Minoru Osada

In situ Raman spectroscopy observation of crystallization process of sol-gel derived Bi4-xLaxTi3O12 films

In situ Raman spectra measurements have been employed to reveal the crystallization process of sol-gel derived Bi4-xLaxTi3O12 (BLT) thin films. The Raman spectra of BLT films with a La composition x of 0.75 were measured with increasing temperature up to 800°C in air and N2 ambient. It is demonstrated that Bi2O2 layered structures form first at 500°C and perovskite structures start to crystallize at 600°C or higher.

Property design of Bi4Ti3O12-based thin films using a site-engineered concept

Abstract A property design concept involving the substitution of each crystal site in the pseudoperovskite in Bi 4 Ti 3 O 12 structure, a site-engineered concept, is proposed for Bi 4 Ti 3 O 12 -based thin films. This concept is based on the selective substitution of each crystal site contributing to the ferroelectric properties. By using this concept, a novel material, i.e. (Bi,Nd) 4 (Ti,V) 3 O 12 was developed. A remnant polarization (Pr) value above 16xa0μC/cm 2 at a process temperature of 540°C was obtained together with orientation control of the films. This Pr value is comparable to that of the widely used Pb-based ferroelectrics, such as Pb(Zr,Ti)O 3 .

Characterization of Sol-gel Derived Bi4-xLaxTi3O12 Films

Ferroelectric Bi4Ti3O12 (BIT) and Bi4-xLaxTi3O12(BLT) (x=0.25, 0.5, 0.75) films were prepared on Pt/Ti/SiO2 substrates by the sol-gel technique. The P–E hysteresis loop, whose squareness was enhanced more than that of a BIT films, was obtained for a BLT film with a La composition of 0.75 annealed at 650°C or higher temperatures. A remanent polarization of 13 µC/cm2 and a coercive electric field of 80 kV/cm were obtained. Raman scattering measurements reveal that the crystallization temperature at which a perovskite structure forms decreased with increasing La content. In addition, it is shown that the crystallization process depends on the La content.

Crystal Structure and Ferroelectric Property of Tungsten-substituted Bi4Ti3O12 Thin Films Prepared by Metal-Organic Chemical Vapor Deposition

W-substituted Bi4Ti3O12 (BIT), Bi4(Ti3-xWx)O12, films were prepared on (111)Pt/IrO2/SiO2/Si substrates at 600°C by metal-organic chemical vapor deposition (MOCVD). The degree of (117) orientation and the lattice spacing of 117 Bi4(Ti3-xWx)O12 increased with increasing W content up to x=0.25. Ferroelectricity was hardly observed in as-deposited films but confirmed in the films after annealing at 700°C. The coercive field (Ec) monotonically increased with increasing W content, but the remanent polarization (Pr) showed a maximum value at x=0.11. Pr and Ec of Bi4(Ti2.89W0.11)O12 thin film after heat treatment was 13 µC/cm2 and 160 kV/cm, respectively. W-substituted films showed high fatigue resistance against continuous switching up to 8×1010 cycles. These results show that W substitution for BIT film is effective in improving the ferroelectricity.

Effect of La substitution on Electrical Properties of Highly Oriented Bi4Ti3O12 Films Prepared by Metalorganic Chemical Vapor Deposition

The effect of La substitution on the electrical properties of Bi4Ti3O12 was investigated by using epitaxially grown Bi4Ti3O12 (BIT) films and highly oriented (Bi3.25La0.75)Ti3O12 [BLT] films with (001)-, (118)- and (104)(014)-orientations. By characterizing the leakage current density for films with different orientations, it was found that the leakage current density along the a–b plane was decreased by La substitution. The spontaneous polarization (Ps) of the BLT film evaluated from the P–E hysteresis properties of the (001)- and (118)-oriented films was 3.5 and 38 µC/cm2 along the c-axis and the a-axis, respectively. They were smaller than those of the BIT films: 4.0 and 50 µC/cm2. Based on the fact that La preferentially substitutes for the Bi site in the pseudoperovskite layer, the large remanent polarization widely reported for polycrystalline (Bi3.25La0.75)Ti3O12 films is related to the reduction of the number of defect complexes in the pseudoperovskite layer that possibly pin the domain motion.

