Reproducible Resistive Switching Research Articles

Evaluation of Nanostructured NiS2 Thin Films from a Single-Source Precursor for Flexible Memristive Devices.

Herein, we report the first demonstration of a single-step, in situ growth of NiS2 nanostructures from a single-source precursor onto a flexible substrate as a versatile platform for an effective nonvolatile memristor. The low temperature, solution-processed deposition of NiS2 thin films exhibits a wide band gap range, spherical-flower-like morphology with high surface area and porosity, and negligible surface roughness. Moreover, the fabricated Au/NiS2/ITO/PET memristor device reveals reproducible bipolar resistive switching (RS) at low operational voltages under both flat and bending conditions. The flexible device shows stable RS behavior for multiple cycles with a good memory window (∼102) and data retention of up to 104 s. The switching of a device between a high-resistance state and a low-resistance state is attributed to the filamentary conduction based on sulfur ion migration and sulfur vacancies and plays a key role in the outstanding memristive performance of the device. Consequently, this work provides a simple, scalable, solution-processed route to fabricate a flexible device with potential applications in next-generation neuromorphic computing and wearable electronics.

Definition of a Localized Conducting Path via Suppressed Charge Injection in Oxide Memristors for Stable Practical Hardware Neural Networks.

Oxide-based memristors have been demonstrated as suitable options for memory components in neuromorphic systems. In such devices, the resistive switching characteristics are caused by the formation of conductive filaments (CFs) comprising oxygen vacancies. Thus, the electrical performance is primarily governed by the CF structure. Despite various approaches for regulating the oxygen vacancy distributions in oxide memristors, controlling the CF structure without modifying the device configuration related to material compatibility is still a challenge. This study demonstrates an effective strategy for localizing CF distributions in memristors by suppressing charge injection during the formation of conducting paths. As the injected charge quantity is reduced in the electroforming process of the oxide memristor, the CF distributions become narrower, leading to more reproducible and stable resistive switching characteristics in the device. Based on these findings, a reliable hardware neural network comprising oxide memristors is constructed to recognize complex images. The developed memristor has been employed as a synaptic memory component in systems without degradation for a long time. This promising concept of oxide memristors acting as stable synaptic components holds great potential for developing practical neuromorphic systems and their expansion into artificial intelligent systems.

Highly uniform resistive switching characteristics of KGM resistive memory through air plasma technology

In this study, a highly stable resistive switching behavior was obtained by utilizing konjac glucomannan, a biomaterial, as a dielectric layer through a solution-based process, and the ITO bottom electrode was modified by using an air plasma technique. Electrical results showed that the pretreatment device did not observe significant switching characteristics (∼101). The device, which underwent additional plasma treatment, was recognized for its significant improvements in this area, demonstrating a reduced set voltage (Vset = 0.37 V) with a 6% coefficient of variation, more than 200 consecutive cycles, and up to 90% electrical yield. Moreover, the treated device exhibited a dramatically higher memory window (>104) due to the relatively low off-state current. According to the XPS and UPS analysis, the work function increased from an untreated 4.3 to 5.79 eV as the treatment time increased to 180 s. The conductive filaments, which stemmed from oxygen vacancies, were introduced through plasma treatment to enhance the proportion of oxygen vacancies in the ITO films, thereby creating a stable and consistent filament path. Consequently, the systematic and reproducible resistive switching phenomenon was intensified. The reported results confirmed that the reliability and uniformity in bioelectronic devices can be accomplished through a simple and effective plasma technique. This approach paved the way for alternative applications of these devices.

Epitaxial La0.5Sr0.5MnO3‐δ Bipolar Memristive Devices with Tunable and Stable Multilevel States

AbstractValence change memories are novel data storage devices in which the resistance is determined by a reversible redox reaction triggered by voltage. The oxygen content and mobility within the active materials of these devices play a crucial role in their performance. Therefore, materials which present fast oxygen migration properties and can accommodate variable oxygen stoichiometry are promising candidates. In this work, the perovskite La0.5Sr0.5MnO3‐δ (LSM50) as memristive material is studied, which presents a more facile oxygen vacancy formation and faster oxygen migration compared to other strontium‐substituted manganites. For the first time reproducible resistive switching is reported in epitaxial LSM50‐based devices with active Ti electrodes, which show large operating window and stable multilevel states. Based on the structural, chemical, and electrical results, a simple phenomenological description of the resistive switching phenomena taking place in these novel LSM50‐based memristive devices is proposed.

