Ferroelectric BaTiO3 Thin Films Research Articles

Integration Of Solution‐Processed BaTiO3 Thin Films with High Pockels Coefficient on Photonic Platforms

AbstractThe heterogeneous integration of ferroelectric BaTiO3 thin films on silicon (Si) and silicon nitride (SiN)‐based platforms for photonic integrated circuits (PICs) plays a crucial role in the development of future nanophotonic thin film modulators. Since the electro‐optic (EO) properties of ferroelectric thin films strongly depend on their crystal phase and texture, the integration of BaTiO3 thin films on these platforms is far from trivial. So far, a conventional integration route using a SrTiO3 template film in combination with high vacuum deposition methods has been developed, but it has a low throughput, is expensive and requires monocrystalline substrates. To close this gap, a cost‐efficient, high‐throughput and scalable method for integrating highly textured BaTiO3 films is needed. Therefore, an alternative method for the integration of highly textured BaTiO3 films using a La2O2CO3 template film in combination with a chemical solution deposition (CSD) process is presented. In this work, the structural and EO properties of the solution‐processed BaTiO3 film are characterized and its integration into an optical ring resonator is evaluated. The BaTiO3 film exhibits a fiber texture, has a large Pockels coefficient (reff) of 139 pm V−1, and integration into a ring resonator‐based modulator shows a VπL of 1.881 V cm and a bandwidth of > 40 GHz. This enables low‐cost, high‐throughput, and flexible integration of BaTiO3 films on PIC platforms and the potential large‐scale fabrication of nanophotonic BaTiO3 thin‐film modulators.

Artificial synaptic properties of zirconium-doped barium titanate film for neuromorphic computing

The traditional Von Neumann Architecture relies on the separation of memory and CPU, leading to bottlenecks like slow data transfer and high energy consumption. This is especially challenging when addressing the demands of large-scale parallel computing tasks such as artificial intelligence. Devices capable of simulating synaptic properties have gotten significant attention. Neuromorphic computing combines those devices with methods that work with neural networks. They are considered a viable approach to overcoming the Von Neumann Architecture bottleneck. This paper presents a Zr-doped BaTiO3 ferroelectric thin film prepared by a sol-gel process, which mimics the synaptic properties of the nerve: Short-Term Synaptic Plasticity (STP), paired-pulse facilitated (PPD), Long-Term Synaptic Plasticity (LTP) and Spike-Timing-Dependent Plasticity (STDP). An enhanced stochastic adaptive method is used to a built-in Convolutional Neural Network (CNN) to evaluate the device's neuromorphic computing capabilities. According to the results, the device obtains recognition accuracies of 96.5 % and 75.1 % for the MNIST and Fashion-MNIST datasets, respectively. This significantly contributes to the advancement of ferroelectric materials in neuromorphic computing applications.

Fabrication and photonic applications of Si-integrated LiNbO3 and BaTiO3 ferroelectric thin films

Silicon, renowned for its applications in electronic circuits, also offers significant advantages in the realm of integrated optics. While silicon does have inherent limitations in fabricating active devices, these challenges can be overcome through heterogeneous integration with other materials. Ferroelectric materials like lithium niobate (LN) and barium titanate (BTO) exhibit exceptional electrical and nonlinear optical properties, making them ideal candidates for integrated silicon photonics. Over the past few decades, the LN platform has made substantial progress, surmounting obstacles such as manufacturing difficulties, high waveguide losses, and low modulation bandwidth caused by wide electrode spacing. Consequently, LN-based devices have found widespread application in various fields. With the advent of high-quality, large-size LN on insulator wafers, LN photonics has become a burgeoning research area. At the same time, the ferroelectric material BTO, with its tremendous electro-optic response, shows promise for the next generation of integrated photonics. Recent studies have highlighted the remarkable performance of BTO-based modulators, which offer smaller device sizes compared to LN by achieving lower half-wave voltages. This comprehensive review presents various fabrication methods for integrating LN and BTO thin films on silicon, focusing on recent advancements. We discuss their electro-optic responses and provide an overview of the diverse applications enabled by these platforms. Finally, we summarize the current state of integrated LN and BTO photonics research and offer insights into future directions.

