中嶋 誠二

A intermediate multidomain state and large crystallographic tilting of 1.78° for the (hh0)pc planes of a (001)pc-oriented single-domain Mn-doped BiFeO3 (BFMO) thin film were found when an electric field was applied along the [110]pc direction. The anomalous crystallographic tilting was caused by ferroelastic domain switching of the 109° domain switching. In addition, ferroelastic domain switching occurred via an intermediate multidomain state. To investigate these switching dynamics under an electric field, we used in situ fluorescent X-ray induced Kossel line pattern measurements with synchrotron radiation. In addition, in situ inverse X-ray fluorescence holography (XFH) experiments revealed that atomic displacement occurred under an applied electric field. We attributed the atomic displacement to crystallographic tilting induced by a converse piezoelectric effect. Our findings provide important insights for the design of piezoelectric and ferroelectric materials and devices.

Bismuth ferrite (BiFeO3: BFO) is a multiferroic material that exhibits ferroelectricity, antiferromagnetism, and ferroelasticity simultaneously at RT. BFO holds great promise as a ferroelectric semiconductor because of its ability to alter conductivity by reversing its spontaneous polarization. Moreover, BFO thin films doped with transition metals such as Mn or V can modulate their conductivity. Nevertheless, the mechanism of this conductivity change remains unclear because the effects of dopants on the local atomic structure of BFO are not fully understood. In this study, we investigated the local atomic structure around the Fe site in a V-doped BFO thin film by X-ray fluorescence holography. Reconstructed atomic structures from the Fe Kα hologram patterns revealed that the atomic structure stability of the V-doped BFO thin film differs from that of previously reported Mn-doped BFO thin films. The results provide important insights into the mechanism of controlling the conductivity of BFO thin films by dopants.

HfO2-based ferroelectric materials do not necessarily require high-temperature annealing for crystallization, making them attractive for applications in transparent electronic devices on plastic or glass substrate. In this study, (Hf, Zr)O2 (HZO) films prepared via non-heating sputtering are investigated and their application to ferroelectric-gate thin-film transistors (TFTs) is demonstrated. The internal tensile stress induced by (In, Sn)O x (ITO) top-electrode deposition is found to promote the crystallization of HZO from the amorphous state to the ferroelectric phase. ITO/HZO (15-25 nm)/ITO capacitors prepared via the non-heating process exhibit ferroelectric hysteresis loops with remanent polarizations of 6-9 μC cm-2 and coercive fields of 0.6-1.1 MV cm-1. Ferroelectric-gate TFTs with a 10 nm thick ITO channel are also fabricated via the non-heating process. These TFTs show nonvolatile operation with an on/off ratio of ∼10. These findings demonstrate the potential of HZO for transparent devices on substrates with low thermal resistance prepared via the non-heating process.

Recently, ferroelectric semiconductors has become a subject of interest with regard to potential applications in novel electronic and opto-electric devices. One of the most important aspects of employing these materials is band modulation based on spontaneous polarization to generate polarization charges acting as quasi-dopants at metal/ferroelectric and ferroelectric/ferroelectric interfaces. The present study fabricated graphene/Mn-doped BiFeO3 (BFMO)/SrRuO3/SrTiO3(001) capacitor structures with the BFMO having either upward or downward polarization. Band modulation at the graphene/BFMO interface as a result of polarization charges was evaluated using photoemission electron microscopy on the BL17SU beamline at the SPring-8 facility, Japan. The chemical shifts observed in Bi 4f and C 1s XPS spectra indicated that positive (negative) polarization charges acted as quasi-dopants for electron (hole) doping of the BFMO and graphene.

Ferroelectric gate-all-around (GAA) transistors with a nanowire (NW) channel standing vertically on the substrate would be a potential breakthrough to overcome limitations in the high-integration of ferroelectric memories. In the present study, we fabricated vertical ferroelectric GAA NW transistors (VFGAANWTs) with ZnO NWs (average diameters: 53-193 nm) as the channel, (Hf,Zr)O2 film (average thicknesses: 9.3-58 nm) as the gate ferroelectric, and Ti as the gate electrode. The channel length was 100-300 nm. The VFGAANWTs showed n-channel operation with on/off ratios of ∼10, and their on/off states were retained for at least 2 min by the ferroelectric polarization of (Hf,Zr)O2 after the gate bias was removed, demonstrating the nonvolatile electric field effect in the VFGAANWTs. The results confirm the strong potential of the VFGAANWTs in high-density ferroelectric memory applications.

This work demonstrates an enhancement of the external quantum efficiency (η ex) associated with the bulk photovoltaic effect (BPVE) resulting from the ballistic conduction of hot carriers in indium tin oxide/Mn-doped BiFeO3 (BFMO)/SrRuO3 capacitors in response to a blue-violet laser (λ = 405 nm). The effect of the BFMO film thickness on the short circuit current showed an abnormal trend in the case of films less than 100 nm thick. This result established that ballistic conduction was the dominant BPVE mechanism, although an effect of the BFMO/electrode interface on the photovoltaic properties may also have been a factor. BFMO films with thicknesses less than 100 nm also exhibited enhanced external quantum efficiency, demonstrating that the ballistic conduction of hot carriers enhanced η ex.

