Country：Japan

Country：Japan

Country：Japan

Country：Japan

Country：Japan

Country：United States

Country：United States

Country：Japan

Country：Japan

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

This study applies Bayesian super-resolution to X-ray photoelectron spectroscopy (XPS), achieving up to a 20-fold reduction in measurement time while preserving data quality. Traditional XPS, crucial for surface analysis, typically requires extensive measurement durations. Our methodology significantly accelerates the process, as demonstrated with glass and Polytetrafluoroethylene samples, where we reduced measurement times by up to 1/20th without compromising spectral accuracy. This approach decreases noise levels and retains spectral integrity, offering a highly efficient solution for XPS. This innovation is particularly valuable in material science, enabling rapid, reliable surface analysis.

DOI： 10.35848/1347-4065/ad3140

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

Abstract

Silicon carbide (SiC) is widely used in power semiconductor devices; however, basal plane dislocations (BPDs) degrade device performance, through a mechanism called bipolar degradation. Recently, we proposed that proton implantation could suppress BPD expansion by reducing BPD mobility. We considered three potential mechanisms: the hydrogen presence around BPDs, point defects induced by implantation, and carrier lifetime reduction. In this study, we discuss the mechanisms of proton implantation and its applicability to SiC power device production.

DOI： 10.35848/1347-4065/ad1779

Web of Science

Other Link： https://iopscience.iop.org/article/10.35848/1347-4065/ad1779/pdf

Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/acs.cgd.3c01476

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Scopus

Authorship：Lead author,　Corresponding author   Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japanese Association for Crystal Growth  

<p>  The automation of manufacturing processes and improvement of productivity are key challenges in the manufacturing industry. In the Floating Zone (FZ) method, a process used for the growth of crystals in the manufacturing of semiconductor wafers, operators adaptively control input parameters based on the state of the crystal growth process. The operational dynamics of crystal growth using the FZ method are complex, making automation a challenging task. This study aims to automate the control of FZ crystal growth using reinforcement learning, employing dynamics predicted by a Gaussian Mixture Model (GMM) based on a limited amount of operational data. The results from studies using an emulator program for FZ crystal growth demonstrated that the constructed control model can achieve ideal crystal shapes more effectively than human operation.</p>

DOI： 10.19009/jjacg.51-1-04

CiNii Research

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：日本結晶学会  

DOI： 10.5940/jcrsj.65.s19

CiNii Research

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

Non-destructive characterization of crystalline defects in SiC wafers is important for manufacturing high-performance SiC power devices with high productivity. The present study shows that the edge component of the Burgers vector can be identified by birefringence imaging under conditions deviating slightly from the crossed-Nicols condition and by calculation of the in-plane shear stress distribution. It is also found that the combination of birefringence observation and X-ray topography allows the discrimination of the family of threading screw dislocations. The present results will further the understanding of the relationship between defect structure and the properties of SiC power devices.

DOI： 10.1016/j.diamond.2023.110192

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1002/adts.202300218

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

For the solution growth of silicon carbide, solvent inclusions are significant technological issues, and methods to suppress the formation of solvent inclusions are investigated in this study. Experimental observations show that solvent inclusions are formed behind the cellular structures. A phase field model is used to reproduce the formation process of cellular structures and solvent inclusions. Simulation results indicate that slight perturbations of the step front can convert into cellular structures in the case of insufficient supply of carbon, and the overdeveloped cellular structures consequently result in solvent inclusions. Accordingly, several schemes can be suggested by the simulation model to suppress the formation of cellular structures by enhancing the carbon supply. By increasing the carbon diffusion coefficient, cellular structures can be suppressed. Moreover, the step height and the solution flow direction also play an important role in suppressing the cellular structures. This study provides a comprehensive understanding of the formation process of cellular structures and solvent inclusions. A growth process with a high diffusion coefficient and opposite solution flow to the step flow direction was proposed to suppress the formation of cellular structures. The proposed numerical model could be applied in other solution crystal growth methods.

DOI： 10.1021/acs.cgd.2c01512

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Scopus

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

Characterization of defects in semiconductor wafers is essential for the development and improvement of semiconductor devices, especially power devices. X-ray topography (XRT) using synchrotron radiation is a powerful methods used for defect characterization. To achieve detailed characterization of large-size semiconductor wafers by synchrotron XRT, we have developed nuclear emulsion plates reaching a high-resolution and wide dynamic range. We have shown that higher-resolution XRT images could be obtained using emulsions with smaller iodobromide crystals, and demonstrated clear observation of threading edge dislocations in a SiC epitaxial layer having small contrast. Furthermore, we demonstrated XRT image acquisition for almost all of a 150-mm SiC wafer with one plate. Our development will contribute to advances in electronic materials, especially in the field of power electronics, in which defect characterization is important for improving the performance and yield of devices.

DOI： 10.1007/s11664-023-10270-8

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Scopus

Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

Thermal conduction was investigated in titanium-chromium oxide natural superlattices with an ordered arrangement of crystallographic shear planes, which were nearly pristine interfaces for thermal phonons. Thermal conductivity exhibited a clear maximum value at around 50 K, which indicates interface roughness as small as 20 pm based on the calculation of thermal conductivity by a modified Debye-Callaway model. With an ordered arrangement of nearly pristine interfaces at nanoscale, the wave contribution to phonon transport was estimated to be predominant at lower temperatures (less than 100 K) and as much as 30 % even at room temperature. We found that the interface roughness greatly influences the thermal conduction in natural superlattice systems, which may lead to variations in the reported value of thermal conductivity. We have demonstrated that the wave contribution of thermal conductivity become apparent in bulk crystal having nano scale periodic structure with pristine interfaces.

DOI： 10.1016/j.jallcom.2022.167915

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Scopus

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing Ltd  

Bipolar degradation in SiC bipolar devices, in which stacking faults (SFs) expand to accommodate the movement of partial dislocations during forward bias application, is one of the critical problems impeding the widespread implementation of SiC power devices. Here we clearly demonstrate that the movement of partial dislocations can be suppressed by proton implantation, which has good compatibility with semiconductor processing, through investigation of the contraction behavior of SFs in SiC epitaxial layers subjected to proton implantation.

DOI： 10.35848/1882-0786/acb585

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

In the solution growth of the SiC crystal, macrosteps with sufficient height on an off-axis substrate are required to reduce defects and achieve a high-quality grown layer. However, over-developed macrosteps can induce new defects and adversely affect the crystal quality. To better understand and control the behavior of macrosteps corresponding to the control parameters of the growth system, a simulation method that consists of a global two-dimensional computational fluid dynamic (CFD) model, a local three-dimensional CFD model near the growth front, and a kinetics model that describes the movement of macrosteps on the crystal surface is proposed. The simulation method is first applied to investigate the effect of the crystal rotation speed on macrostep morphology. Although the results indicate that a higher crystal rotation speed results in less step bunching, constantly rotating the crystal in one direction is demonstrated to be incapable of yielding a uniform macrostep distribution on the whole surface. Accordingly, a sophisticated control pattern is designed by periodically switching the flow direction underneath the crystal surface, where the proposed simulation method is critical to determine detailed control-parameter values. When the control pattern suggested by the simulation is used, a grown crystal with a uniform macrostep morphology and ideal step height on the whole surface is obtained in the practical experiment.

DOI： 10.1021/acs.cgd.2c01194

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

We analyzed the carrier lifetime in a drift layer of 1.2 kV-class SiC p-n diodes to suppress bipolar degradation. According to the device simulation results, the required carrier lifetime in the drift layer was estimated to be shorter than 10 ns with a current density of 300 A/cm2 if the threshold hole density for the expansion of the stacking fault at the interface of the drift layer and substrate was 2 × 1016 cm−3. Numerical analysis revealed that to ensure a carrier lifetime shorter than 10 ns, the 1/e2 lifetime obtained by microwave photoconductivity decay (μ-PCD) measurements should be shorter than 7.2 ns. Experimental μ-PCD measurements showed that 1/e2 lifetimes obtained from the as-received epiwafer were much longer than 7.2 ns, and even after H+ implantation, high-temperature annealing, or electron irradiation, 1/e2 lifetimes were still long. Therefore, carrier lifetime control in the drift layer is not sufficient to suppress bipolar degradation, and a combination of carrier lifetime control with other methods is necessary for the fabrication of 1.2 kV-class SiC p-n diodes for the complete suppression of bipolar degradation at a current capacity of 300 A/cm2.

DOI： 10.1016/j.mssp.2022.107126

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Informa UK Limited  

DOI： 10.1080/27660400.2022.2107884

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Authorship：Lead author,　Corresponding author   Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japan Synchrotron Radiation Research Institute  

DOI： 10.18957/rr.10.6.521

CiNii Research

Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)  

4H-SiC has been commercialized as a material for power semiconductor devices. However, the long-term reliability of 4H-SiC devices is a barrier to their widespread application, and the most important reliability issue in 4H-SiC devices is bipolar degradation. This degradation is caused by the expansion of single Shockley stacking-faults (1SSFs) from basal plane dislocations in the 4H-SiC crystal. Here, we present a method for suppressing the 1SSF expansion by proton implantation on a 4H-SiC epitaxial wafer. PiN diodes fabricated on a proton-implanted wafer show current–voltage characteristics similar to those of PiN diodes without proton implantation. In contrast, the expansion of 1SSFs is effectively suppressed in PiN diodes with proton implantation. Therefore, proton implantation into 4H-SiC epitaxial wafers is an effective method for suppressing bipolar degradation in 4H-SiC power-semiconductor devices while maintaining device performance. This result contributes to the development of highly reliable 4H-SiC devices.

