Ferroelectric Thin Films Research Articles

Thermodynamic analysis of the inverse electrocaloric effect in ferroelectric thin films

Abstract Inverse electrocaloric (i-EC) effect occurs in ferroelectrics when the applied electric field aligns anti-parallel to polarization. In this study, the dependence of the i-EC effect on temperature and misfit strain is formulated and applied to clamped BaTiO3 thin films using the Landau–Ginzburg–Devonshire formalism. It is found that an interplay exists between the pyroelectric coefficient and the maximum possible inverse electric field. We demonstrate that the temperature change is strongly dependent on the inverse field amplitude and is maximal at lower temperatures away from the ferroelectric-paraelectric transition for a given misfit strain. Such an outcome is opposite to the direct electrocaloric effect, where it is desirable to remain near the transition temperature for the maximum electrocaloric temperature change. The fact that the i-EC effect can be maximum at lower temperatures could allow for the potential tailoring of this effect in strained films at moderate temperatures for device applications.

Ultra-fine nano-crystalline optimize electrostatic energy storage

Grain size pays a crucial role in the properties of ferroelectrics and dielectrics. Reducing the grain size is considered to be an effective mean for enhancing dielectric energy storage. In this work, high recovered energy storage density and efficiency were achieved in three-layered Aurivillius thin films by ultra-fine grain nano-crystalline engineering. The ultra-low remanent polarization can be attributed to the emergence of polar nano-regions due to the disruption of macroscopic continuity of ferroelectric domains by ultra-fine nano-grains. At the same time, all thin films have high dielectric breakdown strength due to the presence of extremely high grain boundary density. Thus, a high recovered energy storage density of 71 J/cm3 and an efficiency of 76% were achieved, and the thin film capacitors show good fatigue endurance and temperature stability. The results suggest that ultra-fine nano-crystalline engineering can expand the application of traditional ferroelectric thin films in energy storage devices.

Surface polarization profile of ferroelectric thin films probed by X-ray standing waves and photoelectron spectroscopy

Understanding the mechanisms underlying a stable polarization at the surface of ferroelectric thin films is of particular importance both from a fundamental point of view and to achieve control of the surface polarization itself. In this study, we demonstrate that the X-ray standing wave technique allows the surface polarization profile of a ferroelectric thin film, as opposed to the average film polarity, to be probed directly. The X-ray standing wave technique provides the average Ti and Ba atomic positions, along the out-of-plane direction, near the surface of three differently strained \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mathrm {BaTiO_3}$$\\end{document} thin films. This technique gives direct access to the local ferroelectric polarization at and below the surface. By employing X-ray photoelectron spectroscopy, a detailed overview of the oxygen-containing species adsorbed on the surface is obtained. The different amplitude and orientation of the local ferroelectric polarizations are associated with surface charges attributed to different type, amount and spatial distribution of the oxygen-containing adsorbates.

Effect of Extrapolation Length on Electrical Properties for Ba0.8Sr0.2TiO3 Ferroelectric Thin Films

Effect of Extrapolation Length on Electrical Properties for Ba0.8Sr0.2TiO3 Ferroelectric Thin Films

Engineering of ferroelectricity in thin films using lattice chemistry: A perspective

Engineering of ferroelectricity in thin films using lattice chemistry: A perspective

Multiferroic properties and local ferroelectricity of polycrystalline LuFeO3 thin films with triangular grains on glass substrates

Polycrystalline LuFeO3 (LFO) thin films were prepared on multilayer Pt/Ta/glass substrates. The ferroelectric hysteresis loop of the LFO thin film exhibited a remanent polarization of about 5.7 μC/cm2. The ferroelectric hysteresis loop of the LFO thin film deposited at a relatively slow rate provides valuable information about its recoverable energy density (8.8 J/cm3) and loss energy density (9.3 J/cm3), resulting in an estimated energy storage efficiency of around 49%. The LFO thin film had triangular grains with a diameter of approximately 90 nm, which well reflects the crystallinity of hexagonal LFO. From the local piezoelectric d33 hysteresis loops of a triangular grain, it was confirmed that the edge of a triangular grain had a remanent d33 value larger than the center of a triangular grain. Additionally, a mosaic domain structure of the LFO thin film was well correlated with the AFM image. By applying the Landau, Lifshitz, and Kittel (LLK) scaling law, it was confirmed that the LFO thin films have a domain wall energy lower than that of PbTiO3 thin films.

