Multilevel Storage Research Articles

Switching-enhanced RRAM for reliable synaptic simulation and multilevel data storage

In this study, silkworm body fluid (SF), a natural biological material, was compounded with gold nanoparticles (Au NPs) to form a dielectric layer, and the biodegradable resistive random access memory (RRAM) of Al/SF:Au NPs/indium tin oxide (ITO) was fabricated. The experimental results show that embedding Au NPs increases the ON/OFF current ratio of the Al/SF:Au NPs/ITO device to 5.22 × 105. The resistances of the high resistance and low resistance states of the Al/SF:Au NPs/ITO device are well maintained, reaching 104 s. These devices can be easily fabricated using solution-based processes on substrates at room temperature, and they have reliable data storage capability. Because the compliance current is controlled, a single memory cell of a device based on SF:Au NPs has a multilevel (8 levels, 3 bits) resistance state for high-density storage, enabling multilevel data storage. Application of the device in simulating neurobehavior is further explored, and potentiation and depression of the simulated synapse can be achieved by applying pulses to the device. SF:Au NP-based RRAM shows great potential in multilevel data storage and neuromorphic computing. RRAM based on natural biomaterials has the advantages of biocompatibility, biodegradability and low cost, and it provides an important step toward the future development of bioelectronic devices.

Forming-free Pt/Al2O3/HfO2/HfAlOx/TiN memristor with controllable multilevel resistive switching and neuromorphic characteristics for artificial synapse

Controllable multilevel resistive switching (RS) and neuromorphic characteristics emerges as a promising paradigm to build power-efficient computing hardware for high density data storage memory and artificial intelligence. Nevertheless, the current nonvolatile memory still endures from reliability and variability of the memristors. In this work, Pt/Al2O3/HfO2/HfAlOx/TiN multilayer memristor was prepared by using atomic layer deposition (ALD) to examine the well-regulated multilevel RS and neuromorphic properties. The memristor was found to demonstrate admirable RS properties, including forming-free, low operating voltage (Set/Reset), high switching ratio (>100), multi-level retention time (104 s), and good durability (1000 switching cycles). Furthermore, seven and four resistance states can be accomplished by modulating CC through set-operation and stop-voltage during the reset-operation. By modulating the multi-level resistance state, the electronic synapse can simulate synaptic plasticity, such as potentiation/depression, paired pulse facilitation (PPF) and spike-timing-dependent plasticity (STDP). Results show that a multilayer memristor has potential in the application of multilevel data storage memory and bionic portable electronic devices.

Multilevel Switching in Phase‐Change Photonic Memory Devices

Multilevel storage in chalcogenide‐based phase‐change materials is one of the desired characteristics to design neuromorphic and in‐memory computing applications. However, precisely controlling the crystalline and amorphous fraction to achieve reliable multilevel states is one of the key challenges in multilevel switching. Herein, multilevel switching is focused on the aspect of optical domain, where it enjoys the benefits of higher bandwidth with low delay connectivity suitable for non‐von Neumann architecture. The essential requirements for multilevel optical switching are discussed in terms of programming techniques, novel device structures, and emerging materials for its better optimization. Furthermore, the impact of nature of crystallization mechanism on the multilevel switching for different families of phase‐change materials is reviewed. In addition, the multilevel switching for neuromorphic engineering and in‐memory computation based on the integrated photonic memory devices are assessed. Finally, several challenges and different strategies to improve the performance of multilevel switching in phase‐change materials are discussed and thereby signify its importance for the design of future on‐chip phase‐change photonic memory devices.

