TiO3 Films Research Articles

Hydrothermal growth of c-axis oriented ferroelectric (Bi1/2K1/2)TiO3 films on metal substrates

Ferroelectric films fabricated on metal substrates are desirable for use in piezoelectric vibration energy harvesters. In this report, we demonstrate that dense films of lead-free ferroelectric (Bi1/2K1/2)TiO3 (BKT) with preferred c-axis orientation can be grown on nickel-based alloy substrates by utilizing the hydrothermal synthesis method. A buffer layer of LaNiO3 (LN) was formed on the substrates by the sol-gel spin coating method, and then BKT was grown by a hydrothermal reaction at a low temperature of 150 °C. The surface structure and thickness of the final BKT films were found to strongly depend on the numbers of the deposition cycles for LN and BKT, respectively. As a result, highly c-axis oriented BKT films with a smooth surface and a thickness up to 830 nm were obtained. Although the as-deposited BKT films were suggested to contain hydroxyl groups in their crystal lattice, a post-annealing process at 650 °C in air was found to be effective to remove them. The polarization response of the annealed BKT film proved the ferroelectric polarization switching in the film.

Combinatorial study of the phase development of sputtered Pb(Zr,Ti)O3 films

For a comprehensive study of the growth mechanisms of Pb(Zr,Ti)O3 (PZT), we employed a combinatorial sputtering method that enabled us to fabricate films with composition and substrate temperature gradients in-plane. The gradient of the amount of Pb supply was created using PZT ceramic and Pb3O4 powder targets. The substrate temperature gradient was generated in an orthogonal direction by inserting a shadow mask between the heater and substrate. The PZT film was deposited on uniformly deposited (001)LaNiO3/Si in a substrate temperature range of 490 °C–570 °C. It was determined that although the growth condition of PZT films has a relatively large process window, the orientation is easily changed by the temperature and the amount of Pb supply. In addition, the range of the growth temperature, where the films show ferroelectricity decreases with decreasing the amount of Pb supply.

Crystal structure, ferroelectric and piezoelectric properties of epitaxial (1−x)(Bi0.5Na0.5)TiO3–x(Bi0.5K0.5)TiO3 films grown by hydrothermal method

The composition dependencies of crystal structure, and electrical and electromechanical properties were systematically investigated for epitaxial films of (1−x)(Bi0.5Na0.5)TiO3–x(Bi0.5K0.5)TiO3 [(1−x)BNT–xBKT (x = 0 –1)] deposited on (100)cSrRuO3//(100)SrTiO3 substrates by a hydrothermal method. An In-plane crystal lattice was clamped by that of underlying SrRuO3 irrespective of the x value. The dielectric constant decreased monotonously with increasing x. The polarization–electric field hysteresis loops were imprinted due to the preferential polarization in the films. In addition, the increase of tetragonality with increasing x value contributes to the increase of saturation and remnant polarization (Psat. and Pr, respectively), coercive field (Ec) as well as Pr/Psat. values. The effective piezoelectric coefficient (near E = 0 kV cm−1) improved considerably with increasing x, reaching a maximum of 62 pm V−1 in the BKT film. A comprehensive and systematic study has been performed on (1−x)BNT–xBKT epitaxial films in this work, which lays the foundation for further research of this system in the future.

Electrophysical Properties Investigation of Ba0.8Sr0.2TiO3 Ferroelectric Films in Paraelectric State

Phenomenological analytical expressions relating the ferroelectric film capacity to the coefficients of free energy decomposition in accordance to the degrees of substance polarization in paraelectric state are obtained. High-frequency capacitance-voltage characteristics of metal–Ba0.8Sr0.2TiO3–metal structures have been measured at the temperatures above the transition point from ferroelectric phase to paraelectric one. The comparison of theoretical and experimental data has been resulted in plotting the dependences of Ba0.8Sr0.2TiO3 film polarization on external voltage and determining the values of the parameters of the theory of phase transitions of second kind of Ginsburg–Landau for the objects under investigation.

