Direct Epitaxial Growth of Polar (1 - x)HfO2-(x)ZrO2 Ultrathin Films on Silicon

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.

Recent progress on defect-engineering in ferroelectric HfO2: The next step forward via multiscale structural optimization

The discovery of unconventional scale-free ferroelectricity in HfO2-based fluorite thin films has attracted great attention in recent years for their promising applications in low-power logic and nonvolatile memories. The ferroelectricity of HfO2 is intrinsically originated from the widely accepted ferroelectric metastable orthorhombic Pca21 phase. In the last decade, defect-doping/solid solution has shown excellent prospects in enhancing and stabilizing the ferroelectricity via isovalent or aliovalent defect-engineering. Here, the recent advances in defect-engineered HfO2-based ferroelectrics are first reviewed, including progress in mono-ionic doping and mixed ion-doping. Then, the defect-lattice correlation, the point-defect promoted phase transition kinetics, and the interface-engineered dynamic behaviour of oxygen vacancy are summarized. In addition, thin film preparation and ion bombardment doping are summarized. Finally, the outlook and challenges are discussed. A multiscale structural optimization approach is suggested for further property optimization. This article not only covers an overview of the state-of-art advances of defects in fluorite ferroelectrics, but also future prospects that may inspire their further property-optimization via defect-engineering.

An Aqueous Route to Oxygen-Deficient Wake-Up-Free La-Doped HfO2 Ferroelectrics for Negative Capacitance Field Effect Transistors

The crucial role of nanocrystalline morphology in stabilizing the ferroelectric orthorhombic (o)-phase in doped-hafnia films is achieved via chemical solution deposition (CSD) by intentionally retaining carbonaceous impurities to inhibit grain growth. However, in the present study, large-grained (>100 nm) La-doped HfO2 (HLO) films are grown directly on silicon by adopting engineered water-diluted precursors with a minimum carbonaceous load and excellent shelf life. The o-phase stabilization is accomplished through a well-distributed La dopant, which generates uniformly populated oxygen vacancies, eliminating the need for oxygen-scavenging electrodes. These oxygen-deficient HLOs show a maximum remnant polarization of 37.6 μC/cm2 (2Pr) without wake-up and withstand large fields (>6.2 MV/cm). Furthermore, CSD-HLO in series with Al2O3 improves switching of MOSFETs (with an amorphous oxide channel) based on the negative capacitance effect. Thus, uniformly distributed oxygen vacancies serve as a standalone factor in stabilizing the o-phase, enabling efficient wake-up-free ferroelectricity without the need for nanostructuring, capping stresses, or oxygen-reactive electrodes.

On the switching dynamics of epitaxial ferroelectric CeO2-HfO2 thin film capacitors

Epitaxial layers of ferroelectric orthorhombic HfO₂ are frequently investigated as model systems for industrially more relevant polycrystalline films. The recent success in stabilizing the orthorhombic phase in the solid-solution cerium oxide - hafnium oxide system allows detailed investigations of external influences during fabrication. This report analyzes the ferroelectric properties of two thin film capacitors, which were post-deposition annealed in N₂ and O₂ atmospheres to achieve the orthorhombic phase after room temperature deposition. The samples, which exhibit very similar constituent phase, appear identical in conventional polarization-field hysteresis measurements. However, a significant switching speed difference is observed in pristine devices. Continued field cycling reduces the difference. Deeper analysis of switching transients based on the Nucleation Limited Switching model suggests that the O₂ heat treatment atmosphere results in an altered oxygen vacancy profile, which is reverted during ferroelectric cycling.

Thin-Film Ferroelectrics

Over the last 30 years, the study of ferroelectric oxides has been revolutionized by the implementation of epitaxial-thin-film-based studies, which have driven many advances in the understanding of ferroelectric physics and the realization of novel polar structures and functionalities. New questions have motivated the development of advanced synthesis, characterization, and simulations of epitaxial thin films and, in turn, have provided new insights and applications across the micro-, meso-, and macroscopic length scales. This review traces the evolution of ferroelectric thin-film research through the early days developing understanding of the roles of size and strain on ferroelectrics to the present day, where such understanding is used to create complex hierarchical domain structures, novel polar topologies, and controlled chemical and defect profiles. The extension of epitaxial techniques, coupled with advances in high-throughput simulations, now stands to accelerate the discovery and study of new ferroelectric materials. Coming hand-in-hand with these new materials is new understanding and control of ferroelectric functionalities. Today, researchers are actively working to apply these lessons in a number of applications, including novel memory and logic architectures, as well as a host of energy conversion devices.

