Takanori Mimura

<jats:title>Abstract</jats:title><jats:p>Materials with tunable thermal properties enable on-demand control of temperature and heat flow, which is an integral component in the development of solid-state refrigeration, energy scavenging, and thermal circuits. Although gap-based and liquid-based thermal switches that work on the basis of mechanical movements have been an effective approach to control the flow of heat in the devices, their complex mechanisms impose considerable costs in latency, expense, and power consumption. As a consequence, materials that have multiple solid-state phases with distinct thermal properties are appealing for thermal management due to their simplicity, fast switching, and compactness. Thus, an ideal thermal switch should operate near or above room temperature, have a simple trigger mechanism, and offer a quick and large on/off switching ratio. In this study, we experimentally demonstrate that manipulating phonon scattering rates can switch the thermal conductivity of antiferroelectric PbZrO<jats:sub>3</jats:sub> bidirectionally by −10% and +25% upon applying electrical and thermal excitation, respectively. Our approach takes advantage of two separate phase transformations in PbZrO<jats:sub>3</jats:sub> that alter the phonon scattering rate in different manners. In this study, we demonstrate that PbZrO<jats:sub>3</jats:sub> can serve as a fast (&lt;1 second), repeatable, simple trigger, and reliable thermal switch with a net switching ratio of nearly 38% from ~1.20 to ~1.65 W m<jats:sup>−1</jats:sup> K<jats:sup>−1</jats:sup>.</jats:p>

Abstract The asymmetry in the capacitance–voltage (C–V) curves obtained from a ferroelectric material can provide information concerning the internal microstructure of a specimen. The present study visualized nanoscale switching of a HfO₂-based ferroelectric thin film in real space based on assessing asymmetry using a local C–V mapping method. Several parameters were extracted from the local C–V curves at each point. The parameter V_i, indicating the lateral shift of the local C–V curve, was employed as an indicator of local imprint. In addition, the differences in the areas between the C–V curves for the forward and reverse sweeps, S_f − S_r, provided another slightly different indicator of nanoscale switching asymmetry. These parameters obtained from asymmetric C–V curves are thought to be related to internal electric fields and local stress caused by defects in the film. The work reported here also involved a cluster analysis of the extracted parameters using the k-means method.

<jats:p> Electronic conduction pathways in dielectric thin films are explored using automated experiments in scanning probe microscopy (SPM). Here, we use large field of view scanning to identify the position of localized conductive spots and develop an SPM workflow to probe their dynamic behavior at higher spatial resolution as a function of time, voltage, and scanning process in an automated fashion. Using this approach, we observe the variable behaviors of the conductive spots in a 20-nm-thick ferroelectric Hf<jats:sub>0.54</jats:sub>Zr<jats:sub>0.48</jats:sub>O<jats:sub>2</jats:sub> film, where conductive spots disappear and reappear during continuous scanning. There are also fresh conductive spots that develop during scanning. The automated workflow is universal and can be integrated into a wide range of microscopy techniques, including SPM, electron microscopy, optical microscopy, and chemical imaging. </jats:p>

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.

Direct integration of transition metal dichalcogenides on a ferroelectric such as hafnium zirconium oxide (HZO) using an industrially scalable technique is important for realizing various ferroelectric-based device architectures. The interface formed due to the processing conditions during direct deposition is the focus of the current study. In this work, molecular beam epitaxy (MBE) is used to directly deposit WSe2 on HZO substrates, and the effects of the MBE growth conditions, specifically high temperature and a high Se flux, are examined. Anneals of HZO under a Se flux, which serve to replicate the conditions during actual WSe2 deposition, result in the crystallization of amorphous as-deposited HZO substrates and incorporation of Se into the HZO. The crystallinity and composition of the HZO substrates affect the degree of Se incorporation. Some of the Se found in the HZO is an adsorbed layer that can be thermally desorbed, but it also has a chemisorbed component fully incorporated within the HZO lattice. Measurement of the electrical properties of the HZO films did not provide evidence that the incorporated Se was detrimental to the functionality of the HZO as a ferroelectric layer.

