Nonvolatile Ferroelectric Memory Research Articles

Compact non-volatile ferroelectric electrostatic doping optical memory based on the epsilon-near-zero effect.

With the booming development of optoelectronic hybrid integrated circuits, the footprint and power consumption of photonic devices have become the most constraining factors for development. To solve these problems, this paper proposes a compact, extremely low-energy and non-volatile optical readout memory based on ferroelectric electrostatic doping and the epsilon-near-zero (ENZ) effect. The writing/erasing state of an optical circuit is controlled by electrical pulses and can remain non-volatile. The device works on the principle that residual polarization charges of ferroelectric film, which is compatible with CMOS processes, are utilized to electrostatically dope indium tin oxide to achieve the ENZ state. Simulation results show that a significant modulation depth of 10.4dB can be achieved for a device length of 60µm with an energy consumption below 1pJ.

Giant tunneling electroresistance in a 2D bilayer-In2Se3-based out-of-plane ferroelectric tunnel junction.

Ferroelectric tunnel junctions (FTJs) have great potential in nonvolatile memory devices and have been extensively studied in recent years. Compared with conventional FTJs based on perovskite-type oxide materials as the barrier layer, two-dimensional (2D) van der Waals ferroelectric materials are advantageous in improving the performance of FTJs and achieving miniaturization of FTJ devices due to the features such as atomic thickness and ideal interfaces. In this work, we present a 2D out-of-plane ferroelectric tunnel junction (FTJ) constructed using graphene and bilayer-In2Se3. Using density functional calculations combined with the nonequilibrium Green's function technique, we investigate the electron transport properties in the graphene/bilayer-In2Se3 (BIS) vdW FTJ. Our calculations show that the FTJ we constructed can be switched from ferroelectric to antiferroelectric by changing the relative dipole arrangement of the BIS to form multiple nonvolatile resistance states. Since the charge transfer between the layers varies for the four different polarization states, the TER ratios range from 103% to 1010%. The giant tunneling electroresistance and multiple resistance states in the 2D BIS-based FTJ suggest that it has great potential for application in nanoscale nonvolatile ferroelectric memory devices.

Retention Model and Express Retention Test of Ferroelectric HfO2 -Based Memory

Ferroelectric hafnium oxide films are one of the most promising functional materials for a new generation of nonvolatile memory for data storage because of a number of excellent performance features, including nanosecond speed, high endurance, low power consumption, full scalability, and perfect compatibility with Si technology. However, the commercialization of this memory is hindered by its limited retention time, which still does not meet the 10-year electronics-industry standard. To date, much effort has been focused on the improvement of retention via material and interface engineering. Meanwhile, the evaluation and comparison of engineering results have been hampered, since there is still no unified retention test. Indeed, a specific feature of the retention performance is that it cannot be measured directly, because 10 years is required to measure the actual polarization loss. In this paper, we present a physical model that predicts the retention loss in ${\mathrm{Hf}\mathrm{O}}_{2}$-based memory during long-term information storage. The time-dependent polarization loss originates from the phenomenon of polarization imprint, which consists in the temporal development of a preferential polarization state in a poled ferroelectric. For high reliability of the retention calculation, the model takes into account the specific features of imprint evolution in the ferroelectric material ${\mathrm{Hf}\mathrm{O}}_{2}$. Among these is the origin of the imprint in this material, which consists in the interplay of two physical mechanisms, specifically, charge injection into interface traps and migration of mobile charged defects across the functional layer. The second property of ${\mathrm{Hf}\mathrm{O}}_{2}$ taken into account is the different rates of imprint evolution in two types of ${\mathrm{Hf}\mathrm{O}}_{2}$ film that exist within each memory cell. These two types of film originate from two populations of domains that appear during crystallization of the ${\mathrm{Hf}\mathrm{O}}_{2}$ and persist during the whole lifetime of the memory cell. The high accuracy of the retention model makes it possible to implement an express retention test, i.e., to predict the retention time in record short time (1 h) for any particular sample, whereas usually a retention test takes at least several days. For ${\mathrm{Hf}}_{0.5}{\mathrm{Zr}}_{0.5}{\mathrm{O}}_{2}$-based capacitors, the retention model shows excellent agreement with experimental results. The proposed model can serve not only for the implementation of this express retention test, but also as a tool for intelligent material engineering in the field of nonvolatile ferroelectric memory.