Polarization-dependent electrocaloric and pyroelectric effects in ferroelectric BaTiO3 thin films

Herein, we examine the influence of controllable polarization reversal and built-in electric fields on pyroelectric and electrocaloric effects in a BaTiO3 thin film using a modified indirect method. We find that the magnitude of the sample's change in polarization with temperature is sensitive to the degree of polarization reversal. The pyroelectric response is small at low fractions of switched polarization and grows larger by several factors as larger fractions of polarization are reversed. This polarization reversal-sensitive pyroelectric behavior is the result of an internal built-in field, which has the effect of destabilizing low fractions of switched polarization and producing diminished pyroelectric effect. Greater fractions of switched polarization are more stable against backswitching and permit a larger pyroelectric response. Our findings highlight a characterization method for polarization-dependent pyroelectric effects in ferroelectric thin films, where built-in field effects are also present.

Nanoscale Surface Structure of Nanometer-Thick Ferroelectric BaTiO3 Films Revealed by Synchrotron X-ray Scanning Tunneling Microscopy: Implications for Catalytic Adsorption Reactions

Ferroelectric nanomaterials are of interest in catalysis, nonvolatile memory, and neuromorphic computing among other applications because of their switchable structure that can alter the electronic and interface properties of a single material. The investigation of the role of polarization on the surface structure and chemistry of ferroelectric nanomaterials is a longstanding challenge, as it ideally requires a combination of both nanoscale imaging and chemical spectroscopy. In this work, we study a model ferroelectric BaTiO3 thin film by synchrotron X-ray scanning tunneling microscopy (SX-STM), a unique method that integrates nanoscale surface imaging and chemically sensitive spectroscopy. We find that polarization switching from downward to upward in (001) single-crystalline BaTiO3 thin films increases the intensity of X-ray absorption across Ba M, Ti L, and O K edges. Chemical mapping of nanometer-sized domains further demonstrates the modulation of surface structures upon polarization switching, as well as confirming the trends observed in single-point experiments across the surface. We complement these measurements with ab initio computational absorption spectroscopy to elucidate the effect of polarization switching on the core–hole excitations using the Bethe–Salpeter equation approach. Our experimental and theoretical results thus confirm a stronger binding strength for the upward-polarized surface with molecular O2 as a model reactant, offering mechanistic evidence that supports previous reports. This work advances the understanding of the surface chemistry and electronic structure of ferroelectrics, which can ultimately aid strategies to design interfaces with tailored properties.

Neurosynaptic-like behavior of Ce-doped BaTiO3 ferroelectric thin film diodes for visual recognition applications

Brain-like neuromorphic computing networks based on the human brain information processing model are gradually breaking down the memory barriers caused by traditional computing frameworks. The brain-like neural system consists of electronic synapses and neurons. The multiple ferroelectric polarization switching modulated by the external electric field is well suited to simulate artificial neural synaptic weights. Therefore, ferroelectric diodes' (FDs) synapses have great advantages in building highly reliable and energy-efficient artificial neural networks. In this paper, we demonstrate the FDs synapse, which is based on rare-earth metal-doped BaTiO3 ferroelectric dielectric layer materials. This performs short-term and long-term synaptic plasticity behaviors by modulating synaptic weights using pulsed stimuli to polarize or flip ferroelectric films. In addition, convolutional neural networks were constructed on the MNIST dataset and the Fashion-MNIST dataset to check the feasibility of the device in simulating bio-visual recognition. The results expand the application of FDs' devices in the intersection of artificial intelligence and bioscience.

Tip-Induced In-Plane Ferroelectric Superstructure in Zigzag-Wrinkled BaTiO3 Thin Films.

The complex micro-/nanoscale wrinkle morphology primarily fabricated by elastic polymers is usually designed to realize unique functionalities in physiological, biochemical, bioelectric, and optoelectronic systems. In this work, we fabricated inorganic freestanding BaTiO3 ferroelectric thin films with zigzag wrinkle morphology and successfully modulated the ferroelectric domains to form an in-plane (IP) superstructure with periodic surface charge distribution. Our piezoresponse force microscopy (PFM) measurements and phase-field simulation demonstrate that the self-organized strain/stress field in the zigzag-wrinkled BaTiO3 film generates a corresponding pristine domain structure. These domains can be switched by tip-induced strain gradient (flexoelectricity) and naturally form a robust and unique "braided" in-plane domain pattern, which enables us to offer an effective and convenient way to create a microscopic ferroelectric superstructure. The corresponding periodic surface potential distribution provides an extra degree of freedom in addition to the morphology that could regulate cells or polar molecules in physiological and bioelectric applications.

