Multilevel Data Storage Research Articles

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

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.

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.

A flexible nickel phthalocyanine resistive random access memory with multi-level data storage capability

Metal phthalocyanine is considered one of the most promising candidates for the design and fabrication of flexible resistive random access memory (RRAM) devices due to its intrinsic flexibility and excellent functionality. However, performance degradation and the lack of multi-level capability, which can directly expand the storage capacity in one memory cell without sacrificing additional layout area, are the primary obstacles to the use of metal phthalocyanine RRAMs in information storage. Here, a flexible RRAM with pristine nickel phthalocyanine (NiPc) as the resistive layer is reported for multi-level data storage. Due to its high trap-concentration, the charge transport behavior of the device agrees well with classical space charge limited conduction controlled by traps, leading to an excellent performance, including a high on-off current ratio of 107, a long-term retention of 106 s, a reproducible endurance over 6000 cycles, long-term flexibility at a bending strain of 0.6 %, a write speed of 50 ns under sequential bias pulses and the capability of multi-level data storage with reliable retention and uniformity.

Mechanical Manipulation of Nano‐Twinned Ferroelectric Domain Structures for Multilevel Data Storage

AbstractFerroelectric materials feature a switchable spontaneous electric polarization and can enable low‐power logic and nonvolatile memories. These applications require reliable and precise control of ferroelectric domains and domain walls in ferroelectric thin films. Mechanical manipulation is a promising route to engineer ferroelectric domains, but it has proved ineffective when going beyond a critical thickness. Here, multi‐step 90° switching polarization reversal processes in (111)‐oriented PbZr0.2Ti0.8O3 thin films by applying mechanical forces along the direction parallel to the domain bands are reported. By probing the interrelationships between the relevant order parameters, coupled lattice distortion and piezoelectricity is revealed to facilitate domain switching from downward to upward in PbZr0.2Ti0.8O3, a mechanism that is supported by the evolution of domains and electrical performances at different temperatures and under varying pressures, respectively. The multi‐step domain reversal processes render PbZr0.2Ti0.8O3 thin films an excellent candidate for multilevel data storage. The study's results have implications for the manipulation of polarization switching in ferroelectrics and open an avenue to domain reversal driven by mechanical loads for the development of next‐generation ferroelectric devices.

Exploring Phase‐Change Memory: From Material Systems to Device Physics

To deal with the growing demand for data storage and processing, phase‐change memory (PCM) provides one of the most promising candidates for next‐generation nonvolatile data storage and neuromorphic computing applications. A lot of effort has been made toward optimizing the materials and device design; thus, excellent device performances have been achieved including high density, fast switching speed, great endurance, and retention. In addition, the widely tunable optical characteristics of PCMs are irresistibly attractive for optoelectronic or all‐optical applications with unprecedented bandwidth, low energy consumption, and multilevel data storage. Herein, the materials system and switching mechanisms on experimental and modeling methods for PCM designs and applications are discussed. Electric‐domain and optical‐domain PCM‐based artificial synapses/neurons and their applications in neuromorphic computing are also reviewed. Finally, the future prospects and challenges of PCM‐based applications on materials, devices, algorithms, and system levels are highlighted.

Reconfigurable optical memory based on MoS2/QDs mixed-dimensional van der Waals heterostructure

Optical memory unit with the ability to detect and store optical signals is increasingly becoming a crucial part of advanced data communication and image sensing technology. Despite great efforts devoted to develop high-performance optical memory devices based on two-dimensional (2D) material, the photoelectric conversion is still limited to defect-dominant photo-generated carrier trapping/de-trapping process at the interface of 2D materials. Here, a reconfigurable optical memory implanted with photonic programming/electric erasing operation is demonstrated based on MoS2/quantum dots (QDs) mixed-dimensional heterostructure. Unique photoelectric coupling effect between MoS2 and QDs leads to a continuous n-doping on MoS2 channel after light exposure removed, resulting in the generation of persistent photocurrent. Excellent optical memory characteristics such as high programming/erasing ratio, long retention time and stable operation cycles have been achieved and demonstrated to be gate-tunable. Besides, multi-level optical data storage with ten different states is also realized by applying a series of programmable optical signals. Owing to the unique dynamic response of mix-dimensional van der Waals heterostructure to optical and electric signals, our proposed optical memory transistor may pave up a new path to explore photoelectric conversion in low-dimensional system and develop high-performance optoelectronics for a broad range of applications.

