor Schottky contacts. The emitter-base junction uses the thermal ionization properties of a Schottky diode to emit electrons, while the collector-base junction leverages the Schottky barrier to collect electrons. This design enables thermal ionization of electrons into the base region, where they are subsequently accelerated and transferred to the collector region under the influence of the collector-base junction voltage. Our MoS2 BJTs achieves a common-base current gain 0.99 and a remarkable common-emitter current gain of 1967, representing the highest performance reported for BJTs based on 2D semiconductors to date, which is comparable to traditional silicon-based BJTs.

Influence of Varying Recessed Gate Height on Analog/RF Performances of a Novel Normally-Off Underlapped Double Gate AlGaN/GaN-based MOS-HEMT

Current transistor technology has issues with off-state current which reduces power efficiency. The paper presents a novel Normally-off Underlapped Dual Gate (U-DG) AlGaN/GaN MOS-HEMT device to tackle these issues. A novel dual recessed gate technology is implemented in the device which deepens the depletion region, creating a strong barrier that prevents current flow at negative gate voltage. The study investigates various recessed gate heights’ impact on analog and RF performance. Key parameters like drain and gate Currents, transconductance, and transconductance generation factor were analyzed, alongside intrinsic capacitance, gate capacitance, intrinsic resistance and maximum oscillation frequency. Results indicate that a 19 nm recessed gate height offers optimal performance. It is the only one to achieve a normally off state and a substantial 45% increase in transconductance compared to other heights. This research provides crucial insights into designing efficient Normally-off U-DG AlGaN/GaN MOS-HEMT devices, particularly relevant for low-power applications.

Influence of the nature of the distribution of recombination centers in the space charge region of the p–n junction on the parameters of the current–voltage characteristics within the classical Shockley and Shockley–Noyce–Sah models

Abstract The behavior of current–voltage characteristics (CVC) within the classical Shockley and Shockley–Noyce–Sah models depending on the nature of the distribution of recombination centers in the space charge region of the p–n junction was analyzed. Notably, the non-ideality factor determined from experimental CVC cannot be introduced into the exponent of the mathematical model of the CVC because it is unrelated to the physical nature of the exponentials of classical models but indicates the voltage dependence of the pre-exponential factor of the recombination component of the total current. The exponential factor of the model with a non-ideality factor describing the experimental CVC presents only a mathematical approximation of a small portion of it.

Assessment on wire-arc additive manufactured high strength low alloy steel cylindrical component

Large structural parts are widely manufactured through wire-arc additive manufacturing (WAAM) due to its high deposition rate, flexibility, and low cost. There are a variety of industries where high strength low alloy (HSLA) steel is used, including naval, tool and die manufacturing, construction, and offshore structures. The present article examines the mechanical properties and microstructural evolution of a developed HSLA component. The microstructural analysis was done in the component’s top (TR) and bottom (BR) regions. It reveals almost similar microstructural constituents but with more polygonal ferrite in the TR and bainitic structure in the bottom, with acicular ferrite in both regions. The inclusions in both regions were characterized using energy-dispersive X-ray spectroscopy. This demonstrates that manganese (Mn), as a strong deoxidizer, reacts with oxygen and dissolves entirely within the inclusion, forming a Mn depletion zone. Additionally, grain coarsening was observed in the building direction. The average grain size in the TR and BR was 13.12 μm and 6.54 μm, respectively. Within the regions considered, only 2.47%, 4.06%, and 0.65% variations were observed between yield strength, ultimate tensile strength, and elongation to fracture.

