OTANI NAOKI
Faculty of Science and Engineering Department of Electronics
Professor
Last Updated :2020/05/31

Researcher Profile and Settings

Research Interests

  • Light-emitting diodes
  • quantum well
  • quantum dot
  • Photonic devices
  • Dye-sensitized solar cell
  • sol-gel
  • oxides
  • compound semiconductors
  • organic inorganic hybrid materials
  • carbon nitride
  • photocatalyst

Research Areas

  • Manufacturing technology (mechanical, electrical/electronic, chemical engineering)/Electronic devices and equipment

Research History

  • 1994-2001 Research on ultra-fast optical switching devices and photo-physics of semiconductor nano-structures
  • 2001-2005 Research on semiconductor lasers and and photo-physics of semiconductor nano-structures

Academic & Professional Experience

  • Faculty of Science and Engineering, Doshisha University, 2011/04 - Today
  • Doshisha University, 2005/04 - 2011/03
  • ATR Laboratories, Visiting researcher, 1994 - 2001
  • Communications Research Laboratory, Leader of research group, 2001 - 2005

Academic Degrees

  • Doctor of Engineering, Hokkaido University
  • Master of Engineering, Hokkaido University

Association Memberships

  • THE JAPAN SOCIETY OF APPLIED PHYSICS

Published Papers

  • Study of isolation method of green fluorescent pigments contained in cherry tomatoes using column chromatography
    A. Noma, N. Ohtani
    Japanese Journal of Applied Physics, 59(SC)SCCA09-1 - SCCA09-6, Feb. 2020, English, 研究論文(学術雑誌)
  • Fabrication and Characterization of Carbon Nitride Fluorescent Material Using Annealed Melamine
    K. Wada, N. Ohtani
    Physica Status Solidi (b), 256(6)1800521 , Jun. 2019, English, 研究論文(学術雑誌)
  • Optical properties and carrier transport in a biased GaAs/AlAs asymmetric quintuple-quantum-well superlattice
    K. Yoshida, S. Goto, R. Wang, M. Hosoda, K. Akahane, N. Ohtani
    Key Engineering Materials, 777(8)121 - 125, Aug. 2018, English, 研究論文(学術雑誌)
  • All solution-processed quintuple-layer organic light-emitting diodes containing organic-inorganic hybrid active layers fabricated by sol-gel method
    Y. Jitsui, N. Ohtani
    Key Engineering Materials, 777(8)113 - 120, Aug. 2018, English, 研究論文(学術雑誌)
  • Fabrication of aluminum and gallium codoped ZnO multilayer transparent conductive films by spin coating method and discussion about improving their performance
    Y. Morita, N. Ohtani
    Japanese Journal of Applied Physics, 57(2)02CB03-1 - 02CB03-6, Feb. 2018, English, 研究論文(学術雑誌)
  • Enhancement of radiative efficiencies of near-ultraviolet organic light-emitting diodes by localized surface plasmon resonance effect
    A. Matsuoka, A. Emoto, N. Ohtani
    Journal of Physics, 647(1)012058 , Oct. 2015, English, 研究論文(学術雑誌)
  • 半導体超格子における電界ドメインの非線形ダイナミクス
    M. Hosoda, N. Ohtani
    Oyo Butsuri, 84(6)508 , 2015, Japanese
  • Fabrication and optical characterization of organic-inorganic hybrid films using ultraviolet curable silsesquioxanes
    H. Akiyama, N. Ohtani
    Japanese Journal of Applied Physics, 53(2S)02BC18 , Jan. 2014, English, 研究論文(学術雑誌)
  • Observation and numerical analysis of quantum subband energies from photoluminescence spectra of organic multiple quantum wells
    N. Ohtani, M. Murata, T. Yamamoto
    Microelectronics Journal, 40(4-5)867 - 868, Jan. 2009, English, 研究論文(学術雑誌)
  • Experimental study on the condition of formation of electric-field domains in multiple finite superlattices
    N. Ohtani, S. Noma, K. Akahane, M. Hosoda, K. Fujita
    Microelectronics Journal, 40(4-5)818 - 820, Jan. 2009, English, 研究論文(学術雑誌)
  • Influence of co-deposited active layers on carrier transport and luminescent properties in organic light emitting diodes
    M. Murata, T. Yamamoto, M. Haishi, T. Ando, N. Ohtani
    Physica Status Solidi (c), 6(1)330 - 333, Jan. 2009, English, 研究論文(学術雑誌)
  • Quantum size effect on photoluminescence and electroluminescence in organic multiple quantum wells
    T. Yamamoto, M. Murata, M. Haishi, T. Ando, and, N. Ohtani
    Journal of Physics, 109012036 , Apr. 2008, English, 研究論文(学術雑誌)
  • Electric-field-direction dependence of carrier injection into high-energy states in asymmetric GaAs/AlAs multiple-quantum wells embedded in an n-i-n diode
    H. Kitamura, S. Hiratsuka, M. Hosoda, K. Akahane, and, N. Ohtani
    Journal of Physics, 109012021 , Apr. 2008, English, 研究論文(学術雑誌)
  • Improvement of radiative efficiency and wavelength-tuning by TPD-doping in organic light emitting diode using Poly [methylmethacrylate-co- (7- (4-trifluoromethyl) coumarin acrylamide)] (PCA)
    M. Haishi, T. Yamamoto, M. Murata, and, N. Ohtani
    Journal of Physics, 109012011 , Apr. 2008, English, 研究論文(学術雑誌)
  • Various photoluminescence properties due to Gamma-X resonance in type-I GaAs/AlAs multi-quantum wells consisting of quantum wells with different thicknesses
    N. Ohtani, H. Endo, S. Hiratsuka, H. Kitamura, T. Takamatsu, M. Hosoda
    Physica E, 40(6)2016 - 2018, Apr. 2008, English, 研究論文(学術雑誌)
  • Photoluminescence properties affected by carrier transport between X and different first excited states originating from interface imperfection in GaAs/AlAs multi-quantum wells
    H. Endo, S. Hiratsuka, H. Kitamura, T. Takamatsu, M. Hosoda, N. Ohtani
    Japanese Journal of Applied Physics, 47(1)682 - 684, Jan. 2008, English, 研究論文(学術雑誌)
  • Numerical method for coherent electron dynamics with potential-dependent effective-mass distributions in semidonductor heterostructures
    T. Ando, Y. Ohtake, N. Ohtani
    Physical Review E, 73(6)066702 , Jun. 2006, English, 研究論文(学術雑誌)
  • Metamorphic InGaAs/AlAsSb quantum wells grown on GaAs substrates for intersubband devices operating toward short-wavelength region
    S. Gozu, A. Ueta, K. Akahane, N. Yamamoto, N. Ohtani, M. Tsuchiya
    Electronics Letters, 42(10)601 - 602, May 2006, English, 研究論文(学術雑誌)
  • Interface states of AlSb/InAs heterointerfacee with AlAs like interface
    S. Gozu, K. Akahane, N. Yamamoto, A. Ueta, T. Ando, N. Ohtani
    Japanese Journal of Applied Physics, 45(4)3540 - 3543, Apr. 2006, English, 研究論文(学術雑誌)
  • Circular polarization switch by using photo-chemically etched silicon
    N. Yamamoto, N. Ohtani, T. Matsuno, H. Takai
    Japanese Journal of Applied Physics, 44(7A)4749 - 4751, Jul. 2005, English, 研究論文(学術雑誌)
  • Over 1.3 mu m continuous-wave laser emission from InGaSb quantum-dot laser diode fabricated on GaAs substrates
    N Yamamoto, K Akahane, S Gozu, N Ohtani
    A silicon atom irradiation technique is proposed to create high-density InGaSb quantum dots (QDs) on a GaAs substrate. This technique irradiates Si atoms immediately before InGaSb QD growth, and the density of QDs with Si atoms is about 100 times higher than that without Si atoms. The high-density InGaSb QDs show long-wavelength photo-and electroluminescence emissions around 1.3 and 1.5 mu m in the optical communications waveband. An Sb-based QD laser diode (LD) containing an InGaSb QD active layer was fabricated on a GaAs substrate. This Sb-based QD-LD shows a 1.37 mu m continuous-wave laser emission at room temperature. (c) 2005 American Institute of Physics., APPLIED PHYSICS LETTERS, AMER INST PHYSICS, 86(20)203118 , May 2005, English, 研究論文(学術雑誌)
  • Photoluminescence from high Gamma-electron subbands and intersubband electroluminescence using X-Gamma carrier injection in a simple GaAs/AlAs superlattice
    C Domoto, T Nishimura, N Ohtani, K Kuroyanagi, PO Vaccaro, T Aida, H Takeuchi, M Nakayama
    We report the interband photoluminescence from high Gamma-electron subbands and mid-infrared electroluminescence originating from an intersubband transition in a simple GaAs (15.3 nm)/AlAs (4.5 nm) superlattice embedded in a p-i-n structure. Interband photoluminescence properties under applied bias voltages provide conclusive evidence that electrons populate the fourth Gamma (Gamma4) electron subband in the GaAs layer. This electron population results from the carrier injection into the Gamma4 subband from the adjacent X1-subband in the AlAs layer, which is initiated by the X1-Gamma4 resonance. We calculate the overlap integral of the envelope functions for Gamma-electron and heavy-hole subbands in order to discuss the carrier population in high Gamma subbands based on the photoluminescence intensities. The results of analysis suggest that a population inversion can be obtained between the Gamma4 and Gamma4 subbands under the X1-Gamma4 resonant condition. The energy of the,intersubband electroluminescence, 100 meV, agrees with the energy spacing between the Gamma4 and Gamma3 subbands. This demonstrates that the carrier injection into the higher F subband using X-Gamma scattering will be useful for designing of intersubband-emission devices., JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS, INST PURE APPLIED PHYSICS, 41(8)5073 - 5077, Aug. 2002, English, 研究論文(学術雑誌)
  • Intersubband electroluminescence using X-G carrier injection in a GaAs/AlAs superlattice
    C. Domoto, N. Ohtani, K. Kuroyanagi, P. Vaccaro, H. Takeuchi, M. Nakayama, T. Nishimura
    Applied Physics Letters, 77(6)848 - 850, Jun. 2000, English, 研究論文(学術雑誌)
  • Carrier density dependence of transitions between chaotic and periodic photocurrent oscillations in undoped GaAs/AlAs superlattices
    N. Ohtani, N. Egami, H. T. Grahn, K. H. Ploog
    Physica B, 249(1)878 - 881, Apr. 1998, English, 研究論文(学術雑誌)
  • Photocurrent and photoluminescence affected by G-X electron transfer in type-I GaAs-AlAs superlattices
    H. Mimura, M. Hosoda, N. Ohtani, K. Yokoo
    Japanese Journal of Applied Physics, 37(3B)1650 - 1653, Mar. 1998, English, 研究論文(学術雑誌)
  • Avalanche breakdown mechanism originating from G-X-G transfer in GaAs/AlAs superlattices
    M. Hosoda, K. Tominaga, N. Ohtani, K. Kuroyanagi, N. Egami, H. Mimura, K. Kawashima, K. Fujiwara
    Applied Physics Letters, 71(19)2827 - 2829, Nov. 1997, English, 研究論文(学術雑誌)
  • Anomalously large negative differential resistance due to G-X resonances in type-I GaAs/AlAs superlattices
    M. Hosoda, N. Ohtani, K. Tominaga, H. Mimura, T. Watanabe
    Physical Review B, 56(11)6432 - 6435, Sep. 1997, English, 研究論文(学術雑誌)
  • Observation of Gamma-X resonances in type-I GaAs/AlAs semiconductor superlattices: Anomaly in photoluminescence
    M Hosoda, H Mimura, N Ohtani, K Tominaga, K Fujita, T Watanabe, H Inomata, M Nakayama
    We report the direct observation of Gamma-X resonances in type-I GaAs/AlAs semiconductor superlattices under an electric field. Photoluminescence (PL) associated with the first X-z state in the AlAs and the first Gamma heavy-hole state in the GaAs was found in type-I superlattices. The PL strongly increased its intensity due to Gamma-X mixing, by tuning the resonance between the X, state and the second Gamma-electron state in the adjacent GaAs layer. This observation gives clear evidence that Gamma-X scattering and mixing often occur even in type-I superlattices under an electric field., PHYSICAL REVIEW B, AMERICAN PHYSICAL SOC, 55(20)13689 - 13696, May 1997, English, 研究論文(学術雑誌)
  • Stark ladder photoluminescence of X states in GaAs/AlAs type-I superlattices
    N. Ohtani, M. Hosoda, H. Mimura, K. Tominaga, T. Watanabe
    Japanese Journal of Applied Physics, 36(3B)1184 - 1187, Mar. 1997, English, 研究論文(学術雑誌)
  • Influence of G-X resonances on G ground state electron occupation in type-I GaAs/AlAs superlattice
    M. Hosoda, K. Tominaga, N. Ohtani, H. Mimura, M. Nakayama
    Applied Physics Letters, 70(12)1581 - 1583, Mar. 1997, English, 研究論文(学術雑誌)
  • Investigation of X levels in GaAs/InAlAs strained superlattices
    K. Kuroyanagi, N. Ohtani, N. Egami, K. Tominaga, M. Ando, M. Nakayama
    Nonlinear Optics, 18(2)261 - 264, 1997, English, 研究論文(学術雑誌)
  • Evidence for various higher-subband resonances and interferences in a GaAs/AlAs asymmetric quadruple-quantum-well superlattice analyzed from its photoluminescence properties
    Keisuke Hata, Makoto Hosoda, Kouichi Akahane, Naoki Ohtani