Y-doped HfO2 ferroelectric films of approximate to 1 mu m thick are deposited without heating by a radio frequency magnetron sputtering method. {100}-oriented epitaxial films with orthorhombic phase are grown on (100)ITO//(100)YSZ substrates without heating. Their crystal structure is almost unchanged after postheat treatment at 800 degrees C. Ferroelectricity is confirmed for the no-heating-deposited films by polarization-electric field (P- E) curves, and their remanent polarization (P-r) and coercive fields are 12 mu C cm(-2) and 1.5 MV cm(-1), respectively. These values are also almost unchanged after the postheat treatment. However, the postheat-treated films show a lower breakdown electric field compared to the as-deposited films without heat treatment. Approximately 1 itm-thick films are also prepared without heating on (111)ITO/(111)Pt/TiOx/SiO2 /(100)Si and (111)Pt/TiOx/SiO2/(100)Si substrates. Almost pure orthorhombic/tetragonal phase is deposited on both substrates without heating. The P-r value of the film on the (111)ITO/(111) Rt/TiOx/SiO2/(100)Si substrate is about 1.5 times larger than that on the (111)Pt/TiOx/SiO2/(100)Si substrate due to the better crystallinity of the film lattice-matched with the underlying ITO layer. The effective piezoelectric constant (d(33, f)) of the film deposited on the (111)ITO/(111)Pt/TiOx/SiO2/(100)Si substrate without heating is estimated to be 4 pm V-1. The present low process temperature leads us to expect novel applications, especially for low-heat-resistance applications.

Local C-V mapping based on scanning nonlinear dielectric microscopy (SNDM) is a useful tool for characterizing ferroelectric domain dynamics at the nanoscale. In this study, we realized high-precision C-V mapping through an improved measurement system that introduces a digitizer and post-signal processing. Coupled with the high capacitance detection sensitivity of SNDM, nontrivial patterns corresponding to the polarization response were observed even in harmonic images with an order as high as thirty. Using these high-order harmonic components for the C-V curve resynthesis led to improved measurement accuracy. In addition, further improvement was achieved by introducing noise reduction based on principal component analysis. From the C-V curves resynthesized in this manner, parameters that represent the curve features could be extracted and subsequently displayed in two-dimensional maps. Detailed analyses of the datasets obtained with a ferroelectric HfO2 film revealed interesting distributions concerning local polarization switching dynamics.

The thickness dependence of the crystal structure in epitaxial (HfxZr1-x)O-2 (x = 0, 0.5, 1) films is demonstrated. X-ray diffraction measurements suggest that the crystal phase changes from the monoclinic to the orthorhombic and tetragonal phases sequentially as film thickness decreases in HfO2 and (Hf0.5Zr0.5)O-2 films. Both films are very thin at the ferroelectric orthorhombic phase (< 5nm), but the (Hf0.5Zr0.5)O-2 films are thicker than the HfO2 films. In ZrO2 films, the monoclinic phase changes into the tetragonal phase as film thickness decreases. This means that the ferroelectric orthorhombic phase was not observed in the present study. However, ferroelectricity was observed for the present ZrO2 films in the film thickness around the thickness boundary where the tetragonal and monoclinic phases stabilize. We confirmed the field-induced phase transition from the tetragonal to the orthorhombic phase in these ZrO2 films by micro-x-ray diffraction measurement, which suggested that the phase transition takes place due to the small free energy difference between the ferroelectric orthorhombic phase and paraelectric tetragonal phase. Our experimental results agree with theoretical reports of the thickness dependence of the crystal structure in epitaxial films with a surface energy effect.

The mechanisms leading to wake-up and fatigue in ferroelectric hafnium zirconium oxide thin film devices with symmetric RuO2 electrodes are investigated via polarization, relative permittivity, dielectric nonlinearity, pyroelectric coefficient, and microfocus x-ray diffraction (XRD) measurements. The devices are observed to wake-up for up to 10(3) bipolar pulsed field cycles, after which fatigue occurs with polarization approaching zero following 10(8) cycles. Wake-up is accompanied by a decrease in both high-field permittivity and hysteresis loop pinching and an increase in the pyroelectric coefficient, indicating that the wake-up process involves a combination of transformations from the tetragonal to the orthorhombic phase and domain depinning from defect redistribution. Fatigue is observed to coincide with an increase in irreversible domain wall motion and a decrease in pyroelectric coefficient. Finite pyroelectric coefficients are measured on fully fatigued devices, indicating that domain pinning is a strong contributor to fatigue and that fatigued devices contain domain structures that are unable to switch under the fields applied for measurement. Microfocus XRD patterns measured on each device reveal that the phase constitution is qualitatively unaffected by field cycling and resultant polarization fatigue. These data indicate that the wake-up process has contributions from both phase transformations and domain depinning, whereas the fatigue process is driven primarily by domain pinning, and the near-zero measured switchable polarization is actually a poled device with immobile domains. These observations provide insight into the physical changes occurring during field cycling of HfO2-based ferroelectrics while examining a possible oxide electrode material for silicon CMOS device implementation.

