K. Kojima

National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, Japan

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Publications (188)257.95 Total impact

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    ABSTRACT: We have systematically investigated the channel mobility of the 4H-SiC MOSFETs with surface and buried channels in different directions on Si-, a-, and m-faces. We have found that, on a- and m-faces, the in-plane anisotropy of the surface channel mobility is different from that of the buried channel mobility. It has been demonstrated that this anomalous anisotropic behavior can be observed regardless of the gate oxide processes. These results suggest that the dependence of scattering mechanisms or effective masses of carriers on crystal orientations may be modulated near the oxide/SiC interface.
    No preview · Article · Mar 2015 · Applied Physics Letters
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    ABSTRACT: Valence-band and Yb 4d photoemission spectra of YbXCu4 (X=In, Cd, Mg) have been measured with high energy resolution from 10 to 300 K. In the valence-band photoemission spectra of YbInCu4 and YbCdCu4, the structure due to the Yb2+4f7/2 states is clearly observed near the Fermi level (EF) at 10 K. The intensity of the Yb2+4f7/2 structure decreases with increasing temperature and the structure almost disappears at 300 K. The amount of the enhancement from 50 to 107 K is much stronger for YbInCu4 than for YbCdCu4. On the other hand, the Yb2+4f7/2 structure of YbMgCu4 is observed as a broad peak near EF and the spectra show little temperature dependence. In the Yb 4d photoemission spectra of YbInCu4 and YbCdCu4, the structures due to the Yb2+ and Yb3+ states are recognized. The intensity ratio Yb2+/Yb3+ increases with decreasing temperature. In the Yb 4d spectra of YbMgCu4, on the other hand, almost only structures due to Yb2+ states are observed.
    No preview · Article · Jan 2012 · Surface Review and Letters
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    ABSTRACT: We have carried out hard X-ray photoemission spectroscopy on YbCdCu4 and YbSnCu4 at hν∼6keV in order to investigate electronic structure, and compared the results with that of the valence transition compound YbInCu4. Based on fitting analyses of the Yb 3d spectra, the Yb valences of YbCdCu4 and YbSnCu4 are estimated to be 2.96±0.01 and 2.86±0.01 at 300K, respectively, and the valences slightly decrease becoming closer to divalent on cooling. The Cu 2p3/2 level does not follow the chemical trend on going from YbCdCu4, to YbInCu4, and to YbSnCu4. The lowest binding energy of the Cu 2p3/2 level for YbInCu4 suggests that the large number of conduction electrons distributes over the Cu site in YbInCu4 and the excess conduction electrons play an important role for the valence transition of YbInCu4.
    No preview · Article · Apr 2011 · Fuel and Energy Abstracts
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    ABSTRACT: The electric field dependence and anisotropy of the impact ionization coefficients of 4H-SiC are investigated by means of the avalanche breakdown behavior of p+n diodes. The breakdown voltages as a function of doping density and the multiplication factors of a leakage current are obtained using p+n diode fabricated on (0001) and (1120) 4H-SiC epitaxial wafers. The obtained impact ionization coefficients show large anisotropy; the breakdown voltage of a p+n diode on (1120) wafer is 60% of that on (0001) wafer. We have shown that anisotropy of the impact ionization coefficients is attributable to the anisotropy of saturation velocity originated from the electronic structure of 4H-SiC.
    No preview · Article · Jan 2011 · MRS Online Proceeding Library
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    ABSTRACT: The power conversion efficiency of a polysilicon solar cell is nearly 20%, while that of an amorphous silicon solar cell exceeds 15%. However, as silicon material is inorganic, the simple fabrication of a flexible, lightweight and inexpensive inorganic solar cell is difficult. These limitations of inorganic materials have fuelled active study and considerable progress in the domain of organic thin film solar cells on a global scale. After a bulk-hetero junction structure was introduced in an organic thin film solar cell, one of the latter with power conversion efficiency exceeding 6% was reported. In this study, ethanol, a weak solvent, was added to the 0.01w%P3HT, the 0.01w%PCBM, and the mixed solution of the 0.01wt%P3HT respectively, all of which were dissolved in chloroform. From thin films fabricated by these solutions, P3HT grains and PCBM aggregations are observed with, and evaluated by AFM images and UV-vis spectra.
    No preview · Article · Apr 2010 · Proceedings of SPIE - The International Society for Optical Engineering
  • K. Sakai · K. Kojima · T. Mizutani · S. Ochiai
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    ABSTRACT: Organic solar cells have been attracting attention due to their economic and lightweight nature that facilitates processing. However, they also have low power conversion efficiency and short lifespans. Therefore, with practical organic solar cells in mind, solving these problems is important. To improve the power conversion efficiency of organic solar cells, the organic solar cell performance of poly(3-hexylthiophene-2,5-diyl)(P3HT)/PCBM thin film using poly(3-ocxylthiophene) was studied with the bulk heterojunction structure. It was shown that a red shift occurred by adding P3OT to the P3HT/PCBM mixture film at the absorption wavelength area of the active layer. This indicates that the formation of the P3HT grain and the aggregate of the PCBM molecule occur because of the P3OT addition. In the heat-treatment sample, it also emerges that the heat-treatment has an effect similar to the P3OT addition because it causes red shifts in both the P3HT/PCBM and P3OT/P3HT/PCBM thin films. This indicates that the formation of the P3HT grain and the aggregate of the PCBM molecule occur because of the P3OT addition. In the heat-treatment sample, the heat-treatment is understood to have an effect similar to the P3OT addition because it causes red shifts in both the P3HT/PCBM and P3OT/P3HT/PCBM thin film. Under preparatory conditions without thermal or solvent annealing, the power conversion efficiency, short circuit current, and open voltage and fill factor are 0.6%, Isc=3mA/cm2, Voc=0.7V and 0.28, respectively.
