内田 寛

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.

Abstract Aiming at applications of ferroelectric and optical devices, we investigated the fabrication of one-axis-oriented lead zirconate titanate Pb(Zr,Ti)O₃ (PZT) thin films on glass substrates with a processing temperature below the glass-transition point of the substrate. Chemical solution deposition (CSD)-derived PZT thin films with a preferential crystal orientation of (00l)/(h00)PZT were grown on an ITO/glass substrate with a crystalline buffer layer of calcium niobate Ca₂Nb₃O₁₀ (ns-CN). This ns-CN buffer layer lowered the crystallization temperature of the CSD-derived thin films, resulting in crystalline PZT thin films with one-axis (00l) orientation and ferroelectricity of P_r = 4 μC cm⁻² deposited on transparent glass substrates at a crystallization temperature of 500 °C.

Epitaxial films of (K,Na,Li)NbO3 with 10 over line m thickness and various Li contents were fabricated at 240 degrees C on (001)La:SrTiO3 substrates by a hydrothermal method, and their crystal structures and piezoelectric properties were investigated. The film thickness was controlled by varying the deposition time (up to 3.5 h) and the number of deposition cycles. Scanning electron microscopy observations showed that dense thick films were formed. X-ray diffraction (XRD) measurements showed that {001}c-oriented epitaxial films were deposited, and the outof-plane lattice constant changed with an increase in the nominal composition A = [LiOH]/([KOH] + [NaOH] + [LiOH]) of the alkaline source solution. High-temperature XRD measurement revealed that with an increase in A, the Curie temperature increased, while the orthorhombic-tetragonal phase transition temperature decreased from 210 to 120 degrees C. These structural changes indicate that the Li content in the thick films can be controlled by varying A. The dielectric properties depend on the measurement frequency, and the minimum relative dielectric permittivity at all frequencies was observed at A = 0.02. Curves of field-induced strain vs. electric field showed that the maximum normalized strain of Smax/Emax = 40 pm/V was observed at A = 0.03, indicating that Li substitution is an effective way to improve the piezoelectricity of the hydrothermally deposited (K,Na)NbO3 thick films. Interestingly, all thick films exhibited a piezoelectric response despite the applied electric field being lower than the coercive field. Moreover, the Smax/Emax value did not change significantly with an increasing applied electric field. These results suggest that the hydrothermally deposited (K,Na,Li)NbO3 thick films adopt a selfpolarized state without poling treatment. (c) 2022 The Ceramic Society of Japan. All rights reserved.

The aqueous extract of the leaves of Odontonema strictum (OSM) is used in folk medicine for its antihypertensive properties, and it contains a wide range of secondary metabolites, mostly polyphenols such as verbascoside and isoverbascoside, which could play a major role in the preparation of silver nanoparticles. In this study, we aimed to prepare AgNPs for the first time using the OSM leaf extract (OSM-AgNPs) to investigate their free radical-scavenging potency against 1,1-diphenyl-2-picrylhydrazyl (DPPH) and hydrogen peroxide (H2O2). Dynamic light scattering (DLS), UV/Vis, Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), and X-ray photoelectron spectroscopy (XPS) were used to characterize the OSM-AgNPs. With a size around 100 nm and a ζ-potential of -41.1 mV, OSM-AgNPs showed a good stability and a better colloidal property due to electrostatic repulsion and the dispersity. The strong absorption peak at 3 keV in the EDX spectra indicated that silver was the major constituent. Additionally, the existence of silver atoms was confirmed by the Ag 3d5/2 peak around 367 eV in the XPS spectra. IC50 values of 116 μg/mL and 4.4 μg/mL were obtained for the scavenging activities of DPPH and H2O2, respectively. The synthetic OSM-AgNPs can be further exploited as potential antioxidant agents.

