Stable Bipolar Resistive Switching Research Articles

Investigation of Grain Boundary Effects in Sm0.2Ce0.8O2-x Thin Film Memristors.

Cerium-based materials (CeO2-x) are of significant interest in the development of vacancy-modulated resistive switching (RS) memory devices. However, the influence of grain boundaries on the performance of memristors is very limited. To fill this gap, this study explores the influence of grain boundaries in cerium-based thin film resistive random-access memory (RRAM) devices. Sm0.2Ce0.8O2-x (SDC20) thin films were deposited on (100)-oriented Nb-doped SrTiO3 (NSTO) and (110)-oriented NSTO substrates using pulsed laser deposition (PLD). Devices constructed with a Pt/SDC20/NSTO structure exhibited reversible and stable bipolar resistive switching (RS) behavior. The differences in conduction mechanisms between single-crystal and polycrystalline devices were confirmed, with single-crystal devices displaying a larger resistance window and higher stability. Combining the results of XPS and I-V curve fitting, it was confirmed that defects near the grain boundaries in the SDC-based memristors capture electrons, thereby affecting the overall performance of the RRAM devices.

Nonvolatile bipolar resistive switching characteristics of aluminum oxide grown by thermal oxidation processes

This study proposes a bipolar resistive random-access memory (RRAM), which is fabricated using an aluminum oxide (AlO x ) resistive switching (RS) layer. The RRAM shows a large memory window of 106 at a low read voltage of 0.5 V. In addition, high switching speed, long retention time, and superior read-disturb immunity are observed. AlO x layers are prepared by a thermal oxidation growth process. Aluminum metal films deposited on n+-Si wafers are oxidized at O2/(O2 + N2) flow rate ratios of 50%–100%. Al/AlO x /n+-Si device shows no RS behavior when the AlO x is grown in a pure O2 environment. As the O2/(O2 + N2) flow rate ratio decreases to 50%, Al/AlO x :N/n+-Si device reveals stable bipolar RS characteristics. A filamentary mode based on oxygen interstitial and Al vacancy is proposed to explain the difference in electrical characteristics of AlO x devices prepared at different O2 flow rates.

A bioinspired MXene-based flexible sensory neuron for tactile near-sensor computing

Multifunctional near-sensor computing sensory neurons based on the integration of flexible sensors and memristors hold promise to achieve unprecedented development in large-scale neuromorphic perception circuits and human-machine interaction systems. However, the traditional sensory-memory integrated systems are based on the integration of different device modules, which are subjected to redundant analog-to-digital conversion circuits and poor flexibility. Here, an all MXene-based flexible sensory neuron fabricated by a room temperature energy-efficient, and controllable oxidation preparation technique is reported. The near-sensor computing system is seamlessly integrated a MXene-based pressure sensor array for acquisition of tactile signals with a flexible MXene-based memristor array for simulating the implementation of perceived behavior. The tactile sensor array shows a wide range (0.1–100 kPa), high linear sensitivity (23.9 kPa−1), and the memristor array exhibits repeatable and stable bipolar resistive switching behavior (retention >100 cycles, endurance >103 s) and excellent flexibility (R=1.0 mm). As a conceptual demonstration, our devices realize high-precision recognition of handwritten digit recognition (93.21%) and human motion (97.96%), which demonstrate the feasibility for real-time health monitoring based on signals collected by sensors and artificial neural networks constructed by the memristors. The MXene-based sensor-memristor system offers a prominent contribution towards advanced application of intelligence in electronic skin and wearable electronic equipment.

