Physical models of size-dependent nanofilament formation and rupture in NiO resistive switching memories

NiO films display unipolar resistance switching characteristics, due to the electrically induced formation and rupture of nanofilaments. While the applicative interest for possible use in highly dense resistance switching memory (RRAM) is extremely high, switching phenomena pose strong fundamental challenges in understanding the physical mechanisms and models. This work addresses the set and reset mechanisms for the formation and rupture of nanofilaments in NiO RRAM devices. Reset is described in terms of thermally-accelerated diffusion and oxidation processes, and its resistance dependence is explained by size-dependent Joule heating and oxidation. The filament is described as a region with locally-enhanced doping, resulting in an insulator-metal transition driven by structural and chemical defects. The set mechanism is explained by a threshold switching effect, triggering chemical reduction and a consequent local increase of metallic doping. The possible use of the observed resistance-dependent reset and set parameters to improve the memory array operation and variability is finally discussed.     

Vacancy‐Induced Synaptic Behavior in 2D WS2 Nanosheet–Based Memristor for Low‐Power Neuromorphic Computing

Memristors with nonvolatile memory characteristics have been expected to open a new era for neuromorphic computing and digital logic. However, existing memristor devices based on oxygen vacancy or metal‐ion conductive filament mechanisms generally have large operating currents, which are difficult to meet low‐power consumption requirements. Therefore, it is very necessary to develop new materials to realize memristor devices that are different from the mechanisms of oxygen vacancy or metal‐ion conductive filaments to realize low‐power operation. Herein, high‐performance and low‐power consumption memristors based on 2D WS₂ with 2H phase are demonstrated, which show fast ON (OFF) switching times of 13 ns (14 ns), low program current of 1 µA in the ON state, and SET (RESET) energy reaching the level of femtojoules. Moreover, the memristor can mimic basic biological synaptic functions. Importantly, it is proposed that the generation of sulfur and tungsten vacancies and electron hopping between vacancies are dominantly responsible for the resistance switching performance. Density functional theory calculations show that the defect states formed by sulfur and tungsten vacancies are at deep levels, which prevent charge leakage and facilitate the realization of low‐power consumption for neuromorphic computing application.     

Controllable Organic Resistive Switching Achieved by One‐Step Integration of Cone‐Shaped Contact

Conductive filaments (CFs)‐based resistive random access memory possesses the ability of scaling down to sub‐nanoscale with high‐density integration architecture, making it the most promising nanoelectronic technology for reclaiming Moore's law. Compared with the extensive study in inorganic switching medium, the scientific challenge now is to understand the growth kinetics of nanoscale CFs in organic polymers, aiming to achieve controllable switching characteristics toward flexible and reliable nonvolatile organic memory. Here, this paper systematically investigates the resistive switching (RS) behaviors based on a widely adopted vertical architecture of Al/organic/indium‐tin‐oxide (ITO), with poly(9‐vinylcarbazole) as the case study. A nanoscale Al filament with a dynamic‐gap zone (DGZ) is directly observed using in situ scanning transmission electron microscopy (STEM) , which demonstrates that the RS behaviors are related to the random formation of spliced filaments consisting of Al and oxygen vacancy dual conductive channels growing through carbazole groups. The randomicity of the filament formation can be depressed by introducing a cone‐shaped contact via a one‐step integration method. The conical electrode can effectively shorten the DGZ and enhance the localized electric field, thus reducing the switching voltage and improving the RS uniformity. This study provides a deeper insight of the multiple filamentary mechanisms for organic RS effect.     

Physical Mechanism and Performance Factors of Metal Oxide Based Resistive Switching Memory:A Review

This review summarizes the mechanism and performance of metal oxide based resistive switching memory The origin of resistive switching(RS) behavior can be roughly classified into the conducting filament type and the interface type. Here,we adopt the filament type to study the metal oxide based resistive switching memory,which considers the migration of metallic cations and oxygen vacancies,as well as discuss two main mechanisms including the electrochemical metallization effect(ECM) and valence change memory effect(VCM). At the light of the influence of the electrode materials and switching layers on the RS characteristics,an overview has also been given on the performance parameters including the uniformity endurance,the retention,and the multi-layer storage. Especially,we mentioned ITO(indium tin oxide electrode and discussed the novel RS characteristics related with ITO. Finally,the challenges resistive random access memory(RRAM) device is facing,as well as the future development trend,are expressed.     

