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.     

Graphene oxide thin films for flexible nonvolatile memory applications

There has been strong demand for novel nonvolatile memory technology for low-cost, large-area, and low-power flexible electronics applications. Resistive memories based on metal oxide thin films have been extensively studied for application as next-generation nonvolatile memory devices. However, although the metal oxide based resistive memories have several advantages, such as good scalability, low-power consumption, and fast switching speed, their application to large-area flexible substrates has been limited due to their material characteristics and necessity of a high-temperature fabrication process. As a promising nonvolatile memory technology for large-area flexible applications, we present a graphene oxide based memory that can be easily fabricated using a room temperature spin-casting method on flexible substrates and has reliable memory performance in terms of retention and endurance. The microscopic origin of the bipolar resistive switching behavior was elucidated and is attributed to rupture and formation of conducting filaments at the top amorphous interface layer formed between the graphene oxide film and the top Al metal electrode, via high-resolution transmission electron microscopy and in situ X-ray photoemission spectroscopy. This work provides an important step for developing understanding of the fundamental physics of bipolar resistive switching in graphene oxide films, for the application to future flexible electronics.     

Resistive switching characteristics of multilayered (HfO2/Al2O3)n n = 19 thin film

A transparent resistive random access memory used as Indium Tin Oxide (ITO) electrode, ITO/HfO₂/Al₂O₃/…/HfO₂/Al₂O₃/ITO capacitor structure is fabricated on glass substrate by atomic layer deposition. The unipolar resistive switching characteristics can be performed by applying the positive- or negative-bias through top electrode, however, the differences of switching and stability in the two different operations can be observed. The diversities of electrical property are attributed to different oxide/ITO interface materials, which influence the current flow of the injected electrons.     

Development of ferroelectric oxides based resistive switching materials

Resistive random access memory (RRAM) is one of the most promising candidates that satisfies the requirements of new generation non-volatile memories, as a consequence of its high density, outstanding scalability, and low power consumption. The review is based on a summary of recent studies in ferroelectric oxides based resistive switching (RS) materials and devices. It highlights the various ferroelectric oxide materials with RS behaviour and the underlying mechanisms including filament-type and interface-type mechanism. In the end, the challenge in current RRAM for future high-density data storage applications is addressed.     

Improvement of resistive switching characteristics in ZrO2 film by embedding a thin TiOx layer

The stabilization of the resistive switching characteristics is important to resistive random access memory (RRAM) device development. In this paper, an alternative approach for improving resistive switching characteristics in ZrO(2)-based resistive memory devices has been investigated. Compared with the Cu/ZrO(2)/Pt structure device, by embedding a thin TiO(x) layer between the ZrO(2) and the Cu top electrode, the Cu/TiO(x)-ZrO(2)/Pt structure device exhibits much better resistive switching characteristics. The improvement of the resistive switching characteristics in the Cu/TiO(x)-ZrO(2)/Pt structure device might be attributed to the modulation of the barrier height at the electrode/oxide interfaces.     

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.

     

Photocatalytic Reduction of Graphene Oxide–TiO2 Nanocomposites for Improving Resistive‐Switching Memory Behaviors

Graphene oxide (GO)‐based resistive‐switching (RS) memories offer the promise of low‐temperature solution‐processability and high mechanical flexibility, making them ideally suited for future flexible electronic devices. The RS of GO can be recognized as electric‐field‐induced connection/disconnection of nanoscale reduced graphene oxide (RGO) conducting filaments (CFs). Instead of operating an electrical FORMING process, which generally results in high randomness of RGO CFs due to current overshoot, a TiO₂‐assisted photocatalytic reduction method is used to generate RGO‐domains locally through controlling the UV irradiation time and TiO₂ concentration. The elimination of the FORMING process successfully suppresses the RGO overgrowth and improved RS memory characteristics are achieved in graphene oxide–TiO₂ (Go‐TiO₂) nanocomposites, including reduced SET voltage, improved switching variability, and increased switching speed. Furthermore, the room‐temperature process of this method is compatible with flexible plastic substrates and the memory cells exhibit excellent flexibility. Experimental results evidence that the combined advantages of reducing the oxygen‐migration barrier and enhancing the local‐electric‐field with RGO‐manipulation are responsible for the improved RS behaviors. These results offer valuable insight into the role of RGO‐domains in GO memory devices, and also, this mild photoreduction method can be extended to the development of carbon‐based flexible electronics.     

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.     