Graphene oxide-wrapped cobalt-doped oxygen-deficient titanium dioxide hollow spheres clusters as efficient sulfur immobilizers for lithium-sulfur batteries

The design of efficient sulfur host material is of great importance to address the undesirable polysulfide shuttling effect in the lithium-sulfur battery. Herein, we designed and constructed a sulfur immobilizer with multi-level sulfur storage space, which consists of the cluster of Co@TiO2 hollow spheres tightly wrapped by graphene oxide (labeled as Co@TiO2/GO). This structure provided a multi-level sulfur storage space, that is, the sulfur was encapsulated in the cavity of the Co@TiO2 hollow spheres, between Co@TiO2 spheres and GO sheets and in the interlaminar pores of GO. This enabled the inner and outer surfaces of the Co@TiO2 hollow spheres to better participate in the conversion reaction of lithium polysulfides (LPS). Moreover, the incorporated Co nanoparticles and abundance of oxygen vacancies effectively captured LPS and boosted their redox reaction kinetics. Therefore, this multi-level sulfur storage space is very beneficial to the storage and blockade of polysulfides and the improvement of the energy storage performance of the lithium-sulfur battery. Due to these beneficial structural features, the initial discharge capacity of Co@TiO2/GO/S cathode reached 807 mAh g−1 at 1C with a slow decay rate of 0.053% per cycle for 500 cycles, and high rate performance with a capacity of 552 mAh g−1 at 5C.

Bioresistive Random-Access Memory with Gold Nanoparticles that Generate the Coulomb Blocking Effect Can Realize Multilevel Data Storage and Synapse Simulation.

Gold nanoparticles (Au NPs) have good biocompatibility and special quantum effects. In this Letter, we embedded Au NPs into silkworm hemolymph (SH) to improve the performance of the device and fabricated Al/SH:Au NPs/indium tin oxide (ITO)/glass resistive random access memory. The device exhibits a bipolar switching behavior with a retention time of 104 s. Compared with the Al/SH/ITO device without Au NPs, the device has a higher ON/OFF current ratio (>105) and a smaller Vreset. The improvement in device performance is attributed to the fact that Au NPs act as the electron-trapping center in the device; a Coulomb blockade occurs after electrons are trapped, thereby increasing the resistance of the device in the high-resistance state. Using optimized devices can realize multilevel data storage and can also simulate synaptic characteristics such as potentiation and depression. The device is expected to be applied to high-density, low-cost, degradable, and biocompatible storage devices and neuromorphic computing in the future.

Enhancing the Data Reliability of Multilevel Storage in Phase Change Memory with 2T2R Cell Structure.

Multilevel storage and the continuing scaling down of technology have significantly improved the storage density of phase change memory, but have also brought about a challenge, in that data reliability can degrade due to the resistance drift. To ensure data reliability, many read and write operation technologies have been proposed. However, they only mitigate the influence on data through read and write operations after resistance drift occurs. In this paper, we consider the working principle of multilevel storage for PCM and present a novel 2T2R structure circuit to increase the storage density and reduce the influence of resistance drift fundamentally. To realize 3-bit per cell storage, a wide range of resistances were selected as different states of phase change memory. Then, we proposed a 4:3 compressing encoding scheme to transform the output data into binary data states. Therefore, the designed 2T2R was proven to have optimized storage density and data reliability by monitoring the conductance distribution at four time points (1 ms, 1 s, 6 h, 12 h) in 4000 devices. Simulation results showed that the resistance drift of our proposed 2T2R structure can significantly improve the storage density of multilevel storage and increase the data reliability of phase change memory.

