Resistive Switching Performance Research Articles

Redox‐Active Azulene‐based 2D Conjugated Covalent Organic Framework for Organic Memristors

AbstractAs a conjugated and unsymmetric building block composed of an electron‐poor seven‐membered sp2 carbon ring and an electron‐rich five‐membered carbon ring, azulene and its derivatives have been recognized as one of the most promising building blocks for novel electronic devices due to its intrinsic redox activity. By using 1,3,5‐tris(4‐aminophenyl)‐benzene and azulene‐1,3‐dicarbaldehyde as the starting materials, an azulene(Azu)‐based 2D conjugated covalent organic framework, COF‐Azu, is prepared through liquid‐liquid interface polymerization strategy for the first time. The as‐fabricated Al/COF‐Azu/indium tin oxide (ITO) memristor shows typical non‐volatile resistive switching performance due to the electric filed induced intramolecular charge transfer effect. Associated with the unique memristive performance, a simple convolutional neural network is built for image recognition. After 8 epochs of training, image recognition accuracy of 80 % for a neutral network trained on a larger data set is achieved.

Dual Resistive Switching Performance Derived from Ionic Migration in Halide Perovskite Based Memory

Herein, we report an environmentally stable and friendly halide perovskite based resistive random access memory device with an Ag/PMMA/(PMA)2CuBr4/FTO (PMMA = poly(methyl methacrylate); PMA = C6H5CH2NH3) architecture. The device exhibits the coexistence of two bipolar resistive switching modes, including counterclockwise and clockwise switching characteristics. The devices with both switching modes show stable endurance (>100 cycles) and long retention performance (>104 s). By applying a suitable electrical stimulation, the counterclockwise and clockwise switching behaviors are interconvertible. Furthermore, the Au/PMMA/(PMA)2CuBr4/FTO and Ag/(PMA)2CuBr4/FTO devices were fabricated to verify the origin of dual resistive switching behaviors. The similar dual resistive switching behaviors after electroforming processes of three types of memory devices suggest that the interconvertible dual resistive switching characteristics could be attributed to the ionic migration in the (PMA)2CuBr4 perovskite layer.

Novel ternary organic resistive switching memory doped with bipolar materials

Organic resistive switching memory (ORSM) shows great potential for neotype memory devices due to the preponderances of simple architecture, low power consumption, high switching speed and feasibility of large-area fabrication. Herein, solution-processed ternary ORSM devices doped with bipolar materials were achieved with high ON/OFF ratio and outstanding device stability. The resistive switching performance was effectively ameliorated by doping two bipolar materials (DpAn-InAc and DpAn-5BzAc) in different blending concentration into the PVK:OXD-7 donor-accepter system. Compared with the binary system (PVK: 30 wt% OXD-7), the ON/OFF ratios of the ternary devices doped with 6 wt% DpAn-5BzAc were greatly increased from 7.91 × 102 to 4.98 × 104, with the operating voltage (∣V set–V reset∣) declined from 4.90 V to 2.25 V, respectively. Additionally, the stability of resistance state and uniformity of operating voltage were also significantly optimized for the ternary devices. For comparison, ternary devices doped with DpAn-InAc have been explored, which also achieved improved resistive switching behavior. A detailed analysis of electrical characteristics and the internal charge transfer properties of ORSM was performed to unveil the performance enhancement in ternary devices. Results indicate that the use of bipolar materials favors the efficient operation of OSRMs with proper energy level alignment and effective charge transfer.

Bipolar Resistive Switching in TiO2 Artificial Synapse Mimicking Pavlov's Associative Learning.

Memristive devices are among the most emerging electronic elements to realize artificial synapses for neuromorphic computing (NC) applications and have potential to replace the traditional von-Neumann computing architecture in recent times. In this work, pulsed laser deposition-manufactured Ag/TiO2/Pt memristor devices exhibiting digital and analog switching behavior are considered for NC. The TiO2 memristor shows excellent performance of digital resistive switching with a memory window of order ∼103. Furthermore, the analog resistive switching offers multiple conductance levels supporting the development of the bioinspired synapse. A possible mechanism for digital and analog switching behavior in our device is proposed. Remarkably, essential synaptic functions such as pair-pulse facilitation, long-term potentiation (LTP), and long-term depression (LTD) are successfully realized based on the change in conductance through analog memory characteristics. Based on the LTP-LTD, a neural network simulation for the pattern recognition task using the MNIST data set is investigated, which shows a high recognition accuracy of 95.98%. Furthermore, more complex synaptic behavior such as spike-time-dependent plasticity and Pavlovian classical conditioning is successfully emulated for associative learning of the biological brain. This work enriches the TiO2-based resistive random-access memory, which provides information about the simultaneous existence of digital and analog behavior, thereby facilitating the further implementation of memristors in low-power NC.

