BaTiO3 Films Research Articles

High Power Density Pyroelectric Energy Conversion in Nanometer-Thick BaTiO3 Films

ABSTRACTSolid-state pyroelectric nanomaterials can be used for thermal-to-electrical energy conversion in the presence of temperature fluctuations. This article reports investigation of energy conversion in a 200 nm thick BaTiO3 film using the pyroelectric Ericsson cycle at cycle frequencies up to 3 kHz. The high cycle frequencies were achieved due to the low thermal mass of the nanometer-scale film, unlike previous studies in which the electrical power output was limited by the rate of heat transfer through the pyroelectric material. A microfabricated platform that allowed precise thermal and electrical cycling enabled us to study the effect of electric field range, temperature oscillation amplitude, and cycle frequency on the electrical power output from pyroelectric Ericsson cycles. We measured a maximum power density of 30 W/cm3 for a temperature range 20–120°C and electric field range 100–125 kV/cm, which represents a significant improvement over past work on pyroelectric cycles. The approach presented in this article could lead to high-power waste heat harvesting in systems with high-frequency temperature oscillations.

Orientation control of barium titanate films using metal oxide nanosheet layer

In the present work, we aim to achieve the preferred crystal orientation of chemical solution deposition (CSD)-derived BaTiO3 films on ubiquitous Si wafers with the assistance of Ca2Nb3O10 nanosheet (ns-CN) template layers. The ns-CN on platinized Si (Pt/Si) substrates aligned the BaTiO3(100) plane to the substrate surface, because of the favorable lattice matching of the ns-CN (001) plane. The CSD process in air required a high crystallization temperature of 900 °C for the preferred crystal orientation of BaTiO3(100) because of the BaCO3 byproduct generated during the combustion reaction of the precursor gel. The processing in vacuum to remove CO2 species enhanced the crystal orientation even at the crystallization temperature of 800 °C, although it can generate oxygen vacancies () that cause distorted polarization behavior under an applied field higher than approximately 150 kV/cm. The relative dielectric constant (εr) of the (100)-oriented BaTiO3 film on the ns-CN-supported Pt/Si substrate (ns-CN/Pt/Si) was generally larger than that of the randomly oriented film on Pt/Si, depending on the degree of crystal orientation.

High ferroelectric polarization in c-oriented BaTiO3 epitaxial thin films on SrTiO3/Si(001)

The integration of epitaxial BaTiO3 films on silicon, combining c-orientation, surface flatness, and high ferroelectric polarization is of main interest towards its use in memory devices. This combination of properties has been only achieved so far by using yttria-stabilized zirconia buffer layers. Here, the all-perovskite BaTiO3/LaNiO3/SrTiO3 heterostructure is grown monolithically on Si(001). The BaTiO3 films are epitaxial and c-oriented and present low surface roughness and high remnant ferroelectric polarization around 6 μC/cm2. This result paves the way towards the fabrication of lead-free BaTiO3 ferroelectric memories on silicon platforms.

Effects of substrate materials on piezoelectric properties of BaTiO3 thick films deposited by aerosol deposition

Piezoelectric properties were evaluated for annealed BaTiO3 (BT) films formed by aerosol deposition on yttria-stabilized zirconia (YSZ) and Fe–Cr–Al-based heat-resistant stainless steel (SS). The piezoelectric constants d31 of BT films annealed at 1200 °C formed on YSZ and SS were −71 and −41 pm/V, respectively. The effects of different substrates on piezoelectric properties were investigated. The grain sizes of the films formed on YSZ and SS were 1.5 and 1.0 µm, respectively. X-ray diffraction analysis using a two-dimensional stress method revealed that the respective residual stresses of the films formed on YSZ and SS were −55 ± 8 and −32 ± 7 MPa, respectively, as compressive stresses. The c-domain structure was formed preferentially in the films on SS because of its larger compressive stress. These results suggest that differences in piezoelectric properties attributable to substrates result from differences in compressive stress magnitude and the volume fraction between the c- and a-domains.