Fabrication and polarization-modulated resistive switching behavior of predominantly (110)-oriented BiFeO3 thin films on indium tin oxide/glass substrates

Predominantly (110)-oriented BiFeO₃ (BFO) thin films were deposited on indium tin oxide/glass substrates using a metal organic decomposition process by controlling the preheating temperature. The structure evolution with film thickness at different preheating temperatures was investigated to clarify the growth mode of (110)-predominant BFO film. The formation of the (110)-oriented BFO film is due to the low-temperature nucleation of (110)-oriented grains preheated at 425 °C. In the Au/BFO(110)/ITO heterostructure, a polarization-modulated bistable resistive switching behavior with high ratio of resistance and large diode current characteristics was observed, which makes the heterostructure attractive for application in resistive ferroelectric memory.     

Resistive switching behavior and multiple transmittance states in solution-processed tungsten oxide

In this work, a tungsten oxide (WO(x)) film is prepared using a thiourea-assisted solution process. We demonstrate a device composed of fluorine doped tin oxide (FTO)-glass/WO(x)/electrolyte/indium-tin oxide (ITO)-glass stacking electrochromic (EC) structure and Al electrodes that are locally patterned and interposed between the WO(x) film and electrolyte, which form an Al(top electrode)/WO(x)/FTO(bottom electrode) resistance random access memory (RRAM) unit. According to transmission electron microscopy and X-ray photoelectron spectroscopy analyses, the WO(x) film contains nanosize pores and metallic-tungsten nanoclusters which are scattered within the tungsten oxide layer and concentrated along the interface between the Al electrode and WO(x) film. With application of voltage to the ITO electrode, multiple transmittance states are achieved for the EC unit due to the different quantity of intercalated Li ions in the WO(x) film. As for the Al/WO(x)/FTO RRAM unit, a bipolar nonvolatile resistive switching behavior is attained by applying voltage on the Al top electrode, showing electrical bistability with an ON/OFF current ratio up to 1 × 10(4).     

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.     

Structural Phase Transition Effect on Resistive Switching Behavior of MoS2‐Polyvinylpyrrolidone Nanocomposites Films for Flexible Memory Devices

The 2H phase and 1T phase coexisting in the same molybdenum disulfide (MoS₂) nanosheets can influence the electronic properties of the materials. The 1T phase of MoS₂ is introduced into the 2H‐MoS₂ nanosheets by two‐step hydrothermal synthetic methods. Two types of nonvolatile memory effects, namely write‐once read‐many times memory and rewritable memory effect, are observed in the flexible memory devices with the configuration of Al/1T@2H‐MoS₂‐polyvinylpyrrolidone (PVP)/indium tin oxide (ITO)/polyethylene terephthalate (PET) and Al/2H‐MoS₂‐PVP/ITO/PET, respectively. It is observed that structural phase transition in MoS₂ nanosheets plays an important role on the resistive switching behaviors of the MoS₂‐based device. It is hoped that our results can offer a general route for the preparation of various promising nanocomposites based on 2D nanosheets of layered transition metal dichalcogenides for fabricating the high performance and flexible nonvolatile memory devices through regulating the phase structure in the 2D nanosheets.     

Resistive switching phenomena of tungsten nitride thin films with excellent CMOS compatibility

We report the resistive switching (RS) characteristics of tungsten nitride (WN_x) thin films with excellent complementary metal-oxide-semiconductor (CMOS) compatibility. A Ti/WN_x/Pt memory cell clearly shows bipolar RS behaviors at a low voltage of approximately ±2.2 V. The dominant conduction mechanisms at low and high resistance states were verified by Ohmic behavior and trap-controlled space-charge-limited conduction, respectively. A conducting filament model by a redox reaction explains the RS behavior in WN_x films. We also demonstrate the memory characteristics during pulse operation, including a high endurance over >10⁵ cycles and a long retention time of >10⁵ s.     

Multistate resistive switching in silver nanoparticle films

Abstract

Resistive switching devices have garnered significant consideration for their potential use in nanoelectronics and non-volatile memory applications. Here we investigate the nonlinear current–voltage behavior and resistive switching properties of composite nanoparticle films comprising a large collective of metal–insulator–metal junctions. Silver nanoparticles prepared via the polyol process and coated with an insulating polymer layer of tetraethylene glycol were deposited onto silicon oxide substrates. Activation required a forming step achieved through application of a bias voltage. Once activated, the nanoparticle films exhibited controllable resistive switching between multiple discrete low resistance states that depended on operational parameters including the applied bias voltage, temperature and sweep frequency. The films’ resistance switching behavior is shown here to be the result of nanofilament formation due to formative electromigration effects. Because of their tunable and distinct resistance states, scalability and ease of fabrication, nanoparticle films have a potential place in memory technology as resistive random access memory cells.     

