Papers by Author: Takaaki Tsurumi

Abstract: We report the growth of Cu₂O thin films on glass and MgO(100) substrates by molecular beam epitaxy. Crystal orientation of Cu₂O thin films on glass substrate were changed from (100) to (111) with increasing the deposition rate. The Cu₂O thin films were epitaxially grown on MgO(100) substrate with an orientation relationship of Cu₂O(110) // MgO(100). The film quality and electrical properties of Cu₂O thin films were changed with deposition rate. The slow deposition rate resulted in high conductivity and mobility, as well as good crystallinity and orientation.

Abstract: The insulation degradation of BaTiO₃-based ceramics under direct-current field is caused by the migration of oxygen vacancies. To improve the reliability of BaTiO₃-based ceramic capacitors, conventional approaches are focusing on dielectrics, mainly working on suppressing the total amount of oxygen vacancies towards the cathode. In this study, we newly focus NOT on the dielectrics but on the electrodes, and tried to improve the reliability of BaTiO₃-based ceramics by suppressing the segregation of migrated oxygen vacancies. La_0.8Sr_0.2MnO₃ (LSM) is a mixed ionic-electronic conductor that generates oxide ions from gas-phase oxygen. We expected that LSM would compensate for migrated oxygen vacancies when using it as the cathode of a BaTiO₃-based ceramic capacitor. The results of our highly-accelerated life tests indicate that the reliability of BaTiO₃-based ceramics improved when LSM porous ceramics was used only as the cathode, (NOT the anode).

Abstract: An effect of addition of CaO on electrical resistivity in BaTiO₃-(Bi_1/2Na_1/2)TiO₃ (BT-BNT) solid solution ceramics was investigated. Three kinds of CaO adding processes were applied. Extra CaO additive against stoichiometric composition is effective to give semiconductive property into the BT-BNT ceramics. However, the ceramics with CaO-inclusive composition such as Ba_0.9(Bi_0.5Na_0.15Ca_0.35)_0.1TiO₃ had a high resistivity. From chemical composition analysis and structure analysis, it was concluded that Ca²⁺ dominantly substituted for Na⁺ in the A site of perovskite-type ABO₃ structure and then that residual Bi³⁺ acts a donor.

Abstract: In order to clarify the origin of the elastic nonlinearity in piezoelectric ceramics, the effect of Mn substitution into Pb (Zr,Ti)O₃-based ceramics on the nonlinear elastic stiffness was investigated by analyzing the transient response after burst excitation. The real part of 3rd-order elastic stiffness drastically decreased with increasing Mn content. It was also found that the appearance of the elastic nonlinearity enhanced the mechanical loss. From the results, the irreversible domain wall motion overcoming the potential barrier due to defect complexes was considered as a possible origin of the elastic nonlinearity.

Abstract: Aluminum substituted La₃Ta_0.5Ga_5.5O₁₄ (LTGA) single crystals were grown via a floating zone (FZ) method in N₂ or O₂ gas flows. Also, LTGA single crystals that were grown using the Czochralski method were annealed in N₂ or O₂ gas flows. The resistivity of these crystals was measured in a temperature range from 300 to 700°C. The LTGA crystals that were grown/annealed in the N₂ atmosphere showed higher resistivity by 1 order of magnitude as compared to those crystals grown/annealed in the O₂ atmosphere. Based on these results, we suggest that the difference in resistivity is due to the concentration of oxygen vacancy and interstitial oxygen ions.

Abstract: The relation between the dielectric tunability and the electro-optic (EO) effect of barium strontium titanate (Ba_0.5Sr_0.5TiO₃) thin film was discussed. The tunability of dielectric permittivity calculated from the complex admittance with planer electrodes reached to 53.1%, and the tunability of birefringence by EO effect was 0.6%. The birefringence change from EO effect was much lower than the tunability of dielectric permittivity. The materials with high tunability do not always exhibit high EO effect. This is concluded to arise due to the ionic mass in the EO materials.

Abstract: Pure Barium titanate (BaTiO₃) ceramics with grain sizes of 0.6, 1.5, and 18 μm on average were prepared by using a two-step sintering method. The wideband dielectric spectra in the range from kHz to THz were measured at 25 and 150 °C to understand the polarization mechanism of fine-grained BaTiO₃ ceramics. It was revealed that the dipole and ionic polarizations decreased with decreasing grain size at 25 and 150 °C when the grain size was smaller than 1 μm. These mechanisms were interpreted as a grain boundary effect.

Abstract: BaTiO₃–(Bi_1/2Na1/2)TiO₃ (abbreviated as BT-BNT) solid solution ceramics, as a lead-free PTC (Positive Temperature Coefficient of resistivity) thermistor material usable over 130°C, were synthesized in air atmosphere. By conventional processes, La-doping was not effective in giving semiconducting property to the BT-BNT ceramics because of volatilization of sodium and bismuth during sintering. By preventing the volatilization by lowering the sintering temperature using very fine TiO₂ powder and by adding calcium oxide to calcined BT-BNT powders, we obtained BT-BNT semiconducting ceramics which shows a resistivity change near Curie temperature.

Abstract: BaTiO₃–(Bi_1/2K_1/2)TiO₃ (abbreviated as BT-BKT) solid solution ceramics, as a lead-free PTC (positive temperature coefficient of resistivity) thermistor material usable over 130°C, has been synthesized by sintering in N₂ atmosphere and after annealing in air over 1200°C. In the BT-BKT ceramics with PTC property, the impedance/modulus spectroscopic plots have revealed that a third resistance-capacitance (RC) response besides grains and grain boundaries. Using the remote electron beam induced current (REBIC) configuration, imaging has revealed EBIC contrast consistent with the presence of negatively charged electrostatic grain boundary barriers in the BT-BKT semiconducting ceramics.

Abstract: An estimation method of the influence of interfaces on properties in perovskite-type oxide thin-film capacitors is presented. We proposed a modified Schottky model that can be employed to explain the electric properties of metal/perovskite-type oxide junctions. The modified model considers the electric field dependence of permittivity and the flow of electrons from metal to defect states located in the band gap of the perovskite oxide. The simulation based on this model could successfully describe the results of capacitance-voltage measurements of junctions between metals (Pt, Au, and Ag) and Nb-doped SrTiO₃, which were not explained by the conventional Schottky model. Additionally, a simulation for a thin-film capacitor with hysteretic behavior could depict an asymmetric capacitance-voltage curve.