Low-temperature processing based on microwave-assisted hydrothermal synthesis was proposed for deposition of piezoelectric oxoate KNbO3 films. The films were deposited on various substrates at reaction temperature below 200 degrees C using Nb2O5 powder and KOH solution. Epitaxial (100)(c) KNbO3 film with similar to 1.3 mu m-thick was deposited at a reaction temperature of 150 degrees C on (100)(c)SrRuO3//(100)SrTiO3 using 12 mol dm(-3) KOH solution for 80 min, which is significantly faster than conventional hydrothermal process. (C) 2022 The Ceramic Society of Japan. All rights reserved.

Rapid processing for hydrothermal deposition of (K,Na)NbO3(KNN) thick films was proposed with the assistance of microwave (MW) heating, for short-time fabrication of ferroelectric/piezoelectric films with thicknesses of several micrometers within 1 h. MW irradiation raised the temperature of water medium up to 240 degrees C within 10 min, whereas conventional heating using hot-air convection required more than 2 h. Epitaxial KNN films were successfully deposited on single-crystal substrates of (100)SrTiO3:Nb and (100)SrRuO3//(100)SrTiO(3)at reaction temperature of 180 degrees C-240 degrees C within 1 h; especially, the films with thicknesses over 10 mu m could be obtained on at 220 degrees C for 40 min. The resulting film exhibited domain structure and dielectric/ferroelectric properties comparable with the KNN films derived from conventional hydrothermal deposition.

Using a hydrothermal method, (K0.88Na0.12)NbO3 films were deposited at 240 degrees C on (100)(c)SrRuO3//(100)SrTiO3 substrates. Moreover, without any poling treatment, direct and inverse transverse piezoelectric coefficients, e(31,f), near 0kV/cm were approximately -5.0C/m(2) for the as-deposited film. This value was nearly unchanged following the application of an electric field and poling treatment, suggesting that as-deposited films are almost in a fully self-polarized state without the application of an electric field. As-deposited films with a thickness of up to 22 mu m showed constant piezoelectricity without any poling treatment. The films did not crack or peel because of substrates due to the small thermal strain originating from the low deposition temperature. The figures of merit (FOM) for the vibration energy harvester [FOM=e(31,f)(2)/(epsilon (0)epsilon (r))] and sensor [FOM=e(31,f)/(epsilon (0)epsilon (r))] were estimated to be good at 32.8GPa and -5.9GV/m, respectively, primarily because of the low relative dielectric constant of similar to 110. Furthermore, the piezoelectric voltage coefficient g(31) [= d(31)/(epsilon (0)epsilon (r))] was estimated and demonstrated a high value of 0.073Vm/N.

The first direct Barkhausen noise measurement in a ferroelectric thin film is presented. The Barkhausen noise energy loop is reconstructed from the measured Barkhausen noise and is closely related to the classic ferroelectric P vs E hysteresis loop. Grain boundaries act as a dominant ferroelectric domain wall pinning site in a polycrystalline thin film based on the calculated domain wall jump distance using the Barkhausen noise frequency. The technique is promising for the measurement of ferroelectric switching dynamics, and provides a physical insight for improving application performance.

This journal is © The Royal Society of Chemistry. Carbon quantum dots (CQDs; luminescent carbon nanoparticles, size < 10 nm) have attracted much attention with respect to their eco-friendliness and multi-functionality. The solvent-dependent photoluminescence of CQDs has been well investigated to optimize the synthesis process and homogeneous dispersion. Although some alkan-1-ol solvents, such as ethanol, have been well utilized empirically as good solvents when synthesizing highly photoluminescent CQDs, the role of alkan-1-ol solvents, particularly long-chain alkan-1-ols (e.g., 1-nonanol, 1-decanol), has not yet been clarified. Herein, we demonstrate a method for the synthesis of strongly yellow emitting CQDs using solvothermal treatment and elucidate the role of alkan-1-ol solvents in the photoluminescence of CQDs. These CQDs have been characterized using theoretical calculations, ex situ morphological observations using transmission electron microscopy (TEM) and dynamic light scattering (DLS), and 500 MHz 1H nuclear magnetic resonance (NMR) and 13C NMR spectroscopy. A comparative study of alkan-1-ol solvents suggests a mechanism for the agglomeration and aggregation of carbon precursors, intermediates, and CQDs, which is expected to lead to further synthesis studies on highly luminescent CQDs.

