Schottky Barrier Height Modulation Research Articles

Analog‐Digital Hybridity of Resistive Switching in Ion‐Irradiated BiFeO3 Memristor for Synergistic Neuromorphic Functionality and Artificial Learning

AbstractMemristors‐based neuromorphic devices represent emerging computing architectures to perform complex tasks by outpacing the traditional Von‐Neumann architectures in terms of speed, and energy efficiency. In this work, the resistive switching (RS) behavior of sol‐gel grown and ion‐irradiated BFO films is investigated under electrical stimulus. The Ag/BFO/FTO memristors emulate a combination of digital and analog RS behavior within a single device. The possible mechanism of analog digital hybridity is addressed by considering the formation of the conducting filament by oxygen vacancies, Ag+ ions and Schottky barrier height modulation. The ion‐irradiated BFO samples are analyzed using the Raman, XRD, and XPS studies. To uphold bioinspired synaptic actions, crucial synaptic functionalities like pair‐pulse facilitation and long‐term potentiation/depression are effectively achieved. More intricate synaptic behaviors are also demonstrated such as spike‐time‐dependent plasticity and Pavlovian classical conditioning, which represent the prominent attributes of both learning and forgetting behavior. Additionally, high pattern recognition accuracy (96.1%) is achieved in an artificial neural network simulation by using the synaptic weights of the memristors. This synergistic effect of digital and analog RS in ion‐irradiated BFO can be beneficial for the emulation of complex learning behavior as well as its incorporation into low‐power neuromorphic computing.

Exploring charge transfer and schottky barrier modulation at monolayer Ge2Sb2Te5-metal interfaces

Monolayer Ge2Sb2Te5exhibits great potential in non-volatile memory technology due to its excellent electronic properties and phase-change characteristics, while the fundamental nature of Ge2Sb2Te5-metal contacts has not been well understood yet. Here, we provide a comprehensiveab initiostudy of the electronic properties between monolayer Ge2Sb2Te5and Pt, Pd, Au, Cu, Cr, Ag, and W contacts based on first-principles calculations. We find that the strong interaction interfaces formed between monolayer Ge2Sb2Te5and Pt, Pd, Cr, and W contacts show chemical bonding and strong charge transfer. In contrast, no apparent chemical bonding and weak charge transfer are observed in the weak interaction interfaces formed with Au, Cu, and Ag. Additionally, our study reveals the presence of a pronounced Fermi level pinning effect between monolayer Ge2Sb2Te5and metals, with pinning factors ofSn=0.325andSp=0.350. By increasing the interlayer distance, an effective transition fromn-type Ohmic contact ton-type Schottky contact is facilitated because the band edge of Ge2Sb2Te5is shifted upwards. Our study not only provides a theoretical basis for selecting suitable metal electrodes in Ge2Sb2Te5-based devices but also holds significant implications for understanding Schottky barrier height modulation between semiconductors and metals.

Ternary Content-Addressable Memory Based on a Single Two-Dimensional Transistor for Memory-Augmented Learning.

Ternary content-addressable memory (TCAM) is promising for data-intensive artificial intelligence applications due to its large-scale parallel in-memory computing capabilities. However, it is still challenging to build a reliable TCAM cell from a single circuit component. Here, we demonstrate a single transistor TCAM based on a floating-gate two-dimensional (2D) ambipolar MoTe2 field-effect transistor with graphene contacts. Our bottom graphene contacts scheme enables gate modulation of the contact Schottky barrier heights, facilitating carrier injection for both electrons and holes. The 2D nature of our channel and contact materials provides device scaling potentials beyond silicon. By integration with a floating-gate stack, a highly reliable nonvolatile memory is achieved. Our TCAM cell exhibits a resistance ratio larger than 1000 and symmetrical complementary states, allowing the implementation of large-scale TCAM arrays. Finally, we show through circuit simulations that in-memory Hamming distance computation is readily achievable based on our TCAM with array sizes up to 128 cells.

Simulation Study of Interfacial Switching Memristor Structure and Neural Network Performance

In this study, the architecture of an interfacial switching memristor, which has a metal-insulatormetal structure of Pt/SrTiO<sub>3</sub>/Nb-SrTiO<sub>3</sub> was investigated. The performance of a neural network that uses memristors as its synapse components was also examined with system-level simulations. A finite element solver, COMSOL Multiphysics, was used to simulate synaptic device characteristics, specifically, the conductance change, using a series of pulses for a given architecture. An open-source software, NeuroSim, was used to simulate the ability of the neural network to recognize and identify handwritten digits. Electrostatics, mass transport, and thermionic emission equations were numerically solved in a fully coupled manner to model the Schottky barrier height modulation at the Pt/SrTiO<sub>3</sub> contact using the applied bias. The barrier height is a function of the oxygen vacancy concentration in the SrTiO<sub>3</sub> near the contact. The gradual change of the oxygen vacancy concentration profile caused by successive pulses results in the gradual change of conductance. Utilizing the simulations, the influences of device structure modification, and more specifically, changing the size of the Schottky contact, on long-term potentiation and depression were analyzed for planar devices. The results show that a smaller Schottky contact yields a higher digit recognition rate. Based on this finding, a three-dimensional device architecture that is vertically stackable was designed.

