Hafnia-Based Ferroelectric Memory: Device Physics Strongly Correlated with Materials Chemistry.

Hafnia-based ferroelectrics and their semiconductor applications are reviewed, focusing on next-generation dynamic random-access-memory (DRAM) and Flash. The challenges of achieving high endurance and high write/read speed and the optimal material properties to achieve them are discussed. In DRAM applications, the trade-off between remanent polarization (Pr), endurance, and operation speed is highlighted, focusing on reducing the critical material property Ec (coercive field). Novel phase formation and interfacial redox chemistry are reviewed as potential game-changers for ferroelectric memories. Regarding Flash operation, the need for an ideal Pr and Ec ratio is emphasized, as excessive Pr can lead to charge trapping, resulting in fatigue and pass disturbance in the NAND array. Achieving the right balance of Pr and Ec for ferroelectric NAND with hafnia-based ferroelectrics remains challenging. This Perspective also recognizes technical advancements in FeFET technology, offering potential solutions for improved performance and casting a positive outlook on the future of ferroelectric memory technology.

Role of oxygen vacancies in ferroelectric or resistive switching hafnium oxide.

HfO2 shows promise for emerging ferroelectric and resistive switching (RS) memory devices owing to its excellent electrical properties and compatibility with complementary metal oxide semiconductor technology based on mature fabrication processes such as atomic layer deposition. Oxygen vacancy (Vo), which is the most frequently observed intrinsic defect in HfO2-based films, determines the physical/electrical properties and device performance. Vo influences the polymorphism and the resulting ferroelectric properties of HfO2. Moreover, the switching speed and endurance of ferroelectric memories are strongly correlated to the Vo concentration and redistribution. They also strongly influence the device-to-device and cycle-to-cycle variability of integrated circuits based on ferroelectric memories. The concentration, migration, and agglomeration of Vo form the main mechanism behind the RS behavior observed in HfO2, suggesting that the device performance and reliability in terms of the operating voltage, switching speed, on/off ratio, analog conductance modulation, endurance, and retention are sensitive to Vo. Therefore, the mechanism of Vo formation and its effects on the chemical, physical, and electrical properties in ferroelectric and RS HfO2 should be understood. This study comprehensively reviews the literature on Vo in HfO2 from the formation and influencing mechanism to material properties and device performance. This review contributes to the synergetic advances of current knowledge and technology in emerging HfO2-based semiconductor devices.

A perspective on the physical scaling down of hafnia-based ferroelectrics.

HfO2-based ferroelectric thin films have attracted significant interest for semiconductor device applications due to their compatibility with complementary metal oxide semiconductor (CMOS) technology. One of the benefits of HfO2-based ferroelectric thin films is their ability to be scaled to thicknesses as low as 10 nm while retaining their ferroelectric properties; a feat that has been difficult to accomplish with conventional perovskite-based ferroelectrics using CMOS-compatible processes. However, reducing the thickness limit of HfO2-based ferroelectric thin films below the sub 5 nm thickness regime while preserving their ferroelectric property remains a formidable challenge. This is because both the structural factors of HfO2, including polymorphism and orientation, and the electrical factors of HfO2-based devices, such as the depolarization field, are known to be highly dependent on the HfO2thickness. Accordingly, when the thickness of HfO2drops below 5 nm, these factors will become even more crucial. In this regard, the size effect of HfO2-based ferroelectric thin films is thoroughly discussed in the present review. The impact of thickness on the ferroelectric property of HfO2-based thin films and the electrical performance of HfO2-based ferroelectric semiconductor devices, such as ferroelectric random-access-memory, ferroelectric field-effect-transistor, and ferroelectric tunnel junction, is extensively discussed from the perspective of fundamental theory and experimental results. Finally, recent developments and reports on achieving ferroelectric HfO2at sub-5 nm thickness regime and their applications are discussed.

Correction: Interfacial engineering of a Mo/Hf0.3Zr0.7O2/Si capacitor using the direct scavenging effect of a thin Ti layer.

