Takanori Kiguchi

Enhanced piezoelectric properties of barium titanate single crystals with different engineered-domain sizes

For tetragonal barium titanate (BaTiO3) single crystals, an electric field (E-field) applied along the [111]c direction can induce an engineered-domain configuration in these crystals. In this study, such engineered-domain configurations of different domain sizes were induced in BaTiO3 single crystals, and their piezoelectric properties were investigated as a function of domain size. Prior to this study, the dependences of the domain configuration on the temperature and E-field were investigated using a polarizing microscope in order to understand the optimum poling condition for fine- and coarse-domain configurations. We found that above the Curie temperature (TC) of 132.2u2009°C, when an E-field above 6.0kV∕cm was applied along the [111]c direction, an engineered domain with a fine-domain configuration appeared. Moreover, it was also found that this fine-domain configuration remained stable at room temperature without the E-field. On the other hand, the coarse-domain configuration was obtained upon poling a...

HRTEM investigation of the 90° domain structure and ferroelectric properties of multi-layered PZT thin films

Multi-layered epitaxial Pb(Zrx,Ti1-x)O3 (PZT) films with x = 0.2-0.5 were deposited on La0.5Sr0.5CoO3-x (LSCO)/(001)STO and LSCO/CeO2/YSZ/(001)Si substrates with buffer layers. We investigated using HRTEM and XRD how the 90° domain structure and the P-E hysteresis character depend on the difference in the thermal expansion coefficient by changing the Zr/Ti composition and the substrate. XTEM analysis showed that large 90° domains 8-30 nm in width penetrate the columnar grain and the PZT layer in the PZT stacked film Zr/Ti=20/80,30/70,40/60. On the other hand, close-packed small 90° domains 4-5 nm in width were present in epitaxial columnar grains in the PZT50/50 stacked film. The P-E hysteresis loops of PZT20/80 stacked films deposited on STO and Si substrates show a remanent polarization of 2Pr = 136 and 80 µC/cm2, respectively. On the other hand, those of PZT50/50 stacked films deposited on STO and Si substrates show a polarization of 2Pr = 125 and 36 µC/cm2, respectively. Thus, the P-E hysteresis loop of PZT50/50 exhibits remarkable differences in 2Pr values between the substrates.

Solution-based fabrication of high-κ dielectric nanofilms using titania nanosheets as a building block

We have demonstrated a novel procedure for fabricating high-κ dielectric nanofilms by using a titania nanosheet as a building block. The layer-by-layer assembly of titania nanosheets using an atomically flat SrRuO3 substrate is advantageous as a means of fabricating atomically uniform multilayer high-κ nanofilms. High-resolution transmission electron microscopy revealed that these multilayer nanofilms are composed of a well-ordered lamellar structure without an interfacial dead layer. These nanofilms exhibited both high dielectric constant (er~125) and low leakage current density (J<10-7 A/cm2) even for thicknesses as low as 10 nm. These results indicate that the titania nanosheet is a very promising candidate as a high-κ nanoblock, and its bottom-up fabrication provides new opportunities for the development of high-κ devices.

Role of Ultra Thin SiOx Layer on Epitaxial YSZ/SiOx/Si Thin Film

The role of an ultra thin SiOx layer for epitaxial growth of a YSZ (Y 2 O 3 stabilized ZrO 2 ) thin film on a (001)Si substrate through the SiOx layer was investigated by high resolution transmission electron microscope (HRTEM), in-situ heating transmission electron microscope and nano-beam diffraction (NBD) methods. It is found that a trace of epitaxial crystallinity remains in an ultra thin SiOx layer within 2 nm from SiOx/Si interface. According to this result, a YSZ layer could epitaxially grow only on an ultra thin 1-2 nm SiOx layer. It is also found that an ultra thin SiOx layer has another effect to relax the crystallization strain at a YSZ/Si interface. These results indicate that an ultra thin SiOx layer plays two important roles: (a) an ultra thin SiOx layer formed just above the Si surface serves as a medium for the epitaxial growth of a YSZ layer, and (b) even in an ultra thin SiOx is able to relax the crystallization strain of YSZ layer at YSZ/Si interface. These are the critical points of the epitaxial growth of YSZ/SiOx/(001)Si thin film together with a role of an oxygen source reported previously.

Activation Energy of Oxygen Vacancy Diffusion of Yttria-Stabilized-Zirconia Thin Film Determined from DC Current Measurements below 150 °C

The current density versus time (J–t) characteristics of yttria-stabilized-zirconia (YSZ) films on Si(001) below 150 °C were measured. By the application of a negative voltage up to 20 V to the aluminum top electrode, a current peak was observed. The peak evolution was considered on the basis of the space-charge-limited current transient of oxygen vacancy (model-1), and the modulation of electronic conductivity upon oxygen vacancy redistribution (model-2). From the point of view of the activation energy and the relative dielectric constant estimation, model-2 gave a more plausible value. It was also observed, based on J–t measurements, that Nb-doping causes the suppression of oxygen vacancies.

