Ultrathin ferroic HfO2-ZrO2 superlattice gate stack for advanced transistors

Novel 2DEG System at the HfO2/STO Interface

Multifunctional integration in a single device has always been a hot research topic, especially for contradictory phenomena, one of which is the coexistence of ferroelectricity and metallicity. The complex oxide heterostructures, as symmetric breaking systems, provide a great possibility to incorporate different properties. Moreover, finding a series of oxide heterostructures to achieve this goal remains as a challenge. Here, taking the advantage of different physical phenomena, we use H2 plasma to pretreat the SrTiO3 (STO) substrate and then fabricate HfO2/STO heterostructures with it. The novel, well-repeatable metallic two-dimensional electron gas (2DEG) is directly obtained at the heterointerfaces without any further complex procedures, while the obvious ferroelectric-like behavior and Rashba spin-orbit coupling are also observed. The understanding of the mechanism, as well as the modified facile preparation procedure, would be meaningful for further development of ferroelectric metal in complex oxide heterostructures.

Deterministic Orientation Control of Ferroelectric HfO2 Thin Film Growth by a Topotactic Phase Transition of an Oxide Electrode

The scale-free ferroelectricity with superior Si compatibility of HfO2 has reawakened the feasibility of scaled-down nonvolatile devices and beyond the complementary metal-oxide-semiconductor (CMOS) architecture based on ferroelectric materials. However, despite the rapid development, fundamental understanding, and control of the metastable ferroelectric phase in terms of oxygen ion movement of HfO2 remain ambiguous. In this study, we have deterministically controlled the orientation of a single-crystalline ferroelectric phase HfO2 thin film via oxygen ion movement. We induced a topotactic phase transition of the metal electrode accompanied by the stabilization of the differently oriented ferroelectric phase HfO2 through the migration of oxygen ions between the oxygen-reactive metal electrode and the HfO2 layer. By stabilizing different polarization directions of HfO2 through oxygen ion migration, we can gain a profound understanding of the oxygen ion-relevant unclear phenomena of ferroelectric HfO2.

Giant energy storage and power density negative capacitance superlattices

Dielectric electrostatic capacitors1, because of their ultrafast charge-discharge, are desirable for high-power energy storage applications. Along with ultrafast operation, on-chip integration can enable miniaturized energy storage devices for emerging autonomous microelectronics and microsystems2-5. Moreover, state-of-the-art miniaturized electrochemical energy storage systems-microsupercapacitors and microbatteries-currently face safety, packaging, materials and microfabrication challenges preventing on-chip technological readiness2,3,6, leaving an opportunity for electrostatic microcapacitors. Here we report record-high electrostatic energy storage density (ESD) and power density, to our knowledge, in HfO2-ZrO2-based thin film microcapacitors integrated into silicon, through a three-pronged approach. First, to increase intrinsic energy storage, atomic-layer-deposited antiferroelectric HfO2-ZrO2 films are engineered near a field-driven ferroelectric phase transition to exhibit amplified charge storage by the negative capacitance effect7-12, which enhances volumetric ESD beyond the best-known back-end-of-the-line-compatible dielectrics (115 J cm-3) (ref. 13). Second, to increase total energy storage, antiferroelectric superlattice engineering14 scales the energy storage performance beyond the conventional thickness limitations of HfO2-ZrO2-based (anti)ferroelectricity15 (100-nm regime). Third, to increase the storage per footprint, the superlattices are conformally integrated into three-dimensional capacitors, which boosts the areal ESD nine times and the areal power density 170 times that of the best-known electrostatic capacitors: 80 mJ cm-2 and 300 kW cm-2, respectively. This simultaneous demonstration of ultrahigh energy density and power density overcomes the traditional capacity-speed trade-off across the electrostatic-electrochemical energy storage hierarchy1,16. Furthermore, the integration of ultrahigh-density and ultrafast-charging thin films within a back-end-of-the-line-compatible process enables monolithic integration of on-chip microcapacitors5, which can unlock substantial energy storage and power delivery performance for electronic microsystems17-19.

Ferroelectric capacitors and field-effect transistors as in-memory computing elements for machine learning workloads

This study discusses the feasibility of Ferroelectric Capacitors (FeCaps) and Ferroelectric Field-Effect Transistors (FeFETs) as In-Memory Computing (IMC) elements to accelerate machine learning (ML) workloads. We conducted an exploration of device fabrication and proposed system-algorithm co-design to boost performance. A novel FeCap device, incorporating an interfacial layer (IL) andHf 0.5 Zr 0.5 O 2 (HZO), ensures a reduction in operating voltage and enhances HZO scaling while being compatible with CMOS circuits. The IL also enriches ferroelectricity and retention properties. When integrated into crossbar arrays, FeCaps and FeFETs demonstrate their effectiveness as IMC components, eliminating sneak paths and enabling selector-less operation, leading to notable improvements in energy efficiency and area utilization. However, it is worth noting that limited capacitance ratios in FeCaps introduced errors in multiply-and-accumulate (MAC) computations. The proposed co-design approach helps in mitigating these errors and achieves high accuracy in classifying the CIFAR-10 dataset, elevating it from a baseline of 10% to 81.7%. FeFETs in crossbars, with a higher on-off ratio, outperform FeCaps, and our proposed charge-based sensing scheme achieved at least an order of magnitude reduction in power consumption, compared to prevalent current-based methods.