2D MoS2 monolayers integration with metal oxide-based artificial synapses

In this study, we report on a memristive device structure wherein monolayers of two-dimensional (2D) molybdenum disulfide (MoS2) are integrated with an ultrathin yttrium oxide (Y2O3) layer to simulate artificial synapses functionality. The proposed physical simulation methodology is implemented in COMSOL Multiphysics tool and is based on the minimization of free energy of the used materials at the applied input voltage. The simulated device exhibits a stable bipolar resistive switching and the switching voltages is significantly reduced by increasing the number of MoS2 layers, which is key to conventional low-power computing and neuromorphic applications. The device is shown to perform synaptic functionalities under various applied bias conditions. The resulting synaptic weight decreases almost linearly with the increasing number of MoS2 layers due to the increase in the device thickness. The simulation outcomes pave the way for the development of optimised metal oxide-based memristive devices through their integration with semiconducting 2D materials. Also, the 2D MoS2 integration can enable the optoelectronic operation of this memory device.

Artificial Tactile Sensing Neuron with Tactile Sensing Ability Based on a Chitosan Memristor.

Owing to the highly parallel network structure of the biological neural network and its triggered processing mode, tactile sensory neurons can realize the perception of external signals and the functions of perception, memory, and data processing by adjusting the synaptic weight. In this paper, a piezoresistive pressure sensor is combined with a memristor to design an artificial tactile sensory neuron. The polyurethane sponge sensor has excellent sensitivity and can convert physical stimuli into electrical signals, and the chitosan-based memristor has stable bipolar resistive switching characteristics, allowing further information to be memorized and processed. The neuron can respond to tactile stimuli of different degrees, durations, and frequencies; realize potentiation/depression modulation, paired-pulse facilitation, and spike-timing-dependent plasticity; exhibit spike-rate-dependent plasticity; and store and erase tactile information through memistor state switching, which has great application potential in biological sensing systems.

Transmission Mechanism and Logical Operation of Graphene-Doped Poly(vinyl alcohol) Composite-Based Memristor.

Memristors are considered the best candidates for nonvolatile memory and advanced computing technologies, and polymer and two-dimensional (2D) materials have been developed as functional layer materials in memristors with high-performance resistive switching characteristics. In this work, a polymer memristor with a graphene (Gr)-doped poly(vinyl alcohol) (PVA) composite acting as the functional layer was prepared. The memristor device exhibited superior performance with good retention and a comparatively large ON/OFF ratio at room temperature. Additionally, excellent logic operations were achieved. These satisfactory properties can be attributed to trap-induced carrier trapping and detrapping. In addition, the device exhibited stable bipolar resistive switching behavior over a moderate temperature range. This work provides insight into the transmission mechanism of polymer-based memristors and the reasons why they become unstable at high temperatures, demonstrating the potential applications of PVA-Gr-based polymer memristors as logic circuit units in integrated chips and artificial intelligence.

Ferroelectric Resistance Switching in Epitaxial BiFeO3/La0.7Sr0.3MnO3 Heterostructures.

BiFeO3/La0.7Sr0.3MnO3 (BFO/LSMO) epitaxial heterostructures were successfully synthesized by pulsed laser deposition on (001)-oriented SrTiO3 single-crystal substrates with Au top electrodes. Stable bipolar resistive switching characteristics regulated by ferroelectric polarization reversal was observed in the Au/BFO/LSMO heterostructures. The conduction mechanism was revealed to follow the Schottky emission model, and the Schottky barriers in high-resistance and low-resistance states were estimated based on temperature-dependent current-voltage curves. Further, the observed memristive behavior was interpreted via the modulation effect on the depletion region width and the Schottky barrier height caused by ferroelectric polarization reversal, combining with the oxygen vacancies migration near the BFO/LSMO interface.

Implementation of Stable Nonvolatile Resistive Switching Behaviors in TiO2 Nanoparticle‐Incorporated 2D Layered Halide Perovskite‐Based Devices

