Papers by Author: Osamu Sakurai

Abstract: Highly oriented and polycrystalline Gd₂O₃ doped CeO₂ thin films were prepared on α-Al₂O₃(0001) substrates by chemical vapor deposition, using Ce(C₅H₄C₂H₅)₃ and Gd(C₅H₄C₂H₅)₃ as precursors. The compositions of the films were controlled by optimizing the vaporization pressure of Gd precursor under the constant vaporization condition of Ce precursor. In-plane electrical conductivities of the films at various temperatures and oxygen partial pressures were evaluated by electrochemical impedance spectroscopy measurements. The activation energy of the film was determined as 0.94 eV, which is comparable with that of pulsed laser deposited films.

Abstract: We investigated an influence of interface layer on a tunability of parallel plate (Ba, Sr)TiO3 thin film capacitors. BST thin film capacitors with top electrode of Pt, Au and Ag were fabricated. BST films had thickness of 40, 60, 80 and 120nm. The tunability increased with increasing the BST film thickness. Considering the interfaces between BST films and electrodes as Schottky junctions, depletion layers were formed in the interfaces depending on the difference of the work function of metal electrodes. Larger external bias voltages were applied to the depletion layer than interior BST film, because the permittivity in the depletion layer was smaller than that in interior BST film. Therefore, the depletion layer lowered the tunability. Tunability decreased with increasing the thickness of the depletion layers.

Abstract: The piezoelectric shear response of 94.0(Ka0.5, Na0.5)NbO3 (KNN) + 5.0LiNbO3 (LN) + 0.5SrTiO3 (ST) + 0.5BiFeO3 (BF) ceramics was investigated by Laser Doppler Vibrometry (LDV) and resonance-antiresonance method. From resonance-antiresonance method, the piezoelectric d15 constant was obtained to be 273 pC/N. The shear strain obtained by LDV　at the frequency of 150kHz showed strong nonlinearity. This suggested that the domain contribution to piezoelectric response in shear mode of KNN-LN-ST-BF ceramics existed at the operational frequency for the shear mode divices.

Abstract: We researched the phenomenon that the permittivity of dielectric layers in multilayer ceramic capacitor (MLCC) increases with the number of dielectric layers. Finite element method (FEM) shows that the internal residual stress in MLCC was generated by the difference of thermal expansion coefficient between internal electrodes and dielectric layers. We developed a electric measurement system with applying external stress for understanding the stress influence on dielectric properties. The compressive stress along electric field increased the polarization. The polar nano regions (PNRs) in shell composition dielectrics were easily influenced by stress. Based on these results, the relationship between the number of dielectric layers and their permittivity in MLCCs was explained.

Abstract: Barium titanate (BaTiO3) ceramics with various grain sizes from 0.7 to 13 μm on average were prepared by a conventional sintering method, a two-step sintering method and a rate controlled two-step sintering method. The permittivity of the ceramics was increased with decreasing grain size to 1.1 μm on average. However, the permittivity of the ceramics was decreased when the grain size was below 1 μm. The field emission scanning electron microscope (FE-SEM) observations revealed that the 90º domain width decreased with decreasing the grain size. By ultrawide range dielectric spectra from kHz to THz range of the BaTiO3 ceramics, the domain contribution to the permittivity was investigated. For the BaTiO3 ceramics with grain sizes over 1 μm, the dipole polarizability and the ionic polarizability were enhanced by high domain-wall density. In contrast, for the BaTiO3 ceramics with grain sizes below 1 μm, these polarizabilities were weakened.

Abstract: We investigated electrical properties of epitaxial Mn doped bismuth ferrite BiFe0.97Mn0.03O3 (BFMO) thin films with different crystal orientations deposited on Si substrates with appropriate buffer layers. Epitaxial SrRuO3 (SRO) thin films with (001), (101), and (111) orientations were grown on CeO2/yttria-stabilized zirconia (YSZ)/Si(001) substrates and YSZ/Si(001), respectively, by the insertion of MgO and TiO2 atomic layers using pulsed-laser deposition (PLD). Using spin coating, we deposited BFMO thin films onto orientated SRO thin films. The BFMO orientation followed the SRO orientation. The Pr values of the BFMO were ordered as follows {111}>{110}>{100}, which is the same as that predicted by crystallographic considerations. The largest Pr value of the {111} orientation is 76 μC/cm2 at 100 kHz, 25°C.