High-density Nonvolatile Memory Research Articles

Sol-Gel Derived ZnO Thin Films with Nonvolatile Resistive Switching Behavior for Future Memory Applications

Herein we report on a facile sol-gel spin-coating technique to fabricate ZnO thin films that grow preferentially along the (002) plane on FTO substrates. By employing the magnetron sputtering technique to deposit a tungsten (W) top metal electrode onto these ZnO thin films, we successfully realize a W/ZnO/FTO memory device that exhibits self-rectifying and forming-free resistive switching characteristics. Notably, the as-prepared device demonstrates impressive nonvolatile and bipolar resistive switching behavior, with a high resistance ratio (RHRS/RLRS) exceeding two orders of magnitude at a reading voltage of 0.1 V. Moreover, it exhibits ultralow set and reset voltages of approximately +0.5 V and −1 V, respectively, along with exceptional durability. In terms of carrier transport properties, the low resistance state of the device is dominated by ohmic conduction, whereas the high resistance state is characterized by trap-controlled space-charge-limited current conduction. This work highlights the potential of the ZnO-based W/ZnO/FTO memory device as a promising candidate for future high-density nonvolatile memory applications.

Developing fatigue-resistant ferroelectrics using interlayer sliding switching.

Ferroelectric materials have switchable electrical polarization that is appealing for high-density nonvolatile memories. However, inevitable fatigue hinders practical applications of these materials. Fatigue-free ferroelectric switching could dramatically improve the endurance of such devices. We report a fatigue-free ferroelectric system based on the sliding ferroelectricity of bilayer 3R molybdenum disulfide (3R-MoS2). The memory performance of this ferroelectric device does not show the wake-up effect at low cycles or a substantial fatigue effect after 106 switching cycles under different pulse widths. The total stress time of the device under an electric field is up to 105 s, which is long relative to other devices. Our theoretical calculations reveal that the fatigue-free feature of sliding ferroelectricity is due to the immobile charge defects in sliding ferroelectricity.

Multilevel resistive switching in hydrothermally synthesized FeWO4 thin film-based memristive device for non-volatile memory application

The memristors offer significant advantages as a key element in non-volatile and brain-inspired neuromorphic systems because of their salient features such as remarkable endurance, ability to store multiple bits, fast operation speed, and extremely low energy usage. This work reports the resistive switching (RS) characteristics of the hydrothermally synthesized iron tungstate (FeWO4) based thin film memristive device. The detailed physicochemical analysis was investigated using Rietveld’s refinement, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM) techniques. The fabricated Ag/FWO/FTO memristive device exhibits bipolar resistive switching (BRS) behavior. In addition, the devices exhibit negative differential resistance (NDR) at both positive and negative bias. The charge-flux relation portrayed the non-ideal or memristive nature of the devices. The reliability in the RS process was analyzed in detail using Weibull distribution and time series analysis techniques. The device exhibits stable and multilevel endurance and retention characteristics which demonstrates the suitability of the device for the high-density non-volatile memory application. The current conduction of the device was dominated by Ohmic and trap controlled-space charge limited current (TC-SCLC) mechanisms and filamentary RS process responsible for the BRS in the device. In a nutshell, the present investigations reveal the potential use of the iron tungstate for the fabrication of memristive devices for the non-volatile memory application.

Fluorinated Quinoxaline-Based Conjugated Polymers for High-Density Nonvolatile Memory Devices

Fluorinated Quinoxaline-Based Conjugated Polymers for High-Density Nonvolatile Memory Devices

Structural and electrical characteristics of Zr-doped HfO2 (HZO) thin films deposited by atomic layer deposition for RRAM applications

In this study, Zr-doped HfO2 (HZO) based resistive random-access memory (RRAM) device were fabricated. The Hf:Zr (1:1) ratio in the HZO films were controlled by changing the HfO2 and ZrO2 cycle ratio during the atomic layer deposition (ALD) process. Next, we studied the structural and electrical properties of the Au/HZO/TiN RRAM device structure. The RRAM devices exhibits an excellent resistance ratio of the high resistance state (HRS) to the low resistance state (LRS) of ~103 A, and as well as good endurance (300 cycles) and retention (>103 s), respectively. Further, the device showed different conduction mechanism in LRS and HRS modes. The lower biased linear region is dominated by ohmic conductivity, whereas the higher biased nonlinear region is dominated by a space charge limited current conduction. This device is suitable for application in future high-density nonvolatile memory RRAM devices.

