Temperature controlled Ru and RuO2 growth via O* radical-enhanced atomic layer deposition with Ru(EtCp)2

Improvement of the Ferroelectric Response of La-Doped Hafnium Zirconium Oxide Employing Tungsten Oxide Interfacial Layer with Back-End-of-Line Compatibility

In this work, the impact of a tungsten oxide (WO3) seed and capping layer for ferroelectric La-doped (Hf, Zr)O2 (La:HZO) based capacitors, designed with back-end-of-line (BEOL) compatibility, is systematically investigated. The WO3 capping layer supplies oxygen to the La:HZO layer throughout the fabrication process and during device cycling. This facilitates the annihilation of oxygen vacancies (Vo) within the La:HZO layer, thereby stabilizing its ferroelectric orthorhombic phase and resulting in an increase of the remanent polarization (Pr) value in the capacitor. Moreover, the effectiveness of the WO3 capping layer depends on the seed layer of the HZO film, suggesting that proper combination of the seed and capping layers should be employed to maximize the ferroelectric response. Finally, a TiN/TiO2 seed layer/La:HZO/WO3 capping layer/TiN capacitor is successfully fabricated and optimized by a complete set of atomic layer deposition (ALD) processes, achieving a superior 2Pr value and endurance value of more than 109 cycles at an electric field of 2.5 MV/cm. The WO3 capping layer is anticipated to offer a viable solution for doped HZO capacitors with reduced thickness, addressing the challenge of elevated Vo levels that favor the tetragonal phase and result in low 2Pr values.

Improving the ferroelectric properties of Lu doped Hf0.5Zr0.5O2thin films by capping a CeOxlayer

Lu doped Hf0.5Zr0.5O2(HZO) ferroelectric films were prepared on Pt/TiN/SiO2/Si substrate by chemical solution deposition method, and an interfacial engineering strategy for improving the ferroelectric property was explored by capping the Lu doped HZO films with a cerium oxide layer. Compared with the Lu doped HZO film without the CeOxcoating layer, the Lu doped HZO film with the CeOxcoating layer has a larger remanent polarization (2Pr= 34.72µC cm-2) and presents weaker wake-up behavior, which result from the higher orthogonal phase ratio and the lower oxygen vacancy of the CeOxcoated Lu doped HZO film. In addition, the CeOxcoating can remarkably improve the fatigue resistance and retention performance of the Lu doped HZO films. It is hoped that the results can provide an effective approach for the realization of high-performance and highly reliable hafnium oxide based ferroelectric thin films.

Robust multiferroicity and magnetic modulation of the ferroelectric imprint field in heterostructures comprising epitaxial Hf0.5Zr0.5O2 and Co

Magnetoelectric multiferroics, either single-phase or composites comprising ferroelectric/ferromagnetic coupled films, are promising candidates for energy efficient memory computing. However, most of the multiferroic magnetoelectric systems studied so far are based on materials that are not compatible with industrial processes. Doped hafnia is emerging as one of the few CMOS-compatible ferroelectric materials. Thus, it is highly relevant to study the integration of ferroelectric hafnia into multiferroic systems. In particular, ferroelectricity in hafnia, and the eventual magnetoelectric coupling when ferromagnetic layers are grown atop of it, are very much dependent on quality of interfaces. Since magnetic metals frequently exhibit noticeable reactivity when grown onto oxides, it is expected that ferroelectricity and magnetoelectricity might be reduced in multiferroic hafnia-based structures. In this article, we present excellent ferroelectric endurance and retention in epitaxial Hf0.5Zr0.5O2 films grown on buffered silicon using Co as the top electrode. The crucial influence of a thin Pt capping layer grown on top of Co on the ferroelectric functional characteristics is revealed by contrasting the utilization of Pt-capped Co, non-capped Co and Pt. Magnetic control of the imprint electric field (up to 40% modulation) is achieved in Pt-capped Co/Hf0.5Zr0.5O2 structures, although this does not lead to appreciable tuning of the ferroelectric polarization, as a result of its high stability. Computation of piezoelectric and flexoelectric strain-mediated mechanisms of the observed magnetoelectric coupling reveal that flexoelectric contributions are likely to be at the origin of the large imprint electric field variation.

