Kazuki Okamoto

Piezoelectric thin films have been widely used in microelectromechanical systems (MEMS), where Pb(Zr _x Ti _{1‐ x} )O ₃ is a representative material. To achieve significant piezoelectric responses, multidomain structural changes under electric fields must be understood via domain‐lattice observations. Here, (100)/(001)‐oriented tetragonal Pb(Zr _x Ti _{1‐ x} )O ₃ epitaxial films with a small (001)‐oriented domain fraction were grown on (100) Si substrates for piezo‐MEMS. Although previous studies have primarily focused on out‐of‐plane piezoelectric responses (33‐mode actuation), in‐plane (31‐mode actuation) is predominantly utilized in piezo‐MEMS devices. Therefore, we investigate the microscopic structural responses associated with both modes. Operando synchrotron x‐ray diffraction under pulsed electric fields applied perpendicular to the film surface was used to probe out‐of‐plane and in‐plane lattice evolution. Domain switching from (100) to (001) was accompanied by in‐plane lattice elongation and out‐of‐plane shrinkage due to substrate‐clamping. Time‐resolved measurements revealed out‐of‐plane lattice dynamics between domain orientations during pulse application. This study provides an operando correlation between ferroelastic domain switching and both out‐of‐plane and in‐plane lattice responses relevant to 33‐ and 31‐mode actuation in Si‐compatible multidomain Pb(Zr _x Ti _{1‐ x} )O ₃ films, providing key insights for optimizing the piezoelectric properties of clamped epitaxial films for MEMS applications.

The effects of deposition temperature on constituent phases, orientation, and ferroelectric and piezoelectric properties were investigated for 200 nm- and 400 nm-thick HfO ₂ –CeO ₂ films on (111)ITO//(111)YSZ substrates. The constituent phases and their orientations were primarily governed by the deposition temperature. An orthorhombic/tetragonal phase was obtained at deposition temperatures up to 400 ^o C, while a monoclinic phase coexisted with it at temperatures above 500 ^o C. A mixture (111)+{110} orientation transitioned to a single (111) orientation when the temperature increased from without heating (room temperature; R.T.) to 400 ^o C. For deposition temperatures up to 700 °C, the degree of this (111) orientation steadily improved with increasing deposition temperature. Ferroelectricity and piezoelectricity were observed up to 500 ^o C. A maximum remanent polarization value of 10–12 µC cm ⁻² was obtained at around 400 ^o C –500 ^o C, while the coercive field decreased with increasing deposition temperature. A large effective piezoelectric constant (>30 pm V ⁻¹ ) was observed for 400 nm-thick films deposited without heating and at 400 ^o C.

The effect of x = Ce/(Hf+Ce) (0.15–0.25) on dielectric, ferroelectric, and piezoelectric properties was investigated together with the crystal structure for 500 nm-thick x CeO ₂ −(1− x ) HfO ₂ films. The films were deposited on (111) (indium tin oxide, ITO)//(111) (yttrium-stabilized zirconia, YSZ) substrates using the sputtering method without heating, followed by a post-heat treatment at 1000 °C. X-ray diffraction analysis showed that the films showed {200} diffraction peaks of tetragonal and/or orthorhombic (T./O.) phase. Lattice spacing of the {200} _T./O . linearly increased with x value increasing. The relative dielectric constant reached a maximum at x = 0.22, while dielectric loss was below 0.07 irrespective of the x value. Ferroelectric property and piezoelectric response originating from the ferroelectricity were observed for the films with x = 0.15–0.22. Largest remnant polarization of 10 μC cm ⁻² and effective vertical piezoelectric constant, d _{33, f} , of 16 pm V ⁻¹ were obtained at x = 0.22 and 0.21, respectively.

Ferroelectric wurtzite-structure films serve as pivotal building blocks for next-generation microelectronics. However, their inherently high coercive fields severely limit their implementation in low-power device architectures. Clarifying polarization switching mechanisms and identifying key performance-regulating parameters are, therefore, critical to advancing wurtzite-type ferroelectric materials. In this paper, we discuss the switching kinetics across atomic, nano, and macroscopic levels. To improve the switching performance of scaled (Al0.9Sc0.1)N films, we investigate the effect of a bottom electrode modified via post-deposition annealing on polarization switching characteristics. Our results reveal that enhanced electrode crystallinity enables precise tuning of the c/a axis ratio and in-plane compressive strain in (Al0.9Sc0.1)N films. Remarkably, the scaled (Al0.9Sc0.1)N films not only preserve the Kolmogorov–Avrami–Ishibashi (KAI) switching mode but also achieve a significant one-order-of-magnitude acceleration in switching speed compared to films grown on as-deposited Pt. Moreover, reduced electrode area promotes faster switching behavior, and thinner (Al0.9Sc0.1)N layers deposited on annealed Pt electrodes show superior switching speed relative to those on pristine Pt electrodes. The present study establishes optimization of bottom electrode quality as an effective strategy to overcome scaling challenges in (Al0.9Sc0.1)N films, laying a reliable foundation for their integration into high-density, high-speed ferroelectric microelectronic devices.

