BaTiO3 Films Research Articles

Thickness-dependent thermal conductivity of ultrathin (< 100 nm) barium titanate films

Thin film barium titanate (BaTiO3) is a promising material in the electronics and ceramics industry owing to its compelling dielectric properties. A number of works have investigated its dielectric and structural properties, but less studied are its thermal properties particularly at sub-100 nm thicknesses. Here, we measure the room-temperature thermal conductivity of ultrathin (< 100 nm), pulsed laser deposited BaTiO3 films. The measured thermal conductivities are thickness-dependent, and this trend is consistent with the thickness-dependent crystallinity of the films. Transmission electron microscopy analysis of the films reveals the presence of an initial amorphous layer ~ 60 nm thick from the growth interface and the subsequent formation of columnar grains of width ~ 12 nm that are embedded within an amorphous matrix. For a region that incorporates grains with columnar morphology, we find that cross-plane heat conduction may be favored by 30–40% over in-plane heat conduction due to the columnar morphology of grains.

Synchrotron x-ray microdiffraction study of residual stresses in BaTiO3 films deposited at room temperature by aerosol deposition

The residual stress through the thickness of a BaTiO3 ceramic film deposited on steel substrate at room temperature with aerosol deposition has been analyzed using synchrotron x-ray microdiffraction. A gradient in stress distribution was evident through the film and the maximum biaxial compressive stress of −800 MPa was observed at the film-substrate interface. Heat-treatment was found to relax the internal compressive stress, due to thermal expansion mismatch between the film and substrate. Variation in ferroelectric response was correlated to the change in stress state by thermal treatment. This analysis is crucial for development of micro-and nanoelectronic devices with AD films.

BaTiO&lt;sub&gt;3&lt;/sub&gt;/ Teflon Nanocomposite Ferroelectric Thin Films for Low Voltage Electrowetting Systems

We present a simple and cost effective method for the design and fabrication of electrowetting devices using a nanocomposite thin film of BaTiO3 and Teflon-AF as the dielectric layer to achieve low voltage operation. The nanocomposite film is prepared by using Teflon-AF as matrix and BaTiO3 nanoparticles as the filler material. The solution is spin coated to deposit thin film on metal electrodes. The characterization results show that the nanocomposite thin film can serve as the dielectric for EWOD with a high dielectric constant and a crack free hydrophobic film. To test the electrowetting effect, the variation of droplet contact angle achieved with DC voltage, AC voltage and AC frequency change are fully experimented. The EWOD device with nanocomposite dielectric layer also manipulates water droplet at low driving voltages. This study shows the potential of using ferroelectric nanocomposite film as the dielectric layer in high-performance EWOD devices.

BaZrxTi1−xO3 Epitaxial Thin Films for Electrocaloric Investigations

AbstractEpitaxial BaZrxTi1−xO3 (BZT) thin films with a Zr content between x=0 and x=0.3 are grown by pulsed laser deposition on (001)‐oriented SrTiO3 single crystals utilizing a conducting SrRuO3 buffer layer and Pt top electrodes. An undisturbed epitaxial growth with a smooth and homogeneous surface structure is obtained for all Zr containing films, whereas some preferentially growing grains and twins are observed for pure BaTiO3 films. Temperature‐dependent measurements of the relative permittivity suggest a diffuse phase transition. The temperature of the maximum permittivity as well as the electrical polarizability at room temperature decrease with increasing Zr content. The observed frequency dependence suggests relaxor ferroelectric properties for all Zr‐containing films. The indirectly determined electrocaloric responses are considerably lower compared to thick films, which is primarily attributed to the diffuse character of the phase transition. A maximum adiabatic temperature change of about 0.3 K for an electric field change of 170 kV cm−1 is found for BaZr0.2Ti0.8O3.

Control of Polar Orientation and Lattice Strain in Epitaxial BaTiO3 Films on Silicon.

