Nonvolatile Resistive Random Access Memory Research Articles

Non-volatile memory based in-memory computing technology

By integrating the storage and computing functions on the fundamental elements, computing in-memory (CIM) technology is widely considered as a novel computational paradigm that can break the bottleneck of Von Neumann architecture. Nonvolatile memory device is an appropriate hardware implementation approach of CIM, which possess significantly advantages, such as excellent scalability, low consumption, and versatility. In this paper, first we introduce the basic concept of CIM, including the technical background and technical characteristics. Then, we review the traditional and novel nonvolatile memory devices, flash and resistive random access memory (RRAM), used in non-volatile based computing in-memory (nvCIM) system. After that, we explain the operation modes of nvCIM: in-memory analog computing and in-memory digital computing. In addition, the applications of nvCIM are also discussed, including deep learning accelerator, neuromorphic computing, and stateful logic. Finally, we summarize the current research advances in nvCIM and provide an outlook on possible research directions in the future.

Oscillatory Circuits With a Real Non-Volatile Stanford Memristor Model

Stanford memristor model is a widely used model that accurately characterizes real non-volatile metal-oxide resistive random access memory (RRAM) devices with bipolar switching characteristics. The paper studies for the first time the dynamics and bifurcations in a class of nonlinear oscillators with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">real non-volatile memristor devices</i> obeying Stanford model. This is in contrast with papers in the literature considering oscillators with ideal, abstract, or artificial memristor models, that are unable to describe physical memristors implemented in nanotechnology. One main new idea in the paper is to use the memristor as a <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">programmable nonlinear resistor</i> . Namely, two principal modes of operation are considered. 1) Analogue transient phase: the oscillator is designed so that in the transient oscillations the voltage on the memristor is below threshold, hence the main memristor state variable, i.e., the gap of the insulating material, is almost constant and the memristor behaves as a static nonlinear resistor. 2) Programming phase: the nonlinear characteristic of the memristor, which depends on the gap, can be changed via the application of voltages above threshold. The paper studies nonlinear oscillations in the transient phase for a fixed gap as well as the bifurcations phenomena displayed when the gap is varied. The paper also discusses the differences between the approach in the paper and those to design other memristor oscillators with non-volatile memristors.

Modeling of Gate Tunable Synaptic Device for Neuromorphic Applications

The emerging memories are great candidates to establish neuromorphic computing challenging non-Von Neumann architecture. Emerging non-volatile resistive random-access memory (RRAM) attracted abundant attention recently for its low power consumption and high storage density. Up to now, research regarding the tunability of the On/Off ratio and the switching window of RRAM devices remains scarce. In this work, the underlying mechanisms related to gate tunable RRAMs are investigated. The principle of such a device consists of controlling the filament evolution in the resistive layer using graphene and an electric field. A physics-based stochastic simulation was employed to reveal the mechanisms that link the filament size and the growth speed to the back-gate bias. The simulations demonstrate the influence of the negative gate voltage on the device current which in turn leads to better characteristics for neuromorphic computing applications. Moreover, a high accuracy (94.7%) neural network for handwritten character digit classification has been realized using the 1-transistor 1-memristor (1T1R) crossbar cell structure and our stochastic simulation method, which demonstrate the optimization of gate tunable synaptic device.

Polymerized hybrid Hf-based hydroquinone/Al2O3 bilayer structure by molecular/atomic layer deposition for non-volatile resistive random access memory

In this work, we fabricated the Pt/Hf-based hydroquinone (Hf-HQ)/Al2O3/TiN/Si bilayer hybrid memory by molecular layer deposition/atomic layer deposition. The hybrid memory units exhibit electroforming-free bipolar resistive switching (RS) characteristics with tiny fluctuation of operation voltages within 0.2 V, ON/OFF ratio above 102, and good endurance/retention properties. Meanwhile, the multi-state data storage capability is confirmed in hybrid devices. The RS mechanism based on conducting filaments has been proposed. The favorable linkage and rupture of the conducting filament prefer to occur at the interface of the hybrid Hf-HQ layer and Al2O3 layer, resulting in the brilliant performances. Furthermore, flexible hybrid memory devices fabricated on bendable mica show comparable RS behaviors to the Si-based ones at the bending radius of 7.5 mm, indicative of great potential in flexible multilevel resistive random access memory applications.