    In this paper, we present evidence for the resonances between higher subbands in an asymmetric quadruplequantum-well (AQQW) superlattice (SL) by photoluminescence (PL) spectra. Since each QW in an AQQW is separated by thin barriers, subband interferences can easily arise. As a result, various PL branches caused by the subband resonances are observed, including PL emissions from the higher-energy Gamma states, the X state in a barrier with longitudinal optical phonon replica, and long-range Gamma-X transfer with Gamma-X mixing. However, in conventional QW systems, the PL emissions from higher-energy subbands and their interference have not been clearly observed yet; in our system, the interferences between higher-energy subbands can be observed through PL emission, since we achieved effective electron injection into the higher subbands under very thin barriers. From these observations, we exposed the existence of such interferences and transport processes., PHYSICAL REVIEW B, AMER PHYSICAL SOC, 95(7)075309-9 , Feb. 2017, English, 研究論文(学術雑誌)
  • Improvement of photoelectric conversion efficiencies of dye-sensitized solar cells consisting of hemispherical TiO2 films
    Shogo Izumi, Ayame Mizuno, Naoki Ohtani
    To improve the photoelectric conversion efficiency (PCE) of dye-sensitized solar cells (DSSCs), we fabricated hemispherical TiO2 films using a poly(dimethylsiloxane) mold and improved the light harvesting efficiency in DSSCs by a large light-scattering effect caused by the hemispherical structure. In addition, the amount of the adsorption density of the dye in TiO2 increased due to the larger surface area of the hemispherical structure. Consequently, the short-circuit current of DSSCs containing hemispherical TiO2 film increased even though the open-circuit voltage was not changed. As a result, the overall PCE value was improved to about 1.2 times larger than a DSSC containing a flat TiO2 film. In addition, polyethylene glycol-doping in TiO2 effectively improved the DSSC performance comprised of hemispherical TiO2 films. (C) 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 14 NO 6, WILEY-V C H VERLAG GMBH, 14(6)1600186 , 2017, English, 研究論文(国際会議プロシーディングス)
  • Characterization of dopant density dependence on transmittance and resistance of ZnO films fabricated using spin-coating method
    Yusuke Morita, Akira Emoto, Naoki Ohtani
    We fabricated aluminum (Al) and gallium (Ga) co-doped ZnO films by wet-process and evaluated their sheet resistances and transmission spectra. It was found that the sheet resistance becomes minimum when the content ratios of Al and Ga are 0.02 at the annealing temperature of 550 degree. Moreover, we found that the sheet resistance can be improved by performing the spin-coating and annealing twice. In this case, the transmission probability in the visible light regime is more than 90%. This result is a simple method to fabricate fine transparence conductive ZnO films., MOLECULAR CRYSTALS AND LIQUID CRYSTALS, TAYLOR & FRANCIS LTD, 641(1)106 - 110, 2016, English, 研究論文(学術雑誌)
  • Fabrication of silica glass thin films containing organic emissive materials and application to multi-layer organic light-emitting diodes
    Ryo Nakagawa, Yusuke Jitsui, Akira Emoto, Naoki Ohtani
    Organic-inorganic hybrid films are fabricated by applying a sol-gel method using perhydropolysilazane (PHPS), which are then applied on the active layers of quadruple-layer structure organic light-emitting diodes (OLEDs). All organic layers are fabricated using wet-process. In the quadruple-layer OLEDs consisting of the active layer without the sol-gel reaction, electroluminescence (EL) emissions from the active layer is not observed. This is because the electron-transporting layer and the adjacent active layer are mixed. However, if the sol-gel reaction is performed the EL emissions from the active layer are clearly observed. This is because the active layer is not destroyed due to its insolubility., MOLECULAR CRYSTALS AND LIQUID CRYSTALS, TAYLOR & FRANCIS LTD, 641(1)111 - 118, 2016, English, 研究論文(学術雑誌)
  • Improved Photoluminescence Lifetime of Organic Emissive Materials Embedded in Organic-Inorganic Hybrid Thin Films Fabricated by Sol-Gel Method Using Tetraethoxysilane
    Naoki Ohtani, Masashi Tonoi
    We fabricated organic-inorganic hybrid films by a sol-gel method using Tetraethoxysilane (TEOS). It is found that water-soluble organic emissive materials should be used, because water must be added to TEOS for a sol-gel reaction. The acid catalyst is better than the base catalyst when TEOS is used. The organic-inorganic hybrid films reveal a fine antioxidant effect, because most of the organic emissive materials are surrounded by SiO2. This means that hybrid films have fine resistance to moisture and an antioxidant effect. Consequently, they lead to a strong sealing effect, resulting in the long operation lifetime of photoluminescence (PL) emissions., MOLECULAR CRYSTALS AND LIQUID CRYSTALS, TAYLOR & FRANCIS LTD, 599(1)132 - 138, Aug. 2014, English, 研究論文(学術雑誌)
  • Synthesis of uniform and high-density silver nanoparticles by using Peltophorum pterocarpum plant extract
    Matheswaran Balamurugan, Natesan Kandasamy, Shanmugam Saravanan, Naoki Ohtani
    Biologically synthesized nanoparticles have been widely used in the field of medicine. In this present work stable colloidal silver nanoparticles are prepared by a simple cost-effective, facile, and ecofriendly synthesis method to avoid the use of toxic chemicals. Peltophorum pterocarpum (PP) plant extract was employed to serve as both reducing and capping agents in this reaction. The effect of different experimental parameters such as the concentration of silver ions, the dosage of the plant extract, temperature, and reaction time on the synthesis of nanoparticles were studied. The synthesized nanoparticles were characterized using ultraviolet-visible (UV-vis) spectroscopy, X-ray diffractometry (XRD), transmission electron microscope (TEM), and Fourier transform infrared spectroscopy (FT-IR), which revealed the nature of the nanoparticles. The nanoparticles have different diameters ranging from 40 to 85 nm. By optimizing the experimental conditions, more uniform high-density nanoparticles can be achieved. The synthesized silver nanoparticles exhibit significant antibacterial activity on gram-positive Staphylococcus aureus. The silver nanoparticles prepared in this way are uniform and stable, which can be stored at room temperature for more than two months. (C) 2014 The Japan Society of Applied Physics, JAPANESE JOURNAL OF APPLIED PHYSICS, IOP PUBLISHING LTD, 53(5)05FB19 , May 2014, English, 研究論文(学術雑誌)
  • Structural and optical studies of pure and Ni-doped ZnO nanoparticles synthesized by simple solution combustion method
    Murugesan Silambarasan, Shanmugam Saravanan, Naoki Ohtani, Tetsuo Soga
    In this paper, the structural and optical studies of pure and Ni-doped ZnO nanoparticles synthesized by the simple solution combustion method are reported. The structural, morphological, and optical studies are carried out by powder X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM), high resolution-transmission electron microscopy (HR-TEM), Raman, ultraviolet-visible (UV-vis) absorption, and photoluminescence (PL) spectra. The XRD pattern indicates that the prepared particles are in hexagonal wurtzite structure with the average crystalline size is around 30-60 nm. The elemental analyses of pure and doped samples are evaluated by energy dispersive X-ray spectrometry (EDX). Raman spectra of the pure and Ni-doped samples show remarkable effect on the polar as well as non-polar branches. Room temperature PL shows the near band edge related emission and the results are related to several intrinsic defects in the Ni-doped ZnO nanoparticles. Especially the oxygen vacancies are responsible for the long life time of the carriers. c 2014 The Japan Society of Applied Physics, JAPANESE JOURNAL OF APPLIED PHYSICS, IOP PUBLISHING LTD, 53(5)05FB16 , May 2014, English, 研究論文(学術雑誌)
  • Fabrication of near-infrared polymer light-emitting-diodes using dispersed laser dye
    Yusuke Jitsui, Naoki Ohtani
    The purpose of this research is to fabricate polymer light-emitting diodes (PLEDs) operating in the near-infrared (NIR) region. IR-140 is a laser-dye, whose emission wavelength is 870 nm. This NIR dye was dispersed as an emissive dopant within poly(2-methoxy-5-(3'-7'-dimethyloctyloxy)-1,4-phenylenevinylene) (MDMO -PPV) and poly(N-vinylcarbazole) (PVK). In addition, 2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole (PBD) was dispersed in the active layer. We fabricated the following two types of samples: (A) indium tin oxide (ITO)/MDMO-PPV:IR-140/aluminum (Al) and (B) ITO/IR-140:PBD:PVK/Al. Sample A successfully showed NIR emissions. However, sample A also revealed EL signals in the visible light region. In sample B, however, the undesired visible light was successfully eliminated due to the improved carrier-balance in the active region., JOURNAL OF THE KOREAN PHYSICAL SOCIETY, KOREAN PHYSICAL SOC, 60(10)1557 - 1559, May 2012, English, 研究論文(学術雑誌)
  • Excitonic Rabi oscillations in self-assembled quantum dots studied by photon echoes
    Kenta Asakura, Yasuyoshi Mitsumori, Hideo Kosaka, Keiichi Edamatsu, Kouichi Akahane, Naokatsu Yamamoto, Masahide Sasaki, Naoki Ohtani
    We report the excitonic Rabi oscillations in self-assembled quantum dots observed by photon echo spectroscopy. The amplitude of the observed Rabi oscillations increases with the pulse area, which is different from the results of previous reports. A numerical calculation of the Rabi oscillations including the local field effect in a quantum dot well reproduces the experimental oscillations, indicating that the local field effect is one of the important origins of the modification of the Rabi oscillations in quantum dots. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 9, NO 12, WILEY-V C H VERLAG GMBH, 9(12)2513 - 2516, 2012, English, 研究論文(国際会議プロシーディングス)
  • White Organic Light-Emitting Diodes Using Two Phosphorescence Materials in a Starburst Hole-Transporting Layer
    Tomoya Inden, Tomoya Hishida, Masayuki Takahashi, Masato Ohsumi, Naoki Ohtani
    We fabricated two kinds of white organic light-emitting diodes (WOLEDs); one consisted of two emissive materials of red and blue, and the other of three emissive materials of red, green, and blue. The red and blue emissive materials were phosphorescent. We evaluated the thickness dependence of the CIE coordinate, the external quantum efficiency (EQE), and the luminance by changing the thicknesses of the Ir(btp)2acac and FIrpic layers. Samples consisting of three emissive materials revealed the best CIE coordinate and the best EQE in the same sample structure. On the other hand, the samples consisting of two emissive materials revealed the best CIE coordinate and the best EQE in different structures. The best CIE coordinate of (0.33, 0.36) was observed by changing the thicknesses of the stacked active layers. The best EQE was 9.73%, which was observed in the sample consisting of different thickness of stacked active layers., INTERNATIONAL JOURNAL OF PHOTOENERGY, HINDAWI PUBLISHING CORPORATION, 2012703496 , 2012, English, 研究論文(学術雑誌)
  • Excitonic Rabi oscillations in semiconductor quantum dot observed by photon echo spectroscopy
    K. Asakura, Y. Mitsumori, H. Kosaka, K. Edamatsu, K. Akahane, N. Yamamoto, M. Sasaki, N. Ohtani
    Optical properties in single-layer InGaAlAs/GaAlAs semiconductor quantum dots were studied by two-pulse photon echo technique. From the decay time, the optical coherence time of the excitons was estimated to be 2.5 ns, which is independent of the excitation intensity. We also observed the excitonic Rabi oscillations by measuring the photon echo intensity as a function of the excitation pulse area. The observed oscillatory structure is not expected from two-level systems., PHYSICS OF SEMICONDUCTORS: 30TH INTERNATIONAL CONFERENCE ON THE PHYSICS OF SEMICONDUCTORS, AMER INST PHYSICS, 1399529 - 530, 2011, English, 研究論文(国際会議プロシーディングス)
  • Fabrication of Organic Light-Emitting Diodes Using Photosynthetic Pigments Extracted from Spinach
    Naoki Ohtani, Natsuko Kitagawa, Takashi Matsuda