Conventional strain engineering of epitaxial ferroelectric oxide thin films is based on the selection of substrates with a suitable lattice parameter. Here, we show that the variation of oxygen pressure during pulsed laser deposition is a flexible strain engineering method for epitaxial ferroelectric BaTiO3 films either on perovskite substrates or on Si(001) wafers. This unconventional growth strategy permits continuous tuning of strain up to high levels (ε > 0.8%) in filmsgreater than onehundred nanometersthick, as well as selecting the polar axis orientation to be either parallel or perpendicular to the substrate surface plane.

Control of misfit strain in ferroelectric BaTiO3 thin-film capacitors with SrRuO3-based electrodes on (Ba, Sr)TiO3-buffered SrTiO3 substrates

We investigate ferroelectric BaTiO3 capacitors with SrRuO3-based electrodes employing a (BaxSr1−x)TiO3 (BST) buffer layer on (100) SrTiO3 substrates prepared by pulsed laser deposition. Structural analysis by high-resolution X-ray diffraction reciprocal space mapping shows that the BST (x = 0.5 and 0.7) layers are relaxed and have their bulk in-plane lattices in the upper part owing to strain relief. The bottom electrodes on the buffer layers grow heteroepitaxially but cannot withstand a tensile stress and then show a diminution of the in-plane lattice. On the BST (x = 0.7) layer, compared with the capacitor with an SrRuO3 electrode, that with a Ba0.1Sr0.9RuO3 one has a reduced in-plane lattice relaxation of the BaTiO3 film and then exhibits a larger remanent polarization (Pr) of 34 μC/cm2 associated with a smaller shift of the hysteresis loop. The polarization hysteresis shift is attributed to a flexoelectric effect stemming from the coupling between out-of-plane polarization and a strain gradient in the BaTiO3 film. We conclude that a reduced misfit strain relaxation in the ferroelectric film achieved with the Ba0.1Sr0.9RuO3 electrode on the thicker BST (x = 0.7) buffer layer is the origin of an enhanced Pr with a smaller hysteresis shift.

Combination of Solid-State and Electrochemical Impedance Spectroscopy To Explore Effects of Porosity in Sol–Gel-Derived BaTiO3 Thin Films

BaTiO3 thin films were deposited onto polycrystalline Pt using a dip-coating technique, with annealing temperatures of 750–900 °C. To avoid film imperfections such as cracking or pinholes, key conditions, including aging periods, water content, and stirring speeds, were refined to produce a pinhole-free, uniform film with some porosity. Whereas those coated a single time short circuited during electrical characterization, this could be avoided in films produced by multiple coating cycles. The effective permittivity of a 600 nm BaTiO3 film was measured at 290 by fitting solid-state impedance data in the frequency range of 100 Hz to 1 MHz. Electrochemical impedance with an aqueous electrolyte allowed evaluation of the porosity, which remained fairly constant between 1 and 5 coating cycles. Using this method, it was possible to estimate the effective permittivity of the BaTiO3 itself as 374 and hence to evaluate the increase in the effective permittivity that could be achieved by minimizing porosity.

Solid-solution thin films of ternary BaTiO3–Bi(Mg1/2Ti1/2)O3–BiFeO3 system epitaxially grown on SrRuO3//SrTiO3 substrates via chemical solution process

Lead (Pb)-free perovskite thin films of the ternary BaTiO3–Ba(Mg1/2Ti1/2)O3–BiFeO3 (BT–BMT–BF) solid solution system with preferential crystal orientation were fabricated and the dependence of their polarization behavior on crystal anisotropy was evaluated. The chemical solution deposition (CSD) technique was utilized for the film deposition while controlling the chemical composition. The films fabricated on single-crystal perovskite substrates of (100)SrRuO3//(100)SrTiO3 and (111)SrRuO3//(111)SrTiO3 exhibited preferential (100)pc and (111)pc orientations, respectively, depending on the crystallographic feature on the substrate surface. Excess Bi addition into the precursor solution to compensate for the volatile Bi component enhanced the polarization behavior of the resulting films heat-treated at 750 °C to induce crystallization. The (111)pc-oriented BT–BMT–BF film had a larger remanent polarization (Pr) of approximately 35 µC/cm2 than the (100)pc-oriented film (Pr = approximately 20 µC/cm2).