Society is demanding clean energy to substitute greatly polluting carbon-based fuels. As an alternative, the use of green hydrogen produced by electrocatalysis constitutes a nice strategy as its products and reactants are not toxic to the environment. However, the use of scarce materials and high overpotentials to accomplish the oxygen evolution reaction (OER) make electrocatalysis an uncompetitive process. To solve these challenges, a low-cost procedure for the preparation of earth-abundant Ni, Fe and NiFe decorated electrodes has been developed. For this purpose, pencil graphite rods have been selected as highly porous substrates. A reasonable performance is achieved when they are employed for the OER. Furthermore, for the first time, a detailed analysis of impedance spectroscopy allows the association of the Ni redox transitions Ni2+/Ni3+and Ni3+/Ni4+(including the identification of the hydrated α-γ and the non-hydrated β phases) with an electrochemical redox capacitance response. Additionally, the Ni3+/Ni4+redox peak capacitance together with a quick decrease in the charge transfer resistance indicates the implication of Ni4+in the OER. These results show the utility of impedance spectroscopy as a non-destructive and non-invasive technique to study these electrochemical systems in detail under operating conditions.

© 2020, The Author(s). The bulk photovoltaic effect (BPVE) is a mechanism of recent focus for novel solar cells that exceed the power conversion efficiency of p–n junction solar cells because of the quantum mechanical effect to generate photocurrent known as shift current. Ferroelectrics are receiving attention again because of their high voltage generation by the BPVE and converse piezoelectric effect to realize high performance optical actuators. We have investigated the BPVE in ferroelectric BiFeO3 (BFO) single crystal thin films, whereby the photovoltage was enhanced by Mn doping, and 852 V generation was demonstrated at 80 K. The enhancement mechanism was also investigated using soft and hard X-ray photoelectron spectroscopy (SXPES, HAXPES), and soft X-ray absorption spectroscopy with synchrotron radiation. This report reveals a way to new voltage source applications employing the BPVE for high impedance devices with ferroelectrics. Important aspects for designing ferroelectric materials by impurity doping are also discussed.

© 2020 Author(s). The charging effect often complicates photoemission spectroscopy and x-ray absorption spectroscopy of an insulating material. Here, monolayer graphene was used as a conductive layer to prevent the charging effect of insulating substrates such as glass and LiNbO3. Charging-free spectra were obtained with various photon energies ranging from vacuum ultraviolet light to hard x-rays. This method could also be applied to photoemission spectroscopy of epoxy adhesives and to photoemission electron microscopy of an insulating film. Photoelectron transmissivities for the transferred graphene film were evaluated over a wide kinetic energy range from 29 to 7910 eV. A minimum transmissivity of ∼0.1 was found at a kinetic energy of ∼60 eV, which rose to 0.86 at 7910 eV. In terms of the kinetic energy dependence of the transmissivity, this method is especially suitable for conventional and hard x-ray photoelectron spectroscopy.

Copyright © 2020 American Chemical Society. An inexpensive, simple, and high-activity catalyst preparation method has been introduced in this work. Pt and RuOx catalysts were fabricated by soaking inexpensive graphite electrodes (pencil-lead graphite rod: PGR) in catalyst precursor solutions and using a simple flame-annealing method, which results in lower amount of Pt and RuOx catalyst layers. From X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure analysis, it has been found that platinum and ruthenium were deposited as zero-valence metal (Pt) and oxide (RuOx), respectively. Catalytic activities of Pt/PGR and RuOx/PGR for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) were evaluated using neutral 1 M Na2SO4 aqueous electrolyte, respectively. Although HER and OER currents using PGR without catalysts were -16 mA cm-2 (at -1.5 V vs Ag/AgCl) and +20 mA cm-2 (at +2.0 V vs Ag/AgCl), they were improved to -110 and +80 mA cm-2 with catalysts (Pt and RuOx), respectively. Such an inexpensive and rapid catalyst electrode preparation method on PGR using flame-annealing is a very significant method in the initial catalyst activity evaluation requiring a large amount of trial and error.

© 2019 The Japan Society of Applied Physics. This study experimentally investigated the atomic structure of a single-crystalline Mn-doped BiFeO3 (BFO) thin film using X-ray fluorescence holography with synchrotron radiation. Bi atoms can be reconstructed from both Fe Kβ and Mn Kα fluorescence X-ray holograms, indicating Mn doping at B sites in ABO3-type perovskite structure. The fluorescence intensity of the 1st to 4th nearest Bi atoms around target Fe atoms was lower than those around Mn atoms, confirming the selective formation of Bi vacancies around Fe atoms. In contrast, the 5th to 8th nearest Bi atoms around Fe atoms showed higher intensities than those around Mn atoms, indicating that the Mn-doped BFO thin film had a double-perovskite-like structure. These results provide important information that helps elucidate the mechanism by which Mn doping modifies the conductivity of BFO.

© 2019 The Japan Society of Applied Physics. Highly c-axis oriented (Bi3.25Nd0.65Eu0.10)Ti3O12 (BNEuT) thin films were deposited on the Pt(100)/MgO(100) substrates by high-temperature sputtering. The substrate temperature was varied from 550 °C to 650 °C to examine its effect on the structural, dielectric, ferroelectric, and piezoelectric characteristics of the films, and consequently find the optimal substrate temperature for heteroepitaxial growth of BNEuT thin films. All the films deposited at 580 °C-650 °C exhibited a high degree of c-axis orientation [α (00l)] of ≥97%. All the films grown heteroepitaxially on Pt(100)/MgO(100) substrates was rotated by ±45° with respect to the underlying substrates and had a mainly upward polarization, based on data observed by piezoresponse force microscopy. Judging from the structural, dielectric, ferroelectric, and piezoelectric characteristics, it is shown that the optimal substrate temperature for heteroepitaxial growth of BNEuT films with a high α (00l) of >97% and a comparatively large remanent polarization of 2.0 μC cm-2 is 580 °C.