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.

Grain size effects on electromechanical properties and voltage-driven ferroelastic domain wall motion are a well-known phenomenon in polycrystalline ferroelectrics. Here, the origin of the grain size effects on voltage-driven ferroelastic domain wall motion is presented with the direct observation of ferroelastic domain evolution with applied DC voltage by piezoelectric force microscopy and polarization hysteresis loop. It is demonstrated that the microstructure parameter for controlling the voltage-driven ferroelastic domain wall motion is the number of colonies of stripe domains in a grain rather than the grain size. Single colony grains do not show considerable out-of-plane (001) domain width change whereas multiple colony grains exhibit significant domain width increase with an applied DC voltage. No independent grain size effect on ferroelastic domain wall motion is observed in the grain size range 0.6-1.6 mu m.

Thin films of BaCe0.8Y0.2O3−δ (BCYO) and SrZr0.8Y0.2O3-δ (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.

{100}c-, {110}c- and {111}c-oriented epitaxial (K, Na)NbO3 thin films were grown at 240 °C on (100)cSrRuO3//(100)SrTiO3, (110)cSrRuO3//<br /> (110)SrTiO3, and (111)cSrRuO3//(111)SrTiO3 substrates by the hydrothermal method. Their film thicknesses increased with the deposition time and then eventually saturated at longer deposition times. Their saturated film thicknesses were mainly determined by their orientation and the order was {100}c-, {110}c- and {111}c-orientation regardless of any experimental conditions. These films consisted of grains with characteristic morphologies. All of the films exhibited similar ferroelectric and piezoelectric properties irrespective of the film orientation. The remnant polarizations (Pr) and coercive fields (Ec) of the {100}c-, {110}c- and {111}c-oriented films at the maximum electric field of 500 kV cm−1 were 31 μC cm−2 and 111 kV cm−1, 27 μC cm−2 and 94 kV cm−1, and 25 μC cm−2 and 110 kV cm−1, respectively, while their effective values of piezoelectric coefficient (d33) were approximately 31–33 pm V−1. Similar films are associated with its mixed domain structure.

Ta substituted (K,Na)NbO3 films have been deposited at 200 °C on (001)La:SrTiO3 substrates by hydrothermal method. Film composition continuously changed with increasing the concentration of Ta2O5 in the raw material powder, and X-ray diffraction measurement showed that {001}c-oriented epitaxial (KxNa1−x)(Nb1−yTay)O3 films were obtained for all compositions. Microstructural analysis using scanning electron microscopy and transmission electron microscopy revealed that Ta-substitution made {001}c planes dominate on the surface of the film, leading to an improvement in average surface roughness. Film thickness increased with deposition time and eventually saturated. However, the saturated film thickness decreased with increasing Ta/(Nb + Ta) ratio. By evaluating the residual powders synthesized simultaneously with the films, it was found that the formation of the perovskite phase had already started in the very early stage of the deposition process, provided that Ta2O5 powder was present in the source solution.

(K,Na)NbO3 thick films were grown at 240 degrees C on Ni-based metal substrates by repeated hydrothermal method. The metal substrates were covered with two types of buffer layers; SrRuO3/LaNiO3 and SrRuO3. Film thickness monotonically increased with increasing number of deposition cycles. The 27 mu m-thick film was obtained on the metal substrate with SrRuO3/LaNiO3 by four cycles. The obtained films tended to show {100}(c) orientation and their degree of orientation increased with increasing number of deposition cycles. Films deposited on SrRuO3/LaNiO3 -covered metal substrates showed more highly {100}(c) orientation compared with those on SrRuO3-covered metal substrates. Remnant polarization and coercive field measured at 5 kHz were 12 mu C/cm(2) and 70 kV/cm, while their effective values of piezoelectric coefficient (d(33)) was 35-40 pm/V for both films. These properties remained unchanged irrespective of a number of deposition cycles despite the orientation change of films. These results show that repeated hydrothermal deposition technique is one of the effective ways to prepare thick (K,Na)NbO3 films on metal substrates. (C) 2019 The Ceramic Society of Japan. All rights reserved.

