Multistate Resistive Switching Research Articles

Nanoscale multistate resistive switching in WO3 through scanning probe induced proton evolution

Multistate resistive switching device emerges as a promising electronic unit for energy-efficient neuromorphic computing. Electric-field induced topotactic phase transition with ionic evolution represents an important pathway for this purpose, which, however, faces significant challenges in device scaling. This work demonstrates a convenient scanning-probe-induced proton evolution within WO3, driving a reversible insulator-to-metal transition (IMT) at nanoscale. Specifically, the Pt-coated scanning probe serves as an efficient hydrogen catalysis probe, leading to a hydrogen spillover across the nano junction between the probe and sample surface. A positively biased voltage drives protons into the sample, while a negative voltage extracts protons out, giving rise to a reversible manipulation on hydrogenation-induced electron doping, accompanied by a dramatic resistive switching. The precise control of the scanning probe offers the opportunity to manipulate the local conductivity at nanoscale, which is further visualized through a printed portrait encoded by local conductivity. Notably, multistate resistive switching is successfully demonstrated via successive set and reset processes. Our work highlights the probe-induced hydrogen evolution as a new direction to engineer memristor at nanoscale.

Remotely tuned multistate resistive switching in MoS2/NiMnIn thin film heterostructure for highly flexible ReRAM application

In this article, the Cu/MoS2/NiMnIn memory structure was fabricated over stainless steel substrate for flexible electronics applications. The bipolar resistive switching characteristics were observed in two different fabricated devices (i) without sulfur vacancies (D1) and (ii) with sulfur vacancies (D2) in MoS2 thin film. Three different resistance states such as the high resistance state (HRS), intermediate resistance state (IRS) and low resistance state (LRS) have been detected for the D2 memory device. It could be ascribed to the two filamentary models based on the movement of sulfur vacancies and the formation of Cu metallic filament with applied bias voltage. Moreover, the Ohmic and modified space charge limited conduction mechanisms clearly explain the current conduction in different resistance states of device D2. The fabricated MoS2 thin film-based memory structure exhibits stable resistive switching behavior with a high OFF/ON ratio of ∼3.6 × 103, good consistency of ∼3600 endurance cycles and excellent data retention capability up to 3000 s. Moreover, the remote tuning of device D2 was thoroughly investigated and an appreciable change in SET voltage was detected with external temperature and magnetic field. Additionally, the external magnetic field altered the switching states and enhanced the multi-bit data storage capability of the memory device. This can be ascribed to the effect of the Lorentz force on ionic movement in the presence of the external magnetic field. The mechanical flexibility of the memory structure was tested for 1000 bending cycles at various bending angles in both the tensile and compressive bending modes. Hence, the present study opens up new ways for the futuristic flexible device for high data storage and neuromorphic computing applications.

Highly Flexible and Asymmetric Hexagonal‐Shaped Crystalline Structured Germanium Dioxide‐Based Multistate Resistive Switching Memory Device for Data Storage and Neuromorphic Computing

AbstractWith the increase of big data and artificial intelligence (AI) applications, fast and energy‐efficient computing is critical in future electronics. Fortunately, nonvolatile resistive memory devices can be potential candidates for these issues due to their in‐computing and neuromorphic computational abilities. Hence, the paper proposes a highly flexible and asymmetric hexagonal‐shaped crystalline structured germanium dioxide‐based Ag/GeO2/ITO device for high data storage and neuromorphic computing. The proposed device shows the highly asymmetric memristor behavior at low operating voltage to block backward current. The operational behaviors are observed by modulating the applied amplitude, current compliance, and varying the frequency, which shows excellent stability and repeatability in electrical characterizations. Furthermore, the neuromorphic device exhibits synaptic learning properties such as potentiation‐depression, pulse amplification, and spike time‐dependent plasticity rules (STDP). Here, the weights update of the memristive synaptic device is analyzed using a multilayer perceptron convolutional neural network (CNN) by optimizing the learning rate, training epochs, and algorithm to achieve higher accuracy for pattern recognition using CIFAR‐10 data. Undoubtedly, the demonstrated results suggest that the proposed device is a promising candidate to develop high‐density storage and neuromorphic computing technology for wearable and AI electronics.