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.     

Interface-modified unipolar resistive random access memory (RRAM) structure for low-power application

An interface-engineered resistive random access memory (RRAM) using bilayer transition metal oxide (TMO) is presented for improving unipolar resistive-switching characteristics. The experiment and simulation data show that better resistive switching characteristics and superb uniformity can be realized by inserting a thin AIOx insertion layer between the Ir/NiO interface. To elucidate the uniformity improvement of our bilayer structure, the conducting-defect effects in the resistive cell were also investigated using a random circuit breaker (RCB) simulation model. It has been verified that the forming and set characteristics are more effectively improved because the conducting-defect ratio in the insertion layer region is low, therefore making it more advantageous for a filament path controllability. Using the optimal oxygen contents in both the insertion layer and the resistive cell, it was confirmed that a significant reduction of up to 0.15 mA of the reset current (I(RESET)) is possible compared to the conventional cell. These results indicate that new AI insertion has a large contribution to the reset and forming processes.     

Multilevel resistive switching and synaptic plasticity of nanoparticulated cobaltite oxide memristive device

Multilevel resistive switching(RS)is a key property to embrace the full potential of memristive devices for non-volatile memory and neuromorphic computing applications.In this study,we employed nanopar-ticulated cobaltite oxide(Co3O4)as a model material to demonstrate the multilevel RS and synaptic learning capabilities because of its multiple and stable redox state properties.The Pt/Co3O4/Pt memris-tive device exhibited tunable RS properties with respect to different voltages and compliance currents(CC)without the electroforming process.That is,the device showed voltage-dependent RS at a higher CC whereas CC-dependent RS was observed at lower CC.The device showed four different resistance states during endurance and retention measurements and non-volatile memory results indicated that the CC-based measurement had less variation.Besides,we investigated the basic and complex synap-tic plasticity properties using the analog current-voltage characteristics of the Pt/Co3O4/Pt device.In particular,we mimicked the potentiation-depression and four-spike time-dependent plasticity(STDP)rules such as asymmetric Hebbian,asymmetric anti-Hebbian,symmetric Hebbian,and symmetric anti-Hebbian learning rules.The results of the present work indicate that the cobaltite oxide is an excellent nanomaterial for both multilevel RS and neuromorphic computing applications.     

Biodegradable Natural Pectin‐Based Flexible Multilevel Resistive Switching Memory for Transient Electronics

Transient electronics that can physically vanish in solution can offer opportunities to address the ecological challenges for dealing with the rapidly growing electronic waste. As one important component, it is desirable that memory devices combined with the transient feature can also be developed as secrecy information storage systems besides the above advantage. Resistive switching (RS) memory is one of the most promising technologies for next‐generation memory. Herein, the biocompatible pectin extracted from natural orange peel is introduced to fabricate RS memory devices (Ag/pectin/indium tin oxides (ITO)), which exhibit excellent RS characteristics, such as forming free characteristic, low operating voltages (≈1.1 V), fast switching speed (<70 ns), long retention time (>10⁴ s), and multilevel RS behaviors. The device performance is not degraded after 10⁴ bending cycles, which will be beneficial for flexible memory applications. Additionally, instead of using acid solution, the Ag/pectin/ITO memory device can be dissolved rapidly in deionized water within 10 min thanks to the good solubility arising from ionization of its carboxylic groups, which shows promising application for green electronics. The present biocompatible memory devices based on natural pectin suggest promising material candidates toward enabling high‐density secure information storage systems applications, flexible electronics, and green electronics.     

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.     

Analog memory and spike-timing-dependent plasticity characteristics of a nanoscale titanium oxide bilayer resistive switching device

We demonstrated analog memory, synaptic plasticity, and a spike-timing-dependent plasticity (STDP) function with a nanoscale titanium oxide bilayer resistive switching device with a simple fabrication process and good yield uniformity. We confirmed the multilevel conductance and analog memory characteristics as well as the uniformity and separated states for the accuracy of conductance change. Finally, STDP and a biological triple model were analyzed to demonstrate the potential of titanium oxide bilayer resistive switching device as synapses in neuromorphic devices. By developing a simple resistive switching device that can emulate a synaptic function, the unique characteristics of synapses in the brain, e.g. combined memory and computing in one synapse and adaptation to the outside environment, were successfully demonstrated in a solid state device.