DOI： 10.1038/s41598-022-23691-y

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Scopus

PubMed

Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

The macroscopic distribution of fluid flows, which affect the quality of final products for various kinds of materials, is often difficult to describe in mathematical formulae and hinders the implementation of empirical knowledge in scaling up. In the present study, the characteristics of the flow distribution in silicon carbide (SiC) solution growth are described by using the position of the saddle point and the solution growth conditions are optimized by computational fluid dynamics simulation, machine learning, and a genetic algorithm. As a result, the candidates of the optimal condition for the solution growth of 6-in. SiC crystals are successfully obtained from the empirical knowledge gained from 3-in. crystal growth, by adding the topological description to the objective function. The present design of the objective function using the topological description can possibly be applied to other crystal growth or materials processing problems and to overcome scale-up difficulties, which can facilitate the rapid development of functional materials such as SiC wafers for power device applications.

DOI： 10.1002/adts.202200302

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

Real-time prediction and dynamic control systems that can adapt to an unsteady environment are necessary for material fabrication processes, especially crystal growth. Recent studies have demonstrated the effectiveness of machine learning in predicting an unsteady crystal growth process, but its wider application is hindered by the large amount of training data required for sufficient accuracy. To address this problem, this study investigates the capability of transfer learning to predict geometric evolution in an unsteady silicon carbide (SiC) solution growth system based on a small amount of data. The performance of transferred models is discussed regarding the effect of the transfer learning method, training data amount, and time step length. The transfer learning strategy yields the same accuracy as that of training from scratch but requires only 20% of the training data. The accuracy is stably inherited through successive time steps, which demonstrates the effectiveness of transfer learning in reducing the required amount of training data for predicting evolution in an unsteady crystal growth process. Moreover, the transferred models trained with relatively more data (no more than 100%) further improve the accuracy inherited from the source model through multiple time steps, which broadens the application scope of transfer learning.

DOI： 10.1002/adts.202200204

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Scopus

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

SiC bipolar degradation, which is caused by stacking fault expansion from basal plane dislocations in a SiC epitaxial layer or near the interface between the epitaxial layer and the substrate, is one of the critical problems inhibiting widespread usage of high-voltage SiC bipolar devices. In the present study, we investigated the stacking fault expansion behavior under UV illumination in a 4H-SiC epitaxial layer subjected to proton irradiation. X-ray topography observations revealed that proton irradiation suppressed stacking fault expansion. Excess carrier lifetime measurements showed that stacking fault expansion was suppressed in 4H-SiC epitaxial layers with proton irradiation at a fluence of 1 × 1011 cm−2 without evident reduction of the excess carrier lifetime. Furthermore, stacking fault expansion was also suppressed even after high-temperature annealing to recover the excess carrier lifetime. These results implied that passivation of dislocation cores by protons hinders recombination-enhanced dislocation glide motion under UV illumination.

DOI： 10.1038/s41598-022-17060-y

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Scopus

PubMed

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

For the nondestructive characterization of SiC wafers for power device application, birefringence imaging is one of the promising methods. In the present study, it is demonstrated that birefringence image contrast variation in off-axis SiC wafers corresponds to the in-plane shear stress under conditions slightly deviating from crossed Nicols according to both theoretical consideration and experimental observation. The current results indicate that the characterization of defects in SiC wafers is possible to achieve by birefringence imaging.

DOI： 10.1107/S1600576722006483

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Scopus

PubMed

Language：English   Publishing type：Research paper (scientific journal)  

For the emergence of quantum effects such as plasmonic enhancement, we successfully assembled high-crystallinity nano-gapped particle superlattices composed of sub-10 nm nanoparticles into the solid state through the self-assembly of DNA-programmed nanoparticles (DNA-NPs) in solution followed by dynamic isotropic contraction during dehydration. By this method of isotropic contraction, the crystal symmetry of the self-assembled DNA-NP superlattice in solution was inherited into a dehydrated and contracted one. The optimal isotropic contraction during dehydration was achieved by minimizing the g-factor, which is the index value of the degree of crystallinity. The g-factor was calculated using small-angle X-ray scattering and scanning electron microscopy data for self-assembled DNA-NP superlattices before and after dehydration. The experimental success was driven by our theoretical prediction based on the geometrical calculation regarding the spatial restriction of the nearest-neighbor particle arrangement in a three-dimensional crystal lattice, which was quantified as the particle volume fraction in the lattice, φ. By reducing the nanogap to less than 4 nm through DNA-programmed sub-10 nm nanoparticle crystallization, nanofabrication on a scale exhibiting quantum effects in a bottom-up manner would be feasible.

DOI： 10.1021/acs.cgd.2c00075

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Authorship：Lead author,　Last author,　Corresponding author   Language：Japanese  

Authorship：Last author   Language：Japanese   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.jacl.2022.01.007

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PubMed

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

We have investigated thermal conduction in bulk titanium oxide natural superlattices with crystallographic shear (CS) structures, in which dense planar faults are introduced with different periodicities, prepared by reductive annealing of rutile TiO2 and crystal growth by the floating zone method. High-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) revealed that (132)rutile and (121)rutile CS planes with interspacings of 2.7 and 1.0 nm were introduced in the mother rutile structure. Time-domain thermoreflectance (TDTR) revealed that the thermal conductivity decreased by the introduction of CS planes, but that the decrease is not monotonic with increasing density of CS planes. Calculation of the thermal conductivity and the mean free path for phonons revealed that a crossover from incoherent to coherent thermal conduction took place, and coherent interfaces with nanoscale periodicity were formed as thermodynamically stable phases in bulk titanium oxide natural superlattices.

DOI： 10.1016/j.scriptamat.2021.114326

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

Evaporation of the volatile solute from the liquid phase is a common problem for the crystal grown from solution, especially for the growth under high temperature. In the present study, a numerical model was constructed to quantitatively investigate the evaporation process in crystal growth. This model was applied in top-seeded solution growth (TSSG) of SiC crystal to simulate the transport of aluminum (Al), which is important for crystal surface morphology but easy to vaporize. The transport path of Al in the growth system was determined by analyzing the possible reactions on different boundaries. Accordingly, an improved structure was proposed to suppress the evaporation loss of Al during long-term growth, and was compared with the original case both numerically and experimentally. The simulation results showed that the improved structure could effectively decrease the Al loss by over 70%, and meanwhile had almost no influence on the thermal and flow environment in the solution. For the experimental results, the improved case presented much lower spontaneous nucleation possibilities and higher step height on the crystal surface, which matched well with the features of high Al addition in literatures. Therefore, the improved structure proposed in the present study was proven to be effective to enhance the composition stability of solution during long-term SiC solution growth, and this numerical method could be applied in the growth of other crystals facing the similar problem.

DOI： 10.1016/j.jcrysgro.2021.126448

Scopus

Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

The number of data points of digitally recorded spectra have been limited by the number of multichannel detectors employed, which sometimes impedes the precise characterization of spectral peak shape. Here we describe a methodology to increase the number of data points as well as the signal-to-noise (S/N) ratio by applying Bayesian super-resolution in the analysis of spectroscopic data. In our present method, first, the hyperparameters for the Bayesian super-resolution are determined by a virtual experiment imitating actual experimental data, and the precision of the super-resolution reconstruction is confirmed by the calculation of errors from the ideal values. For validation of the super-resolution reconstruction of spectroscopic data, we applied this method to the analysis of Raman spectra. From 200 Raman spectra of a reference Si substrate with a data interval of about 0.8 cm(-1), super-resolution reconstruction with a data interval of 0.01 cm(-1) was successfully achieved with the promised precision. From the super-resolution spectrum, the Raman scattering peak of the reference Si substrate was estimated as 520.55 (+0.12, -0.09) cm(-1), which is comparable to the precisely determined value reported in previous works. The present methodology can be applied to various kinds of spectroscopic analysis, leading to increased precision in the analysis of spectroscopic data and the ability to detect slight differences in spectral peak positions and shapes.

DOI： 10.1007/s11664-021-09326-4

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER  

For solution growth of silicon carbide, it is significant to understanding the evolutionary mechanism of step bunching. This study infers that solute's incorporation into steps and transport together determines step bunching progress. The occurrence of step bunching is due to the depletion of solute in the region with high step density, caused by a high step kinetic coefficient. On the other hand, by promoting the transport of the solute in the solution, the step speed becomes uniform, thereby the step bunching can be prevented. Furthermore, we proposed a non-dimensional Damko center dot hler number for crystal growth in step-flow mode. It correlates incorporation rates with bulk diffusion rates and can build a phase map of growth rates and step bunching stability. Several solvents are located in the phase map, demonstrating the possible usage of the phase map as a pointer for solvent designing.

DOI： 10.1016/j.jcrysgro.2021.126425

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

In this study, an algorithm was constructed for the automatic detection of basal plane dislocations (BPDs) propagating in SiC epitaxial layers in photoluminescence images, and its performance was evaluated. The BPDs are the origin of the degradation of SiC bipolar devices caused by the expansion of stacking faults. The present automatic detection algorithm, based on the template-matching method, was confirmed to have high accuracy and precision, and we succeeded in visualizing the BPD density in 150-mm SiC epitaxial wafers. We confirmed that the template-matching method is applicable for the detection of crystalline defects with geometrically fixed shapes such as BPDs in SiC epitaxial layers.

DOI： 10.1007/s11664-021-09284-x

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：CERAMIC SOC JAPAN-NIPPON SERAMIKKUSU KYOKAI  

Aluminum nitride (AlN) possesses excellent thermal conductivity and electrical resistivity, which makes it an ideal candidate for high-power, high-speed integrated circuit substrates. Its low fracture toughness requires the manufacturing of thicker substrates. However, this leads to degraded heat dissipation performance. Herein, we investigated a new strategy, combining the addition of AlN whiskers and tape-casting, to overcome the low fracture toughness disadvantage. The sintered AlN with AlN whiskers addition induced the formation of a highly anisotropic microstructure with aligned rod-like grains, effectively enhancing the fracture toughness to 6.7 MPa m1/2, while maintaining thermal conductivity but degrading strength. (c) 2022 The Ceramic Society of Japan. All rights reserved.

DOI： 10.2109/jcersj2.21143

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CiNii Research

Language：English   Publishing type：Research paper (scientific journal)   Publisher：2022 IEEE International Meeting for Future of Electron Devices, Kansai, IMFEDK 2022  

Polarized light observation is one of the promising methods for the non-destructive characterization of semiconductor wafers for power devices. We have visualized the distribution of defects in SiC wafers by birefringence imaging.