Bipolar resistance switching characteristics in TiN/ZnO:Mn/Pt junctions developed for nonvolatile resistive memory application

As conventional flash memory is approaching its fundamental scaling limit, there is an urgent demand for an alternative nonvolatile memory technology at present. Resistance-switching random access memory has attracted extensive interests due to its nonvolatile nature, good scalability, and simple structure. In this work, TiN/ZnO:Mn/Pt junctions, which employ a conductive compound TiN as the top electrode to replace regular metal electrodes, were fabricated and investigated for nonvolatile resistive memory applications. These junctions exhibit bistable resistance state at room temperature, and the devices can be reproducibly switched between the two resistance states by applying bidirectional voltage biases. Moreover, both resistance states are demonstrated to retain for more than 10(4) s without electrical power, demonstrating a nonvolatile nature of the memory device. The mechanism of resistance switching effects in TiN/ZnO:Mn/Pt junctions is interpreted in terms of the drift of oxygen vacancies and the resultant formation/annihilation of local conductive channels through ZnO:Mn/Pt Schottky barrier.     

Improved performance of ZnO-based resistive memory by internal diffusion of Ag atoms

An Ag/ZnO/Pt memory device, which has much better resistive switching behaviour than Pt/ZnO/Pt device was demonstrated. The detailed resistive mechanisms for the Pt/ZnO/Pt and the Ag/ZnO/Pt systems are proposed and investigated. Microstructures are observed by transmission electron microscope (TEM), indicating that the formation of conducting path for both systems is different. For the Pt/ZnO/Pt device, the conductive filament path is constructed by the oxygen vacancies from top to bottom electrodes under a larger enough bias at a forming process. For the Ag/ZnO/Pt device, the filament path was grown by oxygen vacancies combined with an internal diffusion of Ag atoms under a large bias and can provide the lowest energy barrier for electrons transported between two electrodes during set and reset processes, which reduces formation of other conducting paths after each switching. Accordingly, the stable switching performance of the Ag/ZnO/Pt device can be achieved over 100 cycles even the thickness of ZnO film <25 nm.     

Multi‐Nonvolatile State Resistive Switching Arising from Ferroelectricity and Oxygen Vacancy Migration

Resistive switching phenomena form the basis of competing memory technologies. Among them, resistive switching, originating from oxygen vacancy migration (OVM), and ferroelectric switching offer two promising approaches. OVM in oxide films/heterostructures can exhibit high/low resistive state via conducting filament forming/deforming, while the resistive switching of ferroelectric tunnel junctions (FTJs) arises from barrier height or width variation while ferroelectric polarization reverses between asymmetric electrodes. Here the authors demonstrate a coexistence of OVM and ferroelectric induced resistive switching in a BaTiO₃ FTJ by comparing BaTiO₃ with SrTiO₃ based tunnel junctions. This coexistence results in two distinguishable loops with multi‐nonvolatile resistive states. The primary loop originates from the ferroelectric switching. The second loop emerges at a voltage close to the SrTiO₃ switching voltage, showing OVM being its origin. BaTiO₃ based devices with controlled oxygen vacancies enable us to combine the benefits of both OVM and ferroelectric tunneling to produce multistate nonvolatile memory devices.     

Nanometer‐Scale Phase Transformation Determines Threshold and Memory Switching Mechanism

Creation of nanometer‐scale conductive filaments in resistive switching devices makes them appealing for advanced electrical applications. While in situ electrical probing transmission electron microscopy promotes fundamental investigations of how the conductive filament comes into existence, it does not provide proof‐of‐principle observations for the filament growth. Here, using advanced microscopy techniques, electrical, 3D compositional, and structural information of the switching‐induced conductive filament are described. It is found that during in situ probing microscopy of a Ag/TiO₂/Pt device showing both memory‐ and threshold‐switching characteristics, a crystalline Ag‐doped TiO₂ forms at vacant sites on the device surface and acts as the conductive filament. More importantly, change in filament morphology varying with applied compliance currents determines the underlying switching mechanisms that govern either memory or threshold response. When focusing more on threshold switching features, it is demonstrated that the structural disappearance of the filament arises at the end of the constricted region and leads to the spontaneous phase transformation from crystalline conductive state into an initial amorphous insulator. Use of the proposed method enables a new pathway for observing nanosized features in a variety of devices at the atomic scale in three dimensions.     

