Uchida Hiroshi

Abstract This study systematically investigated the role of Ba(OH)₂ concentration on the properties of thin films. Films of (001)-oriented barium titanate (BaTiO₃) were grown on (100) La-doped SrTiO₃ substrates at 200°C by hydrothermal synthesis. X-ray diffraction analysis confirmed the films showed epitaxial and highly (100)-oriented growth with no secondary phases. The as-deposited films exhibited a significant unit cell volume expansion relative to bulk BaTiO₃, which is attributed to the incorporation of hydroxyl (OH^-) groups inherent in the hydrothermal process. Scanning electron microscopy revealed that a high Ba(OH)₂ concentration (2.0 mol dm^⁻3) was essential to achieving a uniform film approximately 150 nm thick, lower concentrations resulted in discontinuous growth. The dense film exhibited a clear ferroelectric domain structure and good dielectric properties. These results demonstrate that precise control over precursor concentration is a key factor in fabricating high-quality epitaxial ferroelectric oxide thin films via low-temperature hydrothermal methods.

Abstract The piezoelectric and ferroelectric applications of heterovalent ternary materials are not well explored. Epitaxial MgSiN₂ films are grown at 600 °C on (111)Pt//(001)Al₂O₃ substrates by the reactive sputtering method using metallic Mg and Si under the N₂ atmosphere. Detailed X‐ray diffraction measurements and transmission electron microscopy observations revealed that the epitaxially grown films on the substrates have a hexagonal wurtzite structure with c‐axis out‐of‐plane orientation. The random occupation of this structure by Mg and Si differs from that of the previously reported structure in which these two cations periodically occupy the cationic sites. However, the lattice spacings closely approximate those that are previously reported, irrespective of the ordering, and they are almost comparable with those of (Al_0.8Sc_0.2)N. The wide bandgap of >5.0 eV in deposited MgSiN₂ is compatible with that of AlN and suggests durability against the application of strong external electric fields, possibly to induce polarization switching. In addition, MgSiN₂ is shown to have piezoelectric properties with an effective d₃₃ value of 2.3 pm V⁻¹ for the first time. This work demonstrates the compositional expansion of hexagonal wurtzite to heterovalent ternary nitrides for novel piezoelectric materials, whose ferroelectricity is expected.

Abstract This study examines the effect of tensile stress on the ferroelectric properties of Pb(Zr0.4Ti0.6)O3 thin film, with a focus on Barkhausen noise, observed for the first time under such conditions. Tensile stress significantly alters domain wall motions, affecting Barkhausen noise more than average polarization. Frequency analysis identifies grain boundaries as primary pinning sites, consistent across stress levels. A non-linear relationship between stress, domain wall mobility, and polarization is found, where increased stress initially enhances pinning and polarization changes, but this effect diminishes at higher stress levels, indicating a shift in behavior.