Ultrafast and accurate prediction of polycrystalline hafnium oxide phase-field ferroelectric hysteresis using graph neural networks

Polycrystalline hafnium oxide emerges as a promising material for the future of nanoelectronic devices. While phase-field modeling stands as a primary choice tool for forecasting domain structure evolution and electromechanical properties of ferroelectric materials, it suffers from a high computational cost, which impedes its applicability to real-size systems. Here, we propose a Graph Neural Network (GNN) machine-learning framework to predict the ferroelectric hysteresis of polycrystalline hafnium oxide, with the goal of significantly accelerating computations in contrast to high-fidelity phase-field methods. By leveraging the inherent graph structure of the polycrystalline system and incorporating edge-level feature properties through graph attentional layers, our approach accurately predicts hysteresis behaviors across a broad range of polycrystalline structures, grain numbers, and Landau coefficients. The GNN framework exhibits high accuracy, with an average relative error of ∼4%, and demonstrates remarkable computational efficiency with respect to ground truth phase-field simulations, offering speed-ups exceeding a million-fold. Furthermore, we showcase the transferability of our model to efficiently scale predictions in polycrystals comprising up to a thousand grains, paving the way for effective simulations of real-sized systems. Our approach, by overcoming computational limitations in polycrystalline hafnium oxide, opens doors for accelerating discovery and design in ferroelectric materials.

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.

HfO2-ZrO2 Ferroelectric Capacitors with Superlattice Structure: Improving Fatigue Stability, Fatigue Recovery, and Switching Speed

HfO2-ZrO2 ferroelectric films have recently gained considerable attention from integrated circuit researchers due to their excellent ferroelectric properties over a wide doping range and low deposition temperature. In this work, different HfO2-ZrO2 superlattice (SL) FE films with varying periodicity of HfO2 (5 cycles)-ZrO2 (5 cycles) (SL5), HfO2 (10 cycles)-ZrO2 (10 cycles) (SL10), and HfO2 (15 cycles)-ZrO2 (15 cycles) (SL15) were studied systematically. The HfZrOx (HZO) alloy was used as a comparison device. The SL5 film demonstrated improved ferroelectric properties compared to the HZO film, with the 2 times remnant polarization (2Pr) values increasing from 41.4 to 48.6 μC/cm2 at an applied voltage of 3 V/10 kHz. Furthermore, the first-order reversal curve diagrams of different SL and HZO capacitors at different states (initial, wake-up, fatigue, and recovery) were measured. The SL capacitors were found to effectively suppress the diffusion of defects during P-V cycling, resulting in improved fatigue stability characteristics and fatigue recovery capability compared to the HZO capacitor. Moreover, an improved switching speed of the SL films compared to the HZO capacitor was concluded based on the inhomogeneous field mechanism (IFM) model. These results indicate that the SL structure has a high potential in future high-speed ferroelectric memory applications with excellent stability and recovery capability.

Room-Temperature Quantum Diodes with Dynamic Memory for Neural Logic Operations

The pursuit of high-performance, next-generation nanoelectronics is fundamentally reliant on exploiting quantum phenomena such as tunneling at room temperature. However, quantum tunneling and memory dynamics are governed by two conflicting parameters: the presence or absence of defects. Therefore, the integration of both attributes within a single device presents substantial challenges. Nevertheless, successful integration has the potential to prompt crucial breakthroughs by emulating biobrain-like dynamics, in turn enabling sophisticated in-material neural logic operations. In this work, we demonstrate that a conformal nanolaminate HfO2/ZrO2 structure on silicon enables high-performing (>106 s) Fowler-Nordheim tunneling at room temperature. In addition, the device exhibits unipolar dynamic hysteresis loop opening (on/off ratio >102) with high endurance (>104 cycles) along with negative differential resistance, which is attributed to the collective ferroelectric and capacitive effects and is utilized to emulate synaptic functions. Further, proof-of-concept logic gates based on voltage-dependent plasticity and time-domain were developed using a single device, offering in-material neural-like data processing. These findings pave the way for the realization of high-performing and scalability tunneling devices in advanced nanoelectronics, which mark a promising route toward the development of next-generation, fundamental neural logic computing systems.

