Ferroelectric Orthorhombic Phase Research Articles

Oxygen vacancy contributions to the electrical stress response and endurance of ferroelectric hafnium zirconium oxide thin films

Despite its scalability and CMOS process compatibility, the limited endurance and sub-optimal stress response of ferroelectric Zr-substituted hafnia [(Hf,Zr)O2] have been one of the key impediments toward its integration into practical device and technology applications. Here, using electrical measurements complemented by photoluminescence spectroscopy, we investigate the underlying mechanisms behind this behavior in 10 nm thick W/Hf0.5Zr0.5O2/W capacitors. Analyzing the evolution of leakage current with stress cycles and the spectroscopic response of the stress-induced leakage current, we attribute the behavior to defect levels, which lie at 0.6 eV from the conduction band edge of the ferroelectric. Photoluminescence spectroscopy, in turn, further corroborates the defect level's position within the bandgap while enabling its attribution to the presence of oxygen vacancies. This work helps to identify oxygen vacancies as the key factor responsible for the degraded endurance and stress response in (Hf,Zr)O2 and subsequently motivates the exploration of methods to reduce the oxygen vacancy concentrations without destabilizing the ferroelectric orthorhombic phase.

Ferroelectric Orthorhombic ZrO2 Thin Films Achieved Through Nanosecond Laser Annealing.

A new approach for the stabilization of the ferroelectric orthorhombic ZrO2 films is demonstrated through nanosecond laser annealing (NLA) of as-deposited Si/SiOx /W(14nm)/ZrO2 (8nm)/W(22nm), grown by ion beam sputtering at low temperatures. The NLA process optimization is guided by COMSOL multiphysics simulations. The films annealed under the optimized conditions reveal the presence of the orthorhombic phase, as confirmed by X-ray diffraction, electron backscatter diffraction, and transmission electron microscopy. Macroscopic polarization-electric field hysteresis loops show ferroelectric behavior, with saturation polarization of 12.8µC cm-2 , remnant polarization of 12.7µCcm-2 and coercive field of 1.2MVcm-1 . The films exhibit a wake-up effect that is attributed to the migration of point defects, such as oxygen vacancies, and/or a transition from nonferroelectric (monoclinic and tetragonal phase) to the ferroelectric orthorhombic phase. The capacitors demonstrate a stable polarization with an endurance of 6.0× 105 cycles, demonstrating the potential of the NLA process for the fabrication of ferroelectric memory devices with high polarization, low coercive field, and high cycling stability.

Ferroelectricity and pseudo-coherent growth in HfO2/SrHfO3 nanolaminates

Ferroelectricity in thin films of HfO2 has been the subject of extensive studies in materials science as well as device applications. The emergence of ferroelectricity is attributable to the orthorhombic phase (Pca21) of HfO2, stabilized in the films by metal-element doping, strains from substrates and electrode films, and oxygen deficiency. Recently, ferroelectricity has been reported in nanolaminates of HfO2 with other oxides such as ZrO2 and Al2O3, implying that nanolaminates are another effective way to bring about ferroelectricity in HfO2. However, the mechanism of orthorhombic phase stabilization in nanolaminates is not fully understood. In this study, we demonstrated that ferroelectricity emerges in nanolaminates consisting of undoped HfO2 and perovskite SrHfO3 deposited on Sn-doped In2O3 bottom electrodes, when the thickness of HfO2 layers was ≥6 nm. For nanolaminates in which the thickness of the HfO2 layers was ≤5 nm, ferroelectricity was remarkably suppressed due to Sr-incorporation into the HfO2 layers at the interface. In those nanolaminates, the crystal orientations of HfO2 grains were well aligned throughout the HfO2 layers, indicating that the HfO2 layers grew in a pseudo-coherent manner. This study aids to understand the stabilization of the ferroelectric orthorhombic phase in nanolaminates in terms of their structural properties.