Strain engineering of Bi2OS2 ultrathin films: electronic and ferroelectric properties

The non-monotonous relationship of ferroelectric polarization with strain can be attributed to distinct atomic coordination environments in Bi2OS2, which is different from a nearly monotonous trend of ferroelectricity-stabilized energy.

High tunneling electroresistance in ferroelectric tunnel junctions based on two-dimensional α-In2Se3/MoTe2 van der Waals heterostructures.

Ferroelectric polarization-controlled band alignment can be realized in van der Waals heterostructures (vdWHs), which can be used to create new types of ferroelectric tunnel junctions (FTJs). In this work, we design six probable configurations of two-dimensional vdWHs based on a two-dimensional α-In2Se3 ferroelectric material which has two opposite polarization states P↑ and P↓, and the semiconductor MoTe2. First-principles calculations show robust ferroelectric polarization-controlled switching behavior between the high conductance state in configuration AA-P↓ and the low conductance state in configuration AA-P↑ in the most stable AA stacked vdWHs. Based on this vdWH, a two-dimensional transverse FTJ with AA-P↓ or AA-P↑ as the tunneling barrier and (In0.5Sn0.5)2Se3 monolayers (n-type doped) as electrodes is designed. The tunneling electroresistance ratio of the FTJs at the Fermi level reaches 1.22 × 104% when the tunneling barrier contains two repeating units N = 2 and can be greatly increased by increasing the thickness of the ferroelectric layer. Analysis of the work function, charge redistribution, and local density of states is performed to interpret the above phenomena. The findings suggest the great potential of the AA stacked α-In2Se3/MoTe2 vdWH in the design of high-performance FTJs and application in high-density non-volatile memory devices.

Ultra-High-Density Ferroelectric Array Formed by Sliding Ferroelectric Moiré Superlattices.

2D van der Waals (vdW) materials offer infinite possibilities for constructing unique ferroelectrics through simple layer stacking and rotation. In this work, we stack nonferroelectric GeS2 and ferroelectric CuInP2S6 to form heterostructures by combining sliding ferroelectric polarization with displacement ferroelectric polarization to achieve multiple polarization states. First-principles calculations reveal that the polarization reversal of the CuInP2S6 component in the GeS2/CuInP2S6/GeS2 heterostructure can simultaneously drive the switching of sliding ferroelectric polarization, displaying a robust coupling of the two polarizations and leading to the overall polarization switching. Based on this, ferroelectric arrays with a density of 6.55 × 1012 cm-2 (equivalent to a storage density of 0.7 TB cm-2) were constructed in a moiré superlattice, and the polarization strength of array elements was 11.77 pC/m, higher than that of all reported 2D vdW out-of-plane ferroelectrics. High density, large polarization, and electrically switchable array elements in ferroelectric arrays provide unprecedented opportunities to design 2D high-density nonvolatile ferroelectric memories.

Self‐assembled vapor-transport-deposited SnS nanoflake-based memory devices with synaptic learning properties

The most salient features of resistive switching (RS) devices are low energy consumption, fast switching speed, and high-density integration, which render them promising candidates for realizing non-volatile memory and artificial synaptic devices. However, the growth of functional switching layers for RS devices needs innovative deposition techniques. Herein, we utilize a high-throughput vapor-transport-deposition (VTD) technique for synthesizing self-assembled tin-sulfide (SnS) nanoflakes, which are then used as a switching layer to fabricate an RS device. First principle calculations are conducted to understand the optoelectronic properties of SnS by employing density functional theory. The proposed Ag/SnS/Pt memory device exhibits substantial merits, including low-switching voltages (VSET: 0.22 V and VRESET: −0.20 V), suitable ON/OFF ratio (∼259), excellent endurance (106), and extended memory retention (106 s) characteristics. In addition, RS stochasticity is modeled using statistical time-series analysis via Holt’s exponential smoothing. Interestingly, the device can emulate multiple synaptic functionalities, including potentiation, depression, paired-pulse facilitation, paired-pulse depression, excitatory postsynaptic current, inhibitory postsynaptic current, and advanced spike-timing dependent plasticity rules. Moreover, the proposed synaptic device can detect the edge of images by utilizing a convolutional neural network. The unique and efficient VTD-SnS-based device will be a potential candidate for high-density non-volatile memory and neuromorphic computing applications.

