Nonvolatile Resistive Random Access Memory Research Articles

Multi-terminal memtransistors from polycrystalline monolayer molybdenum disulfide.

Memristors are two-terminal passive circuit elements that have been developed for use in non-volatile resistive random-access memory and may also be useful in neuromorphic computing. Memristors have higher endurance and faster read/write times than flash memory and can provide multi-bit data storage. However, although two-terminal memristors have demonstrated capacity for basic neural functions, synapses in the human brain outnumber neurons by more than a thousandfold, which implies that multi-terminal memristors are needed to perform complex functions such as heterosynaptic plasticity. Previous attempts to move beyond two-terminal memristors, such as the three-terminal Widrow-Hoff memristor and field-effect transistors with nanoionic gates or floating gates, did not achieve memristive switching in the transistor. Here we report the experimental realization of a multi-terminal hybrid memristor and transistor (that is, a memtransistor) using polycrystalline monolayer molybdenum disulfide (MoS2) in a scalable fabrication process. The two-dimensional MoS2 memtransistors show gate tunability in individual resistance states by four orders of magnitude, as well as large switching ratios, high cycling endurance and long-term retention of states. In addition to conventional neural learning behaviour of long-term potentiation/depression, six-terminal MoS2 memtransistors have gate-tunable heterosynaptic functionality, which is not achievable using two-terminal memristors. For example, the conductance between a pair of floating electrodes (pre- and post-synaptic neurons) is varied by a factor of about ten by applying voltage pulses to modulatory terminals. In situ scanning probe microscopy, cryogenic charge transport measurements and device modelling reveal that the bias-induced motion of MoS2 defects drives resistive switching by dynamically varying Schottky barrier heights. Overall, the seamless integration of a memristor and transistor into one multi-terminal device could enable complex neuromorphic learning and the study of the physics of defect kinetics in two-dimensional materials.

Bioinspired Tribotronic Resistive Switching Memory for Self-Powered Memorizing Mechanical Stimuli.

Haptic memory, from the interaction of skin and brain, can not only perceive external stimuli but also memorize it after removing the external stimuli. For the mimicry of human sensory memory, a self-powered artificial tactile memorizing system was developed by coupling bionic electronic skin and nonvolatile resistive random access memory (RRAM). The tribotronic nanogenerator is utilized as electronic skin to transform the touching signal into electric pulse, which will be programmed into the artificial brain: RRAM. Because of the advanced structural designs and accurate parameter matching, including the output voltages and the resistances in different resistive states, the artificial brain can be operated in self-powered mode to memorize the touch stimuli with the responsivity up to 20 times. For demonstrating the application potential of this system, it was fabricated as an independently addressed matrix to realize the memorizing of motion trace in two-dimensional space. The newly designed self-powered nonvolatile system has broad applications in next-generation high-performance sensors, artificial intelligence, and bionics.

Memory effect behavior with respect to the crystal grain size in the organic-inorganic hybrid perovskite nonvolatile resistive random access memory

The crystal grain size of CH3NH3PbI3 (MAPbI3) organic-inorganic hybrid perovskite (OHP) film was controllable in the range from ~60 nm to ~600 nm by non-solvents inter-diffusion controlled crystallization process in dripping crystallization method for the formation of perovskite film. The MAPbI3 OHP non-volatile resistive random access memory with ~60 nm crystal grain size exhibited >0.1 TB/in2 storage capacity, >600 cycles endurance, >104 s data retention time, ~0.7 V set, and ~−0.61 V re-set bias voltage.

(Invited) Understanding Resistive Switching in Manganite-Based Memristive Devices