Phonon dynamics in lead free perovskite (1-x)KNN-xBAN (x = 0.0–0.1): a temperature dependent raman study

K0.5Na0.5NbO3 (KNN) has been under extensive focus in recent times due to it being an alternative to lead-free multifunctional materials and properties like room temperature ferroelectricity, stability in air and technology friendly applications. In this work, we report a comprehensive temperature dependent, compositional change induced structural variation and polarization dependent Raman spectroscopy on BaAl0.5Nb0.5O3 (BAN) doped KNN in the spectral range of 10–1000 cm−1. Multiple phase transitions are marked by the strong renormalization of the phonon self-energy parameters, i.e. mode frequencies and linewidths, in the temperature range of 83 K to 623 K. Change in the phase transition temperature is tracked via phonon anomalies with varying doping concentration, x = 0.0, 0.02, 0.05, 0.07 and 0.1, and is found to be as large as ∼100 K for orthorhombic to tetragonal phase transition for x = 0.1. Raman extracted phase diagram shows that the stability of the ferroelectric orthorhombic phase in the vicinity of room temperature increases with the increasing doping concentration. Also, from our polarization-dependent measurements we could decipher the symmetry of the observed phonon modes for KNN system.

Controlling ferroelectric properties in Y-doped HfO2 thin films by precise introduction of oxygen vacancies

Thin-film ferroelectric doped hafnia has emerged as a promising candidate for non-volatile computer memory devices due to its CMOS compatibility. The ferroelectricity in thin-film HfO2 is defined by the polar orthorhombic phase, whose stabilization depends on various parameters, such as doping species, stress, thickness, crystallization annealing temperature, etc. The concentration of oxygen vacancies is yet another parameter affecting the stabilization of the ferroelectric phase in HfO2 thin films. Here, we report on the effect of oxygen vacancies introduced in Y-doped HfO2 (HYO) films during reactive pulsed laser deposition on their ferroelectric properties, which we systematically study by correlating structural and electrical properties. Among different techniques, near-edge x-ray absorption fine structure analysis is successfully employed to distinguish between structurally similar ferroelectric orthorhombic and paraelectric tetragonal phases. It is shown that oxygen vacancies introduced at a certain concentration in HYO films can be used as a tool to control the phase composition as well as to decrease the formation energy (crystallization temperature) of the ferroelectric phase. Based on these results, we demonstrate a back-end-of-line compatible ferroelectric HYO capacitor device with competitive functional properties.

Potassium–Sodium Ceramics Modified with Metal Oxide Additives: Synthesis, Microstructure, and Properties

Single phase ceramic samples of new compounds (1 − x)(K0.5Na0.5)NbO3−xLa(Ag0.5Sb0.5)O3 (x = 0–0.15), modified by metal oxide additives ZnO, CuO, and MnO2, are prepared via a solid state reaction. The crystal structure, microstructure, and dielectric and ferroelectric properties of the samples are studied. It is established that a phase with a perovskite structure and an orthorhombic unit cell formed in each sample. Ferroelectric phase transitions are confirmed via dielectric spectroscopy. Generation of the second harmonic is observed, along with a drop in the temperature of transitions from the ferroelectric orthorhombic phase to the ferroelectric tetragonal phase, and then to the cubic paraelectric phase.

Anisotropic Strain‐Mediated Symmetry Engineering and Enhancement of Ferroelectricity in Hf0.5Zr0.5O2/La0.67Sr0.33MnO3 Heterostructures

AbstractHafnium‐based binary oxides have attracted considerable attention due to their robust ferroelectricity at the nanoscale and compatibility with silicon‐based electronic technologies. To further promote the potential of Hafnium oxides for practical device applications, it is essential to effectively harness the interplay between structural symmetry, domain configuration, and ferroelectricity. Here, using Hf0.5Zr0.5O2/La0.67Sr0.33MnO3 (HZO/LSMO) heterostructures as a model system, the anisotropic strain‐mediated symmetry engineering and ferroelectricity enhancement are systematically investigated. By growing the heterostructures on (110)‐oriented perovskite substrates, considerable anisotropic strain is imposed on the LSMO bottom electrodes. Such an anisotropically‐strained LSMO layer acts as a structural template and effectively tune the structural symmetry, polar/non‐polar phase ratio, and ferroelectricity of the HZO top layer. Specifically, the anisotropic tensile strain stabilizes the ferroelectric rhombohedral and orthorhombic phases, thus enhancing the remnant polarization (Pr) up to 22 µC cm−2. In contrast, the anisotropic compressive strain facilitates the formation of non‐ferroelectric tetragonal phases, leading to a suppressed Pr down to 8 µC cm−2. These findings provide a guideline for understanding and modulating the intrinsic structure‐ferroelectricity relationship of HZO through anisotropic strain‐mediated symmetry engineering, which may shed light on the development of hafnium‐oxide‐based electronic devices.