Resistive Switching in HfO2-x/La0.67Sr0.33MnO3 Heterostructures: An Intriguing Case of Low H-Field Susceptibility of an E-Field Controlled Active Interface.

High-performance nonvolatile resistive random access memories (ReRAMs) and their small stimuli control are of immense interest for high-speed computation and big-data processing in the emerging Internet of Things (IoT) arena. Here, we examine the resistive switching (RS) behavior in growth-controlled HfO2/La0.67Sr0.33MnO3 (LSMO) heterostructures and their tunability in a low magnetic field. It is demonstrated that oxygen-deficient HfO2 films show bipolar switching with a high on/off ratio, stable retention, as well as good endurance owing to the orthorhombic-rich phase constitution and charge (de)trapping-enabled Schottky-type conduction. Most importantly, we have demonstrated that RS can be tuned by a very low externally applied magnetic field (∼0-30 mT). Remarkably, application of a magnetic field of 30 mT causes RS to be fully quenched and frozen in the high resistive state (HRS) even after the removal of the magnetic field. However, the quenched state could be resurrected by applying a higher bias voltage than the one for initial switching. This is argued to be a consequence of the electronically and ionically "active" nature of the HfO2-x/LSMO interface on both sides and its susceptibility to the electric and low magnetic field effects. This result could pave the way for new designs of interface-engineered high-performance oxitronic ReRAM devices.

Correlation between morphology and resistive switching behaviour of WO3 nanostructures

In recent years, the non-volatile resistive random-access memory (ReRAM) has been investigated for next-generation memory devices to fulfill the limitations of commercial transistor-based memory devices. Every now and then, peculiar nanostructures with unique characteristics are reported. However, the influence of distinct nanostructures of a same metal oxide material on the resistive switching performance are not yet surveyed in detail. In this work, two structurally distinct WO3 nanostructures, namely nanoplates and nanocubes, were synthesized through hydrothermal method. The crystalline nature, morphology, atomic composition, and chemical states of WO3 nanostructures were characterized effectively. The fabricated WO3 ReRAM devices were exhibited bipolar resistive switching and the performance of nanoplate and nanocube devices are distinguished in detail.

Polymer-based non-volatile resistive random-access memory device fabrication with multi-level switching and negative differential resistance state

Resistive random-access memory (RRAM) has been widely considered for its prospective applicability owing to its non-volatile characteristics. In this study, a polymer-based vacuum-free RRAM device fabricated with the conductive polymer, poly(3,4-ethylene-dioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) was proposed. Pristine PEDOT:PSS coated on indium tin oxide (ITO) electrode was used as the active layer, while PEDOT:PSS with 16 vol% ethylene glycol was added for the top electrode. The PEDOT:PSS-based RRAM device demonstrated controlled non-volatile bipolar switching and a good ON/OFF ratio with a negative differential resistance effect in the high-voltage range during the RESET process. Multi-level switching was also accomplished by controlling the voltage, which demonstrated reliable and non-volatile switching. The switching mechanism of this polymer RRAM device can be explained through the electrochemical filamentary formation as well as the current-induced phase segregation of PEDOT:PSS near the anode(ITO)/polymer interface. • Vacuum-free and solution-processed fabrication method of a polymer based RRAM device. • Low cost and low temperature (<120 °C) fabrication. • MLC switching during SET process and NDR effect during RSEST process. • Investigation of detailed switching mechanism from literature survey and I–V curve fitting.

Switchable domains in point contacts based on transition metal tellurides

We report resistive switching in voltage biased point contacts (PCs) based on series of van der Waals transition metals tellurides (TMTs) such as MeTe2 (Me=Mo, W) and TaMeTe4 (Me= Ru, Rh, Ir). The switching occurs between a low resistive "metallic-type" state, which is the ground state, and a high resistive "semiconducting-type" state by applying certain bias voltage (<1V), while reverse switching takes place by applying voltage of opposite polarity. The origin of the effect can be formation of domain in PC core by applying a bias voltage, when a strong electric field (about 10kV/cm) modifies the crystal structure and controls its polarization. In addition to the discovery of the switching effect in PCs, we also suggest a simple method of material testing before functionalizing them, which offers a great advantage in finding suitable novel substances. The new functionality of studied TMTs arising from switchable domains in submicron hetero-structures that are promising, e.g., for non-volatile resistive random access memory (RRAM) engineering.

