研究者詳細 - 石丸　学

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Diffusion and trapping of silicon in wurtzite gallium nitride 査読有り国際誌

Mimura K., Nakamura R., Ishimaru M., Sakane S., Ueoka Y., Mesuda M., Suzuki T.

Materials Science in Semiconductor Processing 204 2026年03月

記述言語：英語掲載種別：研究論文（学術雑誌）

The diffusion behavior of various dopants in gallium nitride (GaN) should be comprehensively understood as essential for materials design and for controlling of the electrical properties towards versatile device applications. This study measures the diffusion coefficient (D) of a typical n-type dopant, silicon (Si), along the c-axis direction in monocrystalline GaN (mc-GaN) films with the wurtzite structure at the relatively low temperature range of 600–900 °C compared to previous reports. Two types of samples are examined using secondary ion mass spectrometry. In the case of a thin Si layer deposited on mc-GaN, the D values are determined by analyzing the Si concentration profiles based on an error-function type equation, yielding values in the range of 5.1 × 10−21 – 3.0 × 10−19 m2 s−1. The activation energy for Si diffusion is 1.07 eV, which is comparable with the data reported in the literature for the measurements above 900 °C. In contrast, no Fickian diffusion of Si is observed in a diffusion couple of an epitaxially-grown GaN film containing Si, GaN(Si), on mc-GaN at 600–900 °C. Cross-sectional transmission electron microscopy reveals that the microstructure of the GaN(Si) layer is highly defective; not only do boundaries between columns exist, but numerous dislocations are also present throughout the layer, serving as trapping sites for Si.

DOI： 10.1016/j.mssp.2025.110272

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イオン照射δ-Sc4Hf3O12に出現する特異な短範囲規則構造査読有り

岩崎将成, 石丸学

まてりあ ( 公益社団法人日本金属学会 ) 65 ( 1 ) 23 - 23 2026年01月

低エネルギーの電子ビームが励起するアモルファスゲルマニウム薄膜の瞬間•広域結晶化査読有り

仲村龍介, 奥川将行, 石丸学

まてりあ ( 公益社団法人日本金属学会 ) 65 ( 1 ) 19 - 19 2026年01月

Diffusion of Oxygen in Amorphous and Monoclinic (Hf,Zr)O2 Films 査読有り国際誌

Motozu Y., Nakamura R., Ishimaru M., Suzuki T.

Physica Status Solidi B Basic Research 263 ( 1 ) 2026年01月

記述言語：英語掲載種別：研究論文（学術雑誌）

The diffusion coefficients of oxygen (D) in thin films of amorphous (a-) and nanocrystalline monoclinic (Hf,Zr)O<inf>2</inf> are determined by profiling the long-range diffusion of the stable oxygen isotope (18O) in the temperature range of 300°C–700°C using secondary-ion mass spectrometry. D at 300°C–400°C in the a-phase and D at 500°C–700°C in the m-phase exhibit a nearly continuous temperature dependence. The activation energy for diffusion in the a-phase is found to be 1.42–2.91 eV, depending on the degree of structural relaxation, and is comparable to or higher than that for volume diffusion in the m-phase (0.48–1.59 eV, including literature data). The pair distribution functions for the a-phase, obtained using electrons as a probe, indicate that the local structure resembles that of an m-phase model from a previous molecular dynamics simulation. Supported by previously reported theoretical calculations, the possible diffusion mechanisms of oxygen in both the a-phase and m-phase are discussed.

DOI： 10.1002/pssb.202500490

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Giant spin-orbit torque induced by spin Hall effect in amorphous Pt(P) alloys 査読有り国際誌

Shashank U., Tomoda T., Mathew A.J., Vashisht G., Imai K., Kusaba Y., Dong C.L., Chen C.L., Horibe Y., Ishimaru M., Awano H., Asada H., Fukuma Y.

Npg Asia Materials 17 ( 1 ) 2025年12月

記述言語：英語掲載種別：研究論文（学術雑誌）

We report a giant spin-orbit torque (SOT) induced by spin Hall effect (SHE) in amorphous Pt(P) alloys, which is confirmed by magnetization switching and spin torque-ferromagnetic resonance measurements. Pt(P) is fabricated by ion-implantation technique using energies from 10 to 30 keV with doses ranging from 2.5 × 1016 to 10 × 1016 ions/cm2. The P-ion implantation process causes distortion and defects in the fcc structure of the as-deposited Pt layer and changes it to an amorphous structure as the accelerating energy as well as the dose increases, leading to a decrease in the electrical conductivity. However, we can obtain a higher spin Hall conductivity, σxySH of 3.62×103ℏ2eΩ−1cm−1 for Pt(P) as compared to that of 1.03×103ℏ2eΩ−1cm−1 for pure Pt. The spin Hall efficiency, θSH is drastically increased up to 1.17 for Pt(P) with 30 keV energy and dose of 7.5 × 1016 ions/cm2. We also observe a significant reduction in the critical switching current density, Jsw of the SOT magnetization switching from 1.5 × 1011 A/m2 for pure Pt to 3.0 × 1010 A/m2 for Pt(P). Such a giant SHE can reduce the power consumption to control the magnetization by using SOT, and therefore our findings may provide an alternate path to enhance θSH by using amorphous materials with a variety of elements, beyond crystalline solids.

DOI： 10.1038/s41427-025-00596-6

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The effective thermal conductivity of a screen-printed thermoelectric film of Bi2Te3 and CsSnI3 査読有り国際誌

Miyazaki K., Mustafa A., Baranwal A.K., Saini S., Watanabe K., Miura A., Yabuki T., Ishimaru M., Kawano S., Wang Q., Iikubo S., Shen Q., Hayase S.

International Journal of Heat and Mass Transfer 250 2025年11月

担当区分：筆頭著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）

In recent years, printed thermoelectric thin films have attracted attention due to their advantages in enabling large-area and flexible designs. We developed printable thermoelectric composites of bismuth telluride and halide perovskite, which were expected to exhibit superior thermoelectric properties compared to other printable materials, such as conductive organic materials. The measured dimensionless figure of merit was about 0.1 at room temperature due to its low thermal conductivity. We assessed the interfacial thermal resistance between bismuth telluride and halide perovskite using multilayered films to understand the measured low thermal conductivity. The calculated thermal conductivity using the measured interfacial thermal resistance was fairly consistent with the effective thermal conductivity observed. We also discussed the interfacial thermal resistance using a phonon transport model based on ab-initio calculations to confirm the validity of our measurements. Additionally, we found that the high interfacial thermal resistance was achieved due to the low group velocities of both materials in the composites. This mechanism can be applied not only to improve thermoelectric materials but also to address thermal issues in composites.

DOI： 10.1016/j.ijheatmasstransfer.2025.127292

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Heterogeneous amorphous structure and bioactivity in silver-doped bioactive glasses synthesized by spray drying methods 査読有り国際誌

Araki F., Lin M.Z., Shih S.J., Ishimaru M.

Advanced Powder Technology 36 ( 11 ) 2025年11月

担当区分：最終著者記述言語：英語掲載種別：研究論文（学術雑誌）

In this study, Ag-doped SiO<inf>2</inf>-CaO-P<inf>2</inf>O<inf>5</inf> bioactive glasses (BGs) were synthesized by a spray drying method, and their atomistic structures were investigated by transmission electron microscopy, X-ray diffraction, and nuclear magnetic resonance. The constituent elements (Si, Ca, P, and O) were homogeneously distributed in the pure (non-doped) BG, but a small Ca- and P-rich layer was present on the surface. The segregation of Ca and P on the surface became more pronounced with increasing Ag concentration. The atomic pair-distribution functions extracted by nanobeam electron diffraction revealed that the Ag-doped BG particles had a heterogeneous amorphous structure: the interior and surface of the BG particles were similar to those of SiO<inf>2</inf> and phosphate-based glasses, respectively. The addition of Ag resulted in less non-bridging oxygen, which may be due to the removal of the network modifier ion (Ca) from the SiO<inf>2</inf> network. In vitro bioactive tests of the pure BG and Ag-doped BG particles revealed that the formation of hydroxyapatite was enhanced by Ag doping, although the amount of non-bridging oxygen decreased. This is attributed to the P- and Ca-rich surface layer, which triggers the nucleation and growth of hydroxyapatite.

DOI： 10.1016/j.apt.2025.105054

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Radiation-induced modifications in ZrN: effect of micro vs. nano crystallites 査読有り国際誌

Beer C., Bahri M., Baldinozzi G., Ishimaru M., Zhang Y., Sharpe M.K., McAleese C.D., Chen X., Bohns F., Manning T., Browning N.D., Whittle K., Springell R., Akhtar R., Patel M.

Acta Materialia 298 2025年10月

記述言語：英語掲載種別：研究論文（学術雑誌）

ZrN is a model system used to understand the irradiation response of isostructural advanced nuclear fuel, UN. UN possesses desirable material properties, such as improved thermal conductivity and fissile uranium density compared to oxides, but understanding damage accumulation and recovery processes in its rocksalt structure remains limited. Micro- and nanocrystalline ZrN were studied under ion irradiation using 600 keV Ar+ ions with fluences varying from 1 × 1015 to 1 × 1017 ions/cm2 at room temperature. Structural parameters, crystallite size, and microstrain of crystalline phases were determined via a unique Rietveld refinement of glancing incidence XRD data. Transmission electron microscopy was used to investigate microstructural modifications, and nanoindentation was used to explore the evolution of mechanical properties. The results show a pronounced increase in the crystallite size of nanocrystalline ZrN, while induced damage degrades the coherence of scattering domains in both ZrN systems post-irradiation. Microcrystalline ZrN displays swelling with increasing fluence, whereas nanocrystalline ZrN contracts before showing a continued unit cell expansion at higher fluences. AFM was used to confirm swelling in mi-crocrystalline ZrN, while oxidation in nanocrystalline ZrN is explored using ToF-ERDA and is attributed to a radiation-induced REDOX reaction. Nanoindentation results confirmed increases in the post-irradiated surface hardness and elastic modulus. The TEM investigation provides evidence of periodic and incommensurate ordering in irradiated regions of microcrystalline ZrN. The formation of a distinct epitaxial layer of cubic ZrO<inf>2</inf> on the surface of microcrystalline ZrN has been attributed to the effects of incident ion interactions.

DOI： 10.1016/j.actamat.2025.121391

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Mode crossover of vortex pinning and excitation in segmented nanorod system 査読有り

Horide T., Torigoe K., Otsubo K., Ishimaru M.

Superconductor Science and Technology 38 ( 8 ) 2025年08月

担当区分：最終著者記述言語：英語掲載種別：研究論文（学術雑誌）

The nanorods are effective in improving the critical current density in YBa<inf>2</inf>Cu<inf>3</inf>O<inf>7</inf> films, but their pinning strength and morphology depend on the fabrication process. To understand the critical current density mechanism, the vortex excitation mode as well as static pinning configuration should be discussed. In this study, we prepared YBa<inf>2</inf>Cu<inf>3</inf>O<inf>7</inf> + BaHfO<inf>3</inf>/YBa<inf>2</inf>Cu<inf>3</inf>O<inf>7</inf> composite multilayers (CMLs). This is a segmented nanorod system where the length of nanorods can be controlled. Two types of CMLs were fabricated to extract the influence of the pinning strength of the nanorods, where the nanorod pinning strength is relatively represented by J<inf>c</inf>(77 K,1T) of a YBa<inf>2</inf>Cu<inf>3</inf>O<inf>7</inf> + BaHfO<inf>3</inf> single layer film: 0.73 MA cm−2 for strong-pinning nanorod and 0.08-0.27 MA cm−2 for weak-pinning nanorod. In the multilayer with strong-pinning nanorods, the critical current density decreased with increasing the pure layer ratio, indicating that the vortex excitation from the nanorods was accelerated by the pure layer. In the multilayer with weak-pinning nanorods, dependence of the critical current density on the pure layer ratio exhibited the peak due to the competition of excitation modes. Surprisingly, when we compare the results in different types of CMLs, the critical current density for high pure layer ratio is almost the same despite different nanorod pinning strength, where the nanorod edges rather than the nanorod core is a dominant pinning center. Design of the excitation mode as well as static configuration is very effective in achieving high critical current density in YBa<inf>2</inf>Cu<inf>3</inf>O<inf>7</inf> films.