The crystal structure and ferroelectric properties of 12- to 18 nm-thick epitaxial YO1.5-HfO2 films with 5-9% YO1.5 on (111)ITO//(111)YSZ substrates are investigated to clarify the formation mechanism of the ferroelectric phase. The ferroelectric orthorhombic phase can be obtained by transformation from the higher symmetric tetragonal phase by surmounting a relatively low energy barrier. The orthorhombic phase is obtained for 6% and 7% YO1.5-doped HfO2 films by heat treatment at 1000 degrees C. Although the 5% YO1.5-doped HfO2 film heat-treated at 1000 degrees C is in a monoclinic phase, the orthorhombic phase was increased by heat treatment at 1200 degrees C because the high temperature promotes the phase transition from the monoclinic phase in as-deposited films to the tetragonal phase. The 8% and 9% YO1.5-doped HfO2 films have a tetragonal structure without the transition to the orthorhombic phase. Nevertheless, the 8% YO1.5-doped HfO2 film exhibits ferroelectricity by polarization-electric field hysteresis measurement. A microarea X-ray diffraction study reveals that the electric-field-induced phase transition can take place in an 8% YO1.5-doped HfO2 film. The comprehensive study of high-temperature X-ray diffraction measurements implies that the tetragonal phase in 8% YO1.5-doped HfO2 is a supercooled state. Therefore, external stimulation, such as application of an electric field, induces the transition from the tetragonal to the orthorhombic phase. The supercooled tetragonal phase can also be reduced by a slower cooling rate. These results reveal that the formed phase in YO1.5-doped HfO2 epitaxial film is not governed by the simple difference in the formation energy; rather, the kinetics is more important for obtaining the ferroelectric orthorhombic phase.

In this paper, we propose local C-V mapping as a novel method for characterizing ferroelectric domain switching on a nanoscale. This method is an extension of scanning nonlinear dielectric microscopy (SNDM), which is one of the ferroelectric domain observation methods. In the conventional SNDM, a small AC bias is applied to the sample to determine the polarization direction by the phase of the response signal of the capacitance variation induced by the AC bias. On the other hand, in the local C-V mapping proposed this time, a large-Amplitude AC bias is applied to the sample, and the capacitance variation is acquired under the condition that polarization switching occurs. When the sample is a ferroelectric material, the observed C-V curves draw characteristic butterfly-shaped loops. By analyzing these C-V butterfly curves, various information on the domain switching dynamics can be obtained. While conventional C-V measurements are generally performed on a macroscale using micro-to millimeter-scale electrodes, the probe microscopy framework and the high capacitance sensitivity of SNDM enable us to measure local C-V curves with a nanoscale probe tip. In this study, we characterized a randomly-oriented HfO2 film as a demonstration of the proposed method. As a result, we succeeded in visualizing how the switchable and unswitchable regions coexist in the real space. In addition, even inside the switchable region, the observed C-V curve shapes varied depending on the position, suggesting the spatial inhomogeneity in domain switching properties. This method also allows us to map parameters extracted from the C-V curve datasets. Such parameter maps provide a wealth of information on the nanoscale distribution in the switchable polarization, coercive field, and local imprint.

The ferroelectric phase transformation from the tetragonal phase to the orthorhombic phase, induced by an electric field, is demonstrated in a 5%YO1.5-doped Hf0.5Zr0.5O2 epitaxial film which is grown on Sn-doped In2O3-covered (111) yttria-stabilized zirconia by the pulsed laser deposition method at room temperature and subsequent heat treatment. Although X-ray diffraction shows the film to consist of a paraelectric tetragonal phase after the heat treatment, polarization-electric field (P-E) measurements reveal a hysteresis loop attributed to the ferroelectricity. To clarify the discrepancy between the crystal structure and electric characteristics, the crystal structure after electric field loading is determined by scanning transmission electron microscopy and synchrotron X-ray diffraction measurements. Both structural characterizations clearly reveal that the application of an electric field promotes the phase transition from the paraelectric tetragonal phase to the ferroelectric orthorhombic phase. This ferroelectric transition is irreversible, as the ferroelectric phase remains after the removal of the electric field. These results facilitate the elucidation of the mechanism by which ferroelectricity is displayed in HfO2-based fluorite ferroelectric materials and imply unimportance of the orthorhombic phase for as-prepared films.