Interdomain Screening Effect in Multidomain Ferroelectric Thin Film and Its Impact on Polarization Switching

Interdomain Screening Effect in Multidomain Ferroelectric Thin Film and Its Impact on Polarization Switching

Superior Piezo-/Ferro-Electricity in Antiferroelectric AgxNbO3-δ Thin Films by Nanopillar Local Structure Design.

Antiferroelectrics are fundamental mother compounds critical in developing innovative lead-free piezoelectrics and ferroelectrics and hold great promise for wide-ranging applications in energy conversion and electronic devices. However, harnessing their superior properties presents a significant challenge due to the delicate balance required between their various states. In this study, through the unique design of nanopillar structures to alleviate the local polar heterogeneity, we have achieved significantly improved piezo-/ferro-electricity in classic lead-free antiferroelectric AgxNbO3-δ (x = 1, 0.9, and 0.8) epitaxial thin films. The effective piezoelectric coefficient reaches 440 pm V-1, 1 order of magnitude larger than the stoichiometric AgNbO3, rivaling classic lead zirconate titanate piezoelectrics. Atomic-scale electron microscopy investigations unravel the underlying mechanisms. The nanopillars, characterized by antisite occupancy of both Ag and Nb atoms and forming out-of-phase boundaries with the matrix, reduce the local crystal symmetry via interphase strain. This leads to the creation of flexible multinanodomain structures that significantly facilitate polarization rotation, thus substantially enhancing the piezoelectric performance. This study demonstrates the feasibility of engineering local heterogeneity through nanopillar design, offering a generally applicable method for property improvement of a wide range of antiferroelectrics.

Crystalline phase control of ferroelectric HfO2 thin film via heterogeneous co-doping

Our study investigates heterogeneous co-doped HfO2 thin films integrated into metal-ferroelectric-metal stacks, achieved by incorporating multiple layers doped with various species during the atomic layer deposition process. This approach creates an artificial crystallization temperature gradient across the HfO2 film, influencing the preferred nucleation sites of HfO2 during rapid thermal processing. Our findings demonstrate that the phase composition of the annealed HfO2 film is primarily determined by heterogeneous or homogeneous crystallization processes. In cases of heterogeneous crystallization, where crystallization initiates from nuclei formed at electrode/HfO2 interfaces, grains predominantly crystallize in the orthorhombic phase. Conversely, grains are more likely to crystallize in the monoclinic phase if they originate from nuclei formed at the center of the HfO2 film. Additionally, we observe correlations between the texture of the HfO2 film and the texture of the electrodes.

Intrinsically Stable Charged Domain Walls in Molecular Ferroelectric Thin Films

AbstractCharged domain walls in ferroelectrics hold great promise for applications in ferroelectric random‐access memory (FeRAM), with advantages such as low energy consumption, high density, and non‐destructive operation. Due to the mechanical compatibility condition, the neutral domain walls are dominant in traditional ferroelectric thin films. Herein, using phase‐field simulations, the formation of intrinsically stable charged domain walls (CDWs) in the molecular ferroelectric films is demonstrated, which can be mainly attributed to the small mechanical stiffness. The switching kinetics are further investigated for the CDWs, showing a lower switching barrier as compared to the neutral counterparts. Moreover, it is indicated that increasing the compressive misfit strain can lead to prolonged switching time, with a significantly increased switching energy barrier. These findings pave the way for the potential applications of metal‐free organic ferroelectric materials in FeRAM devices.

