Nonvolatile Resistive Switching Research Articles

Tunable Resistive Switching Behaviors and Mechanism of the W/ZnO/ITO Memory Cell.

A facile sol-gel spin coating method has been proposed for the synthesis of spin-coated ZnO nanofilms on ITO substrates. The as-prepared ZnO-nanofilm-based W/ZnO/ITO memory cell showed forming-free and tunable nonvolatile multilevel resistive switching behaviors with a high resistance ratio of about two orders of magnitude, which can be maintained for over 103 s and without evident deterioration. The tunable nonvolatile multilevel resistive switching phenomena were achieved by modulating the different set voltages of the W/ZnO/ITO memory cell. In addition, the tunable nonvolatile resistive switching behaviors of the ZnO-nanofilm-based W/ZnO/ITO memory cell can be interpreted by the partial formation and rupture of conductive nanofilaments modified by the oxygen vacancies. This work demonstrates that the ZnO-nanofilm-based W/ZnO/ITO memory cell may be a potential candidate for future high-density, nonvolatile, memory applications.

Nonvolatile resistive switching in lead-free La2CoMnO6-based memory device

Dense and flat La2CoMnO6 films as the resistive switching functional layer were deposited on n-type Si substrates by sol–gel method. Excellent bipolar resistive switching performance was observed in the Au/La2CoMnO6/Si device for the first time. The high and low resistance states with a ratio of ∼105 could be maintained for more than 1.4 × 104 s and 750 cycles without significant degradation at room temperature. The bipolar resistive switching effect could be sustained within a wide temperature range of 220 K∼300 K. Electrical conduction analysis indicated that the resistive switching behavior in La2CoMnO6 films was primarily controlled by space-charge-limited current mechanism which originated from the change of oxygen vacancies. Density functional theory calculations indicated that the conductive ability of La2CoMnO6 films was promoted with increasing concentration of oxygen vacancies. Moreover, it is found that the modulation of the barrier located at the Au/La2CoMnO6 interface was responsible for the resistive switching under applied bias voltages.

Controlled Switching of Bistable Nanophase Domains on a Silicon Surface.

Switching the crystalline phase of a material via electrostatic control is a proven strategy for developing memory devices such as memristors that are based on nonvolatile resistance switching phenomena. However, phase switching in atomic-scale systems is often difficult to control and poorly understood. Here, we explore nonvolatile switching of long 2.3 nm wide bistable nanophase domains in a Sn double-layer structure grown on Si(111), using a scanning tunneling microscope. We identified two mechanisms for this phase switching phenomenon. First, the electrical field across the tunnel gap continuously tunes the relative stability of the two phases and favors one over the other depending on the tunneling polarity. The second mechanism involves carrier injection into empty Sn orbitals. The coupling between these relatively long-lived hot electrons and surface phonons induces a lattice instability at sufficiently large tunneling current and provides access to a hidden metastable state of matter. This hidden state is nonvolatile but can be erased by choosing the appropriate tunneling conditions or raising the temperature. Similar mechanisms could possibly be exploited in phase-change memristor and field effect devices.

Bistable Resistive Memory Switches fabricated by Floating Thin Films of Conjugated Polymers

The layer-by-layer (LBL) fabrication of conjugated polymer (CPs) thin films while preserving their microstructural features by solution processing is highly desired for compact and flexible electronic circuits. However, it is cumbersome and challenging owing to the unavoidable damage to the underlying layers. To circumvent this issue, the unidirectional floating-film transfer method (UFTM) was utilized for the LBL fabrication of oriented CP thin films on the orthogonal liquid surfaces. Further, resistive bistable memory switches were fabricated by sandwiching a layer of metal nanostructures between the LBL-fabricated oriented CP thin films. The resistive switching phenomena were realized by utilizing the applied bias-dependent charge trapping, holding, and recombination on the available states at vacuum-deposited aluminum nanostructures. The effect of CP backbone conformation on the vertical charge transport was also analyzed via a comparative study of three thiophene-based polymers namely RR-P3HT, PBTTT, and PTB-7. It was revealed that CPs with relatively fewer hydrophobic side chains was more favorable for the facile vertical charge transport due to its preferred face-on conformation on the hydrophilic liquid substrate used in UFTM. It was demonstrated that a non-volatile bistable resistive memory switch fabricated using UFTM-processed oriented thin films of PTB-7 exhibited a remarkably high on-off ratio of 1.5 × 106 with high durability.