PLD prepared bioactive BaTiO3 films on TiNb implants

BaTiO3 (BTO) layers were deposited by pulsed laser deposition (PLD) on TiNb, Pt/TiNb, Si (100), and fused silica substrates using various deposition conditions. Polycrystalline BTO with sizes of crystallites in the range from 90nm to 160nm was obtained at elevated substrate temperatures of (600°C–700°C). With increasing deposition temperature above 700°C the formation of unwanted rutile phase prevented the growth of perovskite ferroelectric BTO. Concurrently, with decreasing substrate temperature below 500°C, amorphous films were formed. Post-deposition annealing of the amorphous deposits allowed obtaining perovskite BTO. Using a very thin Pt interlayer between the BTO films and TiNb substrate enabled high-temperature growth of preferentially oriented BTO. Raman spectroscopy and electrical characterization indicated polar ferroelectric behaviour of the BTO films.

Crystal structure and polarization hysteresis properties of ferroelectric BaTiO3 thin-film capacitors on (Ba,Sr)TiO3-buffered substrates

Ferroelectric BaTiO3 (BT) thin-film capacitors with a buffer layer of (Ba1−xSrx)TiO3 (BST) have been fabricated on (001) SrTiO3 (STO) single-crystal substrates by a pulsed laser deposition method, and the crystal structure and polarization hysteresis properties have been investigated. X-ray diffraction reciprocal space mapping shows that the BST buffer effectively reduces the misfit strain relaxation of the BT films on SrRuO3 (SRO) electrodes. The BT capacitor with the SRO electrodes on the BST (x = 0.3) buffer exhibits a well-saturated hysteresis loop with a remanent polarization of 29 µC/cm2. The hysteresis loop displays a shift toward a specific field direction, which is suggested to stem from the flexoelectric coupling between the out-of-plane polarization and the strain gradient adjacent to the bottom interface.

Conductive paths through polycrystalline BaTiO3: Scanning probe microscopy study

The microstructural features determining the leakage current through polycrystalline BaTiO3 films are investigated using Conductive Atomic Force Microscopy. Grain boundaries are found to be the dominant conductive paths compared to the conduction through the grains. Grain boundary currents are observed to reversibly rise with the increase of the applied DC voltages, indicating that the current is controlled by a field-activated charge transport process.

Conformal BaTiO3 Films with High Piezoelectric Coupling through an Optimized Hydrothermal Synthesis.

Two-dimensional (2D) ferroelectric films have vast applications due to their dielectric, ferroelectric, and piezoelectric properties that meet the requirements of sensors, nonvolatile ferroelectric random access memory (NVFeRAM) devices, and micro-electromechanical systems (MEMS). However, the small surface area of these 2D ferroelectric films has limited their ability to achieve higher memory storage density in NVFeRAM devices and more sensitive sensors and transducer. Thus, conformally deposited ferroelectric films have been actively studied for these applications in order to create three-dimensional (3D) structures, which lead to a larger surface area. Most of the current methods developed for the conformal deposition of ferroelectric films, such as metal-organic chemical vapor deposition (MOCVD) and plasma-enhanced vapor deposition (PECVD), are limited by high temperatures and unstable and toxic organic precursors. In this paper, an innovative fabrication method for barium titanate (BaTiO3) textured films with 3D architectures is introduced to alleviate these issues. This fabrication method is based on converting conformally grown rutile TiO2 nanowire arrays into BaTiO3 textured films using a simple two-step hydrothermal process which allows for thickness-controlled growth of conformal films on patterned silicon wafers coated with fluorine-doped tin oxide (FTO). Moreover, the processing parameters have been optimized to achieve a high piezoelectric coupling coefficient of 100 pm/V. This high piezoelectric response along with high relative dielectric constant (εr = 1600) of the conformally grown textured BaTiO3 films demonstrates their potential application in sensors, NVFeRAM, and MEMS.