A study of the structural, optical, and ferroelectric characteristics of Pb-Ge-Te nanocrystalline alloys as potential candidates for memory devices and Near-Infrared (NIR) applications

Pb50-xGexTe50 (x = 15, 20, 25, 30 at. %) nanocrystalline bulk alloys were prepared using solid-state direct reaction. X-ray diffraction and high-resolution transmission electron microscopy (HR-TEM) analysis of the reference structure (Ge = 15 at.%) revealed a slightly distorted cubic structure, with a lattice parameter of 6.43 Å and an inter-axis unit cell angle of 88.69°. Atomic force images' analysis and histograms displayed a homogenous particle size distribution in the nanoscale for all samples. Density measurements showed a gradual decrease from 7.89 to 6.98 g/cm3 with increasing Ge content in agreement with the calculated values. The polarization–field hysteresis behavior verifies the ferroelectric activity of the prepared alloys, suggesting them as potential candidates for non-volatile ferroelectric memory devices (NVFRAMs) applications. Optical properties analyzed using diffuse reflectance measurements exhibited direct transitions with a bandgap decreasing from 1.57 to 1.35 eV with increasing Ge content matching the near-infrared spectrum (NIR) perfectly.

Polarization Switching Kinetics in Thin Ferroelectric HZO Films.

Ferroelectric polycrystalline HfO2 thin films are the most promising material for the implementation of novel non-volatile ferroelectric memories because of their attractive properties, such as compatibility with modern Si technology, perfect scalability, low power consumption and high endurance. However, for the commercialization of ferroelectric memory, some crucial aspects of its operation should be addressed, including the polarization switching mechanism that determines the switching speed. Although several reports on polarization switching kinetics in HfO2-based layers already exist, the physical origin of retardation behavior of polarization switching at the low and medium switching fields remains unclear. In this work, we examine several models of switching kinetics that can potentially explain or describe retardation behavior observed in experimental switching kinetics for Hf0.5Zr0.5O2 (HZO)-based capacitors and propose a new model. The proposed model is based on a statistical model of switching kinetics, which has been significantly extended to take into account the specific properties of HZO. The model includes contributions of the depolarization field and the built-in internal field originating from the charge injection into the functional HZO layer during the read procedure as well as in-plane inhomogeneity of the total electric field in ferroelectric. The general model of switching kinetics shows excellent agreement with the experimental results.

<italic>κ</italic>-Ga<sub>2</sub>O<sub>3</sub>/GaN ferroelectric/polar semiconductor heterojunction for high electron mobility transistors

<p indent="0mm">Polar semiconductor heterostructures offer an alternative strategy to manipulate polarization towards advanced devices with engineered functionality and improved performance. Recently, Prof. Jiandong Ye’s group from Nanjing University reported on a novel <italic>κ</italic>-Ga₂O₃/GaN ferroelectric/polar heterojunction with switchable polarization under external electric field. A type-II band alignment is determined at the <italic>κ</italic>-Ga₂O₃/GaN polar hetero-interface, with charge dipoles induced by spontaneous polarization leading to the observed band bending. Benefiting from the large band discontinuity and ferroelectric manipulation, it is expected to realize the switching of the interfacial two-dimensional electron gas between accumulation and depletion states, which allows the rational design of <italic>κ</italic>-Ga₂O₃ ferroelectric/polar heterojunctions for the application of high electron mobility transistors, ferroelectric non-volatile memories and even ultra-low loss negative capacitance transistors.

