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Ferroelectricity and Curie temperature are demonstrated for epitaxial Y-doped HfO 2 film grown on (110) yttrium oxide-stabilized zirconium oxide (YSZ) single crystal using Sn-doped In 2 O 3 (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/cm 2 . Estimated spontaneous polarization based on the obtained saturation polarization and the crystal structure analysis was 45 μC/cm 2 . 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 HfO 2 -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.

(K0.89Na0.11)(Nb0.85Ta0.15)O3 thick films were epitaxially grown at 200 °C on (001)La:SrTiO3 and (001)cSrRuO3//(001)SrTiO3 substrates by hydrothermal method, and their crystal structures and electrical properties were investigated. Film thickness increased with deposition time and reached 6 µm in 10 h. High-temperature X-ray diffraction measurement showed that successive phase transitions from orthorhombic to tetragonal and from tetragonal to cubic phases take place at 120 and 400 °C, respectively. Microstructure analyses were performed by using electron microscopy, which revealed the existence of two types of stripe patterns with a width of 100 nm or less. In addition, scanning transmission electron microscopy–energy-dispersive X-ray spectroscopy elemental mapping showed that Nb/(Nb + Ta) ratio of the deposited films abruptly changed around 700 nm in thickness. Annealing at 500 °C led to the reduction in leakage current density from 102 to 10–5 A/cm2 at 30 kV/cm, showing that annealing is an effective way to improve insulation. Relative dielectric constant (εr) decreased linearly with increasing frequency, reaching 450 at 10 kHz. Polarization–electric field hysteresis loop and field-induced stain curve were measured by piezoelectric force microscopy, which showed remanent polarization (Pr) of 30 µC/cm2 and piezoelectric constant (d33,PFM) of 70 pm/V. These results demonstrate that (K,Na)(Nb,Ta)O3 thick films with superior electrical properties can be fabricated by the low-temperature deposition technique.

Lattice mismatch-induced biaxial strain effect on the crystal structure and growth mechanism is investigated for the BiFeO 3 films grown on La 0.6 Sr 0.4 MnO 3 /SrTiO 3 and YAlO 3 substrates. Nano-beam electron diffraction, structure factor calculation and x-ray reciprocal space mapping unambiguously confirm that the crystal structure within both of the BiFeO 3 thin films is rhombohedral by showing the rhombohedral signature Bragg’s reflections. Further investigation with atomic resolution scanning transmission electron microscopy reveals that while the ~1.0% of the lattice mismatch found in the BiFeO 3 grown on La 0.6 Sr 0.4 MnO 3 /SrTiO 3 is exerted as biaxial in-plane compressive strain with atomistically coherent interface, the ~6.8% of the lattice mismatch found in the BiFeO 3 grown on YAlO 3 is relaxed at the interface by introducing dislocations. The present result demonstrates the importance of: (1) identification of the epitaxial relationship between BFO and its substrate material to quantitatively evaluate the amount of the lattice strain within BFO film and (2) the atomistically coherent BFO/substrate interface for the lattice mismatch to exert the lattice strain.

Periodically structured nanomaterials such as superlattices have a wide range of applications. Many electronic devices have been fabricated from these materials. The formation of spontaneous layer structures using epitaxial growth has also been reported for many compound semiconductors but for very few ceramics. We demonstrate that strontium titanate (Sr-Ti-O) thin films having an A-site excess composition in the perovskite structure deposited by pulsed laser deposition under a magnetic field show a spontaneously formed superlattice structure. The spontaneous superlattice formation mechanism has been proven to exhibit spinodal decomposition. Preparation of a part of the phase diagram for Sr-Ti-O thin films is reported for the first time. Although SrTiO 3 bulk is quantum paraelectric, previous reports have described that strained SrTiO 3 thin films show room-temperature ferroelectricity, especially along the in-plane direction. However, induced ferroelectricity along the out-of-plane direction has been reported in films with a limited thickness of less than 10 ml. The results show that the Sr-Ti-O thin films with spontaneously formed superlattice structures exhibit room-temperature ferroelectricity even when 300 nm thick. The induced ferroelectricity is brought about by the strain along the out-of-plane direction and is explained based on thermodynamic considerations. Superlattices: crafting magnets using irresistible attractions Applying magnetic fields during growth of ceramic thin films creates a tipping point that spontaneously produces stress-based ferromagnets. While atoms in a normal crystal lattice are spaced closely together, researchers can now make ‘superlattices’ consisting of nanometer-wide strips of different crystals periodically stacked on top of each other. In semiconductors, thin films can be induced to form superlattices by tweaking the deposition conditions. Now, Naoki Wakiya from Shizuoka University in Japan and co-workers have replicated this feat for thin films of strontium titanate, an insulating oxide with magnetic and superconducting capabilities. The team used a pulsed laser to deposit strontium titanate onto a substrate and a strong magnetic field to stretch the usual crystal framework. The compressive stress in the new thin film initiates a transformation to a superlattice state and room-temperature ferroelectric behavior. We demonstrate that strontium titanate (Sr-Ti-O) thin films having A-site excess composition in perovskite structure deposited by pulsed laser deposition (PLD) under magnetic field show a spontaneously formed superlattice structure. The spontaneous superlattice formation mechanism is proved to be spinodal decomposition. Results show that the Sr-Ti-O thin films with spontaneously formed superlattice structure exhibit room-temperature ferroelectricity. The induced ferroelectricity is brought about by the strain along the out-of-plane direction, and is explained based on thermodynamic considerations.