In situ preparation of high quality BaTiO3 dielectric films on Si at 350–500 °C

Reducing thermal budget of functional layers grown on Si substrates has become a necessity for their integration into contemporary manufacturing technology of microelectronics. The work presented here exemplifies one of such efforts by preparing barium titanate (BaTiO3 or BTO) films at temperatures as low as 350 °C on Si substrates via a CMOS-compatible RF-magnetron sputtering process. In this study, X-ray diffraction (XRD) results reveal that use of LaNiO3 (LNO) buffer layer successfully induces BaTiO3 film’s transition from polycrystalline to highly c-axis oriented tetragonal in low temperature range. Moreover, encouraged by BaTiO3 films prepared at 500 °C that shows a nearly uniform (00l) orientation with excellent ferroelectric properties (Pr ~ 2.6 μC/cm2, Ec ~ 100 kV/cm, d33 ~ 150 pm/V), we further push the deposition temperature down to 350 °C. While showing reduced crystallinity, these lower temperature films still possess good dielectric properties which are characterized by a stable dielectric constant of 110 ± 5 and a small dielectric loss between 0.7 and 3 % in the frequency range of [1 kHz, 2 MHz]. We believe the finding of high quality BaTiO3 films achievable at 350 °C is meaningful in that it paves the road for BaTiO3’s real application in Si based CMOS technology.

Growth and optical properties of FePt:BaTiO3 nanocomposite films

FePt:BaTiO3 nanocomposite films consisting of a regular stacking of pure BaTiO3 (BTO) layers and BTO layers embedding FePt nanocrystals were grown on (001) MgO substrates by pulsed laser deposition. Transmission electron microscopy experiments on cross-sectional specimens demonstrated the high quality of the epitaxial growth of the different layers, whatever their composition. The structural analysis also gave evidence of a misfit strain relaxation at the BTO/MgO interface. We report and discuss the optical and pholuminescence properties of the nanocomposite films in comparison with the ones recorded in pure BTO films. Our results emphasize the role of nanostructuration and growth processes on the evolution of the observed properties.

Optical properties of Fe-doped BaTiO3 films deposited on quartz substrates by sol-gel method

Fe-doped BaTiO3 films were prepared on quartz substrates by sol-gel route. Linearly optical constants of the films were obtained by fitting transmittance spectra with Adachi’s model. With increasing Fe composition, refractive index n at 532 nm increases, single oscillator energy Ec and dispersion energy Ed increase, but optical band gap Eg decreases from 3.78 to 3.63 eV. In addition, nonlinear optical properties of the Fe-doped BaTiO3 films were analyzed by Z-scan method. Nonlinear optical responses of the BaTiO3 films were enhanced by Fe doping. For BaTi0.96Fe0.04O3 film, the third-order nonlinear refractive index coefficient γ, the nonlinear refractive index coefficient n2, the nonlinear absorption coefficient β and the third-order nonlinear susceptibility χ(3) are 1.98 × 10−13 m2/W, 1.03 × 10−6 esu, 1.33 × 10−7 m/W, 2.38 × 10−7 esu respectively. The nonlinearity is predominated by the nonlinear refractive behavior. These results suggest that Fe doping may tune the linear and nonlinear optical properties of BaTiO3 films, which indicates that Fe-doped BaTiO3 films are promising materials for applications in optical devices.

Structure Dependent Room Temperature Ferromagnetism In Co, Nb Co-doped BaTiO3 Thin Films Prepared By RF Sputtering

Thin films of BaTiO3 and Co, Nb co-doped BaTiO3 on glass and Si (100) substrates were deposited by RF sputtering (at 350 oC), and annealed. The amorphous and crystalline phases were observed for the as-deposited and annealed samples, respectively from the X-ray diffraction (XRD) studies. The magnetic behaviour of the pure and doped BaTiO3 films was studied by vibrating sample magnetometry (VSM). In this study, the ferromagnetic behaviour at room temperature was observed in the Co, Nb co-doped BaTiO3 of both amorphous and crystalline films. The annealed polycrystalline Co, Nb co-doped BaTiO3 films have the larger saturation magnetization and coercivity than the amorphous films. The room temperature ferromagnetic responses were also observed by the Magneto-optical Kerr effect (MOKE) measurements for both as-deposited and annealed samples. Copyright © 2016 VBRI Press.

