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Journal Articles

Impact of post-nitridation annealing in CO$$_{2}$$ ambient on threshold voltage stability in 4H-SiC metal-oxide-semiconductor field-effect transistors

Hosoi, 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:30.40(Physics, Applied)

The combination of NO annealing and subsequent post-nitridation annealing (PNA) in CO$$_{2}$$ ambient for SiO$$_{2}$$/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$$_{2}$$ side of the SiO$$_{2}$$/SiC interface incorporated by NO annealing, which are plausible cause of charge trapping sites, could be selectively removed by CO$$_{2}$$-PNA at 1300$$^{circ}$$C without oxidizing the SiC. CO$$_{2}$$-PNA was also effective in compensating oxygen vacancies in SiO$$_{2}$$, resulting high immunity against both positive and negative bias-temperature stresses.

Journal Articles

Impact of nitridation on the reliability of 4H-SiC(11$$bar{2}$$0) MOS devices

Nakanuma, 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:6 Percentile:69.05(Physics, Applied)

The leakage current and flat-band voltage (VFB) instability of NO-nitrided SiC (11$$bar{2}$$0) (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$$^{-1}$$, leading to pronounced leakage current. Synchrotron X-ray photoelectron spectroscopy revealed that the nitridation reduces the conduction band offset at the SiO$$_{2}$$/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.

Journal Articles

High reactivity of H$$_{2}$$O vapor on GaN surfaces

Sumiya, Masatomo*; Sumita, Masato*; Tsuda, Yasutaka; Sakamoto, Tetsuya; Sang, L.*; Harada, Yoshitomo*; Yoshigoe, Akitaka

Science and Technology of Advanced Materials, 23(1), p.189 - 198, 2022/00

 Times Cited Count:4 Percentile:46.76(Materials Science, Multidisciplinary)

GaN is an attracting material for power-electronic devices. Understanding the oxidation at GaN surface is important for improving metal-oxide-semiconductor (MOS) devices. In this study, the oxidation at GaN surfaces depending on the GaN crystal planes (+c, -c, and m-plane) was investigated by real time XPS and DFT-MD simulation. We found that H$$_{2}$$O vapor has the highest reactivity due to the spin interaction between H$$_{2}$$O and GaN surfaces. The bond length between the Ga and N on the -c GaN surface was increased by OH attacking the back side of three-fold Ga atom. The chemisorption on the m-plane was dominant. The intense reactions of oxidation and Al$$_{x}$$Ga$$_{1-x}$$N formation for p-GaN were observed at the interface of the Al$$_{2}$$O$$_{3}$$ layer deposited by ALD using H$$_{2}$$O vapor. This study suggests that an oxidant gas other than H$$_{2}$$O and O$$_{2}$$ should be used to avoid unintentional oxidation during Al$$_{x}$$Ga$$_{1-x}$$N atomi layer deposition.

Journal Articles

Exploring the development of cesium removal processes via nanoscale chemical state analysis of cesium-containing clay minerals; Application of soft X-ray synchrotron radiation photoelectron microscopy to observe insulating materials

Yoshigoe, Akitaka

Hoshako Riyo No Tebiki, p.130 - 138, 2019/02

The educational book on next-generation synchrotron radiation usage is partially written. This book is being planned by several professors at Tohoku University. In this book, the contents which have already reported in our recent paper [Appl. Phys. Lett. 112 (2018) 021603.] are mainly written. The future perspective on photoemission electron microscopy using next generation synchrotron radiation and its possibility to analyze environmental samples and insulating functional materials are also described.