Evaluation of Cellulose-MXene Composite Hydrogel Based Bio-Resistive Random Access Memory Material as Mimics for Biological Synapses.

We report a memory device based on organic-inorganic hybrid cellulose-Ti3C2TX MXene composite hydrogel (CMCH) as a switching layer sandwiched between Ag top and FTO bottom electrodes. The device (Ag/CMCH/FTO) was fabricated by a simple, solution-processed route and exhibits reliable and reproducible bipolar resistive switching. Multilevel switching behavior was observed at low operating voltages (±0.5 to ±1 V). Furthermore, the capacitive-coupled memristive characteristics of the device were corroborated with electrochemical impedance spectroscopy and this affirmed the filamentary conduction switching mechanism (LRS-HRS). The synaptic functions of the CMCH-based memory device were evaluated, wherein potentiation/depression properties over 8 × 103 electric pulses were observed. The device also exhibited spike time-dependent plasticity-based symmetric Hebbian learning rule of a biological synapse. This hybrid hydrogel is expected to be a potential switching material for low-cost, sustainable, and biocompatible memory storage devices and artificial synaptic applications.

A Study of the Electroforming Process in 1T1R Memory Arrays

For reproducible resistive switching in memristive devices, electroforming is a crucial process. However, a deeper understanding of the electroforming process is still lacking due to unavailability of a proper simulation tool. Here, we propose a physics-based compact model for the electroforming of valence change mechanism (VCM) memristive devices. The developed JART VCM Forming model is experimentally validated with the ZrOx-based memristive device. Furthermore, the electroforming process in different 1T1R memristive arrays is simulated with this model. The study shows that the electrical characteristics of each device in the array after the forming process are influenced by word/bit line series resistance. In addition, control effects depending on the channel width and applied gate voltage of transistor in the 1T1R cell are also investigated with the compact model simulation.

High-performance and humidity robust multilevel lead-free all-inorganic Cs3Cu2Br5 perovskite-based memristors

Halide perovskites have attracted surge of interest in the memristor field due to their superior electrical property and corresponding remarkable device performances. However, the issues of toxicity and unstable properties still severely restrict their potential applications. Here, the lead-free all-inorganic perovskite Cs3Cu2Br5 films are adopted as the switching layer to fabricate memristors with Al/Cs3Cu2Br5/ITO structure. The prepared Al/Cs3Cu2Br5/ITO memristors exhibit typical reproducible bipolar resistive switching (RS) behavior with striking characteristics, including ultralow operating voltages (0.45, −0.39 V), moderate high resistance state/low resistance state ratio (≈102), and remarkable retention time (>104 s). In addition, the multilevel storage capability can be achieved by controlling compliance current. The RS effect, stemming from the formation/rupture of both localized conductive Br vacancy/Al atom filaments, is proposed to illustrate the memristors. More importantly, the RS behavior of Al/Cs3Cu2Br5/ITO memristors maintains robustness in harsh environments with humidity up to 80%, enabling secure hardware applicable in extreme environments. This work demonstrates the opportunity for exploring the next-generation nonvolatile memories based on lead-free all-inorganic halide perovskites in future environmental-friendly and humidity robust electronics.

A Ti‐Doped Chemical Vapor Deposition Diamond Device as Artificial Synapse for Neuromorphic Applications

AbstractThe great demand of multifunctional portable electronic products in daily life and the need of a large integration of memories with logic devices and sensors, have increased the interest in the identification of suitable materials for neuromorphic computing applications. Major innovations in this direction have been achieved by exploring materials belonging to different fields of applications and taking advantage of already consolidated deposition methods. Despite the great interest in the field and the large use in complementary applications such as sensing electrodes, neural and cellular interfaces, the use of diamond‐like materials in neuromorphic applications is still limited to a few examples. Herein, the development of a synaptic element based on high‐quality polycrystalline diamond layers containing Ti inclusions showing a marked and reproducible resistance switching behavior is reported. Realized by means of a hybrid chemical vapor deposition‐powder flowing technique, this titanium doped diamond shows a 3D polycrystalline organization that is characterized by globular grains of a few microns. The coupling of Raman spectroscopy, X‐ray photoelectron spectroscopy, and X‐ray diffraction analyses confirms the good quality of diamond phase and convincingly points out the inclusion of the titanium species within the diamond lattice.