Effects of electrolyte on micro-structure and properties of BaxSr(1−x)TiO3 films prepared by micro-arc oxidation

BaxSr(1−x)TiO3(BST) films are deposited on titanium substrate by micro-arc oxidation (MAO) technique under three electrolytes. The micro-structure and electrical properties of the BST thin films are analyzed and compared. The results show that the BST films obtained with these three electrolytes are all mainly composed of tetragonal BaxSr(1−x)TiO3. Compared with electrolyte without additive, both PVP30 and EDTA, as additive, significantly inhibit the growth of BST films, and the BST films become flat and dense. Moreover, the dielectric and ferroelectric properties of BST thin films are positively correlated with their micro-structure, and flat and dense BST film possesses good dielectric and ferroelectric properties. The residual polarization intensity of BST film prepared with the electrolyte involving PVP30 or EDTA is about 18 and 13 times of that of BST film prepared without additive, respectively.

Ferroelectric and photoluminescent properties of Eu3+-doped Bi4Ti3O12 films prepared via the spin-coating method

In this study, the films of Bi4−xEuxTi3O12 (x = 0, 0.2, 0.4, 0.6, 0.8) were prepared on substrates (Pt(111)/Ti/SiO2/Si(100)) by the spin-coating method. The results indicate that the films of Bi4−xEuxTi3O12 annealed at 700 °C consist of Bi4Ti3O12 and Pt from the substrates. This indicates that Eu3+ has dissolved into the lattice of Bi4Ti3O12. The doping of Eu3+ to the films of Bi4Ti3O12 enhances ferroelectric properties as well as photoluminescent properties. The films of Bi4−xEuxTi3O12 show two orange emission perks at 590 nm and 595 nm, and a red emission at 613 nm. These three peaks are ascribed to 5D0 → 7F0, 5D0 → 7F1, and 5D0 → 7F2, respectively. The bandgaps of the films of Bi4−xEuxTi3O12 range from 3.07 eV to 2.86 eV with the increase of the doping concentration of Eu3+. Bi3.4Eu0.6Ti3O12 film possesses maximal remanent polarization, minimal leakage current density, and greatest emission intensity.

Oxygen diffusion barriers for epitaxial thin-film heterostructures with highly conducting SrMoO3 electrodes

Transition metal perovskite oxide SrMoO3 with a Mo4+ 4d2 electronic configuration exhibits a room-temperature resistivity of 5.1 μΩcm in a single-crystal form and, therefore, is considered a prominent conducting electrode material for all-oxide microelectronic devices. Stabilization of the unfavorable Mo4+ valence state in SrMoO3 thin films necessitates reductive growth conditions that are often incompatible with a highly oxidative environment necessary to grow epitaxial heterostructures with fully oxygenated functional layers (e.g., tunable dielectric BaxSr1−xTiO3). Interestingly, only a few unit cells of the perovskite titanate capping layers SrTiO3, BaTiO3, and Ba0.5Sr0.5TiO3 act as an efficient oxygen barrier and minimize SrMoO3 oxidation into electrically insulating SrMoO4 in the broad range of the thin-film growth parameters. The Mo valence state in SrMoO3, determined by x-ray photoelectron spectroscopy, is used to analyze oxygen diffusion through the capping layers. The lowest level of oxygen diffusion is observed in Ba0.5Sr0.5TiO3. A Ba0.5Sr0.5TiO3 film with a thickness of only 6 unit cells preserves the Mo4+ oxidation state in the SrMoO3 underlayer up to the oxygen partial pressure of 8 mTorr at the temperature of 630 °C. Results, therefore, indicate that SrMoO3 films covered with atomically thin Ba0.5Sr0.5TiO3 remain conducting in an oxygen environment and can be integrated into all-oxide thin-film heterostructures with other functional materials.

Phenomenological modeling of anisotropic dielectric properties in epitaxial (Pb, Sr)TiO3 thin films

Strain tuning of polarization states and dielectric properties in (001) epitaxial (Pb, Sr)TiO3 (PST) films is investigated using a thermodynamic phenomenological model. We find that our calculations of anisotropic dielectric properties and their electric field tunability at various in-plane strained states are in good agreement with relevant experimental data for epitaxial Pb0.35Sr0.65TiO3 films when material constants are properly determined. Our modeling further suggests that dielectric tunability can be optimized by combined control of epitaxial strain and PST composition.