Fermi-level depinning in Mo/Si junctions by insertion of amorphous Si-rich Mo silicide film formed via gas-phase reactions of MoF6 with SiH4

We demonstrate the reduction of the electron Schottky barrier height (SBH) to 0.48 eV at Mo/n-type Si junctions through the insertion of a semimetal Si-rich Mo silicide (MoSi n , n = 7.9) film. Raman scattering measurements elucidated the persistence of the same amorphous structure within the MoSi n film even when subjected to temperatures as high as 900 °C. This excellent thermal stability yielded a notable result: preservation of the SBH modulation effect even following annealing at 700 °C. Moreover, we investigated the capacitance−voltage characteristics of MOS capacitors, revealing that MoSi n film has a remarkably low effective work function, measuring 4.1 eV when deposited onto SiO2. The deposition of MoSi n film was accomplished with an excellent coverage by using MoF6 and SiH4 gases. Thus, MoSi n film is a promising contact material in advanced CMOS transistors.

Interface resistance-switching with reduced cyclic variations for reliable neuromorphic computing

As a synaptic device candidate for artificial neural networks (ANNs), memristors hold great promise for efficient neuromorphic computing. However, commonly used filamentary memristors normally exhibit large cyclic variations due to the stochastic nature of filament formation and ablation, which will inevitably degrade the computing accuracy. Here we demonstrate, in nanoscale Ag2S-based memristors that resistance-switching (RS) at the contact interface can be a promising solution to reduce cyclic variations. When the Ag2S memristor is operated with a filament-free interface RS via Schottky barrier height modification at the contact interface, it shows an ultra-small cycle-to-cycle variation of 1.4% during 104 switching cycles. This is in direct contrast to the variation of (28.9%) of the RS filament extracted from the same device. Interface RS can also emulate synaptic functions and psychological behavior. Its improved learning ability over a filament RS, with a higher saturated accuracy approaching 99.6%, is finally demonstrated in a simplified ANN.

Finite-element simulation of interfacial resistive switching by Schottky barrier height modulation

Finite-element simulation of interfacial resistive switching by Schottky barrier height modulation

Two-dimensional α-In2Se3-based ferroelectric semiconductor junction for reconfigurable photodetectors

The proliferation of visual information promoted in part by the Internet of Things is increasing demand for high-quality imaging, which in turn imposes more stringent physical requirements on photodetectors. Given that dark current is a significant figure of merit for photodetectors, we report herein a vertical ferroelectric semiconductor junction based on two-dimensional α-In2Se3 that suppresses the dark current and, thereby, enhances photodetection sensitivity. By utilizing the tight coupling between the ferroelectric and semiconductor properties of α-In2Se3, the two-terminal graphene-ferroelectric semiconductor–graphene crossbar structure demonstrates typical memristive behavior. The conductance, reflecting the dark current, is effectively regulated by modulating the height of the out-of-plane ferroelectric polarization-induced Schottky barrier height modulation between α-In2Se3 and graphene. As a result, the dark current is suppressed to 14 nA when α-In2Se3 is polarized down, which is a 50-fold decrease from 660 nA of dark current when α-In2Se3 is polarized up. Furthermore, α-In2Se3 exhibits excellent optoelectronic properties, demonstrating a high responsivity of 4.3 × 104 A/W, a fast response speed of 43 μs, and a broadband response spectrum from the visible to 980 nm. The combination of semiconductor and ferroelectric properties means that such devices may be used in self-powered, broadband, and highly integrated optoelectronic platforms.

Design‐Dependent Switching Mechanisms of Schottky‐Barrier‐Modulated Memristors based on 2D Semiconductor

AbstractFor Schottky barrier‐modulated memristors based on 2D semiconductors, it has, to date, not been possible to achieve control over defect type and concentration as the measured switching characteristics vary considerably even under similar fabrication conditions. In this work, four distinct types of memristors are identified based on the combination of low and high resistance sequences, as well as volatile and nonvolatile characteristics. All these four types of memristors were previously observed experimentally by different research labs. It is found that the specific behavior of each memristor type can be explained by the Schottky barrier height modulation and current rectification arising from the concerted effects of the concentration, charge polarity and mobility of defects. The conditions required to realize the four types of 2D semiconductor‐based memristors are analyzed and design guidelines for fabricating each of these four types of memristors are provided.