Observation of Resistive Switching Behavior in Crossbar Core–Shell Ni/NiO Nanowires Memristor

The crossbar structure of resistive random access memory (RRAM) is the most promising technology for the development of ultrahigh‐density devices for future nonvolatile memory. However, only a few studies have focused on the switching phenomenon of crossbar RRAM in detail. The main purpose of this study is to understand the formation and disruption of the conductive filament occurring at the crossbar center by real‐time transmission electron microscope observation. Core–shell Ni/NiO nanowires are utilized to form a cross‐structure, which restrict the position of the conductive filament to the crosscenter. A significant morphological change can be observed near the crossbar center, which results from the out‐diffusion and backfill of oxygen ions. Energy dispersive spectroscopy and electron energy loss spectroscopy demonstrate that the movement of the oxygen ions leads to the evolution of the conductive filament, followed by redox reactions. Moreover, the distinct reliability of the crossbar device is measured via ex situ experiments. In this work, the switching mechanism of the crossbar core–shell nanowire structure is beneficial to overcome the problem of nanoscale minimization. The experimental method shows high potential to fabricate high‐density RRAM devices, which can be applied to 3D stacked package technology and neuromorphic computing systems.     

Local resistance switching at grain and grain boundary surfaces of polycrystalline tungsten oxide films

Resistance switching behavior has been investigated in as-prepared and oxygen-annealed polycrystalline tungsten oxide films using conductive atomic force microscopy. The oxygen-annealed film appeared more insulative than the as-prepared films. The local current distributions demonstrated the lower conductivity at the grain boundaries than at the grains in the oxygen-annealed films. Reversible resistance switching behavior only occurred at the grain surface region of the oxygen-annealed films and the resistance switching process was described by the local valence change of tungsten ions induced by electrochemical migration of protons or oxygen vacancies. This different resistance switching behavior between the grain and grain boundary surface was attributed to the different oxygen vacancy density caused by the post-annealing process. The present results would be especially meaningful for the fabrication of nanoscale resistive nonvolatile memory devices.     

Anomalous Resistance Hysteresis in Oxide ReRAM: Oxygen Evolution and Reincorporation Revealed by In Situ TEM

The control and rational design of redox‐based memristive devices, which are highly attractive candidates for next‐generation nonvolatile memory and logic applications, is complicated by competing and poorly understood switching mechanisms, which can result in two coexisting resistance hystereses that have opposite voltage polarity. These competing processes can be defined as regular and anomalous resistive switching. Despite significant characterization efforts, the complex nanoscale redox processes that drive anomalous resistive switching and their implications for current transport remain poorly understood. Here, lateral and vertical mapping of O vacancy concentrations is used during the operation of such devices in situ in an aberration corrected transmission electron microscope to explain the anomalous switching mechanism. It is found that an increase (decrease) in the overall O vacancy concentration within the device after positive (negative) biasing of the Schottky‐type electrode is associated with the electrocatalytic release and reincorporation of oxygen at the electrode/oxide interface and is responsible for the resistance change. This fundamental insight presents a novel perspective on resistive switching processes and opens up new technological opportunities for the implementation of memristive devices, as anomalous switching can now be suppressed selectively or used deliberately to achieve the desirable so‐called deep Reset.     

Resistive Switching Behavior in Ferroelectric Heterostructures

Resistive random‐access memory (RRAM) is a promising candidate for next‐generation nonvolatile random‐access memory protocols. The information storage in RRAM is realized by the resistive switching (RS) effect. The RS behavior of ferroelectric heterostructures is mainly controlled by polarization‐dominated and defect‐dominated mechanisms. Under certain conditions, these two mechanisms can have synergistic effects on RS behavior. Therefore, RS performance can be effectively improved by optimizing ferroelectricity, conductivity, and interfacial structures. Many methods have been studied to improve the RS performance of ferroelectric heterostructures. Typical approaches include doping elements into the ferroelectric layer, controlling the oxygen vacancy concentration and optimizing the thickness of the ferroelectric layer, and constructing an insertion layer at the interface. Here, the mechanism of RS behavior in ferroelectric heterostructures is briefly introduced, and the methods used to improve RS performance in recent years are summarized. Finally, existing problems in this field are identified, and future development trends are highlighted.     