Journal Articles

Biodegradability of disulfide-organosilica nanoparticles evaluated by soft X-ray photoelectron spectroscopy; Cancer therapy implications

Mekaru, Harutaka*; Yoshigoe, Akitaka; Nakamura, Michihiro*; Doura, Tomohiro*; Tamanoi, Fuyuhiko*

ACS Applied Nano Materials (Internet), 2(1), p.479 - 488, 2019/01

 Times Cited Count:40 Percentile:84.08(Nanoscience & Nanotechnology)

Organosilica nanoparticles are attractive for use as drug delivery systems for cancer therapy. Ideally, the nanoparticles need to degrade in the body after drug delivery to minimize their risk. However, the biodegradation of organosilica nanoparticles has not been thoroughly investigated. In this study, we used X-ray photoelectron spectroscopy and field-emission scanning electron microscopy to investigate the degradation of organosilica nanoparticles by glutathione, a peptide found in cells. Our results indirectly indicate that glutathione is able to reduce the disulfide bonds present in the network of one type of the nanoparticles, promoting dissociation of these nanoparticles. The dissociated nanoparticles form large maple leaf-shaped structures under certain conditions. We consider the formation mechanism of these symmetrical structures.

Journal Articles

Current status and future developments of hard X-ray photoelectron spectroscopy

Kobata, Masaaki; Okane, Tetsuo; Kobayashi, Keisuke*

Bunko Kenkyu, 67(4), p.161 - 162, 2018/08

We introduce hard X-ray photoelectron spectroscopy, which has been rapidly introduced and developed in synchrotron radiation facilities. In particular, in order to realize electronic state analysis by hard X-ray photoelectron spectroscopy of insulators, the developed charge neutralization method was described. As an example, we showed adsorption behavior of cesium to nuclear reactor structure assuming Fukushima Daiichi Nuclear Power Plant accident. Finally, future prospects of hard X-ray photoelectron spectroscopy will be described.

Journal Articles

SiO$$_{2}$$/AlON stacked gate dielectrics for AlGaN/GaN MOS heterojunction field-effect transistors

Watanabe, Kenta*; Terashima, Daiki*; Nozaki, Mikito*; Yamada, Takahiro*; Nakazawa, Satoshi*; Ishida, Masahiro*; Anda, Yoshiharu*; Ueda, Tetsuzo*; Yoshigoe, Akitaka; Hosoi, Takuji*; et al.

Japanese Journal of Applied Physics, 57(6S3), p.06KA03_1 - 06KA03_6, 2018/06

 Times Cited Count:10 Percentile:45.14(Physics, Applied)

The advantage of SiO$$_{2}$$/AlON stacked gate dielectrics over SiO$$_{2}$$, AlON and Al$$_{2}$$O$$_{3}$$ single dielectric layers was demonstrated. Our systematic research revealed that the optimized stacked structure with 3.3-nm-thick AlON interlayer is beneficial in terms of superior interface quality, reduced gate leakage current and C-V hysteresis for next-generation high frequency and high power AlGaN/GaN MOS-HFETs.

Journal Articles

Implementation of atomic layer deposition-based AlON gate dielectrics in AlGaN/GaN MOS structure and its physical and electrical properties

Nozaki, Mikito*; Watanabe, Kenta*; Yamada, Takahiro*; Shih, H.-A.*; Nakazawa, Satoshi*; Anda, Yoshiharu*; Ueda, Tetsuzo*; Yoshigoe, Akitaka; Hosoi, Takuji*; Shimura, Takayoshi*; et al.

Japanese Journal of Applied Physics, 57(6S3), p.06KA02_1 - 06KA02_7, 2018/06

 Times Cited Count:19 Percentile:66.48(Physics, Applied)

We fabricated AlON dielectric films by repeating thin AlN deposition and in situ O$$_{3}$$ oxidation for AlGaN/GaN MOS-HFETs. Uniform nitrogen distribution is achievable by the proposed ALD-based process and that nitrogen concentration can be precisely controlled by changing AlN thickness (ALD cycle number) in each step. It was found that AlON films grown by ALD system offers significant advantages in terms of practical application while keeping superior Vth stability and electrical properties at the insulator/AlGaN interface in AlGaN/GaN MOS-HFETs.

Journal Articles

Physical and electrical characterizations of AlGaN/GaN MOS gate stacks with AlGaN surface oxidation treatment

Yamada, Takahiro*; Watanabe, Kenta*; Nozaki, Mikito*; Shih, H.-A.*; Nakazawa, Satoshi*; Anda, Yoshiharu*; Ueda, Tetsuzo*; Yoshigoe, Akitaka; Hosoi, Takuji*; Shimura, Takayoshi*; et al.