Thermal conductivity of ultrathin BaTiO3 films grown by plasma-assisted atomic layer deposition

We report the first measurements of the room temperature thermal conductivity of ultrathin (12 and 24 nm) barium titanate (BaTiO3) films prepared by plasma-enhanced atomic layer deposition (PEALD). The measured thermal conductivities of as-deposited films are relatively low as compared to values reported previously. We further investigate the effects of a post-deposition remote oxygen plasma treatment on the crystallinity and thermal conductivity of our PEALD BaTiO3 films. We find that the thermal conductivity slightly decreases with increasing duration of plasma treatment, which is most likely due to increased phonon scattering at interfaces between amorphous and crystalline phases within the films.

Sol-gel template synthesis of BaTiO3 films with nano-periodic structures

BaTiO3 films with novel nano-periodic structures were prepared by a sol-gel template synthesis. A through-hole type anodic aluminum oxide film was used as a template. The nano-periodic structure was drastically modified by applying a spray cleaning process during spin-coating of the template with a BaTiO3 sol. The formation mechanism was discussed from the viewpoint of liquid bridge force and surface tension applied during the drying process. The novel structures are promising for various functional devices, in which the nano-periodicity improves their performance.

Effect of surface charges on the polarization of BaTiO3 thin films investigated by UHV‐SPM

AbstractNanostructured BaTiO3 polar thin films are increasingly critical to the function of future multilayer ceramic capacitors and related oxide‐based electronic devices. The effect of surface charges on BaTiO3 polarization behavior is therefore investigated by ultra‐high vacuum scanning probe microscopy (UHV‐SPM) for 3 distinct morphologies—epitaxial, polycrystalline, and nanocrystalline films. Regardless of the film morphology, Kelvin probe force microscopy reveals that BaTiO3 thin film surfaces exhibit positive charging after contact scanning by various noble AFM probes due to the work function difference between tip and specimen. According to piezoresponse force microscopy, these positive charges uniformly stabilize downward polarized domains. However, the hysteresis and concomitant surface charging behavior are strongly sensitive to microstructure and defects. In particular, the stability and switching behavior are influenced by bulk and interfacial defect distributions and hence correlated to film deposition methods and grain size. Such morphology dependent properties for BaTiO3 films are revealed only through UHV measurements where screening charges from the ambient can be minimized, demonstrating the importance of UHV‐SPM for understanding ferroelectric thin films and nanostructures.

Two steps hydrothermal growth and characterisations of BaTiO3 films composed of nanowires

Barium titanate (BaTiO3) films composed of nanowires have gained considerable research interest due to their lead-free composition and strong energy conversion efficiency. BaTiO3 films can be developed with a simple two steps hydrothermal reactions, which are low cost effective. In this research, BaTiO3 films were fabricated on titanium foil through two steps hydrothermal method namely, the growth of TiO2 and followed by BaTiO3 films. The structural evolutions and the dielectric properties of the films were investigated as well. The structural evolutions of titanium dioxide (TiO2) and BaTiO3 nanowires were characterized using X-ray diffraction and scanning electron microscopy. First step of hydrothermal reaction, TiO2 nanowires were prepared in varied temperatures of 160 °C, 200 °C and 250 °C respectively. Second step of hydrothermal reaction was performed to produce a layer of BaTiO3 films.

Thin Film Coating with Highly Dispersible Barium Titanate-Polyvinylpyrrolidone Nanoparticles.

Thin BaTiO3 (BT) coating layers are required in various multilayer ceramic technologies, and fine nanosized BT particles with good dispersion in solution are essential for this coating process. In this work, cubic and tetragonal phase monodispersed BT nanoparticles—which were referred to as LBT and HBT-PVP coated on their surface by polyvinylpyrrolidone (PVP) polymer—were prepared by low temperature synthesis (LTS) and hydrothermal method (HT) at 80 and 230 °C, respectively. They were applied for the thin film coating on polyethylene terephthalate (PET) and Si wafer substrates by a simple bar coating. The thickness of BT, LBT-PVP, and HBT-PVP films prepared by their 5 wt % coating agent on Si are around 268, 308, and 263 nm, and their surface roughness are 104.6, 91.6, and 56.1 nm, respectively. The optical transmittance of BT, LBT-PVP, and HBT-PVP films on PET are 55, 66, and 73% at 550 nm wavelength and the haze values are 34.89, 24.70, and 20.53% respectively. The mechanism of dispersant adsorbed on the BT surface for densification of thin film during the drying process of the film was discussed.