We experimentally show that the thermal conductance across confined solid-solution crystalline thin films between parent materials does not necessarily lead to an increase in thermal resistances across the thin-film geometries with increasing film thicknesses, which is counterintuitive to the notion that adding a material serves to increase the total thermal resistance. Confined thin epitaxial Ca0.5Sr0.5TiO3 solid-solution films with systematically varying thicknesses in between two parent perovskite materials of calcium titanate and (001)-oriented strontium titanate are grown, and thermoreflectance techniques are used to accurately measure the thermal boundary conductance across the confined solid-solution films, showing that the thermal resistance does not substantially increase with the addition of solid-solution films with increasing thicknesses from similar to 1 to similar to 10 nm. Contrary to the macroscopic understanding of thermal transport where adding more material along the heat propagation direction leads to larger thermal resistances, our results potentially offer experimental support to the computationally predicted concept of vibrational matching across interfaces. This concept is based on the fact that a better match in the available heat-carrying vibrations due to an interfacial layer can lead to lower thermal boundary resistances, thus leading to an enhancement in thermal boundary conductance across interfaces driven by the addition of a thin "vibrational bridge" layer between two solids.

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 Delta T. This result clearly shows that the phase transition of the ferroelectric HfO2-based film is first order. The Delta T value of 40-210 degrees 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 Delta T 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.

Y-doped HfO2 films with thicknesses of 150-1000 nm were prepared on Pt/TiOx/SiO2/Si substrates by the sputtering method and subsequent heat treatment at 800 degrees C. XRD analysis showed that the films consisted of an almost pure orthorhombic/tetragonal phase. Hysteresis loops originating from the ferroelectricity were observed in the polarization-electric field relationship; the remnant polarization and coercive field were about 12 mu C cm(-2) and 1.2 MV cm(-1), respectively. Piezoelectricity was also confirmed from the strain-electric field curves for 1 mu m thick films, and the apparent piezoelectric coefficient, d(33,f), near 0 MV cm(-1) was estimated to be about 2.5 pm V-1. Taking account of the relatively low dielectric constant of about 23, the piezoelectric responses from 1 mu m thick films prepared by the sputtering method are useful for piezoelectric microelectromechanical system applications, especially for sensor applications, since the performance of such applications is proportional not only to the piezoelectric response but also to the inverse of the relative dielectric constant.

Detailed analysis of local polarization switching will promote the further development of a wide range of applications using ferroelectrics. Here, we propose a localC-V mapping technique using scanning nonlinear dielectric microscopy (SNDM) that enables visualization of dynamic ferroelectric switching behavior in real space. Using this method,C-V butterfly curves characteristic of ferroelectrics can be measured on a scanning probe microscopy platform with nanoscale resolution by virtue of the high capacitance-detection sensitivity of SNDM. This provides real-space mapping of the net switchable polarization, the switching voltage, and the local imprint with a short measurement time (e.g., 10min or less for256 x 256pixels). Furthermore, the proposed method will be useful for study of the electric-field response of domain walls. In this paper, we present some examples of experiments with LiTaO3 single crystals and HfO2-based ferroelectric thin films and give an overview of what kind of evaluation is possible with the localC-V mapping technique.

Y-doped HfO2 films with various thicknesses were prepared on (100)-oriented [10 wt.% Sn-doped In2O3, ITO]//(100) [yttria-stabilized zirconia, YSZ], and (111)ITO//(111)YSZ substrates by a radio-frequency magnetron sputtering method. Almost a single phase of orthorhombic symmetry was obtained for all films. {100}-oriented epitaxial films were obtained on (100)ITO//(100)YSZ substrates, while the film orientation changed from {111} to {100} with increasing film thickness on (111)ITO//(111)YSZ substrates. {100}-oriented epitaxial Y-doped HfO2 films were also obtained on (100)-oriented epitaxial ITO layers on (100)YSZ//(001)Si substrates. Ferroelectricity was observed for all films. Their remanent polarization (P-r) and coercive fields (E-c) were about 5 mu C/cm(2) and 1MV/cm, respectively, indicating that Pr and Ec were almost independent of the film thickness and kind of substrate. (C) 2020 The Ceramic Society of Japan. All rights reserved.