Epitaxial (K0.88Na0.12)NbO3 films were deposited at 240 degrees C onto (1 0 0)(c)SrRuO3//(1 0 0)SrTiO3 substrates utilizing a hydrothermal method. Both crystalline and amorphous niobium oxide powders were utilized as starting niobium sources with KOH and NaOH as potassium and sodium sources, respectively. Film thickness increased with deposition time, but eventually became saturated. The saturated film thickness of (K0.88Na0.12)NbO3 films prepared from the amorphous niobium source was greater than that from the crystalline source for mixtures of 6 and 7 mol/L KOH-NaOH solutions. This is found to be due to the suppression of the (K,Na)NbO3 powder simultaneously prepared with films by selecting amorphous niobium source. Their electrical and piezoelectric properties were nearly identical after following heat treatment at 600 degrees C for 10 min in an atmospheric O-2 flow irrespective of the starting niobium sources; The average values of remanent polarization and effective longitudinal piezoelectric constant, d(33), were 5-6 mu C/cm(2) and 45 pm/V, respectively.

Ca-Mg-Si films were firstly prepared on (001)Al2O3 substrates by RF-magnetron sputtering method from Mg disc target together with Ca and Si chips. The composition of the deposited films was controlled by adjusting deposition temperature and Ca/Si area ratio of Ca and Si chips on Mg disk target. Ca0.32Mg0.33Si0.35 film deposited at 610 K consisted of a single phase of CaMgSi and this CaMgSi phase was stable after heat treated at 770 K under an atmospheric Ar with 5% -H-2. As-deposited film shows the semiconductor behavior and have a power factor of 50 mu W/(mK(2)) at 670 K. while annealed one showed the metallic behavior and its power factor down below 10 mu W/(mK(2)) at 320-770 K. On the other hand, Ca0.27Mg0.51Si0.2 film deposited at 590 K showed no obvious crystalline phase but became single phase of Ca7Mg7.25Si14 after heat treatment at 770 K under an atmospheric Ar with 5% -H-2. As deposited film had a large power factor of 100 mu W/(mK(2)) at 670 K. However, power factor decreased below 1 mu W/(mK(2)) at 320-770K after the heat treatment at 770 K under an atmospheric Ar with 5% -H-2.

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.

In this study, the effect of Fe doping on the crystal structure and electrical properties in 20-nm-thick HfO2 epitaxial thin films were systematically investigated. X-ray diffraction measurements revealed that undoped HfO2 films were composed of a paraelectric monoclinic phase. On the other hand, the formation of non-centrosymmetric orthorhombic phase was observed in Fe doped HfO2 films and was most promoted at an optimum doping concentration. In addition, high-temperature X-ray diffraction measurements showed that an orthorhombic phase to a highly symmetric phase transition occurs between 500 and 600 degrees C. Microstructural analysis using scanning transmission electron microscopy revealed multidomain structure consisting of orthorhombic and monoclinic phases. Ferroelectricity depended on Fe doping concentration, and the maximum remanent polarization value was 8.8 mu C/cm(2). These results indicate that Fe doped HfO2 thin films can be applied as nanoscale ferroelectrics. (C) 2018 The Japan Society of Applied Physics