DOI： 10.1109/IMFEDK56875.2022.9975432

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japanese Association for Crystal Growth  

<p>  In this paper, machine learning and optimization based on simulation data of crystal growth are discussed from the viewpoint of informatics application, introducing our application to solution growth of SiC crystal. First, general aspects of crystal growth process simulation and its informatics applications are described. Next, after an overview of the solution growth of SiC crystal, the process optimization of solution growth of SiC crystal using machine learning is described, including the prediction model of temperature and flow of the solution in the crucible, the optimization of geometry conditions, and the optimization of process conditions corresponding to time evolution. Next, application to other materials is described. Finally, this paper is summarized with future prospects.</p>

DOI： 10.19009/jjacg.49-1-06

CiNii Research

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.5551/jat.63151

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PubMed

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCI LTD  

The expansion of double Shockley stacking faults (DSFs) in an n-type 4H-SiC substrate with a nitrogen concentration of 3.9 x 10(19) cm(-3) was investigated using in situ synchrotron X-ray topography. DSF expansion was observed to be suppressed and immobilized above 1590 K, along with the partial dislocation (PD) shape being changed from a straight to zig-zag configuration. For a different heating process (higher heating rate), the PDs could continue to expand, even above 1590 K. Ex situ topography experiments revealed that the DSFs close to the specimen surface expanded widely, although those expanding toward the specimen interior became immobile. One possible mechanism for this immobilization was proposed, where the core structural changes from a Si-core to the C-core by non-conservative motion induced by the interaction between the PDs and point defects (C interstitials).

DOI： 10.1016/j.mtla.2021.101246

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER  

A more sophisticated simulation model is developed based on the principle of ray-tracing to simulate the grazing-incidence synchrotron X-ray topographic contrast of dislocations lying on the basal plane including basal plane dislocations and deflected threading screw and mixed dislocations in off-axis 4H-SiC crystals. The model incorporates effects of surface relaxation as well as the photoelectric absorption to predict dislocation contrast. Compared to conventional ray-tracing images, surface relaxation effects dominate dislocation contrast for diffraction near the crystal surface. The simulated dislocation contrast gradually weakens with increasing depth of the diffracted beam position within the crystal due to photoelectric absorption. The distinctive features of the net simulated dislocation images obtained by aggregating through the effective penetration depth correlate well with contrast features observed on the experimental topographic images. Depth analysis reveals that in some cases the diffracted X-rays from regions below the dislocation can contribute additional contrast features previously not considered.

DOI： 10.1016/j.mseb.2021.115281

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Applied Physics  

DOI： 10.11470/jsapmeeting.2021.2.0_3410

CiNii Research

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

DNA strands are powerful tools as ligand molecules that bind nanoparticles to each other via programmable self-assembly for colloidal crystallization. We found that hydrated DNA-functionalized nanoparticle (DNA-NP) superlattices with a properly controlled volume fraction and spatial arrangement of nanoparticles successfully maintained their crystallinity even after dehydration, which involves drastic contraction. A detailed study of the structural changes was performed for the self-assembled DNA-NP sample using small-angle X-ray scattering (SAXS) after dehydration. Then, an optimal volume fraction of nanoparticles in the superlattice, φ, which minimized the level of distortion of the dehydrated superlattice, was found for each bcc and fcc structure. By acquiring clear SAXS diffraction patterns showing crystal symmetries for dehydrated DNA-NP superlattices, their lattice distortion was evaluated using our analysis technique, which is based on Hosemann's paracrystalline theory and involves SAXS and scanning electron microscopy data. Geometrical calculations substantiated the ease of movement of a nanoparticle under the influence of repulsions from adjacent particles that mainly affect the dehydration stability. These results suggest that it is possible to design the crystal structure of solid nanoparticle superlattices via DNA-guided nanoparticle assembly under a near-equilibrium state in solution as the first step, followed by dehydration under nonequilibrium conditions as the second step.

DOI： 10.1021/acs.cgd.1c00398

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Authorship：Lead author,　Corresponding author   Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

The atomic structure of titanium-chromium oxide natural superlattices with different compositions having an ordered arrangement of crystallographic shear (CS) planes was investigated by high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) and electron diffraction, using polycrystalline ceramics prepared by arc-melting and heat treatment. Analysis of the electron diffraction patterns revealed that not only the interspacing but also the direction of planar faults changed depending on the chromium concentration. HAADF-STEM observations showed that the atomic arrangements of CS planes deviating from (121)rutile are random arrangements of (121)rutile CS planes and (011)rutile antiphase boundaries. Using a random walk model and by calculating the specularity parameter, we found that the CS planes in titanium-chromium oxides behave as coherent interfaces for phonons based on the estimation of interface roughness.

DOI： 10.1021/acs.jpcc.1c04516

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1021/acs.jpcc.1c01831

Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.35848/1347-4065/abde29

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Acta Materialia  

DOI： 10.1016/j.actamat.2021.116746

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1039/d1ce00106j

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：CrystEngComm  

DOI： 10.1039/d0ce01824d

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.35848/1347-4065/abc7a1

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japanese Association for Crystal Growth  

<p>  We have been developing a SiC crystal growth technique using the solution method. As a result, we have achieved the growth of ultra-high quality crystals with extremely low dislocation density. The key to this is the reduction of dislocation density by utilizing the macro-step dislocation conversion phenomenon and the suppression of surface morphology roughness by controlling the flow in the solution. In order to put these technologies to practical use, we have developed a new machine learning technique for optimizing crystal growth conditions for large-diameter crystals. In this method, a model is constructed in the computer that reproduces the actual experiment quickly and accurately, and then hundreds of thousands or millions of trials are performed using the model to derive the experimental conditions with high efficiency. This means that optimization by surrogate models, which is one of the methods of process informatics, has been realized in crystal growth. By using these techniques, we were able to achieve 6-inch crystal growth in a very short time.</p>

DOI： 10.19009/jjacg.48-3-04

CiNii Research

Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Applied Physics  

DOI： 10.11470/jsapmeeting.2020.2.0_2128

CiNii Research

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.actamat.2020.04.019

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1002/pssb.201900553

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1002/pssb.201900527

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Authorship：Lead author,　Last author  

Publishing type：Research paper (scientific journal)   Publisher：ACS Applied Materials and Interfaces  

DOI： 10.1021/acsami.9b22157

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Publishing type：Research paper (scientific journal)   Publisher：CrystEngComm  

DOI： 10.1039/c9ce01338e

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Publishing type：Research paper (scientific journal)  

DOI： 10.7567/1347-4065/ab0acf

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Scopus

Authorship：Lead author,　Corresponding author  

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (scientific journal)   Publisher：Materials Science Forum  

DOI： 10.4028/www.scientific.net/MSF.963.80

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japanese Association for Crystal Growth  

<p>  Metal anodes are promising materials for rechargeable batteries because of the higher capacity. However, low cycle efficiency due to non-uniform deposition prevents them from realization. In this review, we report the crystal orientation dependence of the deposition shape of Li and Zn, which are known as attractive materials for anodes. The result shows that the both of Li and Zn morphology has crystal orientation dependence.</p>

DOI： 10.19009/jjacg.46-1-08

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Vacuum and Surface Science  

<p>The prediction model of the result of computed fluid dynamics simulation in SiC solution growth was constructed on neural network using machine learning. Utilizing the prediction model, we can optimize quickly crystal growth conditions. In addition, the real-time visualization system was also made using the prediction model.</p>

DOI： 10.1380/vss.62.136

CiNii Books

CiNii Research

Language：English   Publishing type：Research paper (international conference proceedings)  

© 2019 CS Mantech. All rights reserved. We are developing solution growth technique for high-quality SiC bulk crystal. In actual, we have achieved high-quality crystal with very-low-density of threading dislocations grown by controlling the surface morphology. In order to apply this technique to large-scale crystal growth, it is necessary to control supersaturation at growth surface, flow rate and flow direction of solvent in detail. However, there are many growth parameters which should be optimized. Simulation technique based on computational fluid dynamics (CFD) is often used. However, it is still difficult to optimize growth condition by utilizing simulation technique since the calculation speed of CFD simulation is not enough to optimize the growth conditions, exhaustively. In recent, informatics including machine learning is applied to various fields including materials science. In this study, we tried to apply machine learning to the analysis of the results of CFD. We could make the model to optimize the crystal growth parameters based on a neural network model. Using the model, the optimization time became 10000 times faster. This is just a trial of “Process Informatics”.

Scopus

Publishing type：Research paper (scientific journal)   Publisher：CS MANTECH 2019 - 2019 International Conference on Compound Semiconductor Manufacturing Technology, Digest of Papers  

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.jcrysgro.2018.09.002

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1039/c8ce00977e

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.7567/APEX.11.111001

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1063/1.5042561

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Applied Physics  

DOI： 10.11470/jsapmeeting.2018.2.0_3248

CiNii Research

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1021/acs.cgd.8b00420

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1063/1.5038189

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1063/1.5021116

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Applied Physics  

We succeeded in measuring the density and direction of the edge component of threading dislocations (TDs) in c-plane (0001) GaN by micro- Raman spectroscopy mapping. In the micro-Raman spectroscopy mapping of the E2 H peak shift between 567.85 and 567.75 cm-1, six different contrast images are observed toward directions of h1-100i. By comparing X-ray topography and etch pit images, the E2 H peak shift is observed where the edge component of TDs exists. In contrast, the E2 H peak is not observed where the screw component of TDs exists.

DOI： 10.7567/APEX.11.061002

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Scopus

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Trans Tech Publications Ltd  

The dislocation behavior during bulk crystal growth on the 4H-SiC (000-1) C-face using the solution method was investigated. A 2 inch wafer with a 4 degree off angle was fabricated from a bulk crystal grown by the TSSG method, and the dislocations in the crystal were evaluated using synchrotron X-ray topography and TEM observation. From the topograph images, it was found that the TSD density remarkably decreased as the growth progressed. Furthermore, the TEM observation suggested that TSD decreases as the threading dislocations convert to in-plane defects toward the center of the crystal. Conventionally, it was considered that conversion of threading dislocations hardly occurs in solution growth on the C-face. However, it is thought that this phenomenon was not observable because the conversion efficiency is remarkably low. We speculate that dislocations may be converted by suddenly making macrosteps during bulk growth.