High energy density in artificial heterostructures through relaxation time modulation

Electrostatic capacitors are foundational components of advanced electronics and high-power electrical systems owing to their ultrafast charging-discharging capability. Ferroelectric materials offer high maximum polarization, but high remnant polarization has hindered their effective deployment in energy storage applications. Previous methodologies have encountered problems because of the deteriorated crystallinity of the ferroelectric materials. We introduce an approach to control the relaxation time using two-dimensional (2D) materials while minimizing energy loss by using 2D/3D/2D heterostructures and preserving the crystallinity of ferroelectric 3D materials. Using this approach, we were able to achieve an energy density of 191.7 joules per cubic centimeter with an efficiency greater than 90%. This precise control over relaxation time holds promise for a wide array of applications and has the potential to accelerate the development of highly efficient energy storage systems.

High-Performance Ferroelectric Thin Film Transistors with Large Memory Window Using Epitaxial Yttrium-Doped Hafnium Zirconium Gate Oxide

Preventing ferroelectric materials from losing their ferroelectricity over a low thickness of several nanometers is crucial in developing multifunctional nanoelectronics. Epitaxially grown 5 at. % yttrium-doped Hf0.5Zr0.5O2 (YHZO) thin films exhibit an atomically smooth surface, an ability to maintain ferroelectricity even at a thickness of 10 nm, and excellent insulating properties, making them suitable for use as gate oxides in ferroelectric thin film transistors (FeTFTs). Through the epitaxial growth of a YHZO/La0.67Sr0.33MnO3 (LSMO)/SrTiO3 (STO) heterostructure, YHZO effectively retains its ferroelectricity and orthorhombic single phase, leading to enhancing electron mobility (∼19.74 cm2 V-1 s-1) and memory window (3.7 V) in the amorphous InGaZnO4 (a-IGZO)/YHZO/LSMO/STO FeTFTs. These FeTFTs demonstrate a consistent memory function with remarkable endurance (∼106 cycles) and retention (∼104 s). Furthermore, they sustain a constant memory window even under ±6 V bias stress for 104 s and exhibit excellent stability even under ±6 V/1 ms pulse cycling for 107 cycles. For comparison, a transistor with the same structure was fabricated using epitaxial nonferroelectric LaAlO3 (LAO) and epitaxial undoped Hf0.5Zr0.5O2 (HZO) as alternatives to YHZO. This study presents a novel approach to exploit the potential of YHZO in FeTFTs, contributing to the development of next-generation logic-in-memory.

Ferroelastically protected reversible orthorhombic to monoclinic-like phase transition in ZrO2 nanocrystals

Robust ferroelectricity in nanoscale fluorite oxide-based thin films enables promising applications in silicon-compatible non-volatile memories and logic devices. However, the polar orthorhombic (O) phase of fluorite oxides is a metastable phase that is prone to transforming into the ground-state non-polar monoclinic (M) phase, leading to macroscopic ferroelectric degradation. Here we investigate the reversibility of the O-M phase transition in ZrO2 nanocrystals via in situ visualization of the martensitic transformation at the atomic scale. We reveal that the reversible shear deformation pathway from the O phase to the monoclinic-like (M') state, a compressive-strained M phase, is protected by 90° ferroelectric-ferroelastic switching. Nevertheless, as the M' state gradually accumulates localized strain, a critical tensile strain can pin the ferroelastic domain, resulting in an irreversible M'-M strain relaxation and the loss of ferroelectricity. These findings demonstrate the key role of ferroelastic switching in the reversibility of phase transition and also provide a tensile-strain threshold for stabilizing the metastable ferroelectric phase in fluorite oxide thin films.

Anti-Ambipolar Heterojunctions: Materials, Devices, and Circuits

Anti-ambipolar heterojunctions are vital in constructing high-frequency oscillators, fast switches, and multivalued logic (MVL) devices, which hold promising potential for next-generation integrated circuit chips and telecommunication technologies. Thanks to the strategic material design and device integration, anti-ambipolar heterojunctions have demonstrated unparalleled device and circuit performance that surpasses other semiconducting material systems. This review aims to provide a comprehensive summary of the achievements in the field of anti-ambipolar heterojunctions. First, the fundamental operating mechanisms of anti-ambipolar devices are discussed. After that, potential materials used in anti-ambipolar devices are discussed with particular attention to 2D-based, 1D-based, and organic-based heterojunctions. Next, the primary device applications employing anti-ambipolar heterojunctions, including anti-ambipolar transistors (AATs), photodetectors, frequency doublers, and synaptic devices, are summarized. Furthermore, alongside the advancements in individual devices, the practical integration of these devices at the circuit level, including topics such as MVL circuits, complex logic gates, and spiking neuron circuits, is also discussed. Lastly, the present key challenges and future research directions concerning anti-ambipolar heterojunctions and their applications are also emphasized.