(001)c-oriented (K0.86Na0.14)NbO3 thick films were prepared at 240°C on (100)cSrRuO3//(100)SrTiO3 substrates by repeated hydrothermal deposition technique. The film thickness was found to increase linearly with the number of deposition cycles, and 60 ¯m-thick film was obtained after nine repetitions of the deposition. The K/(K+Na) ratio of the deposited thick films, measured by X-ray fluorescence spectroscopy, showed constant values regardless of the number of deposition cycles. Cross-sectional scanning electron microscopy images revealed uniformity of the obtained dense films with no obvious micro cracks and pores. Structural characterization based on X-ray diffraction, XRD 2ª-½ patterns and X-ray pole figure measurement, showed that the epitaxial relationship between the films and substrates with a (001)c orientation was maintained throughout the deposition cycles. In addition, cross-sectional Raman spectra showed that 60 ¯m-thick (K0.86Na0.14)NbO3 film had an orthorhombic structure. The dielectric constant, ¾r, and tan ¤ showed frequency dependence. The average remanent polarization measured at 100 Hz was 8 ¯C/cm2

A method for controlling the conduction-type in Mg2Si films without doping is investigated. Mg2Si films exhibit p-type conduction after a post-heat treatment up to 500 °C in atmospheric He. However, covering the films with Mg ribbon during a subsequent heat treatment at 500 °C converts the conduction to n-type, demonstrating that the heat treatment atmosphere can control the conduction type. Based on the reported first principles calculations suggesting that interstitial Mg and Mg vacancies in Mg2Si are the origins of n-type and p-type conduction, respectively, the post-heat treatment in He induces Mg vacancies due to the evaporation of Mg from the film, resulting in p-type conduction. The subsequent heat treatment when the film is covered with Mg ribbon fills the Mg vacancies and the additional interstitial Mg is incorporated, resulting in n-type conduction. These observations differ from the reported data for heat treatment of stable n-type conduction in non-doped Mg2Si-sintered bodies and may realize a novel control method for the conduction type in Mg2Si films.

In this work, the effect of the film thickness on the crystal structure and ferroelectric properties of (Hf0.5Zr0.5)O-2 thin films was investigated. The thin films were deposited on (111) Pt-coated SiO2, Si, and CaF2 substrates with thermal expansion coefficients of 0.47, 4.5, and 22 x 10(-6)/degrees C, respectively. From the X-ray diffraction measurements, it was found that the (Hf0.5Zr0.5)O-2 thin films deposited on the SiO2 and CaF2 substrates experienced in-plane tensile and compressive strains, respectively, in comparison with the films deposited on the Si substrates. For films deposited on all three substrates, the volume fraction of the monoclinic phase increased with increasing film thickness, with the SiO2 substrate having the lowest monoclinic phase volume fraction at all film thicknesses tested. The grain size of the films, which is an important factor for the formation of the ferroelectric phase, remained almost constant at about 10 nm in diameter regardless of the film thickness and type of substrate utilized. Ferroelectricity was observed for the 17 nm-thick films deposited on SiO2 and Si substrates, and the maximum remanent polarization (P-r) value of 9.3 mu C/cm(2) was obtained for films deposited on the SiO2 substrate. In contrast, ferroelectricity with P-r=4.4 mu C/cm(2) was observed only for film on SiO2 substrate in case of 55 nm-thick films. These results suggest that the films under in-plane tensile strain results in the larger ferroelectricity for 17 nm-thick films and have a ferroelectricity up to 55 nm-thick films.

(111)-oriented epitaxial (K0.5Na0.5)NbO3 films of 2 mu m thickness were deposited at 240 degrees C on (111)(c)SrRuO3//(111)SrTiO3 substrates by a hydrothermal method. Scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy (STEM-EDS) mapping revealed that a K-rich interfacial region existed in the obtained films. Although the as-deposited films were annealed between 240 and 600 degrees C in O-2 atmosphere, the crystal structure did not markedly change. On the other hand, the leakage current density drastically decreased from 10(-2)-10(0) to 10(-6)A/cm(2) at +/- 80 kV/cm after annealing above 240 degrees C. In addition, the relative dielectric constant (epsilon(r)), remanent polarization (P-r), and piezoelectric constant (d(33),(PFM)) increased with increasing annealing temperature from 300 to 600 degrees C, and their values were respectively 840, 5.7 mu C/cm(2), and 64 pm/V for the films annealed at 600 degrees C. (C) 2017 The Japan Society of Applied Physics