    No preview · Article · Apr 2010 · Proceedings of SPIE - The International Society for Optical Engineering
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    ABSTRACT: We experienced a case where the radioactive seed migrated to the right ventricle after 125I brachytherapy for prostate cancer. We could not estimate seed migration to either the coronary artery or right ventricle on non-CE conventional CT and chest X-ray. However, on coronary CT using 64 MDCT, we diagnosed the correct region of migration. It is considered important to evaluate the coronary artery using 64 or more MDCT synchronized with the electrocardiogram, if the seed exists at the level of the left lower lung on chest X-ray.
    No preview · Article · Jun 2009 · Journal of JASTRO
  • K Kojima · K Nose · M Kambara · T Yoshida
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    ABSTRACT: We have achieved in situ magnesium doping into nanocrystalline cubic boron nitride thin films during sputter deposition. It was clarified, through x-ray diffraction, Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy, that less than 1 at% magnesium doping does not significantly affect the growth of the cubic phase. Magnesium-doped films deposited in pure argon showed electric conductivity up to 104 times higher than the undoped film. Although the Hall voltage was not reproduced reliably for most of the cases, the films containing 2.1 at% magnesium clearly revealed p-type conduction with carrier concentration and mobility of 4 × 1014 cm−3 and 6 cm2 V−1 s−1, respectively, at 380 K. It was also found that the increase in electric conductivity with magnesium concentration was accompanied by the decrease in the relative amount of nitrogen in the film.
    No preview · Article · Feb 2009 · Journal of Physics D Applied Physics
  • S. Ochiai · J. Ramajothi · K. Kojima · T. Mizutani
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    ABSTRACT: The third-order nonlinear optical properties of regioregular poly(3-hexylthiophene) (RR-P3HT) thin films prepared on fused glass substrate were evaluated. The surface modification by hexamethyldisilazane (HMDS) and octadecyltrichlorosilane (ODTS) was performed on the glass substrate to enhance the optical property of the RR-P3HT thin films. The third-order optical susceptibilities chi(3) (-3omega; omega, omega, omega) of spin coated P3HT thin films on quartz glass substrate were estimated from optical third-harmonics (TH) intensity measurement using the Maker-fringe method. The orientation, alignment and crystallinity of the RR-P3HT thin films were evaluated using X-ray diffraction and UV-vis absorption spectra. An Nd:YAG laser with a wavelength of 1064 nm, a pulse width of 5 ns and a repetition frequency of 10 Hz was used to evaluate the TH intensity. The effect of surface modification of quartz glass substrate by HMDS and ODTS on the RR-P3HT film structure was also investigated. The third-order nonlinear optical susceptibility (chi(3)) of RR-P3HT thin films on quartz glass surface modified by ODTS is 1.90 times 10-8 esu.
    No preview · Article · Sep 2008
  • J. Ramajothi · S. Ochiai · K. Kojima · T. Mizutani
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    ABSTRACT: Organic field effect transistors (OFETs) were fabricated with high-kappa titanium dioxide (TiO2) as gate insulator and regioregular poly(3-hexylthiophene-2,5-diyl) (RR-P3HT) as electronically active semiconductor. The dielectric material was prepared by sol-gel method and the gate insulator layer was fabricated by spin-coating method. The thickness of the thin films was measured using surface profile measuring system. The fabricated thin films structure was analyzed by atomic force microscopy (AFM), X-ray diffraction (XRD) and UV-visible absorption spectra. The X-ray result shows that the drop-cast RR-P3HT thin film has high crystallinity on the TiO2 surface and leads to high field effect mobility of the OFET. Good characteristics performances were obtained with low threshold voltage (+3 V) and the field effect mobility is 3.73 times 10-3 cm2/Vs.
    No preview · Article · Sep 2008
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    ABSTRACT: We have investigated the electronic structure of the optimally doped trilayer cuprate superconductor Bi2Sr2Ca2Cu3O10+δ by angle-resolved photoemission spectroscopy. We have found that the d-wave like superconducting gap opens below Tc and the pseudogap in the antinodal region persists at slightly above Tc, consistent with the previous report. The energy position of the kink in the quasi-particle dispersion was nearly isotropic below Tc, but became momentum dependent above Tc, consistent with the scenario that the antinodal states are coupled to the B1g phonon mode and the nodal states to the half-breathing mode.
    No preview · Article · Apr 2008 · Journal of Physics Conference Series
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    ABSTRACT: Low-energy excited photoemission spectroscopy (LEPES) has been carried out at hν = 7 eV for the Y-doped YbInCu4, which exhibits a valence transition with the low Y concentration. In the LEPES spectra of YxYb1−xInCu4 (x = 0.1, 0.2), the structure appears at 20–60 meV below the valence transition temperature (TV) and its energy position shifts toward to the Fermi level with increasing x. The energy shift is qualitatively consistent with the decrease of the Kondo temperature below TV suggested from the magnetic susceptibility measurements. The Kondo peak in the high temperature phase is not detected still for x = 0.25 with no valence transition, suggesting also the decreases of the Kondo temperature above TV.
    No preview · Article · May 2007 · Journal of Electron Spectroscopy and Related Phenomena
  • K. Hiraoka · H. Yabuta · K. Kojima · K. Oikawa · T. Kamiyama
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    ABSTRACT: The crystal structure of a YbInCu4 sample grown by the Bridgman technique was studied by neutron powder diffraction at room temperature. The site disorder for Yb and In atoms was not observed by Rietveld refinement of the crystal structure. Combining with electron probe X-ray microanalysis, it was found that Cu atoms partially occupy the 4a sites, and the partial occupation causes the increase of the valence transition temperature.