UV light modulated synaptic behavior of MoTe2/BN heterostructure

Electrical synaptic devices are the basic components for the hardware based neuromorphic computational systems, which are expected to break the bottleneck of current von Neumann architecture. So far, synaptic devices based on three-terminal transistors are considered to provide the most stable performance, which usually use gate pulses to modulate the channel conductance through a floating gate and/or charge trapping layer. Herein, we report a three-terminal synaptic device based on a two-dimensional molybdenum ditelluride (MoTe2)/hexagonal boron nitride (hBN) heterostructure. This structure enables stable and prominent conductance modulation of the MoTe2 channel by the photo-induced doping method through electron migration between the MoTe2 channel and ultraviolet (UV) light excited mid-gap defect states in hBN. Therefore, it is free of the floating gate and charge trapping layer to reduce the thickness and simplify the fabrication/design of the device. Moreover, since UV illumination is indispensable for stable doping in MoTe2 channel, the device can realize both short- (without UV illumination) and long- (with UV illumination) term plasticity. Meanwhile, the introduction of UV light allows additional tunability on the MoTe2 channel conductance through the wavelength and power intensity of incident UV, which may be important to mimic advanced synaptic functions. In addition, the photo-induced doping method can bidirectionally dope MoTe2 channel, which not only leads to large high/low resistance ratio for potential multi-level storage, but also implement both potentiation (n-doping) and depression (p-doping) of synaptic weight. This work explores alternative three-terminal synaptic configuration without floating gate and charge trapping layer, which may inspire researches on novel electrical synapse mechanisms.

Solution-Processed Photoinduced Multilevel Resistive Switching Devices Based on Lead-Free All-Inorganic Perovskite

The letter reports lead free all-inorganic perovskite based resistive switching (RS) memory devices. The cesium tin bromide (CsSnBr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) perovskite films were prepared by the solution deposition technique. The memory devices exhibit bipolar resistive RS characteristic and light assisted multilevel storage capability. The proposed devices show a maximum ON/OFF ratio of ~ 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> , data retention of ~ 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> s, and endurance of 400 cycles. We explain the resistive switching effect on lights of the formation and rupture of a conducting path under an electric field. This work may provide an opportunity to develop transparent, flexible, and multibit storage capacity photonic resistive memory devices based on all-inorganic perovskites for successful logic application.

Electric Field‐Induced Area Scalability toward the Multilevel Resistive Switching

AbstractNonvolatile memory devices based on resistive switching with fast switching speed and high density have driven extensive research that is potentially ideal for data‐centric applications such as neuromorphic computing. However, due to uncontrolled filament formation, resistive random access memories (RRAM) still suffer from instability and reproducibility. In this study, NiO layer is added to the WO3‐based RRAM layer to confine the conducting path for multilevel resistive switching. The current‐voltage characteristics show the multilevel resistive switching, which is further confirmed by conductive atomic force microscopy measurements at nanoscale. At low voltages, few localized conducting channels are formed (in small area) within the oxide film while at the higher voltages they are distributed throughout the film and the active surface area increases from 4.5 to 75%. Kelvin probe force microscopy was employed to confirm nanoscale variations in surface potential, which are responsible for spatial current variation. Furthermore, it is found that the charge trapping/detrapping is the main governing mechanism and the conducting paths are formed gradually with increasing applied bias. We propose a strategy to achieve stable and reproducible electric field‐induced multilevel memory storage which will be utilized for the development of ultrahigh density multilevel nonvolatile storage and neuromorphic computing.

Tunable Multilevel Data Storage Bioresistive Random Access Memory Device Based on Egg Albumen and Carbon Nanotubes.

In this paper, a tuneable multilevel data storage bioresistive memory device is prepared from a composite of multiwalled carbon nanotubes (MWCNTs) and egg albumen (EA). By changing the concentration of MWCNTs incorporated into the egg albumen film, the switching current ratio of aluminium/egg albumen:multiwalled carbon nanotubes/indium tin oxide (Al/EA:MWCNT/ITO) for resistive random access memory increases as the concentration of MWCNTs decreases. The device can achieve continuous bipolar switching that is repeated 100 times per cell with stable resistance for 104 s and a clear storage window under 2.5 × 104 continuous pulses. Changing the current limit of the device to obtain low-state resistance values of different states achieves multivalue storage. The mechanism of conduction can be explained by the oxygen vacancies and the smaller number of iron atoms that are working together to form and fracture conductive filaments. The device is nonvolatile and stable for use in rewritable memory due to the adjustable switch ratio, adjustable voltage, and nanometre size, and it can be integrated into circuits with different power consumption requirements. Therefore, it has broad application prospects in the fields of data storage and neural networks.

