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ambient on threshold voltage stability in 4H-SiC metal-oxide-semiconductor field-effect transistorsHosoi, Takuji*; Osako, Momoe*; Moges, K.*; Ito, Koji*; Kimoto, Tsunenobu*; Sometani, Mitsuru*; Okamoto, Mitsuo*; Yoshigoe, Akitaka; Shimura, Takayoshi*; Watanabe, Heiji*
Applied Physics Express, 15(6), p.061003_1 - 061003_5, 2022/06
Times Cited Count：2 Percentile：34.67(Physics, Applied)The combination of NO annealing and subsequent post-nitridation annealing (PNA) in CO ambient for SiO/SiC structures has been demonstrated to be effective in obtaining both high channel mobility and superior threshold voltage stability in SiC-based metal-oxide-semiconductor field-effect transistors (MOSFETs). N atoms on the SiO side of the SiO/SiC interface incorporated by NO annealing, which are plausible cause of charge trapping sites, could be selectively removed by CO-PNA at 1300
C without oxidizing the SiC. CO-PNA was also effective in compensating oxygen vacancies in SiO, resulting high immunity against both positive and negative bias-temperature stresses.
RhSiYamauchi, Hiroki; Sari, D. P.*; Watanabe, Isao*; Yasui, Yukio*; Chang, L.-J.*; Kondo, Keietsu; Ito, Takashi; Ishikado, Motoyuki*; Hagihara, Masato*; Frontzek, M. D.*; et al.
Communications Materials (Internet), 1, p.43_1 - 43_6, 2020/07
High-temperature short-range order is discovered up to 720 K in MnRhSi by complementary use of neutron scattering and muon spin relaxation measurements.
Yamada, Takahiro*; Ito, Joyo*; Asahara, Ryohei*; Watanabe, Kenta*; Nozaki, Mikito*; Nakazawa, Satoshi*; Anda, Yoshiharu*; Ishida, Masahiro*; Ueda, Tetsuzo*; Yoshigoe, Akitaka; et al.
Journal of Applied Physics, 121(3), p.035303_1 - 035303_9, 2017/01
Times Cited Count：66 Percentile：91.84(Physics, Applied)Initial oxidation of GaN(0001) epilayers and subsequent growth of thermal oxides in dry oxygen ambient were investigated by means of X-ray photoelectron spectroscopy, spectroscopic ellipsometry, atomic force microscopy and X-ray diffraction measurements. It was found that, whereas initial oxide formation tends to saturate at temperatures below 800C, selective growth of small oxide grains proceeds at dislocations in the epilayers, followed by noticeable grain growth leading to rough surface morphology at higher oxidation temperatures. This indicates that oxide growth and its morphology are crucially dependent on the defect density in the GaN epilayers. Structural characterizations also revealed that polycrystalline 
- and 
-phase GaO
grains in an epitaxial relation with the GaN substrate are formed from the initial stage of the oxide growth. On the basis of these experimental findings, we also developed a comprehensive model for GaN oxidation mediated by nitrogen removal and mass transport.
Asahara, Ryohei*; Nozaki, Mikito*; Yamada, Takahiro*; Ito, Joyo*; Nakazawa, Satoshi*; Ishida, Masahiro*; Ueda, Tetsuzo*; Yoshigoe, Akitaka; Hosoi, Takuji*; Shimura, Takayoshi*; et al.
Applied Physics Express, 9(10), p.101002_1 - 101002_4, 2016/10
Times Cited Count：40 Percentile：83.77(Physics, Applied)The superior physical and electrical properties of AlON gate dielectrics on AlGaN/GaN substrates in terms of thermal stability, reliability, and interface quality were demonstrated by direct AlON deposition and subsequent annealing. Nitrogen incorporation into alumina was proven to be beneficial both for suppressing intermixing at the insulator/AlGaN interface and reducing the number of electrical defects in AlO films. Consequently, we achieved high-quality AlON/AlGaN/GaN metal-oxide-semiconductor capacitors with improved stability against charge injection and a reduced interface state density as low as 1.2
10
cm
eV
. The impact of nitrogen incorporation into the insulator was discussed on the basis of experimental findings.