A stable rhombohedral phase in ferroelectric Hf(Zr)1+xO2 capacitor with ultralow coercive field

Hafnium oxide-based ferroelectric materials are promising candidates for next-generation nanoscale devices because of their ability to integrate into silicon electronics. However, the intrinsic high coercive field of the fluorite-structure oxide ferroelectric devices leads to incompatible operating voltage and limited endurance performance. We discovered a complementary metal-oxide semiconductor (CMOS)-compatible rhombohedral ferroelectric Hf(Zr)1+xO2 material rich in hafnium-zirconium [Hf(Zr)]. X-ray diffraction combined with scanning transmission electron microscopy reveals that the excess Hf(Zr) atoms intercalate within the hollow sites. We found that the intercalated atoms expand the lattice and increase the in-plane and out-of-plane stresses, which stabilize both the rhombohedral phase (r-phase) and its ferroelectric properties. Our ferroelectric devices, which are based on the r-phase Hf(Zr)1+xO2, exhibit an ultralow coercive field (~0.65 megavolts per centimeter). Moreover, we achieved a high endurance of more than 1012 cycles at saturation polarization. This material discovery may help to realize low-cost and long-life memory chips.

Yttrium Doping Effects on Ferroelectricity and Electric Properties of As-Deposited Hf1-xZrxO2 Thin Films via Atomic Layer Deposition

Hf1-xZrxO2 (HZO) thin films are versatile materials suitable for advanced ferroelectric semiconductor devices. Previous studies have shown that the ferroelectricity of HZO thin films can be stabilized by doping them with group III elements at low concentrations. While doping with Y improves the ferroelectric properties, there has been limited research on Y-HZO thin films fabricated using atomic layer deposition (ALD). In this study, we investigated the effects of Y-doping cycles on the ferroelectric and electrical properties of as-deposited Y-HZO thin films with varying compositions fabricated through ALD. The Y-HZO thin films were stably crystallized without the need for post-thermal treatment and exhibited transition behavior depending on the Y-doping cycle and initial composition ratio of the HZO thin films. These Y-HZO thin films offer several advantages, including enhanced dielectric constant, leakage current density, and improved endurance. Moreover, the optimized Y-doping cycle induced a phase transformation that resulted in Y-HZO thin films with improved ferroelectric properties, exhibiting stable behavior without fatigue for up to 1010 cycles. These as-deposited Y-HZO thin films show promise for applications in semiconductor devices that require high ferroelectric properties, excellent electrical properties, and reliable performance with a low thermal budget.

HfO2-based ferroelectric thin film and memory device applications in the post-Moore era: A review

The rapid development of 5G, big data, and Internet of Things (IoT) technologies is urgently required for novel non-volatile memory devices with low power consumption, fast read/write speed, and high reliability, which are crucial for high-performance computing. Ferroelectric memory has undergone extensive investigation as a viable alternative for commercial applications since the post-Moore era. However, conventional perovskite-structure ferroelectrics (e.g., PbZr x Ti1- x O3) encounter severe limitations for high-density integration owing to the size effect of ferroelectricity and incompatibility with complementary metal-oxide-semiconductor technology. Since 2011, the ferroelectric field has been primarily focused on HfO2-based ferroelectric thin films owing to their exceptional scalability. Several reviews discussing the control of ferroelectricity and device applications exist. It is believed that a comprehensive understanding of mechanisms based on industrial requirements and concerns is necessary, such as the wake-up effect and fatigue mechanism. These mechanisms reflect the atomic structures of the materials as well as the device physics. Herein, a review focusing on phase stability and domain structure is presented. In addition, the recent progress in related ferroelectric memory devices and their challenges is briefly discussed.

