Multilevel Resistive Switching Behavior Research Articles

Tunable Resistive Switching Behaviors and Mechanism of the W/ZnO/ITO Memory Cell.

A facile sol-gel spin coating method has been proposed for the synthesis of spin-coated ZnO nanofilms on ITO substrates. The as-prepared ZnO-nanofilm-based W/ZnO/ITO memory cell showed forming-free and tunable nonvolatile multilevel resistive switching behaviors with a high resistance ratio of about two orders of magnitude, which can be maintained for over 103 s and without evident deterioration. The tunable nonvolatile multilevel resistive switching phenomena were achieved by modulating the different set voltages of the W/ZnO/ITO memory cell. In addition, the tunable nonvolatile resistive switching behaviors of the ZnO-nanofilm-based W/ZnO/ITO memory cell can be interpreted by the partial formation and rupture of conductive nanofilaments modified by the oxygen vacancies. This work demonstrates that the ZnO-nanofilm-based W/ZnO/ITO memory cell may be a potential candidate for future high-density, nonvolatile, memory applications.

Multilevel resistive switching and synaptic behaviors in MnO-based memristor

Exploring new synaptic electronic devices that combine computing and memory is a promising strategy that fundamentally approaches intelligent machines. In this study, the multilevel resistive switching and synaptic behaviors of a MnO-based device is studied. The device is composed of Al/MnO/Ni sandwich structure, has stable resistance switching characteristics, has continuous nonvolatile memory state, can be used as electrically programmable and erasable analog memory. The gradual conductance modulation is realized by changing the compliance current and the maximum scanning voltage. The Al/MnO/Ni devices successfully mimic the basic functions of synapses, including the paired-pulse facilitation, spike-rate-dependent plasticity, excitatory postsynaptic current, short-term plasticity, long-term plasticity, and sike-timing-dependent plasticity.

Flexible and degradable resistive switching memory fabricated with sodium alginate* *Project supported by the Fund from the Ministry of Science and Technology of China (Grant No. 2018YFE0118300), the National Natural Science Foundation of China (Grant Nos. 11974072, 51902048, 61774031, and 61574031), the “111” Project, China (Grant No. B13013), the China Postdoctoral Science Foundation, China (Grant No. 2019M661185), the Fundamental Research

Transient electronics has attracted interest as an emerging technology to solve electronic-waste problem, due to its physically vanishing ability in solution. Here in this work, we demonstrate a flexible and degradable transient resistive switching (RS) memory device with simple structure of Cu/sodium alginate (SA)/ITO. The device presents excellent RS characteristics as well as high flexibility, including low operating voltage (< 1.5 V) and multilevel RS behavior. No performance degradation occurs after bending the device 50 times. Moreover, our device can be absolutely dissolved in deionized water. The proposed SA-based transient memory device has great potential for the development of green and security memory devices.

Study on Multilevel Resistive Switching Behavior With Tunable ON/OFF Ratio Capability in Forming-Free ZnO QDs-Based RRAM

Resistive random access memories (RRAMs) have attracted tremendous attention due to their miniaturized size, low power consumption, high response speed, and simple fabrication requirements, which are regarded as a promising candidate for future nonvolatile memories. However, the traditional binary storage RRAMs fail to meet the high-density requirement in the area of big data. One of the ways to overcome this obstacle is to realize multilevel state transitions in a unit RRAM. Herein, we propose a facile spin-coating method to develop a multilevel RRAM based on Al/poly(methyl methacrylate) (PMMA)/ZnO quantum dots (QDs)/PMMA/ZnO QDs/PMMA/fluorine-doped tin oxide (FTO) structure, with the significant merits of typical forming-free bipolar resistive switching behavior, low-power operating voltage, stable endurance, and excellent retention. In specific, the ON/OFF ratio of the proposed ZnO QDs-based device can be adjusted by tuning the concentration of the ZnO QDs. Moreover, a multilevel RRAM performance with distinct four-level resistance states is obtained by varying the compliance current, which demonstrates that the proposed device has a specific application in multilevel data storage. The conductive filament model, combining the enhanced internal field and robust formation of the conductive filament, is introduced to explain the mechanism of the resistive switching behavior and multilevel performance. This work paves a new way for its prospect in constructing high-density data storage RRAMs.