In recent years manganites with tailored functional properties have attracted special attention for their use in miniaturized devices such as resistive switching (RS) memories, spintronic sensors, micro solar cells and micro solid oxide fuel cells [1]. When prepared in the form of thin films (from a few to around a hundred nanometers) their functional properties can largely vary in comparison to their intrinsic bulk properties. There is thus a large interest in understanding and controlling the influence of different parameters, such as epitaxy, substrate-induced strain and grain boundary effects, for the envisioned use of manganites in applied functional devices, and in particular for non-volatile resistive random access memories (ReRAM). In this work we have focused on the study of LaMnO3±δ(LMO) as switching sandwiched material for new oxide memristors. This perovskite oxide, in which both oxygen and cation vacancies can exist, presents a rich phase diagram. A change from orthorhombic to rhombohedral structure can be achieved by increasing the temperature or varying the oxygen stoichiometry (from δ<0.06 to δ>0.09) [2]. Furthermore, the resistivity and the insulator-to-metal transition temperature can be tuned by the oxygen, making this manganite a very interesting candidate for resistive switching devices. Epitaxial and polycrystalline LaMnO3+δ films of different thickness were prepared by pulsed injectionmetal-organic chemical vapor deposition (PI-MOCVD) and integrated in thermomechanically stable metal-insulator-metal (MIM) structures. The LMO oxygen content is adjusted either ex situ or in situ, by thermal anneals under Ar or O2atmospheres in the PI-MOCVD reactor. By controlling the substrate, deposition conditions and anneal treatments, either the orthorhombic or the rhombohedral structure could be obtained. The resistive switching behavior of the films was tuned as a function of the metallic nature of the top electrode, showing very promising results and a large resistance window for operation. Three metals were investigated as top electrodes, i.e. Ag, Ti and Au. In addition, with the aim of relating the functional properties of the films to their structural characteristics, a fundamental study was carried out combining in-situ X-ray diffraction and Raman spectroscopy under different oxygen partial pressures. The structural, compositional and electronic properties of LMO films showed a remarkable influence of the type of defects (cation or anion vacancies), while the La/Mn ratio is kept close to 1. A rhombohedral phase is obtained for films treated under O2, pointing out to a 3+d composition (i.e. cation vacancies) whereas films treated under Ar showed an orthorhombic phase, linked to a 3-d oxygen content (i.e. VO ..). The best RS response is obtained for LaMnO3+δ(oxygen over-stoichiometric), which shows a reproducible switching behaviour with a high programming window (up to 3 orders of magnitude) using low set/reset voltages for all three top metal electrode configurations.

Control of Switching Modes and Conductance Quantization in Oxygen Engineered HfOx based Memristive Devices

Hafnium oxide (HfOx)‐based memristive devices have tremendous potential as nonvolatile resistive random access memory (RRAM) and in neuromorphic electronics. Despite its seemingly simple two‐terminal structure, a myriad of RRAM devices reported in the rapidly growing literature exhibit rather complex resistive switching behaviors. Using Pt/HfOx/TiN‐based metal–insulator–metal structures as model systems, it is shown that a well‐controlled oxygen stoichiometry governs the filament formation and the occurrence of multiple switching modes. The oxygen vacancy concentration is found to be the key factor in manipulating the balance between electric field and Joule heating during formation, rupture (reset), and reformation (set) of the conductive filaments in the dielectric. In addition, the engineering of oxygen vacancies stabilizes atomic size filament constrictions exhibiting integer and half‐integer conductance quantization at room temperature during set and reset. Identifying the materials conditions of different switching modes and conductance quantization contributes to a unified switching model correlating structural and functional properties of RRAM materials. The possibility to engineer the oxygen stoichiometry in HfOx will allow creating quantum point contacts with multiple conductance quanta as a first step toward multilevel memristive quantum devices.

Ultra-low power non-volatile resistive crossbar memory based on pull up resistors

To realize a flexible and ultra-low power non-volatile resistive random access memory (NVRRAM), we propose a 3 × 3 resistive memory array comprised of crossbar memristors and a pull-up resistor connected to each column bar (1R-CM). This architecture forms a voltage divider network, which directly reads a parallel data of a row in the form of voltage instead of current. In the proposed structure, the optimum value of the pull-up resistor was found to be 10 MΩ. Poly(4-vinylphenol) (PVP) material is utilized in the memristor to achieve high OFF/ON resistance ratio of ∼1000 as a high resistance state (HRS) is 10 GΩ and a low resistance state (LRS) is 10 MΩ. The operating voltage for writing and reading are ±2 V and 0.5 V in current bound of 10–100 nA, respectively, and consumes ultra-low power (<8.33 nW) during operation. The proposed memory demonstrates bendability down to 10 mm with bending endurance of 1000 cycles and retention time for more than 180 days. It is fabricated on a plastic substrate by using direct-printing technique electrohydrodynamic (EHD) at ambient conditions that can be used in wearable electronics.

Reversible voltage dependent transition of abnormal and normal bipolar resistive switching.