© 2019 The Japan Society of Applied Physics. This work demonstrated the artificial introduction of charged domain walls (CDWs) into a ferroelectric BiFeO3 (BFO) thin film by domain structure control using a pit-patterned SrTiO3 (STO) (001) surface. The pattern consisted of 1 × 1 μm square holes with sloped sides, fabricated on the STO (001) surface by electron beam lithography and Ar+ ion etching. Scanning electron and atomic force microscopy analyses demonstrated that the pit slopes had angles of 6.1°-7.6°, which were sufficient to limit the in-plane growth direction of the BFO at step edges on the STO surface, and thus control the domain structure. Lateral and vertical piezoresponse scanning force and transmission electron microscopy confirmed the artificial introduction of CDWs in the pit and showed that the sign of the CDWs could be reversed via ferroelectric polarization switching. This domain control technique based on a pit pattern provides a simple approach to the integration of ferroelectric DWs into functional devices.

© 2019 The Japan Society of Applied Physics. We investigated the compositional self-regulation and seeding effects of an Fe2O3 layer during the metalorganic chemical vapor deposition of epitaxial BiFeO3 thin films. The growth of the BiFeO3 films was explained in terms of the prior deposition of Fe2O3 and the subsequent incorporation of Bi2O3 into Fe2O3. The self-regulation of film composition was achieved by supplying excess Bi at high growth temperature. The introduction of an Fe2O3 seed layer as thin as 5 nm promoted homogeneous thin film growth and consequently reduced the leakage and its fluctuation as well as the compositional fluctuation. The seed layer also served to control the interfacial composition between BiFeO3 and SrRuO3.

© 2018 The Japan Society of Applied Physics. Pt/1-μm-thick Bi1.0Fe0.99Mn0.01O3 (BFMO)/Pt coplanar capacitors with an interelectrode distance of 260 μm have been fabricated on vicinal SrTiO3 (STO) (001) substrates by radio frequency magnetron sputtering. The bulk photovoltaic effect of a BFMO thin film has been investigated by measuring the current-voltage (I-V) characteristics of the Pt/BFMO/Pt coplanar capacitors under blue-violet laser illumination (λ = 405 nm). The I-V characteristics show the light-polarization-dependent open-circuit voltage (V OC) with a maximum of -209 V. In addition, the optical strain of a BFMO/STO cantilever with a size of 15 mm × 1.3 mm × 70 μm has been investigated by measuring the displacement of the edge of the cantilever under blue-violet laser illumination. The edge displacement depended on light polarization, indicating that the optical strain is due to the coupling between the bulk photovoltaic effect and the inverse piezoelectric effect.

© 2017 The Japan Society of Applied Physics. Recently, new functionalities of ferroelectric domain walls (DWs) have attracted much attention. To realize novel devices using the functionalities of the DWs, techniques to introduce the DWs at arbitrary positions in the ferroelectric thin films are necessary. In this study, we have demonstrated the introduction of the DWs at arbitrary positions in epitaxial BiFeO3(BFO) thin films using the patterned surface of the SrTiO3(STO) single-crystal substrate. On the slope pattern of the STO surface, the in-plane orientation of BFO has changed because the in-plane orientation of BFO can be controlled by the step propagation direction of the patterned surface. From the piezoresponse scanning force microscopy and X-ray diffraction reciprocal space mapping results, charged 109° DWs have been introduced into the BFO thin film at the bottom and top of the slope pattern of the STO surface. In addition, the conductivity modulation of the positively charged DW has been observed by current-sensitive atomic force microscopy imaging.

© 2017 The Japan Society of Applied Physics. Metalorganic chemical vapor deposition (MOCVD) is one of the suitable techniques for practical applications of BiFeO3films. To develop the potential of MOCVD as a device fabrication process, we investigated the relationship between the film and gas compositions, and the growth under highly oxidizing conditions using O2 and O3gases. In the growth of epitaxial BiFeO3thin films on SrRuO3-covered 4° vicinal SrTiO3(001) at 620 °C, the self-regulation of the film composition was achieved for Bi and Fe precursor supply ratios between 62.1 to 78.5% under O2and 56.1 to 73.2-under 5 wt-O3-mixed O2 atmospheres. The leakage was very sensitive to the precursor supply ratio and oxidizing gas. 150-nm-thick MOCVDBiFeO3films grown using O2+O3gas showed the minimum leakage current density of 2.3 × 10-7A/cm2at +1 V. The highly oxidizing growth conditions using O3can suppress the leakage while precise composition control is required.

© 2017 Author(s). Time-resolved X-ray diffraction (XRD) with synchrotron radiation while applying continuous voltage pulses was employed to investigate the electric-field-induced lattice distortion of an epitaxial BiFeO3(BFO) thin film in a Pt/BFO (1 μm)/SrRuO3(50 nm)/vicinal SrTiO3(001) structure. XRD-reciprocal space maps based on the BFO 003, 114, and 1 14 diffraction spots with and without the application of +15 V (150 kV/cm) to the capacitor demonstrated simultaneous electric-field-induced lattice distortion and crystallographic rotation in the BFO thin film. In response to the application of +15 V, the BFO lattice elongated by 0.08% along the [001]BFOdirection and compressed by 0.05% along the [110]BFOdirection. In addition, the BFO crystals were rotated by 0.01° along the [110]STOdirection as a result of electric-field-induced lattice distortion under epitaxial strain along the vertical direction at the step edges of the vicinal substrate.