Multistate Resistive Switching with Self-Rectifying Behavior and Synaptic Characteristics in a Solution-processed ZnO/PTAA Bilayer Memristor

The progress of artificial synaptic devices is vital to the realization of the consciousness of effectual bioinspired neuromorphic computing systems. In this study, we fabricated an ITO/ZnO/PTAA/Ag artificial synaptic memristor based on a low-cost solution process. The fabricated device exhibited uniform gradual bipolar resistive switching with excellent endurance and self-rectifying behavior owing to the bilayer heterojunction structure of ZnO/PTAA. The growth of the conducting filament can be efficiently controlled by modulating the current compliance and voltage during the SET process. Modification of conductance states was also observed by simulations to stimuli, which are essential for synaptic function in neuromorphic computing. Various pulse measurements were performed to mimic synaptic behaviors, including long-term potentiation, long-term depression, spike-rate-dependent plasticity, paired-pulse facilitation, and post-tetanic potentiation. Moreover, we reveal that the real device shows an approximately similar pattern recognition rate as the ideal device owing to a more uniform conductance update.

Ionic liquid multistate resistive switching characteristics in two terminal soft and flexible discrete channels for neuromorphic computing

By exploiting ion transport phenomena in a soft and flexible discrete channel, liquid material conductance can be controlled by using an electrical input signal, which results in analog neuromorphic behavior. This paper proposes an ionic liquid (IL) multistate resistive switching device capable of mimicking synapse analog behavior by using IL BMIM FeCL4 and H2O into the two ends of a discrete polydimethylsiloxane (PDMS) channel. The spike rate-dependent plasticity (SRDP) and spike-timing-dependent plasticity (STDP) behavior are highly stable by modulating the input signal. Furthermore, the discrete channel device presents highly durable performance under mechanical bending and stretching. Using the obtained parameters from the proposed ionic liquid-based synaptic device, convolutional neural network simulation runs to an image recognition task, reaching an accuracy of 84%. The bending test of a device opens a new gateway for the future of soft and flexible brain-inspired neuromorphic computing systems for various shaped artificial intelligence applications.

Forming‐Free Pt/Ti/AlOx/CeOx/Pt Multilayer Memristors with Multistate and Synaptic Characteristics

Due to their unique electrical performance and simple structure, memristors exhibit excellent application prospects for future information technology. In this work, we fabricated Pt/Ti/AlOx/CeOx/Pt memristors demonstrating electroforming‐free bipolar resistive switching behavior with low operating voltage (−1 to 1 V), stable endurance, and retention. Space‐charge limited conduction (SCLC) as well as the formation and rupture of conductive filaments are responsible for the resistive switching behavior. Increasing the magnitude of the RESET voltage could generate multistate resistive switching. We studied the synaptic characteristics of the device by obtaining multilevel conductance states and investigating the relationship between the device resistance, pulse amplitude, pulse width, and pulse number. By applying programmed pre and postsynaptic spiking pulses, spike‐timing dependent plasticity was observed. This study shows that the device is suitable for multivalue storage and can be used as an electronic synapse device in artificial neural networks.

Soft and flexible: core-shell ionic liquid resistive memory for electronic synapses

The human brain is the most efficient computational and intelligent system, and researchers are trying to mimic the human brain using solid-state materials. However, the use of solid-state materials has a limitation due to the movement of neurotransmitters. Hence, soft memory devices are receiving tremendous attention for smooth neurotransmission due to the ion concentration polarization mechanism. This paper proposes a core-shell soft ionic liquid (IL)-resistive memory device for electronic synapses using Cu/Ag@AgCl/Cu with multistate resistive behavior. The presence of the Ag@AgCl core shell in the liquid electrolyte significantly helps to control the movement of Cu2+ ions, which results in multistate resistive switching behavior. The core-shell IL soft memory device can open a gateway for electronic synapses.

Resistive Switching Effect of Multiferroic Complex Oxide Solid Solution Thin Films

Resistive switching in multiferroics has attracted increasing attention due to the potential application for next-generation nonvolatile memory and could lead to forms of computing. However, the resistive switching mechanism in solid solution oxides is unclear. In this article, we have successfully fabricated binary 0.72BiTi0.27Fe0.46Mg0.27O3-0.28LaFeO3 (BTFM-LFO) and ternary 0.625BiTi0.27Fe0.46Mg0.27O3-0.25LaFeO3-0.125La2MgTiO6 (BTFM-LFO-LMT) thin films with precise component control by the spin coating method. The solid solution films exhibit obvious ferroelectricity and magnetism at room temperature. Both binary and ternary solid solution films show switchable polarization, weak magnetism, and reversible and repeatable resistive switching effects. Multistage resistive switching behavior was observed. Four resistive switching states were obtained in the binary film; the highest Ion/off is up to 106. The influence of the film composition on the resistive switching effect was discussed. It is considered that the oxide vacancy/valance exchange-induced defects play a dominant role in the resistive switching effect of complex oxide thin films. This work provides an alternative pathway to explore the resistive switching in multiferroic oxides by fabrication of complex solid solution films.