Nozaki, Mikito*; Ito, Joyo*; Asahara, Ryohei*; Nakazawa, Satoshi*; Ishida, Masahiro*; Ueda, Tetsuzo*; Yoshigoe, Akitaka; Hosoi, Takuji*; Shimura, Takayoshi*; Watanabe, Heiji*
Applied Physics Express, 9(10), p.105801_1 - 105801_4, 2016/10
Times Cited Count：5 Percentile：24.79(Physics, Applied)Interface reactions between Ti-based electrodes and n-type GaN epilayers were investigated by synchrotron radiation X-ray photoelectron spectroscopy. Metallic Ga and thin TiN alloys were formed at the interface by subsequently depositing Al capping layers on ultrathin Ti layers even at room temperature. By comparing results from stacked Ti/Al and single Ti electrodes, the essential role of Al capping layers serving as an oxygen-scavenging element to produce reactive Ti underlayers was demonstrated. Further growth of the metallic interlayer during annealing was observed. A strategy for achieving low-resistance ohmic contacts to n-GaN with low-thermal-budget processing is discussed.
Ezure, Toshiki; Ito, Kei; Kameyama, Yuri*; Kamide, Hideki; Kunugi, Tomoaki*
Nihon Genshiryoku Gakkai Wabun Rombunshi, 15(3), p.151 - 158, 2016/09
no abstracts in English
Ezure, Toshiki; Ito, Kei; Kamide, Hideki; Kunugi, Tomoaki*
Thermal Science and Engineering, 24(3), p.31 - 38, 2016/07
Ezure, Toshiki; Ito, Kei; Kameyama, Yuri*; Kurihara, Akikazu; Kunugi, Tomoaki*
Konsoryu, 30(2), p.189 - 196, 2016/06
Ito, Kei; Kunugi, Tomoaki*; Ohno, Shuji; Kamide, Hideki; Ohshima, Hiroyuki
Journal of Computational Physics, 273, p.38 - 53, 2014/09
Times Cited Count：17 Percentile：71.23(Computer Science, Interdisciplinary Applications)Ito, Kei; Kunugi, Tomoaki*; Ohshima, Hiroyuki; Kawamura, Takumi*
Computers & Fluids, 88, p.250 - 261, 2013/12
Times Cited Count：24 Percentile：69.6(Computer Science, Interdisciplinary Applications)The PLIC (Piecewise Linear Interface Calculation) algorithm is one of the volume-of-fluid algorithms. However, the PLIC algorithm needs complicated geometrical calculations for the reconstructions of the interfaces in interfacial cells, and therefore, the PLIC algorithm on unstructured meshes have not been studied enough. In this paper, the authors propose a new PLIC algorithm on unstructured meshes with arbitrary-shaped cells. In the proposed PLIC algorithm, several methods are developed or implemented. In addition, the volume fraction transport is improved by developing a volume-conservative algorithm in which gas and liquid volumes are conserved perfectly during the transport. As a result of the verification and validation tests, it is confirmed that the proposed PLIC algorithm enables the numerical simulations of interfacial dynamic behaviors with high accuracy even on unstructured meshes with highly-irregular cell arrangement.
Kamiji, Yu; Akai, Naoki*; Ozaki, Koichi*; Ito, Chikara
JAEA-Research 2013-018, 18 Pages, 2013/11
JAEA-Research-2013-018.pdf:2.1MB
Preliminary examination of image processing was conducted using existing reactor inside images through a fiber scope in order to confirm applicability of 3D-mapping in a reactor as a part of development of 3D-measurement technology. The Upright SURF (Speeded Up Robust Features) was used to find corresponding points between two captured images. In case of images showing many similar textures or lack of texture, it was difficult to find corresponding points using SURF only. By coupling with a canny algorithm to detect edges of the inside structure, it was found that 3D-structure could be measured for rectilinear objects. Also, image acquisition model was examined in order to clarify camera specs.