Preparation of AlxZn1-xO resistive switching film by sol–gel method and its corrosion behavior in 3.5 wt% NaCl solution

AlxZn1-xO films were prepared on SS304 by using sol–gel method combined with spinning and heat treatment. The surface morphology, composition and structure of films were observed and characterized by scanning electron microscopy, X-ray diffractometer and Fourier transform infrared spectroscopy. The oxygen vacancy content and semiconductor type of AlxZn1-xO films were determined by X-ray photoelectron spectroscopy, electronic para-magnetic resonance spectroscopy, Mott-Schottky curve and density function theoretical calculation. The corrosion resistance and resistive switching performance of AlxZn1-xO films were measured by using electrochemical methods and immersion-polarisation experiments. The results show that the prepared AlxZn1-xO films are in n-type semiconductor material with a homogeneous and dense surface, the oxygen vacancy content is decreased with the increasing of Al dopping content. The corrosive resistance of AlxZn1-xO film was increased with the increasing of Al doping content with the best corrosion resistance at the doping content of 4% Al. During the immersion process, the corrosion resistance of AlxZn1-xO film was firstly increased and then decreased. The corrosion resistance of AlxZn1-xO films could be switched in the high and low resistance states under the immersion-polarization process by adjusting of the oxygen vacancy content in film.

Improved resistive switching performance and realized electric control of exchange bias in a NiO/HfO2 bilayer structure.

The fluctuation of switching parameters is unavoidable in conductive filaments (CFs)-type resistive switching (RS) devices, which restricts the application in resistive random-access memory. Here, we employed an uninsulated antiferromagnetic (AFM) NiO layer adhered to a well-insulating HfO2 layer to effectively suppress the RS fluctuation by achieving forming-free, narrower set voltage distribution, a more stable on/off ratio, and better endurance in comparison with single-HfO2-layer based RS devices. The conduction scaling behavior indicates that the NiO/HfO2 bilayer has a smaller scale parameter S0 (lateral dimension of the bottleneck for the CFs). Besides this, considering some preexisting conductive paths in the NiO layer, the electric fields and the formation/rupture of CFs can be highly localized, leading to reduced switching fluctuation and improved RS performance in the NiO/HfO2-based RS devices. Moreover, asymmetric I-V curves measured in a high resistance state (HRS) in positively and negatively biased regions and the electric modulation of exchange bias (EB) arising from the Co-NiO interfacial coupling are favorable for revealing the inherent mechanism for RS. The coexistence of RS and EB is also useful to the design of novel multifunctional memory devices.

Tuning the photocurrent responsive and resistive switching performances of stilbazolium/iodoargentate hybrids by modulating the aggregation mode among chromophores

D–π–A stilbazolium-type dyes with substitutes ranging from –C2H5CN to –C2H5OH were incorporated with iodoargentates to generate three hybrids, whose photo/electrical performance will be enhanced greatly by modulating aggregation modes.

Improved resistive switching performance through donor–acceptor structure construction in memristors based on covalent organic framework films

The impact of acceptors on memory performance was explored based on two COF film-based devices with different donor–acceptor structures.

Introduction of defects in hexagonal boron nitride for vacancy-based 2D memristors.

Two-dimensional (2D) materials have become potential resistive switching (RS) layers to prepare emerging non-volatile memristors. The atomically thin thickness and the highly controllable defect density contribute to the construction of ultimately scaled memory cells with stable switching behaviors. Although the conductive bridge random-access memory based on 2D hexagonal boron nitride has been widely studied, the realization of RS completely relying on vacancies in 2D materials has performance superiority. Here, we synthesize carbon-doped h-BN (C-h-BN) with a certain number of defects by controlling the weight percentage of carbon powder in the source. These defects can form a vacancy-based conductive filament under an applied electric field. The memristor displays bipolar non-volatile memory with a low SET voltage of 0.85 V and shows a long retention time of up to 104 s at 120 °C. The response times of the SET and RESET process are less than 80 ns and 240 ns, respectively. The current mapping by conductive atomic force microscopy demonstrates the electric-field-induced current tunneling from defective sites of the C-h-BN flake, revealing the defect-based RS in the C-h-BN memristor. Moreover, C-h-BN with excellent flexibility can be applied to wearable devices, maintaining stable RS performance in a variety of bending environments and after multiple bending cycles. The vacancy-based 2D memristor provides a new strategy for developing ultra-scaled memory units with high controllability.