Tellurium vacancy in two-dimensional Si2Te3 for resistive random-access memory

2D materials show exciting prospects in the application of nonvolatile resistive random-access memory (RRAM). Here, by using first-principles calculations based on density functional theory, we demonstrate that the experimentally observed resistive switching behavior in layered Si2Te3 could arise from the formation and diffusion of Te vacancies (VTe). Specifically, we find that VTe0 and VTe2− are the most stable vacancies under both Te-rich/poor growing conditions. Moreover, the interlayer and intralayer diffusion energy barriers of VTe are analyzed by the climbing image nudged elastic band (cNEB) calculation, the results show that VTe with −2 charge is easier to mobile through the lattice. In order to investigate the possibility of effective tuning of the properties of VTe, we apply uniaxial strains along a or b axis of Si2Te3. It is found that the formation energy and diffusion energy barrier of VTe varies upon tensile or compressive strains, pointing out a feasible way to further promote the mobility of Te vacancies in Si2Te3.

Light‐Assisted Resistive Switching Memory Device with Reduced SET/RESET Voltage Using Sol–Gel TiO2 on Al‐Doped ZnO Electrode

Herein, light‐assisted resistive switching (RS) characteristic of low‐cost sol–gel TiO2 film‐based metal–insulator–metal (MIM) configuration using Al‐doped ZnO (AZO) as the bottom electrode is demonstrated. Unprecedentedly, instead of the costly transparent conducting electrode such as tin‐doped indium oxide, AZO electrode is exploited in this study due to its dual role as a transparent and an additional oxygen vacancy (VO)‐rich electrode. The Al/TiO2/AZO device shows electroforming free bipolar RS characteristics, which can be explained by the formation (rupture) of conducting filaments composed of VO. The memory window is increased three times under the blue light illumination compared to dark conditions. Interestingly, the SET and RESET voltages are decreased from 5.2 to 4.5 V and −3.9 to −3.3 V, respectively, under the blue light illumination with respect to the dark condition. The enhancement in the RS effect under the blue light illumination is explained in terms of the photoresponsive properties of TiO2. The MIM device exhibits good switching repeatability with stable retention characteristics, making them attractive for next‐generation nonvolatile resistive random access memory devices.

Inkjet printed BiFeO3 thin films with non-volatile resistive switching behaviors

BiFeO3 (BFO) is the only material known to have both magnetism and strong ferroelectricity at room temperature. BFO inks prepared by the sol-gel method are deposited on FTO substrates, dense and stable BFO thin films are formed by multiple printing and pyrolysis processes. The effects of the inkjet voltage on surface and structure properties are characterized, and obvious bipolar resistive switching characteristics are obtained in Al/BFO/FTO devices. The resistance switching mechanism is primarily explained by the space charge limited conduction mechanism controlled by local traps. The switching between high resistance state and low resistance state is reproducible, reversible, and controllable. This result may enhance the application of BFO-based composites in non-volatile resistive random access memory.

Effect of Light and Heat on Polymer‐Based Resistive Random Access Memory

Highly repeatable, photosensitive nonvolatile resistive random access memory (ReRAM) made of a Si/SiO2/Cu/PRPC/Ta stack with a photoresponsive polymer composite (PRPC) as the active medium is presented. The device shows errorless bipolar resistive switching with more than 100 cycles repeatability and stable SET and RESET voltages. It changes its resistive states in response to white light and recovers completely after removal of the light source. It also changes its state in response to heat and recovers randomly after the removal of the heat source. The phenomenon is explained by valance charge transfer along with a space‐charge‐limited conduction (SCLC) mechanism. The initial state of the device is low resistance state (LRS) because of electric‐field‐induced formation of valance charge transfer states. It switches to a high resistance state (HRS) due to electric‐field‐induced dissociation of the charge transfer states.

Resistive Switching of the Tetraindolyl Derivative in Ultrathin Films: A Potential Candidate for Nonvolatile Memory Applications.

