Takahisa Shiraishi

Epitaxial (Bi, K)TiO3 films with 800- to 900-nm thicknesses were grown hydrothermally at 200 °C on SrTiO3 substrates covered with SrRuO3 layers. Perfectly (001)-oriented films grew on (100)SrTiO3 due to good lattice matching. Films on (110)SrTiO3 had mixed orientations of dominant (110) and minor (101), while three types of (111) orientations with in-plane 120° rotation were observed for the film on (111)SrTiO3. The (101) and (110) orientations of the film deposited on (110)SrTiO3 were tilted by approximately 2.6° and 1.6°, respectively, from surface normal due to the formation of a 90° domain with a twinning plane. The plane-view measurement for the film deposited on (111)SrTiO3 showed nine spots. These are explained by the presence of 15 possible spots resulting from the relaxed 90° domain combination and by overlapping. The lattice parameters of these films explain the tilting angles of these domains. These results reveal the formation of perfectly relaxed 90° domain structures for films grown on (110) and (111)SrTiO3. This differs from our previous data for tetragonal Pb(Zr,Ti)O3 films grown above TC on (110) and (111)SrTiO3 because the present films directly grow the ferroelectric films below TC without phase change. The tilting angle of the polar-axis and the volume fraction of the 90° domain can explain the piezoelectric responses of these films assuming that films have purely an up-state. This suggests that these films show almost pure up-state polarization without 180° domains, at least along surface-normal directions. These data show that these films have domain structures different from the well-known ones for the tetragonal Pb(Zr,Ti)O3 films.

Ferroelectric (Al, Sc)N thin films have the potential for use in low-power memory applications. This study demonstrates the thickness scalability of ferroelectricity down to an approximately 20 nm-thick (Al0.8Sc0.2)N film sandwiched between microfabricable TiN electrodes. The impact of the deposition gas atmosphere during the sputtering process and the top electrode materials on the crystal structures and ferroelectric properties was investigated for 20–30 nm-thick (Al0.8Sc0.2)N thin films deposited on Si substrates covered with a TiN layer by radio frequency magnetron sputtering. The deposition atmosphere did not strongly affect the crystal structures of the 30 nm-thick (Al0.8Sc0.2)N films but significantly affected their ferroelectric properties. The leakage current density decreased for films deposited under pure N2 gas compared to the films deposited under a gas mixture of 0.67Ar + 0.33N2. The ferroelectric properties of 20 nm-thick (Al0.8Sc0.2)N films were changed by the top electrode materials; both the switching electric field and its maximum applicable electric field increased for the TiN top electrodes compared with the Pt top electrodes, improving the saturation characteristics of the remnant polarization (Pr) against the applied electric field. Consequently, the 20 nm-thick (Al0.8Sc0.2)N film sandwiched between the microfabricable TiN top and bottom electrodes showed ferroelectricity without noticeable degradation with decreasing film thickness; the film maintained large Pr values of over 100 μC/cm2 in the temperature range from room temperature to 150 °C. The present data open the door to scalable ferroelectric random-access memories using almost thickness-degradation-free thin (Al, Sc)N films with microfabricable TiN electrodes.

In this study, the switching kinetics in a (001)-out-of-plane-oriented (Al0.8Sc0.2)N thin film were systematically investigated as a function of temperature and frequency. The switching behavior followed the Kolmogorov–Avrami–Ishibashi model within the temperature and write-pulse width ranges of 300–473 K and 10 Hz–600 kHz, respectively. The switching speed increased linearly with the electric field, even at high electric fields. The frequency dependence of the coercive field (Ec) evaluated at room temperature revealed that the scaling regime of the domain-wall propagation did not transition to the flow regime, and the switching velocity of the domain increased linearly with the electric field even at high frequencies up to 600 kHz. The invariant domain-wall motion regime implies the potential of using (Al,Sc)N films for high-frequency applications. In addition, a formula for estimating the coercive field of an (Al,Sc)N film was derived by fitting the parameters obtained from the study of switching behavior. The formula indicated that the switching electric field tends to become insensitive to frequency as temperature increases. Moreover, the frequency dependency of the field was minimal even at room temperature. This is a useful index for estimating the drive voltage at the operating frequency and temperature.

Ferroelectric hafnium and zirconium oxides have undergone rapid scientific development over the last decade, pushing them to the forefront of ultralow-power electronic systems. Maximizing the potential application in memory devices or supercapacitors of these materials requires a combined effort by the scientific community to address technical limitations, which still hinder their application. Besides their favorable intrinsic material properties, HfO2–ZrO2 materials face challenges regarding their endurance, retention, wake-up effect, and high switching voltages. In this Roadmap, we intend to combine the expertise of chemistry, physics, material, and device engineers from leading experts in the ferroelectrics research community to set the direction of travel for these binary ferroelectric oxides. Here, we present a comprehensive overview of the current state of the art and offer readers an informed perspective of where this field is heading, what challenges need to be addressed, and possible applications and prospects for further development.

