Shoichiro Nakao
University of Tokyo
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Featured researches published by Shoichiro Nakao.
Applied Physics Letters | 2007
Taro Hitosugi; Atsuki Ueda; Shoichiro Nakao; Naoomi Yamada; Yutaka Furubayashi; Y. Hirose; Toshihiro Shimada; Tetsuya Hasegawa
Nb-doped anatase TiO2 [Ti0.94Nb0.06O2 (TNO)] films with high electrical conductivity and transparency were fabricated on nonalkali glass using pulsed laser deposition and subsequent annealing in a H2 atmosphere. The amorphous films as deposited on unheated substrates were found to crystallize, forming polycrystalline films at around 350°C. The films annealed at 500°C showed resistivity down to 4.6×10−4Ωcm at room temperature and optical transmittance of 60%–80% in the visible region, which are comparable to those of epitaxial films. These results indicate that TNO films have the potential to be practical transparent conducting oxides that could replace indium tin oxide.
Applied Physics Express | 2008
Taro Hitosugi; Hideyuki Kamisaka; Koichi Yamashita; Hiroyuki Nogawa; Yutaka Furubayashi; Shoichiro Nakao; Naoomi Yamada; Akira Chikamatsu; H. Kumigashira; Masaharu Oshima; Yasushi Hirose; Toshihiro Shimada; Tetsuya Hasegawa
We have investigated electronic band structure of a transparent conducting oxide, Nb-doped anatase TiO2 (TNO), by means of first-principles band calculations and photoemission measurements. The band calculations revealed that Nb 4d orbitals are strongly hybridized with Ti 3d ones to form a d-nature conduction band, without impurity states in the in-gap region, resulting in high carrier density exceeding 1021 cm-3 and excellent optical transparency in the visible region. Furthermore, we confirmed that the results of valence band and core-level photoemission measurements are consistent with prediction by the present band calculations.
Journal of Applied Physics | 2009
Naoomi Yamada; Taro Hitosugi; Junpei Kasai; Ngoc Lam Huong Hoang; Shoichiro Nakao; Yasushi Hirose; Toshihiro Shimada; Tetsuya Hasegawa
This paper proposes a novel sputter-based method for the direct growth of transparent conducting Ti1−xNbxO2 (TNO) polycrystalline films on glass, without the need for any postdeposition treatments, by the use of an initial seed-layer. Anatase TNO epitaxial films grown on LaAlO3 (100) substrates under a reducing atmosphere exhibited a low resistivity (ρ) of (3–6)×10−4Ωcm. On glass, however, highly resistive rutile phase polycrystalline films (ρ∼100Ωcm) formed preferentially under the same conditions. These results suggest that epitaxial stabilization of the oxygen-deficient anatase phase occurs on lattice-matched substrates. To produce a similar effect on a glass surface, we deposited a seed-layer of anatase TNO with excellent crystallinity under an increased oxygen atmosphere. As a result, anatase phase TNO polycrystalline films could be grown even under heavily reducing atmospheres. An optimized film exhibited ρ=1.1×10−3Ωcm and optical absorption lower than 10% in the visible region. This ρ value is more...
Applied Physics Express | 2008
Ngoc Lam Huong Hoang; Naoomi Yamada; Taro Hitosugi; Junpei Kasai; Shoichiro Nakao; Toshihiro Shimada; Tetsuya Hasegawa
We present a low-temperature (~300 °C) process for preparing transparent conducting anatase Nb-doped TiO2 (TNO) polycrystalline films by sputtering. We first deposited amorphous films composed of an oxygen-rich bottom layer and oxygen-deficient top layer at room temperature. These films were then crystallized in a reducing atmosphere. The oxygen-rich bottom layer behaved as a seed layer during crystallization of the top layer, resulting in significant improvement of crystallinity and reduction of crystallization temperature. We obtained TNO polycrystalline films showing a resistivity of 6.4×10-4 Ω cm and absorption below 10% in the visible region by post-deposition annealing at 400 °C. The developed low-temperature process was applied to fabricating TNO films on plastics and glass with low glass-transition temperature.
