Next-generation Nonvolatile Memory Devices Research Articles

Fabrication of high quality lead-free double perovskite Cs2AgBiBr6 thin film and its application in memristor with ultralow operation voltage

Recently, the lead-free double perovskite Cs2AgBiBr6 has been considered as a promising candidate for next-generation nonvolatile memory and artificial synapse devices due to its high stability and low toxicity compared to its lead-based counterparts. In this work, we developed a simple and effective method to produce high-quality lead-free double perovskite Cs2AgBiBr6 thin films without pinholes and particles by applying a low-pressure assisted method under ambient condition with a relative humidity (RH) of about 45%. The formation of pinholes and Ag precipitation in the perovskite Cs2AgBiBr6 films is effectively suppressed by the proper ratio of N,N-dimenthylformamide (DMF) mixed in dimethyl sulfoxide (DMSO) solvents. Furthermore, the grain size of the Cs2AgBiBr6 films can be significantly increased by increasing the post-annealing temperature. Finally, a sandwiched structure memristor with an ITO/Cs2AgBiBr6/Ta configuration was successfully demonstrated, featuring ultralow operation voltage (V Set ∼ 57 ± 23 mV, V Reset ∼ −692 ± 68 mV) and satisfactory memory window (the ratio of R HRS/R LRS ∼ 10 times), which makes it suitable for low-power consumption information storage devices.

Thermally-induced nanoscale phase change in chalcogenide glass Cr2Ge2Te6 revealed by scanning tunneling microscopy

Cr2Ge2Te6 (CrGT) is viewed as an important phase change material (PCM) for next-generation nonvolatile memory devices because of its superior properties, e.g., high thermal stability and low operation energy, compared to conventional PCMs. However, the phase change mechanism of CrGT remains unsolved, especially at the nanoscale. Here, we investigated thermally induced nanoscale phase changes of CrGT thin films using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). By performing statistical analysis of the measured STM topographic and STS data, we evaluated the inhomogeneity and distribution of the phase change characteristics of CrGT thin films. We also related the nanoscale phase change properties of CrGT to macroscopic phase changes by comparing the STM and STS results with experimental data from Raman spectroscopy.

(Invited) Fabrication Technique of Ferroelectric HfxZr1−XO2 Thin Films Using ALD-ZrO2 Nucleation Layers