The dependence of the output power of piezoelectric vibration energy harvesters on film thickness was systematically investigated using {100} _c -oriented epitaxial (K,Na)NbO₃ films prepared on single-crystal SrTiO₃ substrates by the hydrothermal deposition technique. First, we measured the vibration energy harvesting properties by a unimorph Pt/(K,Na)NbO₃//SrRuO₃//SrTiO₃ cantilever. Based on the fact that the elastic stiffness of (K,Na)NbO₃ is lower than that of SrTiO₃ substrate, it can be understood that increasing film thickness leads to decreasing Q _m, resulting in the decrease in output power as film thickness increased under the same input acceleration condition. In the next step, these cantilevers were attached to an Al plate having a much larger volume than the Pt/(K,Na)NbO₃//SrRuO₃//SrTiO₃ cantilever. The output power of this type of harvester was more than 400% greater when the film thickness was increased from 3.5 to 22.3 μm due to the almost constant Q _m value.

The composition dependence of ferroelectric properties was investigated for (111)-oriented epitaxial HfO ₂ -CeO ₂ solid solution films. Twenty nanometer thick films with different compositions were prepared on (111)ITO//(111)YSZ substrates at room temperature by pulsed laser deposition and subsequent heat treatment at 1000 °C under atmospheric N ₂ or O ₂ gas flow. All the films had fluorite structures, and their crystal symmetries changed from monoclinic through orthorhombic to tetragonal/cubic phases as x increased for the (Hf _{1− x} Ce _x )O ₂ ( x = 0.12–0.25) films. The orthorhombic phase was confirmed by X-ray diffraction analysis for films with x = 0.15 and 0.17. On the other hand, ferroelectric properties were observed in films with x = 0.15–0.20, suggesting that a field-induced phase transition takes place for films with x = 0.20. The film composition showing ferroelectricity was the widest range of doping concentration for reported epitaxial HfO ₂ -based films. Their remanent polarization ( P _r ) and coercive field ( E _c ) were almost identical, at 17–19 μ C cm ⁻² and 2.0–3.0 MV cm ⁻¹ . This wide ferroelectric composition range with relatively similar ferroelectricity is due to the solid solution of the same fluorite structure of HfO ₂ and CeO ₂ with monoclinic and cubic symmetries, that are respectively lower and higher crystal symmetries of the ferroelectric orthorhombic phase.

Two types of vibrators using hydrothermally synthesized ferroelectric (K, Na)NbO ₃ films were fabricated on SrTiO ₃ substrates. Cantilever piezoelectric vibrators with film thicknesses of 3.5–22.3 μ m were used under low-frequency and no resonance drive. The obtained output displacement increased almost linearly as the film thickness increased under both unipolar and bipolar drives. The obtained transverse piezoelectric coefficient, e _31,f , was almost independent of film thickness, ≈−5 C m ⁻² . On the other hand, a longitudinal piezoelectric vibrator was used under the resonance of half-length longitudinal vibration. The electromechanical coupling factor, k _eff ² , and the mechanical quality factor, Q _m , were evaluated. k _eff ² and Q _m respectively increased from 0.14% to 1.23% and decreased from 556 to 224 as the film thickness increased from 2.5 to 26 μ m. By applying a large voltage of 50 V _0-p , a large vibration velocity of 1.5 m s ⁻¹ was obtained for the vibrator with a film thickness of 26 μ m.

In this study, the bulk and surface acoustic waves (BAW and SAW) propagation properties of (K,Na)NbO ₃ (KNN) films deposited by hydrothermal synthesis or RF magnetron sputtering methods were evaluated to investigate the applicability of such films to high-frequency devices. For the {100} _c -oriented KNN epitaxial films deposited by the hydrothermal synthesis method, a BAW phase velocity of 6900 m s ⁻¹ and an electromechanical coupling coefficient k _t ² of 8.4% were obtained. From the measured Rayleigh-type SAW properties, a large electromechanical coupling coefficient K ² of 4.0% in the 1st mode was obtained in the {110} _c -oriented KNN epitaxial films. On the other hand, for the preferentially {100} _c -oriented KNN film deposited on Pt(111) by RF magnetron sputtering, a BAW phase velocity of 7850 m s ⁻¹ and k _t ² of 7.4% were obtained. For the 0th mode of the Rayleigh-type SAW, a propagation loss of 0.13 dB/ λ ( λ : wavelength) at 440 MHz and a temperature coefficient of frequency of –42 ppm °C ⁻¹ were obtained for the {100} _c -oriented KNN epitaxial film deposited on STO(100) by the RF magnetron sputtering method.