Kaori Nishizawa

Ferro- and piezoelectric properties of polar-axis-oriented CaBi4Ti4O15 films

Polar-axis-oriented CaBi4Ti4O15 (CBTi144) films were fabricated on Pt foils using a complex metal alkoxide solution. The 500-nm-thick film showed the columnar structure and consisted of well-developed grains. The a/b-axis orientation of the ferroelectric films is considered to be associated with the preferred orientation of Pt foil. The film showed good ferro- and piezoelectric properties. The Pr and Ec were 25 μC/cm2 and 306 kV/cm, respectively, at an applied voltage of 115 V. The d33 was characterized as 30 pm/V by piezoresponse force microscopy. The values were twice as large as those of the CBTi144 thin film with random orientation. The polar-axis-oriented CBTi144 films would open up possibilities for devices as Pb-free piezoelectric materials.

Microstructure Control and Dielectric/Piezoelectric Properties of Alkoxy-Derived Ba(Ti,Zr)O3 Thin Films

Lead-free ferroelectric Ba(Ti1-xZrx)O3 (BTZ, x=0.00–0.50) thin films were fabricated on Pt(111)/Ti/SiO2/Si(100) substrates by the chemical solution deposition (CSD) process using complex alkoxide solutions. When the Zr content x was increased, the crystallinity of BTZ thin films was changed from the typical random orientation to the (111) preferred orientation. From the composition dependence of the dielectric constant er and piezoelectric constant d33, the pinching region at room temperature of BTZ thin films consisting of nano-crystals appeared around x=0.20. For the BTZ thin film (x=0.20) fabricated using an alkoxide solution with partial hydrolysis, the grain and crystallite size were increased, and the er and d33 were improved to 253 and 8.9 pm/V, respectively.

Grain Size Effect on Dielectric and Piezoelectric Properties of Alkoxy-Derived BaTiO3-Based Thin Films

Lead- and bismuth-free Ba(Ti1-xZrx)O3 (BTZ) thin films were fabricated on Pt(111)/Ti/SiO2/Si(100) substrates by the chemical solution deposition (CSD) process. The BaTiO3 and BTZ005 thin films fabricated by the conventional process were observed to have a remarkable difference in grain size along the depth of the thin film. The microstructure of the BaTiO3 and BTZ005 thin films was improved by additional sintering process, and their grain sizes were increased to about 60 and 40 nm, respectively, by additional sintering at 800°C for 1 h. The dielectric constant er and piezoelectric constant d33 of the BaTiO3 thin film consisting of large grains by the additional sintering at 800°C for 1 h were found to be about 375 and 14.5 pm/V. But, er and d33 of the BTZ005 thin film changed negligibly with increasing of the grain size.

Platinum-assisted phase transition in bismuth-based layer-structured ferroelectric CaBi4Ti4O15 thin films

The phase transition of nonferroelectric pyrochlore to ferroelectric perovskite in CaBi4Ti4O15 thin films depends on platinum bottom electrodes. Rather than the strain and crystallinity of the bottom electrode, matching of the atomic arrangement to the Ca–Bi–Ti–O thin films is predominant. CaBi4Ti4O15 thin films crystallized on (200)-oriented platinum at 650 °C showed c-axis orientation. In contrast, thin films crystallized on highly crystalline (111)-oriented platinum at the same temperature contained pyrochlore grains which were about several tens of nanometers in diameter and located in the interface region. They showed P–V hysteresis loops. The remanent polarization and coercive electric field depended on platinum top electrode size.

Preparation of (Y,Yb)MnO3/Y2O3/Si (MFIS) Structure by Chemical Solution Deposition Method

(Y,Yb)MnO3 films were prepared on Y2O3-buffered n-type Si(111) substrates using alkoxy-derived precursor solutions and rapid thermal annealing at 750°C in Ar. The (Y,Yb)MnO3 films crystallized to hexagonal phase and had high crystallinity and high degrees of c-axis orientation. The hexagonal (Y,Yb)MnO3 films consisted of uniform grains and had smooth surfaces. The electrical properties of the 200-nm-thick Y0.5Yb0.5MnO3 films crystallized on the Y2O3/Si(111) substrates at 750°C in Ar were investigated. The leakage current density of the Pt/Y0.5Yb0.5MnO3/Y2O3/Si capacitor was 1.0×10-8 A/cm2 at an applied voltage of 5 V. The counterclockwise capacitance-voltage (C-V) hysteresis induced by ferroelectric polarization switching was observed in the Pt/Y0.5Yb0.5MnO3/Y2O3/Si capacitor.