Diode-less bilayer oxide (WO(x)-NbO(x)) device for cross-point resistive memory applications

The combination of a threshold switching device and a resistive switching (RS) device was proposed to suppress the undesired sneak current for the integration of bipolar RS cells in a cross-point array type memory. A simulation for this hybrid-type device shows that the matching of key parameters between switch element and memory element is an important issue. Based on the threshold switching oxides, a conceptual structure with a simple metal-oxide?1-oxide?2-metal stack was provided to accommodate the evolution trend. We show that electroformed W-NbO(x)-Pt devices can simultaneously exhibit both threshold switching and memory switching. A qualitative model was suggested to elucidate the unique properties in a W-NbO(x)-Pt stack, where threshold switching is associated with a localized metal-insulator transition in the NbO(x) bulk, and the bipolar RS derives from a redox at the tip of the localized filament at the WO(x)-NbO(x) interface. Such a simple metal-oxide-metal structure, with functionally separated bulk and interface effects, provides a fabrication advantage for future high-density cross-point memory devices.     

Low power and stable resistive switching in graphene oxide-based RRAM embedded with ZnO nanoparticles for nonvolatile memory applications

The present study reports the role of zinc oxide nanoparticles (ZnO NPs) embedded in graphene oxide (GO)-based RRAM for non-volatile memory applications. GO thin film embedded with different concentrations of ZnO NPs was deposited on bottom electrode, i.e., indium tin oxide (ITO) coated glass. Thermal evaporation technique was used for the fabrication of top electrodes for electrical measurements. Structural and morphological studies of synthesized GO and ZnO NPs were done by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). Switching characteristics of the RRAM devices were investigated using electrical measurements. It has been observed that the optimized concentration of ZnO NPs (20%) shows stable switching behavior with low SET (??0.61 V) and RESET (+?0.65 V) voltages as compared to pure GO devices. The switching of the fabricated memory devices from high resistance state (HRS) to low resistance state (LRS) has been found due to conductive filament formed between top and bottom electrodes. This conductive filament has been confirmed by the change in resistance as a function of temperature. The Al/GO-ZnO(20%)/ITO devices show stable endurance behavior for >?50 cycles and retention behavior for >?4?×?10³ s. In HRS, the dominated conduction mechanism was found to be space-charge limited conduction (SCLC), whereas in LRS, the Ohmic conduction mechanism was observed. The incorporation of ZnO NPs increased the number of oxygen vacancies in switching layer which eventually enhanced the formation of conductive filament. This phenomenon has been confirmed using XPS characterization of the switching layer. These optimized concentrations of ZnO embedded in GO switching layers provide a way for future low power non-volatile memory devices.

     

Filament growth dynamics in solid electrolyte-based resistive memories revealed by in situ TEM

Solid electrolyte based-resistive memories have been considered to be a potential candidate for future information technology with applications in non-volatile memory, logic circuits and neuromorphic computing. A conductive filament model has been generally accepted to be the underlying mechanism for the resistive switching. However, the growth dynamics of such conductive filaments is still not fully understood. Here, we explore the controllability of filament growth by correlating observations of the filament growth with the electric field distribution and several other factors. The filament growth behavior has been recorded using in situ transmission electron microscopy. By studying the real- time recorded filament growth behavior and morphologies, we have been able to simulate the electric field distribution in accordance with our observations. Other factors have also been shown to affect the filament growth, such as Joule heating and electrolyte infrastructure. This work provides insight into the controllable growth of conductive filaments and will help guide research into further functionalities of nanoionic resistive memories.     

Intrinsic mechanisms of memristive switching

Resistive switching (RS) memory effect in metal-oxide-metal junctions is a fascinating phenomenon toward next-generation universal nonvolatile memories. However the lack of understanding the electrical nature of RS has held back the applications. Here we demonstrate the electrical nature of bipolar RS in cobalt oxides, such as the conduction mechanism and the switching location, by utilizing a planar single oxide nanowire device. Experiments utilizing field effect devices and multiprobe measurements have shown that the nanoscale RS in cobalt oxides originates from redox events near the cathode with p-type conduction paths, which is in contrast with the prevailing oxygen vacancy based model.     