Ferroelectric HfxZr1−xO2 (HZO) thin films have attracted considerable attention towards high-density and low-power ferroelectric memory devices of ferroelectric random access memory (FeRAM) and ferroelectric field-effect transistors (FeFETs) [1]. To obtain ferroelectric orthorhombic (O) phase of HZO films, several techniques have been investigated, such as the TiN stressor layer technique [2] and controlled annealing conditions [3]. On the other hand, we pay attention to the ZrO2 nucleation layer technique because ZrO2 films deposited by atomic layer deposition (ALD) method formed a nanocrystalline structure and consisted dominantly of O, tetragonal (T), and cubic (C) phases, whose lattice constants are considerably close to that of ferroelectric O phase for HfO2 [4]. In this paper, we report the crystallinity and ferroelectricity of HZO films fabricated using the ZrO2 nucleation layer technique for metal-ferroelectric-metal (MFM) and metal-ferroelectric-semiconductor (MFS) capacitors.First, four types of MFM capacitors of TiN/HZO/TiN (MFM w/o), TiN/HZO/bottom-ZrO2/TiN (MFM B-ZrO2), TiN/top-ZrO2/HZO/TiN (MFM T-ZrO2), and TiN/top-ZrO2/HZO/bottom-ZrO2/TiN (MFM D-ZrO2) were systematically investigated for the FeRAM applications [5]. The thicknesses of the ALD-grown HZO and ZrO2 films were 10 and 2 nm, respectively. The post-deposition annealing was performed at 600°C. The remanent polarization (2P r = P r + − P r −) of the MFM capacitors increased in the following order: MFM w/o (12 µC/cm2) < MFM B-ZrO2 (15 µC/cm2) < MFM T-ZrO2 (23 µC/cm2) < MFM D-ZrO2 (29 µC/cm2), while all capacitors showed almost the same coercive electric field (E c) value of around 1.2 MV/cm. Therefore, maximum 2P rvalue of the MFM capacitor was obtained using the top- and bottom-ZrO2 nucleation layers. For the MFM capacitors with ZrO2 nucleation layers, epitaxial-like grain growth of the HZO film occurred on the nanocrystalline top- and bottom-ZrO2 layers. Moreover, the relative ratio of O/T/C phases in the ferroelectric film, which was estimated using the peak area ratio of O(111)/T(101)/C(111)/{M(−111)+ O(111)/T(101)/C(111)+M(111)}, increased in the following order: MFM w/o < MFM B-ZrO2 < MFM T-ZrO2 < MFM D-ZrO2. Therefore, the top- and bottom-ZrO2 layers play an important role as nucleation layers for the crystallization of the HZO film, resulting in a larger relative ratio of O/T/C phases in the ferroelectric film compared to w/o. Moreover, these results indicate that the top-ZrO2 layer was more effective to form the ferroelectric phase compared to the bottom-ZrO2 layer.Next, two types of MFS capacitors of TiN/HZO/Si (MFS w/o) and TiN/top-ZrO2/HZO/Si (MFS T-ZrO2) were fabricated by low-temperature processes at 300°C of ALD and post-metallization annealing (PMA) for the FeFET applications [6]. The thicknesses of both HZO and ZrO2 films were 10 nm. Both MFS w/o and MFS T-ZrO2 were found to form an extremely thin SiO2 interfacial layer with a thickness of one or two monolayers between the HZO film and Si substrate due to a low temperature fabrication process of 300°C. The HZO film of the MFS T-ZrO2 formed O/T/C phases even with a low thermal budget of 300°C, while that of the MFS w/o had an amorphous structure. This result is attributed to the beneficial role of the nanocrystalline top-ZrO2 nucleation layer for the crystallization of an amorphous HZO film during the PMA process. Therefore, a higher 2P r value of 15 µC/cm2 was obtained for the MFS T-ZrO2 than that (2.2 µC/cm2) for the MFS w/o. Moreover, the endurance properties of the MFS T-ZrO2 were free from the wake-up effect and exhibited less degradation of switching polarization (P sw). This could be attributed to the insertion of a ZrO2 film at the TiN/HZO interface, which promoted the preferential formation of the ferroelectric orthorhombic phase and prevent the formation of oxygen vacancies at the ZrO2/HZO interface. Thus, we concluded that superior ferroelectricity with wake-up free properties and higher fatigue resistance of HZO-based MFS capacitors were achieved by using the ZrO2 nucleation layer technique.These results suggest that the fabrication technique of ferroelectric HZO films using nanocrystalline ALD-ZrO2 nucleation layers is one of the promising processes for the next-generation non-volatile memory device applications.This work was supported by JSPS KAKENHI (Nos. JP21J01667, JP20H02189, and JP18J22998) and MEXT Leading Initiative for Excellent Young Researchers (No. JPMXS0320220213).[1] T. Mikolajick et al., J. Appl. Phys. 129, 100901 (2021).[2] S. J. Kim, et al., Appl. Phys. Lett. 111, 242901 (2017).[3] S. Migita et al., Jpn. J. Appl. Phys. 58, SBBA07 (2019).[4] T. Onaya et. al., Appl. Phys. Express. 10, 081501 (2017).[5] T. Onaya et al., APL Mater. 7, 061107 (2019).[6] T. Onaya et al., APL Mater. 10, 051110 (2022).

