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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.
/4H-SiC(11
0) interfacesNakanuma, Takato*; Iwakata, Yu*; Watanabe, Arisa*; Hosoi, Takuji*; Kobayashi, Takuma*; Sometani, Mitsuru*; Okamoto, Mitsuo*; Yoshigoe, Akitaka; Shimura, Takayoshi*; Watanabe, Heiji*
Japanese Journal of Applied Physics, 61(SC), p.SC1065_1 - SC1065_8, 2022/05
Times Cited Count：7 Percentile：77.62(Physics, Applied)Nitridation of SiO/4H-SiC(110) interfaces with post-oxidation annealing in an NO ambient (NO-POA) and its impact on the electrical properties were investigated. Sub-nm-resolution nitrogen depth profiling at the interfaces was conducted by using a scanning X-ray photoelectron spectroscopy microprobe. The results showed that nitrogen atoms were incorporated just at the interface and that interface nitridation proceeded much faster than at SiO/SiC(0001) interfaces, resulting in a 2.3 times higher nitrogen concentration. Electrical characterizations of metal-oxide-semiconductor capacitors were conducted through capacitance-voltage (
) measurements in the dark and under illumination with ultraviolet light to evaluate the electrical defects near the conduction and valence band edges and those causing hysteresis and shifting of the curves. While all of these defects were passivated with the progress of the interface nitridation, excessive nitridation resulted in degradation of the MOS capacitors. The optimal conditions for NO-POA are discussed on the basis of these experimental findings.
0) MOS devicesNakanuma, Takato*; Kobayashi, Takuma*; Hosoi, Takuji*; Sometani, Mitsuru*; Okamoto, Mitsuo*; Yoshigoe, Akitaka; Shimura, Takayoshi*; Watanabe, Heiji*
Applied Physics Express, 15(4), p.041002_1 - 041002_4, 2022/04
Times Cited Count：5 Percentile：48.5(Physics, Applied)The leakage current and flat-band voltage (VFB) instability of NO-nitrided SiC (110) (a-face) MOS devices were systematically investigated. Although NO nitridation is effective in improving the interface properties, we found that it reduces the onset field of Fowler-Nordheim (F-N) current by about 1 MVcm
, leading to pronounced leakage current. Synchrotron X-ray photoelectron spectroscopy revealed that the nitridation reduces the conduction band offset at the SiO/SiC interface, corroborating the above finding. Furthermore, systematical positive and negative bias stress tests clearly indicated the VFB instability of nitrided a-face MOS devices against electron and hole injection.
Okamoto, Yoshihiro; Osugi, Takeshi; Shiwaku, Hideaki; Akabori, Mitsuo*
Insights Concerning the Fukushima Daiichi Nuclear Accident, Vol.4; Endeavors by Scientists, p.285 - 294, 2021/10
The transfer behavior of cesium adsorbed on some clay minerals in aqueous solution was investigated by X-ray absorption fine structure (XAFS) analysis of the Cs K-edge. The sample was prepared by mixing Cs-adsorbed mineral with a different pure clay mineral in water. The XAFS results of the dried mixture powder were compared with those obtained before mixing. It was recognized from the XAFS analysis for three kinds of clay minerals illite, kaolinite and vermiculite, that cesium was transferred from kaolinite to illite and vermiculite, and from illite to vermiculite. It can be concluded that cesium is transferred to and accumulated in vermiculite.
Okamoto, Yoshihiro; Osugi, Takeshi; Akabori, Mitsuo; Kobayashi, Toru; Shiwaku, Hideaki
Journal of Molecular Liquids, 232, p.285 - 289, 2017/04
Times Cited Count：3 Percentile：12.21(Chemistry, Physical)High energy XAFS measurement using cerium K-edge was performed to study the chemical state of cerium in high-temperature molten slag (SiO-CaO-FeO
-CeO). It was found from the change in the nearest Ce-O distance obtained from EXAFS analysis and the energetic shift of the white line peak observed in XANES analysis that oxidation state of cerium was tetravalent in the molten state and trivalent in solid state. The Debye-Waller factor of the nearest Ce-O pair in solid slag was very large even at room temperature, and the change in its value upon heating and melting was very small. This result suggests that cerium is highly disordered and stable in solid slag.