DOI： 10.1088/1361-6668/adfa4a

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Control of threading dislocation formation in epitaxially grown YBa2Cu3O7 thin films and their pinning properties 査読有り国際誌

Voulhoux M., Igarashi T., Horide T., Ishimaru M.

Journal of Applied Physics 137 ( 17 ) 2025年05月

担当区分：最終著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）

To improve the critical current density of c-axis oriented YBa<inf>2</inf>Cu<inf>3</inf>O<inf>7</inf> (YBCO) thin films, much effort has been devoted to the introduction of pinning centers that disrupt vortex migration under magnetic fields. Threading dislocations are anticipated to act as one-dimensional pinning centers without reducing the superconducting regions. In this study, YBCO thin films were prepared on a SrTiO<inf>3</inf> (STO) + xCeO<inf>2</inf> (x = 0%, 30%, and 50%) buffer layer by pulsed-laser deposition, and their nanostructures were analyzed by transmission electron microscopy. The interface between the buffer layer and the YBCO thin film was flat for the pure STO, while it was roughened by numerous nanoparticles for the CeO<inf>2</inf>-doped buffer layer. Due to these nanoparticles, threading dislocations were introduced more densely in the YBCO films on the STO + CeO<inf>2</inf> buffer layer than in those on the pure STO buffer layer, depending on the volume percent of CeO<inf>2</inf>. Investigations of the critical current density as a function of the magnetic field revealed that the threading dislocations effectively act as pinning centers.

DOI： 10.1063/5.0249007

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Asymmetric self-organization from a symmetric film by phase separation 査読有り国際誌

Horide T., Usuki M., Ishimaru M., Horibe Y.

Nanoscale 17 ( 8 ) 4740 - 4749 2025年01月

記述言語：英語掲載種別：研究論文（学術雑誌）

Self-organization realizes various nanostructures to control material properties such as superconducting vortex pinning and thermal conductivity. However, the self-organization of nucleation and growth is constrained by the growth geometric symmetry. To realize highly controlled three-dimensional nanostructures by self-organization, nanostructure formation that breaks the growth geometric symmetry thermodynamically and kinetically, such as tilted or in-plane aligned nanostructures, is a challenging issue. A vertically aligned nano-checkerboard is typically formed from ZnMnGaO<inf>4</inf> with the twin domain vertically aligned by the stress from the MgO substrate. The change in the template structure is promising to form a different type of nanostructure. The cubic ZnMnGaO<inf>4</inf>/MgO films were annealed to form nanoscale tetragonal domains in the tilted direction from the surface, which is determined by lattice mismatch, lattice symmetry, and atomic bonding. On the other hand, as a result of free deformation, in-plane aligned twin domains were formed on the SrTiO<inf>3</inf> substrate with a thin MgO buffer layer, which does not induce stress in the ZnMnGaO<inf>4</inf> film. By annealing the ZnMnGaO<inf>4</inf>/MgO/SrTiO<inf>3</inf> film, the nano-checkerboard with a size of ∼10 nm and a length of ∼200 nm is elongated to the in-plane [100] or [001] direction. This study demonstrates the possibility of fabricating a nanostructure that breaks the growth geometric symmetry, which is not achieved by the previous self-organization. The phase separation with controlled template opens more complicated three-dimensional structures by self-organization.

DOI： 10.1039/d4nr04343j

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Short-range order in ion irradiated fluorite structural derivatives in Sc2O3–HfO2 査読有り国際誌

Izumi T., Iwasaki M., Patel M.K., Baldinozzi G., Grygiel C., Ishimaru M.

Acta Materialia 284 2025年01月

担当区分：最終著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）

Knowledge of radiation-induced structural changes in complex oxides is of technological importance for the development of container materials for the immobilization of nuclear waste. Fluorite structural derivatives are candidates for container materials, but their disordering processes under radiation environments are still unclear. In the present study, ion irradiation was performed on the Sc<inf>2</inf>O<inf>3</inf>–HfO<inf>2</inf> pseudo-binary system in which fluorite-related structural compounds such as β–Sc<inf>2</inf>Hf<inf>7</inf>O<inf>17</inf>, γ–Sc<inf>2</inf>Hf<inf>5</inf>O<inf>13</inf>, and δ–Sc<inf>4</inf>Zr<inf>3</inf>O<inf>12</inf> exist, and the short-range ordered structures were investigated by transmission electron microscopy (TEM). The pristine long-range ordered rhombohedral phase was found to transform into a long-range disordered cubic oxygen-deficient fluorite phase upon ion irradiation. The atomic arrangements are not completely disordered, but a more ordered structure was present in the disordered fluorite matrix. A characteristic diffuse scattering was observed in the electron diffraction patterns, and dark-field TEM observations revealed that it is due to microdomains, defined as small regions where the degree of order is higher than that of the disordered matrix. Although the parent phase was different before irradiation, the short-range organization is the same. A comparison of diffuse scattering with the location of superlattice reflections suggested that the microdomains have a structure similar to the δ-phase. With increasing Sc<inf>2</inf>O<inf>3</inf> concentration, the microdomains of δ-type structures were suggested to change to short-range ordered bixbyite structures, which are not present in the equilibrium phase diagram.

DOI： 10.1016/j.actamat.2024.120581

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Fabrication of YBa<inf>2</inf>Cu<inf>3</inf>O<inf>7</inf> Superconducting Nanocomposite Film on Silicon Substrate 査読有り

Bertrand A., Voulhoux M., Kuroki M., Ishimaru M., Horide T.

ACS Applied Electronic Materials 6 ( 6 ) 4601 - 4607 2024年06月

記述言語：英語掲載種別：研究論文（学術雑誌）

Fabrication of YBa2Cu3O7 nanocomposite films on silicon substrates is required for superconducting electromagnetic devices. Low-temperature deposition is effective in suppressing the chemical reaction between YBa2Cu3O7 and Si, while sufficient diffusion is required to form the well-defined nanocomposite structure. The fabrication of the YBa2Cu3O7 nanocomposite films on Si substrates should simultaneously satisfy these conflicting requirements. A YSZ (yttrium stabilized zirconia) buffer layer was epitaxially grown on Si substrates to suppress the chemical reaction. Then, the YBa2Cu3O7 + Ba2YbNbO6 films were fabricated on the YSZ/Si. The nanorods with diameters of 9-17 nm were elongated along the c-axis even at the low deposition temperature. The YBa2Cu3O7 + Ba2YbNbO6 nanocomposite film exhibited a critical temperature of 86.0 K, which is comparable to the critical temperature of 85.6 K in the pure film. The irreversibility temperature was slightly improved by the nanorods. Thus, we demonstrate the formation of nanorods in the YBa2Cu3O7 films on Si substrates without lowering the critical temperature. This opens the superconducting electromagnetic devices integrated with high-temperature superconductors and semiconductors.

DOI： 10.1021/acsaelm.4c00612

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Stability of ion-beam-induced bixbyite phase in δ-Sc<inf>4</inf>Hf<inf>3</inf>O<inf>12</inf> under heat treatments and electron beam irradiations 査読有り

Iwasaki M., Patel M.K., Baldinozzi G., Sickafus K.E., Ishimaru M.

Journal of the European Ceramic Society 44 ( 5 ) 3131 - 3138 2024年05月

担当区分：最終著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）

Oxygen-deficient compounds A4B3O12 have a δ-type structure with a regular long-range arrangement of oxygen vacancies induced by compensation of the charge difference between A3+ and B4+ cations, while A and B cations do not order long-range and only display weak short-range correlations. The δ-type compounds in the Sc2O3-HfO2 and Sc2O3-ZrO2 systems exhibit excellent resistance to radiation-induced amorphization, but the long-range ordered δ-phase was transformed into a short-range ordered bixbyite phase. Since this structural change was not predicted from the phase diagram, the validity of the formation of the bixbyite phase and its stability are still unclear. In the present study, the changes of radiation-induced microstructures in δ-Sc4Hf3O12 under heat treatments and electron beam irradiations were examined by ex-situ and in-situ transmission electron microscopy. It was found that the ion-beam-induced metastable bixbyite phase is transformed into the δ and fluorite phases by the rearrangement of the oxygen vacancies.

DOI： 10.1016/j.jeurceramsoc.2023.12.063

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Metastable Substitution of an Isovalent Anion Element in SnSe Films to Control the Thermoelectric Property 査読有り

Yamaguchi K., Ishimaru M., Horide T.

ACS Applied Electronic Materials 6 ( 2 ) 1071 - 1077 2024年02月

記述言語：英語掲載種別：研究論文（学術雑誌）

To improve the thermoelectric properties of SnSe films, carrier control is required, but elemental doping is difficult due to the thermodynamic solubility limit. Isovalent elements may generate holes or electrons not in a direct manner but in the manner to form point defects. In this study, the Sn(Se,Te) films were fabricated by a pulsed laser deposition (PLD) method to control the carrier concentration by substituting isovalent Te for Se. The coexistence of the orthorhombic and cubic phases at the tens of nanometer scale in the SnSe0.5Te0.5 film was clarified by the structural observation, which is consistent with the equilibrium phase diagram. In spite of the phase coexistence, the lattice parameters linearly increased with an increase in the Te content in the Sn(Se,Te) films. This demonstrates the metastable composition situation for each phase, namely, the carrier control beyond the thermodynamic limit due to the nonequilibrium growth in PLD. As a result, the Seebeck coefficient decreased, and the electrical conductivity increased to increase the power factor, especially in a low temperature near room temperature. The Te substitution in the nonequilibrium PLD increases the hole concentration beyond the thermodynamic solubility limit and thus is effective in controlling the carrier in the SnSe films where carrier doping is difficult.

DOI： 10.1021/acsaelm.3c01490

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Carrier Control of Bi-Doped SnSe Films for Fabrication of π-Type Thermoelectric Film Modules 査読有り国際誌

Horide T., Nakamura K., Ishimaru M.

ACS Applied Energy Materials 7 ( 1 ) 346 - 352 2024年01月

担当区分：最終著者記述言語：英語掲載種別：研究論文（学術雑誌）

Thermoelectric energy conversion is promising for high-efficiency power generation. The p- and n-type thermoelectric materials are required to fabricate high-performance thermoelectric modules. SnSe is one of the highest-performance thermoelectric materials, and SnSe films should be fabricated for small-scale applications. While the p-type SnSe films can be easily prepared without precise control of the doping, it is difficult to control the electron concentration in the n-type SnSe films. In this study, the Bi-doped SnSe films were fabricated at low deposition temperatures by using the self-buffer layer, namely, the SnSe-based buffer layers. The low-temperature deposition on the buffer layer increased the concentration of doped Bi, although the contribution of the buffer layer to the electronic conduction was observed. The π-type thermoelectric module consisting of p-type SnSe and n-type SnSe/buffer layers was fabricated. The output power of 0.09 μW and the open-circuit voltage of 0.11 V were obtained at 155 °C at the cold end and 255 °C at the hot end. The feasibility of fabricating the π-type SnSe film module was demonstrated. By optimizing the buffer layer thickness and increasing the number of legs, we expect the module performance to be further improved.