Multivalued Memory via Freezing of Super-Hard Magnetic Domains in a Quasi 2D-Magnet.

The design of high-density non-volatile memories is a long-standing dream, limited by conventional storage "0" or "1" bits. An alternative paradigm exists in which regions within candidate materials can be magnetized to intermediate values between the saturation limits. In principle, this paves the way to multivalued bits, vastly increasing storage density. Single-molecule magnets, are good examples offering transitions between intramolecular quantum levels, but require ultra-low temperatures and limited relaxation time between magnetization states. It is showed here that the quasi 2D-Ising compound BaFe2 (PO4 )2 overcomes these limitations. The combination of giant magneto-crystalline anisotropy, strong ferromagnetic exchange, and strong intrinsic pinning creates remarkably narrow magnetic domain walls, collectively freezing under Tf ≈15K. This results in a transition from a soft to a super-hard magnet (coercive force > 14T). Any magnetization can then be printed and robustly protected from external fields with an energy barrier >9T at 2K.

Field-free spin-orbit torque switching assisted by in-plane unconventional spin torque in ultrathin [Pt/Co

Electrical manipulation of magnetization without an external magnetic field is critical for the development of advanced non-volatile magnetic-memory technology that can achieve high memory density and low energy consumption. Several recent studies have revealed efficient out-of-plane spin-orbit torques (SOTs) in a variety of materials for field-free type-z SOT switching. Here, we report on the corresponding type-x configuration, showing significant in-plane unconventional spin polarizations from sputtered ultrathin [Pt/Co]N, which are either highly textured on single crystalline MgO substrates or randomly textured on SiO2 coated Si substrates. The unconventional spin currents generated in the low-dimensional Co films result from the strong orbital magnetic moment, which has been observed by X-ray magnetic circular dichroism (XMCD) measurement. The x-polarized spin torque efficiency reaches up to −0.083 and favors complete field-free switching of CoFeB magnetized along the in-plane charge current direction. Micromagnetic simulations additionally demonstrate its lower switching current than type-y switching, especially in narrow current pulses. Our work provides additional pathways for electrical manipulation of spintronic devices in the pursuit of high-speed, high-density, and low-energy non-volatile memory.

Controllable coexistence of threshold and non-volatile crosspoint memory for highly linear synaptic device applications

A highly reliable and versatile resistive memory device that demonstrates threshold and non-volatile memory (NVM) switching behaviour depending on the compliance current (CC) modulation was utilised by doping a semiconducting (Si) material into a high-k (HfO x ) film with highly linear synaptic behaviour. The device shifted towards volatile switching at a CC less than 1 µA and exhibited NVM behaviour at a CC limit above 10 µA. A 3-bit/cell data storage capability on RESET voltage modulation was implemented for high-density memory application. The device exhibited excellent programming linearity of potentiation/depression responses up to 10 000 pulses compatible with fast pulse (100 ns) with good I ON/I OFF ratio (>103), stable data retention capability (105 s) at 85 °C and high WRITE endurance (∼107 cycles) with a pulse width of 200 ns. The neuromorphic applications were successfully emulated through neural network simulations using the experimentally calibrated data of the Si-doped HfO x resistive cross-point devices. Simulation results revealed a low nonlinearity of 0.03 with 98.08% pattern recognition accuracy. The estimated results revealed the potential of the device as a low-power selector and high-density NVM storage in large-scale crossbar array in future neuromorphic computing applications.

Magnetic-field-controlled spin valve and spin memory based on single-molecule magnets

A single-molecule magnet is a long-sought-after nanoscale component because it can enable us to miniaturize nonvolatile memory storage devices. The signature of a single-molecule magnet is switching between two bistable magnetic ground states under an external magnetic field. Based on this feature, we theoretically investigate a magnetic-field-controlled reversible resistance change active at low temperatures in a molecular magnetic tunnel junction, which consists of a single-molecule magnet sandwiched between a ferromagnetic electrode and a normal metal electrode. Our numerical results demonstrate that the molecular magnetism orientation can be manipulated by magnetic fields to be parallel/antiparallel to the ferromagnetic electrode magnetization. Moreover, different magnetic configurations can be “read out” based on different resistance states or different spin polarization parameters in the current spectrum, even in the absence of a magnetic field. Such an external magnetic field-controlled resistance state switching effect is similar to that in traditional spin valve devices. The difference between the two systems is that one of the ferromagnetic layers in the original device has been replaced by a magnetic molecule. This proposed scheme provides the possibility of better control of the spin freedom of electrons in molecular electrical devices, with potential applications in future high-density nonvolatile memory devices.