    No preview · Article · Mar 2007 · Journal of Magnetism and Magnetic Materials
  • K. Kojima · S. Kuroda · H. Okumura · K. Arai
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    ABSTRACT: Homoepitaxial growth on a 4H-SiC carbon face (C-face) was investigated. The C-face showed a similar site-competition phenomenon to that seen on a 4H-SiC Si-face. This site-competition phenomenon was distinctly sensitive to growth pressure. The surface morphology of the C-face epitaxial layer was sensitive to surface imperfections on the substrate. The growth window of 4° off-axis, C-face substrates was shown to be wider than that of the corresponding Si-face. A specular surface morphology without a bunched step structure was obtained over a whole 2 inch C-face substrate with a vicinal off-angle of 0.5°.
    No preview · Article · Sep 2006 · Chemical Vapor Deposition
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    ABSTRACT: Yb 4f electronic structure of YbXCu4 (X=Cd, Mg, Zn and Sn) has been investigated by means of high-resolution soft X-ray photoemission spectroscopy (SX-PES) with the excitation energy of hν=800eV, in comparison with the results of the valence transition compound YbInCu4. The SX-PES spectra of YbCdCu4 exhibit the temperature dependence similar to YbInCu4, while those of the other YbXCu4 compounds almost no temperature dependence. The X-dependence of the Yb 4f states suggests that the low In 5p density of states is important for the valence transition of YbInCu4.
    No preview · Article · Feb 2006 · Journal of Alloys and Compounds
  • K. Kojima · N. Suefuji · R. Matono · T. Abe · N. Hayabuchi
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    ABSTRACT: We describe CT and MRI findings of normal anatomy and pathologic conditions of the parotid gland and submandibular gland. It is important to know where to find the parotid gland tumors in relation to the facial nerve. When reading CT and MRI of salivary gland' s tumor, it is important to confirm whether there are features of common tumor (pleomorphic adenoma. Warthin's tumor) or not at first.
    No preview · Article · Jan 2006
  • S. Mototani · S Ochiai · H Ito · G. Sawa · Y Uchida · A Ohashi · K Kojima · T Mizutani
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    ABSTRACT: The third-order nonlinearity of nonlinear optical materials is of great importance in achieving all-optical switching. However, the relationship between the morphology and third-order nonlinear optical characteristics of metal-phthalocyanine thin film is unclear. In this paper, the thickness dependence of the morphologies and the nonlinear optical properties of titanylphthalocyanine (TiOPc) and vanadyl-phthalocyanine (VOPc) thin films prepared on a KBr substrate are investigated by using UV/Vis spectra, X-ray diffraction (XRD), and third-order harmonics (TH) and second-order harmonics (SH) detected using the Maker fringe method. TiOPc and VOPc thin films having different thicknesses were prepared on a KBr substrate using a molecular beam epitaxy (MBE) method. From the Vis/UV spectra of VOPc thin films fabricated on KBr substrate, the VOPc thin film epitaxially grows until the thickness of 90nm and VOPc thin films that are thicker than 90 nm change to a pseudomorphic layer. In the incident angle dependence of TH intensity, the maximum TH intensity of the pseudomorphic layers is not proportional to the square of film thickness. This indicates that defects inside the pseudomorphic layers are closely related to the maximum TH intensity not being proportional to the square of film thickness. These defects may be due to the misfit between the VOPc and TiOPc molecules and the KBr substrate. On the other hand, within the thickness of the TiOPc thin films fabricated on KBr substrate, the TiOPc thin films mainly grow with epitaxy. However, the maximum TH intensity measured with the TiOPc thin film is lower than that of the VOPc thin film. This is related to the molecular packing density of VOPc thin film being larger than that of TiOPc thin film.
    No preview · Conference Paper · Oct 2005
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    ABSTRACT: Direct excitation of two and three magnons is observed in mid-infrared absorption and Raman scattering spectra of α-Fe2O3 crystals. These polarization characteristics and the spectra themselves are shown to be understood from group-theoretical point of view. The microscopic mechanism of three-magnon excitation is proposed in addition to that of well-known two-magnon excitation process.
    No preview · Article · Aug 2005 · Low Temperature Physics
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    ABSTRACT: Low-energy excited photoemission spectra of YbXCu4 (X = Cd, Mg, Zn) with high-energy resolution have been measured, in comparison with those of the valence transition compound YbInCu4. The energy position of the peak due to the conduction-band states hybridized with the Yb2+ 4f(7/2) states, qualitatively reflects the X-dependent Kondo temperature. A comparison between the PES spectra of YbInCu4 and YbCdCu4, suggests the In 5p states play an important role for the valence transition of YbInCu4. (c) 2005 Elsevier B.V. All rights reserved.
    No preview · Article · Jun 2005 · Journal of Electron Spectroscopy and Related Phenomena
  • K. Kojima · S. Nishizawa · S. Kuroda · H. Okumura · K. Arai
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    ABSTRACT: In this study, we studied the effect of growth conditions on micropipe (MP) filling during 4H–SiC epitaxial growth. We found that an MP in an on-axis substrate was filled during epitaxial growth and that this MP was filled with a spiral growth. The MP filling on on-axis substrates had a high probability and was independent of growth conditions. On the other hand, the probability of MP filling of 8° off-axis substrates showed a strong dependence on the growth pressure and the growth rate. The probability of MP filling increased with decreasing growth pressure or increasing growth rate. The probability of MP filling of the C-face was higher than that of the Si-face. From a comparison of a numerical simulation and experimental results, we found that the concentration of Si species just above the substrate was a crucial factor for MP filling.
    No preview · Article · Feb 2005 · Journal of Crystal Growth