Clear understanding the mechanism of resistive switching is the important prerequisite for the realization of high performance nonvolatile resistive random access memory. In this paper, binary metal oxide MoOx layer sandwiched by ITO and Pt electrodes was taken as a model system, reversible transition of abnormal and normal bipolar resistive switching (BRS) in dependence on the maximum voltage was observed. At room temperature, below a critical maximum voltage of 2.6 V, butterfly shaped I-V curves of abnormal BRS has been observed with low resistance state (LRS) to high resistance state (HRS) transition in both polarities and always LRS at zero field. Above 2.6 V, normal BRS was observed, and HRS to LRS transition happened with increasing negative voltage applied. Temperature dependent I-V measurements showed that the critical maximum voltage increased with decreasing temperature, suggesting the thermal activated motion of oxygen vacancies. Abnormal BRS has been explained by the partial compensation of electric field from the induced dipoles opposite to the applied voltage, which has been demonstrated by the clear amplitude-voltage and phase-voltage hysteresis loops observed by piezoelectric force microscopy. The normal BRS was due to the barrier modification at Pt/MoOx interface by the accumulation and depletion of oxygen vacancies.

Non-Volatile ReRAM Devices Based on Self-Assembled Multilayers of Modified Graphene Oxide 2D Nanosheets.

2D nanomaterials have been actively utilized in non-volatile resistive switching random access memory (ReRAM) devices due to their high flexibility, 3D-stacking capability, simple structure, transparency, easy fabrication, and low cost. Herein, it demonstrates re-writable, bistable, transparent, and flexible solution-processed crossbar ReRAM devices utilizing graphene oxide (GO) based multilayers as active dielectric layers. The devices employ single- or multi-component-based multilayers composed of positively charged GO (N-GO(+) or NS-GO(+)) with/without negatively charged GO(-) using layer-by-layer assembly method, sandwiched between Al bottom and Au top electrodes. The device based on the multi-component active layer Au/[N-GO(+)/GO(-)]n /Al/PES shows higher ON/OFF ratio of ≈105 with switching voltage of -1.9 V and higher retention stability (≈104 s), whereas the device based on single component (Au/[N-GO(+)]n /Al/PES) shows ≈103 ON/OFF ratio at ±3.5 V switching voltage. The superior ReRAM properties of the multi-component-based device are attributed to a higher coating surface roughness. The Au/[N-GO(+)/GO(-)]n /Al/PES device prepared from lower GO concentration (0.01%) exhibits higher ON/OFF ratio (≈109 ) at switching voltage of ±2.0 V. However, better stability is achieved by increasing the concentration from 0.01% to 0.05% of all GO-based solutions. It is found that the devices containing MnO2 in the dielectric layer do not improve the ReRAM performance.

Modeling and experimental study of resistive switching in vertically aligned carbon nanotubes

Model of the resistive switching in vertically aligned carbon nanotube (VA CNT) taking into account the processes of deformation, polarization and piezoelectric charge accumulation have been developed. Origin of hysteresis in VA CNT-based structure is described. Based on modeling results the VACNTs-based structure has been created. The ration resistance of high-resistance to low-resistance states of the VACNTs-based structure amounts 48. The correlation the modeling results with experimental studies is shown. The results can be used in the development nanoelectronics devices based on VA CNTs, including the nonvolatile resistive random-access memory.

Electrical Characteristics of TiO(2-x)/TiO2 Resistive Switching Memory Fabricated by Atomic Layer Deposition.

The rewritable low-power operated nonvolatile resistive random access memory device composed of Al(top)/TiO(2-x)/TiO2/Al(bottom) are demonstrated. The active component, the TiO2 layer of the device, is fabricated by atomic layer deposition. The oxygen vacancy TiO(2-x)/TiO2 layer annealed at 600 degrees C using rapid thermal annealing and it was proven to be in the rutile phase by X-ray diffraction analysis. The device exhibits nonvolatile memory behavior consistent with resistive switching properties, demonstrates an ON/OFF ratio of approximately 1,000:1, requires range of low voltage less than 0.4 V, and is still operational more than 120 times.

비-휘발성 저항 변화 메모리 응용을 위한 WOx물질의 전기적 특성 연구

비-휘발성 저항 변화 메모리 응용을 위한 WO_x물질의 전기적 특성 연구

A physical-based simulation for the dynamic behavior of photodoping mechanism in chalcogenide materials used in the lateral programmable metallization cells

Photodoping of chalcogenide glass (ChG) is an important process that has been used in several technology applications such as submicron lithography, diffraction gratings, and holographic recording. Today, one of the primary uses of photodoping is in the production of nonvolatile cation-based resistive random access memory (RRAM). Cation RRAM operates through electronic control of metal concentration within a ChG film, which alters the material between high resistance (HRS) and low resistance states (LRS). The process of photodoping is performed after the fabrication of ChG-based RRAM in order to introduce active metal into the film and lower switching energy. In spite of recent advances in the control of photodoping in ChG materials during manufacturing, the physical principles governing the dynamics of the process are still not fully understood. In this paper, we present a physical-based simulation for photodoping in ChG materials used in the lateral programmable metallization cells. The analysis structure is a lateral device that uses GeSe binary film as the ChG material with electrically active Ag anode and neutral Ni cathode contacts.