Emergence and transformation of polar skyrmion lattices via flexoelectricity

As analogies to magnetic skyrmions, polar skyrmions in ferroelectric superlattices and multilayers have garnered widespread attention for their non-trivial topology and novel properties like negative capacitance and nonlinear optical effect. So far, they have only been theoretically predicted to be able to assemble ordered hexagonal skyrmion lattices (SkLs) in ferroelectric thin films. Here, based on phase-field simulations, we report the critical roles of flexoelectricity playing in the stabilization and transformation of polar SkLs. Different polar SkL patterns can emerge in the ferroelectric thin films, including tetragonal-SkL, and hexagonal-SkLs with diverse orientations, as summarized by phase diagrams. These emergent SkL states are attributed to the material anisotropy modified by the flexoelectric effect. Interestingly, we further found that the hexagonal-SkLs can be rotated by applying strain gradient or in-plane electric field to the films. Moreover, a nonreciprocal bending response of tetragonal-SkL is also induced by the flexoelectric effect. Our results provide useful guidelines for the implementation of polar skyrmion lattices in experiments.

The fractional solitary wave profiles and dynamical insights with chaos analysis and sensitivity demonstration

This work investigates the dynamics of solitary waves in a thin-film ferroelectric material model, an essential component of optical systems. The study recognizes the limitations of the inverse scattering transform in solving the Cauchy problem for this conformable thin-film ferroelectric material model and applies a new Kudryashov approach and Bernoulli’s equation technique to ordinary differential equations. We apply these methods to the present model and obtain closed form analytical solutions. A variety of traveling wave and soliton solutions are constructed, including kink, bell, bright, and dark soliton solutions. It is acknowledged that the nonlinear evolution equation can be transformed into an ordinary differential equation by applying traveling wave transformations. Suitable physical parameters are chosen to create 2D, 3D, and contour plots that graphically depict the graphical dispersion of obtained fractional soliton solutions. The impact of the conformable fractional parameter is further demonstrated by the graphical representation of solitons propagation. Furthermore, the study develops the Hamiltonian function of the dynamical system to discuss the system’s total energy in terms of momentum and position. While a chaos analysis describes chaotic, quasi-periodic, and periodic behaviors, a sensitivity analysis demonstrates how responsive the model is to various initial conditions. Due to their dual-purpose nature as nonlinear ferroelectric and dielectric materials, thin ferroelectric films are widely used in modern electrical devices.

Tunable Ferroionic Properties in CeO2/BaTiO3 Heterostructures.

Ferroionic materials combine ferroelectric properties and spontaneous polarization with ionic phenomena of fast charge recombination and electrodic functionalities. In this paper, we propose the concept of tunable polarization in CeO2-δ (ceria) thin (5 nm) films induced by built-in remnant polarization of a BaTiO3 (BTO) ferroelectric thin film interface, which is buried under the ceria layer. Upward and downward fixed polarizations at the BTO thin film (10 nm) are achieved by the lattice termination engineering of the SrO or TiO2 terminated Nb:SrTiO3 (NSTO or STN) substrate. We find that the ceria layer punctually replicates the polarization of the BTO interface via a dynamic reconfiguration of its intrinsic defects, i.e., oxygen vacancies and small polarons. Tunable oxidative or reducing properties (redox) also arise at the surface from the built-in polarization. Opposite polarities at the ceria termination tune the chemo-physical dynamics toward water molecule adsorbates. The inversion of the surface potential leads to a modulation of the water adsorption-desorption equilibrium and water ionization (splitting) redox overpotentials within ±400 mV at room temperature, depending on the ceria termination's charges. Such tunability opens up the perspectives of using ferroionics for wireless electrochemically enhanced catalysis.

Atomic-Scale Scanning of Domain Network in the Ferroelectric HfO2 Thin Film.