Resistive switching mechanisms in random access memory devices incorporating transition metal oxides: TiO2, NiO and Pr0.7Ca0.3MnO3

Resistance change random access memory (RRAM) cells, typically built as MIM capacitor structures, consist of insulating layers I sandwiched between metal layers M, where the insulator performs the resistance switching operation. These devices can be electrically switched between two or more stable resistance states at a speed of nanoseconds, with long retention times, high switching endurance, low read voltage, and large switching windows. They are attractive candidates for next-generation non-volatile memory, particularly as a flash successor, as the material properties can be scaled to the nanometer regime. Several resistance switching models have been suggested so far for transition metal oxide based devices, such as charge trapping, conductive filament formation, Schottky barrier modulation, and electrochemical migration of point defects. The underlying fundamental principles of the switching mechanism still lack a detailed understanding, i.e. how to control and modulate the electrical characteristics of devices incorporating defects and impurities, such as oxygen vacancies, metal interstitials, hydrogen, and other metallic atoms acting as dopants. In this paper, state of the art ab initio theoretical methods are employed to understand the effects that filamentary types of stable oxygen vacancy configurations in TiO(2) and NiO have on the electronic conduction. It is shown that strong electronic interactions between metal ions adjacent to oxygen vacancy sites results in the formation of a conductive path and thus can explain the 'ON' site conduction in these materials. Implication of hydrogen doping on electroforming is discussed for Pr(0.7)Ca(0.3)MnO(3) devices based on electrical characterization and FTIR measurements.     

Memristive switching mechanism for metal/oxide/metal nanodevices

Nanoscale metal/oxide/metal switches have the potential to transform the market for nonvolatile memory and could lead to novel forms of computing. However, progress has been delayed by difficulties in understanding and controlling the coupled electronic and ionic phenomena that dominate the behaviour of nanoscale oxide devices. An analytic theory of the 'memristor' (memory-resistor) was first developed from fundamental symmetry arguments in 1971, and we recently showed that memristor behaviour can naturally explain such coupled electron-ion dynamics. Here we provide experimental evidence to support this general model of memristive electrical switching in oxide systems. We have built micro- and nanoscale TiO2 junction devices with platinum electrodes that exhibit fast bipolar nonvolatile switching. We demonstrate that switching involves changes to the electronic barrier at the Pt/TiO2 interface due to the drift of positively charged oxygen vacancies under an applied electric field. Vacancy drift towards the interface creates conducting channels that shunt, or short-circuit, the electronic barrier to switch ON. The drift of vacancies away from the interface annilihilates such channels, recovering the electronic barrier to switch OFF. Using this model we have built TiO2 crosspoints with engineered oxygen vacancy profiles that predictively control the switching polarity and conductance.     

Using post-breakdown conduction study in a MIS structure to better understand the resistive switching mechanism in an MIM stack

We apply our understanding of the physics of failure in the post-breakdown regime of high-κ dielectric-based conventional logic transistors having a metal-insulator-semiconductor (MIS) structure to interpret the mechanism of resistive switching in resistive random-access memory (RRAM) technology metal-insulator-metal (MIM) stacks. Oxygen vacancies, gate metal migration and metal filament formation in the gate dielectric which constitute the chemistry of breakdown in the post-breakdown stage of logic gate stacks are attributed to be the mechanisms responsible for the SET process in RRAM technology. In this paper, we draw an analogy between the breakdown study in logic devices and filamentation physics in resistive non-volatile memory.     

Phosphorene/ZnO Nano‐Heterojunctions for Broadband Photonic Nonvolatile Memory Applications

High‐performance photonic nonvolatile memory combining photosensing and data storage with low power consumption ensures the energy efficiency of computer systems. This study first reports in situ derived phosphorene/ZnO hybrid heterojunction nanoparticles and their application in broadband‐response photonic nonvolatile memory. The photonic nonvolatile memory consistently exhibits broadband response from ultraviolet (380 nm) to near infrared (785 nm), with controllable shifts of the SET voltage. The broadband resistive switching is attributed to the enhanced photon harvesting, a fast exciton separation, as well as the formation of an oxygen vacancy filament in the nano‐heterojunction. In addition, the device exhibits an excellent stability under air exposure compared with reported pristine phosphorene‐based nonvolatile memory. The superior antioxidation capacity is believed to originate from the fast transfer of lone‐pair electrons of phosphorene. The unique assembly of phosphorene/ZnO nano‐heterojunctions paves the way toward multifunctional broadband‐response data‐storage techniques.