High-performance van der Waals antiferroelectric CuCrP2S6-based memristors

Layered thio- and seleno-phosphate ferroelectrics, such as CuInP2S6, are promising building blocks for next-generation nonvolatile memory devices. However, because of the low Curie point, the CuInP2S6-based memory devices suffer from poor thermal stability (<42 °C). Here, exploiting the electric field-driven phase transition in the rarely studied antiferroelectric CuCrP2S6 crystals, we develop a nonvolatile memristor showing a sizable resistive-switching ratio of ~ 1000, high switching endurance up to 20,000 cycles, low cycle-to-cycle variation, and robust thermal stability up to 120 °C. The resistive switching is attributed to the ferroelectric polarization-modulated thermal emission accompanied by the Fowler–Nordheim tunneling across the interfaces. First-principles calculations reveal that the good device performances are associated with the exceptionally strong ferroelectric polarization in CuCrP2S6 crystal. Furthermore, the typical biological synaptic learning rules, such as long-term potentiation/depression and spike amplitude/spike time-dependent plasticity, are also demonstrated. The results highlight the great application potential of van der Waals antiferroelectrics in high-performance synaptic devices for neuromorphic computing.

Collective Control of Potential-Constrained Oxygen Vacancies in Oxide Heterostructures for Gradual Resistive Switching.

Filamentary resistive switching in oxides is one of the key strategies for developing next-generation non-volatile memory devices. However, despite numerous advantages, their practical applications in neuromorphic computing are still limited due to non-uniform and indeterministic switching behavior. Given the inherent stochasticity of point defect migration, the pursuit of reliable switching likely demands an innovative approach. Herein, a collective control of oxygen vacancies is introduced in LaAlO3 /SrTiO3 (LAO/STO) heterostructures to achieve reliable and gradual resistive switching. By exploiting an electrostatic potential constraint in ultrathin LAO/STO heterostructures, the formation of conducting filaments is suppressed, but instead precisely control the concentration of oxygen vacancies. Since the conductance of the LAO/STO device is governed by the ensemble concentration of oxygen vacancies, not their individual probabilistic migrations, the resistive switching is more uniform and deterministic compared to conventional filamentary devices. It provides direct evidence for the collective control of oxygen vacancies by spectral noise analysis and modeling by Monte-Carlo simulation. As a proof of concept, the significantly-improved analog switching performance of the filament-free LAO/STO devices is demonstrated, revealing potential for neuromorphic applications. The results establish an approach to store information by point defect concentration, akin to biological ionic channels, for enhancing switching characteristics of oxide materials.

A Facile Hydrothermal Synthesis and Resistive Switching Behavior of α-Fe2O3 Nanowire Arrays.

A facile hydrothermal process has been developed to synthesize the α-Fe2O3 nanowire arrays with a preferential growth orientation along the [110] direction. The W/α-Fe2O3/FTO memory device with the nonvolatile resistive switching behavior has been achieved. The resistance ratio (RHRS/RLRS) of the W/α-Fe2O3/FTO memory device exceeds two orders of magnitude, which can be preserved for more than 103s without obvious decline. Furthermore, the carrier transport properties of the W/α-Fe2O3/FTO memory device are dominated by the Ohmic conduction mechanism in the low resistance state and trap-controlled space-charge-limited current conduction mechanism in the high resistance state, respectively. The partial formation and rupture of conducting nanofilaments modified by the intrinsic oxygen vacancies have been suggested to be responsible for the nonvolatile resistive switching behavior of the W/α-Fe2O3/FTO memory device. This work suggests that the as-prepared α-Fe2O3 nanowire-based W/α-Fe2O3/FTO memory device may be a potential candidate for applications in the next-generation nonvolatile memory devices.

Effect of frequency on the classical and relaxor ferroelectric behavior of substituted titanate Ba0.7Er0.16Ca0.05Ti0.91Sn0.09O3.