Gate-tunable plasticity in artificial synaptic devices based on four-terminal amorphous gallium oxide memristors

This paper reports on the demonstration of gate-tunable plasticity in artificial synaptic devices based on four-terminal planar memristors with amorphous gallium oxide as a memristive material. Reproducible resistance switching properties were obtained by applying voltages to the four terminals, indicating two-dimensional modulation of oxygen vacancy distribution. Based on the resistive switching properties, gate-tunable synaptic plasticity was successfully implemented by assigning read/write and gate roles to two pairs of diagonally arranged electrodes. Multilevel modulation of conductance change efficiency was demonstrated, mimicking neural functions of both excitatory principal neurons and inhibitory interneurons required for homeostatic plasticity in biological neural networks.

Flexible Memristor Devices Using Hybrid Polymer/Electrodeposited GeSbTe Nanoscale Thin Films.

We report on the development of hybrid organic-inorganic material-based flexible memristor devices made by a fast and simple electrochemical fabrication method. The devices consist of a bilayer of poly(methyl methacrylate) (PMMA) and Te-rich GeSbTe chalcogenide nanoscale thin films sandwiched between Ag top and TiN bottom electrodes on both Si and flexible polyimide substrates. These hybrid memristors require no electroforming process and exhibit reliable and reproducible bipolar resistive switching at low switching voltages under both flat and bending conditions. Multistate switching behavior can also be achieved by controlling the compliance current (CC). We attribute the switching between the high resistance state (HRS) and low resistance state (LRS) in the devices to the formation and rupture of conductive Ag filaments within the hybrid PMMA/GeSbTe matrix. This work provides a promising route to fabricate flexible memory devices through an electrodeposition process for application in flexible electronics.

High‐Performance and Environmentally Robust Multilevel Lead‐Free Organotin Halide Perovskite Memristors

AbstractWith a striking explosion of digital information, organic–inorganic halide perovskite (OHP) memristors have been regarded as a promising solution to break the von Neumann bottleneck as the nonvolatile computing‐in‐memory architecture. However, toxicity and stability under ambient conditions are two critical issues for practical applications. Here, lead‐free MASnI3 (MASI) perovskites are reported with improved resistive switching (RS) performance via defects passivating by introducing PEACl. In‐Sn/PEACl‐MASI/PEDOT:PSS/ITO memristor arrays (10 × 10) exhibit reproducible RS with low SET/RESET voltage (VSET/VRESET, +0.58/−0.49 V), ultrahigh ON/OFF ratio (8.5 × 103), stable endurance (2 × 103 cycles), excellent retention (2 × 104 s), high device yield (88%), and multilevel storage. RS remains robust in harsh environments, including high temperature and humidity, long‐term light irradiation, and open‐air conditions. Importantly, a complete physical simulation model that quantitatively and accurately describes RS in vacancy concentration, temperature, and electric potential is proposed originating from conductive filaments formation/rupture. Additionally, PEACl‐MASI memristors are successfully fabricated on polyethylene terephthalate (PET) and Si substrates to explore the potential for large‐scale application on different substrates. The PEACl‐MASI memristors provide an opportunity to develop the next generation of high‐performance and environmentally robust multilevel information storage devices.

Unraveling the Effect of the Water Content in the Electrolyte on the Resistive Switching Properties of Self-Assembled One-Dimensional Anodized TiO2 Nanotubes.

The applied potential, time, and water content are crucial factors in the electrochemical anodization process because the growth of one-dimensional nanotubes can be accelerated by enhancing the corrosive effect. We investigated the effect of the water content on the resistive switching (RS) properties of Ti foils by anodizing the foils and varying the water content in an electrolyte (1-10 vol %). By increasing the water content, we facilitated a slow transition from nanopores to nanotubes and realized an increase in the tube wall diameter and tube length. All of the fabricated memristive devices exhibited a reliable and reproducible bipolar resistive switching effect. The optimized device exhibited bipolar RS properties with good dc endurance (104 cycles) and data retention capability (105 s). Our results suggest that as the water content increases to 5 vol %, the RS process improves; further increases in the water content impair the RS process.