For the application of electronic devices using ferroelectric/piezoelectric components, one-axis-oriented tetragonal Pb(Zr0.40Ti0.60)O-3 (PZT) films with thicknesses of up to 1 mu m were fabricated with the aid of a Ca2Nb3O10 nanosheet (ns-CN) template for preferential crystal growth for evaluating their polarization switching behavior. The ns-CN template was supported on ubiquitous silicon (Si) wafer by a simple dip coating technique, followed by the repetitive chemical solution deposition (CSD) of PZT films. The PZT films were grown successfully with preferential crystal orientation of PZT(100) up to the thickness of 1020 nm. The (100)-oriented PZT film with similar to 1 mu m thickness exhibited unique polarization behavior of ferroelectric polarization, i.e., a marked increase in remanent polarization (P-r) up to approximately 40 mu C/cm(2) induced by domain switching under high electric field, whereas the film with a lower thickness showed only a lower Pr of approximately 11 mu C/cm(2) even under a high electric field. The ferroelectric property of the (100)-oriented PZT film after domain switching on ns-CN/Pt/Si can be comparable to those of (001)/(100)-oriented epitaxial PZT films. (C) 2017 The Japan Society of Applied Physics

A strongly {110}-oriented perovskite-type thin film of tetragonal Pb(Zr0.4Ti0.6)O-3 (PZT) was successfully obtained on a (100) Si substrate using a {101} PdO//{111} Pd thin film as a buffer layer. The {101} PdO//{111} Pd thin film buffer layer was obtained by oxidizing {111} Pd after depositing {111} Pd on a {111} Pt/TiOx/SiO2/{100} Si substrate. Using this buffer layer, a {110}(c)-oriented SrRuO3 (SRO) thin film was deposited by sputtering as a bottom electrode of PZT thin films. Subsequently, the {110}-oriented PZT thin film can be deposited on a (110)(c)SRO thin film by metal-organic chemical deposition (MOCVD) and its properties can be compared with those of PZT thin films with other orientations of {100} and {111}. Among the {100}, {110}, {111}-oriented PZT films, the {100}-oriented one showed the largest remnant polarization, which is in good agreement with those of the PZTs epitaxially grown in the (100), (110), and (111) directions. The other properties, i.e., piezoelectricity and dielectric constants, also showed similar anisotropic tendencies, which is in good agreement with the data reported in the epitaxially grown PZTs. (C) 2017 The Japan Society of Applied Physics

We have been proposed a new orientation controlling method using a (101)PdO//(111)Pd buffer layer to deposit (110)-one-axis-oriented perovskite oxide thin films on various kinds of substrates. The (101)-oriented PdO can be obtained by oxidizing (111)Pd and (110)(Ba0.5Sr0.5)TiO3 (BST) was obtained on it by an RF-magnetron sputtering method. The (110)BST thin films fabricated on various substrates, i.e., SrTiO3, Si, and CaF2, showed the difference in their dielectric behaviors, which originated from internal strains of the BST films. This internal strain is compress or tensile, which is caused by the difference of thermal expansion coefficients between BST and substrates during cooling them from deposition temperature (700 °C) to room temperature. The internal strains of the (110)-one-axis-oriented BST films affect the dielectric constants and its tunability. The largest tunability was derived on CaF2, i.e., compressive side, suggesting the internal strain of the BST films affects its capacitance behavior, which is similar behavior reported in (100)-one-axis-oriented-BST films.