Nanocrystallite Seeding of Metastable Ferroelectric Phase Formation in Atomic Layer-Deposited Hafnia-Zirconia Alloys.

Hafnia-based ferroelectric thin films are promising for semiconductor memory and neuromorphic computing applications. Amorphous, as-deposited, thin-film binary alloys of HfO2 and ZrO2 transform to the metastable, orthorhombic ferroelectric phase during post-deposition annealing and cooling. This transformation is generally thought to involve formation of a tetragonal precursor phase that distorts into the orthorhombic phase during cooling. In this work, we systematically study the effects of atomic layer deposition (ALD) temperature on the ferroelectricity of post-deposition-annealed Hf0.5Zr0.5O2 (HZO) thin films. Seed crystallites having interplanar spacings consistent with the polar orthorhombic phase are observed by a plan-view transmission electron microscope in HZO thin films deposited at an elevated ALD temperature. After ALD under conditions that promote formation of these nanocrystallites, high-polarization (Pr > 18 μC/cm2) ferroelectric switching is observed after rapid thermal annealing (RTA) at low temperature (350 °C). These results indicate the presence of minimal non-ferroelectric phases retained in the films after RTA when the ALD process forms nanocrystalline particles that seed subsequent formation of the polar orthorhombic phase.

Wake-Up Free La-Doped HfO2-ZrO2 Ferroelectrics Achieved With an Atomic Layer-Specific Doping Technique

In this letter, wake-up free La-doped HfO2-ZrO2 (HZO) ferroelectrics are experimentally fabricated using an atomic layer-specific doping (ALSD) technique, which selectively introduces La dopants at effective locations to facilitate the formation of ferroelectric orthorhombic phase. With this technique, robust La-doped HZO ferroelectric devices that are mostly free from the undesired wake-up effect and anti-ferroelectricity were demonstrated over a large La doping range (from 2.9 to 11.5%). Moreover, large remanent polarization and good endurance were achieved at the same time (>107 cycles). The outstanding performances can be attributed to the effective stabilization of ferroelectric orthogonal phase and the suppression of tetragonal phase, through controlling of the dopants’ local environment. Thus, ALSD is an effective approach to achieve reliable, wake-up free and CMOS-compatible HZO-based ferroelectrics.

Modulating the microscopic lattice distortions through the Al-rich layers for boosting the ferroelectricity in Al:HfO2 nanofilms

Combining the experimental characterization with the large-scale density functional theory calculations based on finite-element discretization (DFT-FE), we address the stabilization of polar orthorhombic phases (o-HfO2) in Al:HfO2 nanofilms by means of the atomic registry distortions and lattice deformation caused by Al substitutional defects (AlHf) and Schottky defects (2AlHf + VO) in tetragonal phases (t-HfO2) or monoclinic phases (m-HfO2). The phase transformation directly from the t-HfO2 into polar o-HfO2 are also elucidated within a heterogeneous distribution of Al dopants in both t-HfO2 bulk crystal structure and Al:HfO2 nanofilm. It is revealed using large-scale DFT calculations that the Al substitutional defects (AlHf) or the Schottky defect (2AlHf + VO) could induce the highly extended atomic registry distortions or lattice deformation in the t- and m-HfO2 phases, but such effects are greatly diminished in ferroelectric orthorhombic phase. By purposely engineering the multiple AlHf defects to form dopant-rich layers in paraelectric t-HfO2 nanofilm or bulk crystal, the induced extended lattice distortions surrounding the defect sites exhibit the shearing-like atomic displacement vector field. The large-scale DFT calculations further predicted that the shearing-like microscopic lattice distortions could directly induce the phase transformation from the t-HfO2 into polar orthorhombic phase in both Al:HfO2 bulk crystal and nanofilms, leading to the large remanent polarization observed in Al:HfO2 nanofilms with the presence of Al-rich layers. The current study demonstrates that the ferroelectricity of HfO2 bulk crystal or thin film can be optimized and tuned by delicately engineering both the distribution and concentration of Al dopants in atomic layer deposition without applying the top capping electrode, providing the extra flexibility for designing the HfO2 based electronic devices in the future.