Biocompatible AlOOH-based memristor with biomimicking synaptic functions for artificial nociceptor applications

Electronic devices with biomimetic synaptic characteristics are one of the most critical components in health monitoring and artificial intelligence. The design of neuromorphic responsive devices, however, yet remains a ground challenge. Herein, a biocompatible and electrically responsive boehmite material (γ-AlOOH) is utilized to fabricate artificial memristors to realize mimicking biological synapsefunctions. The obtained AlOOH memristor is flexible, highly transparent, and biocompatible, exhibiting nonvolatile resistive switching behaviours with low threshold voltages, high ON-OFF ratios (>103), good switching durability (>104 cycles), and long retention time (>104 s). Specifically, the device can mimic the biosynaptic functions, such as paired-pulse facilitation (PPF), long-term potentiation/depression (LTP/LTD), amplitude-dependent plasticity (SADP), and spike time dependent plasticity (STDP), and the typical sensitization, desensitization, hyperalgesia, and allodynia behaviours of the biological nociceptors. This study reveals that the AlOOH memristor with excellent artificial synapses is resulted by the high proton mobility along with the OH bond length variation of AlOOH subjected to applied stimuli, such as voltages, pulse width, and pulse intervals. Therefore, the biocompatible AlOOH-based memristors with artificial synapse behaviour have promising potential for the applications in high-performance neuromorphic devices and future artificial intelligence.

Implementation of threshold- and memory-switching memristors based on electrochemical metallization in an identical ferroelectric electrolyte

The use of an identical electrolyte in electrochemical metallization (ECM)-based neuron and synaptic devices has not yet been achieved due to their different resistive-switching characteristics. Herein, we describe ECM devices comprising the same ferroelectric PbZr0.52Ti0.48O3 (PZT) electrolyte, which can sustain both neuron and synaptic behavior depending on the identity of the active electrode. The Ag/PZT/La0.8Sr0.2MnO3 (LSMO) threshold switching memristor shows abrupt and volatile resistive switching characteristics, which lead to neuron devices with stochastic integration-and-fire behavior, auto-recovery, and rapid operation. In contrast, the Ni/PZT/LSMO memory switching memristor exhibits gradual, non-volatile resistive switching behavior, which leads to synaptic devices with a high on/off ratio, low on-state current, low variability, and spike-timing-dependent plasticity (STDP). The divergent behavior of the ECM devices is attributed to greater control of cation migration through the ultrathin ferroelectric PZT. Thus, ECM devices with an identical ferroelectric electrolyte offer promise as essential building blocks in the construction of high-performance neuromorphic computing systems.

Characteristics of Synaptic Function of Mesoporous Silica-Titania and Mesoporous Titania Lateral Electrode Devices.

In this paper, we have fabricated non-volatile memory resistive switching (RS) devices and analyzed analog memristive characteristics using lateral electrodes with mesoporous silica-titania (meso-ST) and mesoporous titania (meso-T) layers. For the planar-type device having two parallel electrodes, current-voltage (I-V) curves and pulse-driven current changes could reveal successful long-term potentiation (LTP) along with long-term depression (LTD), respectively, by the RS active mesoporous two layers for 20~100 μm length. Through mechanism characterization using chemical analysis, non-filamental memristive behavior unlike the conventional metal electroforming was identified. Additionally, high performance of the synaptic operations could be also accomplished such that a high current of 10-6 Amp level could occur despite a wide electrode spacing and short pulse spike biases under ambient condition with moderate humidity (RH 30~50%). Moreover, it was confirmed that rectifying characteristics were observed during the I-V measurement, which was a representative feature of dual functionality of selection diode and the analog RS device for both meso-ST and meso-T devices. The memristive and synaptic functions along with the rectification property could facilitate a chance of potential implementation of the meso-ST and meso-T devices to neuromorphic electronics platform.