In the present work, we synthesized the perovskite Ba0.70Er0.16Ca0.05Ti0.91Sn0.09O3 compound (BECTSO) by a solid-state reaction and sintering at 1200 °C. The effects of doping on the structural, electrical, dielectric, and ferroelectric characteristics of the material are examined in this work. X-ray powder diffraction analysis shows that BECTSO crystallizes in a tetragonal structure with space group P4mm. A detailed study of the dielectric relaxation of the BECTSO compound has been reported for the first time. Classical low-frequency ferroelectric and high-frequency relaxor ferroelectric behaviors have been studied. The study of the real part of the permittivity (ε') as a function of temperature demonstrated a high dielectric constant and identified a phase transition from the ferroelectric phase to the paraelectric phase at Tc = 360 K. The analysis of conductivity curves shows two behaviors: semiconductor behavior for f < 106 Hz and metallic behavior for f >106 Hz. The relaxation phenomenon is dominated by the short-range motion of the charge carriers. The BECTSO sample could be considered as a potential lead-free material for next-generation non-volatile memory devices and wide-temperature range capacitor applications.

Robustly Stable Ferroelectric Polarization States Enable Long-Term Nonvolatile Storage against Radiation in HfO2-Based Ferroelectric Field-Effect Transistors.

The ferroelectric field-effect transistors (FeFETs) with HfO2-based ferroelectric layers in the gate stacks are emerging as one of the most promising candidates for the next-generation nonvolatile memory devices due to their scalability and compatibility with conventional Si technology. Moreover, owing to the high radiation hardness of the HfO2-based ferroelectric thin films, HfO2-based FeFETs have attracted great interest in the fields of radiation-hard (rad-hard) memory. However, the reliability of their memory states under irradiation, which represents the validity of the stored information, has not been investigated. Here, we focus on the impact of the total ionizing dose (TID) on erased and programmed states of HfO2-based FeFETs. The TID radiation (X-ray) characteristics of erased and programmed HfO2-based FeFETs are characterized using an on-site read operation. Both the erased and programmed states show robust stability under irradiation at a dose rate of 90 rad(Si)/s, and even at 230 rad(Si)/s, only the erased state shows a slight variation. The possible factors contributing to memory state degradation are discussed. Through the analysis of the threshold voltage shift and subthreshold swing evolution, as well as studies of ferroelectric polarization stability under radiation, it is revealed that the erased state degradation is caused by oxide-trapped charges rather than interface degradation or polarization switching. The physical mechanism of the difference in radiation-induced oxide-trapped charges buildup in programmed and erased FeFETs is analyzed to explain different TID radiation characteristics between them. Our work suggests that the HfO2-based FeFETs have great potential in radiation environment applications.

Preparation and incorporation of NiSe@MoSe2 nano arrays in PVA matrix for resistive switching memory

In the present work, we have synthesized Nickel selenide@Molybdenum diselenide (NiSe@MoSe2) heteronanostructured arrays by a one-step hydrothermal method. The structural and optical analysis of nanopowder is done through Field emission scanning electron microscopy (FESEM), elemental mapping, Energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), UV–vis, and Photoluminescence (PL) spectroscopy. XRD reveals the formation of a hexagonal structured system with a crystallite size of 166 nm. The bandgap of NiSe@MoSe2 nanosheets is found to be 1.71 eV, calculated from UV–vis spectra. FESEM showed the homogenous growth of NiSe@MoSe2 nanosheets while electronic transitions are investigated through PL. Further, the polymer nanocomposites of NiSe@MoSe2/PVA are synthesized by the ex-situ method at six different NiSe@MoSe2 concentrations in the PVA matrix. XRD shows good interaction between NiSe@MoSe2 and PVA polymer. The crystallinity increases as concentrations of NiSe@MoSe2 in PVA increases. FESEM shows the uniform distribution of NiSe@MoSe2 nanosheets in the PVA matrix. The two-terminal Ag/NiSe@MoSe2-PVA/FTO devices have been fabricated and characterized electrically by I-V measurements. I-V shows hysteresis behavior and also, bipolar switching is observed with a maximum Ion/Ioff ratio of ~103 at 2 wt. % NiSe@MoSe2 concentration in the PVA matrix. The good stability and reliability are checked through endurance and write-read-erase-read cyclic sequences. The obtained results open up the possibility of a next-generation non-volatile resistive switching memory device.