MODELING OF THE CHEMICAL X-PARAMETER IN PSEUDOBINARY ALLOYS FROM THE SHIFT OF THE X-RAY BRAGG ANGLE: APPLICATION TO CUBIC BaxSr1-xTiO3 FILMS

Pseudobinary alloys of the type AxB1-xC are expected to obey the Vegard’s law that states a linear relationship between the chemical parameter x and the lattice parameter. However, in many cases, deviations from the Vegard’s are found, and the calculation of x requires compositional characterization, often complicated in the presence of secondary phases of the same material. In the present work, 𝐵𝑎𝑥𝑆𝑟(1−𝑥)𝑇𝑖𝑂3 (BSTO) films were deposited by RF co-sputtering from 𝐵𝑎𝑇𝑖𝑂3 and 𝑆𝑟𝑇𝑖𝑂3 targets to obtain differently 𝐵𝑎⁄𝑆𝑟 ratios considering the different RF-power applied to each target. We describe a mathematical model based on the Boltzmann equation to calculate the 𝑥 parameter of the cubic phase of the BSTO films in the 0 ≤ 𝑥 ≤ 1 range. That was made by following the 2𝜃 diffraction angular shift of the plane (111) upon the increase of substitutional Ba2+ in the pseudobinary alloy, as a function of the RF applied power and fitting the interplanar distances and lattice parameter in the alloys were calculated onto the fitted sigmoid trajectory. The model showed the region where the lattice parameter a vs x obeys the Vegard’s law and correctly fitted “a” in the entire 0<x<1 range. The model can be applicable to other pseudobinary systems with deviations from the Vegard’s law, from a simple X-ray diffraction analysis, providing fast chemical information, complementary with further compositional characterization.

Application of Ba0.5Sr0.5TiO3 (Bst) Film Doped with 0%, 2%, 4% and 6% Concentrations of RuO2 as an Arduino Nano-Based Bad Breath Sensor

Ba0.5Sr0.5TiO3 (BST) film doped with variations in RuO2 concentration (0%, 2%, 4%, and 6%) has been successfully grown on a type-p silicon substrate (100) using the chemical solution deposition (CSD) method and spin-coating at a speed of 3000 rpm for 30 s. The film on the substrate was then heated at 850 °C for 15 h. The sensitivity of BST film + RuO2 variations as a gas sensor were characterized. The sensitivity characterization was assisted by various electronic circuitry with the purpose of producing a sensor that is very sensitive to gas. The responses from the BST film + RuO2 variation were varied, depending on the concentration of the RuO2 dope. BST film doped with 6% RuO2 had a very good response to halitosis gases; therefore, this film was applied as the Arduino-Nano-based bad-breath detecting sensor. Before it was integrated with the microcontroller, the voltage output of the BST film was amplified using an op-amp circuit to make the voltage output from the BST film readable to the microcontroller. The changes in the voltage response were then shown on the prototype display. If the voltage output was ≤12.9 mV, the display would read “bad breath”. If the voltage output >42.1 mV, the display would read “fragrant”. If 12.9 mV < voltage output ≤ 42.1 mV, the display would read “normal”.

Strain effects on conductivity and charge transport in La-doped BaTiO3 thin films

We investigate how epitaxial strain affects the conductivity and charge transport of Ba0.6La0.4TiO3 (BLTO) films deposited by pulsed laser deposition. Depositing BLTO films with thicknesses varying from 14.1 to 95.9 nm gives differing epitaxial strains, with a maximum strain up to ~6% for the thinnest film of 14.1 nm. Transport measurements demonstrate that film thickness (i.e. epitaxial strain) affects the conductivity, carrier mobility, and concentration. Measurements of resistivity versus temperature demonstrate that all the BLTO films undergo a distinct semiconductor-metal phase transition, and the transition temperature TSM also depends clearly on strain. For all the films, the charge transport follows the small-polaron hopping mechanism below TSM and the thermal phonon scattering mechanism above TSM. These results prove that strain in epitaxial films strongly affects the conductivity and charge transport of traditional insulating ferroelectric oxide films.