Okamoto, Yoshihiro; Shiwaku, Hideaki; Nakada, Masami; Komamine, Satoshi*; Ochi, Eiji*; Akabori, Mitsuo
Journal of Nuclear Materials, 471, p.110 - 115, 2016/02
Times Cited Count：6 Percentile：49.29(Materials Science, Multidisciplinary)Extended X-ray Absorption Fine Structure (EXAFS) analyses were performed to evaluate REDOX (REDuction and OXidation) state of platinoid elements in simulated high-level nuclear waste glass samples prepared under different conditions of temperature and atmosphere. At first, EXAFS functions were compared with those of standard materials such as RuO
. Then structural parameters were obtained from a curve fitting analysis. In addition, a fitting analysis used a linear combination of the two standard EXAFS functions of a given elements metal and oxide was applied to determine ratio of metal/oxide in the simulated glass. The redox state of Ru was successfully evaluated from the linear combination fitting results of EXAFS functions. The ratio of metal increased at more reducing atmosphere and at higher temperatures. Chemical form of rhodium oxide in the simulated glass samples was RhO unlike expected RhO
. It can be estimated rhodium behaves according with ruthenium when the chemical form is oxide.
Okamoto, Yoshihiro; Osugi, Takeshi; Shiwaku, Hideaki; Akabori, Mitsuo
Nihon Genshiryoku Gakkai Wabun Rombunshi, 13(3), p.113 - 118, 2014/09
Transfer behavior of cesium adsorbed on some clay minerals in aqueous solution was investigated by X-ray absorption fine structure (XAFS) analysis of Cs K-edge. The sample was prepared by mixing Cs-adsorbed mineral with another different kind of pure clay mineral in water. The XAFS results of the dried mixture powder were compared with those obtained before the mixing. It was recognized from the XAFS analysis for three kinds of clay minerals; Illite, Kaolinite and Vermiculite, that cesium was transferred from Kaolinite to Illite and vermiculite, and from Illite to Vermiculite. It can be concluded that cesium is transferred to and accumulated in Vermiculite.
Umeda, Takahide*; Okamoto, Mitsuo*; Arai, Ryo*; Sato, Yoshihiro*; Kosugi, Ryoji*; Harada, Shinsuke*; Okumura, Hajime*; Makino, Takahiro; Oshima, Takeshi
Materials Science Forum, 778-780, p.414 - 417, 2014/02
Times Cited Count：2 Percentile：72.7(Crystallography)Interface defects of Metal-Oxide-Semiconductors (MOSFETs) fabricated on Carbone (C) face 4H-SiC were investigated by Electrically Detected Magnet Resistance (EDMR). Gate oxide of the MOSFETs was formed by either wet-oxidation and H annealing or dry-oxidation. The values of channel mobility for MOSFETS with wet gate oxide and dry gate oxide are less than 1 and 90 cm
/Vs, respectively. By EDMR measurement under low temperature (less than 20 K), EDMR signals related to C were detected. The peak height of the signals increased with increasing 
-ray doses, and the channel mobility decreased. From this result, it is assumed that hydrogen atoms passivating C dangling bonds are released by -rays and the channel mobility decreases with increasing the C related defects.
Okamoto, Yoshihiro; Nakada, Masami; Akabori, Mitsuo; Komamine, Satoshi*; Fukui, Toshiki*; Ochi, Eiji*; Nitani, Hiroaki*; Nomura, Masaharu*
Denki Kagaku Oyobi Kogyo Butsuri Kagaku, 81(7), p.543 - 546, 2013/07
Times Cited Count：7 Percentile：17.93(Electrochemistry)The molten state of simulated high-level waste glass and the behavior of ruthenium element in the melt were investigated by using synchrotron radiation based X-ray imaging technique. Melting, generating and moving of bubbles, condensation and sedimentation of ruthenium element were observed dynamically in continuous 12-bit gray-scale images from the CCD camera. X-ray intensity was obtained easily by digitizing gray-scale values in the image. The existence of ruthenium element is emphasized as a black color in the CCD image at X-ray energy higher than the Ru K-absorption edge. Position sensitive imaging X-ray absorption fine structure (XAFS) measurement was also performed to clarify the chemical state of ruthenium element in the melt.