Low‐Dimensional Lead‐Free Inorganic Perovskites for Resistive Switching with Ultralow Bias

Abstract3D organic–inorganic and all‐inorganic lead halide perovskites have been intensively pursued for resistive switching memories in recent years. Unfortunately, instability and lead toxicity are two foremost challenges for their large‐scale commercial applications. Dimensional reduction and composition engineering are effective means to overcome these challenges. Herein, low‐dimensional inorganic lead‐free Cs3Bi2I9 and CsBi3I10 perovskite‐like films are exploited for resistive switching memory applications. Both devices demonstrate stable switching with ultrahigh on/off ratios (≈106), ultralow operation voltages (as low as 0.12 V), and self‐compliance characteristics. 0D Cs3Bi2I9‐based device shows better retention time and larger reset voltage than the 2D CsBi3I10‐based device. Multilevel resistive switching behavior is also observed by modulating the current compliance, contributing to the device tunability. The resistive switching mechanism is hinged on the formation and rupture of conductive filaments of halide vacancies in the perovskite films, which is correlated with the formation of AgIx layers at the electrode/perovskite interface. This study enriches the library of switching materials with all‐inorganic lead‐free halide perovskites and offers new insights on tuning the operation of solution‐processed memory devices.

Uniform multilevel switching of graphene oxide-based RRAM achieved by embedding with gold nanoparticles for image pattern recognition

Traditional metal-oxide semiconductor devices are inadequate for use in artificial neural networks (ANNs) owing to their high power consumption, complex structures, and difficult fabrication techniques. Resistive random access memory (RRAM) is a promising candidate for ANNs owing to its simple structure, low power consumption, and excellent compatibility with CMOS. Moreover, it can mimic synaptic functions because of its multilevel resistive switching (RS) behavior. Herein, we demonstrate highly uniform RS and a high on/off ratio of RRAM based on graphene oxide by embedding gold nanoparticles into the device. This allowed reliable multilevel storage. Further, multilevel RRAM based on spike-timing-dependent-plasticity learning rules was used for image pattern recognition. These findings may offer a route to develop reliable digital memristors for ANNs.

Multilevel Resistive Switching in Hf-Based Rram

Resistive random-access memory (RRAM) device has attracted wide attention for next-generation nonvolatile memory (NVM) application due to simple structure, high cycling endurance, good retention, low power consumption and fast switching time (1, 2). In recent years, RRAM devices are extensively researched for multi-level data storage (3, 4). RRAM devices can be driven to various resistance states by applying suitable compliance current values during the switching operation. In this work, multilevel resistive switching behaviors of RRAM devices with three different dielectric materials like HfO2, HfAlO2 and HfZrO2 are investigated with fixed top/bottom electrodes. To control the resistance state, a thin Al2O3 layer (1 nm) was inserted between the bottom electrode and switching dielectric (Ti/TiN/Al2O3/HfAlO2 or HfZrO2 or HfO2/Ti/TiN). The fabrication details of these devices are presented elasewhere (5). Fig. 1 shows DC endurance cycling on the LRS/HRS resistances of the three RRAM structures, (a) HfAlO2, (b) HfZrO2 and (c) HfO2 for different increasing compliance currents. The endurance cycling is performed with 20 cycles per compliance level. As can be seen from the figure, different low resistance states (LRS) and high resistance states (HRS) are well resolved after dc endurance cycling for HfAlO2 and HfZrO2 based RRAM devices. Devices with HfO2, on the other hand, does not show any multilevel LRS or HRS behavior for increase in compliance current. The reason is possibly due to the lack of oxygen vacancies available in the HfO2-RRAM structure. When HfZrO2 was subjected to a post deposition annealing (PDA) in nitrogen environment, the resistance distribution of LRS and HRS was improved, possibly due to improvement of dielectric quality while the oxygen vacancy concentration remained the same. To further improve the resistance window an additional 2 nm thick TiN layer was added before the Ti layer on top electrode of HfZrO2 based RRAM. The cycle to cycle distribution of HRS/LRS resistance of HfZrO2 based RRAM was significantly improved while the compliance current for switching was increased. In summary, process optimization and electrode selection with HfZrO2 as the switching layer can provide significant multilevel storage in RRAM devices.

Associative Memory for Image Recovery with a High‐Performance Memristor Array

AbstractAssociative memory is one of the significant characteristics of the biological brain. However, it has yet to be realized in a large memristor array due to the high requirements on the memristor device. In this work, the multilevel memristor cell is optimized by employing an electro‐thermal modulation layer. Memristor devices show both high resistance, cell‐to‐cell uniformity, and multilevel resistive switching behaviors with good reliability. A Hopfield neural network is experimentally demonstrated on a 1k memristor array that is capable of realizing the associative memory function for emotion image recovery. By using both asynchronous and synchronous refresh schemes, complete emotion images can be recalled from partial information.