The thickness- and orientation- dependences of Curie temperature were investigated for (111) -oriented, (100)-oriented and randomly-oriented 0,07YO(1.5)-0.93HfO(2) films using high-temperature X-ray diffraction to study temperature stability of ferroelectric HfO2 based films. These (111)-, (100)- and randomly-oriented films with various film thickness were prepared at room temperature on (111)YSZ (100)YSZ and Pt/TiO(x)iSiO(2)/Si substrates by pulsedy laser deposition, respectively, and subsequent 1000 degrees C annealing to obtain ferroelectric orthorhombic phase.Although (111)oriented films show that Curie temperature decreases with the decrease in thickness, the temperature was higher than 350 C in 4.6-110 nm thickness range. (100) -oriented films show no thickness dependence of Curie temperature of 550 degrees C in 15-86 nrn thickness range. Moreover, the Curie temperature of 10 nm thick randomly-oriented films was also 550 degrees C. These results revealed that the Curie temperature was above 350 degrees C, which is significantly high for the stable operation of memory application, regardless of the thickness and orientation. (C) 2020 The Japan Society of Applied Physics.

Ferroelectricity has been demonstrated in epitaxial 7%Y-doped HfO2 (0.07YO(1.5)-0.93HfO(2), 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 mu C/cm(2) 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. Published under license by AIP Publishing.

Thin films of BaCe0.8Y0.2O3-delta (BCYO) and SrZr0.8Y0.2O3-delta (SZYO) with a perovskite structure were deposited on (111)Pt//(111)SrTiO3 (STO) single crystal substrates by an RF-magnetron sputtering method. X-ray diffraction revealed that the BCYO and SZYO thin films were polycrystalline but highly (110)-one-axis oriented with local epitaxy on (111)Pt//(111)STO in the < 110 >-direction. However, these films have six-fold symmetric domains and each domain showed in-plane rotations because of the lattice mismatches between (110)BCYO or (110)SZYO and (111)Pt. Especially, the BCYO film showed larger in-plane rotation than that of SZYO because of its larger lattice mismatch. (C) 2019 The Japan Society of Applied Physics

Ferroelectricity has been demonstrated in polycrystalline 7%Y-doped HfO2 (YHO7) films with thicknesses ranging from 10 to 930nm, which were grown on (111)Pt/TiOx/SiO2/(001)Si substrates by pulsed laser deposition at room temperature and subsequent annealing at 1000 degrees 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 (P-r) and the coercive field (E-c) are 14-17 mu C/cm(2) and 1300-1600kV/cm, respectively, at max applied electric fields of similar to 4000kV/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.

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 degrees C and perfectly transformed around 950 degrees C during heating. The change from tetragonal phase to orthorhombic phase was detected at 300 degrees C, corresponding to the Curie temperature under the cooling process. These results clearly suggest that the tetragonal phase was more stable at 1000 degrees 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. (C) 2019 The Japan Society of Applied Physics

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 mu C/cm(2) 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 degrees 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. Published by AIP Publishing.

YO1.5-doped HfO2 films were deposited on yttria-stabilized zirconia substrates by RF magnetron sputtering at room temperature and under various atmosphere conditions. The deposited films were treated by rapid thermal annealing under both O-2 and N-2 flows. Epitaxial films with the orthorhombic phase, which is expected to exhibit ferroelectricity, are obtained under all conditions. The deposition in Ar atmosphere provided good ferroelectricity, while the deposition with O-2 resulted in a low breakdown voltage inhibiting ferroelectricity. Current density-voltage characteristics show a significant increase in leakage current by the deposition in atmosphere containing O-2 and also annealing under O-2 flow. These results indicate that the treatment in atmosphere containing O-2 leads to the degradation of insulation properties. (C) 2018 The Japan Society of Applied Physics

The thickness dependences of the crystal structure and electric properties of (111)-oriented epitaxial 0.07Y(2)O(3)-HfO2 (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 (P-r) and saturation polarization (P-s) were observed. Thickness dependence of the coercive field (E-r) is also small, and this behavior does not resemble that of conventional ferroelectric films such as Pb(Zr,Ti)O-3. Additionally, non-oriented polycrystalline YHO7 films are reported to have similar thickness dependence of E-c and almost the same E-r 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 E-c. Published by AIP Publishing.