Ferroelectric HfO2-based thin films have attracted much interest in the utilization of ferroelectricity at the nanoscale for next-generation electronic devices. However, the structural origin and stabilization mechanism of the ferroelectric phase are not understood because the film is typically nanocrystalline with active yet stochastic ferroelectric domains. Here, electron microscopy is used to map the in-plane domain network structures of epitaxially grown ferroelectric Y:HfO2 films in atomic resolution. The ferroelectricity is confirmed in free-standing Y:HfO2 films, allowing for investigating the structural origin for their ferroelectricity by 4D-STEM, high-resolution STEM, and iDPC-STEM. At the grain boundaries of <111>-oriented Pca21 orthorhombic grains, a high-symmetry mixed-(R3m, Pnm21) phase is induced, exhibiting enhanced polarization due to in-plane compressive strain. Nanoscale Pca21 orthorhombic grains and their grain boundaries with mixed-(R3m, Pnm21) phases of higher symmetry cooperatively determine the ferroelectricity of the Y:HfO2 film. It is also found that such ferroelectric domain networks emerge when the film thickness is beyond a finite value. Furthermore, in-plane mapping of oxygen positions overlaid on ferroelectric domains discloses that polarization is suppressed at vertical domain walls, while it is active when domains are aligned horizontally with subangstrom domain walls. In addition, randomly distributed 180° charged domain walls are confined by spacer layers.

Sub-5 nm Al-doped HfO2 ferroelectric thin films compatible with 3D NAND process

The HfO2-based ferroelectric thin films have provided new opportunity for ferroelectric field-effect transistors (FeFETs) to be the candidate for the next-generation non-volatile memory, because of their compatibility with complementary metal-oxide-semiconductor technology. Benefiting from their similar but simpler structure compared to commercial flash memory, FeFETs can be integrated into three dimensional (3D) NAND architectures, which is named as 3D Fe-NAND, promising for super large storage applications. However, the ultra-thin HfO2-based ferroelectric thin film with tolerance for 850 ℃ thermal treatment has not been realized, which is the prerequisite for the high-density and low-power-consumption 3D Fe-NAND. In this work, we systematically investigated the impact both of thermodynamic and kinetical process on the ferroelectricity and reliability of the Al-doped HfO2 thin films. By carefully designing the dopant concentration and annealing technique, a 4.5-nm-thick HfAlOx film annealed at 850 °C with good ferroelectricity with 2Pr of ∼20 μC/cm² and breakdown field of ∼8.6 MV/cm is achieved. Furthermore, device simulation demonstrates the enhanced memory window at low operation voltage when integrating thinner HfAlOx ferroelectric films into FeFETs, suggesting the competitiveness of ultra-thin HfAlOx ferroelectric films in low-power-consumption operation. This work provides the strategy to design high-performance, ultra-thin HfO2-based ferroelectric thin films compatible with 3D Fe-NAND architecture.

Ferroelectric tunnel junctions based on a HfO2/dielectric composite barrier

The ferroelectric tunnel junction (FTJ), which possesses a simple structure, low power consumption, high operation speed, and nondestructive reading, has attracted great attention for the application of next-generation nonvolatile memory. The complementary metal–oxide–semiconductor-compatible hafnium oxide (HfO2) ferroelectric thin film found in the recent decade is promising for the scalability and industrialization of FTJs. However, the electric performance, such as the tunneling electroresistance (TER) effect, of the current HfO2-based FTJs is not very satisfactory. In this work, we propose a type of high-performance HfO2-based FTJ by utilizing a ferroelectric/dielectric composite barrier strategy. Using density functional theory calculations, we study the electronic and transport properties of the designed Ni/HfO2/MgS/Ni (001) FTJ and demonstrate that the introduction of an ultra-thin non-polar MgS layer facilitates the ferroelectric control of effective potential barrier thickness and leads to a significant TER effect. The OFF/ON resistance ratio of the designed FTJ is found to exceed 4 × 103 based on the transmission calculation. Such an enhanced performance is driven by the resonant tunneling effect of the ON state, which significantly increases transmission across the FTJ when the ferroelectric polarization of HfO2 is pointing to the non-polar layer due to the aroused electron accumulation at the HfO2/MgS interface. Our results provide significant insight for the understanding and development of the FTJs based on the HfO2 ferroelectric/non-polar composite barrier.