Japanese Journal of Applied Physics, 57(6S3), p.06KA07_1 - 06KA07_6, 2018/06

 Times Cited Count:6 Percentile:29.42(Physics, Applied)

Thermal oxidation of AlGaN surface and its impact on the electrical properties of AlGaN/GaN MOS capacitors were investigated by means of synchrotron radiation photoelectron spectroscopy (SR-PES), atomic force microscopy (AFM) and C-V measurements. SR-PES analysis revealed that the AlGaN surface is oxidized even at low temperature of 400$$^{circ}$$C, in contrast to no oxide formation on GaN surface. However, since no noticeable change in the surface morphology was observed at temperatures up to 800$$^{circ}$$C, it can be concluded that an ultrathin oxide overlayer is formed on the AlGaN surface. On the other hand, for the oxidation treatments above 850$$^{circ}$$C, the formation of small oxide grains was observed over the entire area of the AlGaN surface, and the growth of oxide grains significantly degraded the surface morphology. Therefore, the AlGaN/GaN MOS capacitors were fabricated on the AlGaN surface oxidized at moderate temperatures up to 800$$^{circ}$$C. While we have confirmed that relatively good interface properties are obtained for direct AlON deposition without oxidation treatment, it was found that the oxidation treatment at 400$$^{circ}$$C leads to further improvement of interface properties and reduction of C-V hysteresis.

Journal Articles

Control of Ga-oxide interlayer growth and Ga diffusion in SiO$$_{2}$$/GaN stacks for high-quality GaN-based metal-oxide-semiconductor devices with improved gate dielectric reliability

Yamada, Takahiro*; Watanabe, Kenta*; Nozaki, Mikito*; Yamada, Hisashi*; Takahashi, Tokio*; Shimizu, Mitsuaki*; Yoshigoe, Akitaka; Hosoi, Takuji*; Shimura, Takayoshi*; Watanabe, Heiji*

Applied Physics Express, 11(1), p.015701_1 - 015701_4, 2018/01

 Times Cited Count:39 Percentile:84.39(Physics, Applied)

A simple and feasible method for fabricating high-quality and highly reliable GaN-based metal-oxide-semiconductor (MOS) devices was developed on the basis of systematic physical and electrical characterizations. Chemical vapor deposition of SiO$$_{2}$$ films directly onto GaN substrates forming Ga-oxide interlayers was used to fabricate SiO$$_{2}$$/GaO$$_{x}$$/GaN stacked structures. Although well-behaved hysteresis-free GaN-MOS capacitors with extremely low interface state density below 10$$^{10}$$cm$$^{-2}$$eV$$^{-1}$$ were obtained by post-deposition annealing, Ga diffusion into overlying SiO$$_{2}$$ layers severely degraded the insulating property and dielectric breakdown characteristics of the MOS devices. However, this problem was found to be solved by employing rapid thermal processing, leading to superior performance of the GaN-MOS devices in terms of interface quality, insulating property and gate dielectric reliability.

Journal Articles

Nanoscale spatial analysis of clay minerals containing cesium by synchrotron radiation photoemission electron microscopy

Yoshigoe, Akitaka; Shiwaku, Hideaki; Kobayashi, Toru; Shimoyama, Iwao; Matsumura, Daiju; Tsuji, Takuya; Nishihata, Yasuo; Kogure, Toshihiro*; Okochi, Takuo*; Yasui, Akira*; et al.

Applied Physics Letters, 112(2), p.021603_1 - 021603_5, 2018/01

 Times Cited Count:6 Percentile:29.42(Physics, Applied)

A synchrotron radiation photoemission electron microscope (SR-PEEM) was applied to demonstrate pinpoint analysis of micrometer-sized weathered biotite clay particles with artificially adsorbed cesium (Cs) atoms. Despite the insulating properties of the clay, we observed the spatial distributions of constituent elements (Si, Al, Cs, Mg, Fe) without charging issues. We found that Cs atoms were likely to be adsorbed evenly over the entire particle. Spatially-resolved X-ray absorption spectra (XAS) of the Cs M$$_{4,5}$$-edge region showed Cs to be present in a monocation state (Cs$$^{+}$$). Further pinpoint XAS measurements were also performed at the Fe L$$_{2,3}$$-edge to determine the chemical valence of the Fe atoms. Our results demonstrate the utility of SR-PEEM as a tool for spatially-resolved chemical analyses of various environmental substances, which is not limited by the poor conductivity of samples.