Ferroelectric Property of a-/b-Axis-Oriented Epitaxial Sr0.8Bi2.2Ta2O9 Thin Films Grown by Metalorganic Chemical Vapor Deposition

In this study, a-/b-axis-oriented epitaxial Sr0.8Bi2.2Ta2O9 (SBT) thin films were grown on (101)TiO2 and (101)RuO2//(110)Al2O3 substrates by metalorganic chemical vapor deposition (MOCVD). By several X-ray diffraction techniques, the a-/b-axis-oriented epitaxial growth on the substrate with a rutile structure was confirmed. The leakage current density of SBT was characterized along the a-/b-axis and c-axis, and the lower defect density in the pseudoperovskite layer was revealed by comparison with Bi4Ti3O12. The saturation polarization (Ps), remanent polarization (Pr), and coercive field (Ec) were 6.0 µC/cm2, 3.1 µC/cm2 and 54 kV/cm, respectively. The estimated spontaneous polarization along the a-axis corrected by the a-/b-domain volume ratio was 20 µC/cm2, which is identical to previously estimated values. The switching charge versus the repetitive 1.2×1010 switching cycles remained almost constant, indicating a superior fatigue endurance along the direction parallel to the (Bi2O2)2+ layer.

Oxygen Vacancy Migration and Dispersive Photoconductivity in Bi4Ti3O12-d

We investigated the influence of photoexcited nonequilibrium electrons in Bi4Ti3O12-d which contains a high density of oxygen vacancies. With increasing oxygen deficiency, the photocurrent undergoes a rapid enhancement, accompanied by a marked change in kinetics. In heavily oxygen-deficient Bi4Ti3O12-d (d=0.45), we observed a nonexponential relaxation of photogenerated carriers with an increasingly slow decay after termination of visible illumination with energy lower than the band gap. This phenomenon is similar to what has been reported for persistent photoconductivity effects in the layered perovskite YBa2Cu3O7-d and Nd2Ti3O9, both of which resulted from the space charge field established by photoexcited electrons trapped at oxygen vacancies. We argue that oxygen vacancies acting as trapping centers for electrons are linked to dispersive diffusion in the perovskite blocks, which control carrier recombination.

Synthesis and electrical properties of Sr- and Nb-cosubstituted Bi4-xSrxTi3-xNbxO12 polycrystalline thin films

Lead-free ferroelectric Bi4Ti3O12 (BIT) thin films were modified by the cosubstitution of Sr2+ for Bi3+ and of Nb5+ for Ti4+ by spin-coating and decomposition of chemical solutions of metal-alkoxide materials (the nominal compositions of Bi4-xSrxTi3-xNbxO12 (BSTN) are x=0.5, 1.0, 1.5). Single-phase thin films with a BIT-type structure were crystallized above 550?C. At room temperature, the ferroelectric and dielectric properties were found to be Pr=10 ?C/cm2, Ec=100 kV/cm, ?r=300, and tan ?<5% for x=0.5 after annealing at 650?C. The films with x=1.0 and 1.5 did not exhibit ferroelectric hysteresis behavior because of the decrease in Curie temperature (Tc) below room temperature. The x dependence of Tc was studied by considering the soft mode behavior in Raman scattering spectra and the Tc values were 400, -25, and -220?C for x=0.5, 1.0, and 1.5, respectively. The cosubstitution by Sr2+ and Nb5+ is effective for reducing ferroelectric interaction between electrical dipoles leading to a large shift in Tc. Because of its high solubility of Sr2+ and Nb5+ and efficiency for shifting Tc, the BSTN system may find a novel application as a dielectric material rather than as a ferroelectric material.

Growth of Epitaxial Site-Engineered Bi 4 Ti 3 O 12 -Basded Thin Films by Mocvdand Their Characterization

Thin films of BIT, La-substituted BIT (BLT) and La- and V-cosubstituted BIT(BLTV) were epitaxially grown on SrRuO 3 //SrTiO 3 substrates at 850°C by metalorganic chemical vapor deposition (MOCVD), and their electrical properties were systematically compared. All films on (100), (110) and (111)-oriented substrates were epitaxially grown with (001)-, (104)-/(014)-and (118) –preferred orientations, respectively. The leakage current density of the BLTV film was almost the same with that of the BLT film, but was smaller than that of BIT film, suggesting that the La substitution contributed to the decrease of the leakage current density especially in pseudoperovskite layer. Spontaneous polarization of the BLTV film was estimated to be almost the same with the BLT film but was smaller that that of the BIT film. This is explained by the decrease of Tc with the La substitution, while V did not contribute to the change of the Curie temperature ( Tc ). On the other hand, the coercive field ( Ec ) value of the BLTV was smaller than that of the BIT and the BLT films. As a result, La substitution contributed to the decrease of the leakage current density together with the decrease of the spontaneous polarization due to the decrease of the Tc . On the other hand, V substitution contributes to the decrease of the defects that suppress the domain motion and increases the Ec value. Therefore, each substitution of La and V plays different roles and this contribution is remarkable for the films deposited at lower temperature.