Correction for 'Interfacial engineering of a Mo/Hf0.3Zr0.7O2/Si capacitor using the direct scavenging effect of a thin Ti layer' by Se Hyun Kim et al., Chem. Commun., 2021, 57, 12452-12455, https://doi.org/10.1039/D1CC04966F.

From Ferroelectric Material Optimization to Neuromorphic Devices.

Due to the voltage driven switching at low voltages combined with nonvolatility of the achieved polarization state, ferroelectric materials have a unique potential for low power nonvolatile electronic devices. The competitivity of such devices is hindered by compatibility issues of well-known ferroelectrics with established semiconductor technology. The discovery of ferroelectricity in hafnium oxide changed this situation. The natural application of nonvolatile devices is as a memory cell. Nonvolatile memory devices also built the basis for other applications like in-memory or neuromorphic computing. Three different basic ferroelectric devices can be constructed: ferroelectric capacitors, ferroelectric field effect transistors and ferroelectric tunneling junctions. In this article first the material science of the ferroelectricity in hafnium oxide will be summarized with a special focus on tailoring the switching characteristics towards different applications.The current status of nonvolatile ferroelectric memories then lays the ground for looking into applications like in-memory computing. Finally, a special focus will be given to showcase how the basic building blocks of spiking neural networks, the neuron and the synapse, can be realized and how they can be combined to realize neuromorphic computing systems. A summary, comparison with other technologies like resistive switching devices and an outlook completes the paper.

Revival of Ferroelectric Memories Based on Emerging Fluorite-Structured Ferroelectrics.

Over the last few decades, the research on ferroelectric memories has been limited due to their dimensional scalability and incompatibility with complementary metal-oxide-semiconductor (CMOS) technology. The discovery of ferroelectricity in fluorite-structured oxides revived interest in the research on ferroelectric memories, by inducing nanoscale nonvolatility in state-of-the-art gate insulators by minute doping and thermal treatment. The potential of this approach has been demonstrated by the fabrication of sub-30 nm electronic devices. Nonetheless, to realize practical applications, various technical limitations, such as insufficient reliability including endurance, retention, and imprint, as well as large device-to-device-variation, require urgent solutions. Furthermore, such limitations should be considered based on targeting devices as well as applications. Various types of ferroelectric memories including ferroelectric random-access-memory, ferroelectric field-effect-transistor, and ferroelectric tunnel junction should be considered for classical nonvolatile memories as well as emerging neuromorphic computing and processing-in-memory. Therefore, from the viewpoint of materials science, this review covers the recent research focusing on ferroelectric memories from the history of conventional approaches to future prospects.

Reversible transition between the polar and antipolar phases and its implications for wake-up and fatigue in HfO2-based ferroelectric thin film.

Atomic-resolution Cs-corrected scanning transmission electron microscopy revealed local shifting of two oxygen positions (OI and OII) within the unit cells of a ferroelectric (Hf0.5Zr0.5)O2 thin film. A reversible transition between the polar Pbc21 and antipolar Pbca phases, where the crystal structures of the 180° domain wall of the Pbc21 phase and the unit cell structure of the Pbca phase were identical, was induced by applying appropriate cycling voltages. The critical field strength that determined whether the film would be woken up or fatigued was ~0.8 MV/cm, above or below which wake-up or fatigue was observed, respectively. Repeated cycling with sufficiently high voltages led to development of the interfacial nonpolar P42/nmc phase, which induced fatigue through the depolarizing field effect. The fatigued film could be rejuvenated by applying a slightly higher voltage, indicating that these transitions were reversible. These mechanisms are radically different from those of conventional ferroelectrics.

Comparison of balance ability and physical fitness according to the growth period in taekwondo players.