Multibit tribotronic nonvolatile memory based on van der Waals heterostructures

Low power and multifunctional nonvolatile memories are promising candidates for processing massive data in the Internet of Things era. However, the storage states in conventional memory devices are under the restricted control by electrical or optical signals. Herein, a new multibit tribotronic nonvolatile memory (T-NVM) based on a graphene/hexagonal boron nitride/molybdenum disulfide van der Waals heterostructure and triboelectric nanogenerator (TENG) is proposed. The programming/erasing states can be modulated by the triboelectric potential, which is determined by changing the distance between the two triboelectrification layers. Under the modulation of external mechanical actions, the device exhibits a high on/off ratio of 105 via manipulating mechanical distance from − 0.2 mm to + 0.2 mm, a long retention time up to 6000 s, a stable switching behavior for over 100 cycles, and a multilevel data storage capability of 14 stages by different external stimuli. Furthermore, a memory inverter circuit employing the triboelectric potential as input signals can serve as the conversion of logical signals. This work proves the great potential of tribotronic devices for direct interaction with external environment in lower power and broadening diverse applications of human-robot interactions, self-powered wearable devices, and intelligent instrumentation.

Engineering an Indium Selenide van der Waals Interface for Multilevel Charge Storage.

As the continuous miniaturization of floating-gate transistors approaches a physical limit, new innovations in device architectures, working principles, and device materials are in high demand. This study demonstrated a nonvolatile memory structure with multilevel data storage that features a van der Waals gate architecture made up of a partially oxidized surface layer/indium selenide (InSe) van der Waals interface. The key functionality of this proof-of-concept device is provided through the generation of charge-trapping sites via an indirect oxygen plasma treatment on the InSe surface layer. In contrast to floating-gate nonvolatile memory, these sites have the ability to retain charge without the help of a gate dielectric. Together with the layered structure, the surface layer with charge-trapping sites facilitates continual electrostatic doping in the underlying InSe layers. The van der Waals gating effect is further supported by trapped charge-induced core-level energy shifts and relative work function variations obtained from operando scanning X-ray photoelectron spectroscopy and Kelvin probe microscopy, respectively. On modulating the amount of electric field-induced trapped electrons by the electrostatic gate potential, eight distinct storage states remained over 3000 s. Moreover, the device exhibits a high current switching ratio of 106 within 11 cycles. The demonstrated characteristics suggest that the engineering of an InSe interface has potential applications for nonvolatile memory.

Ga2O3 based multilevel solar-blind photomemory array with logic, arithmetic, and image storage functions.

Photomemories offer great opportunities for multifunctional integration of optical sensing, data storage, and processing into one single device. However, little attention has been paid to photomemories working in the solar-blind region so far, which may have unique advantages of insusceptibility to ambient light and higher capacity. Herein, we propose and demonstrate a Ga2O3 based solar-blind photomemory array with logic, arithmetic, and optoelectronic memory functions. The device shows n-type field effect-transistor performance with an on/off ratio as high as 106, a responsivity of 8 × 103 A W-1, and a detectivity of 1.42 × 1014 Jones, all of which are amongst the best values ever reported for Ga2O3 based photodetectors. Based on the trapping and de-trapping process of holes in Ga2O3, multilevel data storage can be realized from the device. Simultaneously, the optical and electrical mixed basic logic of reconfigurable "AND" and "OR" operations have been realized in a single cell through the co-regulation of solar-blind light and the grid voltage. In addition, the photomemory can perform counting and addition operations, and the photomemory array can be utilized to realize solar-blind image storage. The results suggest that Ga2O3 may have potential applications in high-performance information storage, computing, and communications.

Spray-Coated, Volatile and Nonvolatile, Two-Terminal, Resistive Switching Memory Devices Comprising Liquid-Exfoliated Black Phosphorus and Graphene Layers

Resistive switching memory devices fabricated with black phosphorus inks showed volatile memory device characteristics, specifically static random access memory (SRAM) and bipolar resistive switching. A high ON-/OFF-current ratio of 6.5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> was obtained at a reading voltage of 0.5 V with good retention stability (over 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> s). Multilevel data storage performance under different compliance currents was demonstrated. Importantly, a nonvolatile memory device was also fabricated, using an ink that comprised thinner, on average, black phosphorus flakes with a narrower thickness distribution than the ink used in the above described devices. The nonvolatile memory device showed good write/erase operation, like flash memory, during 100 endurance cycles, and good retention stability with 1.9 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> of on/off ratio at 0.5 V. According to these results, we suggest that thin films of liquid-exfoliated black phosphorus deposited by spray coating are suitable for low-cost, solution-processed, two-terminal resistive memory devices, either volatile or nonvolatile ones, the memory device type being controlled by the details of the ink preparation process, and the resulting flake thickness distributions.