General Theory for Bilayer Stacking Ferroelectricity.

Two-dimensional (2D) ferroelectrics, which are rare in nature, enable high-density nonvolatile memory with low energy consumption. Here, we propose a theory of bilayer stacking ferroelectricity (BSF), in which two stacked layers of the same 2D material, with different rotation and translation, exhibit ferroelectricity. By performing systematic group theory analysis, we find all the possible BSF in all 80 layer groups (LGs) and discover the rules about the creation and annihilation of symmetries in the bilayer. Our general theory can not only explain all the previous findings (including sliding ferroelectricity), but also provide a new perspective. Interestingly, the direction of the electric polarization of the bilayer could be totally different from that of the single layer. In particular, the bilayer could become ferroelectric after properly stacking two centrosymmetric nonpolar monolayers. By means of first-principles simulations, we predict that the ferroelectricity and thus multiferroicity can be introduced to the prototypical 2D ferromagnetic centrosymmetric material CrI_{3} by stacking. Furthermore, we find that the out-of-plane electric polarization in bilayer CrI_{3} is interlocked with the in-plane electric polarization, suggesting that the out-of-plane polarization can be manipulated in a deterministic way through the application of an in-plane electric field. The present BSF theory lays a solid foundation for designing a large number of bilayer ferroelectrics and thus colorful platforms for fundamental studies and applications.

ANSA: Adaptive Near-Sensor Architecture for Dynamic DNN Processing in Compact Form Factors

Advanced edge sensing/computing devices, such as AR/VR devices, have a uniquely challenging adaptive baseline workload and camera sensor structure. These devices must process images in real-time from multiple sensors, placing a large burden on a typical centralized mobile SoC processor. Augmenting the sensors with a package-integrated near-sensor processor can improve the device’s processing performance as well as reduce energy consumption. This near-sensor processor must adapt to the dynamic workloads, fit within a limited silicon footprint and energy envelope, and satisfy the real-time requirement. In this work, we present ANSA, a near-sensor processor architecture supporting flexible processing schemes and dataflows to maintain high efficiency for dynamic CNN workloads. ANSA is scalable to sub-mm2 sizes to match the footprint of advanced image sensors. ANSA supports module-level power gating to adapt the compute capacity to dynamic workloads. Finally, ANSA leverages recent advancements in high-density non-volatile memory and 3D packaging to support weight storage within the area constraints of an image sensor. Overall, ANSA achieves inference energy consumption up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$30\times $ </tex-math></inline-formula> lower than a standard SIMD baseline. Additionally, our design’s scalability allows it to achieve up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.76\times $ </tex-math></inline-formula> lower average inference energy at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4.5\times $ </tex-math></inline-formula> lower silicon area compared to competing edge accelerator designs.

Enhanced tunneling electroresistance effect in Pt/BiAlO3/Pt ferroelectric tunnel junctions by a graphene interlayer

Realizing a large tunneling electroresistance (TER) effect with a low ON-state resistance-area product (RAP) is essential for the applications of ferroelectric tunnel junctions (FTJs) in information storage. Herein, taking Pt/BiAlO3/Pt (Pt/BAO/Pt) FTJs as examples, we theoretically demonstrate that a giant TER ratio of about 1458%, which is comparable to most van der Waals tunnel junctions, can be achieved by switching the electric polarization of ferroelectric BAO(0001) barrier due to the reversible metallization of ferroelectric in conjunction with the Schottky barrier. More interestingly, with the effective modulation on barrier height and width via polarization reversal, an enhanced TER effect close to 104% can be obtained by inserting a graphene monolayer into right (or left) Pt/BAO interface of the FTJs and the writing endurance of the FTJ memories is expected to be significantly improved. Furthermore, the calculated ON-state RAPs in all three FTJ devices are not more than 100 Ω μm2. These fascinating findings suggest that the proposed BAO-based FTJs hold great potential for designing next-generation nanoscale high-density and low-power nonvolatile memories with excellent performance.