Low-temperature characteristics of resistive switching memory devices based on reduced graphene oxide-phosphor composites toward reliable cryogenic electronic devices

Low-temperature electronic devices are indispensable for quantum computing and space exploration applications. However, developing memory devices that are an integral part of the control circuits of low-temperature electronic devices is highly challenging. The electrical conductivity of semiconducting materials drops at low temperatures, leading to large variations in SET/RESET voltages. A strategy for the efficient operation of nonvolatile memory devices at low temperatures should be developed. This study investigated the low-temperature resistive switching characteristics of reduced graphene oxide-phosphor composites (GPs). At cryogenic temperatures, the fabricated ITO/GP/Si3N4/p++Si device exhibited a reliable and reproducible bipolar resistive switching performance. The SET/RESET voltages of the devices remained almost constant at all temperatures ranging from 77 to 300 K. The performance of the devices was further investigated by illuminating the devices with blue laser light. The SET/RESET voltages were considerably reduced through light illumination, which is correlated to the lowering of the barrier height at the Si3N4/p++Si interface and enhanced photoconductivity in the GP film. Our studies suggest that light illumination can be a promising tool for investigating issues in ReRAM devices and improving device performance in dark conditions.

A Low Power‐consumption and Transient Nonvolatile Memory Based on Highly Dense All‐Inorganic Perovskite Films

AbstractInorganic perovskite‐based memory devices have attracted tremendous attention due to their higher level of stability, more feasible synthesis conditions and better performance, as compared to organic–inorganic hybrid perovskite‐based devices. However, better film morphology is an essential issue for the performance of these devices. Here, the all‐inorganic halide perovskite cesium lead bromide (CsPbBr3) films are successfully fabricated via a unique solution processable deposition method at ambient conditions. These films possess satisfactory surface morphology with high crystallinity. These are utilized for reproducible resistive switching layers in the gold/CsPbBr3/indium tin oxide/Glass memory applications. A series of resistive switching layers (i.e., CsPbBr3) with varied thicknesses in the range of 200–500 nm are precisely tailored. The resistive switching responses and retention properties of CsPbBr3 based‐memory devices exhibit long‐term retention (exceeding 10000 s), high on/off ratio (up to 10000), low power consumption (set voltage at 0.25V), and good reproducibility. A model for the formation of filaments in the CsPbBr3 layer is proposed to describe the resistive switching mechanism. Furthermore, the devices exhibit excellent transient behavior, which is beneficial for the security of data storage. These characteristics reveal that cesium lead halide based memristor is promising to be utilized as the next‐generation smart memory device.

Highly transparent flexible artificial nociceptor based on forming-free ITO memristor

Designing a flexible transparent electronic device with biological functions is of great interest for the future wearable integrated artificial intelligence equipment. Nociceptor is a vitally important receptor of sensory neuron, which is responsible for providing a warning signal by recognizing noxious stimuli to reduce potential physical injury. Here, a flexible transparent artificial nociceptor device is demonstrated to simulate the biological nociceptor functions based on the indium tin oxide (ITO) memristor, which exhibits forming-free and reproducible threshold resistive switching behaviors. This structurally simple memristor can imitate the key features of biological nociceptor, including “threshold,” “relaxation,” and “no adaptation” behaviors and sensitization phenomena of hyperalgesia and allodynia upon external stimuli. Finally, an alarm system is built to demonstrate the simplicity and feasibility of this artificial nociceptor for future neuromorphic systems. These results indicate a potential application of the ITO memristor in the future flexible invisible neuromorphic cognitive platform.

Memristive devices based on single ZnO nanowires—from material synthesis to neuromorphic functionalities

Memristive and resistive switching devices are considered promising building blocks for the realization of artificial neural networks and neuromorphic systems. Besides conventional top-down memristive devices based on thin films, resistive switching devices based on nanowires (NWs) have attracted great attention, not only for the possibility of going beyond current scaling limitations of the top-down approach, but also as model systems for the localization and investigation of the physical mechanism of switching. This work reports on the fabrication of memristive devices based on ZnO NWs, from NW synthesis to single NW-based memristive cell fabrication and characterization. The bottom-up synthesis of ZnO NWs was performed by low-pressure chemical vapor deposition according to a self-seeding vapor-solid (VS) mechanism on a Pt substrate over large scale (∼cm2), without the requirement of previous seed deposition. The grown ZnO NWs are single crystalline with wurtzite crystal structure and are vertically aligned respect to the growth substrate. Single NWs were then contacted by means of asymmetric contacts, with an electrochemically active and an electrochemically inert electrode, to form NW-based electrochemical metallization memory cells that show reproducible resistive switching behaviour and neuromorphic functionalities including short-term synaptic plasticity and paired pulse facilitation. Besides representing building blocks for NW-based memristive and neuromorphic systems, these single crystalline devices can be exploited as model systems to study physicochemical processing underlaying memristive functionalities thanks to the high localization of switching events on the ZnO crystalline surface.