Material Integration for Flexible Electronics for Energy Application Using Nanoparticulated Dense and Stress-Free Ceramic Thick Film

Material Integration for Flexible Electronics for Energy Application Using Nanoparticulated Dense and Stress-Free Ceramic Thick Film

Electro-Optical Modulation Based on Pockels Effect in BaTiO3With a Multi-Domain Structure

The influence of an in-plane multi-domain structure in BaTiO3 films grown on SrTiO3/Si buffers for highly efficient electro-optic modulation has been analyzed. The modulation performance can be significantly enhanced by rotating a certain angle, the optical waveguide, with respect to the BaTiO3 crystallographic axes. A robust electro-optical performance against variations in the domain structure as well as the lowest $V_{\pi }$ voltage can be achieved by using the rotation angles between 35° and 55°. Our calculations show that $V_{\pi }$ voltages below 1.7 V for a modulation length of 2 mm can be obtained by means of a CMOS compatible hybrid silicon/BaTiO3 waveguide structure.

Dielectric properties of percolative BaTiO3/Ni composite film fabricated by aerosol deposition process

In this paper, the BaTiO3/Ni composite films of 10 μm were fabricated by Aerosol deposition. We investigate the dielectric behavior of BaTiO3/Ni composite films with various Ni contents. The BaTiO3/Ni composite film with Ni content of 9.1 vol% has high dielectric constant at 1 kHz (476) due to the percolation effect. This high dielectric constant was attributed to the microcapacitor structures and Maxwell–Wagner polarization. Although the BaTiO3/Ni composite film was fabricated at room temperature, it had a high dielectric constant of 8 times higher than that of raw BaTiO3 film, and showed a very low leakage current. Therefore, BaTiO3/Ni composite films fabricated by Aerosol deposition can potentially be used as embedded capacitors.

BaTiO3 Film Grown by Water-Based Process

Self-assembled nano-crystalline BaTiO3 films on stainless steel foil substrates, were grown by the water based Streaming Process for Electrodeless Electrochemical Deposition (SPEED). SPEED is an aqueous process that deposits self-assembled nano-crystalline inorganic thin films over large areas, without a vacuum, providing a scalable and manufacturing friendly process to fabricate durable films. The morphology of the ~1 µm thick films comprises single crystals of micron dimensions imbedded in a matrix of nanocrystals. XRD confirms presence of BaTiO3 crystals of hexagonal phase for samples annealed at 500 C. Subsequent annealing at 600 C transforms the film to the cubic phase. Potential applications include dielectric layers, capacitors, waveguides, ferroelectric RAM, pyroelectric infrared detectors, and phosphors. Characterization of infrared pyroelectric response at 10 µm wavelength shows an initially good sensitivity that reversibly decays over a period of days due to water vapor absorption. A short-lived photo-response due to poling of the hydrated sample is also observed.

Dead layer effect and its elimination in ferroelectric thin film with oxide electrodes

Interfacial dead layer effect has been widely noticed in the past and was thought to be responsible for the critical thickness of ferroelectric thin film. Despite extensive studies, the origin is still under fierce debate. The dead layer even exists at the perfect interface without defects and impurities. In this paper, we studied the effects of the electrode/ferroelectric interface on the polarization properties of nano-scale BaTiO3 ferroelectric capacitors by first-principle calculation. A thin layer with reversed polarization is found in the TiO2-teminated LaNiO3/BaTiO3/LaNiO3 capacitor. This pinned domain with reversed polarization at the top interface of ferroelectric film acts as a dead layer and reduces the total polarization. Based on our analyses, this reversed polarization is argued to originate from the intrinsic polarization instability near the top interface of TiO2-teminated ferroelectric thin film and an interfacial electrical field. An interface modification method has been adopted to remove such dead layer effects. Our results show that a LaXO3 (X = Fe, Co) or YNiO3 (Y = Sr, Ba) buffer layer can effectively remove the dead layer effect in BaTiO3 film.