Memristors Based on an Iridium(III) Complex Containing Viologen for Advanced Synaptic Bionics.

Memristors with nonvolatile memory properties are expected to open the era of neuromorphic computing. However, it remains a huge challenge to develop memristors with high uniformity, high stability, and low power consumption for advanced synaptic bionics. Herein, an electroactive iridium(III) complex Ir-vio was designed and synthesized by incorporating a viologen moiety into its N∧N ligand. Complex Ir-vio showed multiple redox states and high sensitivity to an electrical stimulus. Importantly, two-terminal memristors with Ag/Ir-vio/W structure were successfully fabricated by the solution-processable method, which exhibited multilevel storage characteristics with a low switching threshold voltage of 0.5 V and high ON1/ON2/ON3/OFF current ratio of 105/103/102/1 at a low reading bias of 0.05 V. Moreover, the memristors can mimic synaptic plasticity, indicating that they can act as artificial synapses to construct brain-inspired neural networks. The memristive mechanisms can be ascribed to the interconversion among different charge-transfer and redox states under various electrical stimulus. To the best of our knowledge, this work is the first experimental demonstration of memristors based on iridium(III) complexes, opening a new era for the development of synaptic bionic devices based on organometallic compounds.

Emulating synaptic plasticity and resistive switching characteristics through amorphous Ta2O5 embedded layer for neuromorphic computing

Memristors with controllable resistive switching (RS) characteristics gain significant attention for neuromorphic computing applications in high-density memory and artificial synapses. Herein, we present the consequence of an amorphous Ta2O5 embedded layer on RS and synaptic characteristics of ZrO2 memristor. Structural and electronic properties of the memristor without and with-Ta2O5 are exemplified by x-ray diffraction (XRD) and x-ray photoemission spectroscopy (XPS) measurements. Memristor with-Ta2O5 exhibits excellent performance in RS parameters such as lower forming/SET-voltage, improved uniformity of switching cycling, admirable pulse endurance (104), and long retention time (104 s). Moreover, multilevel storage capability is achieved through limiting the current compliance in SET-operation or stop-voltages in RESET-operation. Likewise, diverse synaptic characteristics such as long-term potentiation (LTP), long-term depression (LTD), spike-rate-dependent plasticity (SRDP), paired-pulse facilitation (PPF), and post-tetanic potentiation (PTP), spike-timing-dependent plasticity (STDP) are effectively mimicked, which is regarded as an imperative learning regime of biological synapses. Owing to improved linearity of LTP/LTD behaviors in memristor with-Ta2O5, a solid recognition rate (~85%) is achieved for pattern recognition with the modified national institute of standards and technology database (MNIST) handwritten numbers. Memristor with embedded ~2 nm Ta2O5 barrier layer shows significant potential applications in high-performance multilevel data storage and neuromorphic computing systems.

Multifunctional Optical Polymeric Films with Photochromic, Fluorescent, and Ultra‐Long Room Temperature Phosphorescent Properties

AbstractPhotochromism, fluorescence, and ultra‐long room temperature phosphorescence (URTP) are attractive phenomena in photonics. However, achieving multiple optical properties such as photochromism, fluorescence, and URTP on one material still remains a challenge. Here, the authors report a multifunctional optical polymeric film with photochromic, fluorescent, and URTP properties, fabricated by simply doping the naphthopyran molecule into epoxy polymer with 3D network. When the polymer network is loose, the naphthopyran‐doped film exhibits photochromic property but no fluorescence and phosphorescence. When the polymer network becomes dense, the film shows fluorescent and URTP behaviors but no photochromic property. It is noted that the room temperature phosphorescence lifetime is as high as 604 ms. Based on the excellent optical properties of reversible photochromism, fluorescence, and URTP, an advanced sixfold encryption application has been successfully established. This work not only opens up a new direction in design and fabrication of multifunctional optical materials but also offers principles for developing organic optical materials toward multilevel information encryption and data storage.