Electrocatalytic Hydrolysis‐Modulated Multistate Resistive Switching Behaviors in Memristors

Current rapid development of big data, Internet of Things, and artificial intelligence require exponentially higher data storage capacity. The memristor technology, which stores data by controlling resistance states, demonstrates great prospects in resistive random‐access memory (RRAM), synapse construction, and neuromorphic computing. However, traditional memristor devices can only store 1‐bit of data by tuning two separate resistance states, which limits their storage density. Herein, a water‐coupled Ag/TiO2_few‐layer graphene_TiO2/Al memristor is developed as a multibit data storage system. The high and low resistance state ratio (HRS/LRS) increases from 5 to 44 when water is coupled in the device. An electrocatalytic hydrolysis‐modulated resistive switching mechanism is proposed for the physical phenomenon. Herein, not only a multilevel per cell (MLC) storage device is developed, but also a novel electrocatalysis coupling mechanism for memristor technology is provided.

Multistate resistive switching behaviors for neuromorphic computing in memristor

Conventional Von Neumann computing systems encounter increasing challenges in the big-data era due to the constraints by the separated data storage and processing. Resistive random-access memory provides dual functionalities of data storage and computing at the same position without data transmission. This is one of the most promising candidates for energy efficient neuromorphic computing. The key points to realize neuromorphic computing are the selection of functional materials, the design of multistate devices, and a complete logic function implementing in-memory computing. Here, we demonstrate a memristor device, formed by Al/TiO2–few-layer Graphene–DNA/Pt layers, with stable intermediate multistate resistive switching behaviors. Asynchronous conduction by either oxygen vacancies migration or injected electron transfer is responsible for the multistate resistive switching behaviors. For neuromorphic computing, a pixel data stored and 2-bit parallel logic computations are simulated based on the multistate resistive switching behaviors. Compared with traditional memristor devices, this device can achieve theoretically double the data storage. This work provides a new horizon on the memristive memory and the complete logic hardware.

Excellent Bidirectional Adjustable Multistage Resistive Switching Memory in Bi2FeCrO6 Thin Film.

As an important method to further improve the storage density of resistive memory, multistage resistive switching devices have become an important research direction. However, no stable and controllable multistage resistive switching device has been prepared, and the working mechanism is still unclear. Here, a sandwich-structured device is simply prepared by spin coating, with the work layer is the Bi2FeCrO6 thin film. The device can realize bidirectional controllable multistage resistive switching behavior, the biggest on/off ratio is 104, and it can maintain stability without attenuation at 100 times slow loop and 104 times pulse cycle. The analyzes showed that the charged ions formed by defects in the device migrated under the action of an external electric field lead to the Schottky barrier height reversible changed. Which is the key to cause multistage resistive switching behavior. This work is the first report about the voltage control of bidirectional adjustable multistage resistive switching behavior in the Bi2FeCrO6 thin film. The principle of generation is analyzed, and important ideas and insights are provided for the preparation and treatment of related multistage resistive problems.

Electric-field-induced healing of inanimate topographies: Multistate resistive switching and nano-sized artificial synapse functionality

Formation and dissolution of conductive channels under the influence of applied electric field is widely considered key driver of resistive switching phenomenon, and vital for the emerging development of artificially intelligent synapses. Therefore, a comprehensive understanding of channel formation and knowledge of its real-time dynamics are crucial for conducting engineering at the device level. However, both concepts are poorly understood, especially for oxide-based devices. In this study, we demonstrate that inanimate topographies can be activated for charge transport in a NiO/ZnO heterojunction system by increasing the magnitude of the applied stress or by increasing its duration, which is responsible for multilevel memory storage. The formation and real-time dynamics of localized conducting channels (size > 35 nm) were revealed through current maps obtained using conductive atomic force microscopy. Additionally, bio-synaptic behaviors were mimicked at the nanoscale by applying electrical pulses. Furthermore, Kelvin probe force microscopy was used to confirm nanoscale variations in surface potential, which were attributed to localized defect variations and are responsible for spatial current variation. This study provides an essential breakthrough in enhancing our understanding of nanoscale conductive channel formation, and in doing so, it paves the way for the development of ultrahigh density nonvolatile storage memory applications.