Bipolar resistive switching using organic molecule is very promising for memory applications owing to their advantages, such as simple device structure, low manufacturing cost, stability, and flexibility. Herein we report Langmuir-Blodgett (LB) and spin-coated-film-based bipolar resistive switching devices using organic material 1,4-bis(di(1H-indol-3-yl)methyl)benzene (Indole1). The pressure-area per molecule isotherm (π-A), Brewster angle microscopy (BAM), atomic force microscopy (AFM), and scanning electron microscopy (SEM) were used to formulate an idea about the organization and morphology of the organic material onto thin films. On the basis of the device structure and measurement protocol, it is observed that the device made up of Indole1 shows nonvolatile resistive random access memory (RRAM) behavior with a very high memory window (∼106), data sustainability (5400 s), device yield (86.7%), and repeatability. The oxidation-reduction process and electric-field-driven conduction are the keys behind such switching behavior. Because of very good data retention, repeatability, stability, and a high device yield, the switching device designed using compound Indole1 may be a potential candidate for memory applications.

Electroforming-free flexible organic resistive random access memory based on a nanocomposite of poly(3-hexylthiophene-2,5-diyl) and orange dye with a low threshold voltage

The charge trapping characteristics of an organic nanocomposite based on two polymers, poly(3-hexylthiophene-2,5-diyl) (P3HT) and orange dye (OD), were investigated by fabricating a sandwiched structure on a flexible polyethylene terephthalate substrate. The fabricated flexible organic device with a configuration of silver (Ag)/P3HT–OD/Ag displayed highly stable results consistent with a bipolar non-volatile resistive random access memory (RRAM) device using simple and controllable fabrication technology. It was observed that by the addition of OD, low threshold voltages (V th) of V set = 1.5 V and V reset = −1.5 V were achieved owing to the values of work function and energy bandgap of the materials used. Electrical rewritable effects of the fabricated memory device were tested for 102 voltage sweeps and 104s without any considerable degradation of its bistable resistive states. The switching mechanism was considered to be space charge limited current and this was verified by plotting the double logarithmic I–V curve. This is the first RRAM device based on P3HT polymer whose mechanical robustness has been tested for its mechanical robustness by bending it for 25 bending cycles at a bending diameter ranging from 15 cm to 5 cm without any considerable change in its properties. This finding offers important guidelines for reproducing next-generation flexible organic nanocomposite-based memory devices that are simple to fabricate and have low operating voltages and highly stable memory behavior.

Graphene oxide for nonvolatile memory application by using electrophoretic technique

The experimental work presented here, for the first time using electrophoretic technique to fabricate graphene oxide (GO)-based resistive random access memory (RRAM). By using electrophoretic technique, nonvolatile RRAM devices with Aluminum (Al)/GO/Indium tin oxide (ITO) cross-bar sandwich-like structure were fabricated. The fabricated devices show typical bipolar resistant switching behavior with ON/OFF ratio more than 10, retention time more than 102 s, and transition voltage less than 1.7 V. The switching mechanism for the devices is ascribed to the formation and rupture of the conducting filament induced by the diffusion of oxygen ions. The results show that the electrophoretic technique holds great potential for film manufacturing for RRAM.

In-Memory Computing with Memristor Content Addressable Memories for Pattern Matching.

The dramatic rise of data-intensive workloads has revived application-specific computational hardware for continuing speed and power improvements, frequently achieved by limiting data movement and implementing "in-memory computation". However, conventional complementary metal oxide semiconductor (CMOS) circuit designs can still suffer low power efficiency, motivating designs leveraging nonvolatile resistive random access memory (ReRAM), and with many studies focusing on crossbar circuit architectures. Another circuit primitive-content addressable memory (CAM)-shows great promise for mapping a diverse range of computational models for in-memory computation, with recent ReRAM-CAM designs proposed but few experimentally demonstrated. Here, programming and control of memristors across an 86 × 12 memristor ternary CAM (TCAM) array integrated with CMOS are demonstrated, and parameter tradeoffs for optimizing speed and search margin are evaluated. In addition to smaller area, this memristor TCAM results in significantly lower power due to very low programmable conductance states, motivating CAM use in a wider range of computational applications than conventional TCAMs are confined to today. Finally, the first experimental demonstration of two computational models in memristor TCAM arrays is reported: regular expression matching in a finite state machine for network security intrusion detection and definable inexact pattern matching in a Levenshtein automata for genomic sequencing.