Interface engineering for facile switching of bulk-strong polarization in Si-compatible vertical superlattices

Ferroelectric thin films incorporating different compositional layers have emerged as a promising approach for enhancing properties and performance of electronic devices. In recent years, superlattices utilizing various interactions between their constituent layers have been used to reveal unusual properties, such as improper ferroelectricity, charged domain walls, and negative capacitance in conventional ferroelectrics. Herein, we report a symmetry scheme based on the interface engineering in which the inherent cell-doubling symmetry allowed atomic distortions (phonons) in any vertically aligned superlattice activate novel interface couplings among atomic distortions of different symmetries and fundamentally improve the ferroelectric properties. In a materialized case, the ionic size difference between Hf4+ and Ce4+ in the HfO2/CeO2 (HCO) ferroelectric/paraelectric superlattice leads to these couplings. These couplings mitigate the phase boundary between polar and non-polar phases, and facilitate polarization switching with a remarkably low coercive field ( E c ) while preserving the original magnitude of the bulk HfO2 polarization and its scale-free ferroelectric characteristics. We show that the cell-doubled distortions present in any vertical superlattice have unique implications for designing low-voltage ferroelectric switching while retaining bulk-strong charge storing capacities in Si-compatible memory candidates.

Scalable Synthesis of High-Quality Ultrathin Ferroelectric Magnesium Molybdenum Oxide

The development of ultrathin, stable ferroelectric materials is crucial for advancing high-density, low-power electronic devices. Nonetheless, ultrathin ferroelectric materials are rare due to the critical size effect. Here, a novel ferroelectric material, magnesium molybdenum oxide (Mg2 Mo3 O8 ) is presented. High-quality ultrathin Mg2 Mo3 O8 crystals are synthesized using chemical vapor deposition (CVD). Ultrathin Mg2 Mo3 O8 has a wide bandgap (≈4.4 eV) and nonlinear optical response. Mg2 Mo3 O8 crystals of varying thicknesses exhibit out-of-plane ferroelectric properties at room temperature, with ferroelectricity retained even at a 2 nm thickness. The Mg2 Mo3 O8 exhibits a relatively large remanent polarization ranging from 33 to 52 µC cm- 2 , which is tunable by changing its thickness. Notably, Mg2 Mo3 O8 possesses a high Curie temperature (>980 °C) across various thicknesses. Moreover, the as-grown Mg2 Mo3 O8 crystals display remarkable stability under harsh environments. This work introduces nolanites-type crystal into ultrathin ferroelectrics. The scalable synthesis of stable ultrathin ferroelectric Mg2 Mo3 O8 expands the scope of ferroelectric materials and may prosper applications of ferroelectrics.

Bridging the gap between surface physics and photonics

Use and performance criteria of photonic devices increase in various application areas such as information and communication, lighting, and photovoltaics. In many current and future photonic devices, surfaces of a semiconductor crystal are a weak part causing significant photo-electric losses and malfunctions in applications. These surface challenges, many of which arise from material defects at semiconductor surfaces, include signal attenuation in waveguides, light absorption in light emitting diodes, non-radiative recombination of carriers in solar cells, leakage (dark) current of photodiodes, and light reflection at solar cell interfaces for instance. To reduce harmful surface effects, the optical and electrical passivation of devices has been developed for several decades, especially with the methods of semiconductor technology. Because atomic scale control and knowledge of surface-related phenomena have become relevant to increase the performance of different devices, it might be useful to enhance the bridging of surface physics to photonics. Toward that target, we review some evolving research subjects with open questions and possible solutions, which hopefully provide example connecting points between photonic device passivation and surface physics. One question is related to the properties of the wet chemically cleaned semiconductor surfaces which are typically utilized in device manufacturing processes, but which appear to be different from crystalline surfaces studied in ultrahigh vacuum by physicists. In devices, a defective semiconductor surface often lies at an embedded interface formed by a thin metal or insulator film grown on the semiconductor crystal, which makes the measurements of its atomic and electronic structures difficult. To understand these interface properties, it is essential to combine quantum mechanical simulation methods. This review also covers metal-semiconductor interfaces which are included in most photonic devices to transmit electric carriers to the semiconductor structure. Low-resistive and passivated contacts with an ultrathin tunneling barrier are an emergent solution to control electrical losses in photonic devices.

Physics, Structures, and Applications of Fluorite-Structured Ferroelectric Tunnel Junctions

The interest in ferroelectric tunnel junctions (FTJ) has been revitalized by the discovery of ferroelectricity in fluorite-structured oxides such as HfO2 and ZrO2 . In terms of thickness scaling, CMOS compatibility, and 3D integration, these fluorite-structured FTJs provide a number of benefits over conventional perovskite-based FTJs. Here, recent developments involving all FTJ devices with fluorite structures are examined. The transport mechanism of fluorite-structured FTJs is explored and contrasted with perovskite-based FTJs and other 2-terminal resistive switching devices starting with the operation principle and essential parameters of the tunneling electroresistance effect. The applications of FTJs, such as neuromorphic devices, logic-in-memory, and physically unclonable function, are then discussed, along with several structural approaches to fluorite-structure FTJs. Finally, the materials and device integration difficulties related to fluorite-structure FTJ devices are reviewed. The purpose of this review is to outline the theories, physics, fabrication processes, applications, and current difficulties associated with fluorite-structure FTJs while also describing potential future possibilities for optimization.

