Kazuhide Abe

Ferroelectric properties in epitaxially grown BaxSr1−xTiO3 thin films

Induced ferroelectricity was observed in epitaxially grown BaxSr1−xTiO3 (BST) thin films with thicknesses of about 220 nm. The BST films with various Ba content were deposited at 600 °C on Pt/MgO substrates, by radio frequency magnetron sputtering with double targets, BaTiO3 and SrTiO3. The epitaxial growth of the BST films was confirmed with x‐ray‐diffraction and reflection high‐energy electron‐diffraction analyses. The D–E hysteresis curve and the Curie temperature shift confirmed the BST films with Ba content x≥0.44 had ferroelectricity at room temperature. The mechanism of the ferroelectricity appearance was discussed in relation to the elongation of the c axis in the thickness direction, caused by lattice mismatch between Pt and BST.

Electrical properties and microstructures of Pt/Ba0.5Sr0.5TiO3/SrRuO3 capacitors

Thin film polycrystalline Ba0.5Sr0.5TiO3 capacitors employing conductive perovskite oxide SrRuO3 as a bottom electrode were fabricated on Si substrates by rf-magnetron sputtering. Dielectric constants of these capacitors were observed to be 206, 214, and 146 for 60, 40, and 20 nm Ba0.5Sr0.5TiO3 (BSTO) thicknesses, respectively. The lowest SiO2 equivalent thickness of 0.54 nm was obtained for 20-nm-thick BSTO capacitors; their leakage current density was less than 1×10−7 A/cm2 for ±1.8 V bias. The cross-sectional transmission electron microscope observation revealed that BSTO and SrRuO3 (SRO) formed continuous columnar grains and showed epitaxial growth at the interface of BSTO/SRO within each column; this microstructure of the capacitors was tentatively designated “local epitaxial film.”

Voltage shift phenomena in a heteroepitaxial BaTiO3 thin film capacitor

Voltage shift phenomena of the hysteresis loop were characterized for a c-axis oriented heteroepitaxial BaTiO3 film by means of switching current measurements using various types of pulse sequences. During application of voltage, the hysteresis loop gradually shifted along the voltage axis according to the polarity of the voltage. Even after the application of voltage, while the top and bottom electrodes were short-circuited, the hysteresis loop continued to move. Under certain conditions, a part of the hysteresis loop shifted back, whereas the rest shifted forward. These results were explained, assuming that there is a nonswitching layer between the ferroelectric layer and the bottom electrode, and that the discontinuity of polarization can be compensated by injection of negative charges from the electrode. It was suggested that the nonswitching layer is possibly formed by relaxation of lattice misfit strain in the heteroepitaxial ferroelectric thin film.

Modification of ferroelectricity in heteroepitaxial (Ba, Sr)TiO3 films for non-volatile memory applications

Abstract Modification of ferroelectricity in heteroepitaxial Ba x Sr1-x TiO3(BST) films by making use of lattice misfit was investigated. Theoretical calculation suggested that application of two-dimensional compressive stress of 3 GPa to the ferroelectric crystal would result in Curie temperature increase by 270°C. This theoretical prediction was experimentally confirmed. Heteroepitaxial Ba0.6Sr0.4TiO3 films grown on SrRuO3/SrTiO3 substrates exhibited ferroelectric hysteresis at 200°C, even though the inherent Curie temperature is known to be 0°C. The possibility of heteroepitaxial BST films for ferroelectric non-volatile memory applications and their advantages were pointed out.

RF-MEMS Tunable Capacitor With 3 V Operation Using Folded Beam Piezoelectric Bimorph Actuator

Tunable capacitors with high tuning range and high Q-factors are indispensable for realizing reconfigurable radio-frequency (RF) circuits for multiband/multimode next-generation mobile wireless communication terminals. A novel RF microelectromechanical systems tunable capacitor having a piezoelectric bimorph actuator has been developed using a complementary metal-oxide-semiconductor compatible surface micromachining process. Folded beam piezoelectric actuators were introduced to cancel curling of the actuator beam due to residual stress. The continuous wide tuning ratio of three was realized at an operation voltage of less than 3 V for the first time

Lattice deformation and magnetic properties in epitaxial thin films of Sr1−xBaxRuO3

Crystal structure and magnetic properties in epitaxially grown Sr1−xBaxRuO3 on SrTiO3 substrates were determined. Epitaxial Sr1−xBaxRuO3 exhibits a simple perovskite structure in the whole region of the Ba/Sr ratio, in contrast to the complex hexagonal layered perovskite of Ba-rich bulk Sr1−xBaxRuO3, which has plane-sharing oxygen octahedra. Tetragonal deformation was enhanced from pseudocubic in SrRuO3 to a highly distorted tetragonal lattice in BaRuO3. Electronic properties such as conductivity and magnetization were examined. A metal–insulator transition was not observed in this system, and metallic conductivity was maintained in the whole region of Ba concentration. Ferromagnetic ordering at 160 K seen in bulk SrRuO3 was observed to be suppressed in the Sr1−xBaxRuO3 films with increasing tetragonal deformation and Curie temperatures decreased to 50 K in BaRuO3.

Calculations concerning the phase diagram, dielectric constant and lattice parameters for the Pb(Zn1/3Nb2/3)O3-PbTiO3 solid solution

Abstract The phase diagram, dielectric constant and lattice parameters were calculated for the Pb(Zn1/3Nb2/3)O3-PbTiO3 (PZN-PT) solid solution, using the phenomenological thermodynamic theory. The unknown coefficients for the energy function for PZN were estimated from the PZN-PT phase diagram. In the calculation, linear functions were assumed for the composition dependence of the coefficients. Based on comparisons between the calculation results and reported experimental results, the method was found to be effective in predicting properties of ferroelectric solid solutions.

Ferroelectricity of heteroepitaxial Ba0.6Sr0.4TiO3 ultrathin films

Abstract Remanent polarization (2Pr) larger than 0.4 C/m2 has been obtained in heteroepitaxial Ba0.6Sr0.4TiO3 (BSTO) films, even though the thickness was reduced to as thin as 26 nm. Ferroelectricity is induced by 2-dimensional stress, which is caused by lattice mismatch between the dielectric film and the bottom electrode (SrRuO3). The Curie temperature was confirmed to shift toward higher temperature by more than 200°C. These results constitute a significant advance toward the practical realization of ferroelectric nonvolatile memories with a large capacity of 256 M to 1 G bit.

Fabrication and Performance of Piezoelectric MEMS Tunable Capacitors Constructed with AlN Bimorph Structure

Fabrication process and electrical performances in a novel piezoelectric tunable capacitor have been studied. The piezoelectric bimorph structure of AlN/Al multilayer was fabricated with CMOS compatible process. The voltage dependence of actuation showed a good agreement with those of theoretical calculation, whereas stress measurements showed that suppression of residual stress in AlN layers was most critical for curling in the beams. Suppression of the distance between capacitor electrodes realized by the folded-beam type design resulted in capacitance tuning ratio of over ten at 3V. High frequency measurements up to 20GHz revealed that the tunable capacitor maintained a capacitive property up to 18GHz

Determination of impurities in strontium titanate ceramics by inductively coupled plasma mass spectrometry

The determination of impurities in SrTiO3 by ICPMS was investigated. The sample was decomposed with hydrochloric and hydrofluoric acids in a PTFE pressure vessel. The internal standard method using Au was selected to eliminate an ion count suppression by the Sr and Ti matrix. Impurities at sub μg/ml level in SrTiO3 were determined. The detection limits were in the range of 0.008 to 0.01 μg/g.