(Invited, Digital Presentation) 50 Ns Ultrafast Memory Operation in 2D Heterostructured Non-Volatile Memory Device

2D heterostructures have been extensively investigated as next-generation non-volatile memory (NVM) devices. In the last decade, drastic performance improvements and further advanced functionalities have been demonstrated. However, this progress is not sufficiently supported by the understanding of their operations, obscuring the material and device structure design policy. Here, detailed operation mechanisms are elucidated by exploiting the floating gate voltage (V FG) trajectory measurements [1,2]. Systematic comparisons of MoTe2, WSe2, and MoS2 channel devices revealed that the tunneling behavior between the channel and FG is controlled by three kinds of current-limiting paths, i.e., tunneling barrier, 2D/metal contact, and pn junction in the channel. Based on the understanding of the memory operation mechanism through the V FG trajectory measurement, we propose the best all 2D NVM device structure with a direct tunneling path between source/drain electrodes and floating gate for ultrafast memory operation. Indeed, a 50 ns program/erase operation is successfully achieved [3]. Moreover, we examined the dielectric breakdown strength (E BD) of h-BN under short voltage pulses for the origin for this ultrafast operation, because a high dielectric breakdown strength allows a large tunneling current. Surprisingly, an E BD = 26.1 MV/cm for h-BN is realized under short voltage pulses, largely exceeding the E BD = ~12 MV/cm from the DC measurement. This suggests that the high E BD of h-BN can be the physical origin of the ultrafast operations. In this talk, I would like to discuss the future perspective of 2D NVM application.[1] T. Sasaki, KN, et al., "Understanding the Memory Window Overestimation of 2D Materials Based Floating Gate Type Memory Devices by Measuring Floating Gate Voltage", Small, 2020, 16, 2004907.[2] T. Sasaki, KN, et al., "Material and Device Structure Designs for 2D Memory Devices Based on the Floating Gate Voltage Trajectory", ACS nano, 2021, 15, 6658.[3] T. Sasaki, KN, et al., "Ultrafast Operation of All 2D-Heterostructured Nonvolatile Memory Devices Provided by the Strong Short-Time Dielectric Breakdown Strength of h-BN", Adv. Funct. Mater. (under revision).

Epitaxial growth and phase evolution of ferroelectric La-doped HfO2 films

Hafnium-oxide-based materials are considered a promising candidate for next-generation nonvolatile memory devices owing to their good CMOS compatibility and robust ferroelectricity at the nanoscale. In this work, we synthesize highly (111)-oriented La-doped HfO2 (HLO) ferroelectric thin films via pulsed laser deposition. Furthermore, the effect of La dopant concentration, thickness, and growth temperature on the ferroelectricity of HLO films is investigated in detail. A maximum remnant polarization of ∼9 μC/cm2 is achieved for only the 5-nm-thick 3 mol. % HLO films without a wake-up process. The 180° inversion of the domain, the butterfly-shaped capacitance–voltage curve, and typical ferroelectric displacement current curve further demonstrate the robust ferroelectricity at the nanoscale. Moreover, the phase evolves from the monoclinic to the orthorhombic and subsequently to the cubic phase with increasing La concentration, which is due to the combined action of oxygen vacancy, epitaxial strain, and chemical pressure. Additionally, in the interface configuration of HLO/La0.7Sr0.3MnO3 (LSMO), the MnO20.7− layer is substituted by the HLO layer on the MnO2-terminated surface of LSMO, which can be attributed to the fact that the HLO layer with higher electronegativity replaces the MnO20.7− layer with the same electronegativity in the HLO film. Therefore, this study provides a reliable pathway for directly obtaining a lightly doped HLO ferroelectric thin film, which can help to broaden the understanding of the ferroelectric physical mechanisms with element doping.

Solution-processed light-induced multilevel non-volatile wearable memory device based on CsPb2Br5 perovskite.