A molecular ferroelectric thin film of imidazolium perchlorate on silicon

Molecular ferroelectrics have garnered significant attention due to their structural tunability, low synthesis temperature, and high flexibility. Herein, we successfully synthesized imidazole perchlorate (ImClO4) single crystals and high-quality, highly-oriented thin films on Si substrates. These films demonstrated a high inverse piezoelectric coefficient of 55.7 pm/V. Two types of domain bands were observed: type-I bands tilted ~60° relative to the horizontal axis, and type-II bands positioned perpendicular to the horizontal axis. Under a + 20 V bias, type-I bands showed a reduction and detachment of 180° domain walls to form a needle-like domain. It extended toward the band boundary after applying −20 V bias, which grew along the boundary upon contact. In contrast, type-II bands showed straight domain wall motion and displayed a higher piezoresponse than type-I bands. The growth of high quality molecular ferroelectric thin films on Si substrates paves the way for the development of on-chip devices.

In Situ Modulation of Oxygen Vacancy Concentration in Hf0.5Zr0.5O2−x Thin Films and the Mechanism of Its Impact on Ferroelectricity

Oxygen vacancies play a crucial role in stabilizing the ferroelectric phase in hafnium (Hf) oxide-based thin films and in shaping the evolution of their ferroelectric properties. In this study, we directly manipulated the oxygen vacancy concentration in Hf0.5Zr0.5O2−x (HZO) ferroelectric thin films in situ using oxygen plasma treatment. We scrutinized the variations in the ferroelectric properties of HZO films across different oxygen vacancy concentrations by integrating the findings from ferroelectric performance tests. Additionally, we elucidated the mechanism underlying the influence of oxygen vacancies on the coercive field and polarization properties of HZO ferroelectric films through the first-principles density functional theory (DFT) calculations. Finally, to study the impact of oxygen vacancies on the practical application of HZO ferroelectric synaptic devices, leveraging the plasticity of the ferroelectric polarization, we constructed a multilayer perceptron (MLP) network. We simulated its recognition accuracy and convergence speed under different oxygen vacancy concentrations in the MNIST recognition task.

Multiphase-field modeling of domain structure evolution in ferroelectric thin film

The computational determination of domain structures in ferroelectric films is achieved through the implementation of a multiphase-field methodology. In this framework, domain structures are calculated by minimizing the total energy functional with respect to the multiphase-field order parameter ϕ. This energy functional includes the general interfacial energy, which consists of a multi-obstacle potential and a gradient energy, as well as the phase-dependent bulk energy that incorporates contributions from mechanical forces and electric fields. Using PbTiO3 (PTO) as the chosen model material, we report on the results of investigating domain structures, including an examination of the influence of substrate deformation, variations in misfit strains, different thin film thicknesses, and temperature fluctuations. By implementing the mechanical jump condition approach, the inelastic strain is calculated independently for both the thin film and the substrate, under different conditions. Furthermore, this model demonstrates its ability to investigate domain structures without relying on the Landau potential to characterize the structural stability, providing a valuable reference for studing various ferroelectric thin films that lack higher-order Landau coefficients.

Strain phase equilibria and phase‐field method of ferroelectric polydomain: A case study of monoclinic KxNa1 − xNbO3 thin films

AbstractKnowledge of the thermodynamic equilibria and domain structures of ferroelectrics is critical to establishing their structure–property relationships that underpin their applications from piezoelectric devices to nonlinear optics. Here, we establish the strain condition for strain phase separation and polydomain formation and analytically predict the corresponding domain volume fractions and wall orientations of, relatively low symmetry and theoretically more challenging, monoclinic ferroelectric thin films by integrating thermodynamics of ferroelectrics, strain phase equilibria theory, microelasticity, and phase‐field method. Using monoclinic KxNa1 − xNbO3(0.5 &lt; x &lt; 1.0) thin films as a model system, we establish the polydomain strain–strain phase diagrams, from which we identify two types of monoclinic polydomain structures. The analytically predicted strain conditions of formation, domain volume fractions, and domain wall orientations for the two polydomain structures are consistent with phase‐field simulations and in good agreement with experimental results in the literature. The present study demonstrates a general, powerful analytical theoretical framework to predict the strain phase equilibria and domain wall orientations of polydomain structures applicable to both high‐ and low‐symmetry ferroelectrics and provide fundamental insights into the equilibrium domain structures of ferroelectric KxNa1 − xNbO3 thin films that are of technology relevance for lead‐free dielectric and piezoelectric applications.