The purpose of this study was to investigate and evaluate balance ability, isokinetic moments, and physical fitness according to the growth period from middle school to university in male taekwondo players. Thirty-five taekwondo players (middle school, n=11; high school, n=15; university, n=9) participated in this study. The anteriorposterior and medial-lateral CoP displacement at balance ability was better in university players than in high school and middle school players. The isokinetic knee joint muscle strength of the right extension and flexion % body weight (BW) at 60°/s was higher in university players than in high school players. The average power of right flexion BW at 180°/sec was higher in middle school players than in high school players. Back strength was higher in university players than in middle and high school players. The repeated jump was higher in high school players than in middle school players. The standing broad jump was higher in university players than in middle school players. The side-step was higher in university players than in high school players. The average power and total energy of anaerobic power were higher in university and high school players than in middle school players. The peak drop in anaerobic power was higher in middle school players than in high school players. Based on the results of the present study, the growth period should be considered when applying training such as balance, isokinetic moments, and physical fitness to male taekwondo players.

Interfacial engineering of a Mo/Hf0.3Zr0.7O2/Si capacitor using the direct scavenging effect of a thin Ti layer.

An antiferroelectric Mo/Hf0.3Zr0.7O2/SIOx/Si capacitor was engineered using the direct scavenging effect of a sputtered Ti sacrificial layer. Charge trapping could be mitigated with the oxidized TiO2 layer, and the endurance could be enhanced beyond 109 cycles, which is higher than that of the gate stack of ferroelectric field-effect-transistors by 3-4 orders of magnitude.

Suppressed Stochastic Switching Behavior and Improved Synaptic Functions in an Atomic Switch Embedded with a 2D NbSe2 Material.

We investigated chemical vapor-deposited (CVD) two-dimensional (2D) niobium diselenide (NbSe2) material for the resistive switching and synaptic characteristics. Three different atomic switch devices with Ag/HfO2/Pt, Ag/Ti/HfO2/Pt, and Ag/NbSe2/HfO2/Pt were studied as both memory and neuromorphic devices. Both the inserted Ti and NbSe2 buffer layers effectively control the stochastic Ag-ion diffusion, leading to suppressed variation of switching characteristics, which is a critical issue in an atomic switch device. Especially, the device with the 2D NbSe2 buffer layer strikingly enhanced the device reliability in both endurance and retention. In conjunction with scanning transmission electron microscopy (STEM) and energy-dispersive spectrometry (EDS) analysis of the control of the Ag-ion migration, it was understood that filament connection is interrelated with the SET and RESET processes. Besides resistive behaviors in the memory device, various synapse functions such as spike-rate-dependent plasticity (SRDP), forgetting curve, potentiation, and depression were demonstrated with an atomic switch with the 2D NbSe2 buffer layer. Furthermore, the emulated long-term synaptic property was simulated using the MNIST 28 × 28 pixel database. Using adopting a CVD 2D NbSe2 blocking layer, the stochastic Ag-ion diffusion behavior is well-controlled and therefore stable switching and synapse functions are attained.

Ultra-flexible and rollable 2D-MoS2/Si heterojunction-based near-infrared photodetector via direct synthesis.

Atomic two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted significant attention for application in various optoelectronic devices such as image sensors, biomedical imaging systems, and consumer electronics and in diverse spectroscopic analyses. However, a complicated fabrication process, involving transfer and alignment of as-synthesized 2D layers onto flexible target substrates, hinders the development of flexible high-performance heterojunction-based photodetectors. Herein, an ultra-flexible 2D-MoS2/Si heterojunction-based photodetector is successfully fabricated through atmospheric-pressure plasma enhanced chemical vapor deposition, which enables the direct deposition of multi-layered MoS2 onto a flexible Si substrate at low temperature (<200 °C). The photodetector is responsive to near infrared light (λ = 850 nm), showing responsivity of 10.07 mA W-1 and specific detectivity (D*) of 4.53 × 1010 Jones. The measured photocurrent as a function of light intensity exhibits good linearity with a power law exponent of 0.84, indicating negligible trapping/de-trapping of photo-generated carriers at the heterojunction interface, which facilitates photocarrier collection. Furthermore, the photodetectors can be bent with a small bending radius (5 mm) and wrapped around a glass rod, showing excellent photoresponsivity under various bending radii. Hence, the device exhibits excellent flexibility, rollability, and durability under harsh bending conditions. This photodetector has significant potential for use in next-generation flexible and patchable optoelectronic devices.

