The thickness dependences of the crystal structure and electric properties of (111)-oriented epitaxial 0.07YO1.5-0.93HfO2 (YHO7) ferroelectric films were investigated for the film thickness range of 10–115 nm. The YHO7 films were grown by pulsed laser deposition or sputtering at room temperature and subsequent heat treatment. As a substrate for the epitaxial growth of the YHO7 film, (111)-oriented 10 wt. % Sn-doped In2O3(ITO)//(111) yttria-stabilized zirconia was used. X-ray diffraction measurements confirmed that the main crystal phase of these YHO7 films was ferroelectric orthorhombic for up to 115-nm-thick films. Small film-thickness dependences of remanent polarization (Pr) and saturation polarization (Ps) were observed. Thickness dependence of the coercive field (Ec) is also small, and this behavior does not resemble that of conventional ferroelectric films such as Pb(Zr,Ti)O3. Additionally, non-oriented polycrystalline YHO7 films are reported to have similar thickness dependence of Ec and almost the same Ec value to epitaxial YHO7 films. We suggest that the ferroelectric domain is significantly small for both epitaxial and polycrystalline films. Such small domains remain even in thicker films, giving rise to thickness-independent Ec.

Herein, ferroelastic domain switching from the nonpolar b-axis to the polar c-axis oriented domain in 7%-YO1.5-substituted HfO2 (YHO-7) epitaxial ferroelectric films is demonstrated. Scanning transmission electron microscopy (STEM) indicates that the polarization of a pristine film deposited on a Sn-doped In2O3/(001)YSZ substrate by the pulsed laser deposition method tends to be along the in-plane direction to avoid a strong depolarization field with respect to the out-of-plane direction. Applying an electric field aids in ferroelastic domain switching in YHO-7 films. Such films exhibit ferroelectric characteristics with a relatively large saturated polarization around 30 μC/cm2 by polarization reorientation from the in-plane to the out-of-plane directions and an increased dielectric constant. The synchrotron X-ray diffraction measurements with a focused beam for the pristine and poled area indicate ferroelastic 90° domain switching as the odd number reflection disappears, which is only allowed in the nonpolar b-axis orientation. STEM observations also show a significant increase in the c-axis oriented domain. This observation of ferroelastic domain switching strongly supports the conclusion that the ferroelectricity of HfO2 originates from the non-centrosymmetric orthorhombic phase.

Ferroelectricity has been demonstrated in polycrystalline 7%Y-doped HfO2 (YHO7) films with thicknesses ranging from 10 to 930 nm, which were grown on (111)Pt/TiOx/SiO2/(001)Si substrates by pulsed laser deposition at room temperature and subsequent annealing at 1000 °C. The X-ray diffraction pattern suggested that the major crystal phase consists of orthorhombic/tetragonal phases with a small amount of monoclinic phase even for the 930-nm-thick film despite its thickness. Moreover, the hysteresis loops associated with the ferroelectric orthorhombic phase were clearly observed for all samples including even the 930-nm-thick film. The remnant polarization (Pr) and the coercive field (Ec) are 14–17 μC/cm2 and 1300–1600 kV/cm, respectively, at max applied electric fields of ∼4000 kV/cm for all YHO7 films within the present study. These results indicate that the ferroelectric structure and properties of YHO7 films are insensitive to the film thickness.

One of the general features of ferroelectric systems is a complex nature of polarization reversal, which involves domain nucleation and motion of domain walls. Here, time‐resolved nanoscale domain imaging is applied in conjunction with the integral switching current measurements to investigate the mechanism of polarization reversal in yttrium‐doped HfO2 (Y:HfO2)—currently one of the most actively studied ferroelectric systems. More specifically, the effect of film microstructure on the nucleation process is investigated by performing a comparative study of the polarization switching behavior in the epitaxial and polycrystalline Y:HfO2 thin film capacitors. It is found that although the epitaxial Y:HfO2 capacitors tend to switch slower than their polycrystalline counterparts, they exhibit a significantly higher nucleation density and rate, suggesting that this is a rate‐limiting mechanism. In addition, it is observed that under the external fields approaching the activation field value, the switching kinetics can be described equally well by the nucleation limited switching and the Kolmogorov‐Avrami‐Ishibashi models for both types of capacitors. This signifies convergence of two different mechanisms implying that the polarization reversal proceeds via a homogeneous nucleation process unaffected by the film microstructure, which can be considered as approaching the intrinsic switching limit.