Structure and stability of La- and hole-doped hafnia with/without epitaxial strain

The significance of hafnia in the semiconductor industry has been amplified following the unearthing of its ferroelectric properties. We investigated the structure and electrical properties of La- and hole-doped HfO2with/without epitaxial strain by first-principles calculations. It is found that the charge compensated defect with oxygen vacancy (LaHfVO) and uncompensated defect (LaHf), compared to the undoped case, make the ferroelectric orthorhombicPca21phase (ophase) more stable. Conversely, the electrons compensated defect (LaHf+e) makes the nonpolar monoclinicP21/cphase (mphase) more stable. Furthermore, both pure hole doping (without ions substituent) and compressive strain can stabilize theophase. Our work offers a new perspective on enhancing the ferroelectricity of hafnia.

Structural phase purification of bulk HfO2:Y through pressure cycling

We combine synchrotron-based infrared absorption and Raman scattering spectroscopies with diamond anvil cell techniques and first-principles calculations to explore the properties of hafnia under compression. We find that pressure drives HfO[Formula: see text]:7%Y from the mixed monoclinic ([Formula: see text]) [Formula: see text] antipolar orthorhombic ([Formula: see text]) phase to pure antipolar orthorhombic ([Formula: see text]) phase at approximately 6.3 GPa. This transformation is irreversible, meaning that upon release, the material is kinetically trapped in the [Formula: see text] metastable state at 300 K. Compression also drives polar orthorhombic ([Formula: see text]) hafnia into the tetragonal ([Formula: see text]) phase, although the latter is not metastable upon release. These results are unified by an analysis of the energy landscape. The fact that pressure allows us to stabilize targeted metastable structures with less Y stabilizer is important to preserving the flat phonon band physics of pure HfO[Formula: see text].

Liquid Metal Doping Induced Asymmetry in Two-Dimensional Metal Oxides

The emergence of ferroelectricity in two-dimensional (2D) metal oxides is a topic of significant technological interest; however, many 2D metal oxides lack intrinsic ferroelectric properties. Therefore, introducing asymmetry provides access to a broader range of 2D materials within the ferroelectric family. Here, the generation of asymmetry in 2D SnO by doping the material with Hf0.5 Zr0.5 O2 (HZO) is demonstrated. A liquid metal process as a doping strategy for the preparation of 2D HZO-doped SnO with robust ferroelectric characteristics is implemented. This technology takes advantage of the selective interface enrichment of molten Sn with HZO crystallites. Molecular dynamics simulations indicate a strong tendency of Hf and Zr atoms to migrate toward the surface of liquid metal and embed themselves within the growing oxide layer in the form of HZO. Thus, the liquid metal-based harvesting/doping technique is a feasible approach devised for producing novel 2D metal oxides with induced ferroelectric properties, represents a significant development for the prospects of random-access memories.

Tunable and parabolic piezoelectricity in hafnia under epitaxial strain

Piezoelectrics are a class of functional materials that have been extensively used for application in modern electro-mechanical and mechatronics technologies. The sign of longitudinal piezoelectric coefficients is typically positive but recently a few ferroelectrics, such as ferroelectric polymer poly(vinylidene fluoride) and van der Waals ferroelectric CuInP2S6, were experimentally found to have negative piezoelectricity. Here, using first-principles calculation and measurements, we show that the sign of the longitudinal linear piezoelectric coefficient of HfO2 can be tuned from positive to negative via epitaxial strain. Nonlinear and even parabolic piezoelectric behaviors are further found at tensile epitaxial strain. This parabolic piezoelectric behavior implies that the polarization decreases when increasing the magnitude of either compressive or tensile longitudinal strain, or, equivalently, that the strain increases when increasing the magnitude of electric field being either parallel or antiparallel to the direction of polarization. The unusual piezoelectric effects are from the chemical coordination of the active oxygen atoms. These striking piezoelectric features of positive and negative sign, as well as linear and parabolical behaviors, expand the current knowledge in piezoelectricity and broaden the potential of piezoelectric applications towards electro-mechanical and communications technology.

Out-of-plane ferroelectricity and robust magnetoelectricity in quasi-two-dimensional materials

Thin-film ferroelectrics have been pursued for capacitive and nonvolatile memory devices. They rely on polarizations that are oriented in an out-of-plane direction to facilitate integration and addressability with complementary metal-oxide semiconductor architectures. The internal depolarization field, however, formed by surface charges can suppress the out-of-plane polarization in ultrathin ferroelectric films that could otherwise exhibit lower coercive fields and operate with lower power. Here, we unveil stabilization of a polar longitudinal optical (LO) mode in the n = 2 Ruddlesden-Popper family that produces out-of-plane ferroelectricity, persists under open-circuit boundary conditions, and is distinct from hyperferroelectricity. Our first-principles calculations show the stabilization of the LO mode is ubiquitous in chalcogenides and halides and relies on anharmonic trilinear mode coupling. We further show that the out-of-plane ferroelectricity can be predicted with a crystallographic tolerance factor, and we use these insights to design a room-temperature multiferroic with strong magnetoelectric coupling suitable for magneto-electric spin-orbit transistors.