Formation of All Tin Oxide p–n Junctions (SnO–SnO2) during Thermal Oxidation of Thin Sn Films

Metastable stannous oxide (SnO) phase of p‐type semiconductor and all tin oxides p–n junctions of SnO–SnO2 nanostructures are formed by controlled thermal oxidation of thin tin films. High purity Sn is deposited on quartz substrates using a vacuum‐assisted thermal evaporation technique. Afterwards, controlled thermal oxidation at different temperatures is performed in air ambient condition (150–800 °C). Various surface characterization techniques have been employed to analyze the structure, morphology, chemistry, optical, and electronic properties of these SnOx films. P‐type SnO phase is found to be thermodynamically stable at lower oxidation temperatures (250–400 °C), while n‐type SnO2 phase starts to appear above 500 °C. Highly uniform and dense SnO nanospheres along with few 1D nanorods are observed after oxidation at 400 °C. Mixed oxide phases of p–n junctions with a sudden decrease in electrical conductivity is observed for 500 °C film. Significantly lower surface conductivity of mixed oxide phase indicates the formation of depletion layers between p‐type SnO and n‐type SnO2 nanograins. A transition from SnO layer to SnO2 layer is also observed above 600 °C. Overall, SnOx‐based nanostructures would be a potential candidate for solar cells, p‐channel thin film transistors, p–n junction diodes and gas sensors.

Normally-Off Trench-Gated AlGaN/GaN Current Aperture Vertical Electron Transistor with Double Superjunction

This study proposes an AlGaN/GaN current aperture vertical electron transistor (CAVET) featuring a double superjunction (SJ) to enhance breakdown voltage (BV) and investigates its electrical characteristics via technology computer-aided design (TCAD) Silvaco Atlas simulation. An additional p-pillar was formed beneath the gate current blocking layer to create a lateral depletion region that provided a high off-state breakdown voltage. To address the tradeoff between the drain current and off-state breakdown voltage, the key design parameters were carefully optimized. The proposed device exhibited a higher off-state breakdown voltage (2933 V) than the device with a single SJ (2786 V), although the specific on-resistance of the proposed method (1.29 mΩ·cm−2) was slightly higher than that of the single SJ device (1.17 mΩ·cm−2). In addition, the reverse transfer capacitance was improved by 15.6% in the proposed device.

Reconfigurable Artificial Perception System and Logic Gates Based on Large‐Scale Antiambipolar 2D Heterostructure Array

AbstractVan der Waals (vdWs) heterostructures enable the fabrication of various optoelectronic devices with unprecedented functionalities, particularly antiambipolar transistors. Different methods have been employed to construct such devices; however, their integration and versatility remain significantly constrained. Here, large‐scale antiambipolar vdWs heterostructure arrays capable of implementing self‐adaptive artificial perception systems and reconfigurable logic gates are verified adopting 2D Te/WS2 heterojunctions for the first time. The resulting transistors allow the modulation of band slope and depletion regions in individual constituents through electrostatic gating, delivering a high rectification ratio of 1.8 × 104. Benefiting from the gate‐tunable antiambipolar characteristic and charge capture, such a device can realize reconfigurable modulation of synaptic plasticity between excitatory and inhibitory modes, enabling the emulation of artificial perception system for the adaptive and subjective perception adjustment in various external environments and also a recognition accuracy of 90.5% for image classification through training and inference simulations. In addition, five fundamental logic gate operations with good stability and reliability, as well as low energy consumption, are achieved in the dual‐gate antiambipolar transistor by tuning the input signals. Furthermore, similar antiambipolar characteristics are also observed from other 2D Te‐based heterostructures, defining a simple, effective, and universal method to create multifunctional antiambipolar transistors.

Improved photoresponse performance of self-powered solar-blind UV photodetectors based on n-Si/n-Ga2O3/p-Li:NiO dual-junction