The dependence of applied rectangular pulses with various widths on the crystal structure change was investigated by time-resolved synchrotronbased X-ray diffraction measurement. A (001)-oriented epitaxial Pb(Zr0.5Ti0.5)O-3 film of 2.1 mu m thickness grown on a (100)(c)SrRuO3//(100) LaNiO3//(100)CaF2 substrate by metal organic chemical vapor deposition was investigated. The crystal lattice increased almost linearly with increasing applied electric field up to 230 kV/cm in the case of a 0.3-mu s-width pulse. This elongation with the application of an electric field was ascertained to be almost independent of the pulse width from 0.3 to 2000 mu s at 190 kV/cm. These values were almost consistent with the macroscopic measurements obtained at 5 and 1000 Hz by piezoelectric force macroscopy. The present results show that the time-resolved XRD measurement is very useful for analyzing the frequency dependence of the piezoelectric response in view of the crystal structure change because the crystal structure change under an applied electric field can be systematically investigated by changing the applied pulse width. (C) 2018 The Japan Society of Applied Physics

Local response signals in scanning nonlinear dielectric microscopy during domain switching in ferroelectric materials were studied. Periodic response signals corresponding to domain switching were observed in single-crystal LiTaO3 samples under alternating bias voltage applications. This approach was subsequently applied to ferroelectric HfO2 films, showing different response signals depending on the film orientation and the conditions of film formation. These results suggest that the proposed method is useful for obtaining detailed information concerning domain switching in the nanoscale region, such as the pinning-site effect, backswitching, and 90 degrees switching. (C) 2017 The Japan Society of Applied Physics

BaCe0.9Y0.1O3-delta (BCYO) and SrZr0.8Y0.2O3-delta (SZYO) thin films of perovskite-type oxides were deposited on (111) Pt/TiOx/SiO2/(100) Si substrates. X-ray diffraction patterns showed that the (110)-oriented BCYO and SZYO thin films were grown on (111) Pt/Si substrates directly without using any buffer layers. Thin films of SrRuO3 (SRO), a conductive perovskite-type oxide, were also deposited on those films and highly (110)-oriented SRO thin films were obtained. We believe that this (110)-oriented SRO works as a buffer layer to deposit (110)-oriented perovskite-type ferroelectric oxide thin films as well as a bottom electrode and can modify the ferroelectric properties of the oxide thin films by controlling their crystallographic orientations. (C) 2016 The Japan Society of Applied Physics

0.07YO(1.5)-0.93HfO(2) (YHO7) films were prepared on various substrates by pulse laser deposition at room temperature and subsequent heat treatment to enable a solid phase reaction. (111)-oriented 10 wt.% Sn-doped In2O3(ITO)//(111) yttria-stabilized zirconia, (111) Pt/TiOx/SiO2/(001)Si substrates, and (111) ITO/(111) Pt/TiOx/SiO2/(001)Si substrates were employed for film growth. In this study, X-ray diffraction measurements including theta-2 theta measurements, reciprocal space mappings, and pole figure measurements were used to study the films. The film on (111) ITO//(111) yttria-stabilized zirconia was an (111)-orientated epitaxial film with ferroelectric orthorhombic phase; the film on (111) ITO/(111) Pt/TiOx/SiO2/(001)Si was an (111)-oriented uniaxial textured film with ferroelectric orthorhombic phase; and no preferred orientation was observed for the film on the (111) Pt/TiOx/SiO2/(001)Si substrate, which does not contain ITO. Polarization-hysteresis measurements confirmed that the films on ITO covered substrates had saturated ferroelectric hysteresis loops. A remanent polarization (P-r) of 9.6 and 10.8 mu C/cm(2) and coercive fields (E-c) of 1.9 and 2.0 MV/cm were obtained for the (111)-oriented epitaxial and uniaxial textured YHO7 films, respectively. These results demonstrate that the (111)-oriented ITO bottom electrodes play a key role in controlling the orientation and ferroelectricity of the phase formation of the solid films deposited at room temperature. Published by AIP Publishing.