DOI： 10.4028/www.scientific.net/MSF.924.39

Scopus

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Trans Tech Publications Ltd  

We achieved the growth of extremely-high quality SiC crystal with two-step solution method with specially-designed seed crystals. The two-step growth consists of 1st step growth on Si-face for the reduction of threading dislocations and 2nd step growth on C-face for the reduction of basal plane dislocations and thickening. In this method, we can make the dislocation density extremely low, while the polytype easily changes during growth due to the absence of spiral hillocks originating from threading screw dislocation (TSD). In this study, we prepared specially designed seed crystals for both 1st and 2nd growth steps to provide steps continuously. In the seeds, a few TSDs exist at the upper-side of the step structure. Consequently, we demonstrated the suppression of the polytype transformation during the C-face growth with extremely low-dislocation-density crystal. Accordingly, we successfully obtained extremely low-dislocation density 4H-SiC with TSD, TED and BPD density of 11, 385 and 28 cm-2.

DOI： 10.4028/www.scientific.net/MSF.924.60

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

For understanding of carrier behavior in semiconductors, it is important to measure the carrier relaxation time. In the present study, the relaxation times of inter-valley transition from the valley to the X valley in GaP were evaluated by near-band-gap photoemission spectroscopy of electrons emitted from a surface with a negative electron affinity state. In the energy distribution curves, two peaks, which originate from the electron population accumulated in the valley and the X valley, were observed. From the temperature dependence of the energy of these two peaks, we could successfully evaluate the temperature dependence of the energies of the valley and the X valley. Furthermore, the relaxation times of the inter-valley transition from the valley to the X valley were estimated from the ratio of the electron concentration of the valley and the X valley. The values of the relaxation time are good agreement with the previous studies. These results indicate that the near-band-gap photoemission spectroscopy can directly investigate conduction electrons and also evaluate the carrier dynamics in semiconductor. (C) 2017 Author(s).

DOI： 10.1063/1.4997800

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Applied Physics  

DOI： 10.11470/jsapmeeting.2017.2.0_3506

CiNii Research

Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Applied Physics  

DOI： 10.11470/jsapmeeting.2017.2.0_3378

CiNii Research

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We have investigated the morphology of AlN whiskers on a polycrystalline AlN substrate by using Fe-Al alloy melts under the different synthesis conditions. Formation density of the AlN whiskers increases and the diameter of the whisker decreases with increasing Al content of the Fe-Al alloy melt. Most of the AlN whiskers were zigzag shape with the hexagonal cross section. The longitudinal direction was the [0001] direction. The facet with the zigzag shape was the {1101} or {1101} pyramidal plane and the period of the zigzag facet was almost constant. The average diameter of the whiskers and the period of the zigzag facet decrease with increasing Al content. At the initial stage of the whisker formation, the island of AlN formed on the AlN substrate and the pyramidal facet grows via step-flow growth. From the observation, we discussed the possible mechanism for the formation of the zigzag-shape AlN whiskers.

DOI： 10.1016/j.jcrysgro.2016.11.127

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier B.V.  

We propose a two-step silicon carbide (SiC) solution growth method for dislocation reduction to produce the high-quality silicon carbide SiC crystals. The two-step growth consists of the growth on a Si face and a C face. Firstly, seed crystal with low threading dislocation density was prepared by the growth on a Si face utilizing the threading dislocation conversion. Secondly, the growth on the C face was conducted on the prepared seed crystal with low threading dislocation density to reduce the density of basal plane dislocations and keep smooth growth surface. We demonstrate that the two-step growth leads to the reduction of the density for all types of dislocations by two orders of magnitude compared to the initial density of the seed crystal.

DOI： 10.1016/j.jcrysgro.2016.11.100

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We report the results of microscope measurements examining the phase transition process of a cationic lipid, Dimethyldioctadecylammonium bromide (DDAB) supported lipid bilayer (SLB). Due to lateral fluidity and strong electrostatic interaction with DNA, SLB serves as a fluid substrate for assembling 2D lattices of DNA functionalized nanoparticles (DNA-NPs): lipid molecules work as carriers for transporting DNA-NPs. By fluorescence microscopy and atomic force microscopy (AFM), two types of phase transitions, which correspond to liquid crystalline-gel and liquid crystalline-interdigitated gel (L beta I) ones, were observed in DDAB SLB during cooling. In thermal equilibrium at room temperature both gel and L beta I phases have enough adsorbed amounts of DNA-NPs which indicate that both domains have enough surface charge densities for adsorbing DNA-NPs, however, during phase transition DNA-NPs preferably distributed into L beta I phase.

DOI： 10.1016/j.jcrysgro.2016.09.063

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

DOI： DOI:10.4028/www.scientific.net/MSF.897.24

Language：English   Publishing type：Research paper (scientific journal)  

DOI： DOI: 10.4028/www.scientific.net/MSF.897.32

Language：English   Publishing type：Research paper (scientific journal)  

DOI： DOI: 10.4028/www.scientific.net/MSF.897.28

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Dendritic growth at the Li anode during charging is caused by a morphological inhomogeneity of Li electrodeposition. In this study, we investigate the dependence of the morphology of Li electrodeposited on a polycrystalline Cu current collector on the Cu grain orientation both experimentally and by numerical analysis. The experimental results show that the Li precipitates that form on Cu grains that have close to (111) orientations are the smallest and the most uniform in size. Such a morphology is expected to be effective for the suppression of dendrite; growth. Numerical analysis indicates that the initial stage of electrodeposition plays an important role in determining morphological variation, and this is due to the crystal orientation dependence of the adatom concentration at equilibrium.

DOI： 10.1021/acs.cgd.6b01710

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Applied Physics  

DOI： 10.11470/jsapmeeting.2017.1.0_2741

CiNii Research

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

For solution growth of 4H-SiC with Si0.6-x-yCr0.4AlxSny solvents, the changes in surface morphology and polytype induced by the addition of Sn and Al to the Si0.6Cr0.4 solvent were investigated. Growth with Si0.6Cr0.4 solvents resulted in a rough surface covered with large macrosteps that were several micrometers high, and the polytype of the grown layer transformed to 6H and 15R-SiC. The surface roughening and polytype instability were suppressed when more than 2 at% Al was added to the SiCr0.4 solvent. We also found that the combined addition of both 2-4 at% Sn and 0.5-1 at% Al resulted in smooth surface morphology. We discussed the modification of the surface morphology of 4H-SiC caused by the additives in terms of the wetting properties of the solvents. Based on the results of experiments and thermodynamic calculations, the addition of both Sn and Al increased the liquid/solid interfacial energy. Because the two-dimensional nucleation energy increases with the interfacial energy, we conclude that smooth step flow growth of 4H-SiC was achieved by lowering the frequency of two-dimensional nucleation on the growth surface.

DOI： 10.1016/j.jcrysgro.2016.10.045

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

DOI： DOI: 10.1021/acs.cgd.6b01710

Language：English   Publishing type：Research paper (scientific journal)  

DOI： http://dx.doi.org/10.1016/j.jcrysgro.2016.09.063

Language：English   Publishing type：Research paper (scientific journal)  

DOI： http://dx.doi.org/10.1016/j.jcrysgro.2016.10.045

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Trans Tech Publications Ltd  

The relationship between surface morphology and spatial distribution of basal plane dislocations in 4H-SiC crystal grown by top-seeded solution growth on the C face was investigated by the differential interference microscopy as well as X-ray topography. Basal plane dislocations were generated at the boundaries of the domains with the different macrosteps advance directions. On the other hand, at the area where macrosteps advance to the same directions, BPDs were hardly observed. This results suggest that BPD density can be decreased by the suppression of the collision of macrosteps during the solution growth on the C face controlling the surface morphology.

DOI： 10.4028/www.scientific.net/MSF.897.28

Scopus

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Trans Tech Publications Ltd  

In order to design a solvent for high-purity SiC solution growth, the impurity incorporation and the carbon solubility of various solvent materials have been investigated. Among the transition metal elements, the impurity elements of Cr, Ti, V and Hf are more readily incorporate during the solution growth than the other transition metal elements. The thermodynamic calculation revealed that the Y-Si solvent has relatively large carbon solubility, which is comparable to the Cr-Si and Ti-Si solvents often used in the solution growth of bulk SiC crystals. From these results, the Y-Si solvent is expected to be a suitable solvent for the high-purity SiC solution growth. Furthermore, we have demonstrated that the Y-Si solvent can achieve lower incorporation of metal impurity in the grown crystal than the Cr-Si solvent maintaining the growth rate.

DOI： 10.4028/www.scientific.net/MSF.897.32

Scopus

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Trans Tech Publications Ltd  

In order to achieve a high-quality SiC crystal in solution growth, one of the most difficult issues is to grow a thick layer on Si face avoiding polytype transformation. In this case, two-dimensional nucleation, which leads to the polytype transformation, is frequently induced because a density of threading screw dislocations acting as a source of spiral step decreases and wide terraces form by step bunching as growth proceeds. Therefore, it is very difficult to stabilize the polytype of crystals grown with extremely low density of threading screw dislocations. In this study, we tried to overcome these problems by using specially designed seed crystal and optimizing growth temperature and temperature distribution. We successfully grew thick low-threading-dislocation density SiC crystal without polytype transformation under the condition of high growth temperature and homogeneous temperature distribution.