DOI： 10.1021/acsaem.3c02683

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アモルファス炭化ホウ素の電子線動径分布解析招待有り査読有り

石丸学

表面と真空 ( 公益社団法人日本表面真空学会 ) 66 ( 12 ) 719 - 724 2023年12月

担当区分：筆頭著者,　最終著者,　責任著者記述言語：日本語掲載種別：研究論文（学術雑誌）

Short-range and medium-range ordered structures of amorphous materials can be evaluated by radial distribution functions and atomic pair-distribution functions extracted by quantitative analyses of diffraction intensities. Because of the strong interaction of electrons with materials and the short wavelength of high-energy electrons, electron diffraction can produce intensity profiles up to high scattering angles comparable to those of neutron diffraction and synchrotron x-ray diffraction in a short time. Here, we present an example of electron diffraction radial distribution function analysis of amorphous boron carbide by electron diffraction techniques.

DOI： 10.1380/vss.66.719

その他リンク： https://www.jstage.jst.go.jp/article/vss/66/12/66_20181187/_pdf

Modulation of vortex pinning by matrix defects in YBa<inf>2</inf>Cu<inf>3</inf>O<inf>7</inf>nanocomposite film 査読有り

Horide T., Higashi K., Ishimaru M., Okada T., Awaji S., Matsumoto K.

Superconductor Science and Technology 36 ( 10 ) 2023年10月

記述言語：英語掲載種別：研究論文（学術雑誌）

Nanorods are a very effective pinning center in YBa2Cu3O7 films, and the vortex pinning mechanism should be understood to further improve the critical current density (J c) in the nanocomposite films. Matrix defects are naturally formed during nanocomposite growth, and in this study the effect of matrix defects on the J c in the YBa2Cu3O7 films containing nanorods is discussed. YBa2Cu3O7 + BaHfO3 and YBa2Cu3O7 + BaSnO3 films were prepared by varying the laser repetition frequency to control the matrix defects. All the films contained the nanorods. The YBa2Cu3O7 + BaHfO3 film of 2 Hz exhibits high J c for magnetic field parallel to the c-axis (B//c) due to the nanorods and the isolated random point defects. In the YBa2Cu3O7 + BaHfO3 film of 10 Hz, while the short stacking faults and networked random point defects decreased the J c for B//c, the short stacking faults improved the J c for tilted magnetic field. In the YBa2Cu3O7 + BaSnO3 film of 10 Hz, the isolated random point defects improved J c regardless of the magnetic field direction. While the ab-plane correlated defects are analyzed by the structural observation, the network or isolated nature of the random point defects can be discussed by the normal state resistivity. The matrix defects of YBa2Cu3O7 nanocomposite films should be designed considering the correlated/network/isolated nature which is determined by the morphology and defect concentration.

DOI： 10.1088/1361-6668/acecad

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Nano- to microscale structural and compositional heterogeneity of artificial pinning centers in pulsed-laser-deposited YBa<inf>2</inf>Cu<inf>3</inf>O<inf>7−y</inf> thin films 査読有り

Kuroki M., Horide T., Matsumoto K., Ishimaru M.

Journal of Applied Physics 134 ( 4 ) 2023年07月

担当区分：最終著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）

The structure, composition, and spatial distribution heterogeneity of artificial pinning centers affect the critical current density of REBa2Cu3O7−y (RE: rare earth) coated conductors. Nanoscale structures and compositions have been analyzed with transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). However, microscale heterogeneity has been difficult to characterize. Here, YBa2Cu3O7−y thin films doped with double-perovskite Ba2YbNbO6 were prepared via pulsed-laser deposition and characterized with TEM, STEM, and scanning electron microscopy (SEM). Cross-sectional and plan-view TEM/STEM imaging revealed hybrid pinning structures consisting of nanorods, nanoparticles, and planar defects that were formed spontaneously. Nanorods were imaged with high spatial resolution via field-emission SEM of thin-foil specimens. Focused-ion-beam (FIB) micro-sectioning enables SEM imaging of microscale heterogeneity in nanorod spatial distributions. By using TEM/STEM in conjunction with FIB-SEM, the coated conductor inhomogeneity was directly evaluated from the nano- to micrometer scales.

DOI： 10.1063/5.0155145

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Explosive crystallization of amorphous germanium-tin films by irradiation with a 3-keV electron beam 査読有り

Nakamura R., Miyamoto M., Ishimaru M.

Journal of Applied Physics 133 ( 18 ) 2023年05月

担当区分：最終著者記述言語：英語掲載種別：研究論文（学術雑誌）

Much effort has been expended to obtain thin films of metastable solid solutions of germanium (Ge) that contain as high tin (Sn) content as possible because of their excellent electronic and optoelectronic properties. On the basis of our previous study on amorphous Ge, we demonstrated in this study that irradiation of substrate-free films of amorphous Ge100−xSnx (x = 8, 11, and 19 at. %) with a low-energy electron beam of 3 keV at ambient temperature can induce instantaneous wide-area crystallization (explosive crystallization). Characteristic spiral crystal growth associated with explosive crystallization occurred with areas exceeding 50 μm in diameter around a scanned area of the electron beam of 8 × 8 μm2. As a result, solid solutions of GeSn with Sn concentration up to 19 at. % were obtained with the suppression of precipitation of β-Sn. The region of explosive crystallization reduced in size with increasing Sn content. In addition, thermal analyses revealed that the heat released during crystallization of amorphous GeSn films decreased with increasing Sn content. This relationship indicates that the heat release at the growth front plays a key role in the propagation of explosive crystallization of a-GeSn.

DOI： 10.1063/5.0147022

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Planar defects-induced low thermal conductivity in a superhard material SiB<inf>6</inf> 査読有り国際誌

Tanusilp S.a., Kumagai M., Ohishi Y., Ishimaru M., Sadayori N., Kurosaki K.

Journal of Alloys and Compounds 939 2023年04月

記述言語：英語掲載種別：研究論文（学術雑誌）

Generally materials with low thermal conductivity require weaker interatomic bonding, considered as soft materials. Here we demonstrate that SiB6 with planar structural defects shows low thermal conductivity, but uncommonly high strength. Planar defects-induced polycrystalline SiB6 (PD-SiB6) has a relatively large Vickers hardness, which is comparable to those of well-known superhard materials, despite of relatively low thermal conductivity (<10 W m-1 K-1 at 300 K). Transmission electron microscopy observations confirmed the presence of numerous planar defects, which were thought to contribute to the improvement of hardness. Thanks to the special characteristics of a trade-off relationship between low thermal conductivity and high strength, we believe that PD-SiB6 can be used in new strategies of controlling thermal and mechanical properties in various functional materials.

DOI： 10.1016/j.jallcom.2023.168744

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Microstructure and Phase Transformation in Fe<inf>20</inf>Co<inf>20</inf>Ni<inf>20</inf>Cr<inf>20</inf>B<inf>20</inf>1<inf>x</inf>Si<inf>x</inf> Alloys Prepared by Mechanical Alloying 査読有り

Zhang Y., Ishimaru M., Tokunaga T., Era H.

Materials Transactions ( 公益社団法人日本金属学会 ) 64 ( 11 ) 2568 - 2574 2023年01月

記述言語：英語掲載種別：研究論文（学術雑誌）

In this study, microstructure and phase transformation behavior in Fe20Co20Ni20Cr20B201xSix alloys prepared by the mechanical alloying (MA) method were investigated by X-ray diffraction (XRD) measurements, scanning electron microscopy, transmission electron microscopy, and differential scanning calorimetry (DSC). The Fe20Co20Ni20Cr20B201xSix alloys prepared by the melt-spinning method were composed of FCC and compounds, and the FCC and BCC phases predicted by the valance electron concentration parameter were not formed. However, alloy powders prepared by the MA method were revealed to be composed of the FCC and BCC phases. Small amounts of unreacted pure B and Si particles were observed in alloy powders with high and low B content, respectively. XRD and DSC measurements revealed that the BCC phase in MA powder disappeared, and compounds were formed by heating up to 700°C. Especially the compound formation temperature was higher than that of the same alloy prepared by the melt-spinning method, suggesting that the thermal stability of the alloy powders prepared by the MA method was higher than that of the alloy ribbons prepared by the melt-spinning method. [doi:10.2320/matertrans.MT-M2023040]

DOI： 10.2320/matertrans.MT-M2023040

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Impact of supercooled liquid structures on the crystallization processes of amorphous Ge 査読有り

Nagaoka S., Tahara C., Ishimaru M.

Applied Physics Express 16 ( 1 ) 2023年01月

担当区分：最終著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）

The crystallization processes of amorphous Ge as well as the atomistic structures of the growth front were examined by molecular-dynamics simulations. An amorphous Ge network was annealed in a thermal bath with a temperature gradient. Crystallization proceeded via the supercooled liquid, and changed from random nanocrystallization to large-oriented grain growth. The resultant structures qualitatively reproduced the explosive crystallization observed with pulsed-laser irradiation and flash lamp annealing. The supercooled liquid was found to transform from a tetrahedral liquid to a more highly-coordinated liquid with increasing temperature, which was attributed to the change in growth mode.

DOI： 10.35848/1882-0786/aca87a

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Thermoelectric Property of SnSe Films on Glass Substrate: Influence of Columnar Grain Boundary on Carrier Scattering 査読有り国際誌

Horide T., Murakami Y., Ishimaru M., Matsumoto K.

ACS Applied Electronic Materials 4 ( 12 ) 6364 - 6372 2022年12月

記述言語：英語掲載種別：研究論文（学術雑誌）

Thermoelectric SnSe exhibits a very high figure of merit, and the a-axis orientation is needed because a high thermoelectric property is obtained along the bc-plane. Here, in spite of the amorphous nature of glass, a-axis-oriented SnSe films were fabricated using pulsed laser deposition on a glass substrate, which is more practical than single-crystal oxide substrates. Transmission electron microscopy indicated that a-axis-oriented SnSe films with a columnar grain structure grew on amorphous SiO2. The electrical conductivity and the Seebeck coefficient at room temperature showed almost the same trend with respect to the hole concentration in both the SnSe/glass and SnSe/single-crystal-substrate films. The electrical conductivity increased with increasing temperature more slowly in SnSe/glass films than in SnSe/single-crystal-substrate films. This indicates that the grain boundary contribution to carrier scattering is significant at high temperatures, while the grain boundary contribution is as strong as the orthorhombic domain boundary contribution at room temperature. In spite of the grain boundary effect, the power factor in SnSe/glass was as high as that for single-crystal SnSe at high temperatures. Considering the grain boundary effect on electrical conductivity, the structure and process of SnSe films on amorphous substrates should be designed.

DOI： 10.1021/acsaelm.2c01486

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Investigation of Nanoscale Phase Formation in Rapidly Solidified Fe20Co20Ni20Cr20B201xSix Alloys 査読有り国際誌

Zhang Yicheng, Inoue Koji, Ishimaru Manabu, Tokunaga Tatsuya, Era Hidenori

MATERIALS TRANSACTIONS ( 公益社団法人日本金属学会 ) advpub ( 0 ) 1211 - 1216 2022年09月

記述言語：英語掲載種別：研究論文（学術雑誌）

In this study, nanoscale phase formation in rapidly solidified Fe20Co20Ni20Cr20B20−xSix alloys was investigated using atom probe tomography (APT) and transmission electron microscopy (TEM). According to previous X-ray diffraction measurements and scanning electron microscopy with energy-dispersive X-ray spectroscopy results, it was thought that this alloy system was composed of almost a single face-centered cubic (FCC) solid solution phase when the Si content was 5 at.%. However, APT and TEM results at the nanoscale showed that both an FCC solid solution and compound phases formed for all alloy compositions. The rapidly solidified alloy ribbon with a Si content of 5 at.% showed the presence of nanoscale FCC and Cr2B phases. Furthermore, in addition to the nanoscale FCC and Cr2B phases, a nanoscale Cr3Ni5Si2 phase was observed when the Si content exceeded 7.5 at.%.