Impact of sneak paths on in-memory logic design in memristive crossbars

Abstract Resistive Random Access Memory (RRAM), also termed as memristors, is a non-volatile memory where information is stored in memory cells in the form of resistance. Due to its non-volatile resistive switching properties, memristors, in the form of crossbars, are used for storing information, neuromorpic computing, and logic synthesis. In spite of the wide range of applications, memristive crossbars suffer from a so-called sneak path problem which results in an erroneous reading of memristor’s state. Till date, no or very few logic synthesis approaches for in-memory computing have considered the sneak path problem during the realizations of Boolean functions. In other words, the effects of sneak paths on the Boolean function realizations in crossbars still remain an open problem. In this paper, we have addressed this issue. In particular, we study the impacts of function realizations in two memristive crossbar structures: Zero-Transistor-One-Resistor (0T1R) and One-Transistor-One-Resistor (1T1R) in the presence of sneak paths. Experimental analysis on IWLS and ISCAS-85 benchmarks shows that even in the presence of sneak paths, the 1T1R crossbar structures with multiple rows and columns are the most efficient as compared to the 1T1R structures with single row and multiple columns in terms of crossbar size and number of execution cycles.

A Facile Hydrothermal Synthesis and Resistive Switching Behavior of α-Fe2O3 Nanowire Arrays.

A facile hydrothermal process has been developed to synthesize the α-Fe2O3 nanowire arrays with a preferential growth orientation along the [110] direction. The W/α-Fe2O3/FTO memory device with the nonvolatile resistive switching behavior has been achieved. The resistance ratio (RHRS/RLRS) of the W/α-Fe2O3/FTO memory device exceeds two orders of magnitude, which can be preserved for more than 103s without obvious decline. Furthermore, the carrier transport properties of the W/α-Fe2O3/FTO memory device are dominated by the Ohmic conduction mechanism in the low resistance state and trap-controlled space-charge-limited current conduction mechanism in the high resistance state, respectively. The partial formation and rupture of conducting nanofilaments modified by the intrinsic oxygen vacancies have been suggested to be responsible for the nonvolatile resistive switching behavior of the W/α-Fe2O3/FTO memory device. This work suggests that the as-prepared α-Fe2O3 nanowire-based W/α-Fe2O3/FTO memory device may be a potential candidate for applications in the next-generation nonvolatile memory devices.

Perforated Turbostratic Graphene As Active Layer in a Nonvolatile Resistive Switching Memory Device

Perforated turbostratic graphene (PTG) sheets have been synthesized from a natural waste material, dead bougainvillea bracts, using a single-step pyrolysis method, and a resistive switching (RS) memory device has been constructed with it for the very first time. Herein, the edges of these large-area multilayer graphene sheets are highly conducting due to the turbostratic stacking between the adjacent layers of the graphene sheets. These highly conducting PTG sheets embedded inside an insulating polymer matrix can act as an active layer for resistive switching memory devices. This hybrid structure shows nonlinear resistance change between two distinct resistance states by simple bias voltage variation. The trap-assisted space-charge-limited conduction can realize the high resistive state (HRS), whereas the low resistive state (LRS) takes place through direct conduction. To achieve the best performing device, a number of optimizations have been performed, like the variation of polymer matrices, variation of PTG and polymer concentration, active layer thickness variation, and top electrode area variation. The best performing device showed reproducibility of current–voltage data (>200 cycles), low power consumption (SET voltage <1 V), a high ON/OFF ratio (>104), a long retention time (>104 s), and a large number of endurance cycles (>103). High writing-read-erase-read speed and flexibility/bending cycle tests were also carried out on the best-performing device to examine its tenacity. The current PTG-based flexible RS memory device derived from a biowaste, dead bougainvillea bracts, can provide an important step toward developing green electronics.