Sub-Nanosecond Switching of Si:HfO2 Ferroelectric Field-Effect Transistor.

The discovery of ferroelectric doped HfO2 enabled the emergence of scalable and CMOS-compatible ferroelectric field-effect transistor (FeFET) technology which has the potential to meet the growing need for fast, low-power, low-cost, and high-density nonvolatile memory, and neuromorphic devices. Although HfO2 FeFETs have been widely studied in the past few years, their fundamental switching speed is yet to be explored. Importantly, the shortest polarization time demonstrated to date in HfO2-based FeFET was ∼10 ns. Here, we report that a single subnanosecond pulse can fully switch HfO2-based FeFET. We also study the polarization switching kinetics across 11 orders of magnitude in time (300 ps to 8 s) and find a remarkably steep time-voltage relation, which is captured by the classical nucleation theory across this wide range of pulse widths. These results demonstrate the high-speed capabilities of FeFETs and help better understand their fundamental polarization switching speed limits and switching kinetics.

Sensing single domains and individual defects in scaled ferroelectrics.

Ultra-scaled ferroelectrics are desirable for high-density nonvolatile memories and neuromorphic computing; however, for advanced applications, single domain dynamics and defect behavior need to be understood at scaled geometries. Here, we demonstrate the integration of a ferroelectric gate stack on a heterostructure tunnel field-effect transistor (TFET) with subthermionic operation. On the basis of the ultrashort effective channel created by the band-to-band tunneling process, the localized potential variations induced by single domains and individual defects are sensed without physical gate-length scaling required for conventional transistors. We electrically measure abrupt threshold voltage shifts and quantify the appearance of new individual defects activated by the ferroelectric switching. Our results show that ferroelectric films can be integrated on heterostructure devices and indicate that the intrinsic electrostatic control within ferroelectric TFETs provides the opportunity for ultrasensitive scale-free detection of single domains and defects in ultra-scaled ferroelectrics. Our approach opens a previously unidentified path for investigating the ultimate scaling limits of ferroelectronics.

High-Power LiNbO3 Domain-Wall Nanodevices.

Wide band gap semiconductors keep on pushing the limits of power electronic devices to higher switching speeds and higher operating temperatures, including diodes and transistors on low-cost Si substrates. Alternatively, erasable conducting walls created within ferroelectric single-crystal films integrated on the Si platform have emerged as a promising gateway to adaptive nanoelectronics in sufficient output power, where the repetitive creation of highly charged domain walls (DWs) is particularly important to increase the wall current density. Here, we observe large conduction of the head-to-head DW at an optimized inclination angle of 15° within a LiNbO3 single crystal that is 3-4 orders of magnitude higher than that of the tail-to-tail DW. The wall conduction is diode-like with a linear current density of higher than 1 mA/μm and an on/off ratio of larger than 106 under the application of a repetitive switching voltage pulse in time less than 10 ns and an endurance number of higher than 105. The high-power diodes can not only perform direct data processing in high-density nonvolatile DW memories in fast operation speeds and low-energy consumption but also function as sensors in compact electromechanical systems, selectors in phase-change memory and resistive random-access memory, and half-wave/full-wave rectifiers in modern nanocircuits in dimensions approaching the thickness of the depletion layer below which the tradition p-n junction malfunctions.

In-plane ferroelectric tunnel junctions based on 2D α-In2Se3/semiconductor heterostructures

Ferroelectric tunnel junctions (FTJs) have great potential for application in high-density non-volatile memories. Recently, α-In2Se3 was found to exhibit robust in-plane and out-of-plane ferroelectric polarizations at a monolayer thickness, which is ideal to serve as a ferroelectric component in miniaturized electronic devices. In this work, we design two-dimensional van der Waals heterostructures composed of an α-In2Se3 ferroelectric and a hexagonal IV–VI semiconductor and propose an in-plane FTJ based on these heterostructures. Our first-principles calculations show that the electronic band structure of the designed heterostructures can be switched between insulating and metallic states by ferroelectric polarization. We demonstrate that the in-plane FTJ exhibits two distinct transport regimes, tunneling and metallic, for OFF and ON states, respectively, leading to a giant tunneling electroresistance effect with the OFF/ON resistance ratio exceeding 1 × 104. Our results provide a promising approach for the high-density ferroelectric memory based on the 2D ferroelectric/semiconductor heterostructures.