DOI： 10.2320/matertrans.mt-m2022058

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Anomalous structural phase transformation in swift heavy ion-irradiated δ-Sc<inf>4</inf>Hf<inf>3</inf>O<inf>12</inf> 招待有り査読有り国際誌

Iwasaki M., Kanazawa Y., Manago D., Patel M.K., Baldinozzi G., Sickafus K.E., Ishimaru M.

Journal of Applied Physics 132 ( 7 ) 2022年08月

担当区分：責任著者記述言語：英語掲載種別：研究論文（学術雑誌）

Swift heavy ion irradiation was carried out to examine the ionization effects on structural changes of δ-Sc4Hf3O12 in which oxygen vacancies are regularly arranged. The specimens were irradiated at room temperature with 92 MeV xenon ions to fluences ranging from 3 × 1012 to 1 × 1014/cm2 and characterized by grazing (glancing) incidence x-ray diffraction, transmission electron microscopy, and scanning transmission electron microscopy. It was found that the pristine long-range ordered rhombohedral δ-phase undergoes a reconstructive transformation toward a long-range disordered cubic oxygen-deficient fluorite phase promoted by ionization effects. In addition, an ordered phase with a short-range structure different from the δ-type was formed in a layer going from the surface to a depth of ∼4.5 μm in the specimen irradiated to a fluence of 1 × 1014/cm2. It was found that the ordered phase is formed from the disordered cubic fluorite phase. This structural change is anomalous, because it is the opposite process of the usual irradiation-induced structural change, the order-to-disorder phase transformation. Electron diffraction experiments revealed that short-range ordered regions in this layer possess an oxygen-excess bixbyite organization (C-type heavy rare-earth oxides) with randomly filled anion vacant sites to account for the different stoichiometry and a long-range average oxygen-deficient fluorite phase.

DOI： 10.1063/5.0098518

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Self-Organized Nanocomposite Structure Controlled by Elemental Site Occupancy to Improve Vortex Pinning in YBa<inf>2</inf>Cu<inf>3</inf>O<inf>7</inf>Superconducting Films 査読有り国際誌

Horide T., Yoshida Y., Kita R., Gondo M., Ishimaru M., Matsumoto K.

ACS Applied Electronic Materials 4 ( 6 ) 3018 - 3026 2022年06月

記述言語：英語掲載種別：研究論文（学術雑誌）

To fabricate high performance YBa2Cu3O7 superconducting films and tapes, the critical current density (Jc) should be enhanced and the nanocomposite structure should be very effective in controlling the vortex pinning and Jc. Although the nanoscale morphology has been a basic parameter from the early stages of nanorod control, compositional factors should be discussed to effectively control not only the superconducting order parameter distribution but also the thermodynamic parameter relating to crystal growth. In this study, a double perovskite oxide, Ba2RENbO6 (RE = Yb, Lu), was incorporated in YBa2Cu3O7 films, where the site occupancy of the RE element is focused to control the nanocomposite structure. The elongated nanorods were formed in both the YBa2Cu3O7+Ba2YbNbO6 and YBa2Cu3O7+Ba2LuNbO6 films. While Lu mainly existed in the nanorods, Yb was observed not only in the nanorods but also in the matrix. As a result, at the fixed nanorod content, while the matching field was larger for the YBa2Cu3O7+Ba2YbNbO6 film, the Jc in the high magnetic field was larger for the YBa2Cu3O7+Ba2LuNbO6 film. The angular dependence of Jc depended on the RE due to the pinning contribution from the Y-rich nanoparticles. These are unconventional phenomena that are not observed in the YBa2Cu3O7+BaMO3 (M = Zr, Sn, Hf) films. Thus, the site occupancy of the RE element affects the Jc characteristics. The self-organization and the nanocomposite structure can be designed by considering the site occupancy as a variable parameter.

DOI： 10.1021/acsaelm.2c00438

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Impact of annealing on electric and elastic properties of 10-nm Hf<inf>0.5</inf>Zr<inf>0.5</inf>O<inf>2</inf> films prepared on Si by sputtering 査読有り国際誌

Bolotov L., Migita S., Fujio R., Ishimaru M., Hatayama S., Uchida N.

Microelectronic Engineering 258 2022年04月

記述言語：英語掲載種別：研究論文（学術雑誌）

HfO2-based films are important materials used in broad range of electronic applications from high-performance transistors and memory cells, to thermoelectric and energy harvesting elements. By employing scanning probe methods, we made nanoscale comparison of chemical composition, surface morphology, the elastic modulus and the surface potential of bare 10-nm thick Hf0.5Zr0.5O2 films prepared on Si by a carbon-free sputtering process. The composition mapping confirmed uniform distribution of Hf and Zr in the film along wafer size. Suppression of the monoclinic phase in films annealed at 600 - 800 °C had strong impact on spatial variations of film properties. Small surface roughness, large electric domain sizes (50–200 nm at 700 °C) and small fluctuations of the surface potential (40–50 meV) in Si coated with the films are appealing for gate-stack applications. Films annealed at 600-700 °C showed the elastic modulus of about 169 GPa and the ferroelectric polarization reversal at a field of ~1 MV/cm as observed by nanoscale poling with a Pt-coated scanning probe. In contrast, properties of films annealing at 800 °C were affected by growth of thick interfacial oxide layer. The presented nanoscale approach is beneficial in optimizing of physical and mechanical properties of thin dielectric films.

DOI： 10.1016/j.mee.2022.111770

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Electron diffraction radial distribution function analysis of amorphous boron carbide synthesized by ion beam irradiation and chemical vapor deposition 査読有り国際誌

Ishimaru M., Nakamura R., Zhang Y., Weber W.J., Peterson G.G., Ianno N.J., Nastasi M.

Journal of the European Ceramic Society 42 ( 2 ) 376 - 382 2022年02月

担当区分：筆頭著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）

Amorphous boron carbide (a-BxC) networks consist of light elements, and their low atomic scattering factors makes structural analysis by x-ray diffraction difficult. Electron diffraction has an advantage of detecting the light elements, because of the strong interaction between the matter and electrons. We prepared a-BxC by ion beam technologies and plasma-enhanced chemical vapor deposition, and characterized their structures via atomic pair-distribution functions derived from electron diffraction intensity profiles. It was found that a pentagonal pyramid is the most favorable cluster in a-B4C generated by ion irradiation, while C[sbnd]C homonuclear bonds were formed in the deposited a-BxC thin film. X-ray photoemission spectroscopy revealed that the a-BxC thin film possesses more carbon than B4C, which is responsible for the formation of the homonuclear bonds.

DOI： 10.1016/j.jeurceramsoc.2021.10.020

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Aligned Self-Organization Induced by Epitaxial Stress and Shear Deformation in Jahn–Teller Spinel ZnMnGaO<inf>4</inf> 査読有り

Horide T., Morishita K., Horibe Y., Usuki M., Ishimaru M., Matsumoto K.

Journal of Physical Chemistry C 126 ( 1 ) 806 - 814 2022年01月

記述言語：英語掲載種別：研究論文（学術雑誌）

Spinel ZnMnGaO4 exhibits the characteristic self-organization dominated by phase separation and strain to realize an ordered nanostructure. To control the self-organization, ZnMnGaO4 films fabricated using pulsed laser deposition were postannealed. While the ordered nano-checkerboard and the slightly coarse nanolamellar structures were observed for the moderate annealing temperature and the long annealing time, the twin structure was formed in the films annealed at the low temperature for the short annealing time. The cross-sectional observation showed the vertically aligned nanostructure near the surface (twin, nano-checkerboard, or nanolamellar) and the nondefective epitaxial layer near the substrate. The ZnMnGaO4 films exhibited the same in-plane lattice parameter as that of MgO along the MgO[110]//ZMGO[101] regardless of annealing conditions, and the stress analysis indicates that the epitaxial stress at the epitaxial ZnMnGaO4/MgO and the shear stress at the epitaxial ZnMnGaO4/twinned ZnMnGaO4 determined the vertical structure and the twin width. The thermodynamic stability and the Mn and Ga diffusion basically bring about the self-organized nanostructure formation as in bulk, and the epitaxial stress and the interface shear stress align the self-organization in the films. Thus, the highly aligned self-organized nanostructures can be fabricated in the annealed ZnMnGaO4 films by tuning these factors.

DOI： 10.1021/acs.jpcc.1c09324

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Thermoelectric Property of n-Type Bismuth-Doped SnSe Film: Influence of Characteristic Film Defect 査読有り

Horide T., Nakamura K., Hirayama Y., Morishita K., Ishimaru M., Matsumoto K.

ACS Applied Energy Materials 4 ( 9 ) 9563 - 9571 2021年09月

記述言語：英語掲載種別：研究論文（学術雑誌）

Highly aligned SnSe exhibits very high thermoelectric properties, and orientation control in films is very promising for developing high-performance thermoelectric modules. While SnSe films with intrinsic p-type nature have been reported, fabrication of the highly aligned n-type SnSe films is difficult due to thermodynamic restriction on the solubility of the doping elements. Here, highly oriented SnSe films doped with Bi were successfully fabricated using pulsed laser deposition. Characteristic film structures were observed: doped Bi in the SnSe matrix; domain boundaries with a spacing of ∼200 nm; self-organized Bi precipitates with a bimodal size distribution; and stacking faults. The most important result is observation of the n-type Hall resistivity and the n-type Seebeck coefficient. The stacking faults and the doped Bi in the films degraded the room-temperature Seebeck coefficient. While the electron carrier mobility in room temperature was smaller than that in the single crystal due to the domain boundary scattering, the domain boundary scattering was suppressed in the high temperature of 300 °C. Thus, the characteristic film structures significantly affect the thermoelectric properties of SnSe. The doped Bi in the SnSe matrix, the stacking faults, and the domain boundaries should be controlled for further improvement of the thermoelectric properties in the Bi-SnSe films, and the self-organized Bi nanoprecipitates are very promising for achieving high-performance Bi-SnSe with decreased thermal conductivity.

DOI： 10.1021/acsaem.1c01722

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Review of “12th Japanese-Polish joint seminar on micro and nano analysis (August 29-September 1, 2018)” 査読有り

Ishimaru M.

Materials Transactions ( 公益社団法人日本金属学会 ) 62 ( 9 ) 1420 - 1423 2021年09月

担当区分：筆頭著者,　責任著者記述言語：英語掲載種別：記事・総説・解説・論説等（学術雑誌）

“The 12th Japanese-Polish Joint Seminar on Micro and Nano Analysis” was held in Fukuoka, Japan from August 29 to September 1, 2018, and the proceedings were published in May, 2019, as a special issue of Materials Transactions (Vol. 60, No. 5). The main purpose of this seminar is to discuss the structural analysis of materials by electron microscopy techniques. Among the papers presented at the seminar, this article briefly reviews the following topics: observations of dislocations in a thick specimen by ultra-high voltage electron microscopy, suppression of geometric phase shift due to antiphase boundaries in dark-field electron holography, and structural characterization of amorphous materials by electron diffraction techniques.

DOI： 10.2320/matertrans.MT-M2021094

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Explosive crystallization of sputter-deposited amorphous germanium films by irradiation with an electron beam of SEM-level energies 査読有り国際誌

Nakamura R., Matsumoto A., Ishimaru M.

Journal of Applied Physics 129 ( 21 ) 2021年06月

担当区分：最終著者記述言語：英語掲載種別：研究論文（学術雑誌）

The crystallization of sputter-deposited substrate-free films of amorphous germanium was induced by electron irradiation at SEM-level energies of less than 20 keV at ambient temperature using an electron probe microanalyzer. Instantaneous crystallization, referred to as explosive crystallization, occurred consistently at 2-20 keV; the threshold of electron fluxes is 1015-1016 m−2 s−1, which is five to six orders of magnitude lower than those at 100 keV reported previously. This process is expected to be advantageous in the production of polycrystalline Ge films since it is rapid, requires little energy, and results in negligible damage to the substrate.