Journal Articles

Chemical state analysis with soft-X-ray emission spectroscopy based on SEM

Terauchi, Masami*; Imazono, Takashi; Koike, Masato

Hyomen Kagaku, 36(4), p.184 - 188, 2015/04

Electron beam induced soft-X-ray emission spectroscopy (SXES) that uses a grating spectrometer has been introduced to a conventional scanning electron microscope (SEM) for characterizing desired specimen areas of bulk materials. The spectrometer has a grazing-incidence flat-field optics by using aberration corrected (varied-line-spacing) gratings, which has already been applied to transmission electron microscopes. The best resolution was confirmed as 0.13 eV at Mg L-emission (50 eV), which is comparable with that of recent dedicated electron energy-loss spectroscopy instruments. Apparent band structure effects have been observed in Mg-L, Si-L, B-K, and Ti-L emission spectra obtained from bulk materials using SEM-SXES instrument.

Journal Articles

Characterization of surface carbon films on weathered Japanese roof tiles by soft X-ray spectroscopy

Muramatsu, Yasuji; Yamashita, Michiru*; Motoyama, Muneyuki*; Hirose, Mika*; Denlinger, J. D.*; Gullikson, E. M.*; Perera, R. C. C.*

X-Ray Spectrometry, 34(6), p.509 - 513, 2005/11

 Times Cited Count:1 Percentile:5.66(Spectroscopy)

Surface carbon films on the weathered Japanese roof tiles were characterized by soft X-ray spectroscopy. From the X-ray absorption measurements, it was confirmed that the surface carbon films were oxidized by weathering. On the ohterhand, from the X-ray emission measurements, it can be confirmed that the degree of the orientation was kept in the inner carbon films.

Journal Articles

Rapid and quantitative electrolytic preparation and speciation of neptunium ions of various oxidation states using multi-step column electrodes

Aoyagi, Hisao*; Kitatsuji, Yoshihiro; Yoshida, Zenko; Kihara, Sorin*

Analytica Chimica Acta, 538(1-2), p.283 - 289, 2005/05

 Times Cited Count:15 Percentile:41.49(Chemistry, Analytical)

Redox behavior of Np(III), (IV), (V) and (VI) ions in aqueous perchloric, nitric and sulphuric acid solutions was elucidated by using column electrodes connected in series in a flow system. Using a glassy carbon fiber working electrode as the column electrode was found to be very useful for the investigation of current-potential relations of not only reversible redox processes such as Np(VI)/(V) or Np(IV)/(III) but also irreversible processes such as Np(V)/(IV) or Np(V)/(III), the latter not observed by conventional voltammetry which uses a glassy carbon or platinum electrode. Quantitative electrolysis could be executed even if the redox process was completely irreversible by the use of the column electrodes. By taking advantage of column electrode electrolysis, a novel method was developed for the rapid preparation of a neptunium species of a desired oxidation state, including unstable species. The multi-step column electrode system was demonstrated to be useful for the coulometric determination and speciation of Np(IV), (V) and (VI) in aqueous HNO$$_{3}$$ solution as an example.

Journal Articles

Chemical analysis of carbon films on Japanese smoked roof tile "Ibushi Kawara" by soft X-ray spectroscopy using synchrotron radiation

Muramatsu, Yasuji

Rigaku Janaru, 36(1), p.35 - 42, 2005/04

Microstructure and weathering effects of carbon films on the Japanese smoked roof tile "Ibushi-Kawara" were investigated by soft X-ray spectroscopy using synchrotron radiation. The take-off/incident-angle dependence on the X-ray emission/absorption spectra of Kawara shows that the carbon-black-like sp2 carbon atoms partially form a layer structure oriented parallel to the basal clay plane, and that the degree of orientation of the carbon films is estimated to be 50% that of highly oriented pyrolytic graphite (HOPG). X-ray absorption measurements revealed that weathering oxidizes the carbon films and partial carboxy chemical bonding occurs.