© 2016 The Japan Society of Applied Physics. Recently, the bulk photovoltaic effect of BiFeO3(BFO) thin films has attracted much attention because of its above bandgap photovoltage for realizing novel photovoltaic devices. In this study, the epitaxial growth of 1-μm-thick Mn and Zn codoped BFO thin films has been demonstrated, and the effects of Mn and Zn codoping on the ferroelectric and bulk photovoltaic properties of the BFO thin films have been investigated. A 0.5% Mn and 0.5% Zn codoped BFO (BFMZO050) thin film on a SrRuO3-buffered vicinal-SrTiO3(001) substrate showed an atomically flat surface with a step-And-terrace structure, a low leakage current of 1.5 ' 10-6A/cm2at 100 kV/cm, and well-saturated ferroelectric electric displacement-electric field (D-E) hysteresis loops. In addition, a Pt/BFMZO/Pt coplanar capacitor with an interelectrode distance of 260μm illuminated by a violet laser (λ = 405 nm) showed an enhanced photovoltage of 145 V owing to the reduction in photoconductance by Mn and Zn codoping.

© 2016 The Japan Society of Applied Physics. The structural evolution of high-quality 3.3-73.2-nm-thick tetragonal-like BiFeO3(T-BFO) thin films grown on LaAlO3(001) substrates and the bulk photovoltaic effect of the films were investigated. The T-BFO films were grown by rf magnetron sputtering, showing the Peudellösung fringes around the T-BFO (001) diffraction peak in X-ray diffraction 2 patterns. These indicate the structural coherence between the surface and the interface in the surface normal direction of the films. High-resolution synchrotron X-ray diffraction analysis and transmission electron microscopy reveal that the lattice relaxation behavior from the MA monoclinic to MC monoclinic structure occurs as the film thickness increases. The domain structure was partly controlled by using a vicinal Lao (001) substrate along [100]. Regarding the current-voltage characteristics of the Pt/T-BFO/ Pt coplanar capacitor under violet laser illumination, T-BFO films show an anomalous photovoltaic effect with an open-circuit voltage of 6.1V and a short-circuit current of %290 pA along the [100]T-BFO direction.

© 2016 The Japan Society of Applied Physics. HfO2-based thin films are one of the key dielectric and ferroelectric materials in Si-CMOS LSIs as well as in oxide electronic nanodevices. In this study, we demonstrated the fabrication of a ZnO/(Hf,Zr)O2/ZnO-trilayered nanowire (NW) capacitor structure solely by metalorganic chemical vapor deposition (MOCVD). 15-nm-thick dielectric (Hf,Zr)O2and 40-nm-thick top ZnO electrode layers were uniformly grown by MOCVD on a ZnO NW template with average diameter, length, and aspect ratio of 110 nm, 10 μm, and >90, respectively. The diameter and aspect ratio of the resultant trilayerd NWs are 200-300nm and above 30, respectively. The crystalline phase of HfO2and stacked the structure are also discussed.

© 2016 the Owner Societies. The CH3NH3PbI3perovskite solar cells have been fabricated using three-porous-layered electrodes as, 〈glass/F-doped tin oxide (FTO)/dense TiO2/porous TiO2-perovskite/porous ZrO2-perovskite/porous carbon-perovskite〉 for light stability tests. Without encapsulation in air, the CH3NH3PbI3perovskite solar cells maintained 80% of photoenergy conversion efficiency from the initial value up to 100 h under light irradiation (AM 1.5, 100 mW cm-2). Considering the color variation of the CH3NH3PbI3perovskite layer, the significant improvement of light stability is due to the moisture-blocking effect of the porous carbon back electrodes. The strong interaction between carbon and CH3NH3PbI3perovskite was proposed by the measurements of X-ray photoelectron spectroscopy and X-ray diffraction of the porous carbon-perovskite layers.

© 2015 The Japan Society of Applied Physics. Recently, anomalous photovoltaic effects in BiFeO3(BFO) thin films have attracted much attention. To investigate anomalous photovoltaic effects in BFO thin films, the photovoltaic properties of single-domain structured BFO should be determined. In this study, Pt/single-domain-structured BFO/Pt coplanar capacitors have been fabricated on SrTiO3substrates, and the illuminated laser power density, BFO film thickness, measurement direction, polarization direction of light, and sample rotation angle dependences of short-circuit current and open-circuit voltage have been investigated. It is found that such anomalous photovoltaic effects are due to second-order nonlinear optical effects including carrier excitation. Moreover, the values of independent elements in the bulk photovoltaic tensor (β), namely, β33, β31, β15, and β22 have been evaluated.

© 2015 The Japan Society of Applied Physics. For the future realization of nanoelectronic devices using domain walls (DWs) in ferroelectric thin films as a functional element, the artificial introduction and position control of DWs in BiFeO3(BFO) thin films using SrTiO3bicrystal substrates are demonstrated. The BFO thin film follows the bicrystal nature of SrTiO3, and consequently the boundary with an out-of-plane misorientation angle of 8° is formed immediately above that of the substrate. The boundary shows a charged nature induced by a head-to-head polarization configuration. The BFO bicrystal film exhibits a wellsaturated polarization hysteresis loop equivalent to those of single-crystalline films. The piezoresponse scanning force microscopy of polarization switching suggests ferroelastic relaxation-mediated 180° switching through 71° switching. Although the boundary corresponds to a low-angle symmetry boundary with misfit dislocations, experimental results show that it behaves just like normal DWs by crystallographic twinning. Therefore, it is concluded that the artificially introduced boundary can be considered as a charged 101° DW.

