The ferroelectric properties and crystal structure of doped HfO₂ thin films were investigated for different thicknesses, electrode materials, and annealing conditions. Metal-ferroelectric-metal capacitors containing Gd:HfO₂ showed no reduction of the polarization within the studied thickness range, in contrast to hafnia films with other dopants. A qualitative model describing the influence of basic process parameters on the crystal structure of HfO₂ was proposed. The influence of different structural parameters on the field cycling behavior was examined. This revealed the wake-up effect in doped HfO₂ to be dominated by interface induced effects, rather than a field induced phase transition. TaN electrodes were shown to considerably enhance the stabilization of the ferroelectric phase in HfO₂ compared to TiN electrodes, yielding a P_r of up to 35 μC/cm². This effect was attributed to the interface oxidation of the electrodes during annealing, resulting in a different density of oxygen vacancies in the Gd:HfO₂ films. Ab initio simulations confirmed the influence of oxygen vacancies on the phase stability of ferroelectric HfO₂.

Recent research on piezoelectric materials is predominantly devoted to enhancing the piezoelectric coefficient, but overlooks its sign, largely because almost all of them exhibit positive longitudinal piezoelectricity. The only experimentally known exception is ferroelectric polymer poly(vinylidene fluoride) and its copolymers, which condense via weak van der Waals (vdW) interaction and show negative piezoelectricity. Here we report quantitative determination of giant intrinsic negative longitudinal piezoelectricity and electrostriction in another class of vdW solids—two-dimensional (2D) layered ferroelectric CuInP₂S₆. With the help of single crystal x-ray crystallography and density-functional theory calculations, we unravel the atomistic origin of negative piezoelectricity in this system, which arises from the large displacive instability of Cu ions coupled with its reduced lattice dimensionality. Furthermore, the sizable piezoelectric response and negligible substrate clamping effect of the 2D vdW piezoelectric materials warrant their great potential in nanoscale, flexible electromechanical devices.

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 ferroelectricity of (Al_1−xSc_x)N (x = 0–0.34) thin films with various thicknesses was investigated. (Al_1−xSc_x)N films were prepared at 400 °C on (111)Pt/TiO_x/SiO₂/(001)Si substrates by the radio frequency dual-source reactive magnetron sputtering method using Al and Sc targets under pure N₂ gas or a mixture of N₂ and Ar gases. The film deposited under N₂ gas showed larger remanent polarization than those under N₂+ Ar mixture. Ferroelectricity was observed for films with x = 0.10–0.34 for about 140-nm-thick films deposited under N₂ gas. The x = 0.22 films showed ferroelectricity down to 48 nm in thickness from the polarization–electric field curves and the positive-up-negative-down measurement. The ferroelectricity of the 9 nm-thick film also was ascertained from scanning nonlinear dielectric microscopy measurement. These results reveal that ferroelectric polarization can switch for films with much broader compositions and thicknesses than those in the previous study.

Ferroelectricity and Curie temperature are demonstrated for epitaxial Y-doped HfO₂ film grown on (110) yttrium oxide-stabilized zirconium oxide (YSZ) single crystal using Sn-doped In₂O₃ (ITO) as bottom electrodes. The XRD measurements for epitaxial film enabled us to investigate its detailed crystal structure including orientations of the film. The ferroelectricity was confirmed by electric displacement filed – electric filed hysteresis measurement, which revealed saturated polarization of 16 μC/cm². Estimated spontaneous polarization based on the obtained saturation polarization and the crystal structure analysis was 45 μC/cm². This value is the first experimental estimations of the spontaneous polarization and is in good agreement with the theoretical value from first principle calculation. Curie temperature was also estimated to be about 450 °C. This study strongly suggests that the HfO₂-based materials are promising for various ferroelectric applications because of their comparable ferroelectric properties including polarization and Curie temperature to conventional ferroelectric materials together with the reported excellent scalability in thickness and compatibility with practical manufacturing processes.

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.