Multi-level phase-change memory with ultralow power consumption and resistance drift

By controlling the amorphous-to-crystalline relative volume, chalcogenide phase-change memory materials can provide multi-level data storage (MLS), which offers great potential for high-density storage-class memory and neuro-inspired computing. However, this type of MLS system suffers from high power consumption and a severe time-dependent resistance increase (“drift”) in the amorphous phase, which limits the number of attainable storage levels. Here, we report a new type of MLS system in yttrium-doped antimony telluride, utilizing reversible multi-level phase transitions between three states, i.e., amorphous, metastable cubic and stable hexagonal crystalline phases, with ultralow power consumption (0.6–4.3 pJ) and ultralow resistance drift for the lower two states (power-law exponent < 0.007). The metastable cubic phase is stabilized by yttrium, while the evident reversible cubic-to-hexagonal transition is attributed to the sequential and directional migration of Sb atoms. Finally, the decreased heat dissipation of the material and the increase in crystallinity contribute to the overall high performance. This study opens a new way to achieve advanced multi-level phase-change memory without the need for complicated manufacturing procedures or iterative programming operations.

Band‐tailored van der Waals heterostructure for multilevel memory and artificial synapse

AbstractTwo‐dimensional (2D) van der Waals heterostructure (vdWH)‐based floating gate devices show great potential for next‐generation nonvolatile and multilevel data storage memory. However, high program voltage induced substantial energy consumption, which is one of the primary concerns, hinders their applications in low‐energy‐consumption artificial synapses for neuromorphic computing. In this study, we demonstrate a three‐terminal floating gate device based on the vdWH of tin disulfide (SnS2), hexagonal boron nitride (h‐BN), and few‐layer graphene. The large electron affinity of SnS2 facilitates a significant reduction in the program voltage of the device by lowering the hole‐injection barrier across h‐BN. Our floating gate device, as a nonvolatile multilevel electronic memory, exhibits large on/off current ratio (~105), good retention (over 104 s), and robust endurance (over 1000 cycles). Moreover, it can function as an artificial synapse to emulate basic synaptic functions. Further, low energy consumption down to ~7 picojoule (pJ) can be achieved owing to the small program voltage. High linearity (&lt;1) and conductance ratio (~80) in long‐term potentiation and depression (LTP/LTD) further contribute to the high pattern recognition accuracy (~90%) in artificial neural network simulation. The proposed device with attentive band engineering can promote the future development of energy‐efficient memory and neuromorphic devices.image

Cr7Ge33Te60/Hf16Ge6Sb78 Superlattice‐Like Thin Film with Triple‐Phase Transitions for Multilevel Phase‐Change Memory

A multilevel phase‐change memory cell that is based on a CrGeTe(CrGT)/HfGeSb(HfGS) superlattice‐like (SLL) structure is proposed. With increasing temperature in resistance–temperature tests, the SLL thin films exhibit a three‐step phase‐change process that corresponds to the crystallization of Sb, GeTe, and CrGeTe. A stable and reversible three‐step phase change is identified as the key characteristic for realizing multilevel storage. This so‐designed multiple‐crystallization SLL structure is a feasible way to increase the storage density.

Optoelectronic Artificial Synapses Based on Two-Dimensional Transitional-Metal Trichalcogenide.