Multiphase Polarization in Ion‐Intercalation Nanofilms: General Theory Including Various Surface Effects and Memory Applications

AbstractIon concentration polarization (CP, current‐induced concentration gradient adjacent to a charge‐selective interface) has been well studied for single‐phase mixed conductors (e.g., liquid electrolyte), but multiphase CP has been rarely addressed in literature. In our recent publication, we proposed that CP above certain threshold currents can flip the phase distribution in multiphase ion‐intercalation nanofilms sandwiched by ion‐blocking electrodes. This phenomenon is known as multiphase polarization (MP). It is then proposed that MP can further lead to nonvolatile interfacial resistive switching (RS) for asymmetric electrodes with ion‐modulated electron transfer, which theory can reproduce the experimental results of LTO memristors. In this study, a comprehensive 2D phase‐field model is derived for coupled ion‐electron transport in ion‐intercalation materials, with surface effects including electron transfer kinetics, non‐neutral wetting, energy relaxation, and surface charge. Then, the model is used to study MP. Time evolution of phase boundaries is presented, and analyze the switching time, current, energy, and cyclic voltammetry, for various boundary conditions. It is found that the switching performance can be improved significantly by manipulating surface conditions and mean concentration. Finally, the prospects of MP‐based memories and possible extensions of the current model is discussed.

The Effect of Nitrogen Annealing on the Resistive Switching Characteristics of the W/TiO2/FTO Memory Device

In this paper, the effect of nitrogen annealing on the resistive switching characteristics of the rutile TiO2 nanowire-based W/TiO2/FTO memory device is analyzed. The W/TiO2/FTO memory device exhibits a nonvolatile bipolar resistive switching behavior with a high resistance ratio (RHRS/RLRS) of about two orders of magnitude. The conduction behaviors of the W/TiO2/FTO memory device are attributed to the Ohmic conduction mechanism and the Schottky emission in the low resistance state and the high resistance state, respectively. Furthermore, the RHRS/RLRS of the W/TiO2/FTO memory device is obviously increased from about two orders of magnitude to three orders of magnitude after the rapid nitrogen annealing treatment. In addition, the change in the W/TiO2 Schottky barrier depletion layer thickness and barrier height modified by the oxygen vacancies at the W/TiO2 interface is suggested to be responsible for the resistive switching characteristics of the W/TiO2/FTO memory device. This work demonstrates the potential applications of the rutile TiO2 nanowire-based W/TiO2/FTO memory device for high-density data storage in nonvolatile memory devices.

Comparative Study between Sulfurized MoS2 from Molybdenum and Molybdenum Trioxide Precursors for Thin-Film Device Applications.

Two-dimensional (2D) materials have been studied as an emerging class of nanomaterials owing to their attractive properties in nearly every field of science and technology. Molybdenum disulfide (MoS2) is one of the more promising candidates of these atomically thin 2D materials for its technological potential. The facile synthesis of MoS2 remains a matter of broad interest. In this study, MoS2 was synthesized by chemical vapor deposition sulfurization at various temperatures (550 °C, 650 °C, and 750 °C) of either precursor molybdenum metal (Mo) or molybdenum trioxide (MoO3) deposited on silicon/silicon dioxide (Si/SiO2) via e-beam evaporation. Monolayer, bilayer, and few layers sulfurized samples have been grown and verified by Raman, photoluminescence spectroscopy, XRD, XPS, and AFM. MoO3 sulfurization provided monolayer growth in comparison to Mo sulfurization under the same conditions and precursor thicknesses. Optical microscopy showed the homogeneous nature of grown samples. A main finding of this work is that MoO3 sulfurization produced higher quality MoS2 as compared to those grown by an Mo precursor. Device characteristics based on monolayer MoO3 sulfurized MoS2-x include nonvolatile resistive switching with Ion/Ioff ≈ 104 at a relatively low operating bias of ±1 V. In addition, field-effect transistor characteristics revealed p-type material growth with a carrier mobility ∼ 41 cm2 V-1 s-1, which is in contrast to typically observed n-type characteristics.