Reduced fatigue and leakage of ferroelectric TiN/Hf0.5Zr0.5O2/TiN capacitors by thin alumina interlayers at the top or bottom interface

Hf_0.5Zr_0.5O₂(HZO) thin films are promising candidates for non-volatile memory and other related applications due to their demonstrated ferroelectricity at the nanoscale and compatibility with Si processing. However, one reason that HZO has not been fully scaled into industrial applications is due to its deleterious wake-up and fatigue behavior which leads to an inconsistent remanent polarization during cycling. In this study, we explore an interfacial engineering strategy in which we insert 1 nm Al₂O₃interlayers at either the top or bottom HZO/TiN interface of sequentially deposited metal-ferroelectric-metal capacitors. By inserting an interfacial layer while limiting exposure to the ambient environment, we successfully introduce a protective passivating layer of Al₂O₃that provides excess oxygen to mitigate vacancy formation at the interface. We report that TiN/HZO/TiN capacitors with a 1 nm Al₂O₃at the top interface demonstrate a higher remanent polarization (2P_r∼ 42μC cm^-2) and endurance limit beyond 10⁸cycles at a cycling field amplitude of 3.5 MV cm^-1. We use time-of-flight secondary ion mass spectrometry, energy dispersive spectroscopy, and grazing incidence x-ray diffraction to elucidate the origin of enhanced endurance and leakage properties in capacitors with an inserted 1 nm Al₂O₃layer. We demonstrate that the use of Al₂O₃as a passivating dielectric, coupled with sequential ALD fabrication, is an effective means of interfacial engineering and enhances the performance of ferroelectric HZO devices.

Retention Improvement of HZO-Based Ferroelectric Capacitors with TiO2 Insets

The influence of the bottom TiO₂ interfacial layer grown by atomic layer deposition on the ferroelectric properties of the TiN/Hf_0.5Zr_0.5O₂/TiN capacitors is systematically investigated. We show that the integration of the TiO₂ layer leads to an increase in the polar orthorhombic phase content in the Hf_0.5Zr_0.5O₂ film. In addition, the crystalline structure of the Hf_0.5Zr_0.5O₂ film is highly dependent on the thickness of the TiO₂ inset, with monoclinic phase stabilization after the increase of TiO₂ thickness. Special attention in this work is given to the key reliability parameters-retention and endurance. We demonstrate that the integration of the TiO₂ inset induces valuable retention improvement. Using a novel approach to the depolarization measurements, we show that the depolarization contribution to the retention loss is insignificant, which leaves the imprint effect as the root of the retention loss in TiN/TiO₂/Hf_0.5Zr_0.5O₂/TiN devices. We believe that the integration of the insulator interfacial layer suppresses the scavenging effect from the bottom TiN electrode, leading to a decrease in the oxygen vacancy content in the Hf_0.5Zr_0.5O₂ film, which is the main reason for imprint mitigation. At the same time, although the observed retention improvement is very promising for the upcoming technological integration, the field cycling testing revealed the endurance limitations linked to the phase transitions in the TiO₂ layer and the rise of the effective electric field applied to the Hf_0.5Zr_0.5O₂ film.

Oxygen scavenging of HfZrO2-based capacitors for improving ferroelectric properties

HfO2-based ferroelectric (FE) materials have emerged as a promising material for non-volatile memory applications because of remanent polarization, scalability of thickness below 10 nm, and compatibility with complementary metal-oxide-semiconductor technology. However, in the metal/FE/insulator/semiconductor, it is difficult to improve switching voltage (V sw), endurance, and retention properties due to the interfacial layer (IL), which inevitably grows during the fabrication. Here, we proposed and demonstrated oxygen scavenging to reduce the IL thickness in an HfZrO x -based capacitor and the thinner IL was confirmed by cross-sectional transmission electron microscopy. V sw of a capacitor with scavenging decreased by 18% and the same P r could be obtained at a lower voltage than a capacitor without scavenging. In addition, excellent endurance properties up to 106 cycles were achieved. We believe oxygen scavenging has great potential for future HfZrO x -based memory device applications.