Impedance Investigation of MIFM Ferroelectric Tunnel Junction Using a Comprehensive Small-Signal Model

The urge to develop efficient and ultra-low power architectures for modern and future technological needs lead to an increasing interest and investigation of neuromorphic and ultra-low power computing. In this respect, ferroelectric technology is found to be a perfect candidate to guide this technological transition. Elucidating the physical mechanisms occurring during ferroelectric-based devices operations is fundamental in order to improve the reliability of emerging architectures. In this work, we investigate metal/insulator/ferroelectric/metal (MIFM) ferroelectric tunnel junctions (FTJs) consisting of a ferroelectric hafnium zirconium oxide (HZO) layer and an alumina (Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) layer by means of C-f and G-f measurements performed at multiple voltages and temperatures. For a trustworthy interpretation of the measurements results, an innovative small signal model is introduced that goes beyond the state of the art by <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${i}$ </tex-math></inline-formula> ) separating the role played by the leakage in the two layers; <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ii</i> ) including the impact of the series impedance (that depends on the samples layout); <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">iii</i> ) including the frequency dependence of the dielectric permittivity; <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">iv</i> ) accounting for the fact that not the whole HZO volume crystallizes in the orthorhombic ferroelectric phase. The model correctly reproduces measurements taken on different devices in different conditions. Results highlight that the typical estimation method for interface trap density may be misleading.

Improved Endurance of Ferroelectric HfxZr1–xO2 Integrated on InAs Using Millisecond Annealing (Adv. Mater. Interfaces 27/2022)

Improved Endurance of Ferroelectric HfxZr1-xO2 Integrated on InAs The benefits of using a milliseconds flash lamp annealer to crystallize HfxZr1-xO2 into the ferroelectric orthorhombic crystal phase are highlighted in article number 2201038 by Robin Athle, Mattias Borg, and co-workers. By annealing 1000x faster than traditional rapid thermal processing, integration of ferroelectric HfxZr1-xO2 on thermally sensitive substrates such as III-V semiconductors can be achieved. This approach allows for exceptional material integration in emerging memory and neuromorphic applications.

Improvement of ferroelectricity and endurance in Sr doped Hf0.5Zr0.5O2 films

The doping of Hf0.5Zr0.5O2 has attracted increasing attention because of the further regulation of structure and ferroelectric properties. Here, Sr doped Hf0.5Zr0.5O2 (Sr:HZO) ferroelectric films have been prepared by chemical solution deposition on Pt/Ti/SiO2/Si substrate, and the effect of Sr doping on the structure and ferroelectric properties of Sr:HZO films was investigated. It is observed that Sr doping can further promote the formation of ferroelectric orthorhombic phase at low content (≤0.50%), inducing the enhancement of ferroelectricity. And the optimal ferroelectricity with remanent polarization of 14.63 μC/cm2 and coercive field of 1.08 MV/cm is achieved in the 0.50% Sr doped HZO film. The decrease in leakage current density with increasing Sr content is observed, which is mainly ascribed to the reduction of grain boundaries caused by the increase of grain size. Most significantly, Sr doping induces an improvement in the endurance, and a good endurance of 109 cycles without breakdown is achieved in the 0.50% Sr doped sample. These findings indicate that Sr is a potential candidate dopant for improving the structure and ferroelectric properties of HZO films.