A Comparative Study on the Ferroelectric Performances in Atomic Layer Deposited Hf0.5Zr0.5O2 Thin Films Using Tetrakis(ethylmethylamino) and Tetrakis(dimethylamino) Precursors.

The chemical, physical, and electrical properties of the atomic layer deposited Hf0.5Zr0.5O2 thin films using tetrakis(ethylmethylamino) (TEMA) and tetrakis(dimethylamino) (TDMA) precursors are compared. The ligand of the metal-organic precursors strongly affects the residual C concentration, grain size, and the resulting ferroelectric properties. Depositing Hf0.5Zr0.5O2 films with the TDMA precursors results in lower C concentration and slightly larger grain size. These findings are beneficial to grow more ferroelectric-phase-dominant film, which mitigates its wake-up effect. From the wake-up test of the TDMA-Hf0.5Zr0.5O2 film with a 2.8 MV/cm cycling field, the adverse wake-up effect was well suppressed up to 105 cycles, with a reasonably high double remanent polarization value of ~40 µC/cm2. The film also showed reliable switching up to 109 cycles with the 2.5 MV/cm cycling field without involving the wake-up effect but with the typical fatigue behavior.

Influence of Hemodialysis Catheter Insertion on Hemodynamics in the Central Veins.

Central venous catheter (CVC) related thrombosis is a major cause of CVC dysfunction in patients under hemodialysis. The aim of our study was to investigate the impact of CVC insertion on hemodynamics in the central veins and to examine the changes in hemodynamic environments that may be related to thrombus formation due to the implantation of CVC. Patient-specific models of the central veins with and without CVC were reconstructed based on computed tomography images. Flow patterns in the veins were numerically simulated to obtain hemodynamic parameters such as time-averaged wall shear stress (TAWSS), oscillating shear index (OSI), relative residence time (RRT), and normalized transverse wall shear stress (transWSS) under pulsatile flow. The non-Newtonian effects of blood flow were also analyzed using the Casson model. The insertion of CVC caused significant changes in the hemodynamic environment in the central veins. A greater disturbance and increase of velocity were observed in the central veins after the insertion of CVC. As a result, TAWSS and transWSS were markedly increased, but most parts of OSI and RRT decreased. Newtonian assumption of blood flow would overestimate the increase in TAWSS after CVC insertion. High wall shear stress (WSS) and flow disturbance, especially the multidirectionality of the flow, induced by the CVC may be a key factor in initiating thrombosis after CVC insertion. Accordingly, approaches to decrease the flow disturbance during CVC insertion may help restrain the occurrence of thrombosis. More case studies with pre-operative and postoperative modeling and clinical follow-up need to be performed to verify these findings. Non-Newtonian blood flow assumption is recommended in computational fluid dynamics (CFD) simulations of veins with CVCs.

Fluorite-structure antiferroelectrics.

Ferroelectricity in fluorite-structure oxides like hafnia and zirconia have attracted increasing interest since 2011. Two spontaneous polarizations of the fluorite-structure ferroelectrics are considered highly promising for nonvolatile memory applications, with their superior scalability and Si compatibility compared to the conventional perovskite-structure ferroelectrics. Besides, antiferroelectricity originating from a field-induced phase transition between the paraelectric and ferroelectric phases in fluorite-structure oxides is another highly interesting matter. It was suggested that the field-induced phase transition could be utilized for energy conversions between thermal and electrical energy, as well as for energy storage. The important energy-related applications of antiferroelectric fluorite-structure oxides, however, have not been systematically reviewed to date. Thus, in this work, the fluorite-structure antiferroelectrics are reviewed from their fundamentals to their applications based on pyroelectricity as well as antiferroelectricity. Another important application field of the fluorite-structure antiferroelectrics is the semiconductor memory devices. The fluorite-structure antiferroelectrics can be utilized for antiferroelectric random-access-memories, negative capacitance field-effect-transistors, and flash memories. Moreover, the recently reported morphotropic phase boundary (MPB) between the ferroelectric and antiferroelectric phases in this material system marks another significant progress in this material system, and thus, the fundamentals and applications of the MPB phase are also reviewed.