© 2015 The Japan Society of Applied Physics. (Bi3.25Nd0.65Eu0.10)Ti3O12(BNEuT-0.1) films with a- and b-axis orientations and thicknesses of 1.8-2.3μm were sputter-deposited on conductive Nb:TiO2(101) substrates containing 0.79 mass% Nb. The deposition temperature was fixed at 650 °C, and the sputtering gas pressure was varied from 0.4 to 5.0Pa in order to examine its effect on the structural, ferroelectric and piezoelectric properties of the films. The films were found to have a mostly single-phase orthorhombic structure, with a high degree of a- and b-axis orientations (93-98%). The films had a nanoplate microstructure, with the plates being aligned along the [100]/[010] direction, and porosities of 15-25%. A maximum room-temperature remanent polarization (2Pr) of 93 μC/cm2was obtained for a sputtering gas pressure of 5.0 Pa. All the films were strongly a-axis oriented, according to the results of X-ray diffraction measurements and vertical amplitude images in piezoresponse force microscopy. The optimal sputtering gas pressure for heteroepitaxial growth of BNEuT-0.1 nanoplates with a high degree of a-axis orientation of 96.5%, a maximum orthorhombicity of 0.0017, a comparatively large remanent polarization of 2Pr = 66 μC/cm2, and a high porosity of 24% was found to be 0.4 Pa.

© 2015, The Korean Physical Society. Recently, an anomalous photovoltaic effect in a BiFeO<inf>3</inf> (BFO) thin film has attracted much attention. In this research, Pt/BFO/Pt coplanar capacitors have been prepared using striped- and single-domain structured BFO thin films, and the influence of the polarization direction of incident violet laser light (λ = 405 nm) on the photovoltage of these capacitors has been investigated. In a specific geometry, both coplanar capacitors, the one using a striped-domain and the other using a single-domain structured BFO thin film showed anomalous photovoltages above band gap of BFO, indicating that an anomalous photovoltage should be observed under randomly-polarized light illumination such as that by a halogen lamp.

© 2014 The Japan Society of Applied Physics. Sr2Bi4Ti5O18(SBTO) films with a- and b-axis orientations, and thicknesses of 0.9-1.2μm were sputter-deposited on conductive Nb:TiO2(101) substrates containing 0.79 mass% Nb. The deposition temperature was varied from 575 to 700 °C under a fixed gas pressure of 0.4 Pa, and the structural and ferroelectric characteristics of the films were investigated. SBTO films deposited at 625-700 °C had a mostly single-phase orthorhombic structure, with a high degree of a- and b-axis orientations [α(h00)/(0k0)] of 99.0-99.8%. In addition, the full width at half maximum of the (200) diffraction peak was 0.69-0.86°, which indicated good crystallinity. SBTO films deposited at 625-650 °C had a nanoplate-like microstructure with the plates aligned along the [010] direction. The real room-temperature remanent polarization (2P∗r), taking the porosity between the nanoplates into account, exhibited a maximum of 40 μC/cm2at 650 °C. Thus, the optimal deposition temperature for heteroepitaxial growth of SBTO nanoplates with a high α(h00)/(0k0)of ≥99.0% and excellent ferroelectric properties on conductive Nb:TiO2substrates is 650 °C under a gas pressure of 0.4 Pa.

© 2014 The Japan Society of Applied Physics. We evaluated the thicknesses of domain walls (DWs) in rhombohedral BiFeO3thin films epitaxially grown on SrRuO3-covered SrTiO3(100), (110), and (111) single crystals by scanning nonlinear dielectric microscopy (SNDM). The SNDM phase signal revealed an abrupt change in the sign of polarization components normal to the surface at DWs within one or two unit cells. On the other hand, the SNDM amplitude signals gradually changed across DWs, corresponding to the change in the electrostatic potential. The minimum thicknesses estimated from the SNDM amplitude signals are 4, 1, and 2 nm for 71, 109, and 180° DWs, respectively. The relationship between these DW thicknesses and the nature and situation of DWs is discussed taking account of the polarization configuration in adjacent domains.

© 2014 The Japan Society of Applied Physics. Epitaxial BiFeO3(BFO) thin films with striped- and single-domain structures have been grown on SrTiO3(STO) (103) and (113) substrates by radiofrequency planar magnetron sputtering. The domain structure of BFO was controlled by the orientation of the STO substrate. Piezoelectric force microscopy revealed that BFO thin films on STO (103) and STO (113) had a striped-domain structure with 71° domain walls running along o010pSTO, and a single-domain structure, respectively. To confirm the photovoltaic property, rectangular Pt electrodes with widths of 150-200μm were deposited on BFO surfaces with interelectrodes distances of 200-250 μm. I-V characteristics were measured under an illumination of a collimated violet laser (γ = 405nm) with a power density of 380W/cm2. In the striped-domain-structure BFO film with Pt electrodes fabricated along domain walls, above-band-gap open-circuit voltage (VOC) of 29V was observed. In addition, single-domain-structured BFO thin film with Pt electrodes fabricated along h <1¯10> also showed above-band-gap Vocof 26V despite the absence of domain walls. It is considered that these large Vocvalues originated from the photovoltaic effect not at the domain walls but in bulk BFO.

Recently, the semiconducting characteristics of BiFeO3 thin films such as the photovoltaic effect or diode characteristics have been extensively investigated. However, the current conduction mechanism has not been completely clarified yet. In this study, the current conduction mechanism of the ideal BFO thin film, which has a single domain without conduction domain walls, such as 71 and 109° domain walls, has been investigated. The current density-electric field (J-E) characteristics of 100- to 1000-nm-thick BFO thin films and their temperature dependence in the range of 100-260K have been carefully investigated. From these thickness and temperature dependences of the J-E characteristics, it can be concluded that the most probable mechanism of current conduction in the single-domain BFO thin film is space-charge-limited current (SCLC) with a shallow trap. © 2014 The Japan Society of Applied Physics.