    We fabricated organic light-emitting diodes (OLEDs) containing chlorophylls in the active region, which were extracted from spinach using a chemical method. Photoluminescence (PL) cannot be observed in the thin film of the extracted chlorophylls owing to concentration quenching. To overcome the concentration quenching, a host material, poly[(m-phenylenevinylene)-alt-(2,5-dihexyloxy-p-phenylenevinylene)] (PPV) was added in the active region. This leads to the observaton of electroluminescence (EL) signals originating from chlorophyll a. We also evaluated the lifetime of the PL and EL. Consequently, the OLEDs containing carotenoids in the active region exhibit the light-emission much longer time than that without carotenoidos. This is assigned to the antioxidant activities of carotenoids. OLEDs containing a large amount of carotenoids are resistant to the oxidation damage. (C) 2011 The Japan Society of Applied Physics, JAPANESE JOURNAL OF APPLIED PHYSICS, JAPAN SOC APPLIED PHYSICS, 50(1)01BC08 , Jan. 2011, English, 研究論文(学術雑誌)
  • Organic near-infrared photodiodes containing a wide-bandgap polymer and an n-type dopant in the active layer
    Naoki Ohtani, Kiichiro Nakajima, Ken-ichi Bando
    In this research, organic photodiodes (NIR-OPDs) operating in the near-infrared region were fabricated by wet process. Copper(II) 5,9,14,18,23,27,32,36-octabutoxy-2,3-naphthalocyanine (Cu(II)) was used as a photovoltaic material in the active layer, whose lambda max is in the NIR region. To improve the dark current property and external quantum efficiency (EQE), a wide-bandgap polymer and an n-type dopant material were mixed in the active layer. Consequently, the maximum EQE was observed to be 9 % by adjusting the doping ratio of the two materials. In addition, we evaluated ON/OFF ratio as a function of doping densities of the wide-bandgap polymer and the n-type dopant material. As a result, the maximum ON/OFF ratio was observed by adjusting the doping densities. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 8, NO 9, WILEY-V C H VERLAG GMBH, 8(9)2907 - 2910, 2011, English, 研究論文(国際会議プロシーディングス)
  • Improvement of carrier transport and luminous efficiency of organic light emitting diodes by introducing a co-deposited active layer
    Naoki Ohtani, Masaya Murata, Keiichiro Kashiwabara, Kazunori Kurata
    We evaluated carrier transport and luminous efficiency of organic light-emitting diodes (OLEDs) whose active regions consist of a single co-deposited layer. One organic material is a hole transport material N,N'-Bis(3-methylphenyl)-N,N'-diphenylbenzidine (TPD), while the other is an electron transport/emissive material Tris(8-hydroxyquinolinato)-aluminum (Alq3). It was found that the luminous efficiency strongly depends on the thickness and the ratio of the TPD:Alq3 co-deposited layer. This indicates that the carrier balance in the active region can be improved by changing the co-deposited layers. In addition, we performed the dye-doping method to clarify the recombination region. As a result, we found that the radiative recombination is caused in the whole TPD:Alq3 co-deposited layer., 16TH INTERNATIONAL CONFERENCE ON ELECTRON DYNAMICS IN SEMICONDUCTORS, OPTOELECTRONICS AND NANOSTRUCTURES (EDISON 16), IOP PUBLISHING LTD, 1930121060 , 2009, English, 研究論文(国際会議プロシーディングス)
  • 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM)-doping density dependence of luminescence spectra and white emission in polymer light-emitting diodes
    Akinori Inoue, Takeshi Hosokawa, Motoki Haishi, Naoki Ohtani
    We fabricated white polymer light-emitting diodes (PLEDs), in which the active region is doped with a low-weight molecule. The host polymer material is Poly(9,9-didodecyl-fluorenyl-2,7-yleneethylnylene) (PFO), while the guest luminescent low-weight-molecule is 4-(Dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM). The photoluminescence (PL) and electroluminescence (EL) spectra strongly depend on DCM-doping-density. However, the most suitable DCM-doping-densities for white emissions in 11 PL and EL are slightly different. This discrepancy is caused by the difference in excitation efficiencies. The photoexcitation system for PL measurement is better than the current injection for generating carriers efficiently. Thus, the realization of the white EL signal requires the larger DCM-doping-density than that of the white PL signal. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 6, NO 1, WILEY-V C H VERLAG GMBH, 6(1)334 - 337, 2009, English, 研究論文(国際会議プロシーディングス)
  • Photoluminescence properties of annealed InAs quantum dots capped by InGaAs layers
    Shingo Hiratsuka, Shanugam Saravanan, Takahisa Harayama, Naoki Ohtani
    We have investigated the annealing-temperature dependence on photoluminescence (PL) properties of InAs quantum dots (QDs) fabricated on GaAs substrate. The annealing temperatures were varied from 700 to 1000 degrees C. The observed PL spectra were drastically changed by changing the annealing temperature. The PL peaking-wavelength of non-annealed sample is around 1200 nm. However, the peaking-wavelength is drastically blue-shifted with increasing annealing temperature. The strongest PL intensity is observed from the sample annealed at 875 degrees C. This PL intensity is more than seven fold compared with that of the non-annealed sample. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 6, NO 1, WILEY-V C H VERLAG GMBH, 6(1)189 - +, 2009, English, 研究論文(国際会議プロシーディングス)
  • Strong optical emissions from two-dimensional metal photonic crystals with semiconductor multiple quantum wells
    Shin-ichiro Gozu, Akio Ueta, Kouichi Akahane, Naokatsu Yamamoto, Masahiro Tsuchiya, Naoki Ohtani
    We investigated two-dimensional (2D) metal photonic crystals (PhCs) with semiconductor multiple quantum wells as an active material. The 2D metal PhCs were fabricated using only a lift-off process. The lift-off process leads to a simpler fabrication process than the one for conventional PhCs and avoids any process damage induced by dry etching. The 2D metal PhCs exhibited clear photonic band effects: (1) photoluminescence (PL) peak positions changed depending on the lattice constant and (2) the PL intensity from 2D metal PhCs was higher than that without 2D metal PhCs. These effects are suitable for fabricating light-emitting diodes with a high external quantum efficiency. PL intensity depending on PL excitation power revealed slightly improved luminescence efficiency than the sample without 2D metal PhCs. (c) 2007 American Institute of Physics., JOURNAL OF APPLIED PHYSICS, AMER INST PHYSICS, 101(8)086107 , Apr. 2007, English, 研究論文(学術雑誌)
  • Metamorphic molecular beam epitaxy growth and selective wet etching for epitaxial layer lift-off of AlAsSb toward optical waveguides with high optical confinement
    Shin-ichiro Ozua, Kouichi Akahane, Naokatsu Yamamoto, Akio Ueta, Naoki Ohtani, Masahiro Tsuchiya
    In this study, we performed epitaxial layer lift-off (ELO) of AlAs(0.37)Sb(0.63) layer toward realizing AlAs(0.37)Sb(0.63)/In(0.75)Ga(0.25)As systems on SiO(2). This will enable high optical confinement in the AlAs(0.37)Sb(0.63)/In(0.75)Ga(0.25)As systems to be achieved owing to the large refractive index contrast (Delta n similar to 1.6). ELO consists of two processes: the first is adhesive wafer bonding using positive photoresist and the second is two-step selective wet etching. Metamorphic molecular beam epitaxy was used to grow selective etching layers between the substrate and the AlAs(0.37)Sb(0.63). We confirmed that the metamorphic growth did not degrade the crystal quality and ELO was successful. Therefore, ELO improves optical confinement of AlAs(0.37)Sb(0.63) and will be able to improve performance of all optical intersubband switches using the AlAs(0.37)Sb(0.63)/In(0.75)Ga(0.25)As systems. (c) 2006 Elsevier B.V. All rights reserved., JOURNAL OF CRYSTAL GROWTH, ELSEVIER SCIENCE BV, 301(1)955 - 958, Apr. 2007, English, 研究論文(学術雑誌)
  • Growth of InGaSb quantum dot structures on GaAs and silicon substrates
    Naokatsu Yamamoto, Kouichi Akahane, Shin-ichirou Gozu, Akio Ueta, Naoki Ohtani, Masahiro Tsuchiya
    We present a growth technique for InGaSb quantum dots (QDs) as Sb-based QD structures on GaAs and silicon Substrates by molecular beam epitaxy. We successfully fabricated high-quality and high-density (>10(10)/cm(2)) Sb-based QD structures on both substrates by optimizing growth conditions. Additionally, using the Sb-based QDs embedded in a GaAs matrix as an active medium, we demonstrated optical emissions at a wavelength of 1.55-mu m from the Sb-based QDs in an optical microcavity structure fabricated on the GaAs substrate. We also discuss the growth of the Sb-based QDs on a silicon wafer that may become useful materials for silicon photonics technology., JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS, INST PURE APPLIED PHYSICS, 46(4B)2401 - 2404, Apr. 2007, English, 研究論文(学術雑誌)
  • Site control of very low density InAs QDs on patterned GaAs nano-wire surfaces
    Akio Ueta, Kouichi Akahane, Sinichiro Gozu, Naokatsu Yamamoto, Naoki Ohtani, Masahiro Tsuchiya
    Self-organized InAs quantum dots (QDs) were grown on patterned GaAs substrates. A 2.5-ML-thick InAs grown on self-organized GaAs nano-wire surfaces showed QD formation with a very low density of 1.5 x 10(8) cm(-2). The self-organized GaAs nano-wires were grown under low V/III ratio, and nano-wires were parallel to the [1 1 0] direction. On the other hand, the 2.5-ML-thick InAs on smooth (0 0 1) GaAs surfaces formed QDs with a density of 4.4 x 10(9) cm(-2). We grew these low density InAs QDs on patterned GaAs, which had small A-planes. The InAs QDs were formed only on the small A-plane, which was positioned at the boundary between the square-patterned and planar areas. Except for the A-planes, no InAs QD formation was observed in a relatively large area. These results indicate that the combination of GaAs nano-wires and pattern substrates with small A-planes can produce InAs QDs at desired areas. (c) 2006 Elsevier B.V. All rights reserved., JOURNAL OF CRYSTAL GROWTH, ELSEVIER SCIENCE BV, 301(1)846 - 848, Apr. 2007, English, 研究論文(学術雑誌)
  • (In)GaSb/AlGaSb quantum wells grown on Si substrates
    Kouichi Akahane, Naokatsu Yamamoto, Shin-ichiro Gozu, Akio Ueta, Naoki Ohtani
    We have successfully grown GaSb and InGaSb quantum wells (QW) on a Si(001) substrate, and evaluated their optical properties using photoluminescence (PL). The PL emissions from the QWs at room temperature were observed at around 1.55 mu m, which is suitable for fiber optic communications systems. The measured ground state energy of each QW matched well with the theoretical value calculated by solving the Schrodinger equation for a finite potential QW. The temperature dependence of the PL intensity showed large activation energy (similar to 77.6 meV) from QW. The results indicated that the fabricated QW structure had a high crystalline quality, and the GaSb QW on Si for optical devices operating at temperatures higher than room temperature will be expected. (c) 2006 Elsevier B.V. All rights reserved., THIN SOLID FILMS, ELSEVIER SCIENCE SA, 515(10)4467 - 4470, Mar. 2007, English, 研究論文(学術雑誌)
  • Strong optical confinements inside the wavelength-size metal mirror microcavities: New concept for small light emitters using InAsSb QDs microcavities
    Akio Ueta, Sin-ichiro Gozu, Kouichi Akahane, Naokatsu Yamamoto, Masahiro Tsuchiya, Naoki Ohtani
    We studied optical cavity properties of metal mirror microcavities. The mu-PL spectrum for the metal mirror structure showed a sharp emission at the resonance wavelength obtained by the mu-reflection measurement. Small temperature dependences of the cavity resonance were also observed, and the slope of the cavity resonance wavelengths was about 0.05 meV/K. These metal mirror microcavities might be useful for small, high performance optical devices., PHYSICS OF SEMICONDUCTORS, PTS A AND B, AMER INST PHYSICS, 8931145 - +, 2007, English, 研究論文(国際会議プロシーディングス)
  • Various photoluminescence properties observed in a GaAs/AlAs asymmetric double-quantum-well superlattice
    Naoki Ohtani, Makoto Hosoda