Journal of Applied Physics | 2010
J. Kasai; Taro Hitosugi; M. Moriyama; K. Goshonoo; Ngoc Lam Huong Hoang; Shoichiro Nakao; N. Yamada; T. Hasegawa
Anatase Nb-doped TiO2 transparent conducting oxide has been formed on GaN(0001) surfaces using a sputtering method. Amorphous films deposited at room temperature were annealed at a substrate temperature of 500 °C in vacuum to form single-phase anatase films. Films with a thickness of 170 nm exhibited a resistivity of 8×10−4 Ω cm with absorptance less than 5% at a wavelength of 460 nm. Furthermore, the refractive index of the Nb-doped TiO2 was well matched to that of GaN. These findings indicate that Nb-doped TiO2 is a promising material for use as transparent electrodes in GaN-based light emitting diodes (LEDs), particularly since reflection at the electrode/GaN boundary can be suppressed, enhancing the external quantum efficiency of blue LEDs.
Applied Physics Express | 2010
Shoichiro Nakao; Naoomi Yamada; Taro Hitosugi; Yasushi Hirose; Toshihiro Shimada; Tetsuya Hasegawa
High-mobility Ta-doped SnO2 (TTO) thin films were grown on glass substrates by pulsed laser deposition using a seed-layer technique. The use of 10-nm-thick polycrystalline anatase TiO2 seed layers was found to lead to the preferred growth of (200)-oriented TTO films, resulting in a 30% increase in the carrier density and a more than two times increase in mobility, compared to films grown directly on the glass substrates. The highest mobility obtained was 83 cm2 V-1 s-1 with a resistivity of 2.8×10-4 Ω cm, whereas the film with the lowest resistivity of 1.8×10-4 Ω cm had a mobility of 60 cm2 V-1 s-1.
Journal of Applied Physics | 2012
Satoru Mohri; Yasushi Hirose; Shoichiro Nakao; Naoomi Yamada; Toshihiro Shimada; Tetsuya Hasegawa
Transparent conductive anatase TiO2−xFx(F:TiO2) epitaxial thin films were fabricated by reactive pulsed laser deposition with a solid fluorine source. F-doping as high as 1.6 × 1021 cm−3 (TiO1.95F0.05) was achieved under optimal growth conditions, and the obtained anatase TiO1.95F0.05 film had a low resistivity of 1.6 × 10−3 Ωcm and a high internal transmittance of >95%. Furthermore, the refractive index of a TiO1.95F0.05 film was >0.2 less than that of undoped TiO2. By comparing the refractive indices of F:TiO2 and Nb:TiO2, we concluded that the decreased refractive index in F:TiO2 can be attributed to a reduced electronic polarizability due to increased bond ionicity as well as doped electrons.
Physical Review B | 2015
Daichi Oka; Yasushi Hirose; Shoichiro Nakao; Tomoteru Fukumura; Tetsuya Hasegawa
SrVO3 and SrNbO3 are perovskite-type transition-metal oxides with the same d1 electronic configuration. Although SrNbO3 (4d1) has a larger d orbital than SrVO3 (3d1), the reported electrical resistivity of SrNbO3 is much higher than that of SrVO3, probably owing to nonstoichiometry. In this paper, we grew epitaxial, high-conductivity stoichiometric SrNbO3 using pulsed laser deposition. The growth temperature strongly affected the Sr/Nb ratio and the oxygen content of the films, and we obtained stoichiometric SrNbO3 at a very narrow temperature window around 630
Applied Physics Letters | 2012
Chang Yang; Yasushi Hirose; Shoichiro Nakao; Ngoc Lam Huong Hoang; Tetsuya Hasegawa
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Applied Physics Express | 2011
Naoomi Yamada; Tatsuo Shibata; Kenji Taira; Yasushi Hirose; Shoichiro Nakao; Ngoc Lam Huong Hoang; Taro Hitosugi; Toshihiro Shimada; Takayoshi Sasaki; Tetsuya Hasegawa
C. The stoichiometric SrNbO3 epitaxial thin films grew coherently on KTaO3 (001) substrates with high crystallinity. The room-temperature resistivity of the stoichiometric film was