Rhombohedral (R-) and tetragonal-like (T-) BiFeO3(BFO) mixed-phase thin films and single R-phase BFO thin films have been grown on SrRuO3-buffered SrTiO3(STO) (001) substrate by ion beam sputtering. From X-ray diffraction study, it was found that T-BFO and R-BFO mixed-phase thin films can be grown at a low oxygen partial pressure of 1mPa, and the lattice parameters of the T-BFO and R-BFO were a = 0.381 nm, c = 0.461nm (c/a = 1.22), and a = 0.394nm and c = 0.400nm (c/a = 1.02), respectively. High-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) observation and energy dispersive X-ray analysis (EDXA) mappings have revealed a clear T-BFO lattice and a-c domain in the middle of the thin films. However, Bi-rich and Fe-rich secondary phases were also found with the T-BFO phase. Therefore, it can be concluded that secondary phases such as Bi2O3or Bi-poor phase are needed for T-BFO formation on STO (001) substrate, and the a-c domain formation does not occur by stress relaxation but matching of the growth directions of different domains. © 2014 The Japan Society of Applied Physics.

a- and b-axis-oriented (Bi3.25Nd0.75-xEux)Ti3O12(BNEuT, x = 0-0.75) films of 3.0μm thickness were fabricated on conductive Nb:TiO2(101) substrates containing 0.79 mass-Nb by high-temperature sputtering at 650°C, and their structural and piezoelectric characteristics were investigated. The room-temperature remanent polarization (2Pr) and effective piezoelectric coefficient (d33) values for the BNEuT films exhibited maxima of 87 μC/cm2and 15pm/V, respectively, at x = 0.10, which were approximately 1.3 times larger than those (2Pr= 65 μC/cm2and d33= 12pm/V) of the nondoped (Bi3.25Nd0.75)Ti3O12(BNT) nanoplate. The BNEuT film with x = 0.10 had a high a-axis orientation judging from the X-ray diffraction measurement and the observation of the phase image by piezoresponse force microscopy. It is shown that adequate Eu3+doping of BNT nanoplates produces a larger displacement magnitude of the octahedra than that in the nondoped BNT nanoplate, resulting in an improvement of piezoelectric properties in addition to the ferroelectricity. © 2014 The Japan Society of Applied Physics.

Single-domain BFO thin films are prepared on a SrRuO3-buffered SrTiO3 (STO) (001) substrate by RF planar magnetron sputtering. A domain structure is controlled by vicinal direction of the STO substrate. The BFO thin films on vicinal STO along 〈110〉 show single-domain structure without any domain walls. To confirm the influence of epitaxial strain on lattice distortion and ferroelectricity, single-domain BFO thin films with thicknesses ranging from 10-1000 nm are prepared. Synchrotron X-ray diffraction reveals that lattice relaxation and step bunching occur in the thickness range of 50-200 nm. The BFO films with thicknesses over 300 nm are almost free from the influence of the epitaxial strain induced by (001)-oriented substrates. The remanent polarization Pr is almost constant at about 60 μC/cm 2. However, Pr slightly increases in the BFO films with thicknesses less than 200 nm. Even the 100-nm-thick BFO film show fully saturated D-E hysteresis at RT, and the Pr is 65 μC/cm 2. © 2013 The Japan Society of Applied Physics.

a- and b-axis-oriented (Bi3.25Nd0.75-xEu x)Ti3O12 (BNEuT, x = 0-0.75) films of 3.0 μm thickness were fabricated on conductive Nb:TiO2(101) substrates containing 0.79 mass% Nb by high-temperature sputtering at 650 °C, and their structural and ferroelectric characteristics were investigated. All the films had a mostly single-phase orthorhombic structure, with high degrees of a- and b-axis orientations of 99.0-99.8%. The lattice parameters (a-, b-, and c-axis lengths) and the calculated orthorhombic lattice distortion decreased monotonically with increasing Eu content. The microstructure of BNEuT films with x = 0-0.50 was nanoplate-like, whereas that of films with x ≥ 0:60 was significantly more bulklike. The real room-temperature remanent polarization (2Pr*), taking the porosity between the nanoplates into account, had a maximum value of 2Pr* = 87 μC/cm2 at x = 0.10, which was approximately 1.3 times larger than that (65 μC/cm2) of the nondoped BNT film. It is shown that lattice distortion caused by rotation of octahedra in the a-b plane due to the Eu substitution plays a significant role in the improvement of ferroelectricity. © 2013 The Japan Society of Applied Physics.

We investigated the growth of ZnO nanorods on Al2O3(1120) substrates by using metalorganic chemical vapor deposition and demonstrated the ability to control their diameters and surface densities by using a two-step growth method. Following the first step, the ZnO nanorods were found to be densely packed due to random nucleation across the substrate surface, and their diameters and surface densities (numbers per unit area) were interdependent. During the second growth step, because nucleation sites were limited to the tips of existing nanorods, an upper limit was placed on the surface density of the second-layer nanorods. The surface density of the second-layer nanorods was also influenced by the reaction pressure during the second growth step. The diameter of the ZnO nanorods in the second layer was determined by the growth temperature during the second step and could be controlled independently of the surface density. Consequently, ZnO nanorods with small diameters (< 100 nm), low surface densities (< 10 μm-2) and large spacings (> 200 nm), which are difficult grow using a one-step growth method, were successfully produced. The deposition of 180-nm-thick Pb(Zr,Ti)O3coatings onto sparsely-grown ZnO nanorods was also demonstrated. © 2013 The Korean Physical Society.

a- and b-axis-oriented Bi3. 25Nd0. 75Ti3O12(BNT) nanoplates, 3. 0-μm thick, were fabricated on conductive Nb:TiO2(101) substrates with 0. 79 mass% Nb at 650 °C by high-temperature sputtering. Successively, the fabrication of inorganic-organic composites was carried out by introducing an epoxy resin to the spaces between the BNT nanoplates. The fourier transform infrared spectroscopy (FTIR) and the energy dispersive X-ray (EDX) elemental mapping results confirmed that the fabricated composites were inorganic-organic hybridized materials with cured epoxy resin introduced into the spaces between the BNT nanoplates. Piezoelectric response measurements of the fabricated BNT-epoxy resin composites by using piezoresponse force microscopy (PFM) showed that the composites have potential as piezoelectric microelement materials. © 2013 The Korean Physical Society.