Cross‐Point Arrays with Low‐Power ITO‐HfO2 Resistive Memory Cells Integrated on Vertical III‐V Nanowires

AbstractVertical nanowires with cointegrated metal‐oxide‐semiconductor field‐effect‐transistor (MOSFET) selectors and nonvolatile resistive random access memory (RRAM) cells represent a promising candidate for fast, energy‐efficient, cross‐point memory cells. This paper explores indium‐tin‐oxide‐hafnium‐dioxide RRAM cells integrated onto arrays of indium‐arsenide (InAs) vertical nanowires with a resulting area of 0.06 µm2 per cell. For low current operation, an improved switching uniformity over the intrinsic self‐compliant behavior is demonstrated when using an external InAs nanowire MOSFET selector in series. The memory cells show consistent switching voltages below ±1 V and a switching cycle endurance of 106 is demonstrated. The developed fabrication scheme is fully compatible with low‐ON‐resistance vertical III‐V nanowire MOSFET selectors, where operational compatibility with the initial high‐field filament forming is established. Due to the small footprint of a vertical implementation, high density integration is achievable, and with a measured programming energy for 50 ns pulses at 0.49 pJ, the technology promises fast and ultralow power cross‐point memory arrays.

Modelling the switching effect in graphene oxide-based memristors

The two-dimensionality of insulating graphene oxide sheets is attractive for use in nanoscale two-terminal devices. These structures are promising for the next generation of non-volatile resistive random access memories, with high speed, low-power consumption and excellent scalability. In this work, the transport properties of graphene oxide-based metal-insulator-metal structures are addressed. The graphene oxide was synthesized using an eco-friendly modified Hummers method. The obtained graphene oxide sheets, placed between two electrodes, were subjected to an electrical characterization measuring cycling current–voltage (I–V) curves by applying successive forward and reverse voltage sweeps. When the absolute value of the measured current is plotted in logarithmic scale versus the applied voltage, the typical butterfly-shaped curves are observed, which are better interpreted in terms of their first derivative, or differential conductivity, in order to understand the transport mechanism. The experimental I–V curves can be explained in two different ways: by modifying well-known transport models or by using typical flux-controlled memristor models. The connection between both strategies is established in this work. The analytical expressions used for the differential conductance for the low resistance state and the high resistance state lead to simple expressions for the I–V curves. The symmetry of the curves after several cycles is lost when compared to those measured on pristine devices, which might indicate the occurrence of filament forming effect, affecting the bipolar switching.

Electronic mechanism for resistance drift in phase-change memory materials: link to persistent photoconductivity

‘Phase-change’ memory materials, such as the canonical composition Ge2Sb2Te5, are being actively researched for non-volatile resistive random-access memory applications. In these devices, ultra-rapid reversible transformations between metastable highly electrically conducting (degenerate-semiconducting) crystalline and more electrically resistive (semiconducting) glassy phases are produced by the application of appropriate voltage pulses. Multilevel programming, wherein more than two metastable resistance states can be stored in the memory material as different proportions of partially glassy/crystalline regions, allows more than one bit to be stored per memory cell. However, this route to increasing data density, without recourse to device-size down-scaling, is threatened by the phenomenon of ‘resistance drift’, wherein the electrical resistance of the glassy phase slowly increases with time, following a weak power-law dependence, after being written with a voltage pulse. In this paper, we propose an intrinsic electronic mechanism for the resistance drift, particularly valid at ambient temperature and below, by identifying it with the phenomenon of persistent photoconductivity that is commonly observed in a wide range of disordered semiconductors. We develop a model for it in terms of the long-time, deep-trap release and subsequent recombination of charge carriers, akin to that which is believed to be responsible for the long-time photocurrent decay in amorphous semiconductors, such as hydrogenated amorphous silicon. In this case, the parameters controlling the resistance drift are the widths of the (localized) valence- and conduction-band tails in the vicinity of the bandgap. Hence, there is the potential for mitigating resistance drift in the amorphous state of phase-change memory materials by suitable material engineering (e.g. via compositional or fabricational control) to control the extent of band-tailing, thereby facilitating the future introduction of multistate memory.

Solution-processable black phosphorus nanosheets covalently modified with polyacrylonitrile for nonvolatile resistive random access memory

The as-fabricated Al/BP–PAN/ITO device exhibits excellent nonvolatile rewritable memory performance, with a high ON/OFF current ratio exceeding 104 and a small switch-on voltage of −1.45 V.