Sharp Transformation across Morphotropic Phase Boundary in Sub-6 nm Wake-Up-Free Ferroelectric Films by Atomic Layer Technology

Atomic layer engineering is investigated to tailor the morphotropic phase boundary (MPB) between antiferroelectric, ferroelectric, and paraelectric phases. By increasing the HfO2 seeding layer with only 2 monolayers, the overlying ZrO2 layer experiences the dramatic phase transition across the MPB. Conspicuous ferroelectric properties including record-high remanent polarization (2Pr ≈ 60 µC cm-2 ), wake-up-free operation, and high compatibility with advanced semiconductor technology nodes, are achieved in the sub-6 nm thin film. The prominent antiferroelectric to ferroelectric phase transformation is ascribed to the in-plane tensile stress introduced into ZrO2 by the HfO2 seeding layer. Based on the high-resolution and high-contrast images of surface grains extracted precisely by helium ion microscopy, the evolution of the MPB between tetragonal, orthorhombic, and monoclinic phases with grain size is demonstrated for the first time. The result indicates that a decrease in the average grain size drives the crystallization from the tetragonal to polar orthorhombic phases.

0D van der Waals interfacial ferroelectricity

The dimensional limit of ferroelectricity has been long explored. The critical contravention is that the downscaling of ferroelectricity leads to a loss of polarization. This work demonstrates a zero-dimensional ferroelectricity by the atomic sliding at the restrained van der Waals interface of crossed tungsten disufilde nanotubes. The developed zero-dimensional ferroelectric diode in this work presents not only non-volatile resistive memory, but also the programmable photovoltaic effect at the visible band. Benefiting from the intrinsic dimensional limitation, the zero-dimensional ferroelectric diode allows electrical operation at an ultra-low current. By breaking through the critical size of depolarization, this work demonstrates the ultimately downscaled interfacial ferroelectricity of zero-dimensional, and contributes to a branch of devices that integrates zero-dimensional ferroelectric memory, nano electro-mechanical system, and programmable photovoltaics in one.

Ultralow Subthreshold Swing of a MOSFET Caused by Ferroelectric Polarization Reversal of Hf0.5Zr0.5O2 Thin Films

The emergence of complementary metal-oxide semiconductor (CMOS)-compatible HfO2-based ferroelectric materials provides a promising way to achieve ferroelectric field-effect transistors (FeFETs) with a steep subthreshold swing (SS) reduced to below the Boltzmann thermodynamics limit (∼60 mV/dec at room temperature), which has important implications for lowering power consumption. In this work, a metal-oxide-semiconductor field-effect transistor (MOSFET) is connected with Hf0.5Zr0.5O2 (HZO)-based ferroelectric capacitors with different capacitances. By adjusting the capacitance of ferroelectric capacitors, an ultralow SS of ∼0.34 mV/dec in HfO2-based FeFETs can be achieved. More interestingly, by designing the sweeping voltage sequences, the SS can be adjusted to be 0 mV/dec with the drain current ranging over six orders of magnitude, and the threshold voltage for turning on the MOSFET can be further reduced. The manipulated SS could be attributed to the evolution of ferroelectric switching. Our work contributes to understanding the origin of ultralow SS in ferroelectric MOSFETs and the realization of low-power devices.

Scalable integration of hybrid high-κ dielectric materials on two-dimensional semiconductors

Two-dimensional (2D) semiconductors are promising channel materials for next-generation field-effect transistors (FETs). However, it remains challenging to integrate ultrathin and uniform high-κ dielectrics on 2D semiconductors to fabricate FETs with large gate capacitance. We report a versatile two-step approach to integrating high-quality dielectric film with sub-1 nm equivalent oxide thickness (EOT) on 2D semiconductors. Inorganic molecular crystal Sb2O3 is homogeneously deposited on 2D semiconductors as a buffer layer, which forms a high-quality oxide-to-semiconductor interface and offers a highly hydrophilic surface, enabling the integration of high-κ dielectrics via atomic layer deposition. Using this approach, we can fabricate monolayer molybdenum disulfide-based FETs with the thinnest EOT (0.67 nm). The transistors exhibit an on/off ratio of over 106 using an ultra-low operating voltage of 0.4 V, achieving unprecedently high gating efficiency. Our results may pave the way for the application of 2D materials in low-power ultrascaling electronics.

Porous nanorods by stacked NiO nanoparticulate exhibiting corn-like structure for sustainable environmental and energy applications

A porous 1D nanostructure provides much shorter electron transport pathways, thereby helping to improve the life cycle of the device and overcome poor ionic and electronic conductivity, interfacial impedance between electrode-electrolyte interface, and low volumetric energy density. In view of this, we report on the feasibility of 1D porous NiO nanorods comprising interlocked NiO nanoparticles as an active electrode for capturing greenhouse CO₂, effective supercapacitors, and efficient electrocatalytic water-splitting applications. The nanorods with a size less than 100 nm were formed by stacking cubic crystalline NiO nanoparticles with dimensions less than 10 nm, providing the necessary porosity. The existence of Ni²⁺ and its octahedral coordination with O^2- is corroborated by XPS and EXAFS. The SAXS profile and BET analysis showed 84.731 m² g^-1 surface area for the porous NiO nanorods. The NiO nanorods provided significant surface-area and the active-surface-sites thus yielded a CO₂ uptake of 63 mmol g^-1 at 273 K via physisorption, a specific-capacitance (C_S) of 368 F g^-1, along with a retention of 76.84% after 2500 cycles, and worthy electrocatalytic water splitting with an overpotential of 345 and 441 mV for HER and OER activities, respectively. Therefore, the porous 1D NiO as an active electrode shows multifunctionality toward sustainable environmental and energy applications.