Flexible Memristor Constructed by 2D Cadmium Phosphorus Trichalcogenide for Artificial Synapse and Logic Operation

AbstractThe development of advanced microelectronics requires new device architecture and multi‐functionality. Low‐dimensional material is considered as a powerful candidate to construct new devices. In this work, a flexible memristor is fabricated utilizing 2D cadmium phosphorus trichalcogenide nanosheets as the functional layer. The memristor exhibits excellent resistive switching performance under different radius and over 103 bending times. The device mechanism is systematically investigated, and the synaptic plasticity including paired‐pulse facilitation and spiking timing‐dependent plasticity are further observed. Furthermore, based on the linearly conductance modulation capacity of the flexible memristor, the applications on decimal operation are explored, that the addition, subtraction, multiplication, and division of decimal calculation are successfully achieved. These results demonstrate the potential of metal phosphorus trichalcogenide in novel flexible neuromorphic devices, which accelerate the application process of neuromorphic computing.

Enhancement of Resistive Switching Performance in Hafnium Oxide (HfO2) Devices via Sol-Gel Method Stacking Tri-Layer HfO2/Al-ZnO/HfO2 Structures.

Resistive random-access memory (RRAM) is a promising candidate for next-generation non-volatile memory. However, due to the random formation and rupture of conductive filaments, RRMS still has disadvantages, such as small storage windows and poor stability. Therefore, the performance of RRAM can be improved by optimizing the formation and rupture of conductive filaments. In this study, a hafnium oxide-/aluminum-doped zinc oxide/hafnium oxide (HfO2/Al-ZnO/HfO2) tri-layer structure device was prepared using the sol-gel method. The oxygen-rich vacancy Al-ZnO layer was inserted into the HfO2 layers. The device had excellent RS properties, such as an excellent switch ratio of 104, retention of 104 s, and multi-level storage capability of six resistance states (one low-resistance state and five high-resistance states) and four resistance states (three low-resistance states and one high-resistance state) which were obtained by controlling stop voltage and compliance current, respectively. Mechanism analysis revealed that the device is dominated by ohmic conduction and space-charge-limited current (SCLC). We believe that the oxygen-rich vacancy concentration of the Al-ZnO insertion layer can improve the formation and rupture behaviors of conductive filaments, thereby enhancing the resistive switching (RS) performance of the device.

Reliable BiI3-Based Resistive Random-Access Memory Devices with a High On/Off Ratio

In this paper, we report resistive random-access memory (RRAM) with bismuth iodide (BiI3) as the resistance switching layer, which exhibits an on/off ratio of the order of 108. The behaviors of the resistive switching performance of the BiI3 devices were systematically investigated. The chemical states and electronic structures of the resistance switching layer (BiI3) were studied via X-ray photoemission spectroscopy with depth profile. The spectroscopies demonstrate that the filament formation belongs to the metal bridge type, which is dominated by the presence of bismuth atoms resulting from the formation of silver iodide (AgI). X-ray diffraction, the cross section of transmission electron microscopy, scanning electron microscopy, and atomic force microscopy were employed to study the formation process of the metal filament in the BiI3-based RRAM. The results reveal that the upward diffusion of Ag cations will react with iodide anions, and bismuth cations will turn into the metallic state, which results in forming a conductive filament inside the resistance switching layer. Furthermore, the devices with the structure of Ag/polymethylmethacrylate (PMMA)/BiI3/PMMA/Au demonstrate low first reset voltage (approximately −0.66 V), a high retention larger than 5 × 104 s, and 6 × 103 switching cycles. This study not only demonstrates the excellent performance of BiI3-based RRAM devices but also provides fundamental insights into the mechanisms of the metallic filament first reset process in the BiI3 layers.

Supramolecular Framework Constructed by Dendritic Nanopolymer for Stable Flexible Perovskite Resistive Random-Access Memory.