A Highly Conductive Layer at an Interface between Ferroelectric BaSrTiO3 and Ferromagnetic LaMnO3

Properties of the Ba0.8Sr0.2TiO3/LaMnO3 heterostructure are investigated for when LaMnO3 and Ba0.8Sr0.2TiO3 are deposited on a surface SrTiO3. No state of high conductivity is observed. The effect a magnetic field has on the temperature behavior of resistivity is determined for samples of the Ba0.8Sr0.2TiO3/LaMnO3 heterostructure in its “normal” architecture when a Ba0.8Sr0.2TiO3 film is deposited on a LaMnO3 single crystal.

Output Power of Piezoelectric MEMS Vibration Energy Harvesters Under Random Oscillation

The output characteristics of MEMS piezoelectric vibration energy harvesters (PVEHs) under random oscillations are analysed. We fabricated cantilever-type MEMS-PVEHs using Pb(Zr,Ti)O3 films. The autocorrelation function of the transient displacement of the cantilever tip under random oscillations features a narrow-band random vibration. From the power spectral density (PSD) of the output voltage of the PVEHs, the resonance frequency decreases and the full-width at half-maximum increases with increasing vibration acceleration. By comparing output properties under various sinusoidal oscillations, nonlinear effects including the soft-spring effect and nonlinear damping effect clearly influence the output characteristics under random oscillations. The power generation is proportional to the square of the vibration acceleration even in the acceleration region where nonlinear effects become conspicuous.

Dual interfacial modification via anodizing method for achieving enhanced breakdown strength in multi-layer anodized alumina/Sr0.85Bi0.1TiO3 films

Prepared with two layers of anodized alumina (Al2O3), a novel structure of multi-layer anodized alumina/Sr0.85Bi0.1TiO3 (AO/SBT) dielectric capacitor has been designed and fabricated to realize significant improvement in energy density. The first Al2O3 layer (AO1) was artificially introduced through anodic oxidation of aluminum layer in electrolyte, while the second Al2O3 layer (AO2) was automatically generated during the I–V testing process. With ultrahigh breakdown strength, the embedded Al2O3 layers could be taken as the patch to repair the micro-holes located at the interface. In addition, they could be used as barrier layers to dramatically reduce the leakage current. More importantly, it was found that the different location of the AO1 layer has enormous impact on the ultimate breakdown strength. Based on finite element analysis, the simulation results showed the electric field was mainly concentrated on the Al2O3 layer, which could effectively reduce the breakdown probability of SBT films. By dual interfacial modification, the multi-layer thin films with an optimal structure show a remarkable enhancement in breakdown strength from 134.09 to 534.72 MV m−1. As a result, the maximum energy density of 29.71 J cm−3 has been achieved, which represents a ∼741% increase in comparison to the samples without dual interfacial modification. The unique dual interfacial modification technology employed in this work offers a new approach to increase the energy storage density of dielectric capacitors from the perspective of structural design.

Enhanced dielectric performance of BaxSr(1-x)TiO3 films prepared by the direct current micro-arc oxidation in the presence of ethylenediamine tetraacetic acid

BaxSr(1-x)TiO3 (BST) thin-films are fabricated on a Ti-substrate by micro-arc oxidation method in the presence of an aqueous solution of ethylenediamine tetraacetic acid (EDTA) and effects of EDTA, as additive, are investigated on the micro-structure and dielectric properties of BST thin-films. The obtained results show that EDTA delays the micro-arc oxidation reaction and results in a controllable slow-growth of the thin-film. Although, a remarkable thin film is obtained, however, capacitance of the prepared film is prominently increased. When concentration of EDTA is 0.05 M, the best compactness and smoothness for the film are observed. Moreover, the largest dielectric constant and smallest dielectric-loss are attained. In addition, the largest residual polarization strength (2Pr) of 3.4 μC/cm2 is also attained under this condition.

The Effect of Synthesis Temperature on the Microstructure and Electrophysical Properties of BST 80/20 Films

In this paper, we show the effect of synthesis temperature on the microstructure and electrophysical properties of ferroelectric Ba0.8Sr0.2TiO3 films during the formation on silicon substrates with a platinum sublayer. Based on our measurements, we conclude that temperature of synthesis of ferroelectric films affects their electrophysical and topographical properties.