Ito, Kei; Kunugi, Tomoaki*; Ohshima, Hiroyuki
Applied Mathematics, 4(10A), p.17 - 25, 2013/10
In this study, a new high-precision numerical simulation scheme for vortical flows (vortex-based scheme) is proposed. This scheme identifies a vortical flow in each computational cell, and then, reconstructs a vortical velocity distribution based on the Burgers vortex model. In addition, a pressure distribution in the vicinity of the vortex center is also reconstructed. The momentum transfer is calculated with the reconstructed velocity and pressure distributions, and therefore, the vortex-based scheme can simulate vortical flows more accurately than the conventional schemes. In fact, as the simulation result of inviscid vortex attenuation problem, the vortex-based scheme shows lower simulation error compared to the conventional discretization schemes. Moreover, also in the numerical simulation of the quasi-steady vortical flow, the simulation accuracy of the vortex-based scheme is superior to those of the conventional schemes.
Ohshima, Hiroyuki; Tanaka, Nobuatsu*; Eguchi, Yuzuru*; Nishimura, Motohiko*; Kunugi, Tomoaki*; Uchibori, Akihiro; Ito, Kei; Sakai, Takaaki
Nihon Genshiryoku Gakkai Wabun Rombunshi, 11(4), p.316 - 328, 2012/12
It is of importance for stable operations of sodium-cooled fast reactors (SFRs) to prevent gas entrainment (GE) phenomena due to free surface vortices. Therefore, the authors have been developing an evaluation method for GE based on computational fluid dynamics (CFD) methods. In this study, we determine the suitable CFD method for GE phenomena from several candidates through some numerical benchmarks. As the results, we obtain the following guideline for the vortex-induced gas entrainment. Free vortex flow around the vortex core can be correctly evaluated by using the appropriate numerical models such as enough mesh resolution, suitable advection solver, suitable turbulence and free surface modeling.
Ohshima, Hiroyuki; Eguchi, Yuzuru*; Kunugi, Tomoaki*; Kamide, Hideki; Sakai, Takaaki; Ito, Kei
Nihon Genshiryoku Gakkai Wabun Rombunshi, 11(4), p.329 - 339, 2012/12
For the stable operation of fast breeder reactors (FBRs), the occurrences of gas entrainment (GE) phenomena should be suppressed below an allowance level. Therefore, a reliable evaluation method for the GE phenomena is necessary to determine the operating conditions of FBRs. In this paper, the authors propose a GE evaluation method in which free surface vortices are identified from velocity fields by using the second invariant of velocity gradient tensor, and the GE evaluation parameters, e.g. gas core length, are calculated by using the Burgers vortex model. In addition, the standard for prevention of three kinds of the vortex-type GE are shown in consideration of experimental data, evaluation results with the proposed method and the comparison results of them. Finally, it is confirmed that the onset conditions of the vortex-type GE can be evaluated by the proposed method.
Ito, Kei; Kunugi, Tomoaki*; Ohshima, Hiroyuki
Konsoryu, 26(1), p.52 - 59, 2012/03
For the purpose of direct simulations of gas entrainment phenomenon in fast reactors, the authors are developing a high-precision simulation algorithm for gas-liquid two-phase flows on non-orthogonal meshes. In this paper, the calculation method for an interface normal is improved by developing a height function algorithm on non-orthogonal meshes. As a result of basic verification, the height function algorithm succeeds in reproducing rectilinear interfaces on a non-orthogonal triangular mesh. In addition, an interfacial curvature is calculated accurately based on the interface normal calculated by the height function algorithm.
Ito, Kei; Kunugi, Tomoaki*; Ohshima, Hiroyuki
Nihon Kikai Gakkai Rombunshu, B, 78(786), p.254 - 262, 2012/02
The cavitation due to sub-surface vortex, which can damage structural surfaces of a large-scale sodium-cooled fast reactor in Japan, have been studied. The authors are developing a CFD code to simulate the cavitation accurately. In this paper, as the first part of the development, a high-precision simulation scheme for vortical flows is presented. In this scheme, a vortical flow is identified in each cell and a vortical velocity distribution is determined locally to be consistent with the local velocity distribution. Then, the calculations of momentum transport through cell faces are performed in consideration with the vortical velocity distribution. As a fundamental verification of the developed scheme, inviscid vortex attenuation in two-dimensional domain is simulated. As a result, it is confirmed that the developed scheme can simulate vortical flows more accurately than conventional schemes.