Diamagnetism to ferromagnetism in Sr-substituted epitaxial BaTiO3 thin films

We report on the ferromagnetic-like behavior in otherwise diamagnetic BaTiO3 (BTO) thin films upon doping with non-magnetic element Sr having the composition Ba0.4Sr0.6TiO3 (BST). The epitaxial integration of BST (∼800 nm) thick films on Si (100) substrate was achieved using MgO (40 nm) and TiN (20 nm) as buffer layers to prepare BST/MgO/TiN/Si (100) heterostructure by pulsed laser deposition. The c-axis oriented and cube-on-cube epitaxial BST is formed on Si (100) as evidenced by the in-plane and out-of-plane X-ray diffraction. All the deposited films are relaxed through domain matching epitaxy paradigm as observed from X-ray diffraction pattern and A1TO3 mode (at 521.27 cm−1) of Raman spectra. As-deposited BST thin films reveal ferromagnetic-like properties, which persist up to 400 K. The magnetization decreases two-fold upon oxygen annealing. In contrast, as-deposited un-doped BTO films show diamagnetism. Electron spin resonance measurements reveal no evidence of external magnetic impurities. XRD and X-ray photoelectron spectroscopy spectra show significant changes influenced by Sr doping in BTO. The ferromagnetic-like behavior in BST could be due to the trapped electron donors from oxygen vacancies resulting from Sr-doping.

BaTiO3 Film Grown by Water-Based Process

Self-assembled nano-crystalline BaTiO3 films on stainless steel foil substrates, were grown by the water based Streaming Process for Electrodeless Electrochemical Deposition (SPEED). SPEED is an aqueous process that deposits self-assembled nanomaterial inorganic thin films over large areas, without a vacuum, providing a scalable and manufacturing friendly process to fabricate durable films. Water-soluble compounds with complexing agents grow films by heterogeneous reaction on the substrate, with little wasteful homogeneous reaction. Hydrophilic substrates bind hydroxyl ions (OH–), which are attachment sites for nucleation with density exceeding 1012 per square centimeter. The water based precursor, nebulized into 10 to 20 μm droplets (vapor phase SPEED, or VPSPEED), impinges on the substrate, which is at temperatures of ~300 ℃, giving growth rate exceeding 200 nm per minute. The substrate heating provides the reaction activation energy, and decomposes/volatilizes reaction byproducts, but the required temperatures are well below those required for spray pyrolysis. All films were subsequently annealed at 500 C for 1 hour. The morphology of the ~1 mm thick films comprises single crystals of micron dimensions imbedded in a matrix of nanocrystals. XRD confirms presence of BaTiO3 crystals of hexagonal phase. Further annealing at 500 oC increases the hexagonal peak intensity. Subsequent annealing at 600 oC for 1 hour causes a substantial transformation to the cubic phase. Potential applications include dielectric layers, capacitors, waveguides, ferroelectric RAM, and pyroelectric infrared detectors and phosphors. Characterization of pyroelectric properties and application to infrared detection will be presented.

Nanoparticulate BaTiO3 film produced by aerosol-type nanoparticle deposition

Aerosol-type nanoparticle deposition (NPD) is a magical method to form a dense electroceramic film with a fine, nanoscale structure on a substrate surface by depositing ceramic particles through a nozzle at room temperature. This film has the potential to be applied to various electronic, environmental, and energy devices. However, the deposition mechanism and the nanostructure in the film are not understood sufficiently. This study aimed at investigating the crystal structure of an NPD as-deposited film, and compared the crystal structures of the NPD as-deposited film, annealed film, and the raw powders consisting of particles with diameters of 200 and 50 nm, respectively, using barium titanium oxide (BaTiO3). We found that the crystal in BaTiO3 with a disordered phase due to the Ba displacement within the BaTiO3 was responsible for the adhesion between the BaTiO3 crystalline particles having a diameter of approximately 10 nm, as well as with the substrate.