Liquid‐Exfoliated Black Phosphorous Nanosheet Thin Films for Flexible Resistive Random Access Memory Applications

Black phosphorous (BP) is a unique layered p‐type semiconducting material. The successful use of BP nanosheets in field‐effect transistors fueled research on BP atomic layers that focuses on, e.g., the exploration of their optical and electronic properties, and promising applications in (opto)electronics. However, BP films are prone to degradation in ambient conditions, which prevents their commercial application. Here, a route to the application of BP films as an environmental stable nonvolatile resistive random access memory is presented. The BP films, which are prepared from exfoliated BP nanosheets in selected solvents, show solvent‐dependent degradation upon ambient exposure, inducing the formation of an amorphous top degraded layer (TDL). The TDL acts as an insulating barrier just below the Al electrode. This property that was only obtained by degradation, confers a bipolar resistive switching behavior with a high ON/OFF current ratio up to ~3 × 105 and excellent retention ability over 105 s to the flexible BP memory devices. The TDL also prevents propagation of degradation further into the film, ensuring excellent memory performance even after three month of ambient exposure.

Nonvolatile Resistance Random Access Memory Devices Based on ZnO Nanorod Arrays

In this paper, a nonvolatile resistance random access memory (RRAM) device based on ZnO nanorod arrays has been fabricated and characterized. Vertically aligned ZnO nanorod layers (NRLs) were deposited on indium tin oxide (ITO) electrodes using a hydrothermal process/ chemical bath deposition (CBD). It can be found the Ag/ZnO NRL/ITO capacitor exhibits bipolar resistive switching behavior. The resistive switching behavior may be related to the oxygen vacancies and/or zinc interstitials confined on the surface of the ZnO NRs, giving rise to the formation of straight and extensible conducting path along each ZnO NR. Furthermore, superior stability in resistive switching characteristics was also observed. Both growing times and annealing times were investigated and annealing was done in oxygen for 3, 6 and 9 minutes at different temperatures. For ZnO nanorods that had been annealed for 6 minutes the forming voltage was about 6.06V, the Set voltage was about 3.25V and the Reset voltage was -2.78V. The original resistance was 7×10 6 Ω. The resistance in the low-resistance state was 108Ω and in the high-resistance state was 2016Ω, the resistance ratio was 18.7.

Resistive switching properties of epitaxial BaTiO(3-δ) thin films tuned by after-growth oxygen cooling pressure.

BaTiO3-δ, i.e. oxygen-deficient barium titanate (BaTiO3), thin films grown on GdScO3(110) substrates with SrRuO3 conductive electrodes by pulsed laser deposition are studied by X-ray diffraction and conductive AFM to characterize their structure and nanoscale electronic properties. Bias- and time-dependent resistive switching measurements reveal a strong dependence on the oxygen vacancy concentration, which can be tuned by after-growth oxygen cooling conditions of thin films. The results indicate that the resistive switching properties of BaTiO3-δ can be enhanced by controlling oxygen deficiency and provide new insight for potential non-volatile resistive random-access memory (RRAM) applications.

Resistance switching and memory effects in solution-processed BiFeO3/LaNiO3 junctions

Resistance switching and memory effects have been observed in a heterostructure consisting of BiFeO3 (BFO) on a LaNiO3 (LNO) conducting oxide bottom electrode fabricated by chemical solution deposition on quartz substrates. The phase purity and lattice strain of monodispersed thin film nanostructures were confirmed by XRD. Bipolar switching with a resistance switching ratio of one order of magnitude at small bias voltages of +1.3 V and −0.7 V for positive and negative bias respectively has been estimated from the I–V traces measured over a span of more than 6 h. Excellent retention behavior of the BFO–LNO device is confirmed for its possible use as a non-volatile resistance random access memory device. The modeling of the I–V data suggests that in the low resistance state (on) Ohmic conduction is well fitted, whereas in the high resistance state (off) the trap-controlled space-charge-limited current mechanism dominates. The observed resistive switching (RS) is attributed to formation of a metallic filament and its rupture under electric stress conditions. The valence states of Fe ions in the fabricated device structures indicate that oxygen vacancies could also lead to RS in the device.