DOI： 10.4028/www.scientific.net/MSF.897.24

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

DOI： doi:10.1021/acs.cgd.6b01657

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1021/acs.cgd.6b01657

Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

One of the most important issues of SiC solution growth is to increase the growth rate. In our previous study, we reported that increasing the rotational speed of the seed crystal is effective for improving the growth rate in top-seeded solution growth. In the present study, we investigated the origin of the growth rate improvement through a continuous fluid dynamics simulation with a full-scale three-dimensional model of the experimental setup for actual growth experiment. The numerical results indicated that the stagnant layer above the growth interface decreased in thickness and the carbon concentration gradient then became steep with an increase in the rotational speed of the seed crystal. The experimental growth rate was proportional to the calculated carbon concentration gradient, which indicates that the carbon diffusion through the stagnant layer is the rate-determining process. (C) 2016 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.55.125601

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Scopus

Publishing type：Research paper (scientific journal)   Publisher：Crystal Growth and Design  

DOI： 10.1021/acs.cgd.6b01107

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

DOI： doi:10.1021 / acs.cgd.6b01107

Language：English   Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (scientific journal)   Publisher：Crystal Growth and Design  

DOI： 10.1021/acs.cgd.6b00711

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

DOI： doi: 10.1039/c6ce01464j

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

DOI： doi:10.4028/www.scientific.net/MSF.858.57

Language：English   Publishing type：Research paper (scientific journal)  

DOI： doi:10.4028/www.scientific.net/MSF.858.1210

Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The effect of divalent cations on lipid-bilayer-assisted DNA-functionalized nanoparticle (DNA-NP) assembly has been studied. We previously reported the lateral diffusion of DNA-NPs on planar lipid bilayer patches, owing to the mobility of lipid molecules in a supported lipid bilayer (SLB), and the resultant two-dimensional (2D) assembly of DNA-NPs. We here report the structural change of the assembled 2D DNA-NP lattices by magnesium ion concentration control on a successfully formed uniform SLB. In the magnesium-free buffer solution, DNA-NPs on SLB loosely assembled into quasi-hexagonal ordered lattices. In buffer solution containing 1mM magnesium acetate, the interparticle distance of DNA-NPs decreased and the lattice structure became disordered. In buffer solution containing 5mM magnesium acetate, the structure of DNA-NP arrays changed markedly and square lattices appeared. It is suggested that magnesium ions affected DNA molecules, which linked nanoparticles, and enabled the control of the structure of DNA-NP 2D arrays. (C) 2016 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.55.03DF11

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Scopus

Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We report on the relationship between grown polytypes and the activity ratio of silicon to carbon during SiC solution growth using multicomponent solvents. From the thermodynamic calculation as well as crystallization experiments, we revealed that a high activity ratio (a(Si)/a(C)) in the solution leads to the growth of low-hexagonality polytypes, and low a(Si)/a(C) results in the growth of high-hexagonality polytypes. 4H-SiC is stable when a(Si)/a(C) is relatively low (similar to 10(1) > a(Si)/a(C)), 3C-SiC is stable when a(Si)/a(C) is relatively high (similar to 10(4) < a(Si)/a(C)), and 6H-SiC is stable in the intermediate a(Si)/a(C) range (similar to 10(2) < a(Si)/a(C) < similar to 10(3)). From these results, the Cr-Si solvent at high temperatures is expected to be suitable for 4H-SiC growth, and Sc-Si and Fe-Si solvents at relatively low temperatures are expected to be suitable for 3C-SiC growth. (C) 2016 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.55.01AC01

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Scopus

Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Film growth of CaFe2As2 was realized by molecular beam epitaxy on six different substrates that have a wide variation in the lattice mismatch to the target compound. By carefully adjusting the Ca-to-Fe flux ratio, we obtained single-phase thin films for most of the substrates. Interestingly, an expansion of the CaFe2As2 lattice to the out-of-plane direction was observed for all films, even when an opposite strain was expected. A detailed microstructure observation of the thin film grown on MgO by transmission electron microscope revealed that it consists of cube-on-cube and 45 degrees-rotated domains. The latter domains were compressively strained in plane, which caused a stretching along the c-axis direction. Because the domains were well connected across the boundary with no appreciable discontinuity, we think that the out-of-plane expansion in the 45 degrees-rotated domains exerted a tensile stress on the other domains, resulting in the unexpectedly large c-axis lattice parameter, despite the apparently opposite lattice mismatch.

DOI： 10.1088/0953-2048/29/1/015013

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

DOI： doi:10.7567/JJAP.55.01AC01

Language：English   Publishing type：Research paper (scientific journal)  

DOI： doi:10.7567/JJAP.55.03DF11

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

DOI： http://dx.doi.org/10.1016/j.jcrysgro.2016.11.100

Language：English   Publishing type：Research paper (scientific journal)  

DOI： doi:10.1016/j.jcrysgro.2016.11.127

Language：English   Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (scientific journal)   Publisher：ROYAL SOC CHEMISTRY  

We demonstrate that a statistically-significant chiral bias in NaClO3 chiral crystallization can be provoked by inducing nucleation via the optical trapping of Ag nano-aggregates using a continuous wave visible circularly polarized laser (lambda = 532 nm). The laser was focused at the interface between air and an unsaturated NaClO3 aqueous solution containing Ag nanoparticles. The "dominant" enantiomorph was switchable by changing the handedness of the incident circularly polarized laser, indicating that the chiral bias is enantio-selective. Moreover, it has been found that the resulting crystal enantiomeric excess (CEE) reached approximately 25%. The CEE is much higher than the typical enantiomeric excess (EE) in the asymmetric photosynthesis of organic compounds ranging from 0.5 to 2%. The efficient induction of the nucleation and the large chiral bias imply the contribution of localized surface plasmon resonance of the Ag nanoaggregates to chiral nucleation. Our method has potential to offer the benefit for studies on the spatiotemporal nucleation control, optical resolution of chiral compounds and biohomochirality.

DOI： 10.1039/c6ce01464j

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Surface Science Society of Japan  

DOI： 10.14886/sssj2008.36.0_338

CiNii Research

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Trans Tech Publications Ltd  

We investigated the spatial distribution of carrier concentration in n-type 4H-SiC grown by the solution method using the peak frequency of the longitudinal optical phonon-plasmon coupled (LOPC) mode of the Raman spectra on the surface. The carrier concentration at the position of the smooth terrace was higher than the carrier concentration at the position where the macrosteps were formed. This indicates the nitrogen incorporation efficiently occurs on the smooth surface where the density of macrosteps is relatively low. The different incorporation of nitrogen depending on the surface morphology can be understood from the view point of the adsorption time of impurity on the terrace. The present result implies that the uniform surface morphology is necessary to achieve uniform doping concentration in SiC crystal.

DOI： 10.4028/www.scientific.net/MSF.858.57

Scopus

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Trans Tech Publications Ltd  

We achieved high-speed solution growth of AlN single crystal from Cr-Co-Al solvent, which was designed on the basis of thermodynamic calculation. To avoid the unintentional precipitations at the interface between solution and gas as well as to increase the growth rate, we designed the solution growth condition, in which the solution near the interface is undersaturated and the solution near the growth position is supersaturated. To realize this kind of ideal condition, we selected Co and Cr as solvent materials due to their high affinity with nitrogen or aluminum. Finally, we achieved a growth rate as high as 200 μm/h in maximum.

DOI： 10.4028/www.scientific.net/MSF.858.1210

Scopus

Publishing type：Research paper (scientific journal)   Publisher：2015 IEEE 42nd Photovoltaic Specialist Conference, PVSC 2015  

DOI： 10.1109/PVSC.2015.7356192

Scopus

Publishing type：Research paper (scientific journal)   Publisher：Thin Solid Films  

DOI： 10.1016/j.tsf.2015.07.068

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

For the solution growth of 4H-SiC with Si1-xCrx solvents, the change in surface step structure by 4 at% Al addition to the solvent was investigated. Without Al addition, step bunching resulted in the formation of giant macrosteps with height greater than several micrometers, and trench-like surface defects formed with solvent inclusion. The edge of the giant macrosteps composing trench-like defects was faceted into low-index facet planes, (1 (1) over bar 0m) (m=1-4). The formation of the trench-like surface defects is considered to originate from the self-pinning of macrosteps owing to the step-faceting phenomenon, which facilitates further development of macrosteps. On the other hand, the addition of Al to the solvents significantly improved the surface roughening, and suppressed the formation of the trench-like surface defects. In this case, smaller bunched steps with heights from several nanometers to about 10 nm formed on the terraces of the macrosteps, while regularly arranged trains of unit-cell-size steps formed on the terrace of macrosteps in growth without Al addition. The decrease in step stiffness might be a possible cause for the formation of the smaller bunched steps on the terrace in growth with Al addition. (C) 2015 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jcrysgro.2015.04.032

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

DOI： doi:10.1016/j.jcrysgro.2015.04.032

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Trans Tech Publications Ltd  

We realized the growth of 3C-SiC crystal on sapphire by solution growth method. The carbon deposition on a sapphire substrate before growth is the key point for this technology. This carbon layer plays a role to protect the dissolution of sapphire by Si solvent. 3C-SiC was grown on the whole surface of the sapphire substrate and some of the film was 3C-SiC single crystal. Surprisingly, the 3C-SiC layer did not directly grown on the sapphire substrate. The single crystal 3C-SiC layer formed by the reaction between the deposited carbon and the Si wafer that is a solvent material below the melting point of silicon during heating process before the growth. The 3C-SiC grew on Sapphire substrate. In this process, the deposited carbon play another important role.

DOI： 10.4028/www.scientific.net/MSF.821-823.185

Scopus

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Trans Tech Publications Ltd  

Solution growth of SiC has attracted significant attention due to its potential for the production of high-quality SiC wafers. We have recently investigated the dislocation propagation behavior during SiC solution growth with the aim of reducing the dislocation density. Threading dislocations were found to be converted to defects on the basal planes during solution growth. Utilizing this dislocation conversion phenomenon, we have proposed a dislocation reduction process during solution growth and achieved high-quality 4H-SiC crystal growth. Here we confirm the potential of SiC solution growth for the production of high-quality SiC wafers.