Epitaxial BiFeO3(BFO) thin films have been prepared on La-doped (001) SrTiO3substrates (La-STO) by using magnetic-field-assisted pulsed laser deposition. X-ray diffraction patterns of the epitaxial BFO thin films prepared under magnetic fields of 0, 0. 1, and 0. 4 T show only (00ℓ) diffraction peaks without secondary phases. From the results of reciprocal space mapping for all epitaxial BFO thin films, (003) reflections show splitting spots, and asymmetric spots of (-103) and (103) reflections exhibit a rhombohedral structure. The microstructure of the epitaxial thin films was modified by the strength of magnetic field, and a columnar structure was shown in the film prepared under a high deposition rate for a magnetic field of 0. 4 T. The polarization versus electric field hysteresis loops were obtained at room temperature (RT) in all the epitaxial films; in particular, the remanent polarization for the epitaxial film prepared under a magnetic field of 0. 1 T was 46 μC/cm2at RT while the current density was reduced in comparison with those of other epitaxial films. © 2013 The Korean Physical Society.

Polycrystalline BiFeO3(BFO) films on Pt/TiO2/ SiO2/Si substrate have been prepared by a pulsed laser deposition (PLD) method with various repetition rates. The X-ray diffraction (XRD) pattern observes single-phase perovskite structure with a small secondary phase. While the film-thickness is increased with increasing repetition rates, the grain size increases, and the leakage current reduces. Also, good polarization versus electric field (P-E) hysteresis loops of all polycrystalline BFO films were obtained at room temperature (RT). It is seen that the polarization value (Pr) practically remains almost constant for all the BFO films, and the coercive field (Ec) decreases with increasing the film thickness. © 2013 Copyright Taylor and Francis Group, LLC.

磁場印加レーザアブレーション法を用いてPt/TiO2/SiO2/Si基板上に成膜したBiFeO3薄膜は単相ペロブスカイト構造を持ち、XRDパターンのピーク角度は磁場無で成膜した場合に比べると低角側にシフトしていた。また、円柱状で小さいグレインサイズの多結晶構造が得られ、この形状により、残留分極が50μC/cm2、圧電d33定数が100pm/Vと強誘電性や圧電性に影響が現れることがわかった。

Ferroelectric BaTiO3and antiferromagnetic BaFeO3 are deposited alternatively by a pulsed laser deposition method with the aim of fabricating room-temperature multiferroic materials. X-ray diffraction (XRD) analysis reveals that the superlattice structure is fabricated with a designed periodicity and thin film quality is improved by increasing BaFeO3 layers. Ferroelectricity and ferromagnetic properties are examined by the double-wave method (DWM) of D-E hysteresis loops, an XRD dilatometry, and a superconducting quantum interference device magnetometer. Our superlattice thin film shows multiferroicity, i.e., the coexistence of ferroelectricity and ferromagnetism, at room temperature. © 2012 The Japan Society of Applied Physics.

We report a size dependence of switchable polarization in PbTiO 3 nanoislands with heights of 2-7nm and widths of 40-80 nm. An atomic force microscopy (AFM) system that was capable of probing a switching charge from a single PbTiO 3 nanoisland using a conductive AFM tip as a top electrode has been developed. Using the AFM system, the switching charge as small as 10 fC was detected within a tolerance of ±4 fC. The switchable polarization (δP) over 150 μC/cm 2 was observed for the nanoislands as small as 5 nm in height, and δP rapidly decreased with the height below 5 nm. Comparing the height or thickness dependence of δP and stable domain states between the nanoislands and ultrathin films, it was found that the polarization in the nanoislands can be switched by external fields more easily than that in the ultrathin films. The decrease in δP of the nanoislands with the height below 5 nm can be attributed to the intrinsic size effects. © 2012 The Japan Society of Applied Physics.

We describe the selective growth of ZnO nanorods by metalorganic chemical vapor deposition using Pt layers patterned on SiO2/Si by photolithography as nucleation sites, as well as their application to ferroelectric nanorods. The growth of ZnO nanorods on the SiO2and Pt surfaces was quite different in the nucleation stage. Under specific growth conditions, no nucleation was observed on the SiO2surface due to its stable and less reactive nature while nucleation easily occurred on Pt due to its catalytic properties. Using this difference, we achieved selective growth of c-axis oriented ZnO nanorods only on Pt dots with lateral dimensions of 40 μm × 40 μm and diameter of 300 nm on the SiO2/Si substrate. The average diameter and length of selectively grown ZnO nanorods were 70-80 nm and ∼10 μm, respectively. By depositing 100-nm-thick Pb(Zr,Ti)O3layers on the selectively grown ZnO nanorods, Pb(Zr,Ti)O3-covered ZnO nanorods were successfully grown selectively on the patterned Pt layers. These results demonstrate the feasibility of position control of nanorod growth by self-assembly in combination with photolithography for future ferroelectric device applications. © 2012 American Institute of Physics.

We report on switching current measurements of self-assembled PbTiO 3 nanoislands with an average height in the range of 4-5nm and a width of less than 100nm by scanning probe microscopy (SPM). A blunt SPM tip with a flat end-face with a plateau diameter of 1.8 μm was directly contacted to PbTiO 3 nanoislands as a top electrode, which enabled electrical characterization of as-prepared samples without the need for fabricating top electrodes or any preprocessing. Using this SPM system, it was revealed that the nanoislands have switching charge densities as large as 140-170 μC/cm 2, comparable to those of epitaxial thin films. © 2012 The Japan Society of Applied Physics.