Fast Operations of Nonvolatile Ferroelectric Domain Wall Memory with Inhibited Space Charge Injection

The microampere-level domain wall currents in LiNbO3 single crystals have promising applications in nonvolatile ferroelectric domain wall random access memory and logic with high-density integration, ultrafast operation speeds, and almost unlimited switching cycles. For the memory commercialization, the improvements of the reliability and operation speed of the devices are challenging due to the high-field charge injection. The injected charge could compensate the domain-wall boundary charge that screens the domain switching field and reduces the domain wall current. In this work, two kinds of memory nanocells were fabricated on the surfaces of X-cut LiNbO3 single crystals to study the geometry-dependent charge injection. The striped memory cell due to the appearance of the size-driven reconstruction has a smaller coercive field than that of a clamped memory cell without relaxation of the lattice matching stress, which reduces low-frequency charge injection and increases the domain switching speed. At an operating voltage of 5 V, we observed a retention time of more than 1 week and an on/off current ratio of 2 × 104 for a striped-like cell, paving the route to integrate energy-efficient high-density domain wall memory in high reliability.

Aurivillius Phase Bi4V3O12 with d1 Magnetic Cations, Anisotropic and Negative Thermal Expansion, Multiple Structural Transitions, and Low-Dimensional Magnetism.

Aurivillius phases are an important class of inorganic compounds as they often show ferroelectric properties, and some members of this family are used in nonvolatile ferroelectric memories. The majority of Aurivillius phases have nonmagnetic d0 cations in the perovskite block. Bi4Ti3O12 is the best-known and extensively studied compound within this family. Here, using a high-pressure, high-temperature synthesis method, we could successfully prepare a full magnetic analogue, Bi4V3O12, with d1 cations. Bi4V3O12 is unstable in air above about 520 K. However, in an inert atmosphere, Bi4V3O12 demonstrates two first-order reversible structural transitions near 525 and 760 K. The high-temperature prototypical phase is the same in both Bi4V3O12 and Bi4Ti3O12 with tetragonal (T) I4/mmm symmetry and aT = 3.85608(5) Å and cT = 32.6920(8) Å (at 850 K) for Bi4V3O12, while the low-temperature phases are different. Bi4V3O12 shows anisotropic thermal expansion above 300 K and negative volumetric thermal expansion above about 700 K. Magnetic measurements showed a broad maximum near 70 K on magnetic susceptibility, indicating the presence of low-dimensional magnetism with strong antiferromagnetic interactions between V4+ ions with the Curie-Weiss temperature of about -370 K. But no long-range magnetic ordering was found in Bi4V3O12 down to 2 K.

Outdoor Full-color LED Display for Sports Games Based on Non-volatile Ferroelectric Read-write Memory Chip

Outdoor Full-color LED Display for Sports Games Based on Non-volatile Ferroelectric Read-write Memory Chip

How to process P(VDF-TrFE) thin films for controlling short circuits in flexible non-volatile memories

Severe electrical shorting is one of the biggest challenges for the development of flexible non-volatile ferroelectric memories with low operation voltage. In this work, short circuits in flexible ferroelectric capacitors based on copolymer poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)], were explicitly investigated. It was found that processing conditions of P(VDF-TrFE) films have huge effects on electrical shorting conditions. Both deciding factors such as drying temperatures, solvent evaporation rates, and solution concentrations, and plausible factors such as post-drying thermal annealing, were clearly identified. These deciding factors were found to affect short circuits by altering the film porosity, surface roughness, and defect conditions in P(VDF-TrFE) thin films. The detailed correlation of these factors with film porosity were established and explained within the framework of liquid-liquid phase separation. The correlation with the surface roughness and defect conditions were explained with Marangoni convection. The net effects of these factors on electrical shorting conditions are rather complicated because a favorable factor with regards to film porosity may be detrimental in terms of surface roughness and/or defects. So these factors have to been carefully tailored to control short circuits and generate high yields in P(VDF-TrFE) thin film memory devices. Comparative study shows that drying at 80 °C in open air is the optimal condition for processing P(VDF-TrFE) thin films from PGMEA solution. Under this optimal drying condition, a yield of 76% was achieved from large-area devices of about 200 μm × 200 μm with thin P(VDF-TrFE) films of about 200 nm processed with roll-2-roll compatible techniques on plastic substrates. An effective two-step approach to manufacturing P(VDF-TrFE) layers for ferroelectric capacitors with low operation voltage but high yields, is presented. The key information gained from this work on ferroelectric capacitors could be generalized to another type of ferroelectric memories, namely ferroelectric field effect transistors. So it provides researchers and engineers a general guideline on how to control short circuits by processing P(VDF-TrFE) thin films for flexible non-volatile memories.