Absence of critical thickness for polar skyrmions with breaking the Kittel's law

The period of polar domain (d) in ferroics was commonly believed to scale with corresponding film thicknesses (h), following the classical Kittel's law of d ∝ [Formula: see text]. Here, we have not only observed that this relationship fails in the case of polar skyrmions, where the period shrinks nearly to a constant value, or even experiences a slight increase, but also discovered that skyrmions have further persisted in [(PbTiO3)2/(SrTiO3)2]10 ultrathin superlattices. Both experimental and theoretical results indicate that the skyrmion periods (d) and PbTiO3 layer thicknesses in superlattice (h) obey the hyperbolic function of d = Ah + [Formula: see text] other than previous believed, simple square root law. Phase-field analysis indicates that the relationship originates from the different energy competitions of the superlattices with PbTiO3 layer thicknesses. This work exemplified the critical size problems faced by nanoscale ferroelectric device designing in the post-Moore era.

Wurtzite and fluorite ferroelectric materials for electronic memory

Ferroelectric materials, the charge equivalent of magnets, have been the subject of continued research interest since their discovery more than 100 years ago. The spontaneous electric polarization in these crystals, which is non-volatile and programmable, is appealing for a range of information technologies. However, while magnets have found their way into various types of modern information technology hardware, applications of ferroelectric materials that use their ferroelectric properties are still limited. Recent advances in ferroelectric materials with wurtzite and fluorite structure have renewed enthusiasm and offered new opportunities for their deployment in commercial-scale devices in microelectronics hardware. This Review focuses on the most recent and emerging wurtzite-structured ferroelectric materials and emphasizes their applications in memory and storage-based microelectronic hardware. Relevant comparisons with existing fluorite-structured ferroelectric materials are made and a detailed outlook on ferroelectric materials and devices applications is provided.

Polarization Switching and Correlated Phase Transitions in Fluorite-Structure ZrO2 Nanocrystals

Unconventional ferroelectricity in fluorite-structure oxides enables tremendous opportunities in nanoelectronics owing to their superior scalability and silicon compatibility. However, their polarization order and switching process remain elusive due to the challenges of visualizing oxygen ions in nanocrystalline films. In this work, the oxygen shifting during polarization switching and correlated polar-nonpolar phase transitions are directly captured among multiple metastable phases in freestanding ZrO₂ thin films by low-dose integrated differential phase-contrast scanning transmission electron microscopy (iDPC-STEM). Bidirectional transitions between antiferroelectric and ferroelectric orders and interfacial polarization relaxation are clarified at unit-cell scale. Meanwhile, polarization switching is strongly correlated with Zr-O displacement in reversible martensitic transformation between monoclinic and orthorhombic phases and two-step tetrahedral-to-orthorhombic phase transition. These findings provide atomic insights into the transition pathways between metastable polymorphs and unravel the evolution of polarization orders in (anti)ferroelectric fluorite oxides.

Interface-engineered ferroelectricity of epitaxial Hf0.5Zr0.5O2 thin films

Ferroelectric hafnia-based thin films have attracted intense attention due to their compatibility with complementary metal-oxide-semiconductor technology. However, the ferroelectric orthorhombic phase is thermodynamically metastable. Various efforts have been made to stabilize the ferroelectric orthorhombic phase of hafnia-based films such as controlling the growth kinetics and mechanical confinement. Here, we demonstrate a key interface engineering strategy to stabilize and enhance the ferroelectric orthorhombic phase of the Hf_0.5Zr_0.5O₂ thin film by deliberately controlling the termination of the bottom La_0.67Sr_0.33MnO₃ layer. We find that the Hf_0.5Zr_0.5O₂ films on the MnO₂-terminated La_0.67Sr_0.33MnO₃ have more ferroelectric orthorhombic phase than those on the LaSrO-terminated La_0.67Sr_0.33MnO₃, while with no wake-up effect. Even though the Hf_0.5Zr_0.5O₂ thickness is as thin as 1.5 nm, the clear ferroelectric orthorhombic (111) orientation is observed on the MnO₂ termination. Our transmission electron microscopy characterization and theoretical modelling reveal that reconstruction at the Hf_0.5Zr_0.5O₂/ La_0.67Sr_0.33MnO₃ interface and hole doping of the Hf_0.5Zr_0.5O₂ layer resulting from the MnO₂ interface termination are responsible for the stabilization of the metastable ferroelectric phase of Hf_0.5Zr_0.5O₂. We anticipate that these results will inspire further studies of interface-engineered hafnia-based systems.

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.

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.