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.

Lead-Free Dual-Phase Halide Perovskites for Preconditioned Conducting-Bridge Memory.

Organometallic and all-inorganic halide perovskites (HPs) have recently emerged as promising candidate materials for resistive switching (RS) nonvolatile memory due to their current-voltage hysteresis caused by fast ion migration. Lead-free and all-inorganic HPs have been researched for non-toxic and environmentally friendly RS memory devices. However, only HP-based devices with electrochemically active top electrode (TE) exhibit ultra-low operating voltages and high on/off ratio RS properties. The active TE easily reacts to halide ions in HP films, and the devices have a low device durability. Herein, RS memory devices based on an air-stable lead-free all-inorganic dual-phase HP (AgBi2 I7 -Cs3 Bi2 I9 ) are successfully fabricated with inert metal electrodes. The devices with Au TE show filamentary RS behavior by conducting-bridge involving Ag cations in HPs with ultra-low operating voltages (<0.15V), high on/off ratio (>107 ), multilevel data storage, and long retention times (>5 × 104 s). The use of a closed-loop pulse switching method improves reversible RS properties up to 103 cycles with high on/off ratio above 106 . With an extremely small bending radius of 1mm, the devices are operable with reasonable RS characteristics. This work provides a promising material strategy for lead-free all-inorganic HP-based nonvolatile memory devices for practical applications.

Generation, control and erasure of dual LIPSS in germanium with fs and ns laser pulses

Laser-induced periodic surface structures (LIPSS) can readily be fabricated in virtually all types of materials and benefit from an efficient parallel patterning strategy that exploits self-organization. The wide range of different LIPSS types with different spatial scales and symmetries is continuously growing, addressing numerous of applications. Here, we report on the formation of two fundamentally different types of LIPSS on germanium upon exposure to femtosecond laser pulses (λ = 800 nm, 130 fs), featuring different periods and orthogonal orientations. On the one hand, the well-known low-spatial frequency LIPSS (LSFL) with a period ≈ λ and perpendicular orientation to the laser polarization are formed, which can be extended homogeneously in 2D by sample scanning. Additionally, extremely smooth ripples with a period ≈λ/2 and parallel orientation were generated at lower pulse numbers. We show that this new kind of ripples, named parallel high-spatial frequency LIPSS (HSFL-∥), can be superimposed onto LSFL by increasing the pulse number, forming complex dual LIPSS with nanohill-like morphology. While exposure to multiple nanosecond laser pulses is found to trigger also the formation of LSFL, HSFL-∥ cannot be formed under these conditions, which points out the role of ultrafast excitation in the formation of the latter. By performing time-resolved reflectivity measurements, we are able to resolve the melting and solidification dynamics, revealing melting of a very shallow surface layer (<20 nm) and melt durations of a few ns for both pulse durations pulses at the fluences employed for LIPSS formation. Finally, we demonstrate erasure of both types of LIPSS by exposure to single nanosecond pulses at high fluences, which paves the way for erasable multi-level data storage.

Multibit optoelectronic memory using graphene/diamond (carbon sp2-sp3) heterojunctions and its arithmetic functions

This work demonstrates that graphene/diamond (carbon sp2-sp3) heterojunctions can be used as multibit optoelectronic memory, where light information is stored as multilevel resistance in a nonvolatile manner. The carbon heterojunctions exhibit a large memory switching ratio of ∼104 and a retention time of &amp;gt;100 min, which allows for multilevel and nonvolatile data storage of optical information. The carbon heterojunctions also exhibit an apparent response to optical pulses, and the output current (conductivity of the junctions) increased linearly in response to the total number of optical pulses. Simple optical arithmetic operations such as accumulation, subtraction, and counting can be performed by using the multiple resistance states of the heterojunctions. The carbon heterojunctions have light sensing, memory, and arithmetic functions in a single device, and they are expected to pave the way for the production of innovative optical computing devices with multifunctional integration of sensing, memory, and calculation functions.