Multistage resistive switching behavior organic coating films-based of memory devices

Memory devices were prepared by spin-coating process, and the multistage resistive switching behavior has been found in organic coating films-based of memory device. The composite of [6,6]-phenyl C61-butyric acid methyl ester and polyvinyl pyrrolidone were used as active layer materials. The as-prepared device exhibits a typical nonvolatile write-once-read-many- times storage effect, and shows multistate resistive switching behavior, and there are obvious distinctions between different resistance states. The resistance in low resistance state (LRS) and high resistance state (HRS) dependence on temperature is tested, and the resistance of LRS and HRS show metal and semiconductor characteristics, respectively. As well as, the resistance in HRS shows evident dependence on cell size. Hence, the resistive switching mechanism was attributed to the broken processes of carbon-rich conducting filament. Furthermore, conductance quantization during resistive switching processes was analyzed. This work might make it attractive for exploiting high density data storage.

Multistage Resistive Switching Processes in Filament Conduction-Based Organic Memory Devices

The broken processes of carbon-rich conducting filament have been clarified in composite of [6, 6]-phenyl C61-butyric acid methyl ester (PCBM) and poly(4-vinyl phenol) (PVP) based resistive memory devices. Here, the multistage resistive switching processes in Al/PCBM + PVP/Al sandwich construction were reported, which exhibits a typical nonvolatile WORM storage effect, and these several states are well separated by a resistance ratio of 1: (5.01 × 101): (2.08 × 102): (5.45 × 102): (2.42 × 103). The Al/PCBM + PVP/Al memory devices feature multistate data storage, long retention time (1 × 105 s), and large memory window (maximum ON/OFF ratio, ∼104). Furthermore, conductance quantization during resistive switching processes was analyzed. This work might promote and improve the understanding and awareness of resistive switching mechanism for organic-based memory devices.

Low Power and Ultrafast Multi-State Switching in nc-Al Induced Al2O3/AlxOy Bilayer Thin Film RRAM Device

Low power and ultrafast multi-state storage resistive switching memory (RRAM) device had been developed based on Al/Al 2 O 3 /AlxOy/Al structure. Both of Al 2 O 3 and Al nanocrystal (nc-Al) induced AlxOy thin films were deposited by RF sputtering. The nc-Al AlxOy based RRAM device showed typical unipolar switching behavior which was due to conductive filaments (CFs) connected and broke in Al 2 O 3 /AlxOy layers. An additional 30 nm Al 2 O 3 thin film would deposit on AlxOy film to form bi-layer structure, in which the multi-state switching could be observed by applying different voltage pulses on it. In this study, a 15 V pulse with 600 ps width could trigger RRAM device switch from high resistance state(HRS)to next intermediate resistance state (IRS), the device could finally switch to LRS after continuous pulse simulation. Such switching from HRS to LRS was called “writing” process as data would be stored in RRAM device after this process. A longer but lower amplitude voltage pulse was required to make device switch from LRS to HRS which was called “erasing” process, as data would be eliminated after this process. The multi-state switching was corresponding internal switching between these IRSs during “writing” and “erasing” process. The multi-level resistances might be caused by partially formed CFs in Al 2 O 3 /AlxOy layers. The distribution of CFs could be controlled by controlling the shape of pulse voltage to achieve this multi-state storage. This bilayer structured RRAM device had good endurance and retention performances at both room and high temperatures.

Resistive Switching in Atomic Layer Deposited HfO2/ZrO2 Nanolayer Stacks

Resistive switching materials and devices have received unparalleled recent attention due their potential and demonstrated use as resistive random access memory (RRAM) and neuromorphic applications. Resistive switching or memristive materials can serve as two-terminal electrical components where the resistance is a function of the amount and direction of current previously experienced and therefore they can uniquely emulate biological synapse behavior. HfO2 is a well-known dielectric and memristive material that has been previously explored for RRAM applications. Here, to better understand the resistive switching in harfnia in the presence of insulating interfaces, the electrical properties of HfO2/ZrO2 nanolayer stacks are discussed. The stacks with total thickness of 16 nm were prepared on platinized Si substrates using atomic layer deposition (ALD). A capping layer of TiN (30 nm thick) and a Au top electrode (100 nm thick) were sputtered to complete the device. We will discuss the current-voltage behavior, pulse time mode measurements, retention and endurance tests of the devices. Resistive switching was observed in all multilayer samples, with the set voltage (Vset) decreasing with increasing number of layers (i.e., number of hafnia-zirconia interfaces). We will show that grazing incidence x-ray diffraction (GI-XRD) demonstrated the transition of the hafnia phase from monoclinic to orthorhombic structure during the post metallization annealing, and that shifts in the binding energy of the x-ray photoelectron spectroscopy spectra (XPS) imply the existence of hafnia suboxide (HfO2-δ). More importantly, we will show how memristive/insulating nanostructures like nanolayered HfO2/ZrO2 can help modulate the resistive switching of memristive-based devices, to include for example, a two-step electroforming process, and multistate resistive switching.