High-Temperature Ferroelectric Behavior of Al0.7Sc0.3N.

Currently, there is a lack of nonvolatile memory (NVM) technology that can operate continuously at temperatures > 200 °C. While ferroelectric NVM has previously demonstrated long polarization retention and >1013 read/write cycles at room temperature, the largest hurdle comes at higher temperatures for conventional perovskite ferroelectrics. Here, we demonstrate how AlScN can enable high-temperature (>200 °C) nonvolatile memory. The c-axis textured thin films were prepared via reactive radiofrequency magnetron sputtering onto a highly textured Pt (111) surface. Photolithographically defined Pt top electrodes completed the capacitor stack, which was tested in a high temperature vacuum probe station up to 400 °C. Polarization–electric field hysteresis loops between 23 and 400 °C reveal minimal changes in the remanent polarization values, while the coercive field decreased from 4.3 MV/cm to 2.6 MV/cm. Even at 400 °C, the polarization retention exhibited negligible loss for up to 1000 s, demonstrating promise for potential nonvolatile memory capable of high−temperature operation. Fatigue behavior also showed a moderate dependence on operating temperature, but the mechanisms of degradation require additional study.

Emergent ferroelectricity in subnanometer binary oxide films on silicon.

The critical size limit of voltage-switchable electric dipoles has extensive implications for energy-efficient electronics, underlying the importance of ferroelectric order stabilized at reduced dimensionality. We report on the thickness-dependent antiferroelectric-to-ferroelectric phase transition in zirconium dioxide (ZrO2) thin films on silicon. The emergent ferroelectricity and hysteretic polarization switching in ultrathin ZrO2, conventionally a paraelectric material, notably persists down to a film thickness of 5 angstroms, the fluorite-structure unit-cell size. This approach to exploit three-dimensional centrosymmetric materials deposited down to the two-dimensional thickness limit, particularly within this model fluorite-structure system that possesses unconventional ferroelectric size effects, offers substantial promise for electronics, demonstrated by proof-of-principle atomic-scale nonvolatile ferroelectric memory on silicon. Additionally, it is also indicative of hidden electronic phenomena that are achievable across a wide class of simple binary materials.

A Robust Memristor Based on Epitaxial Vertically Aligned Nanostructured BaTiO3 -CeO2 Films on Silicon.

With the exploration of ferroelectric materials, researchers have a strong desire to explore the next generation of non-volatile ferroelectric memory with silicon-based epitaxy, high-density storage, and algebraic operations. Herein, a silicon-based memristor with an epitaxial vertically aligned nanostructures BaTiO3 -CeO2 film based on La0.67 Sr0.33 MnO3 /SrTiO3 /Si substrate is reported. The ferroelectric polarization reversal is optimized through the continuous exploring of growth temperature, and the epitaxial structure is obtained, thus it improves the resistance characteristic, the multi-value storage function of five states is achieved, and the robust endurance characteristic can reach 109 cycles. In the synapse plasticity modulated by pulse voltage process, the function of the spiking-time-dependent plasticity and paired-pulse facilitation is simulated successfully. More importantly, the algebraic operations of addition, subtraction, multiplication, and division are realized by using fast speed pulse of the width ≈50ns. Subsequently, a convolutional neural network is constructed for identifying the CIFAR-10 dataset, to simulate the performance of the device; the online and offline learning recognition rate reach 90.03% and 92.55%, respectively. Overall, this study paves the way for memristors with silicon-based epitaxial ferroelectric films to realize multi-value storage, algebraic operations, and neural computing chip applications.