DOI： 10.1063/5.0052142

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Nanostructures and flux pinning properties in YBa<inf>2</inf>Cu<inf>3</inf>O<inf>7- y </inf>thin films with double perovskite Ba<inf>2</inf>LuNbO<inf>6</inf>nanorods 査読有り国際誌

Gondo M., Yoshida M., Yoshida Y., Ishimaru M., Horide T., Matsumoto K., Kita R.

Journal of Applied Physics 129 ( 19 ) 2021年05月

担当区分：責任著者記述言語：英語掲載種別：研究論文（学術雑誌）

Double perovskite Ba2LuNbO6 (BLNO)-doped YBa2Cu3O7-y (YBCO) thin films are fabricated on a SrTiO3 (001) substrate by pulsed laser deposition, and their nanostructures are characterized by transmission electron microscopy and scanning transmission electron microscopy. Cross-sectional observations and elemental mapping reveal that BLNO self-assembles during thin film deposition, and consequently, nanorods extending straight from the substrate to the surface are formed in the YBCO thin films. It is confirmed that stacking faults perpendicular to the growth direction disturb the formation of BLNO nanorods. Strain maps extracted by geometric phase analysis reveal that the tensile strain occurs in the YBCO matrix around the BLNO nanorods. Misfit dislocations are periodically introduced at the interface between the nanorod and the matrix, which results in the inhomogeneous strain of YBCO around the BLNO nanorods. The superconducting properties of the YBCO + BLNO thin films are compared with those of other previously reported YBCO thin films with normal perovskite and double perovskite nanorods.

DOI： 10.1063/5.0048693

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Enhancement of Thermoelectric Properties of n-Type Bi<inf>2</inf>Te<inf>3- x</inf>Se<inf>x</inf>by Energy Filtering Effect 査読有り

Kawajiri Y., Tanusilp S.A., Kumagai M., Ishimaru M., Ohishi Y., Tanaka J., Kurosaki K.

ACS Applied Energy Materials 4 ( 10 ) 11819 - 11826 2021年01月

記述言語：英語掲載種別：研究論文（学術雑誌）

A pair of n-type Bi2Te3-xSex and p-type (Bi,Sb)2Te3 is known as a promising candidate for assembling a high thermoelectric efficiency module operating at near room temperature. While the dimensionless figure of merit (zT) of p-type (Bi,Sb)2Te3 is commonly larger than 1.0, n-type Bi2Te3-xSex has a lower zT value of μ0.8 at room temperature. Here, we aim to enhance the zT of n-type Bi2Te3-xSex through the energy filtering effect that can improve the Seebeck coefficient by adding Au. In the Bi2Te2.8Se0.2 + 5 at % Au composite, the Seebeck coefficient is significantly improved, and the electrical resistivity does not change significantly, leading to an approximately twice as high power factor as that of the non-Au-added one at near room temperature. Also, the addition of Au suppresses the thermal conductivity. These advantages result in a maximum zT of over 1.0 at room temperature and average zT value of 1.05 at T = 300-400 K, an μ75% improvement, which is comparable to those of the reported n-type Bi2Te3-based materials. Also, Au addition shows no negative effect on the mechanical properties as well as high-temperature stability of Bi2Te3-xSex. These results indicate that Au addition is a promising method to improve the thermoelectric properties of n-type Bi2Te3-xSex used for assembling a practical thermoelectric module operating at near room temperature.

DOI： 10.1021/acsaem.1c02560

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Structure analysis of Hf<inf>0.5</inf>Zr<inf>0.5</inf>O<inf>2</inf> thin films crystallized from amorphous phase by thermal annealing below 500°C 査読有り

Fujio R., Ishimaru M., Migita S., Uchida N.

2021 Silicon Nanoelectronics Workshop, SNW 2021 2021年01月

担当区分：責任著者記述言語：英語掲載種別：研究論文（国際会議プロシーディングス）

Hf0.5Zr0.5O2, (HZO) films are anticipated as a noval insulating material for ferroelectric field-effect transistors. These materials are fabricated by crystallization of amorphous HZO, and therefore knowledge of crystallization processes is required. We examined thermally-induced crystallization processes of amorphous HZO prepared by sputtering using x-ray diffraction and transmission electron microscopy. It was found that a metastable orthorhombic phase is formed in 10 nm thick HZO films after thermal annealing at 500°C.

DOI： 10.1109/SNW51795.2021.00018

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アモルファスGeSnの結晶化におけるSnの振る舞い招待有り査読有り

石丸　学, 仲村龍介

まてりあ 59 ( 12 ) 662 - 668 2020年12月

Combined effect of nanorod and stacking fault for improving nanorod interface in YBa<inf>2</inf>Cu<inf>3</inf>O<inf>7-δ</inf>nanocomposite films 査読有り国際誌

Horide T., Ishimaru M., Sato K., Matsumoto K.

Superconductor Science and Technology 33 ( 11 ) 2020年11月

記述言語：英語掲載種別：研究論文（学術雑誌）

Self-organized nanorods in YBa2Cu3O7-δ films are important pinning centers from the technical and fundamental views. While influence of nanoscale factors (>4-5 nm) on the vortex pinning has already been discussed in detail, the interface control of nanorods (at the smaller scale) has not yet been performed. Here, it is demonstrated that the stacking faults can control the nanorod interface to improve the vortex pinning. The stacking faults were formed across the nanorods by post-annealing the YBa2Cu3O7-δ films containing BaMO3 (M = Zr, Sn, Hf) nanorods. The strong bonding between YBa2Cu3O7-δ and BaMO3 varies the atomic position of YBa2Cu3O7-δ, degrading an interface sharpness and an elementary pinning force of the nanorod. Scanning transmission electron microscopy and density functional theory calculation clarified that the stacking faults broke the strong bonding between YBa2Cu3O7-δ and BaMO3 at the nanorod interface, and reduced the local strain around the nanorods, improving the elementary pinning force. As a result, the critical current density was improved in low temperature and low magnetic field when magnetic field was aligned with the c-axis. Thus, the combined effect of nanoinclusions and crystalline defects, in the present case, nanorods and stacking faults, can control the interface of nanoinclusion pinning centers, opening the interface design to realize the ideal pinning situation.

DOI： 10.1088/1361-6668/abaebf

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Dual-Beam Irradiation Stability of Amorphous Silicon Oxycarbide at 300°C and 500°C 査読有り

Su Q., Greaves G., Donnelly S.E., Mizuguchi S., Ishimaru M., Nastasi M.

JOM 72 ( 11 ) 4002 - 4007 2020年11月

記述言語：英語掲載種別：研究論文（学術雑誌）

The irradiation stability of a material in the temperature range from 300°C to 500°C represents an important step for a variety of nuclear applications. Here we used dual-beam irradiation to investigate the response of an amorphous silicon oxycarbide (SiOC) to extreme environments (temperature and irradiation). The amorphous SiOC was prepared in thin film form by sputtering and then fabricated into cross-sectional transmission electron microscopy (TEM) specimens, and finally irradiated with helium (He) and krypton (Kr) ions inside a TEM. In-situ TEM observations revealed that He bubble and void formation are highly suppressed after irradiation up to 95 dpa with simultaneous He implantation up to 231 at.%. Atomic pair-distribution functions suggested that the amorphous structures are almost the same before and after irradiation, and no crystallization, nor phase separation was detected. This study demonstrates the stability of amorphous SiOC under dual-beam irradiation at nuclear reactor operation temperatures, suggesting that this material is applicable as a structural material for advanced nuclear reactors.

DOI： 10.1007/s11837-020-04332-z

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Simultaneous achievement of high J <inf>c</inf>and suppressed J <inf>c</inf>anisotropy by hybrid pinning in YBa<inf>2</inf>Cu<inf>3</inf>O<inf>7</inf>three-phase-nanocomposite film 査読有り国際誌

Horide T., Torigoe K., Ishimaru M., Kita R., Awaji S., Matsumoto K.

Superconductor Science and Technology 33 ( 10 ) 2020年10月

記述言語：英語掲載種別：研究論文（学術雑誌）

Nanoscale pinning centers are introduced into YBa2Cu3O7 (YBCO) films and tapes to improve vortex pinning and critical current density (J c) for coated conductor application. While pinning with a single type of pinning center has been mainly investigated, hybrid pinning with multiple types of pinning center is expected to be more promising for simultaneously achieving high J c values and suppressing J c anisotropy. In the present study, a three-phase-nanocomposite film consisting of an YBCO matrix, BaHfO3 nanorods and Y-O nanoparticles was fabricated. A global pinning force maximum (F p,max, where F p = J c B) of 1.57 TN m-3 at 4.2 K, which is as high as the record level of F p,max, was achieved by the hybrid pinning of BaHfO3 nanorods and Y-O nanoparticles. Furthermore, the hybrid pinning improved the angular dependence of J c. The pinning contribution from nanoparticles increased with lowering temperature, which can explain the high J c at low temperature. Thus, the present study successfully demonstrates a highly effective structure that can simultaneously achieve suppressed J c anisotropy and a record level of J c.

DOI： 10.1088/1361-6668/aba544

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Dual crystallization modes of sputter-deposited amorphous SiGe films 査読有り

Okugawa M., Nakamura R., Numakura H., Ishimaru M., Yasuda H.

Journal of Applied Physics 128 ( 1 ) 2020年07月

記述言語：英語掲載種別：研究論文（学術雑誌）

The crystallization behavior of sputter-deposited films of amorphous Si (a-Si) and SiGe alloys (a-SiGe) induced by electron irradiation at room temperature and by thermal annealing was investigated by in situ transmission electron microscopy. On electron irradiation at room temperature, extremely rapid crystallization, so-called explosive crystallization, occurred at a higher electron flux but not at a lower electron flux. On in situ thermal annealing, explosive crystallization occurred preferentially and partially at low temperatures in Ge-rich a-SixGe100-x for x < 50 but not for x > 50. These results indicate that the increase of Si content in a-SiGe prevents the occurrence of explosive crystallization. We previously proposed that explosive crystallization can occur in pristine a-Ge films via the interface of a liquid-like, high-density amorphous state at the growth front. An increase in the instability of this high-density amorphous state caused by the increase of Si in a-SiGe apparently gives rise to the suppression of explosive crystallization.

DOI： 10.1063/5.0010202

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Effects of hydrogen on structure and crystallization behavior of sputter-deposited amorphous germanium films 査読有り

Hanya Y., Nakamura R., Okugawa M., Ishimaru M., Oohata G., Yasuda H.

Japanese Journal of Applied Physics 59 ( 7 ) 2020年07月

記述言語：英語掲載種別：研究論文（学術雑誌）

We examined by transmission electron microscopy the structure and crystallization behavior of hydrogenerated and non-hydrogenerated amorphous germanium films (a-Ge:H and a-Ge) prepared by sputtering to figure out the effects of hydrogen on them. In pair-distribution function, first and second peaks at the interatomic distance of 0.26 and 0.40 nm, respectively, were found to be higher and sharper in a-Ge:H than in a-Ge; therefore amorphous structure was organized in a more ordered state within a short range by hydrogeneration. On thermal annealing, fine nanograins (FGs) of about 10 nm in size appeared preferentially at 350 °C and above in a-Ge:H, which is in contrast with the formation of coarse particles of about 100 nm in size at 500 °C in a-Ge. Hydrogeneration would make a certain number of short-range ordered clusters distributed in the amorphous matrix, inducing the preferential formation of FGs at lower temperatures.

DOI： 10.35848/1347-4065/ab9cd9

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Deposition-temperature dependence of vortex pinning property in YBa<inf>2</inf>Cu<inf>3</inf>O<inf>7</inf>+BaHfO<inf>3</inf> films 査読有り

Horide T., Torigoe K., Kita R., Nakamura R., Ishimaru M., Awaji S., Matsumoto K.