Publication Stats

3k Citations
257.95 Total Impact Points

Institutions

  • 2004-2015
    • National Institute of Advanced Industrial Science and Technology
      • Advanced Power Electronics Research Center
      Tsukuba, Ibaraki, Japan
  • 1979-2011
    • Hiroshima University
      • • Graduate School of Integrated Arts and Sciences
      • • Faculty of Integrated Arts and Sciences
      Hirosima, Hiroshima, Japan
  • 1991-2010
    • Aichi Institute of Technology
      Koromo, Aichi, Japan
  • 2008-2009
    • The University of Tokyo
      • • Department of Materials Engineering
      • • Department of Physics
      Tokyo, Tokyo-to, Japan
  • 1990-2004
    • Kurume University
      • Department of Radiology
      Куруме, Fukuoka, Japan
  • 2000
    • Nagoya University
      • Department of Clinical Engineering
      Nagoya, Aichi, Japan
  • 1994-2000
    • Columbia University
      • Department of Physics
      New York City, New York, United States
  • 1988
    • Fukuyama University
      Hukuyama, Hiroshima, Japan
    • Shizuoka General Hospital
      Sizuoka, Shizuoka, Japan
  • 1985
    • Kure National College of Technology
      Kure, Hiroshima, Japan
  • 1980
    • Tohoku University
      • Department of Physics
      Sendai-shi, Miyagi-ken, Japan