Design of GAA Nanosheet Ferroelectric Area Tunneling FET and Its Significance with DC/RF Characteristics Including Linearity Analyses

This work reports an emerging structure of gate-all-around ferroelectric area tunneling field-effect transistor (FATFET) by considering ferroelectric and a n-epitaxial layer enveloped around the overlapped region of the source and channel to succeed with complete area of tunneling probability. To accomplish this, ferroelectric ([Formula: see text]) is exploited and modeled to boost the FATFET performance through internal-voltage ([Formula: see text]) amplification. The corresponding modeling approach to estimate the ferroelectric parameters along with [Formula: see text] calculations of the metal-ferroelectric-insulator (MFIS) option through capacitance equivalent method is addressed. Using these options the proposed device outperforms effectively in delivering superior DC and RF performance among possible options of the [Formula: see text] ferroelectric TFETs. The significance of proposed design is examined with recently reported ferroelectric TFETs. Our results show 10-time advancement on the [Formula: see text], reduced steep or average subthreshold swing (< 25 mV/dec), frequencies higher than 150 GHz, and insignificant to linearity deviations at low bias points. Furthermore, 2-order reduction in energy efficiency is succeeded with the proposed design environment.

Emergent ferroelectricity in subnanometer binary oxide films on silicon

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

Gate-all-around nanowire vertical tunneling FETs by ferroelectric internal voltage amplification

This work illustrates the most effective way of utilizing the ferroelectricity for tunneling field-effect transistors (TFETs). The ferroelectric (Hf_0.5Zr_0.5O₂) in shunt with gate-dielectric is utilized as an optimized metal-ferroelectric-semiconductor (OMFS) option to improve the internal voltage (V_int) for ample utilization of polarization and electric fields of Hf_0.5Zr_0.5O₂across the tunneling region. The modeling ofV_intsignifies 0.15-1.2 nm reduction in tunneling length (λ) than the nominal metal-ferroelectric-insulator-semiconductor (MFIS) option. Furthermore, the TFET geometry with the scaled-epitaxy region as vertical TFET (VTFET), strained Si_0.6Ge_0.4as source, and gate-all-around nanowire options are used as an added advantage for further enhancement of TFET's performance. As a result, the proposed design (OMFS-VTFET) achieves superior DC and RF performances than the MFIS option of TFET. The figure of merits in terms of DC characteristics in the proposed and optimized structure are of improved on-current (=0.23 mAμm^-1), high on-to-off current ratio (=10¹¹), steep subthreshold swing (=33.36 mV dec^-1), and superior unity gain cut-off frequency (≥300 GHz). The design is revealed as energy-efficient with significant reduction of energy-efficiency in both logic and memory applications.

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.

Ferroelectric field-effect transistors based on HfO2: a review

In this article, we review the recent progress of ferroelectric field-effect transistors (FeFETs) based on ferroelectric hafnium oxide (HfO₂), ten years after the first report on such a device. With a focus on the use of FeFET for nonvolatile memory application, we discuss its basic operation principles, switching mechanisms, device types, material properties and array structures. Key device performance metrics such as cycling endurance, retention, memory window, multi-level operation and scaling capability are analyzed. We also briefly survey recent developments in alternative applications for FeFETs including neuromorphic and in-memory computing as well as radiofrequency devices.

Epitaxial Ferroelectric Hf0.5 Zr0.5 O2 with Metallic Pyrochlore Oxide Electrodes

The synthesis of fully epitaxial ferroelectric Hf_0.5 Zr_0.5 O₂ (HZO) thin films through the use of a conducting pyrochlore oxide electrode that acts as a structural and chemical template is reported. Such pyrochlores, exemplified by Pb₂ Ir₂ O₇ (PIO) and Bi₂ Ru₂ O₇ (BRO), exhibit metallic conductivity with room-temperature resistivity of <1 mΩ cm and are closely lattice matched to yttria-stabilized zirconia substrates as well as the HZO layers grown on top of them. Evidence for epitaxy and domain formation is established with X-ray diffraction and scanning transmission electron microscopy, which show that the c-axis of the HZO film is normal to the substrate surface. The emergence of the non-polar-monoclinic phase from the polar-orthorhombic phase is observed when the HZO film thickness is ≥≈30 nm. Thermodynamic analyses reveal the role of epitaxial strain and surface energy in stabilizing the polar phase as well as its coexistence with the non-polar-monoclinic phase as a function of film thickness.