Ito, Kei; Kunugi, Tomoaki*; Ohshima, Hiroyuki
International Journal for Numerical Methods in Fluids, 67(11), p.1571 - 1589, 2011/12
Times Cited Count：9 Percentile：48.25(Computer Science, Interdisciplinary Applications)In this paper, a high-precision unstructured adaptive mesh technique for gas-liquid two-phase flows is developed and verified/validated. In the unstructured adaptive mesh technique, the PLIC algorithm is employed to simulate interfacial dynamic behaviors and, therefore, the reconstruction method for the interfaces in refined cells is developed, which satisfies the gas and liquid volume conservations and geometrical conservations of interfaces. In addition, the physics-based consideration is performed on the momentum calculations near interfaces, and the calculation method with gas and liquid momentum conservations is developed. For verification, the slotted-disk revolution problem is solved and the unstructured adaptive mesh technique succeeds in reproducing the slotted-disk shape accurately. The dam-break problem is also simulated and the momentum conservative calculation method succeeds in providing physically appropriate results, which show good agreements with experimental data.
Ito, Kei; Kunugi, Tomoaki*; Ohshima, Hiroyuki; Kawamura, Takumi*
Progress in Nuclear Science and Technology (Internet), 2, p.114 - 119, 2011/10
The authors have been developed a high-precision and high-performance numerical simulation algorithm for gas-liquid two-phase flows to simulate the gas entrainment (GE) phenomena in fast reactors. In this paper, the developed simulation algorithm is applied to the GE phenomena in a large-scale water experiment. As a result of the unsteady simulation, a typical interfacial dynamic behavior, i.e. the development of a gas core (interfacial dent), is well simulated. Therefore, it is confirmed that the developed simulation algorithm is suitable to evaluate numerically the GE phenomena in the fast reactors.
Ito, Kei; Kunugi, Tomoaki*; Ohshima, Hiroyuki
Computational Fluid Dynamics 2010, p.563 - 567, 2011/05
Times Cited Count：0 Percentile：0.09(Mathematics, Interdisciplinary Applications)The authors are developing a high-precision numerical simulation algorithm for gas-liquid two-phase flows to simulate gas entrainment phenomena in fast reactors. In this study, the calculation method for the unit vector normal to a gas-liquid interface in the high-precision volume-of-fluid algorithm (PLIC-VOF) is improved on two-dimensional unstructured meshes. As the result of a basic verification, the improved calculation method succeeds in reproducing rectilinear interfaces on an unstructured triangular mesh. In addition, it is confirmed that the improved calculation method enhances the simulation accuracy of the well-known slotted-disk revolution problem on an unstructured mesh.
Ito, Kei; Kunugi, Tomoaki*; Ohshima, Hiroyuki; Kawamura, Takumi*
Nihon Kikai Gakkai Rombunshu, B, 77(776), p.978 - 981, 2011/04
To evaluate gas entrainment (GE) phenomena in a large-scale sodium-cooled fast reactor, the authors are developing a high-precision numerical simulation algorithm for gas-liquid two-phase flows based on a volume-of-fluid methodology. It is confirmed that the developed simulation algorithm can reproduce the GE phenomena in a simple experiment. In this study, the simulation algorithm is applied to a real-scale GE test to check the applicability of the algorithm to the GE phenomena in the fast reactor. As a result, transient behaviors of the vortical flows around the pipes and the accompanied GE phenomena are simulated well. In particular, the origin of the flow which induces the GE phenomena is showed clearly. Finally, from the investigation of a lot of GE phenomena observed in the simulation result, it is verified that the GE phenomena by relatively strong vortical flows are important in terms of the GE suppression in the fast reactor.