Reversible transition of volatile to non-volatile resistive switching and compliance current-dependent multistate switching in IGZO/MnO RRAM devices

A controllable and reversible transition of volatile and non-volatile resistive switching is presented in Ag/indium-gallium-zinc oxide (IGZO)/manganese oxide (MnO)/Pt bilayer resistive memory devices. The coexistence of volatile and non-volatile switching characteristics was demonstrated by controlling the current compliance during the SET process. With lower current compliance (<50 μA), the formation of an unstable conducting filament presented typical diode-like non-volatile switching. A reversible transition from volatile to non-volatile switching could be obtained by applying a higher current compliance (≥50 μA). Moreover, highly uniform multistate memory characteristics were achieved by modulating the current compliance in the non-volatile switching region. The coexistence of volatile and multistate non-volatile resistive switching behaviors with a reversible transition demonstrates the capability of developing a selector element for crossbar arrays and the application for next generation multistate high-density storage with the same Ag/IGZO/MnO/Pt device.

Multistate resistive switching characteristics of ZnO nanoparticles embedded polyvinylphenol device

Multistate resistive switching device was demonstrated with organic/inorganic nanocomposite layer of polyvinylphenol (PVP) embedded with zinc oxide nanoparticles (ZnO NPs). Both cap (ITO/ZnO NPs+PVP/Al) and crossbar (Pt/ZnO NPs+PVP/Al) device formats were examined, respectively. Extending range of voltage sweep could show adaptive memory effect, which could roughly emulate analog resistive switching device. In addition, potentiation and depression behaviors were also examined with 50 time consecutive pulsed bias (+2.0V and −2.0V respectively) having 10ms width measured at read voltage of −0.2V. A new possibility of analog resistive switching can be further pursued with the ZnO NPs+PVP nanocomposite.

Resistance switching of La doped SrTiO3 single crystals

To date, there has not been a consensus about the resistance switching mechanism of donor-doped SrTiO3. The La doped STO (LaSTO) single crystal is a donor-doped material and has an N-type conductivity since La3+ could easily substitute Sr2+. In this study, the Pt/LaSTO/In memory device is fabricated based on (100) LaSTO single crystal with 0.5 wt% La doping. Through a series of electrical tests, it is found that the Pt/LaSTO/In memory device has a stable multi-stage resistive switching property, and the maximum switching ratio is 104. The fitting I-V curve at the high resistance state (HRS) shows that there is an interface barrier in the memory device. However, the fitting I-V curve at low resistance state (LRS) is consistent with the characteristic of the electron tunneling model. The spectrum of electron paramagnetic resonance (EPR) indicates that LaSTO single crystal has only one EPR signal of g=2.012. Considering the fact that g=gobs-ge (where gobs is the g factor obtained from the sample, ge=2.0023 is the free electron value) is positive, the signal can be regarded as being due to hole center. The hole center is positively charged and can trap electrons. Comprehensive analysis indicates that the transition between the HRS and LRS of the device can be explained by the modulation of Pt/LaSTO interface barrier, which is caused by the electron trapping and detrapping of interfacial vacancy defects. In addition, it is found that illumination could reduce the low resistance of the Pt/LaSTO/In device. This is due to the photo-generated carriers causing a tunneling current because of the narrow Schottky barrier when the Pt/LaSTO/In device is in the LRS. However, the Schottky barrier plays a leading role in HRS, so the change in carrier concentration, caused by illumination, does not lead to a significant change in current for HRS. The experimental results provide theoretical and technical guidance for the applications of LaSTO single crystals in resistive memory devices.

Microstructure and dynamics of vacancy-induced nanofilamentary switching network in donor doped SrTiO3−x memristors

Donor doping of perovskite oxides has emerged as an attractive technique to create high performance and low energy non-volatile analog memories. Here, we examine the origins of improved switching performance and stable multi-state resistive switching in Nb-doped oxygen-deficient amorphous SrTiO3 (Nb:a-STOx) metal–insulator–metal (MIM) devices. We probe the impact of substitutional dopants (i.e., Nb) in modulating the electronic structure and subsequent switching performance. Temperature stability and bias/time dependence of the switching behavior are used to ascertain the role of substitutional dopants and highlight their utility to modulate volatile and non-volatile behavior in a-STOx devices for adaptive and neuromorphic applications. We utilized a combination of transmission electron microscopy, photoluminescence emission properties, interfacial compositional evaluation, and activation energy measurements to investigate the microstructure of the nanofilamentary network responsible for switching. These results provide important insights into understanding mechanisms that govern the performance of donor-doped perovskite oxide-based memristive devices.