Field-Effect Transistor Based on 2D Microcrystalline MoS2 Film Grown by Sulfurization of Atomically Layer Deposited MoO3

Atomically thin molybdenum disulfide (MoS₂) is a promising channel material for next-generation thin-body field-effect transistors (FETs), which makes the development of methods allowing for its controllable synthesis over a large area an essential task. Currently, one of the cost-effective ways of its synthesis is the sulfurization of preliminary grown oxide- or metallic film. However, despite apparent progress in this field, the electronic quality of the obtained MoS₂ is inferior to that of exfoliated samples, making the detailed investigation of the sulfurized films' properties of great interest. In this work, we synthesized continuous MoS₂ films with a thickness of ≈2.2 nm via the sulfurization of an atomic-layer-deposited MoO₃ layer. X-ray photoelectron spectroscopy, transmission electron microscopy, and Raman spectroscopy indicated the appropriate chemical composition and microcrystalline structure of the obtained MoS₂ films. The semiconductor quality of the synthesized films was confirmed by the fabrication of a field-effect transistor (FET) with an I_on/I_off ratio of ≈40, which was limited primarily by the high contact resistance. The Schottky barrier height at the Au/MoS₂ interface was found to be ≈1.2 eV indicating the necessity of careful contact engineering. Due to its simplicity and cost-effectiveness, such a technique of MoS₂ synthesis still appears to be highly attractive for its applications in next-generation microelectronics. Therefore, further research of the electronic properties of films obtained via this technique is required.

On the Reliability of HZO-Based Ferroelectric Capacitors: The Cases of Ru and TiN Electrodes

Despite the great potential of Hf_0.5Zr_0.5O₂ (HZO) ferroelectrics, reliability issues, such as wake-up, fatigue, endurance limitations, imprint and retention loss, impede the implementation of HZO to nonvolatile memory devices. Herein, a study of the reliability properties in HZO-based stacks with the conventional TiN top electrode and Ru electrode, which is considered a promising alternative to TiN, is performed. An attempt to distinguish the mechanisms underlying the wake-up, fatigue and retention loss in both kinds of stacks is undertaken. Overall, both stacks show pronounced wake-up and retention loss. Moreover, the fatigue and retention loss were found to be worsened by Ru implementation. The huge fatigue was suggested to be because Ru does not protect HZO against oxygen vacancies generation during prolonged cycling. The vacancies generated in the presence of Ru are most likely deeper traps, as compared to the traps formed at the interface with the TiN electrode. Implementing the new procedure, which can separate the depolarization-caused retention loss from the imprint-caused one, reveal a rise in the depolarization contribution with Ru implementation, accompanied by the maintenance of similarly high imprint, as in the case with the TiN electrode. Results show that the mechanisms behind the reliability issues in HZO-based capacitors are very electrode dependent and simple approaches to replacing the TiN electrode with the one providing, for example, just higher remnant polarization or lower leakages, become irrelevant on closer examination.

Synergistic improvement of sensing performance in ferroelectric transistor gas sensors using remnant polarization

Gaseous pollutants, including nitrogen oxides, pose a severe threat to ecosystems and human health; therefore, developing reliable gas-sensing systems to detect them is becoming increasingly important. Among the various options, metal-oxide-based gas sensors have attracted attention due to their capability for real-time monitoring and large response. In particular, in the field of materials science, there has been extensive research into controlling the morphological properties of metal oxides. However, these approaches have limitations in terms of controlling the response, sensitivity, and selectivity after the sensing material is deposited. In this study, we propose a novel method to improve the gas-sensing performance by utilizing the remnant polarization of ferroelectric thin-film transistor (FeTFT) gas sensors. The proposed FeTFT gas sensor has IGZO and HZO as the conducting channel and ferroelectric layer, respectively. It is demonstrated that the response and sensitivity of FeTFT gas sensors can be modulated by engineering the polarization of the ferroelectric layer. The amount of reaction sites in IGZO, including electrons and oxygen vacancy-induced negatively charged oxygen, is changed depending on upward and downward polarization. The results of this study provide an essential foundation for further development of gas sensors with tunable sensing properties.