Journal Articles

Research trends in synchrotron-radiation soft X-ray spectroscopy for chemical analysis

Muramatsu, Yasuji

Advances in X-Ray Chemical Analysis, Japan, 36, p.47 - 62, 2005/03

Research trends in soft X-ray absorption, electron, and soft X-ray emission spectroscopies using synchrotron radiation for chemical analysis are briefly explained. Measurements in the higher-energy region over 2 keV, liquid sample measurements, and MCD measurements have recently been achieved in the soft X-ray absorption spectroscopy. In electron spectroscopy, high-resolution measurements and bulk-sensitive measurements using higher-energy excitation have been achieved. In soft X-ray emission spectroscopy, higher-resolution X-ray spectrometers have been developed and liquid-sample measurements have also been achieved. In addition, chemical analysis of surface carbon films on Japanese smoked roof tiles and electronic structure analysis of boron-doped diamond are demonstrated as the successful analytical studies using soft X-ray spectroscopy.

Journal Articles

Aqueous solutions of uranium(VI) as studied by time-resolved emission spectroscopy; A Round-robin test

Billard, I.*; Ansoborlo, E.*; Apperson, K.*; Arpigny, S.*; Azenha, M.-E.*; Birch, D.*; Bros, P.*; Burrows, H. D.*; Choppin, G. R.*; Couston, L.*; et al.

Applied Spectroscopy, 57(8), p.1027 - 1038, 2003/08

 Times Cited Count:50 Percentile:88.06(Instruments & Instrumentation)

no abstracts in English

Journal Articles

New prompt $$gamma$$-ray analysis using neutron resonance absorption

Matsue, Hideaki

Bunseki, 2002(11), P. 645, 2002/11

no abstracts in English

JAEA Reports

Measurement of neodymium(III) and samarium(III) by spectrophotometer and laser-induced photoacoustic spectroscopic system: A Preliminary study for speciation of aqueous Np(IV) species.

Kitamura, Akira;

JNC TN8400 2001-009, 54 Pages, 2001/01

JNC-TN8400-2001-009.pdf:1.3MB

Spectroscopic measurements of neodymium(III) and samarium(III) were carried out by spectrophotometer and laser-induced photoacoustic spectroscopic (LPAS) system for the investigation of the detection limit of both systems. The absorption spectra and photoacoustic spectra of Nd$$^{3+}$$ and Sm$$^{3+}$$ were obtained with varying the concentration of the ions from 2$$times$$10$$^{-5}$$ to 2$$times$$10$$^{-2}$$ mol$$cdot$$dm$$^{+3}$$. The absorption spectrum of Nd$$^{3+}$$ was also determined by a special spectrophotometer, of which the measurement cell was set in a glove box filled with inert nitrogen gas. For the comparison with these photoacoustic and absorption spectra, the absorption spectra of Nd$$^{3+}$$ and Sm$$^{3+}$$ were determined by an usual spectrophotometer with the light-path lengths of 1 cm and 10 cm. The detection limit of the photoacoustic measurement was reported much lower than that of absorbance measurement by several researchers. However, the present study was concluded that the detection limit of photoacoustic measurement with the present LPAS system was similar to that of absorbance measurement with the light-path length of 10 cm. The detection limits of neptunium(IV,V) were estimated and the possibility of the speciation of neptunium(IV) was discussed from the results of the present study.

Journal Articles

Determination of the axial distribution of CuO$$^{+}$$ or Cu$$^{+}$$ in a superconducting disk sample by flow-coulometry

Sasaki, Yuji

Superconductors, Surfaces and Superlattices (Trans. of Materials Research Soc. Jpn., Vol. 19A), 0, p.301 - 304, 1994/00

no abstracts in English

28 (Records 1-20 displayed on this page)