Room temperature deposited oxygen-deficient CeO2−x layer for multilevel resistive switching memory

Precise switchover of numerous resistance states is desirable to achieve repeatable multilevel data storage memory windows. However, this could be hindered by the inherent random nature of oxygen vacancy creation. In this study, oxygen-deficient CeO2−x thin films were fabricated on indium tin oxide (ITO)-coated/glass substrates using radio-frequency magnetron sputtering at room temperature to obtain simple Ni/CeO2−x/ITO/glass devices. Multilevel bipolar resistive switching characteristics were investigated by controlling the RESET voltage and current compliance. The device could be regarded as four-level switching memory, controlled by the RESET voltage. An increase in the RESET-stop voltage from −1.5 to −2.0 V increased the Schottky barrier height, leading to an improvement in the ON/OFF ratio. The multilevel resistive switching behavior could be attributed to the formation and partial rupture of conductive filaments owing to interfacial oxygen ion migration. This study shows the potentials of the Ni/CeO2−x/ITO/glass devices for multilevel data storage resistive random-access memory applications.

Multilevel resistive switching behaviors of N2-plasma-treated stacked GdOx/SiNx RRAMs

The resistive switching (RS) characteristics of N2-plasma-treated SiNx and stacked GdOx/SiNx resistance random access memories (RRAMs) have been investigated. With the N2 plasma treatment on SiNx films, the operation voltages and resistance ratio of SiNx RRAMs are significantly improved by increasing the concentration of nitrogen vacancies, leading to a reduction in the Schottky barrier height at the Ir/SiNx interface. The device reliabilities of N2-plasma-treated SiNx RRAMs such as a data retention of more than 104 s and an endurance of 100 cycles for a resistance ratio of more than two orders of magnitude are also obtained. Further, the stacked GdOx/SiNx RRAMs present multilevel RS behaviors with an unbalanced formation and rupture of the conductive filaments, resulting from the differences in dielectric permittivity and Gibbs free energy of the stacked RS materials. A resistance ratio of more than one order of magnitude between each resistance state is achieved with a stable cycling endurance and data retention testing. The Schottky emission and space-change-limited-conduction are responsible for the carrier transport mechanism of the stacked GdOx/SiNx RRAMs at a high resistance state and middle resistance state, respectively. Stacked GdOx/SiNx RRAMs with N2 plasma treatment have the potential to be adopted in future high-density nonvolatile memories.

Electroforming-less and multi-level resistive switching characteristics in tungsten oxide thin film

Thermally grown tungsten oxide (WOx) thin film on a tungsten electrode is studied as a potential memristor material. Due to the dislocation formation inside the film, the device could present stable memristive characteristics without electroforming process. In this research, the opportunity of applying the WOx memristor to the multi-level resistive switching behavior based multi-level memory or artificial synaptic device was presented. In particular, the flexibility of the device shows the possibility of a future flexible memory device. Interpreting the memristive characteristics, the resistance switching mechanism was discussed based on the valance change at the device structure interface. Based on the simple fabrication process and electroforming-less operation, the results present the possibility of applying WOx based memristors in current electronic devices at an early stage. This work reveals a useful proposal of WOx based memristors for emerging device applications.

Morphological regulation of all-inorganic perovskites for multilevel resistive switching

Enormous attention has been paid to all-inorganic cesium lead halide perovskites in various photoelectronic fields for their remarkable performances. However, comparing to their analogue organic-inorganic hybrid perovskites, the film morphology of such all-inorganic lead halide perovskites is difficult to control due to the low solubility of cesium salt. Here, we propose a new fabrication routine to control the film morphology of CsPbBr3. A series of CsPbBr3 thin films with big grains (≈800 nm) were successfully prepared. The memristors based on such CsPbBr3 thin films take on typical bipolar resistive switching behavior and remarkable characteristics such as high Ron/Roff ratio (≈105), very low working voltage (≈±1 V), and long data retention (≥104 s). Furthermore, through modulating the film morphology, memristors with multilevel resistive switching behavior can be easily prepared. These advantages demonstrate that the all-inorganic cesium lead halide memristors possess great potential for future application.

The effects of MCNTs on electro-spinning carbonization microstructure of polyimide composite film and resistive switching behavior

Polymer matrix electronic memory devices exhibit many advantages, such as simple structure, good scalability, high mechanical flexibility, and low fabrication cost. In this paper, polyimide nanofibers and composite nanofibers combined by multi-walled carbon nanotubes (MCNTs) were prepared using electro-spinning method, and carbonized to obtain carbon nanofibers (SC-PI) and carbon composite nanofibers (SC-MCNTs). Based on in-situ polymerization method, two novel polyimide composite memory devices were fabricated using composite polyimides doped with those carbon nanofibers. The TEM and SAXS test results confirm that the MCNTs combined into the carbon nanofibers introduces two transition zones (TZs) locating between MCNTs and carbon nanofibers and between carbon nanofibers and PI matrix, resulting in a special multilevel resistive switching behavior of the SC-MCNTs/PI memory device different from the SC-PI/PI memory device, with a high ON/OFF current ratio about 103–104 and an excellent retention > 3000 s. Based on the TZs distribution of the composites, different physical models are proposed to explain the charge transmission mechanism of the resistive switching behaviors. The studies on the effects of the MCNTs on carbon composite microstructures relative to the multilevel resistive switching are possible to further improve the performance of polymer-based memory devices.