2D organic–inorganic layered halide perovskites have received tremendous interest as promising materials in electronics owing to their outstanding physical properties. However, the quantum confinement effect leads to the suppression of ion migration in the out‐of‐plane direction, restricting their applications in resistive switching (RS) memories with vertical structure. Herein, the 2D layered perovskite incorporated with TiO2 nanoparticles, denoted as TiO2–PEA2PbBr4, is employed, as the functional layers to construct RS memory devices with Al/TiO2–PEA2PbBr4/ITO structure. As compared to pure 2D layered perovskite counterparts without switching behaviors, the Al/TiO2–PEA2PbBr4/ITO memories demonstrate stable bipolar RS behaviors, along with striking characteristics, including the relatively centralized set/reset voltages, a large on/off ratio (≈102), and a long retention time (104 s). In addition, the devices also present the multilevel storage capability by controlling the compliance current. Moreover, the RS behaviors are attributable to the formation/rupture of synergy bromine and oxygen vacancy conductive filaments, owing to their migration/distribution of oxygen/bromine ions under electric fields. The oxide nanoparticles incorporation route provides an effective strategy for improving RS performance in 2D layered perovskite‐based RS memories.

Organic-2D composite material-based RRAM with high reliability for mimicking synaptic behavior

The field of artificial intelligence and neural computing has been rapidly expanding due to the implementation of resistive random-access memory (RRAM) based artificial synaptic. However, the low flexibility of conventional RRAM materials hinders their ability to mimic synaptic behavior accurately. To overcome such limitation, organic-2D composites with high mechanical properties are proposed as the active layer of RRAM. Moreover, we enhance the reliability of the device by ZrO2 insertion layer, resulting in stable synaptic performance. The Ag/PVA:h-BN/ZrO2/ITO devices show stable bipolar resistive switching behavior with an ON/OFF ratio of over 5 × 102, a ∼2 400 cycles endurance and a long retention time (>6 × 103s), which are essential for the development of high-performance RRAMs. We also study the possible synaptic mechanism and dynamic plasticity of the memory device, observing the transition from short-term potentiation (STP) to long-term potentiation (LTP) under the effect of continuous voltage pulses. Moreover, the device exhibits both long-term depression (LTD) and paired-pulse facilitation (PPF) properties, which have significant implications for the design of organic-2D composite material RRAMs that aim to mimic biological synapses, representing promising avenues for the development of advanced neuromorphic computing systems.

Role of Defects in the Breakdown Phenomenon of Al1-xScxN: From Ferroelectric to Filamentary Resistive Switching.

Aluminum scandium nitride (Al1-xScxN), with its large remanent polarization, is an attractive material for high-density ferroelectric random-access memories. However, the cycling endurance of Al1-xScxN ferroelectric capacitors is far below what can be achieved in other ferroelectric materials. Understanding the nature and dynamics of the breakdown mechanism is of the utmost importance for improving memory reliability. The breakdown phenomenon in ferroelectric Al1-xScxN is proposed to be an impulse thermal filamentary-driven process along preferential defective pathways. For the first time, stable and robust bipolar filamentary resistive switching in ferroelectric Al1-xScxN is reported. A hot atom damage defect generation model illustrates how filament formation and ferroelectric switching are connected. The model reveals the tendency of the ferroelectric wurtzite-type Al1-xScxN system to reach internal symmetry with bipolar electric field cycling. Defects generated from bipolar electric field cycling influence both the energy barrier between the polarization states and that required for the filament formation.

Neotype kuramite optoelectronic memristor for bio-synaptic plasticity simulations.

Memristive devices with both electrically and optically induced synaptic dynamic behaviors will be crucial to the accomplishment of brain-inspired neuromorphic computing systems, in which the resistive materials and device architectures are two of the most important cornerstones, but still under challenge. Herein, kuramite Cu3SnS4 is newly introduced into poly-methacrylate as the switching medium to construct memristive devices, and the expected high-performance bio-mimicry of diverse optoelectronic synaptic plasticity is demonstrated. In addition to the excellent basic performances, such as stable bipolar resistive switching with On/Off ratio of ∼486, Set/Reset voltage of ∼-0.88/+0.96V, and good retention feature of up to 104s, the new designs of memristors possess not only the multi-level controllable resistive-switching memory property but also the capability of mimicking optoelectronic synaptic plasticity, including electrically and visible/near-infrared light-induced excitatory postsynaptic currents, short-/long-term memory, spike-timing-dependent plasticity, long-term plasticity/depression, short-term plasticity, paired-pulse facilitation, and "learning-forgetting-learning" behavior as well. Predictably, as a new class of switching medium material, such proposed kuramite-based artificial optoelectronic synaptic device has great potential to be applied to construct neuromorphic architectures in simulating human brain functions.