In-plane quasi-single-domain BaTiO3 via interfacial symmetry engineering

The control of the in-plane domain evolution in ferroelectric thin films is not only critical to understanding ferroelectric phenomena but also to enabling functional device fabrication. However, in-plane polarized ferroelectric thin films typically exhibit complicated multi-domain states, not desirable for optoelectronic device performance. Here we report a strategy combining interfacial symmetry engineering and anisotropic strain to design single-domain, in-plane polarized ferroelectric BaTiO3 thin films. Theoretical calculations predict the key role of the BaTiO3/PrScO3{({{{{{boldsymbol{110}}}}}})}_{{{{{{bf{O}}}}}}} substrate interfacial environment, where anisotropic strain, monoclinic distortions, and interfacial electrostatic potential stabilize a single-variant spontaneous polarization. A combination of scanning transmission electron microscopy, piezoresponse force microscopy, ferroelectric hysteresis loop measurements, and second harmonic generation measurements directly reveals the stabilization of the in-plane quasi-single-domain polarization state. This work offers design principles for engineering in-plane domains of ferroelectric oxide thin films, which is a prerequisite for high performance optoelectronic devices.

Magnetoelectric coupling in micropatterned BaTiO3/CoFe2O4 epitaxial thin film structures: Augmentation and site-dependency

The prominent magnetoelectric (ME) effect in ME composites usually originates from an interfacial strain-mediated coupling between ferroelectric and ferromagnetic components. While ME composite bilayer thin films have been proposed for microdevice applications, the ME coupling is usually diminished because of the substrate clamping effect. Here, we apply simple CMOS/MEMS compatible top-down fabrication techniques to ferroelectric BaTiO3 (BTO) and ferromagnetic CoFe2O4 (CFO) bilayer epitaxial thin films to control the substrate clamping. We found augmented ME coupling in micro-patterned bilayer thin films compared to the as-deposited films due to the reduced substrate clamping. In addition, a site-dependent ME coupling within the microstructure was observed. Larger ME coupling was obtained near the edge of the microstructure, and site-dependent ferroelectric imprints were observed on the micropattern. This can be attributed to the non-uniform substrate clamping across the film, and the strain gradient developed in the BTO layer due to the magnetostriction of CFO. Our findings provide additional insights to the design of micro- and nanoscale devices based on 2–2 ME composite thin films.

Large energy-storage density with good dielectric property in bismuth sodium titanate-based thin films

In this study, energy-storage and dielectric performances of x mol% (x = 0, 2, 4, 6) Fe-doped 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 (abbreviated as 0.94BNT-0.06BT-Fex) ferroelectric thin films were prepared through sol-gel process. Fe-doped BNT-BT thin films were found to have slimmer hysteresis with higher maximum polarization (Ps), lower remanent polarization (Pr), higher breakdown strength and much larger dielectric constant. Optimal energy-storage properties were obtained for 0.96BNT-0.04BT-Fe2 thin films, with a breakdown strength, energy-storage density and efficiency of 2500 kV/cm, 33 J/cm3, 67.8%, respectively. Simultaneously, the leakage mechanisms of the 0.96BNT-0.04BT-Fex were investigated, and influences of the oxygen vacancies on the leakage current as well as breakdown strength were discussed. The outstanding properties of our thin film indicate its usage in advanced high-energy-storage capacitors.

Scaling behavior of dynamic hysteresis in epitaxial ferroelectric BaTiO3 thin films

The evolution of the dynamic hysteresis with electrical field amplitude (E) and frequency (f) in the ferroelectric BaTiO3 (BTO) single crystal films prepared by pulsed laser deposition (PLD) has been investigated systematically. A noticeable transition from the low-frequency behavior to the high-frequency one was confirmed by observing the obtained hysteresis loops and the change of the physical properties. In addition, the analysis of the theoretical model shows that the frequency dependence of domain motion is the cause of the dynamic hysteresis loop scaling behavior.