Realizing Electronic Synapses by Defect Engineering in Polycrystalline Two-Dimensional MoS2 for Neuromorphic Computing.

Neuromorphic computing based on two-dimensional transition-metal dichalcogenides (2D TMDs) has attracted significant attention recently due to their extraordinary properties generated by the atomic-thick layered structure. This study presents sulfur-defect-assisted MoS2 artificial synaptic devices fabricated by a simple sputtering process, followed by a precise sulfur (S) vacancy-engineering process. While the as-sputtered MoS2 film does not show synaptic behavior, the S vacancy-controlled MoS2 film exhibits excellent synapse with remarkable nonvolatile memory characteristics such as a high switching ratio (∼103), a large memory window, and long retention time (∼104 s) in addition to synaptic functions such as paired-pulse facilitation (PPF) and long-term potentiation (LTP)/depression (LTD). The synaptic device working mechanism of Schottky barrier height modulation by redistributing S vacancies was systemically analyzed by electrical, physical, and microscopy characterizations. The presented MoS2 synaptic device, based on the precise defect engineering of sputtered MoS2, is a facile, low-cost, complementary metal-oxide semiconductor (CMOS)-compatible, and scalable method and provides a procedural guideline for the design of practical 2D TMD-based neuromorphic computing.

Si-based mid-infrared photodetector with dynamic Schottky barrier height modulation applicable for synchronous detection

We have proposed an electrical shutter operation of a nano-antenna infrared photodetector using Schottky barrier height (SBH) modulation, where the SBH is modulated by a reverse bias voltage V b applied to the detector. The inhomogeneity of the Schottky junction may be dominant in the mechanism of barrier height reduction. Compared to the photocurrent i ph obtained by modulating the frequency of on–off of the incident light, the proposed method yields almost the same value. Therefore, it can be concluded that the proposed method is capable of synchronous detection measurement. In addition, since the proposed method does not require a mechanical shutter, it is a compact system that can be used in very small electrical equipment.

Modulation of Schottky Barrier Height by Nitrogen Doping and Its Influence on Responsivity of Monolayer MoS2 Photodetector

AbstractMonolayer MoS2 flakes are prepared by low‐pressure chemical vapor deposition on p‐type and n‐type silicon substrates and post‐treated under nitrogen (N2)‐rich conditions to incorporate nitrogen atoms in sulfur vacancies. Ultraviolet photoelectron spectroscopy (UPS) shows an increase in work function value by 0.47 eV and 0.53 eV compared to undoped MoS2 when grown on p and n‐type substrates, respectively. Photodetection experiments conducted for doped and undoped MoS2 grown on p‐type substrate reveal a decrease in the value of photo responsivity for N2 doped MoS2 (191 A W−1) compared with undoped MoS2 (572 A W−1). Also, MoS2 crystals grown and doped on an n‐type substrate display an important enhancement of the photoresponsivity from 63 A W−1 for undoped to 606 A W−1 for N2 doped MoS2. The modulation of Schottky barrier height for N2 doped MoS2 on p type substrate decreased whereas for n type substrate the high electric field created due to the difference in the Fermi level allows for greater separation of photogenerated charge carriers. This modulation in the photoresponsivity due to the selection of the type of substrate opens up new avenues of research and engineering of atomically thin optoelectronic devices.

Comparison between modulations of contact and channel potential in nitrogen dioxide gas response of ambipolar carbon nanotube field-effect transistors

Carbon nanotubes (CNTs) are promising materials for gas sensing because of their large specific area and high sensitivity to charge differentiation. In CNT-based field-effect transistors (FETs) for gas sensing, both CNT potential modulation in the channels and Schottky barrier height modulation at the CNT/metal electrode contact influence the current properties. However, researchers have not used Schottky barrier height modulation for gas detection. To investigate and compare the effects of Schottky barrier height modulation and CNT channel potential modulation on NO2 gas exposure, we fabricated ambipolar CNT FETs by the dielectrophoretic assembly. We exposed CNT FET gas sensors to N2 gas containing 100-ppb NO2 and observed two different responses in the electric properties: a steady current shift in the positive direction in the hole-conduction region because of the channel potential modulation, and an abrupt decrease in transconductance in the electron-conduction region because of the Schottky barrier modulation. The CNT channels and CNT/metal contact both contributed to the sensor response, and the modulation rate of the Schottky barrier was higher than that of the CNT potential shift in the channel.