Nanosecond Memristor Based on Oxygen Vacancy Engineering in SrTiO3 Single Crystal

AbstractMemristors are of great significance for the construction of memory devices and neuromorphological computing, which need a fast response time for ultrahigh data storage, ultrafast information processing, and efficient neuromorphic computing. Currently, memristors based on perovskite oxide materials generally have a long response time. In this paper, the electroforming‐free memristive behavior based on Ar+ irradiating induced oxygen vacancy modulation in SrTiO3 (STO) has been investigated. The Ag/STO/(Ta/Pt) device shows a stable bipolar resistive switching (RS) characteristic and can realize ultrafast multi‐resistance switching behavior. More importantly, its conductance can be continuously controlled with short pulse on the nanosecond scale to simulate potentiation and depression of synapse, promising potential application in ultra‐fast biological synapse and high‐efficiency training of neural networks. More importantly, the defect engineering strategy used in this work provides more freedom for the design and optimization of versatile memristor devices.

Resistive switching characteristics of HfO<sub><i>x</i></sub>-based resistance random access memory under photoelectric synergistic regulation

Cu/HfO<sub><i>x</i></sub>/Pt and Cu/HfO<sub><i>x</i></sub>-ZnO/Pt resistance random access memory (RRAM) devices are prepared by magnetron sputtering. The results show that the Cu/HfO<sub><i>x</i></sub>/Pt device has the stable bipolar resistive switching characteristics, good retention (as long as 10<sup>4</sup> s), and a switching ratio greater than 10<sup>3</sup>. The current conduction mechanism of HfO<sub><i>x</i></sub> device is ohmic conduction at low resistance, while space charge limited current (SCLC) mechanism dominates at high resistance, and the conductive filament is composed of oxygen vacancies. Owing to the low content and random distribution of oxygen defects in the HfO<sub><i>x</i></sub> film, the endurance and uniformity of the device are poor. Compared with HfO<sub><i>x</i></sub> device, HfO<sub><i>x</i></sub>-ZnO device exhibits lower operating voltage and better uniformity and stability. The main reason is that ZnO material has smaller formation energy of oxygen vacancy, which can produce more oxygen defects under electric field to participate in the resistive switching behavior of the device, thereby reducing the operating voltage and improving the uniformity of the device. In addition, owing to the existence of the interface between HfO<sub><i>x</i></sub> and ZnO film, the random distribution of oxygen defects is inhibited, that is, the random fracture and formation of conductive filament are inhibited, which is beneficial to improving the uniformity of the device. In addition, the resistive switching behaviors of Cu/HfO<sub><i>x</i></sub>/Pt and Cu/HfO<sub><i>x</i></sub>-ZnO/Pt RRAM devices under different intensities of 255 nm ultraviolet illumination are studied. For Cu/HfO<sub><i>x</i></sub>/Pt device, the light of 255 nm wavelength shows little effect on its resistive switching characteristics. For the Cu/HfO<sub><i>x</i></sub>-ZnO/Pt RRAM device, the operating voltage and stability of the device can be improved by increasing the light intensity. Although the switching ratio of the device decreases with the increase of light intensity, the device can exhibit multiple resistance states by adjusting different light intensities to achieve multi-level storage. Finally, the analysis of the <i>I</i>-<i>V</i> curves of the devices indicates that the two types of devices show similar resistive switching mechanisms under the illumination of light or no light, which can be explained by the resistive switching mechanism of oxygen vacancy conductive filament. Therefore, a physical model based on the oxygen vacancy conductive filament is established to explain the resistive switching behavior of the device in this paper.

Forming-free plant resistive random access memory based on the Coulomb blockade effect produced by gold nanoparticles.