The 3D supramolecular framework (3D-SF) is constructed in this work through the hydrogen bond assisted self-assembly of spherical dendritic nanopolymer to regulate the flexibility, stability, and resistive switching (RS) performance of perovskite resistive random-access memory (RRAM). Herein, the 3D-SF network acts as the perovskite crystallization template to regulate the perovskite crystallization process due to its coordination interaction of functional groups with the perovskite grains, presenting the uniform, pinhole-free, and compact perovskite morphology for stable flexible RRAM. The 3D-SF network in situ stays at the perovskite intergranular boundaries to crosslink the perovskite grains. The RS performance of 3D-SF-modified perovskite RRAM device is evidently improved to the ON/OFF ratio of 105 , the cycle number of 500 times, and the data retention time of 104 s. The 50-days exposure of unencapsulated RRAM device at ambient environment still makes the ON/OFF ratio to be kept at ≈104 , indicating the potential of long-term stable multilevel storage in the high-density data storage. The bending action under different radius also does not change the RS performance due to the excellent bending-resistant ability of 3D-SF-modified perovskite film. This work explores a novel polymer additive strategy to construct the 3D supramolecular framework for stable flexible perovskite optoelectronic devices.

Enhanced Resistive Switching Performance through Air-Stable Cu2AgSbI6 Thin Films for Flexible and Multilevel Storage Application.

Herein, the lead-free halide perovskite films with different Cu-to-Ag ratios (Cu3-xAgxSbI6, x = 0, 1, 2, or 3) have been prepared by a spin-coating method at low temperature. The enhanced resistive switching (RS) performance of more uniform SET/RESET voltages and the endurance up to at least 1600 cycles are found in the RS memory with a device structure of Ag/PMMA/Cu2AgSbI6/ITO. The device performance is not degraded under different bending angles and after 103 bending cycles, which is beneficial for flexible memory applications. The appropriately increased activation energy of the perovskites with the partial substitution of Ag atoms, which would lead to a more robust filament formed, is proposed to explain the enhanced RS mechanism. Importantly, the effective size and number of filaments measured by conductive AFM are introduced to confirm the multilevel storage effect of Cu2AgSbI6. The multilevel storage characteristics with four resistance levels are demonstrated by various compliance currents. Moreover, the Cu2AgSbI6 memory devices still exhibit enhanced RS properties and multilevel storage after 75 days of exposure to ambient conditions. Our study provides a strategy for improving the stability and high-density storage applications of halide perovskite RS memory devices.

Emergence of electric field-induced conducting states in single-crystalline MoTe[formula omitted] nanoflakes and its application in memristive devices

Conductive atomic force microscopy (cAFM) reveals the emergence of conducting states in MoTe2 NFs under applied electric field. In this report, we explore the use of electric field induced conducting states in the single-crystalline MoTe2 nanoflakes (NFs) for nonvolatile resistive memory technologies. The memristive devices are fabricated in vertical metal–insulator (active layer)-metal architecture with aluminum top and fluorine doped tin oxide bottom electrodes. The MoTe2 NFs were embedded in polymethyl methacrylate (PMMA) to form the active layer. The device shows appreciable resistive switching (RS) characteristics while switching from a high resistive state (HRS) to a low resistive state (LRS) and vice versa. The low-frequency conductance noise (LFCN) measurements show that the power spectral density of conductance fluctuation follows the 1/f2 nature in the HRS, while 1/f in LRS. The stochastic analysis of conductance fluctuation data exhibits random telegraphic noise in the HRS, indicating the role of NFs as charge trapping sites. The LFCN measurements and cAFM study confirm that electric field-induced conducting states are pivotal in enhancing the RS performance of the memory device.

High‐Performance and Environmentally Robust Multilevel Lead‐Free Organotin Halide Perovskite Memristors

AbstractWith a striking explosion of digital information, organic–inorganic halide perovskite (OHP) memristors have been regarded as a promising solution to break the von Neumann bottleneck as the nonvolatile computing‐in‐memory architecture. However, toxicity and stability under ambient conditions are two critical issues for practical applications. Here, lead‐free MASnI3 (MASI) perovskites are reported with improved resistive switching (RS) performance via defects passivating by introducing PEACl. In‐Sn/PEACl‐MASI/PEDOT:PSS/ITO memristor arrays (10 × 10) exhibit reproducible RS with low SET/RESET voltage (VSET/VRESET, +0.58/−0.49 V), ultrahigh ON/OFF ratio (8.5 × 103), stable endurance (2 × 103 cycles), excellent retention (2 × 104 s), high device yield (88%), and multilevel storage. RS remains robust in harsh environments, including high temperature and humidity, long‐term light irradiation, and open‐air conditions. Importantly, a complete physical simulation model that quantitatively and accurately describes RS in vacancy concentration, temperature, and electric potential is proposed originating from conductive filaments formation/rupture. Additionally, PEACl‐MASI memristors are successfully fabricated on polyethylene terephthalate (PET) and Si substrates to explore the potential for large‐scale application on different substrates. The PEACl‐MASI memristors provide an opportunity to develop the next generation of high‐performance and environmentally robust multilevel information storage devices.