Mg2Si thin films were deposited at 320 degrees C on (001) Al2O3 and (100) CaF2 substrates by radio-frequency magnetron sputtering. Both films showed a preferential (111) out-of-plane orientation with an in-plane random orientation irrespective of post-heat treatment. Mg2Si films on (001) Al2O3 substrates were under in-plane tensile strain, while those on (100) CaF2 substrates were under in-plane compressive strain both before and after heat treatment. Heat-treated films showed p-type conduction up to 500 degrees C. Their electrical conductivity and Seebeck coefficient were almost independent of the kind of substrate within the limit of the present study, from 0.22% compressive strain to 0.34% tensile strain at room temperature. (C) 2017 The Japan Society of Applied Physics

Mg<inf>2</inf>Si thin films were deposited at 320 °C on (001)Al<inf>2</inf>O<inf>3</inf>and (100)CaF<inf>2</inf>substrates by radio-frequency magnetron sputtering. Both films showed a preferential (111) out-of-plane orientation with an in-plane random orientation irrespective of post-heat treatment. Mg<inf>2</inf>Si films on (001)Al<inf>2</inf>O<inf>3</inf>substrates were under in-plane tensile strain, while those on (100)CaF<inf>2</inf>substrates were under in-plane compressive strain both before and after heat treatment. Heat-treated films showed p-type conduction up to 500 °C. Their electrical conductivity and Seebeck coefficient were almost independent of the kind of substrate within the limit of the present study, from 0.22% compressive strain to 0.34% tensile strain at room temperature.

Ultrathin films of HfO2-ZrO2 system, HfxZr1-xO2, were fabricated for generating ferroelectric phase. Polycrystalline HfxZr1-xO2 films were prepared via chemical solution deposition process, in which precursor films consisting of amorphous phase on platinized silicon wafer were crystallized by post-annealing. Metastable orthorhombic phase, recognized as the "ferroelectric" phase, appeared in the HfxZr1-xO2 films with chemical composition of x = 0.40-0.70 and with film thickness of approximately 40 nm, together with stable monoclinic and/or cubic phases. The dielectric permittivity, epsilon(r), and remanent polarization, P-r, varied with the chemical composition, meaning that constituent phases of the resulting HfxZr1-xO2 films dominate their dielectric and ferroelectric properties. Saturated P-E hysteresis Mop with P-r of 2.1 mu C/cm(2) and coercive field of 580 kV/cm was confirmed for the solid solution film of HfxZr1-xO2 (x = 0.70) at 80 K, whereas no spontaneous polarization was confirmed for pure HfO2 film (x = 1.00). These results imply that the orthorhombic phase stabilized by Zr substitution would contribute to the generation of ferroelectricity in HfO2-based material consequently.

Investigations of defects are important to understand the properties of zinc oxide (ZnO), especially oxygen vacancies, which are intrinsic defects that are easily generated during crystal growth or device processing. In this study, we evaluate the oxygen vacancies in ZnO single crystals and powders using micro-Raman spectroscopy. Reducing ZnO in a hydrogen atmosphere at 400-600 degrees C for 30-240 min changes the amount of oxygen vacancies. Raman spectroscopy reveals a slight shift and a decrease in the E-2(high) phonon mode, which is related to the oxide ion vibration. The peak position of the E-2(high) mode shifts toward a lower frequency and the peak intensity decreases as the oxygen vacancies increase. This behavior can be explained by the existence of oxygen vacancies in ZnO. Because the E-2(high) peak shift and the intensity are scaled in accordance with the amount of oxygen vacancies, these correlations offer a simple and useful probe to evaluate oxygen vacancies in ZnO. (C) 2017 The Ceramic Society of Japan. All rights reserved.

BaCe0.9Y0.1O0.9-δ (BCYO) and SrZr0.8Y0.2O3%δ (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.

Herein, we investigate the angular dependence of the Raman spectra of (001)/(100)-oriented tetragonal and (111)/(111)-oriented rhombohedral epitaxial lead zirconate titanate (PZT) films. The A(1) and E modes have the same angular dependence for the (111)/(111)-oriented rhombohedral epitaxial PZT film, but not for the (001)/(100)-oriented tetragonal epitaxial PZT film. These results agree well with theoretical predictions. On the basis of the fact that the angular dependence of the phonon mode in PZT films is linked to the crystal orientation and structure, the angular dependence of the Raman modes can be used to investigate the orientation and structure of PZT films. (C) 2016 The Japan Society of Applied Physics