Oxidation processes at the surface of BaTiO3 thin films under environmental conditions

Dissociation and adsorption of water on ferroelectric oxide surfaces playimportant role in the processes of screening and switching dynamics of ferroelectric polarization, as well as in catalytic processes which can be additionally coupled with light, temperature or vibration stimuli. In this work,we present XPS study of ferroelectric BaTiO3thin films and determine the entanglement between surface chemistry, polarization direction and stability, by observing changes upon time exposure to environmental conditions, heating in O2atmosphereandirradiation in vacuum. We devote special attention to Ba 3d spectral region and identify two different oxidation states of O atoms in the compounds of Ba. While this second specie was generally attributed to Ba in surface compounds where it has different oxygen coordination than in the bulk, based on the XPS results of oxygen annealed thin films,we demonstrate that this so far neglected component, corresponds to barium peroxide (BaO2)and identify it as important active specie for the study of screening mechanisms closely related with catalytic activity present in this ferroelectric material. Finally, we report on chemically assisted polarization switching in thin films induced byheating in vacuum or exposure to X-Ray radiation due to the formation of positive surface electric field created by oxygen orelectron vacancies, respectively.

Domain fluctuations in a ferroelectric low-strain BaTiO3 thin film

A ferroelectric BaTiO3 thin film grown on a NdScO3 substrate was studied using x-ray photon correlation spectroscopy (XPCS) to characterize thermal fluctuations near the a/b to a/c domain structure transformation present in this low-strain material, which is absent in the bulk. XPCS studies provide a direct comparison of the role of domain fluctuations in first- and second-order phase transformations. The a/b to a/c domain transformation is accompanied by a decrease in fluctuation timescales, and an increase in intensity and correlation length. Surprisingly, domain fluctuations are observed up to 25 degrees C above the transformation, concomitant with the growth of a/c domains and coexistence of both domain types. After a small window of stability, as the Curie temperature is approached, a/c domain fluctuations are observed, albeit slower, potentially due to the structural transformation associated with the ferroelectric to paraelectric transformation. The observed time evolution and reconfiguration of domain patterns highlight the role played by phase coexistence and elastic boundary conditions in altering fluctuation timescales in ferroelectric thin films.

Investigation for the Thermodynamic Polarization Degradation of the BaTiO3 Crystal Thin Films

According to the latest development of memory, it is found that the motion of ferroelectric domain wall is very important for the high-density non-volatile devices of ferroelectric materials. This paper mainly studies the microstructure of barium titanate (BaTiO3) crystal films with the molecular dynamics of crystal domain motion and then models the polarization degradation with the conservation of the nucleation energy-focusing and free energy thermodynamic processes in the crystalline domains. First, by the means of XRD diffractometer and JADE software, the crystal axis orientation of BaTiO3 thin film is analysed, and then the crystal axis orientation is determined by referring the published measurement results. Further, for the ferroelectric crystal BaTiO3 thin film, the thermodynamic process is studied and the temperature dependences of the polarization ratio with respect to the spontaneous polarization is modelled. Numerical simulations have shown the much higher temperature dependence on the nucleation energy than on the free energy in a domain wall system, leading to the possibility to reach a high polarization ratio with the optimal deposition conditions to control the local energy and the nucleation energy cost of crystalline domains.

Skewed electronic band structure induced by electric polarization in ferroelectric BaTiO3

Skewed band structures have been empirically described in ferroelectric materials to explain the functioning of recently developed ferroelectric tunneling junction (FTJs). Nonvolatile ferroelectric random access memory (FeRAM) and the artificial neural network device based on the FTJ system are rapidly developing. However, because the actual ferroelectric band structure has not been elucidated, precise designing of devices has to be advanced through appropriate heuristics. Here, we perform angle-resolved hard X-ray photoemission spectroscopy of ferroelectric BaTiO3 thin films for the direct observation of ferroelectric band skewing structure as the depth profiles of atomic orbitals. The depth-resolved electronic band structure consists of three depth regions: a potential slope along the electric polarization in the core, the surface and interface exhibiting slight changes. We also demonstrate that the direction of the energy shift is controlled by the polarization reversal. In the ferroelectric skewed band structure, we found that the difference in energy shifts of the atomic orbitals is correlated with the atomic configuration of the soft phonon mode reflecting the Born effective charges. These findings lead to a better understanding of the origin of electric polarization.