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.

Fabrication of polymer composite films with carbon composite nanofibers doped MWNTs-OH for multilevel memory device application

Novel carbon composite nanofibers with hierarchical microstructure were synthesized using electro-spinning and carbonization (SC) techniques, and were combined into polyimide matrix based on in-situ polymerization method. The resulted polyimide composites exhibit multilevel resistive switching behavior (MRSB) for memory storage applications, with a favorable ON/OFF current ratio and a high retention time at an operating voltage less than 3 V. A hierarchical microstructure can be observed by SEM, TEM and SAXS tests, which was formed with different sizes between the hydroxyl multi-walled carbon nanotubes (MWNTs-OH), carbon nanofibers and PI matrix, influencing on the MRSB of the composite films. A physical model based on the molecular orbit energy level was utilized to investigate how the microstructures acting as charge transmission layers affect the MRSB. This result provides a new strategy for further improving the high density storage of polymer multilevel switching memory storage device.

Diverse spike-timing-dependent plasticity based on multilevel HfO x memristor for neuromorphic computing

Memristors have emerged as promising candidates for artificial synaptic devices, serving as the building block of brain-inspired neuromorphic computing. In this letter, we developed a Pt/HfO x /Ti memristor with nonvolatile multilevel resistive switching behaviors due to the evolution of the conductive filaments and the variation in the Schottky barrier. Diverse state-dependent spike-timing-dependent-plasticity (STDP) functions were implemented with different initial resistance states. The measured STDP forms were adopted as the learning rule for a three-layer spiking neural network which achieves a 75.74% recognition accuracy for MNIST handwritten digit dataset. This work has shown the capability of memristive synapse in spiking neural networks for pattern recognition application.

Thermal Stability of HfOx-Based Resistive Memory Array: A Temperature Coefficient Study

In HfOx-based RRAM array, the temperature dependent shift of resistance at the low resistance state is statistically distributed. The differences in temperature dependent shifts among cells bring challenge to the reliability of analog or multilevel resistive switching behavior, namely thermal instability, which is characterized by the temperature coefficient ( $\text{T}_{\alpha }$ ) of RRAM. Its effect on RRAM performances was studied by pulse programming and retention measurement. Through atomistic simulation, this letter showed that thermal instability of cells with a positive $\text{T}_{\alpha }$ was related to the arrangement of oxygen vacancy inside the switching layer. Based on this model, the probability of cells with a positive $\text{T}_{\alpha }$ was found to be effectively reduced by introducing a baking process which led to an overall improvement of retention for the 1 k bit array.

Multi-level resistive switching behaviors and retention characteristics in ZnO/Nb:SrTiO3 heterojunction

Epitaxial ZnO thin films were grown on SrTiO3:Nb (NSTO) substrates by rf magnetron sputtering method. The multi-level resistance states were observed by applying different amplitudes and/or polarities of voltage pulses, which is supposed to be related to the drift of oxygen vacancies. Furthermore, the decay of retention is also corresponding to the migration of oxygen vacancies. The retention and cycle stability implies that the ZnO/Nb:SrTiO3 heterojunctions are promising for high density memory application.

B12-O-08Microstructural Investigation of Multi-level Resistive Switching Behavior in Multi-layered Pt/TaOxusing In-situ TEM

B12-O-08Microstructural Investigation of Multi-level Resistive Switching Behavior in Multi-layered Pt/TaO_xusing In-situ TEM

Multiferroic and multilevel resistive switching properties of LaFeO3–PbTiO3 films grown on Nb:SrTiO3 (0 0 1) substrate

Multiferroic properties of LaFeO3–PbTiO3 (LFPTO) epitaxial thin films grown on Nb-doped SrTiO3 (001) (NSTO) substrate by hydrothermal method were investigated at room temperature, which was demonstrated by the ferroelectric (2Pr=3.95μC/cm2, 2Ec=38.6kV/cm at applied electric field 66.8kV/cm) and magnetic (2Mr=12.13emu/cm3, 2Hc=3000Oe at applied magnetic field 1.6T) hysteresis loops. Furthermore, stable multilevel bipolar resistive switching behaviors controlled by the applied reset voltage were observed in the Pt/LFPTO/NSTO/Pt structures. Multiferroic LaFeO3–PbTiO3 thin films combing good resistive switching behaviors have potential application in multifunctional devices.