Preparation and Structure of Lead Magnesium Niobate Titanate Film by Double-Pulse Excitation using Nd:YAG and KrF Excimer Lasers

We report the preparation of lead magnesium niobate titanate films on a buffered silicon substrate using pulsed laser deposition with double-pulse excitation. A Nd:YAG (355 nm) laser and a KrF (248 nm) excimer laser were combined. The film morphology changed drastically concomitant with the delay time. Suppression of droplets was observed when the Nd:YAG (9.0×101 J·cm-2) was used as the first laser and KrF (5.0×10-2 J·cm-2) was used as the second laser with a 0.5 µs time delay. The fluence of the second laser was responsible for the resultant phase and when the fluence of KrF is 9.5×10-1 J·cm-2, a significant amount of pyrochlore type compounds coexisted.

Room-Temperature Electrical-Field Induced Oxygen Diffusion of Aluminum/Yttria-Stabilized Zirconia Thin Film Grown on Si Substrate

A mechanism is proposed for room-temperature “electrical-field-induced oxygen diffusion” based on the interface reaction of aluminum/yttria-stabilized zirconia (YSZ) and the measurements of film leakage properties. The application of a positive electric field (1 MV·cm-1) to a 25-nm-thick and 200-µm-diameter Al top electrode on a YSZ/Si thin film changes the color of the Al top electrode from silver to black in 400 s because of Al oxidation, as confirmed by secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS). Electrically induced diffusion of oxygen ions from the YSZ into the Al electrode causes the oxidation. This phenomenon was not observed when a negative field was applied; nor was it shown by an Al/SiO2/Si thin film when a positive field (50 MV·cm-1) was applied. This method, which might be useful for device applications, can detect the redox reaction between the Al top electrode and the material, thereby indicating oxygen diffusion at low temperatures.

Hrteminvestigation of 90° Domain Configureuration and P-E Hysteresis Loop of Epitaxial PZT Multilayered Thin Films

Multi-layered epitaxial Pb(Zr x ,Ti 1-x )O 3 (PZT) films of x=0.2–0.5 were deposited on La 0.5 Sr 0.5 CoO 3-x (LSCO)/ (001)STO and LSCO/CeO 2 /YSZ/(001)Si substrates with buffer layers. It was investigated how the 90° domain structure and the P-E hysteresis character depend on the difference of the thermal expansion coefficient by changing the Zr/Ti composition and the substrates, using HRTEM and XRD methods. XTEM analysis showed that usual lamella configuration of 90° domains of 8–30nm in width penetrated the columnar grain and the PZT layer in the PZT stacked film of Zr/Ti=20/80, 30/70, 40/60. On the other hand, the close-packed 90° domains of 4–5nm in width existed in a epitaxial columnar grain in the PZT50/50 stacked film. The P-E hysteresis loops of PZT20/80 stacked films deposited on STO and Si substrates show the remanent polarization of 2Pr=136μC/cm 2 , 2Pr=80μC/cm 2 , respectively. On the other hands, those of PZT50/50 stacked films deposited on STO and Si substrates show the polarization of 2Pr=125μC/cm 2 , 2Pr=36μC/cm 2 , respectively. Thus, the P-E hysteresis loop of PZT50/50 has remarkable difference of 2Pr between the substrates.

Role of Ultra Thin SiOx Layer on Epitaxial YSZ/SiOx/Si Thin Film as Multi Functional Buffer Layer by Nano-Probe and In-Situ TEM Investigation

The role of an ultra thin SiOx layer for epitaxial growth of a YSZ (Y stabilized ZrO 2 ) thin film on a (001) Si substrate through the SiOx layer was investigated by high resolution transmission electron microscope (HRTEM), in-situ heating transmission electron microscope and nano-beam diffraction (NBD) methods. It is found that a trace of epitaxial crystallinity remains in an ultra thin SiOx layer within 2nm from SiOx/Si interface. According to this result, a YSZ layer could epitaxially grow only on an ultra thin 1-2nm SiOx layer. It is also found that an ultra thin SiOx layer has another effect to relax the crystallization strain at a YSZ/Si interface. These results indicate that an ultra thin SiOx layer plays two important roles: (a) an ultra thin SiOx layer formed just above the Si surface serves as a medium for the epitaxial growth of a YSZ layer, and (b) even in an ultra thin SiOx is able to relax the crystallization strain of YSZ layer at YSZ/Si interface. These are the critical points of the epitaxial growth of YSZ/SiOx/(001) Si thin film together with a role of an oxygen source reported previously.