Machine learning-assisted design guidelines and performance prediction of CMOS-compatible metal oxide-based resistive switching memory devices

Machine learning (ML) has accelerated the discovery of new materials and properties of electronic devices, reducing development time and increasing efficiency. In this study, ML was used to provide design guidelines and predict the performance of industry-standard resistive switching (RS) memory devices based on HfO2/x, Ta2O5, and TaOx materials. The model building, analyses, and prediction processes were based on a database of peer-reviewed articles published between 2007 and 2020. More than 15,000 property entries were used for our ML tasks. Moreover, supervised and unsupervised ML techniques were used to provide design guidelines for the categorical and continuous feature sets. In addition, a linear model, artificial neural network, and the random forest algorithm were employed to predict the continuous-type features, and gradient boosting was used to understand how device parameters can affect RS performance. Finally, the ML predictions were validated by fabricating the corresponding RS devices. The results indicated that the ML techniques accelerated the discovery and understanding of different RS properties.

Resistive switching characteristics and mechanism of lanthanum yttrium oxide (LaYO3) films deposited by RF sputtering for RRAM applications

LaYO3 thin films are deposited by using RF sputtering and resistive switching properties of LaYO3-based RRAM devices are investigated. Specifically, the effects of sputtering time, top electrode materials, deposition conditions, annealing temperature, and annealing processes on resistive switching characteristics of the devices are studied. The results indicate that conductive filaments are mainly dominated by oxygen vacancies whose concentration can be manipulated by the deposition atmosphere and annealing process. The formation of conductive filaments can be enhanced by the diffusion of In ions from an indium tin oxide substrate into LaYO3 layer during the annealing process. The Schottky barrier height under different top electrodes and deposition atmospheres also have an impact on the formation of filaments. In addition, post-metal anneal can help to form an AlOx interface layer, which plays significant roles in improving resistive switching performance of the LaYO3-based RRAMs. The devices annealed at 400 ºC possess resistive switching characteristics for over 1200 cycles and a Ron/Roff ratio of 103, showing promise for non-volatile memory applications.

Quantized synaptic characteristics in HfO2-nanocrystal based resistive switching memory

We demonstrate the reliable resistive switching performance of nanocrystalline-HfO2 inside amorphous-HfOx in TaN/nc-HfO2/ITO memristor structure. Transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) were utilized to confirm the presence of nc-HfO2 and non-stoichiometric HfOx in the switching layer. In presence of nc-HfO2, quantized conductance was controlled by the narrowing of conductive filaments in an atomic scale applying a very slow voltage sweep. Conductance change under DC voltage shows the quantized conductance states with integer and half-integer multiples of G0 (77.5 μS). Enhanced resistive switching performances with multilevel resistance states behavior were investigated under different current compliance and RESET stop voltages. Short-term plasticity and long-term potentiation, pulse number, and spike rate-dependent plasticity by controlling the magnitude and duration of the input stimulus play a critical role in modulating the post-synaptic conductivity. The combination of nc-HfO2 and amorphous-HfOx in the memristor structure provide promising scope for neuromorphic system applications.

Enhanced resistive switching properties of HfAlOx/ZrO2- based RRAM devices

Resistive Random-Access Memory (RRAM) devices are recognized as potential candidates for next-generation memory devices, due to their smallest cell size, high write/erase speed, and endurance. Particularly, the resistive switching (RS) characteristics in oxide materials have offered new opportunities for developing CMOS-compatible high-density RRAM devices. In this study, the RS behavior of HfAlOx/ZrO2 thin films sandwiched structure between TiN bottom electrode and Au top electrodes were investigated. It was found that Au/HfAlOx/ZrO2/TiN stacks were superior in terms of RS performance when compare to Au/HfAlOx/TiN memory stacks. The devices demonstrated a good resistance ratio of high resistance state and low resistance state ∼103 for Au/HfAlOx/TiN and ∼105 for Au/HfAlOx/ZrO2/TiN stacks, respectively. Both stacks showed good retention characteristics (>104 ​s) and endurance (>103 cycles). The experimental current-voltage characteristics fitted with different conducting mechanisms, the linear lower bias region is dominated by ohmic conductivity, whereas the non-linear higher bias region was dominated by space-charge limited current conduction mechanism.