    We report the observation of various photoluminescence (PL) properties in a biased asymmetric double-quantum-well superlattice. By using the transfer matrix calculation, the PL signals in the shorter wavelength region are caused by carrier injection into higher excited subbands via Gamma-X-Gamma resonance and GaAs LO-phonon replica from the X subband. Novel carrier transport paths leading to the higher subbands produce the various PL properties that comes from the asymmetric conduction band profile including the X subbands., PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 4, NO 2, WILEY-V C H VERLAG GMBH, 4(2)353 - +, 2007, English, 研究論文(国際会議プロシーディングス)
  • Optical cavity properties of metal mirror microcavities with InAsSb quantum dots
    Akio Ueta, Sin-ichiro Gozu, Kouichi Akahane, Naokatsu Yamamoto, Masahiro Tsuchiya, Naoki Ohtani
    The optical properties of metal mirror microcavities with InAsSb quantum dots (QDs) were studied. Metal mirror microcavities, consisting of InAsSb QDs and GaAs, were fabricated using lift-off methods and mounted on a (001) Si substrate. The micro-photoluminescence (mu-PL) spectra of these structures showed a modulation of emission properties at a resonance wavelength determined by mu-reflection measurement. The temperature dependence of cavity resonance wavelength was also investigated. The slope of the cavity resonance at approximately room temperature (RT) was found to be about 0.075 nm/K., JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS, INST PURE APPLIED PHYSICS, 45(11)8650 - 8652, Nov. 2006, English, 研究論文(学術雑誌)
  • Residual carrier density in GaSb grown on Si substrates
    Kouichi Akahane, Naokatsu Yamamoto, Shin-ichiro Gozu, Akio Ueta, Naoki Ohtani
    The relationships between the densities of residual carriers and those of dislocation in GaSb films grown on Si substrates were investigated. Dislocation density was evaluated by cross-sectional transmission electron microscopy (TEM). The TEM images indicated that the dislocation density after a 5-mu m-thick GaSb film was grown was below 1 x 10(8)/cm(2) although the density near the interface between the Si substrate and the GaSb film was about 3 x 10(9)/cm(2). Forming a dislocation loop by growing a thick GaSb layer may decrease the dislocation density. The density and mobility of the residual carrier were investigated by Hall measurement using the van der Pauw method. The residual carriers in GaSb grown on Si substrates were holes, and their densities decreased significantly from 4.2 x 10(18) to 1.4 x 10(17)/cm(3) as GaSb thickness was increased from 500 to 5500 nm. (c) 2005 Elsevier B.V. All rights reserved., THIN SOLID FILMS, ELSEVIER SCIENCE SA, 515(2)748 - 751, Oct. 2006, English, 研究論文(学術雑誌)
  • Nanoscale structure fabrication of multiple AlGaSb/InGaSb quantum wells by reactive ion etching with chlorine-based gases toward photonic crystals
    Shin-ichiro Gozu, Kouichi Akahane, Naokatsu Yamamoto, Akio Ueta, Naoki Ohtani, Masahiro Tsuchiya
    The authors studied nanoscale structure fabrication of multiple AlGaSb/InGaSb quantum wells. The fabrication was successfully conducted using simple resist masks for electron beam lithography and reactive ion etching (RIE) with chlorine-based gases. The etching profile after the RIE showed good vertical shape. Etching damage induced by the RIE was revealed by using photoluminescence measurements. The effects of surface recombination at the etching sidewalls were large, but the effects were suppressed by using HCl and (NH4)(2)S-x treatments. The damage was low enough to create small active optical devices, such as photonic crystals, comparably with current InGaAsP systems. (c) 2006 American Vacuum Society., JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B, A V S AMER INST PHYSICS, 24(5)2291 - 2294, Sep. 2006, English, 研究論文(学術雑誌)
  • Study of InAsN quantum dots on GaAs substrates by molecular beam epitaxy
    A Ueta, K Akahane, S Gozu, N Yamamoto, N Ohtani
    InAsN QDs on (001) GaAs substrates were studied. Each InAsN QD was surrounded by four {011} facets and had a pyramidal shape. However, InAs QDs had a half sphere shape. The InAsN QDs had size distributions with two maxima. This might be explained by the existence of very stable {011} facets and curved surfaces between the {011} facets. The curved surfaces act as the incorporation sites to grow the QD larger. This means that large InAsN QDs will be suppress the further growth, because of the smaller occupations of the curved surfaces to the total surface area of the InAsN QD. Excess In atoms supplied on the {001} plane will accumulate strains and form new small QDs by the Stranski-Krastanow (S-K) growth mode. This turns out two maxima of the size distribution. These results will open the possibility of obtaining the size control and small distributions of the InAsN QDs. We investigated the emission properties of these InAsN QDs. An emission peak of nearly 1300 nm from the InAsN QDs with a plasma power of 350 W was observed. InAsN QDs are an attractive material for achieving emissions in the fiber-optic communication wavelengths., PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, WILEY-V C H VERLAG GMBH, 243(7)1514 - 1518, Jun. 2006, English, 研究論文(学術雑誌)
  • Optical communications waveband lasing from Sb-based quantum dot vertical-cavity laser
    N Yamamoto, K Akahane, SI Gozu, A Ueta, N Ohtani
    Antimonide-based quantum dot (Sb-based QD) vertical-cavity surface-emitting lasers (VCSELs) were developed to operate in the 1.3- and 1.55-mu m optical communications wavebands. The Sb-based QD-VCSELs, which contain high-density InGaSb QDs that are Sb-based material, were fabricated on (001)-oriented GaAs substrates. The high-density InGaSb QDs, used an active media, were created by using a useful method of a silicon atom irradiation. The Sb-based QD-VCSELs in continuous wave operation at room temperature under optically pumped conditions emitted long wavelengths at 1.25 and 1.55 mu m with a low-threshold power. (c) 2006 Elsevier B.V. All rights reserved., PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, ELSEVIER SCIENCE BV, 32(1-2)516 - 519, May 2006, English, 研究論文(学術雑誌)
  • Change in band configuration of In0.57Ga0.43As1-xSbx/AlAs0.48Sb0.52 quantum wells from type-II to type-I by increasing Sb composition x
    S Gozu, K Akahane, N Yamamoto, A Ueta, N Ohtani
    InGaAs/AIAsSb systems lattice matched to InP have two distinguishable features: a high conduction band offset and type-II band configuration. Although, the former results in a large intersubband transition (ISBT) at conduction band, the latter makes it difficult to use the effective interband transition (IBT). To overcome this latter problem, the effect of the Sb was investigated because Sb would act as a band modulator from type-II to type-I. In0.57Ga0.43As1-xSbx/AlAs0.48Sb0.52 single quantum well (SQW) samples with various Sb compositions x, were grown on GaAs substrates via AlAS(0.48)Sb(0.52) buffer layer. Their photoluminescence (PL) properties were examined to identify their band configurations. When the excitation laser power was increased, the PL property of In0.57Ga0.43As SQW sample, showed a larger blue shift than that of In0.57Ga0.43As1-xSbx (x >= 0.13) ones. This indicates that the band configuration modulates from type-II to type-I when the antimonide composition is larger than 0.13. These findings indicate that new functional devices can be fabricated using a combination of IBT and ISBT. (c) 2006 Elsevier B.V. All rights reserved., PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, ELSEVIER SCIENCE BV, 32(1-2)230 - 233, May 2006, English, 研究論文(学術雑誌)
  • 1.5 mu m emission from InAs quantum dots with InGaAsSb strain-reducing layer grown on GaAs substrates
    K Akahane, N Yamamoto, S Gozu, A Ueta, N Ohtani
    InGaAsSb strain-reducing layers (SRLs) are applied to cover InAs quantum dots (QDs) grown on GaAs substrates. The compressive strain induced in InAs QDs from the GaAs is reduced due to the tensile strain induced by the InGaAsSb SRL, because the lattice constant of InGaAsSb is closer to InAs lattice constant than that of GaAs, resulting in a significant red shift of photoluminescence peaks of the InAs QDs. The emission wavelength from InAs QDs can be controlled by changing the Sb composition of the InGaAsSb SRL. The 1.5 mu m band emissions were achieved in the sample with an InGaAsSb SRL whose Sb compositions were above 0.3. The calculation of the electron and the hole wave functions using the transfer matrix method indicates that the electron and the hole were localized around InAs QDs and InGaAsSb SRL. (c) 2006 Elsevier B.V. All rights reserved., PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, ELSEVIER SCIENCE BV, 32(1-2)81 - 84, May 2006, English, 研究論文(学術雑誌)
  • Observation of subband resonances between high-energy states in a series of asymmetric double-quantum-well superlattice systems
    M Hosoda, M Sato, Y Hirose, T Shioji, J Nohgi, C Domoto, N Ohtani
    We report subband resonances related to higher energy subbands in a series of GaAs/AlAs asymmetric double quantum well superlattice (ADQW-SL) systems under electric field. Various phenomena can be observed in these ADQW-SL systems, such as the Gamma-X-Gamma electron resonance and transport, hole subband resonance, phonon replica, and Gamma-Gamma anticrossings. These resonances greatly affect photoluminescence properties, photocurrent-voltage characteristics, and photocurrent impulse responses. We also investigated the X-Gamma transfer mechanism in these systems., PHYSICAL REVIEW B, AMERICAN PHYSICAL SOC, 73(16)165329 , Apr. 2006, English, 研究論文(学術雑誌)
  • Selective formation of self-organized InAs quantum dots grown on patterned GaAs substrates by molecular beam epitaxy
    A Ueta, K Akahane, S Gozu, N Yamamoto, N Ohtani
    We have studied the formation of self-organized InAs quantum dots (QDs) on patterned GaAs substrates. To enhance the migration of the atoms, the InAs QDs were grown by periodic supply epitaxy. For InAs growth on line patterns that were parallel to the [(1) over bar 10] direction, the InAs QDs were selectively formed on the A-planes. Conversely, no selective formation of InAs QDs was observed on line patterns, that were parallel to the [110] direction. This can be explained by the stability of the In atoms on the A-steps; they are more stable because they have only one dangling bond, and the other three bonds are attached to As atoms below them. Therefore, accumulated InAs on the A-steps can form QDs at an early stage. The mu-photoluminescence properties of the InAs QDs were Studied, and strong emissions from the InAs QDs on the patterned areas were observed even at room temperature. Further work on this issue may reveal possibilities for controlling the exact position of a few InAs QDs., JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS, INST PURE APPLIED PHYSICS, 45(4B)3556 - 3559, Apr. 2006, English, 研究論文(学術雑誌)
  • 1.55-mu m-waveband emissions from Sb-based quantum-dot vertical-cavity surface-emitting laser structures fabricated on GaAs substrate
    N Yamamoto, K Akahane, S Gozu, A Ueta, N Ohtani
    We have developed antimonide-based quantum-dot vertical-cavity Surface-emitting laser (Sb-based QD-VCSEL) structures for operation in the 1.3 and 1.55 mu m optical communication wavebands. They were fabricated on a (001)-oriented GaAs Substrate and contain high-density InGaSb QDs as the active medium owing to the use of silicon atom irradiation. Testing demonstrated that they have optical-communication-waveband emission peaks at around 1.55 mu m in cw wave operation at room temperature under conditions of optical pumping and current injection. Additionally, it was found that these Sb-based QD-VCSELs have threshold characteristics in the emission intensities vs both optical-pumping power and current., JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS, INST PURE APPLIED PHYSICS, 45(4B)3423 - 3426, Apr. 2006, English, 研究論文(学術雑誌)
  • All-optical control of the resonant-photon tunneling effect observed in GaAs/AlGaAs multilayered structures containing quantum dots
    N Yamamoto, K Akahane, SI Gozu, N Ohtani