Epitaxial growth of (Bi,K)TiO3-Bi(Mg,Ti)O3 (001) films and their ferroelectric and piezoelectric properties

Epitaxial growth of 0.95(Bi0.5K0.5)TiO3-0.05Bi(Mg0.5Ti0.5)O3(BKT-BMT) films on silicon substrates at the morphotropic phase boundary composition has been achieved for the first time using pulsed laser deposition. Perovskite films with a cube-on-cube relationship grew on the platinum bottom electrode at a substrate temperature of 450 °C–550 °C. Both the Bi/(Mg + Ti) and K/(Mg + Ti) ratios decreased with increasing substrate temperature. A film with an approximately stoichiometric composition was obtained at a substrate temperature of 450 °C. The BKT-BMT film has a high lattice anisotropy parameter (c/a) of 1.048 because a large strain of 0.7%–1.0% is introduced into the film. Its remanent polarization (Pr) and coercive field (Ec) are 60 μC cm−2 and 200 kV cm−1, respectively. The transverse piezoelectric coefficient (e31,f) of the BKT-BMT films is −2.2 C cm−2. The BKT-BMT film has superior displacement linearity with regard to different electric fields.

Time-resolved X-ray diffraction system for study of Pb(Zr, Ti)O3 films under a temporal electric field at BL15XU, SPring-8.

We report on the use of a time-resolved X-ray diffraction system to study a piezoelectric material under a temporal electric field at the BL15XU NIMS beamline, at SPring-8 in Japan. By synchronizing focused X-rays onto a device under an applied electric field with a two-dimensional detector and measurements performed with respect to the synchrotron clock signal, we successfully observed shifts of the 222 Bragg peak of 750-nm-thick Pb(Zr, Ti)O3 films near time zero under a unipolar rectangular wave at 24 V. We expect that this system might be useful for understanding the piezoresponse, lattice dynamics, and domain switching dynamics of functional oxide thin films.

Fabrication of preferentially (001)-oriented Pb(Zr,Ti)O3 films consisting of anisotropic single crystal nanoparticles

Single crystal Pb(Zr,Ti)O3 (PZT) nanoparticles were synthesized via a hydrothermal method using oleic acid. The obtained PZT nanoparticles had a sheet-like structure, and transmission electron microscopy observations revealed that the normal direction of these sheet-like nanoparticles corresponds to the c-axis. PZT films consisting of these nanoparticles were fabricated through the dip-coating method on a Pt/TiOx/SiO2/Si substrate; the sheet-like nanoparticles were aligned with their c-axes parallel to the substrate. Preferentially (001)-oriented PZT film was formed because of the morphology of each single PZT crystal, and this preferential (001)-orientation was maintained even after heat treatment at 600 °C. Local piezoelectric properties were measured via piezoresponse force microscopy with a cantilever as the top electrode, and piezoelectric constant−applied voltage hysteresis loops with high squareness were obtained for the PZT film heated at 600 °C.

Electric field-induced change in the crystal structure of MOCVD-Pb(Zr,Ti)O3 films near the phase boundary

The change in the crystal structure after poling and under an constant electric field was investigated for Pb(ZrxTi1-x)O3 films. The preparation of approximately 1.5 μm thick (100)T/(001)T/(100)R-oriented Pb(ZrxTi1-x)O3 films with a Zr/(Zr + Ti) ratio of 0.54–0.67 took place on (100)CSrRuO3/(100) CLaNiO3/(111)Pt/TiO2/SiO2/Si substrates by pulsed metal organic chemical vapor deposition. Increasing Zr/(Zr + Ti) ratio provides a change in the constituent phases of the films from single-phase tetragonal to a mixture of tetragonal and rhombohedral phases, and finally, to single-phase rhombohedral. The crystal structure of all films changed after the poling process and under an electric field in various ways according to the Zr/(Zr + Ti) ratios of the films; including lattice elongation, domain switching, and phase change. In particular, a single peak from the rhombohedral phase is asymmetrically broadened by both the poling process and an applied electric field suggesting that a (100)-oriented rhombohedral phase presumably changed to a (001)-oriented tetragonal phase.