Ferroelectricity has been demonstrated in epitaxial 7%Y-doped HfO2 (0.07YO1.5–0.93HfO2, YHO7) films grown by the RF magnetron sputtering method at room temperature without any subsequent annealing. The x-ray diffraction patterns of such films suggested that the decrease in RF power and in the partial oxygen pressure changes the crystal structures of the films from the monoclinic phase to the tetragonal/orthorhombic phase. Clear polarization-electric-field (P–E) hysteresis loops were observed for these epitaxial films with the tetragonal/orthorhombic phase. The obtained remanent polarization (Pr) and coercive field (Ec) values were 14.5 and 12.8 μC/cm2 and 2300 and 2200 kV/cm for the epitaxial films on (111) indium tin oxide (ITO)//(111) yttria-stabilized zirconia (YSZ) and (100)ITO//(100)YSZ substrates, respectively. Moreover, ferroelectricity was also observed in room-temperature-deposited polycrystalline YHO7 films prepared on Pt/TiOx/SiO2/(100)Si, crystallized ITO/soda glass, and amorphous ITO/polyethylene terephthalate substrates, namely, crystalline ferroelectric HfO2-based films were prepared at room temperature on various substrates, including organic flexible substrates, by using the RF magnetron sputtering method. The present results open a path to novel applications of ferroelectric HfO2-based films such as ferroelectric flexible memory.

The process of forming the ferroelectric orthorhombic phase was investigated for epitaxial 7% Y-doped (YHO7) films using in situ high-temperature X-ray diffraction. Epitaxial YHO7 films were grown on (111) ITO-coated (111)YSZ substrates by pulsed laser deposition at room temperature and a subsequent heat treatment process. Films deposited at room temperature were crystallized as paraelectric monoclinic phase. The monoclinic phase partially changes to tetragonal phase above 600 °C and perfectly transformed around 950 °C during heating. The change from tetragonal phase to orthorhombic phase was detected at 300 °C, corresponding to the Curie temperature under the cooling process. These results clearly suggest that the tetragonal phase was more stable at 1000 °C for YHO7 films on heating than the other phases, and the formation of this tetragonal phase—the high-temperature paraelectric phase of the ferroelectric orthorhombic phase—is key to the formation of the ferroelectric orthorhombic phase.

The transition between the dielectric tetragonal and ferroelectric orthorhombic phases in 7%Y doped HfO2 and Hf0.5Zr0.5O2 films with various orientations and film thicknesses was investigated by high-temperature x-ray diffraction. All films demonstrate a different phase transition temperature on heating and cooling with thermal hysteresis with a gap of ΔT. This result clearly shows that the phase transition of the ferroelectric HfO2-based film is first order. The ΔT value of 40–210 °C in HfO2-based films is larger than that of other ferroelectric materials but similar to that of martensitic materials with large lattice deformation. This implies that the ferroelectric phase transition of HfO2-based films involves large lattice deformation. Moreover, we show that ΔT is changed by the size and composition effects. Our results are a step toward elucidating the mechanism of phase transition in ferroelectric HfO2-based films.

The presence of the top electrode on hafnium oxide‐based thin films during processing has been shown to drive an increase in the amount of metastable ferroelectric orthorhombic phase and polarization performance. This “Clamping Effect,” also referred to as the Capping or Confinement Effect, is attributed to the mechanical stress and confinement from the top electrode layer. However, other contributions to orthorhombic phase stabilization have been experimentally reported, which may also be affected by the presence of a top electrode. In this study, it is shown that the presence of the top electrode during thermal processing results in larger tensile biaxial stress magnitudes and concomitant increases in ferroelectric phase fraction and polarization response, whereas film chemistry, microstructure, and crystallization temperature are not affected. Through etching experiments and measurement of stress evolution for each processing step, it is shown that the top electrode locally inhibits out‐of‐plane expansion in the HZO during crystallization, which prevents equilibrium monoclinic phase formation and stabilizes the orthorhombic phase. This study provides a mechanistic understanding of the clamping effect and orthorhombic phase formation in ferroelectric hafnium oxide‐based thin films, which informs the future design of these materials to maximize ferroelectric phase purity and corresponding polarization behavior.