DOI： 10.4028/www.scientific.net/MSF.821-823.3

Scopus

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Trans Tech Publications Ltd  

We achieved the convex growth interface shape in top-seeded solution growth of SiC applying non-axisymmetric solution convection induced by non-axisymmetric temperature distribution. The detailed solution flow, temperature distribution and carbon concentration distribution were calculated by 3-dimensional numerical analysis. In the present case, the solution flow below the crystal was unidirectional and the supersaturation was increased along the solution flow direction. By the rotation of the crystal in the unidirectional flow and the temperature distribution, we successfully obtained the crystal with the convex growth interface shape.

DOI： 10.4028/www.scientific.net/MSF.821-823.18

Scopus

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Trans Tech Publications Ltd  

The solution growth of SiC on an off-axis seed is effective on the reduction of threading dislocations. We proposed a novel method to grow a SiC crystal on an off-axis seed by top-seeded solution growth (TSSG). In our previous study, a unidirectional solution flow above a seed crystal is effective to suppress surface roughness in the growth on the off-axis seed. However, it is difficult to apply the unidirectional flow in an axisymmetric TSSG set-up. In this study, the unidirectional flow could be achieved by shifting the rotational axis away from the center of the seed crystal. As a result, the smooth surface was obtained in the wider area where the solution flow direction was opposite to the step-flow direction.

DOI： 10.4028/www.scientific.net/MSF.821-823.31

Scopus

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Trans Tech Publications Ltd  

Surface morphology of the SiC crystal grown on the C face of the 4H-SiC seed crystal by TSSG method using pure Si, Si-1at%Ti-C, Si-5at%Ti-C and Si-20at%Ti-C solvents was investigated. The surface morphology of the crystal grown from pure Si solvent was smooth. By the addition of Ti to the solvent, the surface morphology became rougher. The RMS value is not proportional to the concentration of Ti. The formation of macrosteps with several micrometers was observed when the addition of Ti increased to 5at% indicating the possibility of the threading screw dislocation conversion on the C face of the 4H-SiC crystal.

DOI： 10.4028/www.scientific.net/MSF.821-823.39

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

DOI： doi:10.1088/0953-2048/29/1/015013

Publishing type：Research paper (scientific journal)  

Web of Science

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Visible-light photoemission spectroscopy (VPS) is a novel angle-resolved photoemission spectroscopy developed by us for measuring conduction electrons emitted from a surface with negative electron affinity state. In this study, InGaAs/GaAsP quantum well superlattice was analyzed by the VPS method with changing the excitation photon energy. By comparing with comparison of the energy distribution curves obtained by different excitation energy, the first and second conduction mini-band structures were directly clarified.

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

CiNii Research

Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Threading edge dislocation (TED) conversion during the solution growth of SiC was investigated by synchrotron X-ray topography. TEDs were converted to basal plane dislocations by lateral growth with the advance of macrosteps on the growth surface during the solution growth. TEDs with the Burgers vector parallel to the step-flow direction were converted to basal plane dislocations with a high probability. On the other hand, the conversion ratio of TEDs whose Burgers vector is not parallel to the step-flow direction was much lower. The variation in elastic energy of the basal plane dislocations after the conversion depending on the Burgers vectors led to the different behavior of TED conversion. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.actamat.2014.08.027

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Scopus

Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Letters  

DOI： 10.1063/1.4900423

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Two-dimensional (2D) form of DNA-functionalized gold nanoparticles (DNA-AuNPs) has been studied. To confine the DNA-AuNPs in 2D space, we utilized the strong affinity between DNA molecules and supported lipid bilayers (SLBs). DNA-AuNPs adsorbed onto cationic SLB consisting of Dimethyl dioctadecyl ammonium bromide (DDAB) on mica substrate with high affinity. Owing to the high mobility of lipid molecules, 2D diffusion and transportation of DNA-AuNPs on the SLB was achieved. Using AFM imaging we confirmed a selective adsorption of DNA-AuNPs onto cationic lipid bilayer patches on mica surface, and then we confirmed a drastic formation change of DNA-AuNPs on the SLB with annealing. We observed that the DNA-AuNPs assembled into a densely packed single layered 2D superstructure through DNA hybridization. (C) 2014 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jcrysgro.2014.01.032

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Applied Physics  

DOI： 10.11470/jsapmeeting.2014.2.0_3317

CiNii Research

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1107/S2053273314085842

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Chiral symmetry breaking during the chiral crystallization from a sodium chlorate (NaClO3) aqueous solution is an intriguing phenomenon because it provides insights into the prebiotic process of biohomochirality. However, a mechanism of the emergence and amplification of chirality remains controversial, especially for crystallization from highly supersaturated solution, and one of the hypotheses proposed before is a transition toward the homochiral state during the early stages of crystallization. In this contribution, we directly examined the early stage of crystallization by in situ polarized-light microscopy. The observation revealed that achiral crystals, which appear prior to the formation of chiral crystals, transform to the chiral crystal through two kinds of polymorphic transformations: (1) martensitic transformation (MT) and (2) solution-mediated phase transition (SMPT). The SMPT is remarkably facilitated by contact with a chiral crystal. Notably, the resulting enantiomorph through contact-facilitated SMPT is strongly directed by the contacting enantiomorph. In contrast, the MT yields two enantiomorphs in equal probability. The emergence and amplification of chirality has generally been considered to be a result of direct nucleation of a chiral crystal and its fragmentation. In contrast, our observations provide a possibility that the MT and contact-facilitated SMPT play a role for the emergence and amplification of chirality, respectively.

DOI： 10.1021/cg500527t

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We report a marked reduction in the dislocation density of a 4H-SiC crystal using a high-efficiency dislocation conversion phenomenon. During the solution growth, threading dislocations were efficiently converted to basal plane defects by the step flow of macrosteps. Utilizing this dislocation conversion phenomenon, we achieved the marked reduction of threading dislocation density. Consequently, the threading screw dislocation density was only 30 cm(-2), which was two orders of magnitude lower than that of the seed crystal. The 4H-SiC polytype of the seed crystal was replicated in the grown crystal, which was attributed to the spiral growth owing to a few remaining threading screw dislocations upstream of the step flow. (C) 2014 The Japan Society of Applied Physics

DOI： 10.7567/APEX.7.065501

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

The solubility of the metastable achiral monoclinic phase in NaClO3 crystallization from an aqueous solution, which appears prior to the nucleation of chiral crystals, was successfully measured in the range from 10 degrees C to 23 degrees C. Antisolvent crystallization method was used to obtain metastable crystals for the measurement. The solubility was determined to be about 1.6 times higher than that of the chiral stable cubic phase by observing growth or dissolution of the crystal in aqueous solution at the temperature and concentration of which is predetermined. (C) 2014 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jcrysgro.2014.02.034

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

The nitrogen doping behavior of 4H-SiC was investigated by the top-seeded solution growth technique using Si-Ti solvent. Growth experiments were performed under a mixed gas of helium and nitrogen at atmospheric pressure at 1940°C, in which nitrogen content ranged between 0.17 and 0.5 vol%. The electrical property and structural quality of nitrogen-doped crystals were examined. The nitrogen doping level increased with the increase of nitrogen content in the growth furnace. The most heavily nitrogen-doped SiC with a concentration of 1.1×1020 cm-3 was obtained
however, stacking faults (SFs) were abruptly generated above a nitrogen concentration of 3.0×1019 cm-3. The lowest resistivity of approximately 0.010 Ω cm was obtained with SFs-free. Based on the both undoped and nitrogen-doped growth experimental results, the nitrogen incorporation behavior by employing our solution growth technique was discussed. © 2014 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jcrysgro.2014.01.044

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.4028/www.scientific.net/MSF.778-780.63

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.4028/www.scientific.net/MSF.778-780.67

Language：English   Publishing type：Research paper (scientific journal)  

DOI： DOI: 10.1016/j.actamat.2014.08.027

Language：English  

DOI： DOI:10.1109/PVSC.2014.6925534

Publishing type：Research paper (scientific journal)  

Web of Science

Publishing type：Research paper (scientific journal)   Publisher：The Japanese Association for Crystal Growth  

Silicon carbide forms polytypes due to different stacking arrangements of Si and C. For the use of semiconductor materials, it is necessary to fabricate a single polytype material. In this paper, firstly, determination factors of polytypes are introduced from both experimental and theoretical viewpoints, and the difficulty of the polytype control based on thermal stability is shown. Next, the actual control techniques in sublimation growth method and CVD method are shown to be based on the concept that the polytype sequence of seed crystal continues to the grown crystal. Finally, we propose the kinetic polytype control method based on the growth speed difference among polytypes in SiC solution growth.

DOI： 10.19009/jjacg.40.4_253

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We have proposed a novel angle-resolved photoemission spectroscopy to measure the conduction electrons emitted from a surface with negative electron affinity state. We call this method visible-light photoemission spectroscopy (VPS). In the present study, the conduction miniband structure formed in a superlattice was evaluated by the VPS method. As a result, we succeeded to obtain the dispersion of the miniband. In addition, the effective mass of the miniband was quantitatively determined from the observed dispersion. Therefore, we believe that VPS is a quite powerful tool for the evaluation of intermediate-band solar cells.

DOI： 10.1109/PVSC.2014.6925534

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Chiral symmetry breaking in NaClO3 crystallization from an aqueous solution with perturbations has been of great interest. To understand the mechanism, several models focusing on the early stage of the crystallization have been proposed. However, they are ambiguous because the early stage has been barely explored directly. Here, we investigate the early stages of the crystallization process driven by droplet evaporation using a combination of direct in situ microscopic observations and cryogenic single-crystal XRD experiments. We demonstrate that an achiral crystal having P2(1)/a symmetry, which is newly discovered for a solution growth, first appears in the droplet and then transforms into the chiral crystals. Additionally, determination of the lattice constants by XRD experiments (a = 8.42 angstrom, b = 5.26 angstrom, c = 6.70 angstrom, beta = 109.71 degrees) revealed that the achiral phase should be identical to Phase III (a = 8.78 angstrom, b = 5.17 angstrom, c = 6.83 angstrom, beta = 110 degrees), which is a high-temperature phase from a melt growth of NaClO3. We advocate further assessment of the achiral crystal and a new pathway for the formation of chiral crystals via crystalline phase transition from achiral Phase III.