Facilitation of compliance current for resistive switching and stability of Ta/BaTiO<sub>3</sub>/Al<sub>2</sub>O<sub>3</sub>/ITO

In this work, Ta/BaTiO<sub>3</sub>/Al<sub>2</sub>O<sub>3</sub> multi-layer thin film is deposited on indium tin oxide substrates by using the magnetron sputtering technology. Obvious resistive switching performance can be observed by increasing the compliance current. Ohmic and space charge limited current conduction mechanisms are demonstrated in Ta/BaTiO<sub>3</sub>/Al<sub>2</sub>O<sub>3</sub>. The reproducible and stable resistive switching behaviors in Ta/BaTiO<sub>3</sub>/Al<sub>2</sub>O<sub>3</sub>/ITO device at <i> I</i><sub>cc</sub> = 10<sup>–2</sup> A are reported. The results show that no obvious degradation is found after 365 successive cycles tests.

Single Crystal Halide Perovskite Film for Nonlinear Resistive Memory with Ultrahigh Switching Ratio.

Morre's law is coming to an end only if the memory industry can keep stuffing the devices with new functionality. Halide perovskite acts as a promising candidate for application in next-generation nonvolatile memory. As is well known, the switching ratio is the key device requirement of resistive memory to improve recognition accuracy. Here, the authors introduce an all-inorganic halide perovskite CsPbBr3 single crystal film (SCF) into resistive memory as an active layer. The Ag/CsPbBr3 /Ag memory cells exhibit reproducible resistive switching with an ultrahigh switching ratio (over 109 ) and a fast switching speed (1.8µs). It is studied that the Schottky barrier of metal/CsPbBr3 SCF contact follows the tendency of Schottky-Mott theory, and the Fermi level pinning effect is effectively reduced. The interface S parameter of metal/CsPbBr3 SCF contact is 0.50, suggesting a great interface contact is formed. The great interface contact contributes to the steady high resistance state (HRS), and then the steady HRS leads to an ultrahigh resistive switching ratio. This work demonstrates high performance from halide perovskite SCF-based memory. The introduction of halide perovskite SCF in resistive random access memory provides great potential as an alternative in future computing systems.

Nonvolatile resistive switching memory based on monosaccharide fructose film

In this paper, we report resistive random access memory (RRAM) based on a monosaccharide—fructose for nonvolatile memory in biocompatible and “green” electronics. Fructose thin film acts as the resistive switching layer with Al and Ag top electrodes for comparison. Both devices demonstrated highly reproducible nonvolatile bipolar resistive switching behaviors with a large on/off ratio of ∼106 for the Al electrode and ∼105 for the Ag electrode. The forming voltage, set voltage, and memory window are also larger for the Al electrode than the Ag electrode, but the reset voltages are comparable. Dominant conduction mechanisms of fructose films were proposed. At a high resistance state, both electrodes reveal space charge limited conduction, while at a low resistance state, the governing mechanism is Ohm's law, and in addition, the Ag electrode also shows trap-fill limited conduction when approaching the reset voltage. This observation has yet to be reported in RRAM based on natural bio-organic materials.

Electroforming-free nonvolatile resistive switching of redox-exfoliated MoS2 nanoflakes loaded polystyrene thin film with synaptic functionality

Here, we report robust and highly reproducible nonvolatile resistive switching (RS) devices with artificial synaptic functionalities utilizing redox-exfoliated few-layered 2H-MoS2 nanoflakes. Advantageous polar solvent compatibility of 2D MoS2 from this method were utilized to fabricate thin film devices very easily and cost-effectively using polystyrene as matrix. Prominent RS property of polystyrene thin film devices with varying MoS2 concentrations strongly favors electroforming-free operation. The conduction band position of 2D MoS2 nanosheet in combination with the work functions of chosen electrodes looks alleviating to switch the current from low to high at a suitable positive bias voltage. We further confirmed the mechanism of charge transport through fitting the results with theoretical models, say injection-dominated Schottky emission model for low-conducting states and space-charge-limited current mechanism for the high-conducting state. Interestingly, a relatively high current On/Off ratio 102 was recorded during the pump-probe testing to show resistive random-access memory (ReRAM) application. Finally, artificial synaptic functionalities- the building blocks of neuromorphic computing architectures is also illustrated by considering the robust RS property and ReRAM application.