Comparison of 2-D MoS2 and Si Ferroelectric FET Nonvolatile Memories Considering the Trapped-Charge-Induced Variability

This brief investigates scaled 2-D MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ferroelectric field-effect transistor (FeFET) nonvolatile memories (NVMs) considering the trapped-charge-induced variability with the aid of Technology Computer Aided Design (TCAD) atomistic simulations. Our study indicates that, compared with the Si channel, the monolayer MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> channel with larger electron affinity and bandgap energy can result in an FeFET NVM with larger memory window (MW). Moreover, due to its lower channel permittivity and channel thickness, the 2-D MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> FeFET possesses a superior immunity to trapped-charge-induced variability, and the gap in MW between MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and Si FeFETs enlarges under the presence of trapped charges. Besides, due to its atomically thin channel thickness and superior electrostatic integrity, the scaled 2-D MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> FeFET possesses a remarkably better read margin than the Si counterpart. Our study may provide insights for future scaling of FeFET NVMs.

Novel Fluorite-Structured Materials for Solid-State Refrigeration.

Refrigeration based on the electrocaloric effect can offer many advantages over conventional cooling technologies in terms of efficiency, size, weight, and power source. The discovery of ferroelectric and antiferroelectric properties in fluorite-based materials in 2011 has led to diverse applications related to memory (e.g., ferroelectric tunnel junctions, nonvolatile memory, and field-effect transistors) and energy fields (e.g., energy storage and harvesting, electrocaloric refrigeration, and infrared detection). Fluorite-based materials exhibit several properties not shared by most conventional materials (such as in terms of compatibility with complementary metal-oxide semiconductors and 3D nanostructures, deposition thickness at the nanometer scale, and simple composition). Here, the electrocaloric refrigeration properties of fluorite-based ferroelectric/antiferroelectric materials are reviewed by focusing on the advantages of ZrO2 - and HfO2 -based materials (e.g., relative to conventional perovskite- and polymer-based counterparts). Finally, the recent progress made in this research field are also discussed along with its future perspectives.

Controlling Resistance Switching Performances of Hf0.5Zr0.5O2Films by Substrate Stress and Potential in Neuromorphic Computing

Ferroelectric Hf0.5Zr0.5O2(HZO) thin films have attracted wide attention in terms of potential applications of nonvolatile ferroelectric memories. However, the effect of strain on the resistance switching characteristics of the ferroelectric HZO thin‐film memristors has not been fully studied so far. In this work, the strain effects on the HZO thin‐film memristors are investigated. HZO films with different resistance properties are also prepared by controlling the value of oxygen pressure. Based on the testing results, it is proposed that the resistance switching behavior of HZO films may be caused by the joint participation between ferroelectricity and oxygen vacancy migration. The study also found that HZO films can successfully simulate learning behavior similar to the human brain. The applied pulses with a width of tens of nanoseconds timescale are beneficial to realize fast learning and computing. These results provide a fundamental and deep insight on HZO‐based ferroelectric semiconductor oxide thin‐film memristors and their potential applications in next‐generation artificial electronic synaptic devices.

Wake-up free Hf0.5Zr0.5O2 thin film with enhanced ferroelectricity and reliability synthesized by atomic layer crystallization induced by substrate biasing