Understanding ferroelectric phase formation in doped HfO2 thin films based on classical nucleation theory.

Ferroelectricity in doped HfO2 thin films has attracted increasing attention since 2011. The origin of the unexpected ferroelectric property is now accepted as the formation of the noncentrosymmetric Pca21 orthorhombic phase. However, the mechanism for the ferroelectric phase formation is still under debate. In this paper, the classical nucleation theory is revisited to understand the ferroelectric phase formation in doped HfO2 thin films. From nucleation theory, it can be identified that there is an appropriate doping concentration range for the ferroelectric phase formation. The doping concentration should be sufficiently high to suppress the monoclinic phase formation during the crystallization annealing process. Once the stable monoclinic phase is formed, the transformation into the other metastable phases is improbable. For appropriate doping concentrations, a transition to the second most stable tetragonal phase is kinetically enhanced, whereas that to the most stable monoclinic phase is kinetically suppressed at the annealing temperature. During cooling, the transition of the tetragonal phase to the second most stable orthorhombic phase is kinetically enhanced, whereas that to the most stable monoclinic phase is suppressed near room temperature. However, the doping concentration should not be too high. Otherwise, the tetragonal phase formed during the crystallization annealing process cannot be transformed into the ferroelectric orthorhombic phase during cooling. This is because high doping concentration lowers the transition temperature and makes the transition reaction difficult. The appropriate doping concentration range is dependent on the types of dopants, but the general governing process was the kinetic nucleation of the tetragonal phase during crystallization and its transformation into the ferroelectric orthorhombic phase during cooling.

Morphotropic Phase Boundary of Hf1- xZr xO2 Thin Films for Dynamic Random Access Memories.

The utilization of the morphotropic phase boundary (MPB) between the newly found ferroelectric orthorhombic phase and the tetragonal phase in an HfO2-ZrO2 solid solution is suggested for a high-capacitance dielectric capacitor. Being different from other high- k dielectrics, where the k value decreases with decreasing film thickness, these films (Hf/Zr ratio = 6:4, 5:5, 3:7) showed increasing k values with decreasing film thicknesses in the ∼5-20 nm range. Among them, Hf0.5Zr0.5O2 and Hf0.3Zr0.7O2 films showed 47 and 43 peak k values at 6.5 and 9.2 nm thicknesses, respectively, suggesting the involvement of the MPB phenomenon. For the systematic understanding of this peculiar phenomenon, the phase evolution of the HfO2-ZrO2 solid solution is presented based on experimental observations. The detailed electrical tests of the films with different compositions and thicknesses demonstrated that the characteristic feature of this material system is consistent with the involvement of the MPB depending on the composition and thickness. Through the optimization of the annealing process for crystallization, a 0.62 nm minimum equivalent oxide thickness was reported for the 6.5 nm thick Hf0.5Zr0.5O2 film, which is highly promising for the future dynamic random access memories. This work provided a breakthrough method for overcoming the fundamental limitation of a decreasing k value with a decreasing film thickness of other high- k dielectrics.

Dispersion in Ferroelectric Switching Performance of Polycrystalline Hf0.5Zr0.5O2 Thin Films.

Interests in nanoscale integrated ferroelectric devices using doped HfO2-based thin films are actively reviving in academia and industry. The main driving force for the formation of the metastable non-centrosymmetric ferroelectric phase is considered to be the interface/grain boundary energy effect of the small grains in polycrystalline configuration. These small grains, however, can invoke unfavorable material properties, such as nonuniform switching performance. This study provides an in-depth understanding of such aspects of this material through careful measurement and modeling of the ferroelectric switching kinetics. Various previous switching models developed for conventional ferroelectric thin-film capacitors cannot fully account for the observed time- and voltage-dependent switching current evolution. The accurate fitting of the experimental results required careful consideration of the inhomogeneous field distribution across the electrode area, which could be acquired by an appropriate mathematical formulation of polarization as a function of electric field and time. Compared with the conventional polycrystalline Pb(Zr,Ti)O3 film, the statistical distribution of the local field was found to be three times wider. The activation field and characteristic time for domain switching were larger by more than 1 order of magnitude. It indicates that doped HfO2 is inhomogeneous and "hard" ferroelectric material compared with conventional perovskite-based ferroelectrics.