DOI： 10.1021/cg401324f

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)  

Language：Japanese   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

AlN was directly grown on a sapphire substrate by the solution growth method with the Cu-Si-Al-Ti solvent under a nitrogen gas flow. X-ray diffraction measurements revealed that the grown AlN was single crystal. The AlN layer was epitaxially formed on the sapphire substrate with the orientation relationships: (0001)(AlN) parallel to (0001)(sapphire) and [(1) over bar 100](AlN) parallel to [&lt;(1over bar&gt;120](sapphire). The full widths at half maximum (FWHMs) of X-ray rocking curves for tilt and twist components were 414 and 2031 arcsec, respectively. (C) 2013 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.52.08JE17

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

The control of step bunching by solution flow in 4H-SiC solution growth is proposed. We achieved the solution flow control with the specially designed top-seeded solution growth method as follows: by deviating a seed crystal from the center of a crucible and rotating the crucible in one direction, the solution flow direction was controlled to be parallel or antiparallel to the step-flow direction. After the growth, the widely spaced, accumulated macrosteps were observed and the surface of the grown crystal became rough under the parallel flow. On the other hand, the development of the macrosteps was suppressed under the antiparallel flow. As the growth proceeds, the surface roughness of the growth surface increases under the parallel flow, while the surface roughness decreases under the antiparallel flow. This fact suggests the solution flow control can be an effective method to suppress the step bunching during the solution growth of SiC single crystals. © 2013 American Chemical Society.

DOI： 10.1021/cg400706u

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Institute of Physics Inc.  

Evolution of threading screw dislocation (TSD) conversion during the solution growth of 4H-SiC on a vicinal crystal of 4H-SiC(0001) was investigated by synchrotron X-ray topography. Selecting appropriate X-ray wavelength and g vector, we can change the penetration of X-ray, and the dislocation behaviors with the different depth were successfully observed. Evidently TSDs parallel to the c-axis having c-component Burgers vector were changed into defects on the (0001) basal planes with the same Burgers vector as the TSDs, propagating to the [11\\documentclass[12pt]{minimal}\\begin{document}$\\bar{2}$\\ end{document}2̄0] step-flow direction by advancing macrosteps during the solution growth. The TSD conversions stochastically took place during the growth. The conversion rate was almost uniform and finally almost all TSDs were converted to the basal plane defects. The conversion rate was low at the very early stage of the growth, which implies that the macrosteps were not formed at the initial stage of the solution growth. © 2013 Author(s).

DOI： 10.1063/1.4818357

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Scopus

Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Applied Physics  

<p>The improvement of the substrate quality is important to apply SiC materials to high-performance power devices. A solution growth method is attracting attention as the technique for "ultra-high quality" crystal growth. In our research, it has been revealed that threading dislocations are converted to other defects in the basal planes and they are swept from the crystal during the growth process. Utilizing this conversion phenomenon, we have demonstrated ultra-high quality growth. Recently, reports of improvements in the crystal growth rate and the expansion of the crystal size have also been made using other groups. We have almost established the solution growth as a full-scale quality SiC bulk crystal growth technology.</p>

DOI： 10.11470/oubutsu.82.4_326

Language：Japanese   Publishing type：Research paper (scientific journal)  

Language：Japanese   Publishing type：Research paper (scientific journal)  

Language：Japanese   Publishing type：Research paper (scientific journal)  

Language：Japanese   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.4028/www.scientific.net/MSF.740-742.15

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.4028/www.scientific.net/MSF.740-742.189

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.4028/www.scientific.net/MSF.740-742.311

Publishing type：Research paper (scientific journal)  

Web of Science

Publishing type：Research paper (scientific journal)   Publisher：Conference Record of the IEEE Photovoltaic Specialists Conference  

DOI： 10.1109/PVSC.2013.6744154

Scopus

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Mini-bands formed in superlattice structures are often used for an intermediate-band solar cell. In order to design an optimized structure, it is important to measure the mini-band dispersions which influence transport properties and solar-energy conversion efficiency. In this study, we directly observed the dispersions of mini-bands formed in InGaAs/AlGaAs quantum-well superlattices by angle-resolved photoemission spectroscopy with synchrotron radiation light. The short-period energy dispersions due to the mini-bands around the X-valley were clearly obtained. In addition, the effective mass coefficient of each mini-band could be individually evaluated.

Web of Science

Publishing type：Research paper (scientific journal)  

Web of Science

Publishing type：Research paper (scientific journal)   Publisher：The Japanese Association for Crystal Growth  

Solution growth has a potential to achieve high quality bulk SiC growth. In this paper we review current advances in SiC solution growth specially focusing on the crystal quality. During the solution growth, threading dislocations tend to be converted to basal plane defects by the step-flow of macrosteps. This phenomenon implies that all dislocations can be excluded from the lateral face of the crystal in principle. Actually, high quality SiC crystal with very low threading dislocation density was obtained applying the dislocation conversion during the solution growth.

DOI： 10.19009/jjacg.40.1_25

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Institute of Electrical and Electronics Engineers Inc.  

In this paper we propose an angle-resolved photoemission spectroscopy to observe the conduction electrons emitted from a surface of negative electron affinity state. In actual, we have measured conduction electrons in a GaAs bulk crystal and obtained the electron dispersion around Γ point. In addition, we could also observe hot electrons excited by the light which energy was much larger than band gap energy. These results suggest that the method we proposed is one of the most powerful tools for the evaluation method of the conduction band. © 2013 IEEE.

DOI： 10.1109/PVSC.2013.6744149

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Biophysical Society of Japan General Incorporated Association  

DOI： 10.2142/biophys.53.S92_5

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We realized the polytype-selective growth of 3C-SiC and 6H-SiC on a 6H-SiC (0 0 0 1) seed crystal by controlling the supersaturation. Both 6H-SiC and 3C-SiC grew on the 6H-SiC seed crystal at low supersaturation, but 3C-SiC increased with increasing supersaturation. At high supersaturation, 3C-SiC grew so rapidly that it completely covered the 6H-SiC seed crystal. The growth rates of 3C-SiC and 6H-SiC have different dependences on supersaturation. In the present case, the growth rate of 3C-SiC in 2D nucleation mode is compared with that of 6H-SiC in spiral growth mode. The present kinetic polytype-control technique is based on polytypes having different growth rates and it differs considerably from the conventional technique that is based on "inheritance" of stacking sequence, which is well known as "step-controlled epitaxy". (C) 2011 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jcrysgro.2011.11.041

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Applied Physics  

CiNii Research

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The high-efficiency conversion of threading screw dislocations (TSDs) in 4H-SiC by solution growth provides an efficient method of obtaining ultra high-quality SiC crystals. The behavior of TSDs on on-axis and off-axis 4H-SiC{0001} seed crystals was investigated by synchrotron X-ray topography. Almost all TSDs in the off-axis Si-face seed crystal were converted to Frank-type stacking faults on the basal planes. The conversion ratio of TSDs was highly influenced by the surface polarity of the seed crystal. The stacking faults laterally propagate toward the outside of the crystal. (C) 2012 The Japan Society of Applied Physics

DOI： 10.1143/APEX.5.115501

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Polytype transformations on the 4H-SiC(0001) Si face during top-seeded solution growth have been investigated by transmission electron microscopy and micro-Raman spectroscopy. 4H-, 15R-, and 6H-SiC were grown on the 4H-SiC(0001) Si face via spiral growth. Once a polytype transformation from 4H-SiC to 15R- or 6H-SiC occurs, the polytype rarely returns to 4H-SiC. Just before the polytype transformation, a disturbance in the stacking sequence involving the introduction of stacking faults was observed. A polytype transformation during spiral growth can be understood in terms of the replication of stacking faults due to two-dimensional nucleation of a single bilayer on the spiral steps. This polytype transformation model shows good agreement with observed polytype transformation pathways as well as the disturbance in the stacking sequence at the interface between 4H-SiC and 15R-SiC.

DOI： 10.1021/cg300360h

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Institute of Electronics, Information and Communication Engineers  

CiNii Research

Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Institute of Electronics, Information and Communication Engineers  

Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC APPLIED PHYSICS  

The crystal structure of rhenium silicide with an anomalous ordered arrangement of vacancies has been investigated by utilizing spherical-aberration (C-s)-corrected scanning transmission electron microscopy (STEM). Using C-s-corrected STEM imaging, we directly observe for the first time ordered silicon vacancies in thermoelectric rhenium silicide accompanied by anomalously large local thermal vibration ("rattling" motion) of silicon atoms in their adjacent sites. (C) 2012 The Japan Society of Applied Physics

DOI： 10.1143/APEX.5.035203

Web of Science

Scopus

Publishing type：Research paper (international conference proceedings)   Publisher：TRANS TECH PUBLICATIONS LTD  

Solution growth is considered to be a powerful method for high quality SiC crystals. This work reports that the conversion process from a threading screw dislocation into a few Frank partial dislocations in basal planes was investigated by synchrotron X-ray topography. This process was effectively assisted by step-flow growth on off-oriented (0001) seed crystals. The Frank partials were not extended into the crystal grown toward the [0001] direction perpendicular to the basal plane. Thus, the conclusion of this study suggests the use of off-orientcd seed crystal is important to improve crystal quality.

DOI： 10.4028/www.scientific.net/MSF.717-720.351

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Scopus

Publishing type：Research paper (international conference proceedings)   Publisher：TRANS TECH PUBLICATIONS LTD  

We introduce a method to grow 4H-SiC single polytype stably by controlling the surface morphology. The polytype transition on on-axis 4H-SiC C-face was investigated from a viewpoint of surface morphology of grown layers. At the area where several hillock-like structures grew adjacently, the polytype transition from 4H-SiC to 6H-SiC or 15R-SiC often occurred. Therefore, we tried a vertical seeded solution growth (VSSG) method to suppress the formation of hillock-like structures. As a result, the hillock-like structure on the grown layer was dramatically reduced. Moreover, the ratio of 4H-SiC polytype to the whole grown surface was increased to be almost 100%.