Attentions on HfO2-based ferroelectric thin films have been rapidly raised due to the promising non-volatile memory applications in advanced artificial intelligence. In this study, the novel “atomic layer substrate bias induced crystallization (ALCISB)” technique is proposed to prepare wake-up free and back-end-of-line compatible Hf0.5Zr0.5O2 (HZO) thin films. The adatom migration boosted by the energy transfer from the plasma in the ALCISB process contributes to the film densification and the crystallinity enhancement of the HZO layer. The ferroelectric remnant polarization (2Pr) and the dielectric constant are increased from 3 to 48 μC/cm2 and 21.4 to 27.7, respectively, which are attributed to the crystallization into orthorhombic phase by the ALCISB technique. Furthermore, the ALCISB treatment suppresses the amount of oxygen vacancies, leading to the extension of the endurance up to 109 cycles and the wake-up free operation of the HZO thin film. The retention characteristic is almost free of degradation after extrapolating to 10 years of thermal treatment. The exceptional ferroelectric properties of the HZO thin films synthesized by ALCISB are highly favorable to non-volatile ferroelectric memory devices in the near future.

Conduction mechanisms in thin (0.6)BiFeO3-(0.4)PbTiO3 films

Leakage current is one of the major drawbacks for a proper electrical and dielectric characterization limiting applications of ferroelectric materials, especially considering thin films. In this sense, understanding the leakage current mechanisms in such systems is of utmost importance. Herein, high quality (0.6)BiFeO3–(0.4)PbTiO3 (BFPT) thin films (∼90 nm) on Pt/TiO2/SiO2/Si substrates were synthesized by dip-coating, and electrical, structural, and microstructural properties were investigated. Analysis based on current density measurements revealed that, for BFPT thin films below 100 nm, all related bulk-limited conduction mechanisms are absent. At this thickness regime, only electrode-limited mechanisms should be considered, and the Fowler-Nordheim mechanism was found to be the predominant mechanism for fields above 240 kV/cm. Besides, dielectric spectroscopy investigations revealed real dielectric constants varying from 28 to 26, with low tangent loss (from 8% to 12%) in the range of 10 kHz and 1 MHz. Such results contribute for the further development of a new generation of BFPT thin films looking for appliable materials as electronic devices, non-volatile ferroelectric memories, and energy harvesting.

Evolutionary 2D organic crystals for optoelectronic transistors and neuromorphic computing

Abstract Brain-inspired neuromorphic computing has been extensively researched, taking advantage of increased computer power, the acquisition of massive data, and algorithm optimization. Neuromorphic computing requires mimicking synaptic plasticity and enables near-in-sensor computing. In synaptic transistors, how to elaborate and examine the link between microstructure and characteristics is a major difficulty. Due to the absence of interlayer shielding effects, defect-free interfaces, and wide spectrum responses, reducing the thickness of organic crystals to the 2D limit has a lot of application possibilities in this computing paradigm. This paper presents an update on the progress of 2D organic crystal-based transistors for data storage and neuromorphic computing. The promises and synthesis methodologies of 2D organic crystals (2D OCs) are summarized. Following that, applications of 2D OCs for ferroelectric non-volatile memory, circuit-type optoelectronic synapses, and neuromorphic computing are addressed. Finally, new insights and challenges for the field’s future prospects are presented, pushing the boundaries of neuromorphic computing even farther.

Improving the Scalability of Ferroelectric FET Nonvolatile Memories With High-k Spacers

This paper investigates scaled ferroelectric field-effect transistor (FeFET) nonvolatile memories (NVMs) with high-k spacer device design considering ferroelectric-dielectric random phase variations with TCAD atomistic simulations. Our study indicates that, in addition to raising the orthorhombic phase and reducing the grain size of the ferroelectric, using high-k spacers can serve as another way to enhance the scalability of FeFETs because it improves both the mean memory window (MW) and the worst-case MW. More importantly, these improvements increase with the down-scaling of gate length. In addition, we have investigated the impact of high-k spacers on the critical electric field across interfacial layer (EIL) for the reliability of FeFET NVMs. Our study suggests that, for scaled FeFETs with high-k spacers, the highest EIL during write operation is no longer located near the S/D edge but at the mid channel. Using high-k spacers can reduce the mid-channel EIL, and the reduction increases with decreasing gate length due to the increasing impact of high-k spacers. Our study may provide insights for future high-density FeFET design.