    We observed the resonant-photon tunneling (RPT) effect at the 1.5 mu m optical communications waveband in a GaAs/AlGaAs multilayered structure containing InGaSb quantum dots (QDs) as an active medium by a silicon prism total-reflection measurement. A sharp reflectance dip due to the RPT effect was clearly observed at a resonant-incidence angle in the total reflection region. The resonant-incidence angle clearly shifted when control light was irradiated onto the QD active layer. We demonstrated that the reflectance around the resonant-incidence angle can be controlled all-optically using the control light irradiation. These results clearly indicate the possibility of creating RPT devices for all-optical signal processing in the optical communication wavebands. (c) 2005 American Institute of Physics., APPLIED PHYSICS LETTERS, AMER INST PHYSICS, 87(23)231119 , Dec. 2005, English, 研究論文(学術雑誌)
  • Initial growth stage of GaSb on Si(001) substrates with AlSb initiation layers
    K Akahane, N Yamamoto, S Gozu, A Ueta, N Ohtani
    Although we recently found that a high-quality GaSb film can be grown on Si substrate by using an AlSb initiation layer, the growth mechanism, especially the effects of AlSb initiation layer, was not clear. Therefore, we investigated the initial growth stage of GaSb on Si(0 0 1) substrates with AlSb initiation layers by atomic force microscopy. When small AlSb islands were formed on the Si substrate before the growth of GaSb, two-dimensional GaSb growth occurred. In contrast, without the growth of AlSb small islands, large GaSb islands were formed on the Si substrate. Therefore, the AlSb islands played an important role in preventing excessive surface diffusion of Ga atoms on the Si surface and promoting two-dimensional growth of GaSb. (c) 2005 Elsevier B.V. All rights reserved., JOURNAL OF CRYSTAL GROWTH, ELSEVIER SCIENCE BV, 283(3-4)297 - 302, Oct. 2005, English, 研究論文(学術雑誌)
  • Electron scattering between X and L indirect valleys in type-I GaAs/AlAs semiconductor superlattices
    M Hosoda, J Nohgi, N Ohtani
    We report evidence that an X-L transfer mechanism plays a significant role in carrier transport in semiconductor superlattices (SLs) under an electric field. Experimental observation of anomalously delayed photocurrent and photoluminescence from higher-energy subband states in a GaAs/AlAs type-I SL revealed that a switch of the electron transport path from Gamma-X-Gamma to Gamma-X-L occurred by an electric field induced change of the subband alignment of the ground X state (X-1) in the barrier and the ground L state (L-1) in the adjacent well. The experimental results demonstrate that the electron scattering among the indirect valleys, i.e., the X-1 and L-1 subband states, is a significant transport channel in semiconductor SLs under high electric fields., PHYSICAL REVIEW B, AMER PHYSICAL SOC, 72(3)033317 , Jul. 2005, English, 研究論文(学術雑誌)
  • Characteristics of coupled mode of excitonic quantum beat and coherent longitudinal optical phonon in GaAs/AlAs multiple quantum wells
    T Furuichi, K Mizoguchi, O Kojima, K Akahane, N Yamamoto, N Ohtani, M Nakayama
    We have investigated the coupled mode of excitonic quantum beats and coherent GaAs-like longitudinal optical (LO) phonons in GaAs/AlAs multiple quantum wells with the splitting energy of heavy- and light-hole excitons close to the LO phonon energy. The time-domain signals show the coherent oscillation of the coupled mode between the quantum beat and the LO phonon with a fast dephasing time and the long-lived coherent GaAs-like LO phonon oscillation. The dependence of the coupled-mode frequency on splitting energy exhibits an anticrossing behavior. We demonstrate that the amplitude of the long-lived coherent LO phonon is manipulated by controlling the coupled mode with a double-pulse excitation technique. (c) 2004 Elsevier B.V. All rights reserved., JOURNAL OF LUMINESCENCE, ELSEVIER SCIENCE BV, 112(1-4)142 - 145, Apr. 2005, English, 研究論文(学術雑誌)
  • Strong photoluminescence and laser operation of InAs quantum dots covered by a GaAsSb strain-reducing layer
    K Akahane, N Yamamoto, S Gozu, N Ohtani
    GaAsSb strain-reducing layers (SRLs) are applied to cover InAs quantum dots (QDs) grown on GaAs substrates. The compressive strain induced in InAs QDs is reduced due to the tensile strain induced by the GaAsSb SRL, resulting in a redshift of photoluminescence (PL) peaks of the InAs QDs. A strong PL signal around a wavelength of 1.3 mum was observed even at room temperature. A laser diode containing InAs QDs with GaAsSb SRLs in the active region was fabricated, which exhibits laser oscillation in pulsed operation at room temperature. These results indicate that GaAsSb SRLs have a high potential for fabricating high efficient InAs QDs laser diodes operating at long-wavelength regimes. (C) 2004 Elsevier B.V. All rights reserved., PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, ELSEVIER SCIENCE BV, 26(1-4)395 - 399, Feb. 2005, English, 研究論文(学術雑誌)
  • Anomalous photoluminescence branches observed in an asymmetric double-quantum-well superlattice
    N Ohtani, N Yamamoto, J Nohgi, M Hosoda
    A series of photoluminescence (PL) branches in an asymmetric double-quantum-sell superlattice were observed. These PL spectra revealed an anomalous optical property that is caused by formation of electricfield domains. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim., Physica Status Solidi C - Conferences and Critical Reviews, Vol 2, No 8, WILEY-V C H VERLAG GMBH, 2(8)3006 - 3009, 2005, English, 研究論文(国際会議プロシーディングス)
  • Growth of InAsSb quantum dots on GaAs substrates using periodic supply epitaxy
    A Ueta, SI Gozu, K Akahane, N Yamamoto, N Ohtani
    InAsSb quantum dots (QDs) were studied by introducing periodic supply epitaxy. Photoluminescence spectra from InAsSb QDs showed strong emissions even at room temperature. The InAsSb QDs by periodic supply epitaxy allowed the control of emission wavelengths of optical fiber communication systems. The control of the density of InAsSb QDs in the range from 4.5 x 10(9) to 3.5 x 10(10) cm(-2) without changing emission wavelengths was demonstrated. These results indicate that periodic supply epitaxy is a useful technique for various optical devices such as QD lasers and single photon emitters., JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS & EXPRESS LETTERS, INST PURE APPLIED PHYSICS, 44(20-23)L696 - L698, 2005, English, 研究論文(学術雑誌)
  • High-quality GaSb/AlGaSb quantum well grown on Si substrate
    K Akahane, N Yamamoto, S Gozu, N Ohtani
    We fabricate a GaSb/AlGaSb multi-quantum well (MQW) on a Si substrate. A high-quality GaSb/AlGaSb MQW sample can be Grown on a Si substrate using an AlSb initiation layer. We can obtain a strong emission of 1.55 mu m at room temperature from the sample, which has an 8 nm well width. Furthermore, we can control the emission wavelength by simply changing well width. The emission energy shows a good agreement with the theoretical curve. This indicates that the controllability of this material system is very high even when grown on a Si substrate., JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS & EXPRESS LETTERS, INST PURE APPLIED PHYSICS, 44(1-7)L15 - L17, 2005, English, 研究論文(学術雑誌)
  • Over 1.3 mu m CW laser emission from InGaSb quantum-dot vertical-cavity surface-emitting laser on GaAs substrate
    N Yamamoto, K Akahane, S Gozu, N Ohtani
    An Sb-based quantum-dot vertical-cavity surface-emitting laser (QD-VCSEL) operating in the 1.3 mum optical communication waveband is presented. A QD-VCSEL containing high-density InGaSb QDs in the active region is fabricated on GaAs substrate. The density of InGaSb QDs is drastically increased using a new method of an Si atom irradiation technique. A long-wavelength laser emission at 1.34 mum is successfully achieved on the InGaSb QD-VCSEL in CW operation at room temperature., ELECTRONICS LETTERS, IEE-INST ELEC ENG, 40(18)1120 - 1121, Sep. 2004, English, 研究論文(学術雑誌)
  • Coupled mode of the coherent optical phonon and excitonic quantum beat in GaAs/AlAs multiple quantum wells
    K Mizoguchi, O Kojima, T Furuichi, M Nakayama, K Akahane, N Yamamoto, N Ohtani
    We report on the time-resolved study of the coherent coupled oscillation of excitonic quantum beats and GaAs-like longitudinal optical (LO) phonons in GaAs/AlAs multiple quantum wells. The time-domain signals in GaAs/AlAs multiple quantum wells observed by using a reflection-type pump-probe technique show the coherent coupled oscillation with fast dephasing time and the long-lived coherent GaAs-like LO phonon oscillation. Under the condition that the splitting energy of the heavy-hole and light-hole excitons is close to the energy of the GaAs-like LO phonon, the dependence of the coupled oscillation frequency on the splitting energy exhibits an anticrossing behavior. We discuss the dispersion relation of the coupled oscillation from the viewpoint of the linear coupling of the excitonic quantum beat and the GaAs-like LO phonon through the longitudinal polarization., PHYSICAL REVIEW B, AMER PHYSICAL SOC, 69(23)233302 , Jun. 2004, English, 研究論文(学術雑誌)
  • All-optical switching and memorizing devices using resonant photon tunneling effect in multi-layered GaAs/AlGaAs structures
    N Yamamoto, N Ohtani
    We propose a new structure for all-optical switch (AOS) and all-optical memory (AOM) devices that use the resonant photon tunneling effect in multi-layered GaAs/AlGaAs structures. Numerical calculations revealed a sharp peak in tunneling probability at a resonant incidence angle in double-barrier AOS structures. The resonance angle can be controlled by changing the refractive index of an active layer. During simulation, we demonstrated that the AOS with the new structure acts as an all-optical NOT gate. We also found that optical bi-stability occurs in the dependence of reflectivity on control light intensity. This bi-stability is caused by the penetration of intense input light to the active layer based on the resonant photon tunneling effect, and it is clear that an AOM can be achieved through this bi-stability effect. These results indicate that the resonant photon tunneling effect in multi-layered semiconductor structures can effectively be used to attain efficient all-optical devices., JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS, INST PURE APPLIED PHYSICS, 43(4A)1393 - 1397, Apr. 2004, English, 研究論文(学術雑誌)
  • Formation of electric-field domains in an asymmetric double-quantum-well GaAs/AlAs superlattice
    N Ohtani, Y Hirose, T Nishimura, T Aida, M Hosoda
    We report the observation of electric-field domain formation in an asymmetric double-quantum-well GaAs/AlAs superlattice. Photoluminescence (PL) spectra exhibit several branches with increasing bias voltage. These steeply red-shifted PL branches can be attributed to Stark-ladder transitions between the heavy hole ground state in the narrow quantum well and the electron ground state in the wide quantum well in the high-field domain. According to our calculations, electrons and holes in the high-field domain are separated into different quantum wells, resulting in a large reduction of the radiative recombination energy with increasing electric field. Each PL branch most likely corresponds to the moving of the domain boundary., SEMICONDUCTOR SCIENCE AND TECHNOLOGY, IOP PUBLISHING LTD, 19(4)S89 - S90, Apr. 2004, English, 研究論文(学術雑誌)
  • Uniaxial-strain-induced transition from type-II to type-I band configuration of quantum well microtubes
    N Ohtani, K Kishimoto, K Kubota, S Saravanan, Y Sato, S Nashima, P Vaccaro, T Aida, M Hosoda
    We fabricated quantum well (QW) microtubes by rolling up thin films containing QWs and measured their photoluminescence (PL) properties. Before fabricating microtubes, no PL signal is observed in the QW at all, because their band configuration is indirect, i.e., type-II. However, PL signal is detected in the QW microtubes. This result clearly indicates that the transition from type-II to type-I band configuration of QW microtubes is induced by uniaxial-strain effect. (C) 2003 Elsevier B.V. All rights reserved., PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, ELSEVIER SCIENCE BV, 21(2-4)732 - 736, Mar. 2004, English, 研究論文(学術雑誌)
  • Growth of high-density InGaSb quantum dots on silicon atoms irradiated GaAs substrates
    N Yamamoto, K Akahane, N Ohtani