(Bi,K)TiO3 films were grown on (001) cSrRuO(3) // (100) SrTiO3 substrates by pulsed laser deposition (PLD) at various temperatures. The K/Ti and Bi/Ti ratios of the films were almost the same at 350 and 500 degrees C, but decreased markedly at 650 degrees C. The ferroelectric property was ascertained for epitaxial tetragonal (Bi,K)TiO3 films grown at 500 degrees C from polarization-electric field relationships, and their saturation polarization (P-sat) and coercive field (Ec) at a maximum electric field of 800 kV/cm were 18 degrees C/cm(2) and 300 kV/cm, respectively. To improve the ferroelectric property, the films were heat-treated at 650 degrees C while keeping the films in the PLD chamber after film deposition. These films also maintained a (001)-oriented epitaxy and had a tetragonal symmetry. The P-sat and E-c obtained at an electric field of 800 kV/cm were changed to 22 mu C/cm(2) and 95 kV/cm, respectively. Moreover, these P-sat and E-c values increased to 31 mu C/cm(2) and 165 kV/cm, respectively, at a maximum electric field of 1500 kV/cm. These heat-treated (Bi, K) TiO3 films showed piezoelectricity with an apparent piezoelectric coefficient (d(33(AFM))) of 22pm/V. (C) 2016 The Japan Society of Applied Physics

In the present work, we aim to achieve the preferred crystal orientation of chemical solution deposition (CSD)-derived BaTiO3 films on ubiquitous Si wafers with the assistance of Ca2Nb3O10 nanosheet (ns-CN) template layers. The ns-CN on platinized Si (Pt/Si) substrates aligned the BaTiO3(100) plane to the substrate surface, because of the favorable lattice matching of the ns-CN (001) plane. The CSD process in air required a high crystallization temperature of 900 degrees C for the preferred crystal orientation of BaTiO3(100) because of the BaCO3 byproduct generated during the combustion reaction of the precursor gel. The processing in vacuum to remove CO2 species enhanced the crystal orientation even at the crystallization temperature of 800 degrees C, although it can generate oxygen vacancies (V-O(center dot center dot)) that cause distorted polarization behavior under an applied field higher than approximately 150 kV/cm. The relative dielectric constant (er) of the (100)-oriented BaTiO3 film on the ns-CN-supported Pt/Si substrate (ns-CN/Pt/Si) was generally larger than that of the randomly oriented film on Pt/Si, depending on the degree of crystal orientation. (C) 2016 The Japan Society of Applied Physics

(Bi,K)TiO<inf>3</inf>films were grown on (001)<inf>c</inf>SrRuO<inf>3</inf>// (100)SrTiO<inf>3</inf>substrates by pulsed laser deposition (PLD) at various temperatures. The K/Ti and Bi/Ti ratios of the films were almost the same at 350 and 500 °C, but decreased markedly at 650 °C. The ferroelectric property was ascertained for epitaxial tetragonal (Bi,K)TiO<inf>3</inf>films grown at 500 °C from polarization–electric field relationships, and their saturation polarization (P<inf>sat</inf>) and coercive field (E<inf>c</inf>) at a maximum electric field of 800 kV/cm were 18 µC/cm²and 300 kV/cm, respectively. To improve the ferroelectric property, the films were heat-treated at 650 °C while keeping the films in the PLD chamber after film deposition. These films also maintained a (001)-oriented epitaxy and had a tetragonal symmetry. The P<inf>sat</inf>and E<inf>c</inf>obtained at an electric field of 800 kV/cm were changed to 22 µC/cm²and 95 kV/cm, respectively. Moreover, these P<inf>sat</inf>and E<inf>c</inf>values increased to 31 µC/cm²and 165 kV/cm, respectively, at a maximum electric field of 1500 kV/cm. These heat-treated (Bi,K)TiO<inf>3</inf>films showed piezoelectricity with an apparent piezoelectric coefficient (d<inf>33(AFM)</inf>) of 22 pm/V.