Resistive switching mechanism of MoS2 based atomristor

The non-volatile resistive switching process of a MoS2 based atomristor with a vertical structure is investigated by first-principles calculations. It is found that the monolayer MoS2 with a S vacancy defect () could maintain an insulation characteristic and a high resistance state (HRS) is remained. As an electrode metal atom is adsorbed on the MoS2 monolayer, the semi-conductive filament is formed with the assistance of . Under this condition, the atomristor presents a low resistance state (LRS). The ON state current of this semi-filament is increased close to two orders of magnitude larger than that without the filament. The energy barrier for an Au-atom to penetrate the monolayer MoS2 via is as high as 6.991 eV. When it comes to a double S vacancy (), the energy barrier is still amounted to 3.554 eV, which manifests the bridge-like full conductive filament cannot form in monolayer MoS2 based atomristor. The investigation here promotes the atomic level understanding of the resistive switching properties about the monolayer MoS2 based memristor. The physics behind should also work in atomristors based on other monolayer transition-metal dichalcogenides, like WSe2 and MoTe2. The investigation will be a reference for atomristor-device design or optimization.

High-performance non-volatile memory devices from metallomacrocyclic organic micro/nanostructure: Control via molecular manipulation

Here, we report the synthesis of micro/nanostructures of a metal-macrocyclic organic compound, viz, Iron phthalocyanine (FePc) via cost-effective solvothermal method and consequent fabrication of high-performance resistive memory devices from FePc micro/nanorod embedded in poly(methyl methacrylate) (PMMA) matrix. The devices exhibited bipolar and non-volatile resistive switching characteristics. Interestingly, the performances of the devices (On/Off current ratio) improved by several orders of magnitude when an additional buffer layer of PMMA was deposited prior to the deposition of the active film (FePc embedded in PMMA). The variation in thickness of the PMMA buffer layer indicated that at an optimum thickness ca. 15 nm, the best performances were obtained with switching (Set) voltage ca. 1.6 V, On/Off current ratio ∼ 106 and retention ability of more than 10 h. The conduction mechanism has been discussed by linear fitting of the characteristics curves in both low and high resistance states of the device. The analysis lead to the conclusion that trapping/detrapping of charge carriers at the redox and defect sites within the active film (FePc embedded in PMMA) and at the PMMA-FePc/PMMA interface resulted in the resistive switching of the device and a trap induced space charge limited current was found to dominate in the Off- and On-state of the device.

Multilevel Non-Volatile Memristive Response in e-Textile

Multilevel non-volatile (NV) resistive switching response is reported in electronic textile. The electroconductive textile as a phase change material displayed a typical write-once-read-many (WORM) memory behavior and switched abruptly from OFF to ON under the voltage-sweep mode. Furthermore, current-sweep characterization revealed a large number of intermittent multilevel NV states. A mapping of the single active and multiple passive devices across the 2-D planar structure of the textile allowed to achieve conduction modulation using a remote current-bias write/input stimulus. A comparative study of the wale and course directions in the textile displayed an anisotropic current spreading that may be utilized for selective memory writing/emulating neuron-like behavior. In addition, the textile also displayed the reset WORM or ReWORM effect, enabling it to revert to its initial high resistive state.

The plasticity of synaptic memristor based on 2D-MoS2 thin film prepared in large-scale by a PLD-assisted CVD method

Two-dimensional MoS2 film was directly grown via a two-step chemical vapour deposition (CVD) method in a large-scale on a phosphorus doped silicon substrate, where a precursor of Mo was Pre-deposited by pulsed laser deposition (PLD) to ensure the homogeneity of Mo source. The film was confirmed to be dense, uniform and continuous nanocrystalline of MoS2 by the characterization of XRD, Raman, SEM, TEM and XPS. Then MoS2 film was employed as the channel materials to fabricate the memristor with lateral structure of Cu/MoS2/Cu. Consequently, the devices show stable nonvolatile resistance switching characteristics, such as low operating voltages, repeated cyclic sweeps (600 times) without decrease and long retention time (>104 s). Such mechanism of resistive switching can be attributable to the space-charge limited conduction dominated by defects. Importantly, a variety of biological synaptic functions, such as synaptic potentiation, depression and repetitive learning processes, are effectively simulated. Furthermore, the relationships between the resistance state and the parameters of pulse sequence were explored based on MoS2 thin film memristors. This research provides a feasible method for manufacturing MoS2 based on memristor, which is used for information and computing applications in neural morphological devices.