    We discuss a technique that allows us to grow high-density GaSb and InGaSb quantum dots (QDs) on (001)-oriented GaAs substrates. We study the use of Si atom irradiation on the substrate surface as an anti-surfactant before the QDs fabrication. It is clear that the densities of GaSb and InGaSb QDs are drastically enhanced with the Si atom irradiation. Photoluminescence intensities from these QDs are also increased with the Si atom irradiation. These results indicate that the Si atom irradiation technique is useful to improve the properties of the Sb-based QDs. (C) 2003 Elsevier B.V. All rights reserved., PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, ELSEVIER SCIENCE BV, 21(2-4)322 - 325, Mar. 2004, English, 研究論文(学術雑誌)
  • Long-wavelength light emission from InAs quantum dots covered by GaAsSb grown on GaAs substrates
    K Akahane, N Yamamoto, N Ohtani
    We fabricated InAs quantum dots (QDs) with a GaAsSb strain-reducing layer (SRL) on a GaAs(001) substrate. The wavelength of emission from InAs QD is shown to be controllable by changing the composition and thickness of the SRL. An increase in photoluminescence intensity with increasing compositions of Sb and thickness of the GaAsSb SRL is also seen. The efficiency of radiative recombination was improved under both conditions because the InAs/GaAsSb/GaAs hetero-interface band structure more effectively suppressed carrier escape from the InAs QDs. (C) 2003 Elsevier B.V. All rights reserved., PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, ELSEVIER SCIENCE BV, 21(2-4)295 - 299, Mar. 2004, English, 研究論文(学術雑誌)
  • Heteroepitaxial growth of GaSb on Si(001) substrates
    K Akahane, N Yamamoto, S Gozu, N Ohtani
    We investigated the heteroepitaxial growth of GaSb on Si(001) substrate. We found that high-quality GaSb films can be grown on Si substrate by introducing an AlSb initiation layer. We obtained a narrow X-ray diffraction rocking curve and over-1400-nm emission from GaSb/AlGaSb quantum wells by optimizing the growth temperature and thickness of the AlSb initiation layer. During the growth of GaSb, the AlSb initiation layer played two important roles for improving crystal quality; i.e., it acted as a surfactant and as a buffer layer preventing generation and propagation of dislocations. (C) 2004 Elsevier B.V. All rights reserved., JOURNAL OF CRYSTAL GROWTH, ELSEVIER SCIENCE BV, 264(1-3)21 - 25, Mar. 2004, English, 研究論文(学術雑誌)
  • Self-consistent calculation of subband occupation and electron-hole plasma effects: Variational approach to quantum well states with Hartree and exchange-correlation interactions
    T Ando, H Taniyama, N Ohtani, M Nakayama, M Hosoda
    We present a numerically stable computational method to calculate multiple subband states in quantum wells based on the variational minimization of the total Hamiltonian involving the nonlinear interaction terms such as the Hartree and exchange-correlation interactions. Application of a nonlinear multidimensional minimization procedure to the total Hamiltonian enables us to realize multiple state computations of improved convergence. As an example of the multiple state calculation, subband states of interacting electrons and holes in single quantum wells are computed. We demonstrate that the dependence of the overlap integral of electron and hole wave functions on the electron-hole density is significantly modified by the exchange-correlation interaction. Furthermore, we show the effects of the nonlinear interactions on the carrier population and chemical potential in multiple subbands at various temperatures and carrier concentrations. This method is expected to be a convenient tool for investigating the high-density behavior of carriers in semiconductor low-dimensional structures. (C) 2003 American Institute of Physics., JOURNAL OF APPLIED PHYSICS, AMER INST PHYSICS, 94(7)4489 - 4501, Oct. 2003, English, 研究論文(学術雑誌)
  • Role of A1 in spacer layer on the formation of stacked InAs quantum dot structures on InP(311)B
    K Akahane, N Yamamoto, N Ohtani, Y Okada, M Kawabe
    We have fabricated stacked InAs quantum dot (QD) structures on InP(311)B substrate by using novel strain controlled technique, and investigated the role of At atoms which were used in strain-compensating spacer layers. Ultrahigh density (similar to10(12) cm(-2)) of 20 QDs stacks and significantly improved uniformity of QDs were achieved in samples with 20 nm-thick InGaAlAs strain-compensating spacer layers. From secondary ion mass spectrometry measurement, the composition of InGaAlAs layers was uniform, but a large fluctuation of composition was observed if InGaAs layers were used. We found that Al atoms prevent the segregation of In in InGaAlAs, and InAs QDs grown on these InGaAlAs strain-compensating spacer layers result in an uniform array in three dimensions on InP(311)B substrates. (C) 2003 Elsevier Science B.V. All rights reserved., JOURNAL OF CRYSTAL GROWTH, ELSEVIER SCIENCE BV, 256(1-2)7 - 11, Aug. 2003, English, 研究論文(学術雑誌)
  • Quantum-well microtube constructed from a freestanding thin quantum-well layer
    M Hosoda, Y Kishimoto, M Sato, S Nashima, K Kubota, S Saravanan, PO Vaccaro, T Aida, N Ohtani
    We fabricated and experimentally investigated a nanostructure known as a quantum-well (QW) microtube, which is a fine tube with a micron- or nanometer-order diameter fabricated by rolling a semiconductor GaAs QW. Although the wall thickness is only 40 nm, the system retains the quantum properties of a QW, and photoluminescence from the QW subband can be clearly observed. Even though the QW width is sufficiently small to make the QW subband type-II band-aligned, a type-II to type-I transition caused by uniaxial strain in the microtube allows for optical emission. (C) 2003 American Institute of Physics., APPLIED PHYSICS LETTERS, AMER INST PHYSICS, 83(5)1017 - 1019, Aug. 2003, English, 研究論文(学術雑誌)
  • Photoluminescence property of uniaxial strained GaAs/AlGaAs quantum wells contained in a micro-tube
    N Ohtani, K Kubota, P Vaccaro, T Aida, A Hosoda
    Photoluminescence (PL) properties of uniaxially strained GaAs/AlGaAs quantum wells contained in a micro-tube have been investigated. The PL peak is clearly redshifted due to the strain effect. (C) 2002 Elsevier Science B.V. All, rights reserved., PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, ELSEVIER SCIENCE BV, 17(1-4)391 - 392, Apr. 2003, English, 研究論文(学術雑誌)
  • Growth of InAs quantum dots on a low lattice-mismatched AlGaSb layer prepared on GaAs(001) substrates
    N Yamamoto, K Akahane, S Gozu, N Ohtani
    Optical communication wavelength emissions from the quantum dots (QDs) structures prepared on (001)-oriented GaAs substrates are discussed. A new growth technique of low-stressed InAs QDs on the AlGaSb layer in a low lattice-mismatched (1.3%) InAs/AlGaSb system is presented. The average height and diameter of the 4-ML InAs QDs on AlGaSb are evaluated to 5.8 nm and 45.2 nm respectively with an average density of 2.18 x 10(10)/cm(2) using atomic force microscope (AFM) measurements. There is structural selectivity between the QDs layer and the flat hetero-interface under changing growth conditions in the InAs/AlGaSb system. Long-wavelength PL emissions around 1.3 mum and 1.55 mum can be achieved by embedding InAs QDs in AlGaSb layers. Therefore it is expected that low-stressed InAs QDs grown on a AlGaSb layer prepared on a GaAs substrate will be useful in the fabrication of novel QDs devices for optical-communication networks., FUNCTIONAL NANOMATERIALS FOR OPTOELECTRONICS AND OTHER APPLICATIONS, TRANS TECH PUBLICATIONS LTD, 99-100(1)49 - 52, 2003, English, 研究論文(学術雑誌)
  • Fabrication of ultra-high density InAs-stacked quantum dots by strain-controlled growth on InP(311)B substrate
    K Akahane, N Ohtani, Y Okada, M Kawabe
    We have fabricated stacked InAs quantum dots (QDs) on InP(3 1 1)B substrates. An ultra-high density of QDs was obtained by a method of compensation of strain by controlling the lattice constant of spacer layers without destruction of QDs size uniformity and ordering structure which is characteristic of QDs formation on (3 1 l)B surface. A strong 1.58 mum photoluminescence was observed in this sample at room temperature. (C) 2002 Elsevier Science B.V. All rights reserved., JOURNAL OF CRYSTAL GROWTH, ELSEVIER SCIENCE BV, 245(1-2)31 - 36, Nov. 2002, English, 研究論文(学術雑誌)
  • Numerically stable and flexible method for solutions of the Schrodinger equation with self-interaction of carriers in quantum wells
    T Ando, H Taniyama, N Ohtani, M Hosoda, M Nakayama
    A numerically stable method to calculate the quantum states of carriers based on the variational principle is proposed. It is especially effective for the carriers confined in the quantum wells under the influence of self-interaction of the carriers. In this treatment, a wave function is defined as a set of scalar numbers based on the finite-difference approach. An action defined as the expectation value of a Hamiltonian becomes a multivariate function of the wave function. Application of numerical multidimensional minimization procedures to the action can achieve stable convergence even under the conditions where the conventional self-consistent approach to Schrodinger and Poisson equations fails to give solutions. Application to the calculations of ground states in modulation-doped single quantum wells is demonstrated, and quantitative comparison to the conventional method is also presented. This method has implications not only for numerical procedures, but also for the numerical realization of the variational principle, a fundamental concept in physics., IEEE JOURNAL OF QUANTUM ELECTRONICS, IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 38(10)1372 - 1383, Oct. 2002, English, 研究論文(学術雑誌)
  • Photoluminescence of GaAs/AlGaAs micro-tubes containing uniaxially strained quantum wells
    K Kubota, PO Vaccaro, N Ohtani, Y Hirose, M Hosoda, T Aida
    We fabricated micro-tubes containing two GaAs/AlGaAs quantum wells (QWs) in a section of the tube layer, and studied the optical properties of the embedded QWs. A multilayer structure composed of GaAs and AlGaAs layers and a lattice-mismatched InGaAs layer was epitaxially grown on a GaAs substrate by MBE. This multilayer structure rolled up by the built-in strain when it was freed from the substrate. By measuring the photoluminescence peak shift of the QWs caused by the uniaxial strain, we were able to determine the radial profile of the strain within the micro-tube wall. (C) 2002 Published by Elsevier Science B.V., PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, ELSEVIER SCIENCE BV, 13(2-4)313 - 316, Mar. 2002, English, 研究論文(学術雑誌)
  • Excitation of right- and left-circularly polarized photoluminescence in silicon-based luminescent materials
    N Yamamoto, Hosako, I, M Akiba, N Ohtani, H Takai
    Silicon-based luminescent materials have been investigated widely for use in advanced optoelectronic integrated circuits and display devices. We tested the application of such materials and investigated their basic characteristics. In this study, luminescent silicon was produced by photochemical etching. The polarization of the photoluminescence under a right and left circularly polarized laser (Ar+ laser, 488 nm) irradiation was observed at 20-290 K. We found that the right- and left-circularly polarized photoluminescence intensities were intensified under right- and left-circular excitation, respectively. Information on the circular polarization of the laser excitation is apparently preserved in silicon-based luminescent material. This tendency was observed at room temperature. Additionally, a luminescence mechanism is studied by the spin dynamics of the photon-electron interaction. (C) 2002 American Institute of Physics., JOURNAL OF APPLIED PHYSICS, AMER INST PHYSICS, 91(5)2725 - 2728, Mar. 2002, English, 研究論文(学術雑誌)
  • Electric-field effects on photoluminescence properties in a GaAs/AlAs marginal type-I superlattice
    N Ohtani, C Domoto, N Egami, H Mimura, T Ando, M Nakayama, M Hosoda