Hafnium Oxide (HfO2 ) - A Multifunctional Oxide: A Review on the Prospect and Challenges of Hafnium Oxide in Resistive Switching and Ferroelectric Memories

Hafnium oxide (HfO₂ ) is one of the mature high-k dielectrics that has been standing strong in the memory arena over the last two decades. Its dielectric properties have been researched rigorously for the development of flash memory devices. In this review, the application of HfO₂ in two main emerging nonvolatile memory technologies is surveyed, namely resistive random access memory and ferroelectric memory. How the properties of HfO₂ equip the former to achieve superlative performance with high-speed reliable switching, excellent endurance, and retention is discussed. The parameters to control HfO₂ domains are further discussed, which can unleash the ferroelectric properties in memory applications. Finally, the prospect of HfO₂ materials in emerging applications, such as high-density memory and neuromorphic devices are examined, and the various challenges of HfO₂ -based resistive random access memory and ferroelectric memory devices are addressed with a future outlook.

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.

Nanoscale Doping and Its Impact on the Ferroelectric and Piezoelectric Properties of Hf0.5Zr0.5O2

Ferroelectric hafnium oxide thin films—the most promising materials in microelectronics’ non-volatile memory—exhibit both unconventional ferroelectricity and unconventional piezoelectricity. Their exact origin remains controversial, and the relationship between ferroelectric and piezoelectric properties remains unclear. We introduce a new method to investigate this issue, which consists in a local controlled modification of the ferroelectric and piezoelectric properties within a single Hf_0.5Zr_0.5O₂ capacitor device through local doping and a further comparative nanoscopic analysis of the modified regions. By comparing the ferroelectric properties of Ga-doped Hf_0.5Zr_0.5O₂ thin films with the results of piezoresponse force microscopy and their simulation, as well as with the results of in situ synchrotron X-ray microdiffractometry, we demonstrate that, depending on the doping concentration, ferroelectric Hf_0.5Zr_0.5O₂ has either a negative or a positive longitudinal piezoelectric coefficient, and its maximal value is −0.3 pm/V. This is several hundreds or thousands of times less than those of classical ferroelectrics. These changes in piezoelectric properties are accompanied by either improved or decreased remnant polarization, as well as partial or complete domain switching. We conclude that various ferroelectric and piezoelectric properties, and the relationships between them, can be designed for Hf_0.5Zr_0.5O₂ via oxygen vacancies and mechanical-strain engineering, e.g., by doping ferroelectric films.

Two-step deposition of TiN capping electrodes to prevent degradation of ferroelectric properties in an in-situ crystallized TiN/Hf0.5Zr0.5O2/TiN device