The effects of point defect type, location, and density on the Schottky barrier height of Au/MoS2 heterojunction: a first-principles study

Using DFT calculations, we investigate the effects of the type, location, and density of point defects in monolayer MoS2 on electronic structures and Schottky barrier heights (SBH) of Au/MoS2 heterojunction. Three types of point defects in monolayer MoS2, that is, S monovacancy, S divacancy and MoS (Mo substitution at S site) antisite defects, are considered. The following findings are revealed: (1) The SBH for the monolayer MoS2 with these defects is universally higher than that for its defect-free counterpart. (2) S divacancy and MoS antisite defects increase the SBH to a larger extent than S monovacancy. (3) A defect located in the inner sublayer of MoS2, which is adjacent to Au substrate, increases the SBH to a larger extent than that in the outer sublayer of MoS2. (4) An increase in defect density increases the SBH. These findings indicate a large variation of SBH with the defect type, location, and concentration. We also compare our results with previously experimentally measured SBH for Au/MoS2 contact and postulate possible reasons for the large differences among existing experimental measurements and between experimental measurements and theoretical predictions. The findings and insights revealed here may provide practical guidelines for modulation and optimization of SBH in Au/MoS2 and similar heterojunctions via defect engineering.

Electric Field Screening in Gate‐Tunable van der Waals 2D‐Metal/InSe Junctions

AbstractThe prediction of the dielectric response in 2D metal–semiconductor junctions (MSJs) is challenging since the screening is inhomogeneous. Herein, a generalized model is proposed to uncover the relationship between interfacial electrostatic screening and the modulation of Schottky barrier height (ΦSB) in 2D MSJs. This model is developed based on the effective dielectric constant, interfacial distance, and van der Waals gap that sheds light on the profound difference between dielectric screening in 2D and conventional MSJs. This model can predict the variation of ΦSB for a series of 2D MSJs. Combining thermionic field emission theory with the model, several compatible 2D metals are provided for low‐dimensional electronics, among which the α‐graphyne‐based junction exhibits the largest on/off ratio.

Physical Insights into Vacancy-Based Memtransistors: Toward Power Efficiency, Reliable Operation, and Scalability.

Memtransistors that combine the properties of transistor and memristor hold significant promise for in-memory computing. While superior data storage capability is achieved in memtransistors through gate voltage-induced conductance modulation, the lateral device configuration would not only result in high write bias, which compromises the power efficiency, but also suffers from unsuccessful memory reset that leads to reliability concerns. To circumvent such performance limitations, an advanced physics-based model is required to uncover the dynamic resistive switching behavior and deduce the key driving parameters for the switching process. This work demonstrates a self-consistent physics-based model which incorporates the often-overlooked effects of lattice temperature, vacancy dynamics, and channel electrostatics to accurately solve the interaction between gate potential, ions, and carriers on the memristive switching mechanism. The completed model is carefully calibrated with an ambipolar WSe2 memtransistor and hence enables the investigation of the carrier polarity effect (electrons vs holes) on vacancy transport. Nevertheless, the validity of the model can be extended to different materials by a simple material-dependent parameter modification. Building upon the existing understanding of Schottky barrier height modulation, our study reveals three key insights─leveraging threshold voltage shifts to lower write bias; optimizing lattice temperature distribution and read bias polarity to achieve successful memory state recovery; engineering contact work function to overcome the detrimental parasitic current flow in short channel ambipolar memtransistors. Therefore, understanding the significant correlation between the switching mechanisms, different material systems, and device structures allows performance optimization of operating modes and device designs for future memtransistors-based computing systems.

Revealing the Variation of Photodetectivity in MAPbI3 and MAPb(I0.88Br0.12)3 Single Crystal Based Photodetectors Under Electrical Poling-Induced Polarization

The use of electrical poling to induce polarization potential has been found to increase the photocurrent (Ilight) in hybrid perovskite-based devices; however, the origin of this process has not been fully understood. Here, we study the effect of electrical poling on the photodetection properties of self-powered photodetectors (PDs) based on halide perovskites in two different phase structures (i.e., tetragonal and cubic). Specifically, extensive investigations are performed on the MAPbI3 (tetragonal) and MAPb(I0.88Br0.12)3 (cubic) single crystals (SCs). Our characterization results revealed that the Ilight has increased by 2-fold during forward poling and decreased during reverse poling in both PDs. The improved Ilight is caused by polarization induced ion migration, which builds remanent potential due to ion accumulation near metal electrodes. The effect of this polarization was found to be greater in MAPbI3 PD as compared to MAPb(I0.88Br0.12)3 PD, which influences the interface band bending and reduces Schottky barrier height (SBH). This study highlights that the modification of SBH, which describes the potential energy barrier for electrons formed at a metal–semiconductor junction, can tune the photocurrent and response time of PDs.

