Takahisa Shiraishi

Contribution of oxygen vacancies to the ferroelectric behavior of Hf0.5Zr0.5O2 thin films

The ferroelectric properties of the (Hf0.5Zr0.5)O2 films on Pt/Ti/SiO2/Si substrate are investigated. It is found that the films crystallized by annealing in O2 and N2 atmospheres have similar crystal structures as well as remanent polarization and coercive fields. Weak temperature and frequency dependences of the ferroelectric properties indicate that the hysteretic behavior in HfO2-based films originates not from the mobile defects but rather from the lattice ionic displacement, as is the case of the typical ferroelectric materials.

Study on the effect of heat treatment conditions on metalorganic-chemical-vapor-deposited ferroelectric Hf0.5Zr0.5O2 thin film on Ir electrode

The effect of the heat treatment conditions on the constituent phases and electrical properties of (Hf0.5Zr0.5)O2 films deposited by the metalorganic chemical vapor deposition was investigated. By using a low temperature or short duration for post-heat treatment after the deposition, the volume fraction of the tetragonal phase increases, resulting in a high dielectric constant. On the other hand, the volume fraction of the monoclinic phase increased in the films that were heat-treated at higher temperatures and exposed to longer heat treatment duration. The ferroelectric with and dielectric properties of these films were greatly inferior. Superior ferroelectric properties a significant volume fraction of orthorhombic phase were achieved for intermediate heat treatment conditions. These results give useful information to understand the origin of the ferroelectricity and to control the phases and electrical properties in HfO2-based films.

Impact of mechanical stress on ferroelectricity in (Hf0.5Zr0.5)O2 thin films

To investigate the impact of mechanical stress on their ferroelectric properties, polycrystalline (Hf0.5Zr0.5)O2 thin films were deposited on (111)Pt-coated SiO2, Si, and CaF2 substrates with thermal expansion coefficients of 0.47, 4.5, and 22 × 10−6/ °C, respectively. In-plane X-ray diffraction measurements revealed that the (Hf0.5Zr0.5)O2 thin films deposited on SiO2 and Si substrates were under in-plane tensile strain and that their volume fraction of monoclinic phase decreased as this strain increased. In contrast, films deposited on CaF2 substrates were under in-plane compressive strain, and their volume fraction of monoclinic phase was the largest among the three kinds of substrates. The maximum remanent polarization of 9.3 μC/cm2 was observed for Pt/(Hf0.5Zr0.5)O2/Pt/TiO2/SiO2, while ferroelectricity was barely observable for Pt/(Hf0.5Zr0.5)O2/Pt/TiO2/SiO2/CaF2. This result suggests that the in-plane tensile strain effectively enhanced the ferroelectricity of the (Hf0.5Zr0.5)O2 thin films.

Orientation control and domain structure analysis of {100}-oriented epitaxial ferroelectric orthorhombic HfO2-based thin films

Orientation control of {100}-oriented epitaxial orthorhombic 0.07YO1.5-0.93HfO2 films grown by pulsed laser deposition was investigated. To achieve in-plane lattice matching, indium tin oxide (ITO) and yttria-stabilized zirconia (YSZ) were selected as underlying layers. We obtained (100)- and (001)/(010)-oriented films on ITO and YSZ, respectively. Ferroelastic domain formation was confirmed for both films by X-ray diffraction using the superlattice diffraction that appeared only for the orthorhombic symmetry. The formation of ferroelastic domains is believed to be induced by the tetragonal–orthorhombic phase transition upon cooling the films after deposition. The present results demonstrate that the orientation of HfO2-based ferroelectric films can be controlled in the same manner as that of ferroelectric films composed of conventional perovskite-type material such as Pb(Zr, Ti)O3 and BiFeO3.

Growth of (111)-oriented epitaxial and textured ferroelectric Y-doped HfO2 films for downscaled devices

In this study, the growth of (111)-oriented epitaxial and textured YO1.5-HfO2 (0.07:0.93 ratio) films using the pulsed laser deposition method is presented. Epitaxial films were prepared on ITO//(111)yttria-stabilized zirconia (YSZ) substrates (ITO: Sn-doped In2O3; YSZ: yttria-stabilized zirconia), while textured films were prepared on (111)Pt/TiOx/SiO2//Si substrates with and without an ITO buffer layer via the grain on grain coherent growth. Inserting an ITO layer increased the volume fraction of the ferroelectric orthorhombic phase. Both the epitaxial and uniaxially textured films exhibited similar ferroelectricity with a remanent polarization of around 10 μC/cm2 and a coercive field of 1.9 to 2.0 MV/cm. These results present us with a way of obtaining stable and uniform ferroelectric properties for each grain and device cells consisting of a small number of grains. This opens the door for ultimately miniaturized ferroelectric devices, such as ferroelectric field effect transistors with small gate leng...