Toward van der Waals epitaxy of transferable ferroelectric barium titanate films via a graphene monolayer

Transferable highly (001)-oriented textured ferroelectric BaTiO3 thin films have been achieved on a graphene monolayer for wearable devices like sensors and actuators for future “Internet of Things” era.

High energy storage performance in lead-free BiFeO3-BaTiO3 ferroelectric thin film fabricated by pulsed laser deposition

In this work, the 0.68BiFeO3-0.32BaTiO3 (BFBT) ferroelectric thin film was fabricated with high maximum polarization for energy storage applications. BFBT thin film with pure perovskite phase was deposited on Pt/Ti/SiO2/Si substrates at 600°C by Pulsed Laser Deposition (PLD) method. We measured the ferroelectric hysteresis, dielectric properties and the fatigue performance of the BFBT thin film with thickness of about 200 nm. It was found that the film has a high maximum field-induced polarization value of 86 μC/cm2. Under an applied low electric field of 900 kV/cm, the recoverable energy density (Ure) could reach up to 19 J/cm3, and the energy efficiency (η) is around 51% at room temperature. Furthermore, the film shows outstanding fatigue endurance even after 1×107 cycles. All results suggest that lead-free BFBT ferroelectric thin film is very promising energy storage materials.

Island growth mode in pulsed laser deposited ferroelectric BaTiO3 thin films: The role of oxygen pressure during deposition

Pulsed laser deposition is widely used to grow BaTiO3 thin films. We investigated the influence of oxygen pressure during growth on the topography, microstructure, and roughness of ferroelectric epitaxial BaTiO3 thin films. It also presented an analysis of the epitaxial growth mode and defects throughout the film thickness using aberration-corrected transmission electron microscopy. Although ferroelastic (twin boundary) domain walls are absent, several misfit dislocations were observed and might be the primary cause of the observed island growth mode.

Epitaxial growth of (111) BaTiO3 thin films on (0002) GaN substrates with SrTiO3/TiN buffer layers

High-quality perovskite (111) BaTiO3 (BTO) ferroelectric thin films were epitaxially grown on wurtzite (0002) GaN substrates with the rationally designed SrTiO3 (STO)/TiN buffer layers by pulsed laser deposition. Particularly, TiN thin films with excellent conductivity could also be served as the bottom electrodes. The epitaxial relationship of the BTO/STO/TiN/GaN heterostructures was proved to be (111)[1 $$\bar{1}$$ 0] BTO//(111)[1 $$\bar{1}$$ 0] STO//(111)[1 $$\bar{1}$$ 0] TiN//(0002)[11 $$\bar{2}$$ 0] GaN by reflection high-energy electron diffraction and high resolution X-ray diffraction. Furthermore, the detailed interface structure and epitaxial relationship of the BTO/STO/TiN/GaN heterostructures were identified on atomic scale by high resolution transmission electron microscopy. The epitaxial (111) BTO ferroelectric thin films on GaN substrates exhibited the favorable ferroelectric properties with the remnant polarization of 12.97 μC cm−2. The high-quality epitaxial integration of perovskite BTO thin films on wurtzite GaN substrates could promote the potential applications in the advanced GaN-based integrated ferroelectric devices.

Characterization of domain distributions by second harmonic generation in ferroelectrics

Domain orientations and their volume ratios in ferroelectrics are recognized as a compelling topic recently for domain switching dynamics and domain stability in devices application. Here, an optimized second harmonic generation method has been explored for ferroelectric domain characterization. Combing a unique theoretical model with azimuth-polarization-dependent second harmonic generation response, the complex domain components and their distributions can be rigidly determined in ferroelectric thin films. Using the proposed model, the domain structures of rhombohedral BiFeO3 films with 71° and 109° domain wall, and, tetragonal BiFeO3, Pb(Zr0.2Ti0.8)O3, and BaTiO3 ferroelectric thin films are analyzed and the corresponding polarization variants are determined. This work could provide a powerful and all-optical method to track and evaluate the evolution of ferroelectric domains in the ferroelectric-based devices.