We investigated the growth behavior and the domain structure of PbTiO3 thin films grown on Pt-covered SrTiO3 (100) (STO) substrates by metalorganic chemical vapor deposition. The step height and the terrace width of the single-crystalline Pt films on 1 - 4° vicinal STO substrates along the [100] and the [110] directions were 1 - 6 and 80 - 150 nm, respectively. PbTiO3 films were grown in the Volmer-Weber mode on Pt-covered vicinal STO substrates, and nanosized islands were formed at an early growth stage. Atomic force microscopy revealed that nucleation of PbTiO3 predominantly occurred at ascending steps on the Pt surface and that the lateral growth of PbTiO3 islands was limited by the steps. X-ray diffraction analysis and piezoresponse force microscopy indicated that a-domains and misfit dislocations were anisotropically introduced in PbTiO3 thin films on a Pt/4°-vicinal STO substrate along the [100] direction. On [110]-vicinal STO substrates, no significant influence of the vicinal substrate on the growth behavior or the domain structure of PbTiO3 was observed.

An electric-field-induced strain at 80 K in a 350-nm-thick BiFeO3(BFO) thin film deposited by pulsed laser deposition has been investigated by using time-resolved X-ray diffraction with synchrotron radiation under an electric field. 300-nsec-wide and 804.09-nsec-period voltage pulses were applied to the BFO thin film during the measurements. The time-resolved X-ray diffraction profile around the BFO (001) diffraction peak indicates that the BFO (001) diffraction peak shifts to lower angle when the voltage is applied. The strain estimated from the peak shift increases lineally with increasing amplitude of the voltage pulses. The piezoelectric constant (d33) of the (001)-oriented domains in the polycrystalline BFO thin film at 80 K is 20.2 pm/V. This value is smaller than that measured at RT of 27.8 pm/V in our previous report.

Polycrystalline BiFeO3thin films were prepared on indium-tin-oxide (ITO)-coated glass substrates by using pulsed laser deposition (PLD). The X-ray diffraction (XRD) pattern showed a polycrystalline perovskite phase and a (010) preferred orientation. A higher deposition temperature was required to crystallize for the BFO thin film on an ITO substrate compared to the BFO thin film grown on a Pt substrate, the grain size was smaller than that of the BFO thin film on Pt substrate, and the leakage current density was less. The optical transmittance was about 80% around 780 nm, and the direct band gap (Eg) was 2.72 ± 0.03 eV. The P - E hysteresis loop of the BFO thin film on an ITO substrate was obtained at RT, and the polarization difference at zero electric field (2Pro), corresponding to the double remanent polarization was 140 μC/cm. The M - H hysteresis loop was obtained at RT, and the antiferromagnetic behavior was little affected by using different substrates.

BiFe0.9Co0.1O3 (BFCO) films have been prepared on Pt/TiO2/SiO2/Si substrates with a magnetic field of 0.4 T by pulsed laser deposition. BFCO films prepared under magnetic field show a single-phase perovskite structure with a little secondary phase, and their (010) and (020) peaks shift to lower angles than those of the film prepared without magnetic field. The BFCO film of about 1 μm thickness shows a columnar structure, although the BFCO film of about 350nm prepared without magnetic field shows a grain like structure. The leakage current density of the BFCO film prepared under magnetic field is much lower than that of the BFCO film prepared without magnetic field. The saturated P-E hysteresis loops of both BFCO films are obtained at 80 K, and the M-H hysteresis loops of both BFCO films show a weak ferromagnetism behavior at room temperature. © 2011 The Japan Society of Applied Physics.

BiFeO3 (BFO) thin films with various Bi/Fe ratios have been deposited on SrRuO3/SrTiO3/SrTiO3(001) substrates by dual ion beam sputtering. A Bi2O3 ceramic disk and an α-Fe2O3 powder disk were used as targets, and simultaneously sputtered using a dual ion beam. Bi/Fe ratio has been controlled by adjusting the beam current ratio on the Bi2O 3- and α-Fe2O3-side ion sources. Even a BFO thin film with a Bi/Fe ratio of 0.95 and a smooth surface shows a slightly leaky characteristic. [Fe2+]/(Fe3+]+ [Fe2+]) ratio has been estimated by Auger electron spectroscopy (AES). From the AES profiles, the [Fe2+]/(Fe3+]+ [Fe2+]) ratio of the BFO thin film with a Bi/Fe ratio of 0.95 is estimated to be 0.14. It is considered that not only improving surface roughness but also enhancing oxidization is important for reduction in leakage current. © 2011 The Japan Society of Applied Physics.

BiFe1-xZnxO3(BFZO) thin films (x = 0, 0.05, and 0.1) have been prepared on Pt/TiO2/SiO2/Si substrate by pulsed laser deposition and their multiferoic properties have been investigated. BFZO thin films shows polycrystalline perovskite single phase. The grains become small with increasing the substitution of Zn, and leakage current of BFZO thin film for x = 0.05 was lower than that of BiFeO3thin film. The dielectric constant of BFZO thin films for x = 0, 0.05, and 0.1 are 107, 146, and 170 at room temperature, respectively. P-E hysteresis loops were obtained at room temperature and 80 K, and Prslightly increases at low frequency from 500 Hz to 20 kHz. M-H hysteresis loops show weak ferromagnetic properties at 300 K and 80 K by substitution of Zn. © 2011 Elsevier B.V. All rights reserved.