    We have investigated the electric-field effects on the cw and time-resolved photoluminescence (PL) properties in a marginal type-I GaAs/AlAs superlattice (SL) whose lowest X state (X-1) is situated in the lowest Gamma(Gamma(1)) miniband, In the low bias voltage regime, the PL spectra reveal the transition between type-I and type-II radiative recombination processes caused by Wannier-Stark localization. In contrast, in the high bias voltage regime, the decay time of the time-resolved PL is prolonged. This is because of delayed carrier transport caused by Gamma-X transfer. From these results, it was found that marginal type-I SLs present various interesting phenomena that originate from the competitive carrier transport among the Gamma miniband, the localized Gamma Stark-ladder states, and the X-1 state. (C) 2000 Elsevier Science B.V. All rights reserved., PHYSICA E, ELSEVIER SCIENCE BV, 7(3-4)586 - 589, May 2000, English, 研究論文(学術雑誌)
  • Anomalous photoluminescence originating from resonance between X and intermediate states in GaAs/AlAs superlattices
    N Ohtani, C Domoto, K Kuroyanagi, N Egami, M Hosoda
    We report the observation of a novel radiative recombination process in a biased GaAs/AlAs superlattice. Resonance of X states with an intermediate state, which is located in an energy range between Gamma 1 and Gamma 2 states, leads to anomalous photoluminescence properties. The possible origin of this intermediate state is an impurity state or an L state confined in the GaAs well. (C) 2000 Elsevier Science B.V. All rights reserved., JOURNAL OF LUMINESCENCE, ELSEVIER SCIENCE BV, 87-9(1)415 - 417, May 2000, English, 研究論文(学術雑誌)
  • Photoluminescence and carrier transport properties via the intersubband scattering in a GaAs/AlAs superlattice under applied electric field
    M Ando, M Nakayama, H Takeuchi, H Nishimura, N Ohtani, N Egami, M Hosoda, H Mimura
    We report on the photoluminescence and carrier-transport properties of a GaAs(6.2 nm)AlAs(1.7 nm) superlattice embedded in a p-i-n diode structure from the viewpoint of the intersubband scattering under an applied bias voltage (electric field). We have found the enhancement of the photoluminescence intensity in the reverse-bias region from 4 to 16 V, accompanying a dip structure. The transient photocurrent simultaneously exhibits delayed-transport profiles in the above bias region. The calculated results of the energies of the Gamma 1, Gamma 2 and X1 subbands indicate that the electron scattering between the Gamma and the X subbands causes the above experimental results. (C) 2000 Elsevier Science B.V. All rights reserved., JOURNAL OF LUMINESCENCE, ELSEVIER SCIENCE BV, 87-9(1)411 - 414, May 2000, English, 研究論文(学術雑誌)
  • Carrier transport in GaAs/AlAs type-II superlattices under electric field: Switch from X-X to Gamma-Gamma transfer
    M Hosoda, N Ohtani, K Kuroyanagi, C Domoto
    A switch in carrier transport from X-X to Gamma-X-Gamma is found in a GaAs/AlAs type-II superlattice under an electric field. This phenomenon is caused by an X-Gamma transfer, as demonstrated by the photoluminescence, photocurrent response, and current-voltage characteristics. Under a high electric field, most of the electrons flow through the Gamma path even in type-II superlattices. (C) 2000 American Institute of Physics. [S0003-6951(00)00214-X]., APPLIED PHYSICS LETTERS, AMER INST PHYSICS, 76(14)1866 - 1868, Apr. 2000, English, 研究論文(学術雑誌)
  • Electric-field-induced combination of Wannier-Stark localization and type-I-type-II crossover in a marginal type-I GaAs/AlAs superlattice
    N Ohtani, C Domoto, N Egami, H Mimura, M Ando, M Nakayama, M Hosoda
    We have investigated the influence of carrier transport on photoluminescence (PL) properties in a marginal type-I GaAs/A1As superlattice whose lowest X state (X-1) is situated in the lowest Gamma (Gamma(1)) miniband. By applying a bias voltage, the PL signal from the lower edge of the Gamma(1) miniband disappears, while Stark ladder PL is observed from the X-1 state. With a further increase of the bias voltage, the PL signal from the X state disappears, while the PL signal from the Gamma(1) state reappears as a Stark ladder transition. To confirm the electric-field dependence of the Gamma(1)- and X-1-related transition energies observed in the PL spectra, the corresponding electroreflectance spectra have been also measured. This PL property can be attributed to competitive carrier transport among the Gamma miniband, the localized Gamma Stark ladder states, and the X-1 state, which results in the transition between type-I and type-II radiative recombination processes., PHYSICAL REVIEW B, AMER PHYSICAL SOC, 61(11)7505 - 7510, Mar. 2000, English, 研究論文(学術雑誌)
  • Phase diagram of static and dynamic electric field domain formation in semiconductor superlattices
    N Ohtani, N Egami, HT Grahn, KH Ploog
    We have performed a detailed investigation on photocurrent self-oscillations in a weakly coupled GaAs/AlAs superlattice to derive the phase diagram of static and dynamic domain formation. In the parameter space of the carrier density versus reverse bias voltage, the resulting phase diagram clearly exhibits a boundary between oscillating and static domains. However, as recent calculations have shown, the value of the carrier density at the boundary depends on the reverse bias voltage. (C) 1999 Published by Elsevier Science B.V. All rights reserved., PHYSICA B-CONDENSED MATTER, ELSEVIER SCIENCE BV, 272(1-4)205 - 208, Dec. 1999, English, 研究論文(学術雑誌)
  • Light-hole Stark-ladder photoluminescence induced by heavy-hole-light-hole resonance in a GaAs/InAIAs superlattice
    K Kuroyanagi, N Ohtani, N Egami, K Tominaga, M Hosoda, H Takeuchi, M Nakayama
    We have studied the photoluminescence properties of a GaAs/InAlAs strained superlattice as a function of an applied electric field. We have detected the appearance of light-hole Stark-ladder photoluminescence resulting from heavy-hole-light-hole resonance induced by the applied electric field. From calculated overlap integrals between electron and hole wave functions, it is demonstrated that the light-hole Stark-ladder photoluminescence is observable in a GaAs/InAlAs superlattice. (C) 1999 Elsevier Science B.V. All rights reserved., PHYSICA B, ELSEVIER SCIENCE BV, 272(1-4)198 - 201, Dec. 1999, English, 研究論文(学術雑誌)
  • Controllable bistabilities and bifurcations in a photoexcited GaAs AlAs superlattice
    KJ Luo, SW Teitsworth, H Kostial, HT Grahn, N Ohtani
    APPLIED PHYSICS LETTERS, AMER INST PHYSICS, 74(25)3845 - 3847, Jun. 1999, English, 研究論文(学術雑誌)
  • Influence of strain effects on hole-subband resonances in GaAs/InAlAs superlattices
    K Kuroyanagi, N Ohtani, N Egami, K Tominaga, M Nakayama
    We have investigated the influence of strain effects on hole-subband resonances in GaAs/InAlAs strained superlattices. The samples consist of the same superlattice structure with different buffer layers. We have found that the photocurrent-voltage characteristics exhibit peaky structures due to the hole-subband resonance and that the resonant conditions are modified by the difference in the lattice constant of the buffer layers. The resonant conditions have been discussed from the subband-energy calculation based on an effective-mass approximation taking into account the strain effects. (C) 1999 Elsevier Science B.V. All rights reserved., APPLIED SURFACE SCIENCE, ELSEVIER SCIENCE BV, 142(1-4)633 - 636, Apr. 1999, English, 研究論文(学術雑誌)
  • Gamma-Chi electron transfer in GaAs/AlAs type-I superlattices
    H Mimura, M Hosoda, N Ohtani, K Yokoo
    The photoluminescence (PL) and photocurrent (Pc) were measured for GaAs/AlAs type-I superlattices under an applied electric field to investigate Gamma-X electron transfer. The PL due to the indirect recombination between the ground X state (X-1) and the ground heavy-hole state (HH1) is clearly observed. In addition, the dips in photocurrent, and enhancement of both intensity and lifetime of the direct PL between the ground Gamma (Gamma(1)) and HH1 states are observed at the Gamma(1)-X-n resonant voltages. These results indicate that Gamma-X electron transfer significantly influences the electron transport, even in type-I superlattices. (C) 1999 Elsevier Science B.V. All rights reserved., APPLIED SURFACE SCIENCE, ELSEVIER SCIENCE BV, 142(1-4)624 - 628, Apr. 1999, English, 研究論文(学術雑誌)
  • Electroluminescence in undoped GaAs/AlAs superlattice due to avalanche breakdown
    C Domoto, N Ohtani, K Kuroyanagi, PO Vaccaro, N Egami
    We have observed electroluminescence (EL) in an undoped GaAs/AlAs superlattice (SL) originating from avalanche breakdown. From EL and photocurrent-voltage characteristics, we found that a large number of electrons were injected into Gamma 2 states in the SL by avalanche breakdown under a high electric field. The EL spectra revealed two peaks: one having a longer wavelength originated from the radiative recombination process between Gamma 1 and hh1 states, while the other peak originated from the recombination between Gamma 2 and hh1 states., JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS, JAPAN J APPLIED PHYSICS, 38(4B)2577 - 2579, Apr. 1999, English, 研究論文(学術雑誌)
  • Transition between static and dynamic electric-field domain formation in weakly coupled GaAs/AlAs superlattices
    N Ohtani, N Egami, HT Grahn, KH Ploog, LL Bonilla
    The transition between static and dynamic electric-field domain formation has been investigated in undoped, photoexcited GaAs/AlAs superlattices by varying the carrier density and temperature. The frequency spectra of the photocurrent oscillations exhibit as a function of bias voltage regions without any current oscillations. With decreasing carrier density or increasing temperature, these voltage regions become narrower. These observations indicate that with increasing carrier density the domain formation changes from dynamic to static. However, thermal activation appears to enhance the formation of dynamic domains. [S0163-1829(98)52036-X]., PHYSICAL REVIEW B, AMER PHYSICAL SOC, 58(12)R7528 - R7531, Sep. 1998, English, 研究論文(学術雑誌)
  • Carrier transport affected by Gamma-X transfer in type-I GaAs/AlAs superlattices
    M Hosoda, N Ohtani, H Mimura, K Tominaga, T Watanabe, H Inomata, K Fujiwara
    We studied how T-X transfer affects electron transport in type-I GaAs/AlAs semiconductor superlattices under an electric field,Gamma to X transfer was found to degrade the sweep out of carriers, while X to Gamma resonance was observed to promote it. An anomalously delayed photocurrent was observed under ultrashort optical pulse excitation. This phenomenon is explained by a switch of the electron transport path from Gamma-Gamma to Gamma-X-Gamma-X, caused by a change in the subband alignment between the second Gamma state (Gamma(2)) in the well and the X-z ground state (X-1) in the adjacent barrier by the electric field. The delayed photocurrent could be reproduced by a simple simulation while considering the above carrier transport paths. These results provide evidence that Gamma-X-Gamma real-space transfer considerably affects carrier transport even in type-I superlattices under an electric field., PHYSICAL REVIEW B, AMER PHYSICAL SOC, 58(11)7166 - 7180, Sep. 1998, English, 研究論文(学術雑誌)
  • Photoluminescence detection of the X-electron resonance in a GaAs/AlAs type-II superlattice
    M Nakayama, M Ando, Y Kumamoto, H Nishimura, N Ohtani, N Egami, K Fujiwara, M Hosoda
    We report on a photoluminescence study of the X-electron resonance in a GaAs (3.4 nm)/AlAs (5.1 nm) type-II superlattice embedded in an n-i-n structure. We have measured the photoluminescence spectra of the type-II exciton consisting of the first X-z (X(z)1) electron and the Gamma 1 heavy hole, which are confined in the AlAs and GaAs layers, respectively, as a function of an applied bias (electric field). We find that the electric-field dependence of the type-II photoluminescence intensity reflects various X-electron resonances such as X(z)1-X-xy 1, X(z)1-X(z)2, and optical-phonon-assisted X(z)1-X(xy)1 The resonance conditions are discussed from the X-electron energies calculated in the framework of an effective-mass approximation taking account of lattice-mismatch strain and electric-field effects. In addition, it is demonstrated from the photocurrent-bias-voltage characteristics that the X-electron resonance improves the carrier sweep-out through the superlattice., PHYSICAL REVIEW B, AMER PHYSICAL SOC, 58(11)7216 - 7221, Sep. 1998, English, 研究論文(学術雑誌)