Resistive random access memory has been widely studied as a powerful candidate for building future memories and realizing high-efficiency artificial neuromorphic systems. In this paper, gold nanoparticles (Au NPs) are doped in a leaf solution of Scindapsus aureus (SA) as the active layer to fabricate Al/SA:Au NPs/ITO/glass resistive random access memory. The device exhibits stable bipolar resistance switching characteristics. More importantly, the device's multilevel storage capabilities and synaptic potentiation and depression behaviour have been proven. Compared with the device without doped Au NPs in the active layer, the device has a higher ON/OFF current ratio, which is attributed to the Coulomb blockade effect caused by the Au NPs. The device plays an important role in realizing high-density memory and efficient artificial neuromorphic systems.

Non-Volatile Memory and Synaptic Characteristics of TiN/CeOx/Pt RRAM Devices.

In this study, we investigate the synaptic characteristics and the non-volatile memory characteristics of TiN/CeOx/Pt RRAM devices for a neuromorphic system. The thickness and chemical properties of the CeOx are confirmed through TEM, EDS, and XPS analysis. A lot of oxygen vacancies (ions) in CeOx film enhance resistive switching. The stable bipolar resistive switching characteristics, endurance cycling (>100 cycles), and non-volatile properties in the retention test (>10,000 s) are assessed through DC sweep. The filamentary switching model and Schottky emission-based conduction model are presented for TiN/CeOx/Pt RRAM devices in the LRS and HRS. The compliance current (1~5 mA) and reset stop voltage (-1.3~-2.2 V) are used in the set and reset processes, respectively, to implement multi-level cell (MLC) in DC sweep mode. Based on neural activity, a neuromorphic system is performed by electrical stimulation. Accordingly, the pulse responses achieve longer endurance cycling (>10,000 cycles), MLC (potentiation and depression), spike-timing dependent plasticity (STDP), and excitatory postsynaptic current (EPSC) to mimic synapse using TiN/CeOx/Pt RRAM devices.

MoTe2-based low energy consumption artificial synapse for neuromorphic behavior and decimal arithmetic

Two-dimensional material-based memristors have shown attractive research prospects as brain-like devices for neuromorphic computing. Among them, transition metal dichalcogenides–based memristors have proved to be one of the most promising competitors. In this work, a two-dimensional memristor based on MoTe2 nanosheets was fabricated and demonstrated. The experimental results illustrate that the two-terminal synaptic based on the Ag/MoTe2/ITO structure exhibits stable bipolar and non-volatile resistive switching characteristics attributed to the controllable formation and rupturing of silver conductive filaments. The device can be successively modulated by a pulse train with a minimum pulse width of 40 ns. More interestingly, the energy consumption of the device to complete one write event is only 74.2 pJ. In addition, biological synaptic behaviors, such as excitatory postsynaptic current gain properties, long-term potentiation (LTP)/long-term depression, spike-timing-dependent- plasticity, short-term plasticity, long-term potentiation (LTP), paired-pulse facilitation, post-tetanic potentiation, and learning-experimental behaviors were mimicked faithfully. Finally, the decimal arithmetic application was introduced to the device, and it is confirmed that addition and multiplication functions can be performed. Therefore, the artificial synapse based on MoTe2 nanosheets not only exhibits the stable non-volatile resistive switching behavior but also facilitates the development of low-energy consumption neuromorphic computing chips based on transition metal dichalcogenides.

A memristor based on two-dimensional MoSe2/MoS2 heterojunction for synaptic device application

Two-dimensional materials are promising for exploring memristors with excellent performance. However, the memristor still faces challenges in insufficient reliability due to resistance-switching mechanisms of conductive filaments. In this work, a typical metal/heterojunction/metal structure (Ag/MoSe2/MoS2/Au/Ti) was proposed as the device architecture of the memristor. The device exhibits stable bipolar resistive switching behavior with a high on–off ratio, long retention time, and good endurance. The resistance-switching is achieved by adjusting the interface barrier of the MoSe2/MoS2 due to the band modulation. Moreover, we explore the essential synaptic functions of this memristor device, with outstanding voltage pulse potentiation and depression. Our work highlights the significant prospects of MoSe2/MoS2 heterojunction in artificial synapses and neural networks.