Reconfigurable Low-Voltage Hexagonal Boron Nitride Nonvolatile Switches for Millimeter-Wave Wireless Communications.

Recently, nonvolatile resistive switching memory effects have been actively studied in two-dimensional (2D) transition metal dichalcogenides and boron nitrides to advance future memory and neuromorphic computing applications. Here, we report on radiofrequency (RF) switches utilizing hexagonal boron nitride (h-BN) memristors that afford operation in the millimeter-wave (mmWave) range. Notably, silver (Ag) electrodes to h-BN offer outstanding nonvolatile bipolar resistive switching characteristics with a high ON/OFF switching ratio of 1011 and low switching voltage below 0.34 V. In addition, the switch exhibits a low insertion loss of 0.50 dB and high isolation of 23 dB across the D-band spectrum (110 to 170 GHz). Furthermore, the S21 insertion loss can be tuned through five orders of current compliance magnitude, which increases the application prospects for atomic switches. These results can enable the switch to become a key component for future reconfigurable wireless and 6G communication systems.

Redox-Active Azulene-based 2D Conjugated Covalent Organic Framework for Organic Memristors.

As a conjugated and unsymmetric building block composed of an electron-poor seven-membered sp2 carbon ring and an electron-rich five-membered carbon ring, azulene and its derivatives have been recognized as one of the most promising building blocks for novel electronic devices due to its intrinsic redox activity. By using 1,3,5-tris(4-aminophenyl)-benzene and azulene-1,3-dicarbaldehyde as the starting materials, an azulene(Azu)-based 2D conjugated covalent organic framework, COF-Azu, is prepared through liquid-liquid interface polymerization strategy for the first time. The as-fabricated Al/COF-Azu/indium tin oxide (ITO) mem-ristor shows typical non-volatile resistive switching performance due to the electric filed induced intramolecular charge transfer effect. Associated with the unique memristive performance, a simple convolutional neural network is built for image recognition. After 8 epochs of training, image recognition accuracy of 80% for a neutral network trained on a larger data set is achieved.

Air‐Stable, Eco‐Friendly RRAMs Based on Lead‐Free Cs3Bi2Br9 Perovskite Quantum Dots for High‐Performance Information Storage

Development of lead‐free halide perovskites that are innocuous and stable has become an attractive trend in resistive random access memory (RRAM) fields. However, their inferior memory properties compared with the lead‐based analogs hinder their commercialization. Herein, the lead‐free Cs3Bi2Br9 perovskite quantum dot (PQD)‐based RRAMs are reported with outstanding memory performance, where Cs3Bi2Br9 quantum dots (QDs) are synthesized via a modified ligand‐assisted recrystallization process. This is the first report of applying Cs3Bi2Br9 QDs as the switching layer for RRAM device. The Cs3Bi2Br9 QD device demonstrates nonvolatile resistive switching (RS) effect with large ON/OFF ratio of 105, low set voltage of −0.45 V, as well as good reliability, reproducibility, and flexibility. Concurrently, the device exhibits the notable tolerance toward moisture, heat and light illumination, and long‐term stability of 200 days. More impressively, the device shows the reliable light‐modulated RS behavior, and therefrom the logic gate operations including “AND” and “OR” are implemented, foreboding its prospect in logic circuits integrated with storage and computation. Such multifunctionality of device could be derived from the unique 2D layered crystal structure, small particle size, quantum confinement effect, and photoresponse of Cs3Bi2Br9 QDs. This work provides the strategy toward the high‐performance RRAMs based on stable and eco‐friendly perovskites for future applications.