The memristor is a foundational device for an artificial synapse, which is essential to realize next-generation neuromorphic computing. Herein, an optoelectronic memristor based on a two-dimensional (2D) transitional-metal trichalcogenide (TMTC) is designed and demonstrated. Owing to the excellent optical and electrical characteristics of titanium trisulfide (TiS3), the memristor exhibits stable bipolar resistance switching (RS) as a result of the controllable formation and rupturing of the conductive aluminum filaments. Multilevel storage is realized with light of multiple wavelengths between 400 and 808 nm, and the synaptic properties such as conduction modulation and spiking timing-dependent plasticity (STDP) are achieved. On the basis of the photonic potentiation and electrical habitual ability, Pavlovian-associative learning is successfully established on this TiS3-based artificial synapse. All these results reveal the large potential of 2D TMTCs in artificial neuromorphic chips.

Low power consumption photoelectric coupling perovskite memristor with adjustable threshold voltage

Organic–inorganic halide perovskites (OHPs) have been proven to possess unique optical and electrical properties, and achieved more extensive application as excellent materials for memristors in recent years. Based on the traditional OHP-based memristors, the intermediate layer of the memristor was prepared using yttrium oxide (Y2O3)/OHP stacking structure in this manuscript. The potential barrier between Y2O3 and perovskite is relatively high (ΔE C = 2.13 eV) which leads to comparatively low current of the memristor, thus the power consumption can be reduced. Besides, by changing the external light conditions, one can realize sharp or slow switch between high resistance state (HRS) and low resistance state (LRS), so as to meet the requirement of multilevel data storage, which indicates its promising application prospect in information storage and biological simulation. In addition, based on characteristics of photoelectric coupling, the Y2O3/OHP memristor can also achieve the advantage of adjustable threshold voltage. The transition of HRS and LRS can be realized by changing the illumination condition at any voltage, which means the set and reset voltage are not fixed, so that the memristor with adjustable threshold voltage can adapt to various working conditions.

The optimized memristor performance by utilizing the constrained domain wall motion in Pt/Co–Tb/Ta structure

Spintronic devices can realize multilevel state storage and mimic the properties of the synapse, which enables their potential application in the field of artificial neural networks. In this paper, we demonstrate the existence of a large intermediate transition zone in current-induced magnetization switching curves of Pt/Co–Tb/Ta structures, and the number of states in the transition zone that can be manipulated by changing the Co content. The magneto-optical Kerr microscope imaging indicates that this property is related to the constrained domain wall motion in the Co–Tb films with large Co content. We also demonstrate the multilevel state storage properties of the sample by applying a sequence of current pulses. The synaptic plasticity behaviors were mimicked in these samples through regulating the value of Hall resistance by current pulses. The constrained domain wall motion supplies a simple but effective way to achieve multilevel state storage and show potential applications in neuromorphic computing.

Domains and domain dynamics in fluorite-structured ferroelectrics

Ferroelectricity in fluorite-structured ferroelectrics such as HfO2 and ZrO2 has been attracting increasing interest since its first publication in 2011. Fluorite-structured ferroelectrics are considered to be promising for semiconductor devices because of their compatibility with the complementary metal–oxide–semiconductor technology and scalability for highly dense information storage. The research on fluorite-structured ferroelectrics during the first decade of their conceptualization has been mainly focused on elucidating the origin of their ferroelectricity and improving the performance of electronic devices based on such ferroelectrics. Furthermore, as is known, to achieve optimal performance, the emerging biomimicking electronic devices as well as conventional semiconductor devices based on the classical von Neumann architecture require high operating speed, sufficient reliability, and multilevel data storage. Nanoscale electronic devices with fluorite-structured ferroelectrics serve as candidates for these device systems and, thus, have been intensively studied primarily because in ferroelectric materials the switching speed, reliability, and multilevel polarizability are known to be strongly correlated with the domains and domain dynamics. Although there have been important theoretical and experimental studies related to domains and domain dynamics in fluorite-structured ferroelectrics, they are yet to be comprehensively reviewed. Therefore, to provide a strong foundation for research in this field, herein, domains, domain dynamics, and emerging applications, particularly in neuromorphic computing, of fluorite-structured ferroelectrics are comprehensively reviewed based on the existing literature.