Chemical Approach Using Tailored Liquid Sources for Traditional and Novel Ferroelectric Thin Films

The Ca–Bi–Ti and Sr–Bi–Ti complex alkoxide solutions were used as tailored liquid sources for synthesis of a series of novel Bi-based layer-structured perovskite thin films with the number, n, of oxygen octahedra from 3 to 5. The complex alkoxide solutions had similar local structures, of which the stability for the partial hydrolysis was important to suppress compositional deviation in the resultant thin films. The Ca-containing thin films had the characteristic well-developed columnar structure which was distinct from the refractory stacking-grain structure of the Sr-contained thin films. The relationship between the lattice constants and the number of oxygen octahedra, n, confirmed that the series of thin films prepared using the alkoxide complex solutions had intentionally designed layer structure. Additionally, the dielectric and ferrolectric properties of the thin films were confirmed to depend on the A-site cations and the value of n.

Comparison of Microstructure and Ferroelectric Properties of Alkoxy-Derived MBi4Ti4O15 (M: Ca or Sr) Thin Films

CaBi4Ti4O15 (CBTi144) and SrBi4Ti4O15 (SBTi144) thin films were prepared on Pt-passivated silicon substrate using complex metal alkoxide solutions. The CBTi144 thin films crystallized to a single phase of perovskite at 650°C via a mixture of perovskite and pyrochlore phases. The 650°C-annealed CBTi144 thin films consisted of uniform and isotropic grains and had a closely packed columnar structure. In contrast, the SBTi144 thin films crystallized to the perovskite phase at relatively lower temperature and consisted of smaller and non-uniform grains. The dielectric constants and loss factors of the 650°C-annealed CBTi144 and SBTi144 thin films were 300 and 0.03, and 330 and 0.04, respectively, at 100 kHz. The remanent polarizations and coercive electric fields were 6.0 µC/cm2 and 79 kV/cm, and 2.9 µC/cm2 and 55 kV/cm, respectively, at 9 V. The dielectric and ferroelectric properties depended on both the cation size in the A site and the microstructure.

Thickness Dependence of Electrical Properties of Highly (100)-Oriented BaTiO3 Thin Films Prepared by One-Step Chemical Solution Deposition

Highly (100)-oriented BaTiO3 thin films having different thicknesses of 70–280 nm were deposited on Pt/TiOx/SiO2/Si substrates with LaNiO3 as a buffer layer by one-step chemical solution deposition. X-ray diffraction analyses showed that the LaNiO3 and BaTiO3 layers were all under tensile stress. The tensile stress of the LaNiO3 layer increased after depositing BaTiO3 films. The tensile stress in both the LaNiO3 and BaTiO3 layers decreased with increasing BaTiO3 layer thickness. The thickness dependences of the dielectric and piezoelectric behaviors of the highly (100)-oriented BaTiO3 films were investigated. It was found that the effect of tensile stress on the dielectric and piezoelectric properties is dominant. A reduction in in-plane tensile stress (primarily arising from a thermal expansion mismatch) was considered effective for increasing the dielectric and piezoelectric constants. The measured dielectric constant increased from about 313 for 70 nm films up to 831 for 280 nm films. The local effective piezoelectric coefficient increased from 15 pm/V for 70 nm films up to 45 pm/V for 280 nm films.

Impact of oxygen ambient on ferroelectric properties of polar-axis-oriented CaBi4Ti4O15 films

Polar-axis oriented CaBi4Ti4O15(CBTi144) films were fabricated on Pt foils using a complex metal alkoxide solution. The oxygen ambient during crystallization of the films impacted the crystal perfection, crystallite size, and the ferroelectric properties. The 500mm thick film crystallized in oxygen flow had single columnar structure and in-plane grain size of about 200nm. The Scherrer’s crystallite diameter was calculated as about 110nm. The ferroelectric properties were improved. The Pr and Ec of the film enhanced as 33.6μC∕cm2 and 357kV∕cm, respectively, at an applied voltage of 50V. Voltage applied for full polarization switching was lowered by controlling oxygen stoichiometry of the film. Indeed, the polar-axis-oriented CBTi144 films would open up possibilities for devices as Pb-free ferroelectric materials.

Control of crystallization and crystal orientation of alkoxy-derived SrBi2Ta2O9 thin films by ultraviolet irradiation

A 650 °C annealed thin film was found to be a single phase of SrBi 2 Ta 2 O 9 (SBT) and showed the c-axis orientation without ultraviolet irradiation. The crystallinity and crystal orientation of the thin films were improved by ultraviolet irradiation using an ultrahigh-pressure mercury lamp (UHPML) or a low-pressure mercury lamp (LPML) under three conditions. In particular, the crystal orientation of the 650 °C annealed thin films dramatically changed by ultraviolet irradiation using a LPML at room temperature or using an UHPML at 150 °C. The 650 °C annealed SBT thin film prepared by ultraviolet irradiation using an UHPML at 150 °C showed a remanent polarization (Pr) of 4.3 μC/cm 2 and a coercive electric field (E c ) of 101 kV/cm at 10 V.