Okamoto, Yoshihiro; Nakada, Masami; Akabori, Mitsuo; Komamine, Satoshi*; Fukui, Toshiki*; Ochi, Eiji*; Nitani, Hiroaki*; Nomura, Masaharu*
Proceedings of 4th Asian Conference on Molten Salt Chemistry and Technology & 44th Symposium on Molten Salt Chemistry, Japan, p.47 - 52, 2012/09
The molten state of the simulated high-level waste glass and the behavior of ruthenium element in the melt were investigated by using synchrotron radiation based X-ray imaging technique. Melting, generating and moving of bubbles, condensation and sedimentation of ruthenium element were observed dynamically in continuous 12-bit gray-scale images from the CCD camera. The existence of ruthenium in the X-ray CCD image was emphasized over the energy of Ru K-absorption edge. X-ray intensity was obtained easily by digitalizing gray-scale values in the image. Position sensitive imaging X-ray absorption fine structure (XAFS) measurement was performed to clarify the chemical state of ruthenium element in the melt.
Okamoto, Yoshihiro; Nakada, Masami; Akabori, Mitsuo; Shiwaku, Hideaki; Komamine, Satoshi*; Fukui, Toshiki*; Ochi, Eiji*; Nitani, Hiroaki*; Nomura, Masaharu*
Nihon Genshiryoku Gakkai Wabun Rombunshi, 11(2), p.127 - 132, 2012/06
Distribution and the chemical state of Ru element in the simulated high-level waste glass were examined by using the synchrotron radiation based X-ray imaging technique. In this technique, a direct X-ray CCD camera is used in place of an ion chamber. Position sensitive X-ray absorption spectra were obtained by analyzing gray scale in images of the X-ray CCD camera. At first, we measured a test sample containing RuO and Ru metal powder. We successfully obtained information on the Ru distribution in the sample. In addition, the chemical state (oxide or metal ?) of each small Ru-rich spot was evaluated by the corresponding position sensitive XAFS spectrum. The imaging XAFS technique was applied to some simulated high-level waste glass samples. The Ru distribution of the glass sample and their chemical state were confirmed by image analyses. It can be seen that Ru element scattered in the glass sample exists as oxide RuO.
Am
)O
Nishi, Tsuyoshi; Nakada, Masami; Suzuki, Chikashi; Shibata, Hiroki; Okamoto, Yoshihiro; Akabori, Mitsuo; Hirata, Masaru
Journal of Nuclear Materials, 418(1-3), p.311 - 312, 2011/11
Times Cited Count：20 Percentile：81.61(Materials Science, Multidisciplinary)The XAFS measurements at U-L and Am-L absorption edge of (UAm)O were performed in transmission mode. Moreover, to clarify the valence state of Am in (U,Am)O
, the XANES spectrum of Am-L absorption edge of (UAm)O was verified using those of Am-L absorption edge of AmO and AmO. It was found that the XANES spectrum of the Am-L edge of (UAm)O is in good accordance with that of AmO. Thus, Am in (UAm)O is almost trivalent state.
Umino, Akira; Saito, Mitsuo; Kanazawa, Hiroyuki; Koya, Toshio; Okamoto, Hisato; Sekino, Hajime*; Nishino, Yasuharu
Dekomisshoningu Giho, (32), p.2 - 12, 2005/09
The Research Hot Laboratory (RHL) in Japan Atomic Research Institute (JAERI) was constructed in 1961, as the first one in JAPAN, to perform the examinations of irradiated fuels and materials. RHL with two floors and a basement consists of 10 heavy concrete cells, and 38 lead cells (20 lead cells at present). The RHL had been contributed to research program in JAERI. However, RHL is the one of target 'A middle-range decommissioning plan for the facility in Tokai Research Establishment' as the rationalization program for decrepit facilities in JAERI. Therefore, all PIEs had been finished in March 2003 and the dismantling works of hot cells have been started. The 18 lead cells had been dismantled. The examinations performed in RHL will be succeeded to the RFEF and the WASTEF. The partial area of RHL facility will be used for the temporary storage of un-irradiated fuel samples used for our previous research works and radioactive device generated in proton accelerator facility (called J-PARC).