The solar-blind photodetectors (SBPDs) based on the wide-bandgap semiconductor gallium oxide (Ga2O3) exhibit significant potential for applications in military, civilian, and medical fields. Although multiple structural designs of Ga2O3-based SBPDs have been proposed, their performance typically falls short of commercial standards. However, the photoresponse speed of most self-powered PDs decreases rapidly in the solar-blind region. To address this issue, we first prepared high-quality single-crystal β-Ga2O3 films using RF magnetron sputtering, which exhibit an average transmittance exceeding 85% across the 400–800 nm range and possess a relatively smooth surface. Subsequently, a superior performance self-powered SBPD of vertical structure of n-Si/n-Ga2O3/p-Li:NiO dual-junction was fabricated, which possesses a responsivity of 0.18 mA/W, a photo-to-dark current ratio of 395, rapid rise/decay times of 132/148 ms, and a specific detectivity of 1.57 × 109 Jones at 0 V bias under 254 nm illumination. The photocurrent of the device fully recovered to its initial level after experiencing changes in ambient temperature [from room temperature (RT) to 100 °C and back to RT], demonstrating robust stability in harsh environments. In addition, the valence band structures of p-Li:NiO and n-Ga2O3 were investigated in detail using XPS, and the working mechanism of the devices was analyzed based on the Fermi level alignment. The excellent performance of PDs can be attributed to the increased depletion layer width, which generates more photogenerated carriers. Additionally, the separation and transmission of photo-induced carriers are enhanced by the superposition of a double built-in electric field. Our strategy offers a promising approach for achieving high-performance Ga2O3-based photovoltaic PDs.

PH, pOH shift and electroconvection of strong and weak electrolytes under ion concentration polarization

Ion concentration polarization (ICP) phenomenon near permselective membranes and associated electroconvection contributes greatly to ion enrichment, desalination, and biomolecular separation. Despite extensive studies on ion transport and fluid dynamics near the ion exchange membrane (IEM) surface, the influence of hydrolysis and buffer reactions on pH changes and electroconvection remains unclear. This study investigates the pH variations and electroconvection changes in different electrolytes under ICP, through numerical simulations in strong (NaCl) and weak electrolytes (Na2HPO4). Our findings reveal that the free ions produced by reactions increase the electrical conductivity, significantly boosting the current by approximately 14.82% in weak electrolyte in the overlimiting current (OLC) regime. The vortex velocity generated by electroconvective instability also increases by two times at the threshold voltage of 23 times the thermal voltage (25.8 mV). Additionally, the decrease in ion concentration during the ICP causes a significant pH and pOH surge in the quasi-equilibrium electric double layer (QE-EDL) and extended space charge (ESC) region, causing the water self-ionization constant (pKw) to surpass 14. Hydrolysis reactions release H+, reducing the pH surge by 0.5 and raising the OH− concentration in the diffusion boundary layer (DBL) region, resulting in a pOH of 6.5, which is higher than that of the bulk solution. In Na2HPO4 solution, weak acid dissociation reactions confine pH changes to the QE-EDL and ESC regions, maintaining pH stability in the DBL region. It was also found that at the boundary of the DBL region, the disruption of electro-neutrality results in the highest dissociation reaction rates. This research highlights the interplay of buffer reactions, hydrolysis, pH changes, and electroconvection near the IEM surface, with implications for applications involving ion transport and pH control.

A New TiO2‐Based Cathode Material with Interface‐Dominated Storage Mechanism for Aqueous Zinc‐Ion Batteries

AbstractAqueous zinc‐ion batteries (AZIBs) are highly desirable for large‐scale energy storage applications, yet the lack of suitable cathode materials hinders their deployment. Here, for the first time, a new and promising TiO2‐based cathode material (TOC‐AI) is reported for AZIBs and unveil a novel energy storage mechanism that enables Zn2+ storage predominantly originating from the interface. The TOC‐AI composed of ultrafine TiO2 nanocrystals embedded within an amorphous carbon matrix, featuring abundant atomic interfaces and strong interactions, triggers a decoupled and rapid transport of Zn2+ ions and electrons in the interfacial space charge region, similar to an electrostatic capacitor. Moreover, the presence of titanium vacancies facilitates Zn2+ intercalation into the TiO2 bulks, contributing extra capacity to the composite. Finally, a high capacity of 111 mAh g−1 and excellent cycling performance with 77% capacity retention after 17 000 cycles are achieved. Such interface‐dominated storage mechanism has also been successfully extended to other composite systems, broadening the scope of potential applications for materials traditionally deemed inactive for Zn2+ storage.

Parametric Amplification in Depletion Layer Transduced Microelectromechanical Resonator

Parametric Amplification in Depletion Layer Transduced Microelectromechanical Resonator