Highly-scaled and fully-integrated 3-dimensional ferroelectric transistor array for hardware implementation of neural networks

Hardware-based neural networks (NNs) can provide a significant breakthrough in artificial intelligence applications due to their ability to extract features from unstructured data and learn from them. However, realizing complex NN models remains challenging because different tasks, such as feature extraction and classification, should be performed at different memory elements and arrays. This further increases the required number of memory arrays and chip size. Here, we propose a three-dimensional ferroelectric NAND (3D FeNAND) array for the area-efficient hardware implementation of NNs. Vector-matrix multiplication is successfully demonstrated using the integrated 3D FeNAND arrays, and excellent pattern classification is achieved. By allocating each array of vertical layers in 3D FeNAND as the hidden layer of NN, each layer can be used to perform different tasks, and the classification of color-mixed patterns is achieved. This work provides a practical strategy to realize high-performance and highly efficient NN systems by stacking computation components vertically.

Ferroelectric Nanogap-Based Steep-Slope Ambipolar Transistor

The subthreshold swing (SS) of metal-oxide-semiconductor field-effect transistors is limited to 60 mV dec^-1 at room temperature by the Boltzmann tyranny, which restricts the scaling of the supply voltage. A nanogap-based transistor employs a switchable nanoscale air gap as the channel, offering a steep-slope switching process. Meanwhile, nanogaps featuring even sub-3 nm can efficiently block the current flow, exhibiting the potential for tackling the short-channel effect. Here, an electrically switchable ferroelectric nanogap to construct steep-slope transistors, is exploited. An average SS of 15.9 mV dec^-1 across 5 orders and a minimum SS of 13.23 mV dec^-1 are obtained in the high current density range. The transistor exhibits excellent performance with near-zero off-state leakage current and a maximum on-state current of 202 µA µm^-1 at V_DS  = 0.5 V. In addition, the transistor can turn off with either a positive or negative increase in the gate voltage, exhibiting ambipolar characteristics.

Toward attojoule switching energy in logic transistors

Advances in the theory of semiconductors in the 1930s in addition to the purification of germanium and silicon crystals in the 1940s enabled the point-contact junction transistor in 1947 and initiated the era of semiconductor electronics. Gordon Moore postulated 18 years later that the number of components in an integrated circuit would double every 1 to 2 years with associated reductions in cost per transistor. Transistor density doubling through scaling—the decrease of component sizes—with each new process node continues today, albeit at a slower pace compared with historical rates of scaling. Transistor scaling has resulted in exponential gain in performance and energy efficiency of integrated circuits, which transformed computing from mainframes to personal computers and from mobile computing to cloud computing. Innovations in new materials, transistor structures, and lithographic technologies will enable further scaling. Monolithic 3D integration, design technology co-optimization, alternative switching mechanisms, and cryogenic operation could enable further transistor scaling and improved energy efficiency in the foreseeable future.

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.

Superhigh energy storage density on-chip capacitors with ferroelectric Hf0.5Zr0.5O2/antiferroelectric Hf0.25Zr0.75O2 bilayer nanofilms fabricated by plasma-enhanced atomic layer deposition

Thanks to their excellent compatibility with the complementary metal-oxide-semiconductor (CMOS) process, antiferroelectric (AFE) HfO₂/ZrO₂-based thin films have emerged as potential candidates for high-performance on-chip energy storage capacitors of miniaturized energy-autonomous systems. However, increasing the energy storage density (ESD) of capacitors has been a great challenge. In this work, we propose the fabrication of ferroelectric (FE) Hf_0.5Zr_0.5O₂/AFE Hf_0.25Zr_0.75O₂ bilayer nanofilms by plasma-enhanced atomic layer deposition for high ESD capacitors with TiN electrodes. The effects of the FE/AFE thickness composition and annealing conditions are investigated, revealing that the Hf_0.5Zr_0.5O₂ (1 nm)/Hf_0.25Zr_0.75O₂ (9 nm) bilayer can generate the optimal ESD after optimized annealing at 450 °C for 30 min. This is mainly ascribed to the factor that the introduction of a 1 nm Hf_0.5Zr_0.5O₂ layer enhances the formation of the tetragonal (T) phase with antiferroelectricity in the AFE Hf_0.25Zr_0.75O₂ layer as well as the breakdown electric field of the bilayer while fixing the FE/AFE bilayer thickness at 10 nm. As a result, a ESD as high as 71.95 J cm^-3 can be obtained together with an energy storage efficiency (ESE) of 57.8%. Meanwhile, with increasing the measurement temperature from 300 and 425 K, the capacitor also demonstrates excellent stabilities of ESD and ESE. In addition, superior electrical cycling endurance is also demonstrated. Further, by integrating the capacitor into deep silicon trenches, a superhigh ESD of 364.1 J cm^-3 is achieved together with an ESE of 56.5%. This work provides an effective way for developing CMOS process-compatible, eco-friendly and superhigh ESD three-dimensional capacitors for on-chip energy storage applications.