Schottky Barrier Height Modulation (SBHM) Induced Photon Current Gain in MIS(p) Tunnel Diodes for Low Operation Voltage

Schottky barrier height modulation (SBHM) mechanism is utilized on metal-insulator-semiconductor tunnel diodes (MISTD) to achieve high photon current gain. The photon current gain can reach over 24000 at 2.5 V for the MISTD with oxide thickness 3.11 nm under photon flux of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.03\times 10^{12} cm^{-2}sec^{-1}$ </tex-math></inline-formula> . The operation voltage can go down to below 1 V depending on the oxide thickness of MISTD and the requirement of the gain. For example, the MISTD with oxide thickness 2.89 nm can produce photon current gain above 2800 at 1 V under photon flux of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.03\times 10^{12} cm^{-2}sec^{-1}$ </tex-math></inline-formula> . It is found that the devices can reach an electric field in the order of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{4}-10^{5} V/cm$ </tex-math></inline-formula> under low operation voltage, which is helpful for high-speed applications. MISTD with gate metal and oxide thickness being properly chosen is simple in structure but effective in photon sensing.

Analytical and experimental investigation of dual-mode piezo-gated thin film transistor for force sensors

Piezo-gated thin film transistor (PGTFT) capable of modulating charge transport solely relying on piezo-gating effect plays a pivotal role in developing advanced piezotronic devices. However, most previous PGTFTs were reported to show indistinct piezo-gating effect through piezoelectric-induced modulation of Schottky barrier height in detecting only one-dimensional strain for force sensors. Therefore, we first propose a full-functional PGTFT using ZnO thin film as model piezoelectric semiconducting material that works on dual-mode as depletion and accumulation of charges in detecting strain by both analytically and experimentally to exhibit carrier concentration-dependent behavior. Prior to the PGTFT fabrication, the carrier concentrations of the RF-sputtered ZnO thin films are intentionally varied by varying atmosphere conditions. All the ZnO thin films are fully analyzed regarding morphology, structure and electrical properties to validate the high-quality c-axis oriented with Zn+ terminated crystalline thin films. Finally, two configurations of the PGTFTs are completed with top and bottom electrodes. The I-V curves of the PGTFTs subjected to external forces exhibit opposite force dependence between the top-electrode and bottom-electrode PGTFTs, in agreement with the simulated data. Further, the effect of free carrier concentration on the depletion and accumulation mode through piezo-gating effect is investigated, where an enhancement of around 44.6% in gauge factor is achieved for an order of reduction in the carrier concentration. We provide new insights into the piezo-gating effect by the novel ohmic-contact-based PGTFT, which can be operated in dual mode for acquiring more information as the basis of a multi-dimensional piezotronic force sensor.

Strain-Induced Spatially Resolved Charge Transport in 2H-MoTe2

Strain engineering offers unique control to manipulate the electronic band structure of two-dimensional (2D) materials, resulting in an effective and continuous tuning of the physical properties. Ad hoc straining of 2D materials has demonstrated state of the art photonic devices including efficient photodetectors at telecommunication frequencies, enhanced-mobility transistors, and on-chip single photon sources, for example. However, in order to gain insights into the underlying mechanism required to enhance the performance of the next-generation devices with strain(op)tronics, it is imperative to understand the nano- and microscopic properties as a function of a strong nonhomogeneous strain. Here, we study the strain-induced variation of local conductivity of a few-layer transition metal dichalcogenide using conductive atomic force microscopy. We report a strain characterization technique by capturing the electrical conductivity variations induced by local strain originating from surface topography at the nanoscale, which allows overcoming limitations of existing optical spectroscopy techniques. We show that the conductivity variations parallel the strain deviations across the geometry predicted by molecular dynamics simulation. These results substantiate a variation of the effective mass and surface charge density by 0.026me and 0.03e for every percent of uniaxial strain, respectively, derived using band structure calculation based on the first-principles density functional theory. Furthermore, we demonstrate modulation of the effective Schottky barrier height by quantifying its alteration originating from a gradual reduction of the conduction band minima as a function of tensile strain. Such spatially textured electronic behavior via surface topography-induced strain variations in atomistic-layered materials at the nanoscale opens up exciting opportunities to control fundamental material properties and offers a myriad of design and functional device possibilities, such as for electronics, nanophotonics, flextronics, or smart cloths.