Despite the recent advancements in memory devices, the quest for building materials with low power consumption is still on, with the ultimate focus on the durability of the system and reliability and reproducibility of its performance. Halide perovskites (HPs), which have several intriguing photoelectrical traits, have recently been utilized for memory applications; one of the highlights of these materials is the ionic-motion-based fast switching in their crystal structure. In this work, a CsPb2Br5 film is applied as a switching layer to implement memory devices with a flexible Al/CsPb2Br5/ITO-PET structure. The device exhibits a pronounced bipolar resistive switching (RS) characteristic at low operating voltage. The as-fabricated flexible device presented RS features with no initial forming process, concentrated distributions of high and low resistance states (HRS and LRS), uniform switching, endurance over 100 cycles, and a long retention time of 103 s with a high on/off ratio of around 102. Multilevel data-storage capability was also observed via subtle control of the compliance current (CC). Considering the current demand for smart, wearable, and flexible electronic gadgets, the current-voltage (I-V) characteristics of the as-fabricated all-inorganic halide-perovskite-based memory device were further explored under different bending conditions to determine its electrical reliability and mechanical stability. This flexible device exhibited no discernible difference in device performance under flat or bent conditions, and the performance remained nearly the same even after 500 bending cycles. In addition to control of the RS effect of the device using an electrical field, the performance of the device under light was also explored. Blue light modulates the resistive states by regulating the condition of photo-generated electron-hole pairs, and the SET and RESET voltages are changed from 2.34 to 2.14 V and from -2.04 to -1.90 V, respectively. The observed RS behavior is explained on the basis of the creation and partial annihilation of conductive multifilaments, which is dominated by the migration of bromine ions and their associated vacancies in the HP layer. We believe that this work will offer a new context to understand the intrinsic characteristics of HPs for RS applications at low voltage and validate their potential in the design of next-generation stable and nonvolatile memory devices for future flexible electronic systems.

Nanoscale Multiferroic Properties at Room Temperature of Lead Zirconate Titanate Iron Tantalate for Memory Device Applications

The evolution of device miniaturization and the potential application for next-generation nonvolatile memory devices Pb(Zr0.53Ti0.47)0.60(Fe0.5Ta0.5)0.40O3 (PZTFT) can be used as a barrier in multiferroic tunnel junctions. We report single-phase magnetoelectric multiferroic ultra-thin PZTFT at room temperature. The magnetic and ferroelectric properties have been investigated down to 7 nm thickness which are requisite properties for multiferroic tunnel barriers for multi-states memory applications. In the present study, we show that it remains a switching multiferroic at room temperature down to at least thickness d = 7 nm, which is thin enough for tunnel junctions.

Room temperature tuning of non volatile magnetoelectric memory in Al doped Sr3Co2Fe24O41

Study of magnetoelectric coupling in Type II multiferroics is indeed a fascinating area of research in next-generation nonvolatile data memory devices. To facilitate memory device applications, it is necessary to find a single phase material with substantial magnetoelectric coupling at room temperature. A room temperature nonvolatile memory device in aluminium doped Sr3Co2Fe24O41 is demonstrated here. Aluminium doping in Sr3Co2Fe24O41 not only enhances magnetoelectric coupling but also allows for easy tuning of memory states in the device. Maximum Magnetoelectric coefficient (α) of 59.44 mV/cm-Oe is observed for x = 0.04 in Sr3Co2(Fe1-xAlx)24O41. α could be stored and maintained for memory use for a broad period of time; additionally, the memory states of the device could be switched and tuned with electric pulses, which is an additional benefit of the memory device. This kind of magnetoelectric memory can be beneficial due to its non-destructive read operations, which is the main challenge in FERAM. This conveniently integrable single phase, robust memory system with increased memory density will undoubtedly pave the way for future memory applications.