Effect of Bi3+ and Ti4+ substitution on PbNb2O6 piezoelectric ceramics

In this study, Pb1-2xBi2x(Nb1-xTix)2O6 (PBNTx, x = 0.02–0.08) ceramics with tungsten bronze structure were prepared via the solid-state reaction method. Single orthorhombic ferroelectric phase was detected for all PBNTx ceramics. SEM results indicated fully densified morphology for the samples of x ≥ 0.06. Optimal piezoelectric performance was obtained at the sample of x = 0.08, whose piezoelectric constant (d33) reached to 92 pC/N and Curie temperature (Tc) was up to 593 °C. A notably high electromechanical coupling coefficient (kp) greater than 0.6 was also obtained in this system. The improved piezoelectric performance indicated that Bi3+ and Ti4+ modified PbNb2O6 system is promising for ultrasonic defect detection and transducer applications at high temperatures.

Origin of Ferroelectric Phase Stabilization via the Clamping Effect in Ferroelectric Hafnium Zirconium Oxide Thin Films

AbstractThe presence of the top electrode on hafnium oxide‐based thin films during processing has been shown to drive an increase in the amount of metastable ferroelectric orthorhombic phase and polarization performance. This “Clamping Effect,” also referred to as the Capping or Confinement Effect, is attributed to the mechanical stress and confinement from the top electrode layer. However, other contributions to orthorhombic phase stabilization have been experimentally reported, which may also be affected by the presence of a top electrode. In this study, it is shown that the presence of the top electrode during thermal processing results in larger tensile biaxial stress magnitudes and concomitant increases in ferroelectric phase fraction and polarization response, whereas film chemistry, microstructure, and crystallization temperature are not affected. Through etching experiments and measurement of stress evolution for each processing step, it is shown that the top electrode locally inhibits out‐of‐plane expansion in the HZO during crystallization, which prevents equilibrium monoclinic phase formation and stabilizes the orthorhombic phase. This study provides a mechanistic understanding of the clamping effect and orthorhombic phase formation in ferroelectric hafnium oxide‐based thin films, which informs the future design of these materials to maximize ferroelectric phase purity and corresponding polarization behavior.

Composition dependence of ferroelectric properties in (111)-oriented epitaxial HfO2-CeO2 solid solution films

The composition dependence of ferroelectric properties was investigated for (111)-oriented epitaxial HfO2-CeO2 solid solution films. Twenty nanometer thick films with different compositions were prepared on (111)ITO//(111)YSZ substrates at room temperature by pulsed laser deposition and subsequent heat treatment at 1000 °C under atmospheric N2 or O2 gas flow. All the films had fluorite structures, and their crystal symmetries changed from monoclinic through orthorhombic to tetragonal/cubic phases as x increased for the (Hf1−x Ce x )O2 (x = 0.12–0.25) films. The orthorhombic phase was confirmed by X-ray diffraction analysis for films with x = 0.15 and 0.17. On the other hand, ferroelectric properties were observed in films with x = 0.15–0.20, suggesting that a field-induced phase transition takes place for films with x = 0.20. The film composition showing ferroelectricity was the widest range of doping concentration for reported epitaxial HfO2-based films. Their remanent polarization (P r) and coercive field (E c) were almost identical, at 17–19 μC cm−2 and 2.0–3.0 MV cm−1. This wide ferroelectric composition range with relatively similar ferroelectricity is due to the solid solution of the same fluorite structure of HfO2 and CeO2 with monoclinic and cubic symmetries, that are respectively lower and higher crystal symmetries of the ferroelectric orthorhombic phase.