Lanthanum-Doped Hafnium Oxide: A Robust Ferroelectric Material.

Recently simulation groups have reported the lanthanide series elements as the dopants that have the strongest effect on the stabilization of the ferroelectric non-centrosymmetric orthorhombic phase in hafnium oxide. This finding confirms experimental results for lanthanum and gadolinium showing the highest remanent polarization values of all hafnia-based ferroelectric films until now. However, no comprehensive overview that links structural properties to the electrical performance of the films in detail is available for lanthanide-doped hafnia. La:HfO2 appears to be a material with a broad window of process parameters, and accordingly, by optimization of the La content in the layer, it is possible to improve the performance of the material significantly. Variations of the La concentration leads to changes in the crystallographic structure in the bulk of the films and at the interfaces to the electrode materials, which impacts the spontaneous polarization, internal bias fields, and with this the field cycling behavior of the capacitor structure. Characterization results are compared to other dopants like Si, Al, and Gd to validate the advantages of the material in applications such as semiconductor memory devices.

Understanding the formation of the metastable ferroelectric phase in hafnia-zirconia solid solution thin films.

Hf1-xZrxO2 (x ∼ 0.5-0.7) has been the leading candidate of ferroelectric materials with a fluorite crystal structure showing highly promising compatibility with complementary metal oxide semiconductor devices. Despite the notable improvement in device performance and processing techniques, the origin of its ferroelectric crystalline phase (space group: Pca21) formation has not been clearly elucidated. Several recent experimental and theoretical studies evidently showed that the interface and grain boundary energies of the higher symmetry phases (orthorhombic and tetragonal) contribute to the stabilization of the metastable non-centrosymmetric orthorhombic phase or tetragonal phase. However, there was a clear quantitative discrepancy between the theoretical expectation and experiment results, suggesting that the thermodynamic model may not provide the full explanation. This work, therefore, focuses on the phase transition kinetics during the cooling step after the crystallization annealing. It was found that the large activation barrier for the transition from the tetragonal/orthorhombic to the monoclinic phase, which is the stable phase at room temperature, suppresses the phase transition, and thus, plays a critical role in the emergence of ferroelectricity.

Improved Ferroelectric Switching Endurance of La-Doped Hf0.5Zr0.5O2 Thin Films.

Hf0.5Zr0.5O2 thin films are one of the most appealing HfO2-based ferroelectric thin films, which have been researched extensively for their applications in ferroelectric memory devices. In this work, a 1 mol % La-doped Hf0.5Zr0.5O2 thin film was grown by plasma-assisted atomic layer deposition and annealed at temperatures of 450 and 500 °C to crystallize the film into the desired orthorhombic phase. Despite the use of a lower temperature than that used in previous reports, the film showed highly promising ferroelectric properties-a remnant polarization of ∼30 µC/cm2 and switching cycle endurance up to 4 × 1010. The performance was much better than that of undoped Hf0.5Zr0.5O2 thin films, demonstrating the positive influence of La doping. Such improvements were mainly attributed to the decreased coercive field (by ∼30% compared to the undoped film), which allowed for the use of a lower applied field to drive the cycling tests while maintaining a high polarization value. La doping also decreased the leakage current by ∼3 orders of magnitude compared to the undoped film, which also contributed to the strongly improved endurance. Nonetheless, the La-doped film required a larger number of wake-up cycles (∼106 cycles) to reach a saturated remnant polarization value. This behavior might be explained by the increased generation of oxygen vacancies and slower migration of these vacancies from the interface to the bulk region. However, the maximum number of wake-up cycles was less than 0.01% of the total possible cycles, and therefore, initializing the film to the maximum performance state would not be a serious burden.