Multi-level characteristics of TiOx transparent non-volatile resistive switching device by embedding SiO2 nanoparticles

TiOx-based resistive switching devices have recently attracted attention as a promising candidate for next-generation non-volatile memory devices. A number of studies have attempted to increase the structural density of resistive switching devices. The fabrication of a multi-level switching device is a feasible method for increasing the density of the memory cell. Herein, we attempt to obtain a non-volatile multi-level switching memory device that is highly transparent by embedding SiO2 nanoparticles (NPs) into the TiOx matrix (TiOx@SiO2 NPs). The fully transparent resistive switching device is fabricated with an ITO/TiOx@SiO2 NPs/ITO structure on glass substrate, and it shows transmittance over 95% in the visible range. The TiOx@SiO2 NPs device shows outstanding switching characteristics, such as a high on/off ratio, long retention time, good endurance, and distinguishable multi-level switching. To understand multi-level switching characteristics by adjusting the set voltages, we analyze the switching mechanism in each resistive state. This method represents a promising approach for high-performance non-volatile multi-level memory applications.

Material and Device Structure Designs for 2D Memory Devices Based on the Floating Gate Voltage Trajectory.

Two-dimensional heterostructures have been extensively investigated as next-generation nonvolatile memory (NVM) devices. In the past decade, drastic performance improvements and further advanced functionalities have been demonstrated. However, this progress is not sufficiently supported by the understanding of their operations, obscuring the material and device structure design policy. Here, detailed operation mechanisms are elucidated by exploiting the floating gate (FG) voltage measurements. Systematic comparisons of MoTe2, WSe2, and MoS2 channel devices revealed that the tunneling behavior between the channel and FG is controlled by three kinds of current-limiting paths, i.e., tunneling barrier, 2D/metal contact, and p-n junction in the channel. Furthermore, the control experiment indicated that the access region in the device structure is required to achieve 2D channel/FG tunneling by preventing electrode/FG tunneling. The present understanding suggests that the ambipolar 2D-based FG-type NVM device with the access region is suitable for further realizing potentially high electrical reliability.

Memory characteristics of room-temperature multiferroic hexaferrite Sr3Co2Fe24O41

We prepared the room-temperature magnetoelectric hexaferrite polycrystal Sr3Co2Fe24O41 by the conventional solid-state reaction, the maximum magnetoelectric current of |J| = 8.8 μA/m2 was obtained at about μ0H = 12.9 mT. Furthermore, We obtained a stronger magnetoelectric effect when the remnant magnetization direction was the same with the scanning field than in contrast. The most important, we have demonstrated a type of room-temperature nonvolatile memory based on this characteristic. The magnetoelectric current |J| is employed to store binary information. By applying positive and negative magnetic field pulses, |J| achieved repeatable nonvolatile switching between high and low states. Our work enriched the magnetoelectric effect of multiferroic hexaferrites and provided a new idea to implement next-generation nonvolatile information memory devices.

(Invited) ALD of Phase Change and Threshold Switching Materials for Next-Generation Nonvolatile Memory Devices