Hf0.5Zr0.5O2 (HZO) is an appropriate material for the back-end-of-line (BEOL) process in fabricating ferroelectric TiN/HZO/TiN devices because of its excellent conformality on 3-D nanostructures and a suitable crystallization temperature (≥ 350–400 °C). However, in the semiconductor industry, the depositiontemperature ofTiN isusually higher than 400 °C. Therefore, it is necessary to study the ferroelectric properties of TiN/HZO/TiN devices when the deposition temperature of the TiN top electrode is higher than the HZO film crystallization temperature. In this study, 10-nm-thick TiN top electrodes were deposited at various temperatures on the HZO thin film to investigate the impact of the TiN deposition temperature on the structural features and ferroelectric properties of TiN/HZO/TiN capacitors. Only the sample capped with a TiN top electrode deposited at 400 °C showed ferroelectric properties without subsequent annealing (in situ crystallization). However, this sampleexhibitedan approximately40% reduction inthepolarization value compared with the other samples that were crystallized after the annealing process. This behavior can be ascribed to the formation of a monoclinic nonpolar phase. To prevent the degradation of the polarization value and suppress the formation of the m-phase in the in situ crystallized HZO thin film, a two-step TiN deposition method was carried out. The sample was fabricated by depositing a 5-nm-thick TiN top electrode at room temperature followed by the deposition of a 5-nm-thick TiN layer at 400 °C, which resulted in strong ferroelectric properties comparable to those of the samples capped with TiN grown at relatively low temperatures (room temperature, 200 °C, and 300 °C). These findings can adequately explain the role of the capping layer in achieving the ferroelectric phase, which is closely related not only during the cooling step of any thermal process but also during the heating and crystallization steps.

Impact of La Concentration on Ferroelectricity of La-Doped HfO2 Epitaxial Thin Films

Epitaxial thin films of HfO₂ doped with La have been grown on SrTiO₃(001) and Si(001), and the impact of the La concentration on the stabilization of the ferroelectric phase has been determined. Films with 2-5 at. % La doping present the least amount of paraelectric monoclinic and cubic phases and exhibit the highest polarization, having a remanent polarization above 20 μC/cm². The dopant concentration results in an important effect on the coercive field, which is reduced with increasing La content. Combined high polarization, high retention, and high endurance of at least 10¹⁰ cycles is obtained in 5 at. % La-doped films.

Wake-up Free Ferroelectric Rhombohedral Phase in Epitaxially Strained ZrO2 Thin Films

Zirconia- and hafnia-based thin films have attracted tremendous attention in the past decade because of their unexpected ferroelectric behavior at the nanoscale, which enables the downscaling of ferroelectric devices. The present work reports an unprecedented ferroelectric rhombohedral phase of ZrO₂ that can be achieved in thin films grown directly on (111)-Nb:SrTiO₃ substrates by ion-beam sputtering. Structural and ferroelectric characterizations reveal (111)-oriented ZrO₂ films under epitaxial compressive strain exhibiting switchable ferroelectric polarization of about 20.2 μC/cm² with a coercive field of 1.5 MV/cm. Moreover, the time-dependent polarization reversal characteristics of Nb:SrTiO₃/ZrO₂/Au film capacitors exhibit typical bell-shaped curve features associated with the ferroelectric domain reversal and agree well with the nucleation limited switching (NLS) model. The polarization-electric field hysteresis loops point to an activation field comparable to the coercive field. Interestingly, the studied films show ferroelectric behavior per se, without the need to apply the wake-up cycle found in the orthorhombic phase of ZrO₂. Overall, the rhombohedral ferroelectric ZrO₂ films present technological advantages over the previously studied zirconia- and hafnia-based thin films and may be attractive for nanoscale ferroelectric devices.

Hafnium-zirconium oxide interface models with a semiconductor and metal for ferroelectric devices

Density functional theory (DFT) is employed to investigate ferroelectric (FE) hafnium-zirconium oxide stack models for both metal-insulator-metal (MIM) and metal-insulator-semiconductor (MIS) structures. The role of dielectric (DE) interlayers at the ferroelectric interfaces with metals and semiconductors and the effects of thickness scaling of FE and DE layers were investigated using atomic stack models. A high internal field is induced in the FE and DE layers by the FE polarization field which can promote defect generation leading to limited endurance. It is also shown that device operation will be adversely affected by too thick DE interlayers due to high operating voltage. These DFT models elucidate the underlying mechanisms of the lower endurance in experimental MIS devices compared to MIM devices and provide insights into the fundamental mechanisms at the interfaces.