Materials Transactions ( 日本金属学会 ) 61 ( 3 ) 449 - 454 2020年01月

記述言語：英語掲載種別：研究論文（学術雑誌）

Improvement of critical current density (Jc) in magnetic fields is required in YBa2Cu3O7 films, and process parameters should be optimized for controlling pinning centers. In the present study, a deposition temperature was varied in pulsed laser deposition of YBa2Cu3O7+BaHfO3 films to control the nanorod structure, and its influence on Jc was analyzed. The YBa2Cu3O7+BaHfO3 film deposited at 850°C exhibited pinning force maximum (Fp,max.) as high as 413GN/m3 at 40 K, while the Fp,max. for the deposition temperature of 850°C at 77K was smaller than that in the YBa2Cu3O7+BaHfO3 film deposited at 900°C. A critical temperature decreased and matching field increased with decreasing the deposition temperature. Increase in deposition temperature is effective in improving the Fp,max. in high temperatures, since the critical temperature and matching field dependences of Jc value dominate the Fp,max. On the other hand, low deposition temperature improves the Fp,max. in low temperatures since the Fp shift in accordance with matching field is dominant to the Fp,max. Thus, the deposition temperature should be set in pulsed laser deposition of YBa2Cu3O7 films containing nanorods considering the Jc variation with critical temperature and matching field.

DOI： 10.2320/matertrans.MT-M2019303

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Liquid-mediated crystallization of amorphous GeSn under electron beam irradiation 査読有り

Inenaga K., Motomura R., Ishimaru M., Nakamura R., Yasuda H.

Journal of Applied Physics 127 ( 20 ) 2020年01月

記述言語：英語掲載種別：研究論文（学術雑誌）

Crystallization processes of amorphous germanium–tin (GeSn) under low-energy electron-beam irradiation were examined using transmission electron microscopy (TEM). Freestanding amorphous GeSn thin films were irradiated with a 100 keV electron beam at room temperature. The amorphous GeSn was athermally crystallized by electron-beam irradiation, when the electron flux exceeded the critical value. Heterogeneous structures consisting of nano- and micro-crystallites were formed after crystallization of amorphous GeSn with ∼24 at. % Sn in the as-sputtered amorphous state. In situ TEM observations of structural changes under electron-beam irradiation revealed that random nucleation and growth of nanocrystallites occur at the early stage of crystallization, followed by rapid formation of micro-grains surrounding the nanocrystals. It has been suggested that the growth of micro-grains progresses via supercooled liquid Sn at the amorphous/crystalline interface. The resultant GeSn grains with a size of a few micrometers contained ∼15 at. % Sn, much larger than the solubility limit of Sn in Ge (∼1 at. % Sn).

DOI： 10.1063/5.0006416

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Carrier and heat transport properties of poly-crystalline GeSn films for thin-film transistor applications 査読有り

Uchida N., Hattori J., Lieten R., Ohishi Y., Takase R., Ishimaru M., Fukuda K., Maeda T., Locquet J.

Journal of Applied Physics 126 ( 14 ) 2019年10月

記述言語：英語掲載種別：研究論文（学術雑誌）

© 2019 Author(s). Thin-film transistors (TFTs) on insulator substrates are widely used in applications from liquid crystal displays to sensor devices. However, insulator substrates with low heat conductivity lead to unfavorable self-heating effects in the channel regions. Herein, the carrier and heat transport properties of polycrystalline GeSn films on SiO2/Si substrates were improved by suppressing Sn segregation in the films to fabricate GeSn channel TFTs. Alloying with 5.5% Sn enabled the formation of larger grains than those in poly-Ge films after low-temperature annealing (below 520 °C) without Sn segregation. In addition, the films had a hole mobility of 40 cm2 V-1 s-1 at a hole density of 1.1 × 1018 cm-3 and a thermal conductivity of 12.1 Wm-1 K-1 at room temperature. The temperature dependences of the carrier and heat transport properties of the poly-GeSn films were investigated to accurately simulate a device with a poly-GeSn channel TFT. This was achieved by using the carrier transport measurements and numerical simulations of the heat transport in the Debye model. The simulated device allowed an accurate assessment of the self-heating effects of the TFT and thus provides a design guide for TFTs.

DOI： 10.1063/1.5085470

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Thermoelectric Property in Orthorhombic-Domained SnSe Film 査読有り

Horide T., Murakami Y., Hirayama Y., Ishimaru M., Matsumoto K.

ACS Applied Materials and Interfaces 11 ( 30 ) 27057 - 27063 2019年07月

記述言語：英語掲載種別：研究論文（学術雑誌）

© 2019 American Chemical Society. Single-crystal SnSe exhibits extremely high thermoelectric properties, and fabrication of SnSe films is promising for practical application and basic research on properties. However, the high thermoelectric properties have not yet been reported in SnSe films and their thermoelectric properties and nanostructure have not yet been analyzed in detail. In the present study, a-axis-oriented epitaxial SnSe films were prepared to discuss the thermoelectric properties of the SnSe films. While the electrical conductivity of the films was orders of magnitude smaller than that in the single crystals at room temperature, surprisingly, the thermoelectric property (power factor) of the films was slightly higher than that in the single crystals at high temperatures (∼300 °C). The SnSe films contained orthorhombic domain boundaries with a spacing of several hundred nanometers. The orthorhombic domain boundaries caused carrier scattering and degraded the mobility of the films at room temperature, but their effect decreased with increasing temperature. Thus, the carrier scattering at domain boundaries results in characteristic temperature dependence of thermoelectric properties in the SnSe films. High thermoelectric properties at high temperatures were successfully achieved in the SnSe films in spite of the existence of domain boundaries, demonstrating the possibility of high-performance of SnSe thermoelectric films.

DOI： 10.1021/acsami.9b04868

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Observation of inhomogeneous depinning in YBa<inf>2</inf>Cu<inf>3</inf>O<inf>7</inf> composite multilayers 査読有り

Horide T., Ishimaru M., Matsumoto K.

Superconductor Science and Technology 32 ( 8 ) 2019年06月

記述言語：英語掲載種別：研究論文（学術雑誌）

© 2019 IOP Publishing Ltd. Composite multilayer films (CMLs) comprising YBa2Cu3O7 + BaHfO3 (4.7 vol%)/YBa2Cu3O7 + BaHfO3 (3.1 vol%) were fabricated, and vortex behavior in the CMLs was evaluated to discuss inhomogeneous depinning of vortices. Magnetic field and angular dependences of critical current density (J c) in the CMLs were similar to those in the YBa2Cu3O7 + BaHfO3 (4.7 vol%) single layer film (SL). Variation of the CML structure shifted the matching field effect in global pinning force and irreversibility temperature. The J c in the YBa2Cu3O7 + BaHfO3 (4.7 vol%) SL was smaller than that in the YBa2Cu3O7 + BaHfO3 (3.1 vol%) SL, suggesting that vortices preferentially moved along the low-J c layers in the CMLs. The high-J c layers pinned the vortices in the low-J c layers, resulting in a shift of the matching field effect. These observations are characteristic of inhomogeneous depinning of vortices in the CMLs. The inhomogeneous depinning as well as pinned configuration should be considered for understanding J c and for structural design of pinning centers in YBa2Cu3O7 films.

DOI： 10.1088/1361-6668/ab1d2c

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Direct observations of crystallization processes of amorphous GeSn during thermal annealing: A temperature window for suppressing Sn segregation 査読有り

Higashiyama M., Ishimaru M., Okugawa M., Nakamura R.

Journal of Applied Physics 125 ( 17 ) 2019年05月

記述言語：英語掲載種別：研究論文（学術雑誌）

© 2019 Author(s). The solubility limit of tin (Sn) in germanium (Ge) is very small, and, therefore, it is difficult to synthesize high Sn concentration GeSn crystals by conventional methods. An amorphous phase can contain elements beyond the solubility limit of the crystal state, and, therefore, recrystallization of the amorphous alloy is one of the possible ways to realize materials far from the equilibrium state. To suppress Sn precipitation during thermal annealing, knowledge of crystallization processes is required. In the present study, amorphous GeSn thin films with different Sn concentrations were prepared by sputtering, and their crystallization processes were examined by in situ transmission electron microscopy. It was found that the crystallization temperature decreases with increasing Sn concentration, and it became lower than the eutectic temperature when the Sn concentration exceeded ∼25 at. %. Radial distribution function analyses revealed that phase decomposition occurs in the amorphous state of the specimens which crystallize below the eutectic temperature, and Sn crystallites were simultaneously precipitated with crystallization. On the other hand, no remarkable phase decomposition was detected in amorphous GeSn with <25 at. % Sn. Sn precipitation occurred at a higher temperature than the crystallization in these specimens, and the difference between the crystallization and Sn precipitation temperatures became large with decreasing Sn concentration. Because of the existence of this temperature difference, a temperature window for suppressing Sn segregation existed. We demonstrated that large GeSn grains with high Sn concentration could be realized by annealing the specimens within the temperature window.

DOI： 10.1063/1.5086480

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Compositional effects on radiation tolerance of amorphous silicon oxycarbide 査読有り

Mizuguchi S., Inoue S., Ishimaru M., Su Q., Nastasi M.

Journal of Nuclear Materials 518 241 - 246 2019年05月

記述言語：英語掲載種別：研究論文（学術雑誌）

© 2019 Elsevier B.V. Relationships between amorphous structures and radiation tolerance of silicon oxycarbide (SiOC) glasses with different composition, SiO 2 :SiC = 2:1 (SiO 2 -rich), 1:1 (equiatomic), and 1:2 (SiC-rich), were examined by transmission electron microscopy and atomic pair-distribution function (PDF). These specimens were irradiated with 120 keV helium (He) ions at room temperature. The PDF results indicated that the structure of SiOC alloys are almost the same in irradiation and implantation regions. It was found that SiO 2 -rich specimens show a greater radiation resistance than SiC-rich ones: the formation of He bubbles was highly suppressed in the SiO 2 -rich and equiatomic compositions after even 90 at.% He implantation, whereas pronounced swelling occurred in the SiC-rich composition. The first sharp diffraction peak revealed that the size of network voids formed by connecting the SiO x C 4-x tetrahedra in SiO 2 -rich and equiatomic specimens is much larger than the diameter of He atom, suggesting that He atoms can easily migrate in the amorphous SiOC networks.

DOI： 10.1016/j.jnucmat.2019.03.012

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Nonlocal self-organization of long stacking faults from highly strained nanocomposite film of complex oxide 査読有り

Horide T., Ishimaru M., Sato K., Matsumoto K.

Physical Review Materials 3 ( 1 ) 2019年01月

記述言語：英語掲載種別：研究論文（学術雑誌）

© 2019 American Physical Society. Elastic strain and defects are important key words for controlling structure and properties in films. While epitaxial strain and misfit dislocations have been discussed in conventional films, the evolution of strain and defect can be significantly varied by nanocomposite strain and complicated defects in oxides. In the present study, long stacking faults with a spacing of 5-30 nm and a length of >500nm were self-organized by ex situ annealing highly strained nanocomposite films of YBa2Cu3O7-δ(YBCO)+BaMO3(M=Hf,Sn). It is surprising that the nonlocal nature of stacking faults, namely, the structural correlation over >500nm, was observed in spite of the local configuration of the nanocomposite interface. This kind of structural variation was not observed in the pure YBCO film without nanorods, even when the same annealing was performed. A strain energy analysis showed that the stacking fault formation led to the strain energy minimum by reducing the nanocomposite strain. The layered structure of YBCO stacking faults and the large nanocomposite strain realized the present nonlocal self-organization, which is not observed in the conventional systems with epitaxial strain and misfit dislocations.

DOI： 10.1103/PhysRevMaterials.3.013403

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Application of the Tersoff interatomic potential to pressure-induced polyamorphism of silicon 査読有り国際誌

Mukuno R., Ishimaru M.