Monolithic integration of patterned BaTiO3 thin films on Ge wafers

Titanates exhibit electronic properties highly desirable for field effect transistors such as very high permittivity and ferroelectricity. However, the difficulty of chemically etching titanates hinders their commercial use in device manufacturing. Here, the authors report the selective area in finestra growth of highly crystalline BaTiO3 (BTO) within photolithographically defined openings of a sacrificial SiO2 layer on a Ge (001) wafer by molecular beam epitaxy. After the BaTiO3 deposition, the sacrificial SiO2 can be etched away, revealing isolated nanoscale gate stacks circumventing the need to etch the titanate thin film. Reflection high-energy electron diffraction in conjunction with scanning electron microscopy is carried out to confirm the crystallinity of the samples. X-ray diffraction is performed to determine the out-of-plane lattice constant and crystal quality of the BTO film. Electrical measurements are performed on electrically isolated Pt/BaTiO3/SrTiO3/Ge capacitor devices.

Polarization retention in ultra-thin barium titanate films on Ge(001)

We investigate polarization retention in 10 to 19 nm thick ferroelectric BaTiO3 (BTO) grown on Ge(001) by molecular beam epitaxy. The out-of-plane direction and reversibility of electric polarization were confirmed using piezoresponse force microscopy. After reverse-poling selected regions of the BTO films to a value P with a biased atomic-force microscope tip, we monitored relaxation of their net polarization for as long as several weeks using optical second-harmonic generation microscopy. All films retained reversed polarization throughout the observation period. 10 nm-thick BTO films relaxed monotonically to a saturation value of 0.9 P after 27 days and 19 nm films to 0.75 P after 24 h. Polarization dynamics are discussed in the context of a 1D polarization relaxation/kinetics model.

Laser-synthesized nanocrystalline, ferroelectric, bioactive BaTiO3/Pt/FS for bone implants.

The goal of our study is to design BaTiO3 ferroelectric layers that will cover metal implants and provide improved osseointegration. We synthesized ferroelectric BaTiO3 layers on Pt/fused silica substrates, and we studied their physical and bio-properties. BaTiO3 and Pt layers were prepared using KrF excimer laser ablation at substrate temperature Ts in the range from 200°C to 750°C in vacuum or under oxygen pressure of 10 Pa, 15 Pa, and 20 Pa. The BaTiO3/Pt and Pt layers adhered well to the substrates. BaTiO3 films of crystallite size 60-140 nm were fabricated. Ferroelectric loops were measured and ferroelectricity was also confirmed using Raman scattering measurements. Results of atomic force microscopy topology and the X-ray diffraction structure of the BaTiO3/Pt/fused silica multilayers are presented. The adhesion, viability, growth, and osteogenic differentiation of human osteoblast-like Saos-2 cells were also studied. On days 1, 3, and 7 after seeding, the lowest cell numbers were found on non-ferroelectric BaTiO3, while the values on ferroelectric BaTiO3, on non-annealed and annealed Pt interlayers, and on the control tissue culture polystyrene dishes and microscopic glass slides were similar, and were usually significantly higher than on non-ferroelectric BaTiO3. A similar trend was observed for the intensity of the fluorescence of alkaline phosphatase, a medium-term marker of osteogenic differentiation, and of osteocalcin, a late marker of osteogenic differentiation. At the same time, the cell viability, tested on day 1 after seeding, was very high on all tested samples, reaching 93-99%. Ferroelectric BaTiO3 films deposited on metallic bone implants through a Pt interlayer can therefore markedly improve the osseointegration of these implants in comparison with non-ferroelectric BaTiO3 films.