Enhanced ferroelectricity by strain gradient in few-layer HfO2thin films

Due to the great potential in reducing the size of ferroelectric memory cells and the good compatibility with traditional silicon process, the ferroelectricity of few-layer HfO2 thin films has received huge attention, but its microscopic mechanism is still unclear. Based on first-principles calculations, the ferroelectricity of few-layer HfO2 thin films in the presence of both strain and strain gradient has been investigated systematically. It is shown that the orthorhombic ferroelectric phase exists stably with the strain between −7% and 3%, and the ferroelectric polarization decreases monotonically with the strain increasing, which is consistent with previous studies. In particular, the ferroelectric polarization increases monotonically with the strain gradient increasing within the strain less than −2.5%, which is due to the fact that the strain gradient further separates the positive and negative centers. Our findings provide possible theoretical explanations for recent experimental results and technical guidance for the design of ultra-thin ferroelectric devices.

Independent Effects of Dopant, Oxygen Vacancy, and Specific Surface Area on Crystal Phase of HfO2 Thin Films towards General Parameters to Engineer the Ferroelectricity

Many factors have been confirmed to affect ferroelectric phase formation in HfO2-based thin films but there was still a lack of general view on describing them. This paper discusses the intrinsic parameters to stabilize the ferroelectric phase of HfO2 thin films to approach this general view by investigating the separate effects of dopant, oxygen vacancy (VO), and specific surface area on the crystal phase of the films. It is found that in addition to extensively studied dopants, the ferroelectric orthorhombic phase can also be formed in pure HfO2 films by only introducing sufficient VO independently, and it is also formable by only increasing the specific surface area. By analyzing the common physics behind these factors, it is found that orthorhombic phase formation is universally related to strain in all the above cases with a given temperature. To get a general view, a physical model is established to describe how the strain influences ferroelectric phase formation during the fabrication of HfO2-based films based on thermodynamic and kinetics analysis.

Enhancement of the ferroelectricity by interface engineering observed by in situ transmission electron microscope

Hafnia-based ferroelectrics with excellent scalability and complementary metal–oxide–semiconductor technology compatibility are potential materials for next-generation memory and logic devices. Stabilizing the metastable ferroelectric phase in hafnia-based ferroelectrics is critical for realizing technological applications. Interface engineering is a critical method to stabilize the ferroelectric phase. However, the role played by the interface between the metal electrode and the hafnia-based ferroelectrics oxide remains unclear. In this work, a typical Hf0.5Zr0.5O2 (HZO) ferroelectric oxide film sandwiched between the metal electrode and the silicon substrate was fabricated with and without the interfacial layer. By using the in situ transmission electron microscope, the atomistic structure evolution of the HZO film ferroelectric phase was studied under electrical stimuli. It is found that the phase transition from ferroelectric (FE) orthorhombic phase (O-phase) to dielectric monoclinic phase (M-phase) occurs from the interface between the HZO and the metal electrode. While in the one with Al2O3 as an interfacial layer between the HZO and the metal electrode, the FE O-phase could remain without phase transition. This work shows the microscopic view to enhance the ferroelectric evolution in HfO2-based devices.

Large-Scale Hf0.5 Zr0.5 O2 Membranes with Robust Ferroelectricity.

Hafnia-based compounds have considerable potential for use in nanoelectronics due to their compatibility with complementary metal-oxide-semiconductor devices and robust ferroelectricity at nanoscale sizes. However, the unexpected ferroelectricity in this class of compounds often remains elusive due to the polymorphic nature of hafnia, as well as the lack of suitable methods for the characterization of the mixed/complex phases in hafnia thin films. Herein, the preparation of centimeter-scale, crack-free, freestanding Hf0.5 Zr0.5 O2 (HZO) nanomembranes that are well suited for investigating the local crystallographic phases, orientations, and grain boundaries at both the microscopic and mesoscopic scales is reported. Atomic-level imaging of the plan-view crystallographic patterns shows that more than 80% of the grains are the ferroelectric orthorhombic phase, and that the mean equivalent diameter of these grains is about 12.1nm, with values ranging from 4 to 50nm. Moreover, the ferroelectric orthorhombic phase is stable in substrate-free HZO membranes, indicating that strain from the substrate is not responsible for maintaining the polar phase. It is also demonstrated that HZO capacitors prepared on flexible substrates are highly uniform, stable, and robust. These freestanding membranes provide a viable platform for the exploration of HZO polymorphic films with complex structures and pave the way to flexible nanoelectronics.