PMMA interlayer-modulated memory effects by space charge polarization in resistive switching based on CuSCN-nanopyramids/ZnO-nanorods p-n heterojunction.

Resistive switching (RS) devices are commonly believed as a promising candidate for next generation nonvolatile resistance random access memory. Here, polymethylmethacrylate (PMMA) interlayer was introduced at the heterointerface of p-CuSCN hollow nanopyramid arrays and n-ZnO nanorod arrays, resulting in a typical bipolar RS behavior. We propose the mechanism of nanostructure trap-induced space charge polarization modulated by PMMA interlayer. At low reverse bias, PMMA insulator can block charges through the heterointerface, and and trapped states are respectively created on both sides of PMMA, resulting in a high resistance state (HRS) due to wider depletion region. At high reverse bias, however, electrons and holes can cross PMMA interlayer by Fowler-Nordeim tunneling due to a massive tilt of energy band, and then inject into the traps of ZnO and CuSCN, respectively. and trapped states are created, resulting in the formation of degenerate semiconductors on both sides of PMMA. Therefore, quantum tunneling and space charge polarization lead to a low resistance state (LRS). At relatively high forward bias, subsequently, the trapped states of and are recreated due to the opposite injection of charges, resulting in a recovery of HRS. The introduction of insulating interlayer at heterointerface, point a way to develop next-generation nonvolatile memories.

Resistive Switching Nanodevices Based on Metal–Organic Frameworks

AbstractSURMOF (surface‐anchored metal–organic frameworks) thin films exhibit exciting chemical and physical properties, which can be modulated in a straightforward fashion to achieve specific benefits for numerous applications in future technologies. Here, we report a detailed characterization of resistive switching in crystalline SURMOF films of around ≈10, ≈20 and ≈50 nm thicknesses. These demonstrated switching characteristics combined with the ability to deposit monolithically oriented crystalline HKUST‐1 films with well‐defined thicknesses in the nm‐range on conductive substrates serving as bottom electrodes and to lithographically fabricate top‐electrodes opens up the possibility to employ these metal–organic hybrid materials as solid state devices for potential nonvolatile resistive random access memory (RRAM) memory applications. MOF bipolar switching RRAM devices based on SURMOFs with thicknesses of 10±5 nm, 20±5 nm and 50±5 nm show exceptionally promising performance. The huge flexibility of MOF materials with regards to device applications is demonstrated by loading guest molecules into the pores of these framework materials. In the case of ferrocene infiltration, we show that the already impressive performance of the SURMOF‐RRAM devices can be further improved. The results demonstrate the great potential of SURMOF thin films for the implementation of novel and scalable active materials for the next generation of digital processing and organic‐based microelectronic devices.

Nonpolar resistive memory switching with all four possible resistive switching modes in amorphous LaHoO3 thin films

We studied the resistive memory switching in pulsed laser deposited amorphous LaHoO3 (a-LHO) thin films for non-volatile resistive random access memory applications. Nonpolar resistive switching (RS) was achieved in Pt/a-LHO/Pt memory cells with all four possible RS modes (i.e., positive unipolar, positive bipolar, negative unipolar, and negative bipolar) having high RON/ROFF ratios (in the range of ∼104–105) and non-overlapping switching voltages (set voltage, VON ∼ ±3.6–4.2 V and reset voltage, VOFF ∼ ±1.3–1.6 V) with a small variation of about ±5–8%. Temperature dependent current-voltage (I–V) characteristics indicated the metallic conduction in low resistance states (LRS). We believe that the formation (set) and rupture (reset) of mixed conducting filaments formed out of oxygen vacancies and metallic Ho atoms could be responsible for the change in the resistance states of the memory cell. Detailed analysis of I–V characteristics further corroborated the formation of conductive nanofilaments based on metal-like (Ohmic) conduction in LRS. Simmons-Schottky emission was found to be the dominant charge transport mechanism in the high resistance state.

Improving retention properties by thermal imidization for polyimide-based nonvolatile resistive random access memories

In this study, a polyimide (PI) thin film was synthesized as a resistive layer for creating resistive random access memory (ReRAM). The switch between high- and low-resistance states is caused by the formation and dissociation of dipole direction and Schottky barrier. The impact of imidization on memory properties was evaluated in detail by clarifying the transmission mechanism, and reliability properties including retention and endurance were improved using thermal imidization. In addition, the proposed PI-based ReRAM demonstrated superior performance levels compared with those of electrochemical-metallization-based and valence-change-based ReRAMs, including higher RON/ROFF ratio (>107) and lower operation energy (<0.16MV/cm).