We have studied the strain field around the 90° domains and misfit dislocations in PbTiO 3 /SrTiO 3 (001) epitaxial thin films, at the nanoscale, using the geometric phase analysis (GPA) combined with high-resolution transmission electron microscopy (HRTEM) and high-angle annular dark field--scanning transmission electron microscopy (HAADF-STEM). The films typically contain a combination of a/c-mixed domains and misfit dislocations. The PbTiO 3 layer was composed from the two types of the a-domain (90° domain): a typical a/c-mixed domain configuration where a-domains are 20-30 nm wide and nano sized domains with a width of about 3 nm. In the latter case, the nano sized a-domain does not contact the film/substrate interface; it remains far from the interface and stems from the misfit dislocation. Strain maps obtained from the GPA of HRTEM images show the elastic interaction between the a-domain and the dislocations. The normal strain field and lattice rotation match each other between them. Strain maps reveal that the a-domain nucleation takes place at the misfit dislocation. The lattice rotation around the misfit dislocation triggers the nucleation of the a-domain; the normal strains around the misfit dislocation relax the residual strain in a-domain; then, the a-domain growth takes place, accompanying the introduction of the additional dislocation perpendicular to the misfit dislocation and the dissociation of the dislocations into two pairs of partial dislocations with an APB, which is the bottom boundary of the a-domain. The novel mechanism of the nucleation and growth of 90° domain in PbTiO 3 /SrTiO 3 epitaxial system has been proposed based on above the results.

To realize a high-temperature capacitor, uniaxially (001)-oriented CaBi 4 Ti 4 O 15 films with various film thicknesses were prepared on (100) c SrRuO 3 /Ca 2 Nb 3 O 10 − nanosheet/glass substrates. As the film thickness decreases to 50 nm, the out-of-plane lattice parameters decrease while the in-plane lattice ones increase due to the in-plane tensile strain. However, the relative dielectric constant (ε r ) at room temperature exhibits a negligible degradation as the film thickness decreases to 50 nm, suggesting that ε r of (001)-oriented CaBi 4 Ti 4 O 15 is less sensitive to the residual strain. The capacitance density increases monotonously with decreasing film thickness, reaching a value of 4.5 μF/cm 2 for a 50-nm-thick nanofilm and is stable against temperature changes from room temperature to 400 °C irrespective of film thickness. This behaviour differs from that of the widely investigated perovskite-structured dielectrics. These results show that (001)-oriented CaBi 4 Ti 4 O 15 films derived using Ca 2 Nb 3 O 10 − nanosheets as seed layers can be made candidates for high-temperature capacitor applications by a small change in the dielectric properties against film thickness and temperature variations.

The ferroelectric domain structure of a PbTiO 3 thin film on (100) SrTiO 3 has been investigated by transmission electron microscopy (TEM). Two types of a-domain were found: one extended through the film to the surface and another comprised small a-domains confined within the film. Dark-field TEM (DFTEM) observation revealed that 180° domains formed near the substrate and stopped their growth 100 nm away from the substrate. The DFTEM observation also revealed that 90° domain boundaries had head-to-tail structures. To confirm the polarization direction obtained by experiments, diffracted intensities under a two-beam condition were simulated using the extended Darwin-Howie-Whelan equations. On the basis of the obtained results, a ferroelectric domain structure model of PbTiO 3 thin films on SrTiO 3 is proposed.

Lattice mismatch-induced biaxial strain effect on the crystal structure and growth mechanism is investigated for the BiFeO films grown on La Sr MnO /SrTiO and YAlO substrates. Nano-beam electron diffraction, structure factor calculation and x-ray reciprocal space mapping unambiguously confirm that the crystal structure within both of the BiFeO thin films is rhombohedral by showing the rhombohedral signature Bragg's reflections. Further investigation with atomic resolution scanning transmission electron microscopy reveals that while the ~1.0% of the lattice mismatch found in the BiFeO grown on La Sr MnO /SrTiO is exerted as biaxial in-plane compressive strain with atomistically coherent interface, the ~6.8% of the lattice mismatch found in the BiFeO grown on YAlO is relaxed at the interface by introducing dislocations. The present result demonstrates the importance of: (1) identification of the epitaxial relationship between BFO and its substrate material to quantitatively evaluate the amount of the lattice strain within BFO film and (2) the atomistically coherent BFO/substrate interface for the lattice mismatch to exert the lattice strain.

A well-crystallized Pb(Mg1/3Nb2/3)O3 (PMN) thin film on a Si wafer was prepared using low-temperature processing with a LaNiO3 (LNO) seeding layer. The PMN thin film was crystallized at 550 °C on a LNO/Si stacking structure. Additionally, we attempted to apply the compressive residual stress to PMN layers for a Curie temperature shift. Results show that the ferroelectric property remained at room temperature. The remanent polarization increased concomitantly with increasing annealing temperature.

The crystallization process of yttria-stabilized zirconia (YSZ) gate dielectrics deposited on p-Si (001) and SiOx/p-Si(001) substrates and the growth process of SiOx has been investigated directly using high-temperature in situ cross-sectional view transmission electron microscopy (TEM) method and high-temperature plan-view in-situ TEM method. The YSZ layer is crystallized by the nucleation and growth mechanism at temperatures greater than 573 K. Nucleation originates from the film surface. Nucleation occurs randomly in the YSZ layer. Subsequently, the crystallized YSZ area strains the Si surface. Finally, it grows in the in-plane direction with the strain, whereas, if a SiOx layer of 1.4 nm exists, it absorbs the crystallization strain. Thereby, an ultrathin SiOx layer can relax the strain generated in the Si substrate in thin film crystallization process.