Field‐Polarity‐Dependent Domain Growth in Epitaxial BaTiO3 Films

The growth of circular tip‐induced domains has been studied in epitaxial BaTiO3 (BTO) films as a function of writing voltage and time using piezoresponse force microscopy (PFM). While abundant for Pb(Zr,Ti)O3 (PZT) films, such studies are rare for BTO. Strong relaxation of written domains is observed in the form of reduction of the PFM amplitude inside the area of written domains which occurs additionally to a decrease of the domain radius. The domain wall velocity observed for negative tip voltage is systematically smaller than that for positive tip voltage. Based on maps of the positive and negative switching fields, the effective driving field for both polarities has been estimated. The polarity‐dependent effective field cannot account for the different velocities, indicating a polarity‐dependent mobility of the domain walls.

Multiphase nanodomains in a strained BaTiO3 film on a GdScO3 substrate

Controlling the crystal structure of ferroelectric materials via epitaxial strain, which is a well-known technique in strain engineering, can lead to the formation of unique domain structures generating non-intrinsic phenomena such as electronic conductivity, photovoltages, and enhanced piezoelectric characteristics. Strained BaTiO3 films are promising ferroelectric materials as theoretical modeling predicts that different domain morphologies can introduce additional properties not observed in conventional BaTiO3 ceramics. To rationally design materials for practical application, a thorough understanding of the formation mechanisms and stabilities of different domain structures in strained BaTiO3 films is required. However, there have been very few experimental reports on this topic, and details about the domain structures in strained BaTiO3 films are currently lacking. In this paper, we report multiphase nanodomains in a strained BaTiO3 film deposited on an orthorhombic GdScO3 substrate. The phase-transition behavior of the strained BaTiO3 film reveals that it contains multiple phases at room temperature; the film first undergoes a phase-transition upon heating at around 550 K, and then a paraelectric phase forms at temperatures above 690 K. A picometer-scale analysis of the Ti ion displacements, using an advanced scanning transmission electron microscopy technique, is used to characterize the complex multiphase nanodomains, providing useful insights into the control of domain structures in BaTiO3 films by applying epitaxial strain.

Significant role of antiferromagnetic GdFeO3 on multiferroism of bilayer thin films

Inversion of BaTiO3 and GdFeO3 thin films in bilayer configuration has been deposited by pulsed laser deposition technique. A significant effect of strain on thin film has been observed by X-ray diffraction analysis. Tensile strain of 1.04% and 0.23% has been calculated by X-ray diffraction results. Higher polarization value 70.4 μC cm−2 has been observed by strained BaTiO3 film in GdFeO3/BaTiO3 bilayer film. Strained GdFeO3 film in BaTiO3/GdFeO3 bilayer configuration exhibited ferromagnetic behaviour showed maximum magnetization value of 50 emu gm−1. Magnetoelectric coupling coefficient of bilayer films have been carried out by dynamic method. Room temperature magnetoelectric coupling 2500 mV cm−1-Oe has been obtained for BaTiO3/GdFeO3 bilayer film. The high ME coupling of the BaTiO3/GdFeO3 bilayer film reveals strong interfacial coupling between ferroelectric and ferromagnetic dipoles. On magnetoelectric coupling coefficient effect of ferromagnetic GdFeO3 layer has a significant role. Such high value of ME coupling may be useful in realization of magnetoelectric RAM (MeRAM) application.

Flexocaloric response of epitaxial ferroelectric films

The flexoelectric effect in dielectric materials generates an electric polarization as a result of strain gradient. Here, we show that the flexoelectric response also produces a flexocaloric adiabatic temperature variation in heteroepitaxial ferroelectric films that are either partially or completely relaxed. The flexocaloric temperature change of (001) BaTiO3 films on (001) SrTiO3 substrates is computed as a function of film thickness and temperature. Our calculations predict that a built-in flexocaloric temperature change of 0.61 °C can be realized in 20 nm thick epitaxial BaTiO3 films when compared with the intrinsic electrocaloric response of 0.75 °C for bulk, single-crystal BaTiO3 at 25 °C and applied electric field of 200 kV/cm. This demonstrates that the flexocaloric response can supplement electrocaloric temperature changes in thin film ferroelectrics and may play an important role in potential applications in on-chip solid-state cooling.