A review on morphotropic phase boundary in fluorite-structure hafnia towards DRAM technology

In the present hyper-scaling era, memory technology is advancing owing to the demand for high-performance computing and storage devices. As a result, continuous work on conventional semiconductor-process-compatible ferroelectric memory devices such as ferroelectric field-effect transistors, ferroelectric random-access memory, and dynamic random-access memory (DRAM) cell capacitors is ongoing. To operate high-performance computing devices, high-density, high-speed, and reliable memory devices such as DRAMs are required. Consequently, considerable attention has been devoted to the enhanced high dielectric constant and reduced equivalent oxide thickness (EOT) of DRAM cell capacitors. The advancement of ferroelectric hafnia has enabled the development of various devices, such as ferroelectric memories, piezoelectric sensors, and energy harvesters. Therefore, in this review, we focus the morphotropic phase boundary (MPB) between ferroelectric orthorhombic and tetragonal phases, where we can achieve a high dielectric constant and thereby reduce the EOT. We also present the role of the MPB in perovskite and fluorite structures as well as the history of the MPB phase. We also address the different approaches for achieving the MPB phase in a hafnia material system. Subsequently, we review the critical issues in DRAM technology using hafnia materials. Finally, we present various applications of the hafnia material system near the MPB, such as memory, sensors, and energy harvesters.

Remote Oxygen Scavenging of the Interfacial Oxide Layer in Ferroelectric Hafnium-Zirconium Oxide-Based Metal-Oxide-Semiconductor Structures

Discovery of ferroelectricity in HfO₂ has sparked a lot of interest in its use in memory and logic due to its CMOS compatibility and scalability. Devices that use ferroelectric HfO₂ are being investigated; for example, the ferroelectric field-effect transistor (FEFET) is one of the leading candidates for next generation memory technology, due to its area, energy efficiency and fast operation. In an FEFET, a ferroelectric layer is deposited on Si, with an SiO₂ layer of ∼1 nm thickness inevitably forming at the interface. This interfacial layer (IL) increases the gate voltage required to switch the polarization and write into the memory device, thereby increasing the energy required to operate FEFETs, and makes the technology incompatible with logic circuits. In this work, it is shown that a Pt/Ti/thin TiN gate electrode in a ferroelectric Hf_0.5Zr_0.5O₂ based metal-oxide-semiconductor (MOS) structure can remotely scavenge oxygen from the IL, thinning it down to ∼0.5 nm. This IL reduction significantly reduces the ferroelectric polarization switching voltage with a ∼2× concomitant increase in the remnant polarization and a ∼3× increase in the abruptness of polarization switching consistent with density functional theory (DFT) calculations modeling the role of the IL layer in the gate stack electrostatics. The large increase in remnant polarization and abruptness of polarization switching are consistent with the oxygen diffusion in the scavenging process reducing oxygen vacancies in the HZO layer, thereby depinning the polarization of some of the HZO grains.

Novel Approach to High κ (∼59) and Low EOT (∼3.8 Å) near the Morphotrophic Phase Boundary with AFE/FE (ZrO2/HZO) Bilayer Heterostructures and High-Pressure Annealing

We present herewith a novel approach of equally thick AFE/FE (ZrO₂/HZO) bilayer stack heterostructure films for achieving an equivalent oxide thickness (EOT) of 4.1 Å with a dielectric constant (κ) of 56 in complementary metal-oxide semiconductor (CMOS) compatible metal-ferroelectric-metal (MFM) capacitors using a high-pressure annealing (HPA) technique. The low EOT and high κ values were achieved by careful optimization of AFE/FE film thicknesses and HPA conditions near the morphotropic phase boundary (MPB) after field cycling effects. Stable leakage current density (J < 10^-7 A/cm² at ±0.8 V) was found at 3/3 nm bilayer stack films (κ = 56 and EOT = 4.1 Å) measured at room temperature. In comparison with previous work, our remarkable achievement stems from the interfacial coupling between FE and AFE films as well as a high-quality crystalline structure formed by HPA. Kinetically stabilized hafnia films result in a small grain size in bilayer films, leading to reducing the leakage current density. Further, a higher κ value of 59 and lower EOT of 3.4 Å were found at 333 K. However, stable leakage current density was found at 273 K with a high κ value of 53 and EOT of 3.85 Å with J < 10^-7 A/cm². This is the lowest recorded EOT employing hafnia and TiN electrodes that are compatible with CMOS, and it has important implications for future dynamic random access memory (DRAM) technology.

High Quality Factors in Superlattice Ferroelectric Hf0.5Zr0.5O2 Nanoelectromechanical Resonators

The discovery of ferroelectricity and advances in creating polar structures in atomic-layered hafnia-zirconia (Hf_xZr_1-xO₂) films spur the exploration of using the material for novel integrated nanoelectromechanical systems (NEMS). Despite its popularity, the approach to achieving high quality factors (Qs) in resonant NEMS made of Hf_xZr_1-xO₂ thin films remains unexplored. In this work, we investigate the realization of high Qs in Hf_0.5Zr_0.5O₂ nanoelectromechanical resonators by stress engineering via the incorporation of alumina (Al₂O₃) interlayers. We fabricate nanoelectromechanical resonators out of the Hf_0.5Zr_0.5O₂-Al₂O₃ superlattices, from which we measure Qs up to 171,000 and frequency-quality factor products (f × Q) of >10¹¹ Hz through electrical excitation and optical detection schemes at room temperature in vacuum. The analysis suggests that clamping loss and surface loss are the limiting dissipation sources and f × Q > 10¹² Hz is achievable through further engineering of anchor structure and built-in stress.