DOI： 10.4028/www.scientific.net/MSF.717-720.53

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

Web of Science

Language：English   Publishing type：Research paper (international conference proceedings)  

The crystal structure of thermoelectric rhenium silicide with an ordered arrangement of vacancies is investigated by utilizing spherical aberration (Cs) corrected scanning transmission electron microscopy (STEM) combined with synchrotron X-ray diffraction and conventional transmission electron microscopy. By STEM Cs corrected imaging, we can clearly observe Si vacancies in rhenium silicide, which is impossible without Cs correction. In addition, significantly reduced contrast levels are noted in STEM images for particular Si sites near vacancies. From the STEM image simulation, the reduced contrast levels are concluded to be due to anomalously large local thermal vibration of these Si atoms. The crystal structure of rhenium silicide can be successfully refined by the synchrotron X-ray diffraction starting with the deduced structure model from the STEM images and the occurrence of large local thermal vibration can be qualitatively confirmed. Furthermore, we confirm the validity of the refined crystal structure of rhenium silicide by comparing experimental images with simulated image generating with the refined crystal structure parameters. © 2011 Materials Research Society.

DOI： 10.1557/opl.2011.43

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

The crystal structure and microstructure of ReSi1.75 were investigated by synchrotron X-ray diffraction combined with scanning transmission electron microscopy. ReSi1.75 contains an ordered arrangement of vacancies in Si sites in the underlying tetragonal C11(b) lattice of the MoSi2-type and the crystal structure is monoclinic with the space group Cm. Atomic positions of Si atoms near vacancies are considerably displaced from the corresponding positions in the parent C11(b) structure, and they exhibit anomalously large local thermal vibration accompanied by large values of atomic displacement parameter. There are four differently-oriented domains with two of them being related to each other by the 90 degrees rotation about the c-axis of the underlying C11(b) lattice and the other two being their respective twins. The habit planes for domain boundaries observed experimentally are consistent with those predicted with ferroelastic theory.

DOI： 10.1080/14786435.2011.570278

Web of Science

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Ceramic Society of Japan  

DOI： 10.14853/pcersj.2011F.0.761.0

CiNii Research

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Ceramic Society of Japan  

DOI： 10.14853/pcersj.2011F.0.759.0

CiNii Research

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

The thermoelectric properties of Magneli phase titanium oxides TinO2n-1 (n=2,3,...) have been investigated, paying special attention to how the thermoelectric performance can be altered by changing the microstructure. Dense polycrystalline specimens with nominal composition of TiO2-x (x=0.05, 0.10, 0.15, and 0.20) prepared by conventional hot-pressing are all identified to be one of the Magneli phases, in which crystallographic shear planes are regularly introduced according to the oxygen deficiency. Electrical conduction is n-type for all specimens and the carrier concentration increases with the increase in the oxygen deficiency. The values of lattice thermal conductivity, on the other hand, decrease with the increase in the oxygen deficiency, which can be attributed to phonon scattering at the crystallographic shear plane. The largest value of thermoelectric figure of merit Z, 1.6 X 10(-4) K-1 was obtained at 773 K for the hot-pressed specimen of TiO1.90. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3498801]

DOI： 10.1063/1.3498801

Web of Science

Scopus

Language：English   Publishing type：Research paper (bulletin of university, research institution)  

DOI： 10.1557/PROC-1218-Z01-04

Publishing type：Research paper (scientific journal)   Publisher：Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy  

DOI： 10.2497/jjspm.57.213

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：Japanese  

J-GLOBAL

Language：Japanese  

J-GLOBAL

Language：Japanese  

J-GLOBAL

Language：Japanese  

J-GLOBAL

Language：Japanese  

J-GLOBAL

Language：Japanese  

J-GLOBAL

Language：Japanese  

J-GLOBAL

Language：Japanese  

J-GLOBAL

J-GLOBAL

Language：Japanese   Publishing type：Research paper, summary (national, other academic conference)  

Language：Japanese  

J-GLOBAL

Language：Japanese   Publishing type：Research paper, summary (national, other academic conference)  

J-GLOBAL

Language：Japanese  

J-GLOBAL

J-GLOBAL

Language：English   Publishing type：Rapid communication, short report, research note, etc. (scientific journal)  

J-GLOBAL

Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

CiNii Books

Language：English   Publisher：IEEE  

We have proposed a novel angle-resolved photoemission spectroscopy to measure the conduction electrons emitted from a surface with negative electron affinity state. We call this method visible-light photoemission spectroscopy (VPS). In the present study, the conduction miniband structure formed in a superlattice was evaluated by the VPS method. As a result, we succeeded to obtain the dispersion of the miniband. In addition, the effective mass of the miniband was quantitatively determined from the observed dispersion. Therefore, we believe that VPS is a quite powerful tool for the evaluation of intermediate-band solar cells.

DOI： 10.1109/PVSC.2014.6925534

Web of Science

Language：Japanese   Publishing type：Rapid communication, short report, research note, etc. (scientific journal)  

Language：Japanese   Publishing type：Rapid communication, short report, research note, etc. (scientific journal)  

Language：Japanese   Publishing type：Rapid communication, short report, research note, etc. (scientific journal)  

Language：Japanese  

J-GLOBAL

Language：Japanese  

J-GLOBAL

Language：Japanese  

J-GLOBAL

Language：English   Publishing type：Rapid communication, short report, research note, etc. (scientific journal)  

Language：English   Publisher：JAPAN SOC APPLIED PHYSICS  

The crystal structure of rhenium silicide with an anomalous ordered arrangement of vacancies has been investigated by utilizing spherical-aberration (C-s)-corrected scanning transmission electron microscopy (STEM). Using C-s-corrected STEM imaging, we directly observe for the first time ordered silicon vacancies in thermoelectric rhenium silicide accompanied by anomalously large local thermal vibration ("rattling" motion) of silicon atoms in their adjacent sites. (C) 2012 The Japan Society of Applied Physics

DOI： 10.1143/APEX.5.035203

Web of Science

Scopus

J-GLOBAL

Language：English  

The crystal structure of thermoelectric rhenium silicide with an ordered arrangement of vacancies is investigated by utilizing spherical aberration (Cs) corrected scanning transmission electron microscopy (STEM) combined with synchrotron X-ray diffraction and conventional transmission electron microscopy. By STEM Cs corrected imaging, we can clearly observe Si vacancies in rhenium silicide, which is impossible without Cs correction. In addition, significantly reduced contrast levels are noted in STEM images for particular Si sites near vacancies. From the STEM image simulation, the reduced contrast levels are concluded to be due to anomalously large local thermal vibration of these Si atoms. The crystal structure of rhenium silicide can be successfully refined by the synchrotron X-ray diffraction starting with the deduced structure model from the STEM images and the occurrence of large local thermal vibration can be qualitatively confirmed. Furthermore, we confirm the validity of the refined crystal structure of rhenium silicide by comparing experimental images with simulated image generating with the refined crystal structure parameters. © 2011 Materials Research Society.

DOI： 10.1557/opl.2011.43

Scopus

Language：English   Publisher：TAYLOR & FRANCIS LTD  

The crystal structure and microstructure of ReSi1.75 were investigated by synchrotron X-ray diffraction combined with scanning transmission electron microscopy. ReSi1.75 contains an ordered arrangement of vacancies in Si sites in the underlying tetragonal C11(b) lattice of the MoSi2-type and the crystal structure is monoclinic with the space group Cm. Atomic positions of Si atoms near vacancies are considerably displaced from the corresponding positions in the parent C11(b) structure, and they exhibit anomalously large local thermal vibration accompanied by large values of atomic displacement parameter. There are four differently-oriented domains with two of them being related to each other by the 90 degrees rotation about the c-axis of the underlying C11(b) lattice and the other two being their respective twins. The habit planes for domain boundaries observed experimentally are consistent with those predicted with ferroelastic theory.

DOI： 10.1080/14786435.2011.570278

Web of Science

Scopus

Language：English   Publisher：AMER INST PHYSICS  

The thermoelectric properties of Magneli phase titanium oxides TinO2n-1 (n=2,3,...) have been investigated, paying special attention to how the thermoelectric performance can be altered by changing the microstructure. Dense polycrystalline specimens with nominal composition of TiO2-x (x=0.05, 0.10, 0.15, and 0.20) prepared by conventional hot-pressing are all identified to be one of the Magneli phases, in which crystallographic shear planes are regularly introduced according to the oxygen deficiency. Electrical conduction is n-type for all specimens and the carrier concentration increases with the increase in the oxygen deficiency. The values of lattice thermal conductivity, on the other hand, decrease with the increase in the oxygen deficiency, which can be attributed to phonon scattering at the crystallographic shear plane. The largest value of thermoelectric figure of merit Z, 1.6 X 10(-4) K-1 was obtained at 773 K for the hot-pressed specimen of TiO1.90. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3498801]

DOI： 10.1063/1.3498801

Web of Science

Scopus

Language：English  

Thermoelectric properties of a homologous series of Magnéli phase titanium oxides TinO2n-1 (n = 2, 3..) have been investigated. Dense polycrystalline specimens with nominal composition of TiO2-x (x = 0.10, 0.20) have been prepared by conventional hot-pressing. X-ray diffraction analysis has revealed that prepared specimens are slightly reduced during hot-pressing. Electrical conduction is of n-type for all prepared titanium oxides and electrical resistivity and absolute values of Seebeck coefficient decrease with increasing oxygen deficiency. The carrier concentration of Magnéli phase titanium oxide increases with increasing oxygen deficiency. Lattice thermal conductivity decreases with increasing oxygen deficiency by more than 60% at room temperature and 40% at 773K compared to TiO2, which can be due to the presence of dense planar defects. The largest thermoelectric figure of merit Z, 1.6 × 10-4 K -1 at 773K, was obtained in TiO1.90 hot pressed specimen. © 2010 Materials Research Society.

DOI： 10.1557/PROC-1218-Z01-04

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