To investigate the impact of mechanical stress on their ferroelectric properties, polycrystalline (Hf_0.5Zr_0.5)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 (Hf_0.5Zr_0.5)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/(Hf_0.5Zr_0.5)O₂/Pt/TiO₂/SiO₂, while ferroelectricity was barely observable for Pt/(Hf_0.5Zr_0.5)O₂/Pt/TiO₂/SiO₂/CaF₂. This result suggests that the in-plane tensile strain effectively enhanced the ferroelectricity of the (Hf_0.5Zr_0.5)O₂ thin films.

The influence of ion modification using rare-earth cations on crystal structures, along with the insulating and ferroelectric properties of BiFeO3 (BFO) thin films was investigated. Rare-earth-substituted BFO films with chemical compositions of (Bi1.00−xREx)Fe1.00O3 (x=0–0.15, RE=La and Nd) were fabricated on (111)Pt∕TiO2∕SiO2∕(100)Si substrates using a chemical solution deposition technique. A crystalline phase of rhombohedral BFO was obtained by heat treatment in a N2 atmosphere at 500°C for 5min. The crystal anisotropy and the Curie temperature of BFO were degraded continuously with increasing contents of La3+ or Nd3+ cations. Ion modification using La3+ and Nd3+ cations up to x=0.05 lowered the leakage current density of the BFO film at room temperature from approximately 10−3 down to 10−6A∕cm2. A polarization (P)-electrical field (E) hysteresis loop measured at 10K revealed that the intrinsic remanent polarization of La3+- and Nd3+-substituted BFO films with x=0.05 (44 and 51μC∕cm2, respectively) was smaller than that of a nonsubstituted BFO film (79μC∕cm2), which is ascribed to the degradation of crystal anisotropy and the Curie temperature of the BFO crystal.

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.

Thin films of ${\mathrm{Nd}}^{\text{3+}}$ -substituted bismuth titanate, $({\mathrm{Bi}}_{\text{4.00−y}},{\mathrm{Nd}}_y){\mathrm{Ti}}_{\text{3.00}}{\mathrm{O}}_{12}$ (BNT), ${\mathrm{Nd}}^{\text{3+}}/{\mathrm{V}}^{\text{5+}}$ -cosubstituted bismuth titanate, $({\mathrm{Bi}}_{\text{4.00−y}},{\mathrm{Nd}}_y\mathrm{)(}{\mathrm{Ti}}_{\text{3.00−x}}{\mathrm{V}}_x){\mathrm{O}}_{12}$ (BNTV), and ${\mathrm{La}}^{\text{3+}}$ -substituted bismuth titanate, $({\mathrm{Bi}}_{\text{3.25}},{\mathrm{La}}_{\text{0.75}}){\mathrm{Ti}}_{\text{3.00}}{\mathrm{O}}_{12}$ (BLT) were fabricated on the $\text{(111)}{\mathrm{Pt/Ti/SiO}}_2\text{/(100)}\mathrm{Si}$ substrates by a chemical solution deposition technique. These films possessed random-oriented polycrystalline structure. The BNT film had larger remnant polarization ${\mathrm{(P}}_r)$ than the BLT film; $P_r$ and coercive field ${\mathrm{(E}}_c)$ of the BNT film with $\text{y=0.50}$ were 32 μC/cm² and 126 kV/cm, respectively. Furthermore, ${\mathrm{V}}^{\text{5+}}$ substitution improved the $P_r$ value of the BNT film up to 37 μC/cm² (BNTV film; $\text{y=0.50,}$ $\text{x=0.02),}$ while the BNTV film had an $E_c$ value of approximately 119 kV/cm which was similar to that of the BNT film. Ferroelectric properties of the Pb-free polycrystalline BNT and BNTV films are comparable with those of conventional Pb-based ferroelectric films like a lead zirconate titanate.

In this study, the growth of (111)-oriented epitaxial and textured YO_1.5-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/TiO_x/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 length and resistive random access memory using ferroelectric tunnel junctions.

Orientation control of {100}-oriented epitaxial orthorhombic 0.07YO_1.5-0.93HfO₂ 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.