Improved Performance of the Al2O3-Protected HfO2-TiO2 Base Layer with a Self-Assembled CH3NH3PbI3 Heterostructure for Extremely Low Operating Voltage and Stable Filament Formation in Nonvolatile Resistive Switching Memory.

Herein, we report intriguing observations of an extremely stable nonvolatile bipolar resistive switching (NVBRS) memory device fabricated using HfO2-TiO2 topologically protected by Al2O3 as a stacked base layer for a CH3NH3PbI3 (MAPI) electrolyte layer sandwiched between Ag and fluorine-doped tin oxide (FTO) electrodes. MAPI has been successfully synthesized by a rapid microwave-solvothermal (MW-ST) method within 10 min at 120 °C without requiring any inert gas atmosphere using low-cost precursors and solvents. Subsequently, MAPI powders are dissolved in aprotic solvents (DMF/DMSO = 8:2), and a spin-coated thin film is allowed to recrystallize upon annealing at 120 °C via a solution-based nanoscale self-assembly process. The fabricated memory device with the Ag/MAPI/Al2O3/TiO2-HfO2/FTO configuration shows an enhanced resistance ratio of 105 for >104 s at an extremely lower operating voltage (SET +0.2 V, RESET -0.2 V) when compared to that of the pristine MAPI device (±1 V, 102, 104 s). We show that the memory device also exhibits a remarkable endurance of ≥3500 cycles due to the Al2O3 robust coating on the HfO2-TiO2 layer, facilitating prompt heterojunction formation. Thus, the adopted innovative strategies to prepare structurally and optically stable (∼1.5 years) MAPI under high-humid conditions could offer enhanced performance of NVBRS memory devices for medical, security, internet of things (IoT), and artificial intelligence (AI) applications.

Resistive Switching and Synaptic Behavior of Perovskite Lanthanum Orthoferrite Thin Film for Neuromorphic Computing

A resistance random access memory (RRAM), based on a metal oxide thin film with resistive switching behavior, has been explored as an emerging candidate for their application as nonvolatile memories, due to their various advantages, such as simple device configuration, long data retention, high switching speed, and low operating voltage. Various metal oxides have been explored for resistive switching applications including, e.g., binary and ternary compounds. Among all metal oxides, the perovskites have attracted considerable interest due to their potential to be used for information storage and neuromorphic application. In this work, we demonstrate stable bipolar resistive switching devices based on the sputtered LaFeO3 thin film on fluorine doped tin oxide (FTO)-coated glass with circular-shaped silver contacts. The memory performance of fabricated devices was characterized as a function of the thickness of the LFO thin layer. The resistive switching properties are investigated using macroscopic <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${I}$ </tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}$ </tex-math></inline-formula> measurements, showing low-voltage switching with a high ON–OFF ratio ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\approx 300$ </tex-math></inline-formula> ) and long retention (≥9000 s). The fabricated devices demonstrate the stable, low voltage and high-speed switching. Furthermore, in this work, we demonstrate the synaptic behavior of the LFO thin-film memory devices, as it exhibits analog memory characteristics, potentiation, and depression.

Multiphotoconductance Levels of the Organic Semiconductor of Polyimide-Based Memristor Induced by Interface Charges.

A memristor with Au/polyimide (PI)/Au structure is prepared by magnetron sputtering to investigate the multiphotoconductance resistive switching (RS) memory behavior. The PI-based memristor presents stable bipolar RS memory and is sensitive to visible light. Four discrete conductance states in both high-resistance state (HRS) and low-resistance state (LRS) are obtained when illuminating by 365, 550, 590, and 780 nm light. Electron trapping and detrapping from the defects distributed at interfaces and the PI switching layer are responsible for the observed RS memory behavior. The enhanced trapping and detrapping process by light illumination is responsible for the multiconductance states. This work provides the possibility for further development of neuromorphic vision sensors using an organic semiconductor-based memristor.