Japanese Journal of Applied Physics 58 ( 10 ) 2019年01月

担当区分：責任著者記述言語：英語掲載種別：研究論文（学術雑誌）

© 2019 The Japan Society of Applied Physics. Molecular-dynamics simulations of the pressure-induced structural changes of amorphous Si have been performed using the Tersoff interatomic potential to examine the validity of this potential. Amorphous Si with a tetrahedral network was prepared by melt-quenching methods, and it was then compressed under isothermal-isobaric conditions. The changes of the atomic pair-distribution functions and static structure factors with increasing pressure were in agreement with those observed experimentally. The pressure-induced amorphous structures contained a short-range order similar to the β-tin and Imma structures. These results suggest that the Tersoff potential is suitable for describing the structural changes of amorphous Si under high pressure.

DOI： 10.7567/1347-4065/ab42f3

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特集にあたって査読有り

石丸学

顕微鏡 ( 公益社団法人日本顕微鏡学会 ) 54 ( 3 ) 109 - 109 2019年01月

低エネルギー電子線照射によるアモルファスGeSnの結晶化査読有り

石丸学, 仲村龍介

顕微鏡 ( 公益社団法人日本顕微鏡学会 ) 54 ( 3 ) 126 - 130 2019年01月

担当区分：筆頭著者記述言語：日本語掲載種別：記事・総説・解説・論説等（学術雑誌）

材料が照射場に曝されると欠陥が導入され，欠陥蓄積に伴い構造変化が引き起こされる．照射環境下における材料の損傷過程は，「はじき出し効果」と「電子励起効果」によるものに大別される．電子線照射においては，加速電圧が高いときには前者が，低いときには後者が顕著になる．これらの効果は構造解析に対して負の影響を与えるため，電子顕微鏡観察に際しては極力避けるべきものである．一方，照射効果を利用することにより，通常の手法では実現することが難しい平衡状態から離れた材料の創製が可能である．本稿では，低エネルギー電子線照射によるアモルファスの結晶化を利用して，次世代フレキシブルディスプレイとしての応用が期待されている多結晶GeSn薄膜を，非熱的過程により作製した例を紹介する．

DOI： 10.11410/kenbikyo.54.3_126

その他リンク： https://ci.nii.ac.jp/naid/130007783749

スパッタリング法によるアモルファスFe-B合金薄膜の作製と透過型電子顕微鏡による構造解析査読有り

仲村龍介, 半谷祐樹, 石丸学, 和田武

鉄と鋼 ( 日本鉄鋼協会 ) 105 ( 10 ) 1017 - 1021 2019年01月

記述言語：日本語掲載種別：研究論文（学術雑誌）

Amorphous Fe-B thin films of the concentration range between 12 and 25 at.% B were prepared by radio frequency sputtering techniques, and their local structures were examined by transmission electron microscopy. Pair-distribution-function analyses of Fe-12, -18 and -25 at.%B for a region of 300 nm in diameter revealed a systematic trend according to the concentration; with the increase in B concentration, the intensity of the weak shoulder peak at 0.205 nm for the Fe-B pair increases whereas those of main peaks for Fe-Fe pair decrease. In electron diffraction patterns of the samples taken by a probe size of 2 nm, the specific spots of iron-borides, such as Fe3B and Fe23B6, were detected, suggesting the atomic arrangement of the compounds in the amorphous matrices.

DOI： 10.2355/tetsutohagane.TETSU-2019-032

その他リンク： https://ci.nii.ac.jp/naid/130007719957

Deposition-temperature dependence of vortex pinning property in YBa<inf>2</inf>Cu<inf>3</inf>O<inf>7</inf>+BaHfO<inf>3</inf> Film 査読有り

Horide T., Torigoe K., Kita R., Nakamura R., Ishimaru M., Awaji S., Matsumoto K.

Nippon Kinzoku Gakkaishi/Journal of the Japan Institute of Metals ( 公益社団法人日本金属学会 ) 83 ( 9 ) 320 - 326 2019年01月

記述言語：英語掲載種別：研究論文（学術雑誌）

© 2019 The Japan Institute of Metals and Materials. Improvement of critical current density (Jc) in magnetic fields is required in YBa2Cu3O7 films, and process parameters should be optimized for controlling pinning centers. In the present study, a deposition temperature was varied in pulsed laser deposition of YBa2Cu3O7+BaHfO3 films to control the nanorod structure, and its influence on Jc was analyzed. The YBa2Cu3O7+BaHfO3 film deposited at 850°C exhibited pinning force maximum(Fp,max) as high as 413 GN/m3 at 40 K, while the Fp,max for the deposition temperature of 850°C at 77 K was smaller than that in the YBa2Cu3O7+BaHfO3 film deposited at 900°C. A critical temperature decreased and matching field increased with decreasing the deposition temperature. Increase in deposition temperature is effective in improving the Fp,max in high temperatures, since the critical temperature and matching field dependences of Jc value dominate the Fp,max. On the other hand, low deposition temperature improves the Fp,max in low temperatures since the Fp shift in accordance with matching field is dominant to the Fp,max. Thus, the deposition temperature should be set in pulsed laser deposition of YBa2Cu3O7 films containing nanorods considering the Jc variation with critical temperature and matching field.

DOI： 10.2320/jinstmet.JA201902

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Chalcopyrite ZnSnSb<inf>2</inf>: A Promising Thermoelectric Material 査読有り

Nomura A., Choi S., Ishimaru M., Kosuga A., Chasapis T., Ohno S., Snyder G., Ohishi Y., Muta H., Yamanaka S., Kurosaki K.

ACS Applied Materials and Interfaces 10 ( 50 ) 43682 - 43690 2018年12月

記述言語：英語掲載種別：研究論文（学術雑誌）

© 2018 American Chemical Society. Ternary compounds with a tetragonal chalcopyrite structure, such as CuGaTe2, are promising thermoelectric (TE) materials. It has been demonstrated in various chalcopyrite systems, including compounds with quaternary chalcopyrite-like structures, that the lattice parameter ratio, c/a, being exactly 2.00 to have a pseudo-cubic structure is key to increase the degeneracy at the valence band edge and ultimately achieve high TE performance. Considering the fact that ZnSnSb2 with a chalcopyrite structure is reported to have c/a close to 2.00, it is expected to have multiple valence bands leading to a high p-type zT. However, there are no complete investigations on the high temperature TE properties of ZnSnSb2 mainly because of the difficulty of obtaining a single-phase ZnSnSb2. In the present study, pure ZnSnSb2 samples with no impurities are synthesized successfully using a Sn flux-based method and TE properties are characterized up to 585 K. Transport properties and thermal analysis indicate that the structure of ZnSnSb2 remains chalcopyrite with no order-disorder transition and clearly show that ZnSnSb2 can be made to exhibit a high zT in the low-to-mid temperature range through further optimization.

DOI： 10.1021/acsami.8b16717

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Structure of crystallized particles in sputterdeposited amorphous germanium films 査読有り

Okugawa M., Nakamura R., Hirata A., Ishimaru M., Yasuda H., Numakura H.

Journal of Applied Crystallography 51 ( 5 ) 1467 - 1473 2018年10月

記述言語：英語掲載種別：研究論文（学術雑誌）

© 2018 International Union of Crystallography. Pristine thin films of amorphous Ge prepared by sputtering are unstable and form coarse crystalline particles of 100 nm in size upon crystallization by electron irradiation. These crystalline particles exhibit unusual diffraction patterns that cannot be understood from the diamond cubic structure. The structure has previously been assumed to be a metastable hexagonal form. In the present work, the structure of the coarse crystalline particles has been analysed in detail by transmission electron microscopy, considering the possibility that those diffraction patterns might occur with the diamond cubic structure if the particle consists of thin twin layers. By high-resolution lattice imaging the particles have been shown to be of the diamond cubic structure containing a high density of twins and stacking faults parallel to {111}. With such defects, diffraction patterns can be complex because of the following effects: superposition of two or more diffraction patterns of the same structure but of different orientations, double diffraction through twin crystals, and streaks parallel to the thin crystal which give rise to extra diffraction spots. It is found that diffraction patterns taken from various orientations can be explained in terms of these effects.

DOI： 10.1107/S1600576718012153

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Molecular-dynamics simulations of solid phase epitaxy in silicon: Effects of system size, simulation time, and ensemble 査読有り

Kohno K., Ishimaru M.

Japanese Journal of Applied Physics 57 ( 9 ) 2018年09月

記述言語：英語掲載種別：研究論文（学術雑誌）

© 2018 The Japan Society of Applied Physics. Solid phase epitaxial (SPE) recrystallization of amorphous Si on a Si(001) substrate was examined by large-scale (6144-129024 Si atoms), longtime (up to 2000 ns) molecular-dynamics (MD) simulations using the empirical Tersoff interatomic potential. We particularly focused on the effects of the MD cell size, simulation time, and ensemble on the SPE growth rate. We found that the simulations under the isothermal-isochoric conditions (NVT ensemble) show a higher crystallization rate than those under the isothermal-isobaric conditions (NPT ensemble). The system size dealt with in the present MD simulation, i.e., >6144 Si atoms, was enough to estimate the SPE growth rate. The Arrhenius plot of the growth rate between 1300 and 1600 K exhibited a single activation energy, >2.4 eV, which is in agreement with the experimental value (>2.7 eV). However, the growth rate at temperatures below 1300 K deviated from the extrapolated ones from 1300 to 1600 K, which is because recrystallization does not reach a steady state: long-time MD simulations are required to estimate the growth rate at low temperature. The structure analysis of amorphous/ crystalline interfaces suggested that the braking of atomic bonds parallel to the interface becomes a rate-limiting step of the SPE growth.

DOI： 10.7567/JJAP.57.095503

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Geometric and compositional factors on critical current density in YBa<inf>2</inf>Cu<inf>3</inf>O<inf>7-δ</inf>films containing nanorods 査読有り

Horide T., Nagao S., Izutsu R., Ishimaru M., Kita R., Matsumoto K.

Superconductor Science and Technology 31 ( 6 ) 2018年05月

記述言語：英語掲載種別：研究論文（学術雑誌）

© 2018 IOP Publishing Ltd. Critical current density (Jc) was investigated in YBa2Cu3O7-δfilms containing nanorods prepared with various nanorod materials, with variation of nanorod content, substrate temperature, and oxidization condition. Three types of compositional situation were realized: films containing strain induced oxygen vacancies; fully oxidized films containing cation compositional deviation; and oxygen deficient films. Normalized Jc-B behavior was determined via the matching field, which is a geometric factor, regardless of the compositional details. A Jc-critical temperature (Tc) relation depending on distribution and fraction of compositional deviation (cation compositional deviation and strain induced oxygen vacancies) was found: the Jcvalues decreased with decreasing Tcdue to the effect of Tcon nanorod pinning strength in the fully oxidized films; Jcdecreased with decreasing oxygen pressure in the film cooling process after film deposition in spite of Tcremaining almost the same, due to reduction of the effective area for current flow in the oxygen deficient films. Thus, a Jclandscape based on geometric and compositional factors was obtained. The study highlights the importance of the Jc-Tcanalysis in the understanding of Jcin YBa2Cu3O7-δfilms containing nanorods.

DOI： 10.1088/1361-6668/aabe68

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第12回ミクロ・ナノ解析に関する日本・ポーランド合同セミナー開催報告査読有り

石丸学

まてりあ ( 公益社団法人日本金属学会 ) 57 ( 12 ) 631 - 631 2018年01月

Proton-Driven Intercalation and Ion Substitution Utilizing Solid-State Electrochemical Reaction 査読有り

Fujioka M., Wu C., Kubo N., Zhao G., Inoishi A., Okada S., Demura S., Sakata H., Ishimaru M., Kaiju H., Nishii J.