Memory technology continues to be at a crossroads. Established devices such as DRAM, Flash, and SRAM remain dominant, but each are facing increasingly formidable challenges in scaling, cost, and performance. Emerging memory technologies are attempting to displace these three or to find application spaces between those they traditionally support. However, no emerging memory has been able to demonstrate significant advantages over the incumbents, resulting in a proliferation of options such as phase change memory (PCM), filamentary resistive memory (ReRAM), ferroelectric memory (FERAM), magnetic memory (e.g. STT-MRAM and SOT-MRAM), and several others. Of these, the most successful has been PCM as evidenced by the commercial introduction of PCM based crosspoint memory in 2017. PCM based crosspoint memory has had limited market penetration in part due to the high manufacturing cost; the current layer-by-layer approach to PCM fabrication makes scaling difficult. In order to be cost competitive, crosspoint arrays will need to be manufactured in a style similar to that of 3D V-NAND where the active layers are deposited conformally inside deep features simultaneously forming hundreds of junctions. Chemical Vapor Deposition or Atomic Layer Deposition of these films is likely required to make this a reality. Some progress in materials for the PCM memory element has been reported; however, a remaining concern for crosspoint memory is the selector device, used in suppressing the sneak path current, consisting of a non-linear two-terminal device to properly select each memory element. In addition to high non-linearity, a selector device also requires high on-state current density JON, fast switching time, high endurance, stability, high scalability and back-end-of-line (BEOL) integration. Several selector devices have been proposed to date, e.g., mixed-ionic-electronic-conduction (MIEC), field-assisted superlinear threshold (FAST), and amorphous silicon (a-Si) selectors, but Ovonic Threshold Switch (OTS) composed of chalcogenide multinary materials clearly appears as the best candidate to cover all the mentioned requirements for high-density crosspoint applications.Currently, chalcogenide materials are deposited by using physical vapor deposition (PVD) that allows for fine tuning of the film stoichiometry, but at the same time limits its conformality and homogeneity on a large scale. To address this challenge, atomic layer deposition (ALD) is desirable; unfortunately, only ALD OTS As-free compositions with high leakage and limited endurance have been developed so far. In this report we describe the process and materials for GeSbTe PCM and a novel quaternary GeAsSeTe ALD OTS composition. We show a physical characterization of the ALD films and provide a comprehensive electrical characterization of integrated devices in DC and pulsed regimes. The OTS selector performance, in particular, shows excellent results (See Figure 1) - good selectivity (> 104), low threshold voltage (Vth) drift (< 40 mV/dec), fast switching (< 10 ns), excellent endurance (> 109 cycles) and robust stability. These results pave the way for future development of ALD chalcogenide-based selectors as a leading technology for multiple-stack integration of crosspoint memory arrays. To provide a physical understanding of the devices under study, we propose an analytical model for the subthreshold conduction mechanism, allowing for future development and optimization of the composition for 3D crosspoint memory applications.Figure 1. Electrical performance of an OTS selector over one billion cycles a) Pulsed-IV taken at each decade of the 1e9 endurance cycles. b) Sub-threshold current measured at each decade of the 1e9 endurance cycles. c) Evolution of the threshold voltage throughout the endurance test. Each point is measured three times, and all three measurements are reported d) Selector window at each decade of the endurance test. Figure 1

Resistive switching in FTO/CuO–Cu 2 O/Au memory devices

Memristors are considered to be next-generation non-volatile memory devices owing to their fast switching and low power consumption. Metal oxide memristors have been extensively investigated and reported to be promising devices, although they still suffer from poor stability and laborious fabrication process. Herein, the authors report a stable and power-efficient memristor with novel heterogeneous electrodes structure and facile fabrication based on cupric oxide (CuO)–cuprous oxide (Cu2O) complex thin films. The proposed structure of the memristor contains an active complex layer of CuO and Cu2O sandwiched between fluorine-doped tin oxide (FTO) and gold (Au) electrodes. The fabricated memristors demonstrate bipolar resistive switching (RS) behaviour with a low working voltage (∼1 V), efficient power consumption, and high endurance over 100 switching cycles. The authors suggest the RS mechanism of the proposed device is related to the formation and rupture of conducting filaments inside the memristor. Moreover, they analyse the conduction mechanism and electron transport in the active layer of the device during the RS process. Such a facile fabricated device has a promising potential for future memristive applications.

Improvement of durability by incorporating nanocrystals in ZnO-based resistive random access memory devices

The rapid expansion of data information has necessitated higher requirements for the operational speed, low energy consumption and high density integration. Resistive random access memory (RRAM), which can realize all of the mentioned advantages, has the potential to be the next-generation nonvolatile memory device. However, it is not easy to design an RRAM device with both high performance and stability. Herein, we improve the performance of the RRAM devices by incorporating Cu nanocrystals (NCs) into the ZnO oxide layer. Afterwards, many resistive parameters of the NC-RRAM devices are markedly improved, such as longer retention time over 106 s, with consecutive 103 switching cycles. It is inferred that the switching mechanism is due to the migration of Cu cations and the corresponding electrochemical metallization, and the Cu NCs enhance the electric field intensity, which boosts the formation and rupture of the conductive filaments.