Okamoto, Yoshihiro; Akabori, Mitsuo; Ito, Akinori; Ogawa, Toru
Journal of Nuclear Science and Technology, 39(Suppl.3), p.638 - 641, 2002/11
We report local structural features of molten UCl with LiCl-KCl eutectic probed by the U L-edge XAFS(X-ray absorption fine structure). The XAFS measurements were performed in a transmission mode at the BL27B station of the Photon Factory(High Energy Accelerator Organization, Tsukuba, JAPAN). Sample prepared by chlorination of uranium hydride and then reduction with zinc powder was sealed in a quartz cell under reduced pressure. The nearest U
-Cl
distance and the coordination number of Cl around U ion were obtained by a curve fitting of the 1st shell XAFS function k
(k). The pair potential in the U-Cl correlation was evaluated from XAFS simulation by combinational use of the MD and the FEFF8. In addition, valence state of uranium in the melt was evaluated by XANES(X-ray absorption near edge structure) spectra.
Okamoto, Yoshihiro; Akabori, Mitsuo; Motohashi, Haruhiko*; Ito, Akinori; Ogawa, Toru
Nuclear Instruments and Methods in Physics Research A, 487(3), p.605 - 611, 2002/07
Times Cited Count：27 Percentile：83.24(Instruments & Instrumentation)Measurement system for the high temperature XAFS was developed for investigating the local structure of hygroscopic molten salts like rare earth halides. Solid sample was enclosed in the upper tank of the quartz cell having a sandglass shape under reduced pressure to avoid oxygen and moisture. The measurement was carried out at the electric furnace of the highest arrival temperature 1273K. After melting, the sample runs down through the melt path with 0.1mm(or 0.2mm) thickness to the lower tank. The minimum energy which could be measured was about 10keV mainly due to the absorption of the quartz. It was confirmed that the measurement of the expensive hygroscopic sample became possible in the present work.
Takano, Masahide; Ito, Akinori; Akabori, Mitsuo; Ogawa, Toru; Numata, Masami; Okamoto, Hisato
Journal of Nuclear Materials, 294(1-2), p.24 - 27, 2001/04
Times Cited Count：13 Percentile：66.91(Materials Science, Multidisciplinary)no abstracts in English
Xe ionsWatanabe, Satoshi; Osa, Akihiko; Sekine, Toshiaki; Ishioka, Noriko; Koizumi, Mitsuo; Kojima, Takuji; Hasegawa, A.*; Yoshii, M.*; Okamoto, E.*; Aoyagi, K.*; et al.
Applied Radiation and Isotopes, 51(2), p.197 - 202, 1999/08
Times Cited Count：7 Percentile：49.7(Chemistry, Inorganic & Nuclear)no abstracts in English
Kobayashi, Fumiaki; Ogawa, Toru; Okamoto, Yoshihiro; Akabori, Mitsuo
Journal of Alloys and Compounds, 271-273, p.374 - 377, 1998/00
Times Cited Count：8 Percentile：52.63(Chemistry, Physical)no abstracts in English
compoundAkabori, Mitsuo; R.G.Haire*; J.K.Gibson*; Okamoto, Yoshihiro; Ogawa, Toru
Journal of Alloys and Compounds, 257, p.268 - 272, 1997/00
Times Cited Count：2 Percentile：29.46(Chemistry, Physical)no abstracts in English
Akabori, Mitsuo; R.G.Haire*; J.K.Gibson*; Okamoto, Yoshihiro; Ogawa, Toru
Journal of Nuclear Materials, 247, p.240 - 243, 1997/00
Times Cited Count：2 Percentile：23.04(Materials Science, Multidisciplinary)no abstracts in English