Brain-like spatiotemporal information processing with nanosized second-order synaptic emulators; “solid-state memory visualizer”

Abstract Emulating bio-counterpart artificial intelligence abilities at the hardware level requires high-density integration of artificial synapses. For that, not only nanosize artificial synapses with reliable multilevel functionality is needed but visualization of critical information processing within it is also essential. Here, we demonstrate nanosize second-order synaptic emulator and observed the dynamics of neuromorphic spatiotemporal information processing using local probe force microscopy. Particularly, the device shows stable analog hysteresis loop opening and multilevel memory storage, which was confirmed by current mapping. All versatile and necessary synaptic features like shot-, long-term memory, and corresponding dynamics, long term potentiation, and depression and pulse pair facilitation are demonstrated. Further, both Bienenstock, Cooper, and Munro learning rules e.g., frequency-dependent synaptic weight change and sliding threshold characteristics ‒ spike rate-dependent plasticity are confirmed. Based on Kelvin probe force microscopy measurements, the observed results are quantitatively explained as a dynamic of charge trapping/detrapping. Benefited from high-density integration and multilevel data storage along with processing, our approach exhibit an attractive future to build advance neuromorphic circuits at nanoscale.

The realization of insulator-metal transition in a p-type metastable ZnSb by dual-phase nanostructure

Insulator-metal transition (IMT) in a metastable phase of ZnSb-based films has been explored systematically. It is found that the reduction of thickness contributes to the precipitation of metastable structure and induces a wide controllable temperature span for the presence of metastable ZnSb phase, resulting in thickness dependent IMT of p-type ZnSb film during annealing process. The metastable structure can be obtained without any void generation after erbium-filling by the formation of crystalline/crystalline dual-phase nanostructure, which can improve the electrical transport and trigger IMT behavior with the temperature of 430 °C, providing a strategy for realizing multi-level data storage.

Control of binary states of ferroic orders in bi-domain BiFeO3 nanoislands

Understanding switching mechanisms in multiferroics such as BiFeO3 (BFO) is an important challenge to control ferroic orders (ferroelectric or ferroelastic) as it could lead to the design of non-volatile memories based on magnetoelectric coupling. Here, we demonstrate an alternative way to control the binary states of ferroic orders by locally applying pressure and electric field in ferroelectric bi-domains confined in single BFO nanoislands. The study of the electronic transport properties and domain orientations using atomic force microscopy (AFM) based techniques enabled us to determine the electric and mechanical parameters at which ferroelectric and ferroelastic resistive switching can be observed. Nanoislands exhibited binary high and low resistance states without scaling effect, with high performance switching characteristics. Positive-forward rectifying behavior at high tip force was interpreted by the formation of a subsurface non-conductive interface due to the strain gradient. Ferroelastic switching at the surface was associated with a symmetry-breaking induced by electromechanical coupling between the AFM tip and the BFO thin film. It led to out-of-plane polarization pinning that allows performing only in-plane switching accompanied by nucleation and propagation of a conductive domain wall. The control of ferroic binary states by the electric field and pressure may pave the way for multilevel data storage devices.

Structural Transitions in Ge2Sb2Te5 Phase Change Memory Thin Films Induced by Nanosecond UV Optical Pulses.

Ge-Sb-Te-based phase change memory alloys have recently attracted a lot of attention due to their promising applications in the fields of photonics, non-volatile data storage, and neuromorphic computing. Of particular interest is the understanding of the structural changes and underlying mechanisms induced by short optical pulses. This work reports on structural changes induced by single nanosecond UV laser pulses in amorphous and epitaxial Ge2Sb2Te5 (GST) thin films. The phase changes within the thin films are studied by a combined approach using X-ray diffraction and transmission electron microscopy. The results reveal different phase transitions such as crystalline-to-amorphous phase changes, interface assisted crystallization of the cubic GST phase and structural transformations within crystalline phases. In particular, it is found that crystalline interfaces serve as crystallization templates for epitaxial formation of metastable cubic GST phase upon phase transitions. By varying the laser fluence, GST thin films consisting of multiple phases and different amorphous to crystalline volume ratios can be achieved in this approach, offering a possibility of multilevel data storage and realization of memory devices with very low resistance drift. In addition, this work demonstrates amorphization and crystallization of GST thin films by using only one UV laser with one single pulse duration and one wavelength. Overall, the presented results offer new perspectives on switching pathways in Ge-Sb-Te-based materials and show the potential of epitaxial Ge-Sb-Te thin films for applications in advanced phase change memory concepts.