Journal of the American Chemical Society 139 ( 49 ) 17987 - 17993 2017年12月

記述言語：英語掲載種別：研究論文（学術雑誌）

© 2017 American Chemical Society. The development of an unconventional synthesis method has a large potential to drastically advance materials science. In this research, a new synthesis method based on a solid-state electrochemical reaction was demonstrated, which can be made available for intercalation and ion substitution. It was referred to as proton-driven ion introduction (PDII). The protons generated by the electrolytic dissociation of hydrogen drive other monovalent cations along a high electric field in the solid state. Utilizing this mechanism, Li + , Na + , K + , Cu + , and Ag + were intercalated into a layered TaS 2 single crystal while maintaining high crystallinity. This liquid-free process of ion introduction allows the application of high voltage around several kilovolts to the sample. Such a high electric field strongly accelerates ion substitution. Actually, compared to conventional solid-state reaction, PDII introduced 15 times the amount of K into Na super ionic conductor (NASICON)-structured Na 3-x K x V 2 (PO 4 ) 3 . The obtained materials exhibited a thermodynamically metastable phase, which has not been reported so far. This concept and idea for ion introduction is expected to form new functional compounds and/or phases.

DOI： 10.1021/jacs.7b09328

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Helium Irradiation and Implantation Effects on the Structure of Amorphous Silicon Oxycarbide 査読有り

Su Q., Inoue S., Ishimaru M., Gigax J., Wang T., Ding H., Demkowicz M., Shao L., Nastasi M.

Scientific Reports 7 ( 1 ) 2017年12月

記述言語：英語掲載種別：研究論文（学術雑誌）

© 2017 The Author(s). Despite recent interest in amorphous ceramics for a variety of nuclear applications, many details of their structure before and after irradiation/implantation remain unknown. Here we investigated the short-range order of amorphous silicon oxycarbide (SiOC) alloys by using the atomic pair-distribution function (PDF) obtained from electron diffraction. The PDF results show that the structure of SiOC alloys are nearly unchanged after both irradiation up to 30 dpa and He implantation up to 113 at%. TEM characterization shows no sign of crystallization, He bubble or void formation, or segregation in all irradiated samples. Irradiation results in a decreased number of Si-O bonds and an increased number of Si-C and C-O bonds. This study sheds light on the design of radiation-tolerant materials that do not experience helium swelling for advanced nuclear reactor applications.

DOI： 10.1038/s41598-017-04247-x

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Formation of metastable phases in Zr-ion-irradiated Al<inf>2</inf>O<inf>3</inf>upon thermal annealing 査読有り

Oka N., Ishimaru M., Tane M., Sina Y., McHargue C., Sickafus K., Alves E.

Microscopy 66 ( 6 ) 388 - 396 2017年12月

記述言語：英語掲載種別：研究論文（学術雑誌）

© The Author 2017. Published by Oxford University Press on behalf of The Japanese Society of Microscopy. All rights reserved. Formation of metastable phases in Zr-ion-irradiated corundum alumina (Al 2 O 3 ) upon thermal annealing was examined using transmission electron microscopy. A metastable cubic spinel phase was formed in the topmost layer of the as-irradiated microstructure. During thermal annealing at temperatures ranging from 1073 to 1273 K, this spinel layer grew in extent via an unusual corundum-to-spinel phase transformation. A normal spinel-to-corundum phase transformation was observed at post-irradiation annealing temperatures greater than 1473 K. In addition, ZrO 2 nanocrystals embedded in α-Al 2 O 3 were observed to form at these higher temperatures. High-resolution transmission electron microscopy observations and electron diffraction experiments revealed that the structure of the ZrO 2 precipitates observed in this study are consistent with a highpressure metastable orthorhombic phase of ZrO 2 known as the Ortho-I phase.

DOI： 10.1093/jmicro/dfx028

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Low-temperature synthesis of crystalline GeSn with high Sn concentration by electron excitation effect 査読有り

Kimura T., Ishimaru M., Okugawa M., Nakamura R., Yasuda H.

Japanese Journal of Applied Physics 56 ( 10 ) 2017年10月

記述言語：英語掲載種別：研究論文（学術雑誌）

© 2017 The Japan Society of Applied Physics. The low-temperature synthesis of high-Sn-concentration GeSn is challenging in realizing flexible thin-film transistors and solar cells. Because of athermal processes, irradiation with energetic particles is anticipated to significantly reduce the processing temperature for device fabrication. Here, we demonstrated that polycrystalline Ge with 30 at.% Sn can be realized at room temperature by the electron-beam-induced recrystallization of amorphous GeSn. We found that inelastic electronic stopping, the so-called electron excitation effect, plays an important role in the recrystallization of amorphous GeSn.

DOI： 10.7567/JJAP.56.100307

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Inhomogeneous crystallization of sputter-deposited amorphous Ge films 査読有り

Nakamura R., Okugawa M., Ishimaru M., Yasuda H., Numakura H.

AM-FPD 2017 - 24th International Workshop on Active-Matrix Flatpanel Displays and Devices: TFT Technologies and FPD Materials, Proceedings 276 - 278 2017年08月

記述言語：英語掲載種別：研究論文（国際会議プロシーディングス）

Crystallization of amorphous germanium films induced by electron irradiation was investigated by in-situ transmission electron microscopy under a low electron-eneigy of 125 keV with various pre-Aging treatments at room temperature. Different microstructures were observed depending on the aging time: coarse and spherical nanoparticles grew rapidly in samples aged for less than 3 months, while a homogeneous nanociystalline structure with a grain size of about 10 nm appealed in samples aged for over 7 months. Analyses by the pair distribution lunction revealed that the initial structure is inhomogeneous, consisting of a mixture of amorphous regions and some ordered arrangements, but turned spontaneously to homogeneous random amorphous structures during aging. Such inhomogeneity could be an origin of the rapid growth of nanoparticles in the samples not subjected to the long-Time aging.

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Strong c-axis correlated pinning and hybrid pinning in YBa<inf>2</inf>Cu<inf>3</inf>O<inf>7-δ</inf> films containing BaHfO<inf>3</inf> nanorods and stacking faults 査読有り

Horide T., Otsubo K., Kita R., Matsukida N., Ishimaru M., Awaji S., Matsumoto K.

Superconductor Science and Technology 30 ( 7 ) 2017年06月

記述言語：英語掲載種別：研究論文（学術雑誌）

© 2017 IOP Publishing Ltd. High critical current density (J c ) was achieved in low temperatures of 40 and 20 K in YBa 2 Cu 3 O 7-δ films containing elongated BaHfO 3 nanorods and stacking faults, which were prepared using pulsed laser deposition and post-oxygen-annealing. The YBa 2 Cu 3 O 7-δ + BaHfO 3 film exhibited global pinning force maximum (F p,max ) of 312 GNm -3 at 40 K and 619 GNm -3 at 20 K due to strong pinning by nanorods. These Fp,max values are comparable to those in the previously-reported high-J c films with almost the same matching field. Thus, the present study demonstrated that YBa 2 Cu 3 O 7-δ superconducting matrix realized high J c in low temperatures like other REBa 2 Cu 3 O 7-δ (RE = Gd, Sm, (Gd, Y)). Furthermore, high J c was obtained also for magnetic field parallel to the ab-plane, indicating hybrid pinning of nanorods and stacking faults. Thus, the stacking faults significantly affected the J c properties in the YBa 2 Cu 3 O 7-δ films containing nanorods.The present film structure with strong c-axis correlated pinning and hybrid pinning is effective in simultaneous control of J c values and J c anisotropy in YBa 2 Cu 3 O 7-δ films.

DOI： 10.1088/1361-6668/aa70d3

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Pin potential effect on vortex pinning in YBa<inf>2</inf>Cu<inf>3</inf>O<inf>7-δ</inf> films containing nanorods: Pin size effect and mixed pinning 査読有り

Horide T., Matsukida N., Ishimaru M., Kita R., Awaji S., Matsumoto K.

Applied Physics Letters 110 ( 5 ) 2017年01月

記述言語：英語掲載種別：研究論文（学術雑誌）

© 2017 Author(s).The pin size effect and mixed pinning of nanorods and matrix defects are discussed for YBa2Cu3O7-δ films containing nanorods. BaSnO3 nanorods with a diameter of 11 nm and BaHfO3 nanorods with a diameter of 7 nm were prepared, and critical current density (Jc) and resistivity were measured in the films. When the coherence length was larger than the nanorod size at high temperatures near the critical temperature, the trapping angle and activation energy of the vortex flow depended on the nanorod diameter. At a moderate temperature of 65−77 K, the pin size effect on Jc disappeared since the coherence length became smaller than the nanorod size. At a low temperature of 20 K, the contribution from matrix pinning became comparable to that of nanorods in a high magnetic field due to the small coherence length. Thus, the temperature-dependent coherence length caused the pin potential situation to vary significantly, namely, the pin size effect and mixed pinning, which strongly affected vortex pinning in YBa2Cu3O7-δ containing nanorods.

DOI： 10.1063/1.4975300

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アモルファスGeにおける構造緩和過程の分子動力学法による解析査読有り

Imabayashi S., Ishimaru M.

日本金属学会誌 81 ( 2 ) 66 - 70 2017年01月

記述言語：日本語掲載種別：研究論文（学術雑誌）

© 2016 The Japan Institute of Metals and Materials.Structural relaxation of amorphous germanium has been examined by molecular dynamics simulations based on the empirical Tersoff interatomic potential. Although the Tersoff potential overestimated both the melting and glass transition temperatures, it was found that this potential can reproduce the behavior of structural relaxation. The potential energy decreased with thermal annealing below the glass transition temperature, but it sometimes increased during structural relaxation. The mean square displacement of atoms also increased at the corresponding period. These changes were attributed to the cooperative atomic motion during structure relaxation. Atomic trajectories revealed that structural changes are induced by spatially- and temporally-inhomogeneous atomic motions: atomic mobile and immobile regions coexisted during structural relaxation.

DOI： 10.2320/jinstmet.J2016051

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Molecular dynamics study on structural relaxation processes in amorphous germanium 査読有り

Imabayashi S., Ishimaru M.

Materials Transactions 58 ( 6 ) 857 - 861 2017年01月

記述言語：英語掲載種別：研究論文（学術雑誌）

© 2017 The Japan Institute of Metals and Materials.The structural relaxation of amorphous germanium was examined by molecular dynamics simulations based on the empirical Tersoff interatomic potential. Although the Tersoff potential overestimated both the melting and glass transition temperatures, it was able to reproduce the structural relaxation behavior. The potential energy decreases with thermal annealing below the glass transition temperature, but occasionally increases during structural relaxation. The mean square displacement of atoms also increases in these periods. These changes were attributed to cooperative atomic motion during the structural relaxation. Atomic trajectories revealed that structural changes are induced by spatially- and temporally-inhomogeneous atomic motions: atomically mobile and immobile regions coexist during structural relaxation.

DOI： 10.2320/matertrans.M2017036

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透過電子顕微鏡法による照射誘起構造変化の解析招待有り査読有り

石丸学

顕微鏡 ( 公益社団法人日本顕微鏡学会 ) 52 ( 1 ) 24 - 28 2017年01月

担当区分：筆頭著者記述言語：日本語掲載種別：記事・総説・解説・論説等（学術雑誌）

照射場を利用した材料創製では，通常の手法では得ることが難しい平衡状態から離れた準安定構造を実現することが出来る．また，機能性材料および構造用材料の照射誘起構造変化に関する研究は，宇宙空間や原子炉等の極限環境下で使用される材料の信頼性を知る上で重要である．本稿では，照射場と材料の相互作用による構造変化およびそれを利用した材料創製に関する最近の研究成果について報告する．準安定構造の形成に関しては，イオン照射アルミナの熱処理過程において見出された，通常の熱処理では起こらないコランダム→スピネル型構造相変態について報告する．一方，耐照射性材料の開発に関する一連の研究では，ナノ構造制御により炭化ケイ素の耐照射性を向上させた例について紹介する．

DOI： 10.11410/kenbikyo.52.1_24