This study investigates the control of the domain configuration in tetragonal Pb(Zr _0.3 Ti _0.7 )O ₃ (PZT) thin films based on strain using Sr(Zr _y Ti _{1- y} )O ₃ (SZTO) buffer layers grown on (100) [(LaAlO ₃ ) _0.3 -(SrAl _0.5 Ta _0.5 O ₃ ) _0.7 ] substrates. The high-temperature heat treatment of SZTO layers was used to improve crystallinity and increase thermal stability during the subsequent high-temperature deposition of the bottom electrode layer and PZT films. Conductive La–SrSnO ₃ and (Ba _0.4 Sr _0.6 )RuO ₃ bottom electrodes with lattice parameters compatible with SZTO were introduced between the PZT film and SZTO layer for electrical characterization and strain control. Reciprocal space mapping revealed that purely a -domain [(100) orientation] and c -domain [(001) orientation] structures were achieved by controlling the strain of the film growth using these buffer layers. The present approach can be applied to a wide range of ferroelectric thin films by tuning the lattice parameters of the strain-control layers.

Displacive ferroelectrics exhibit a strong coupling between lattice anisotropy and intrinsic structural distortions, and epitaxial constraints in thin films make it essential to directly monitor strain-driven structural responses under controlled conditions. Here, we report in situ high-temperature x-ray diffraction of hydrothermally grown PbTiO3 epitaxial films with negligible residual strain on (100) Nb-doped SrTiO3 substrates, where minimized substrate-induced lattice and thermal strains during deposition enable ferroelectric-phase growth without a growth-induced phase transition. Upon heating, the films undergo a discontinuous ferroelectric-to-paraelectric transition, during which partial relaxation is enabled by thermally active defect migration, likely associated with misfit dislocations. Upon cooling, the structure evolves relatively continuously, with a domains nucleating at the transition temperature to relieve anisotropic misfit strain, followed by gradual relaxation through domain wall evolution. These findings highlight the importance of determining the strain relaxation mediated by domain formation and, together with theoretical predictions, offer a framework for distinguishing strain–structure relationships in epitaxial thin films from those in bulk counterparts.

Nanocrossbar ferroelectric tunnel junctions (FTJs) with TiO _x /Y-doped HfO ₂ /Pt were fabricated at 25 nm. Direct tunneling was observed in the high-resistance state, achieving a TER ratio of 2.2 × 10 ³ for non-volatile memory for CMOS back-end integration.

The total thickness scaling of electrodes and the (Al _0.9 Sc _0.1 )N layer is evaluated for ferroelectric memory applications. The good crystal orientation and large remanent polarization ( P _r ) above 100 µC cm ⁻² are obtained for 160‐nm‐thick (Al _0.9 Sc _0.1 )N films even when the Pt bottom electrode is reduced to a thickness of 5 nm. The (Al _0.9 Sc _0.1 )N film integrated on a 5‐nm‐thick Pt bottom electrode maintains a saturated P _r value above 100 µC cm ⁻² at a thickness of 30 nm. Furthermore, the (Al _0.9 Sc _0.1 )N film exhibits improved crystal orientation by the post‐heat‐treatment of the Pt bottom electrode, and a saturated large P _r value above 100 µC cm ⁻² is obtained at a thickness of 20 nm. Moreover, a P _r value of 90 µC cm ⁻² is obtained at a (Al _0.9 Sc _0.1 )N thickness of 10 nm. No significant degradation in the P _r characteristics is detected when the thickness of the Pt top electrode is reduced to 5 nm. Finally, the total thickness is successfully scaled down to 30 nm for a Pt (top electrode, 5 nm)/(Al _0.9 Sc _0.1 )N (20 nm)/Pt (bottom electrode, 5 nm) stack. This work presents guidelines for the thickness scaling of the (Al,Sc)N‐based stacks for ferroelectric memory applications, including ferroelectric thin films and electrodes.

The ferroelectric properties and structural phase transition behaviors of fluorite-type CeO₂-HfO₂-ZrO₂ films were investigated. The epitaxial films on indium tin oxide (ITO) (111)/yttria-stabilized zirconia (YSZ) (111) substrates were grown through pulsed laser deposition at room temperature and subsequently heat-treated at 1000 °C under a N₂ gas flow. The crystalline phases and Curie temperatures of the films were investigated by X-ray diffraction. An increase in the Ce or Zr content in the films led to a higher crystallographic symmetry, such as orthorhombic or tetragonal. In addition, electrical characterization revealed that the orthorhombic films and some of the tetragonal films displayed ferroelectricity. This was due to the field-induced phase transition from the tetragonal to ferroelectric orthorhombic phase in the films, where the Curie temperatures were relatively low. The tetragonal metastable phase was kinetically frozen and could not change into the stable orthorhombic phase at such a low temperature. The critical electric field where the field-induced phase transition occurred was below 0.8 MV/cm, which was sufficiently small compared to the coercive field. These results evidence the kinetic driving force that causes a field-induced phase transition from the paraelectric tetragonal phase to the ferroelectric orthorhombic phase in HfO₂-based ferroelectrics. They also enhance our understanding of the thermodynamic phase stabilities of HfO₂-based material polymorphs.