Growth of Epitaxial 100-Oriented KNbO3?NaNbO3 Solid Solution Films on (100)cSrRuO3?(100)SrTiO3 by Hydrothermal Method and Their Characterization

Films of solid solution in KNbO3–NaNbO3 (KNN) were deposited at 240 °C on (100)cSrRuO3∥(100)SrTiO3 substrates by the hydrothermal method. (KxNa1-x)NbO3 films with x = 0–1.0 were synthesized by changing the fraction of KOH in a solution of KOH and NaOH. The x in (KxNa1-x)NbO3 continuously changed with the volume fraction of KOH, while the deposition amount strongly depended on x. Epitaxial films with 100 orientation were obtained in the entire composition range and their out-of-plane lattice spacing changed with x. All the films showed ferroelectricity and their remanent polarization became larger than what above x = 0.58.

Crystal Structure Analysis of Hydrothermally Synthesized Epitaxial (KxNa1-x)NbO3 Films

Epitaxial (KxNa1-x)NbO3 films with various compositions x were deposited on Nb-doped (100)SrTiO3 single crystal substrates at 240 °C by the hydrothermal method. Detailed crystal structures were analyzed by X-ray diffraction. All the (KxNa1-x)NbO3 films consisted of a perovskite phase without any impurity phase. High-temperature XRD data showed that (KxNa1-x)NbO3 films with x = 0.03 and 0.86 transformed to a single cubic phase at 600 °C through the intermediate phase. On the other hand, (KxNa1-x)NbO3 films with x = 0.24 showed two diffraction peaks at 600 °C originating from individual phases having different unit cell volumes. The result shows that these films consisted of a mixture of two perovskite phases with K- and Na- rich (KxNa1-x)NbO3 composition.

Large irreversible non-180° domain switching after poling treatment in Pb(Zr, Ti)O3 films

(11 1¯)/(111)-oriented rhombohedral Pb(Zr0.65Ti0.35)O3 films with different domain fractions were epitaxially grown on various single crystals. The volume fraction of (111)-polar-axis oriented domains in as-deposited films, Vpol.(as-depo.), was controlled by selecting a single crystal substrate with a different thermal expansion coefficient. Applying an electric field, referred to as “poling treatment”, resulted in irreversible non-180° domain switching from the (11 1¯)-oriented domain (non-polar-axis) to the (111)-oriented domain (polar-axis), which was observed by synchrotron X-ray diffraction. Remanent polarization (Pr) values were higher than those estimated using the proportional relationship with Vpol.(as-depo.). However, the experimental Pr values were in good agreement with the values estimated using the volume fraction of (111)-oriented domains after applying the poling treatment. In rhombohedral Pb(Zr0.65Ti0.35)O3 films, 30%−50% of the (11 1¯)-oriented domains switched irreversibly to (111)-orie...

Ferroelectric and piezoelectric properties of KNbO3 films deposited on flexible organic substrate by hydrothermal method

KNbO3 films were synthesized at 120–240 °C by a hydrothermal method. The deposition amount of KNbO3 films decreased with decreasing deposition temperature because the deposition rate decreased together with the increase in the starting time of precipitation. The KNbO3 phase was ascertained at temperature as low as 120 °C in films deposited on (100)cSrRuO3//(100)SrTiO3 substrates. On the other hand, powders prepared at 150 °C included the Nb2O5 phase as well as the KNbO3 phase. In addition, powders prepared at 120 °C consist of the K3Nb7O19 phase instead of the KNbO3 phase. KNbO3 films were deposited at 150 °C on LaNiO3/Pt/Ti/polysulfone substrates. Films consisting of the polycrystalline KNbO3 phase were obtained, and their ferroelectricity and piezoelectricity were observed. The remanent polarization and piezoelectric coefficient were 4.2 µC/cm2 and 20–32 pm/V, respectively.

Crystal structure and magnetism in κ-Al2O3-type AlxFe2-xO3 films on SrTiO3(111)

We prepared κ-Al2O3-type structured AlxFe2-xO3 films in the range of x = 0 – 1.70 deposited on SrTiO3(111) substrates and investigated their crystal structures and magnetic properties. All films could be stabilized in the κ-Al2O3-type orthorhombic phase, and the lattice parameters were found to be monotonically decreased with an increase in the Al content. Neel temperature of AlxFe2-xO3 films was found to decrease with an increase in Al content, until the Al1.70Fe0.30O3 film showed paramagnetic behavior. On the other hand, saturation magnetization showed a maximum 0.79 μB/Fe at 10 K in the Al0.91Fe1.09O3 film, manifesting the preferential occupation of Al in the tetrahedral site. Cross sectional TEM observation has revealed the columnar growth of AlxFe2-xO3 films with an average width of ∼10 nm on the bottom layer that may have a similar cation arrangement with a bixbyite-type structure.