A review of ultra-thin ferroelectric films

Ultrathin ferroelectrics are of great technological interest for high-density electronics, particularly non-volatile memories and field-effect transistors. With the rapid development of micro-electronics technology, there is an urgent requirement for higher density electronic devices, which need ultra-thin ferroelectric materials films. However, as ferroelectric films have becomes thinner and thinner, electrical spontaneous polarization signals have been found in a few atomic layers or even monolayer structures. The mechanisms of detection and formation of these signals are not well understood and various controversial interpretations have emerged. In this review, we summarized the recent research progress in the ultra-thin film ferroelectric material, such as HfO₂, CuInP₂S₆, In₂Se₃, MoTe₂and BaTiO₃. Various key aspects of ferroelectric materials are discussed, including crystal structure, ferroelectric mechanism, characterization, fabrication methods, applications, and future outlooks. We hope this review will offer ideas for further improvement of ferroelectric properties of ultra-thin films and promotes practical applications.

Defects in ferroelectric HfO2

The discovery of ferroelectricity in polycrystalline thin films of doped HfO2 has reignited the expectations of developing competitive ferroelectric non-volatile memory devices. To date, it is widely accepted that the performance of HfO2-based ferroelectric devices during their life cycle is critically dependent on the presence of point defects as well as structural phase polymorphism, which mainly originates from defects either. The purpose of this review article is to overview the impact of defects in ferroelectric HfO2 on its functional properties and the resulting performance of memory devices. Starting from the brief summary of defects in classical perovskite ferroelectrics, we then introduce the known types of point defects in dielectric HfO2 thin films. Further, we discuss main analytical techniques used to characterize the concentration and distribution of defects in doped ferroelectric HfO2 thin films as well as at their interfaces with electrodes. The main part of the review is devoted to the recent experimental studies reporting the impact of defects in ferroelectric HfO2 structures on the performance of different memory devices. We end up with the summary and perspectives of HfO2-based ferroelectric competitive non-volatile memory devices.

Influence of Applied Stress on the Ferroelectricity of Thin Zr-Doped HfO2 Films

HfO2-based ferroelectric materials have been widely studied for their application in ferroelectric FETs, which are compatible with conventional CMOS processes; however, problems with the material’s inherent fatigue properties have limited its potential for device application. This paper systematically investigates the effects of tensile stress and annealing temperature on the endurance and ferroelectric properties faced by Zr-doped HfO2 ferroelectric film. The remnant polarization (Pr) shows an increasing trend with annealing temperature, while the change in the coercive electric field (Ec) is not obvious in terms of the relationship with tensile stress or annealing temperature. In addition, the application of tensile stress does help to improve the endurance characteristics by about two orders of magnitude for the ferroelectric material, and the endurance properties show a tendency to be negatively correlated with annealing temperature. Overall, although the effect of stress on the ferroelectricity of a HZO material is not obvious, it has a great influence on its endurance properties and can optimize the endurance of the material, and ferroelectricity exhibits a higher dependence on temperature. The optimization of the endurance properties of HZO materials by stress can facilitate their development and application in future integrated circuit technology.

Interplay between oxygen defects and dopants: effect on structure and performance of HfO2-based ferroelectrics

A review on ferroelectric phase formation and reliability in HfO₂-based thin films and semiconductor devices.

High polarization, endurance and retention in sub-5 nm Hf0.5Zr0.5O2 films

Ferroelectric HfO2 is a promising material for new memory devices, but significant improvement of its important properties is necessary for practical application. However, previous literature shows that a dilemma exists between polarization, endurance and retention. Since all these properties should be simultaneously high, overcoming this issue is of the highest relevance. Here, we demonstrate that high crystalline quality sub-5 nm Hf0.5Zr0.5O2 capacitors, integrated epitaxially with Si(001), present combined high polarization (2Pr of 27 μC cm-2 in the pristine state), endurance (2Pr > 6 μC cm-2 after 1011 cycles) and retention (2Pr > 12 μC cm-2 extrapolated at 10 years) using the same poling conditions (2.5 V). This achievement is demonstrated in films thinner than 5 nm, thus opening bright possibilities in ferroelectric tunnel junctions and other devices.