  • Carrier transport and photoluminescence affected by Gamma-X resonance in GaAs-AlAs type-I superlattices
    H Mimura, M Hosoda, N Ohtani, K Fujiwara, K Yokoo
    We observed two peaks (fast and slow components) in time-resolved photocurrents for type-I GaAs/AlAs superlattices (SLs). The calculations of the envelop function for the SLs showed that the X-1 state in the AlAs barriers resonates with the Gamma(2) state in the adjacent GaAs wells at the bias voltage where the slow components become larger. The phololumines-- cence measurements revealed that some electrons transfer from the Gamma(1) to X-1 states and flow through the X-1-Gamma(2)-X-1-Gamma(2) channel at the near X-1-Gamma(2) resonance voltage. The slow component in the transient photocurrents originates from the X-1-Gamma(2)-X-1-Gamma(2) electron transport. (C) 1998 Elsevier Science B.V. All rights reserved., PHYSICA E, ELSEVIER SCIENCE BV, 2(1-4)308 - 312, Jul. 1998, English, 研究論文(学術雑誌)
  • Photocurrent self-oscillations in undoped GaAs/AlAs superlattices modulated by an external ac voltage
    N Ohtani, N Egami, K Fujiwara, HT Grahn
    We report the observation of bifurcation patterns in the frequency spectra of current self-oscillations in an undoped, photoexcited GaAs/AlAs superlattice modulated by an external ac driving voltage. The ac frequency was adjusted to the golden mean times the fundamental frequency of the current self-oscillations. These bifurcation patterns exhibit transitions between periodic and chaotic oscillations as a function of the amplitude of the external driving voltage. A change in the laser intensity results in a very different bifurcation pattern indicating that these patterns are also strongly dependent on the carrier density. (C) 1998 Elsevier Science Ltd. All rights reserved., SOLID-STATE ELECTRONICS, PERGAMON-ELSEVIER SCIENCE LTD, 42(7-8)1509 - 1513, Jul. 1998, English, 研究論文(学術雑誌)
  • Wavefunction delocalization of strongly-localized Stark-ladder states in a GaAs/AlAs superlattice
    M Ando, M Nakayama, H Nishimura, M Hosoda, N Ohtani, N Egami, K Fujiwara
    We have investigated the wavefunction-localization properties as a function of electric field from similar to 30 to similar to 300 kV/cm in a GaAs(6.8 nm)/AlAs(0.8 nm) superlattice by using electroreflectance spectroscopy to observe Stark-ladder transitions. It has been found from the electric-field dependence of the electroreflectance spectra that the electron wavefunctions are delocalized after they are fully localized in individual quantum wells in a high electric-field regime. We conclude that the wavefunction delocalization results from the tunneling effect induced by the interaction between the spatially-separated first and second Stark-ladder states from the analysis of the eigenstates based on a transfer-matrix method in the framework of an envelope-function approximation. (C) 1998 Elsevier Science Ltd. All rights reserved., SOLID-STATE ELECTRONICS, PERGAMON-ELSEVIER SCIENCE LTD, 42(7-8)1499 - 1503, Jul. 1998, English, 研究論文(学術雑誌)
  • Influence of Gamma-x resonance on photocurrent-voltage characteristics in GaAs/InAlAs strained superlattices
    K Kuroyanagi, N Ohtani, N Egami, K Tominaga, M Nakayama
    We have investigated the influence of Gamma-X resonance on the photocurrent-voltage characteristics in GaAs/InAlAs type-I strained superlattices. To explain the experimental results, we have calculated the energies of the Gamma and X electron states, which are confined in the GaAs and InAlAs layers, respectively, using an effective-mass approximation taking into account the strain effect. We conclude that the Gamma 1-X1(xy) resonance causes peaky profiles in the photocurrent-voltage characteristics., JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS, JAPAN J APPLIED PHYSICS, 37(3B)1650 - 1653, Mar. 1998, English, 研究論文(学術雑誌)
  • Influence of Gamma-X mixing on carrier transport and photoluminescence in GaAs/AlAs type-I superlattices
    N Ohtani, M Hosoda, H Mimura, K Tominaga, T Watanabe, K Fujiwara
    We report on the effects of field-induced Gamma-X resonances on carrier transport and optical properties of GaAs/AlAs type-I short-period superlattices (SLs). We have observed an anomalously delayed photocurrent and Gamma 2-hhl photoluminescence originating from X1-Gamma 2 mixing. These observations clearly suggest that electrical conductivity and optical properties even in type-I SLs are seriously affected by X states in the applied electric field. (C) 1998 Academic Press Limited., SUPERLATTICES AND MICROSTRUCTURES, ACADEMIC PRESS LTD, 23(1)19 - 22, 1998, English, 研究論文(学術雑誌)
  • Current self-oscillations in undoped, photoexcited GaAs/AlAs type-I superlattices
    N Ohtani, N Egami, K Kuroyanagi, M Ando, M Hosoda, HT Grahn, KH Ploog
    Self-sustained current oscillations have been observed in undoped; photoexcited GaAs/AlAs type-I superlattices with different well and barrier thicknesses. The oscillations originate from a periodic motion of a space charge layer forming the boundary between low- and high-field regions of electric-field domains. When undamped photocurrent oscillations are detected, the high-field domain is found to originate from Gamma-X resonant transfer. This observation implies that through the T-X resonance the recombination lifetime for electrons and holes is prolonged so that undamped photocurrent oscillations in the MHz regime can even be observed in undoped type-I superlattices., PHYSICA STATUS SOLIDI B-BASIC RESEARCH, AKADEMIE VERLAG GMBH, 204(1)489 - 492, Nov. 1997, English, 研究論文(学術雑誌)
  • Photoluminescence from the barrier-X state in GaAs/InAlAs strained superlattices under applied-bias voltages
    K Kuroyanagi, N Ohtani, N Egami, K Tominaga, M Ando, M Nakayama
    We have studied the photoluminescence properties of GaAs/InAlAs strained superlattices as a function of applied-bias voltage. We have detected the appearance of the photoluminescence from the first X-electron state confined in the InAlAs barriers, resulting from the Gamma-X electron resonance induced by the applied bias. From the calculations of the energies of the Gamma and X states based on an effective-mass approximation including strain effects, it is demonstrated that the above PL is applicable to probe the barrier-X state., PHYSICA STATUS SOLIDI B-BASIC RESEARCH, AKADEMIE VERLAG GMBH, 204(1)187 - 190, Nov. 1997, English, 研究論文(学術雑誌)
  • Subband-resonance oscillations in undoped GaAs/AlAs type-II superlattices under photoexcitation
    H Mimura, M Hosoda, N Ohtani, HT Grahn, K Yokoo
    Subband-resonance oscillations have been observed in photoexcited, undoped type-II GaAs-AlAs superlattices. The frequency can be tuned over a wide range either by the applied voltage or the photoexcited carrier density. The oscillations can be explained by an oscillating space-charge which forms the domain boundary between two electric-field regions, which are attributed to negative differential velocity originating from X-related resonant tunneling., APPLIED SURFACE SCIENCE, ELSEVIER SCIENCE BV, 113(1)85 - 89, Apr. 1997, English, 研究論文(学術雑誌)
  • Photocurrent self-oscillations in a direct-gap GaAs-AlAs superlattice
    N Ohtani, M Hosoda, HT Grahn
    Undamped photocurrent self-oscillations have been observed in a direct-gap GaAs-AlAs superlattices. The oscillations in the MHz regime appear over a wide voltage range, where the time-averaged I-V characteristic exhibits a strong negative differential conductivity. The frequency distribution is strongly dependent on the applied voltage and the laser intensity. (C) 1997 American Institute of Physics., APPLIED PHYSICS LETTERS, AMER INST PHYSICS, 70(3)375 - 377, Jan. 1997, English, 研究論文(学術雑誌)
  • Structural and optical investigations of high quality InGaAs/InAlAs short period superlattices grown on an InGaAs quasisubstrate
    K Tominaga, M Hosoda, N Ohtani, T Watanabe, H Inomata, K Fujiwara
    Unstrained InGaAs (4.5 nm)/InAlAs (1.0 nm) short-period superlattices grown on a (100) GaAs substrate were studied. To achieve this growth, an In-composition-graded buffer layer and a thick InGaAs buffer layer were adopted. Structural properties were investigated by x-ray diffraction, atomic force microscopy, and a compositional analysis by the thickness fringe method. X-ray diffraction patterns showed clear periodicity in the superlattices and atomic-force spectroscopy images showed cross-hatch morphology for the main ridge along the (01(1) over bar) direction. Clear thickness fringes in the bright-field electron microscope images for the superlattice region and ambiguous fringes for the graded buffer layer indicate that misfit dislocation due to lattice mismatch concentrates in the graded buffer and a high-quality superlattice is successfully grown in spite of the large lattice mismatch between the superlattice and the substrate. Optical characteristics measured by photocurrent spectroscopy reveal a clear Wannier-Stark localization effect at room temperature. The experimental absorption energies agree well with calculated values by a transfer matrix method using parameters for bulk InGaAs and InAlAs. (C) 1996 American Institute of Physics., JOURNAL OF APPLIED PHYSICS, AMER INST PHYSICS, 80(10)5915 - 5920, Nov. 1996, English, 研究論文(学術雑誌)
  • Electric-field domain formation in type-II superlattices
    H Mimura, M Hosoda, N Ohtani, K Tominaga, K Fujita, T Watanabe, HT Grahn, K Fujiwara
    Static electric-field domains as well as current self-oscillations due to domain-wall oscillations have been observed in undoped, type-II GaAs-AlAs superlattices under photoexcitation. Photoluminescence measurements clearly demonstrate the coexistence of low- and high-field domains in the static and oscillating domain voltage regime. A comparison with the calculated energy level distribution indicates that the static domains are connected with negative differential velocity (NDV) due to resonant transfer between X(2) in the AlAs layer and Gamma(1) in the GaAs layer, while the oscillating domains are attributed to NDV originating from resonant tunneling between X(1) and X(2) in the AlAs layers. This observation demonstrates the importance of transport channels due to the indirect band structure of type-II superlattices., PHYSICAL REVIEW B, AMER PHYSICAL SOC, 54(4)R2323 - R2326, Jul. 1996, English, 研究論文(学術雑誌)
  • Current self-oscillations in photoexcited type-II GaAs-AlAs superlattices
    M Hosoda, H Mimura, N Ohtani, K Tominaga, T Watanabe, K Fujiwara, HT Grahn
    Self-oscillations of the photocurrent have been observed in type-II GaAs-AlAs superlattices. In addition to the fundamental frequency, several higher harmonics are present. The frequency of the oscillations can be tuned for a fixed carrier density from 15 to 120 MHz by simply changing the applied bias. The frequency distribution within a certain voltage range can be varied by changing the density of photoexcited carriers. For larger carrier densities, higher frequencies are observed in a different voltage range. This system could therefore be used as a high-frequency oscillator, which can be controlled by two external parameters, the applied voltage and the light intensity. (C) 1996 American Institute of Physics., APPLIED PHYSICS LETTERS, AMER INST PHYSICS, 69(4)500 - 502, Jul. 1996, English, 研究論文(学術雑誌)
  • Influence of type-I to Type-II transition by an applied electric field on photoluminescence and carrier transport in GaAs/AlAs type-I short-period superlattices
    N Ohtani, H Mimura, M Hosoda, K Tominaga, T Watanabe, K Fujiwara
    We have investigated the influence of type-I to type-II transition by an applied electric field on photoluminescence and carrier transport by time-of-flight experiments in GaAs/AlAs short-period superlattices. Type-I to type-II transition was confirmed by observing degenerated photoluminescence spectra caused by Gamma-X mixing. The rise time of the time-resolved photocurrent was found to increase by the type-I to type-II transition. This result clearly shows that the tunneling time through type-II alignment is longer than that through type-I alignment. The delayed carrier transport is most likely caused by Gamma-X-Gamma transfer. The results suggest that in type-II superlattices, carriers prefer Gamma-X-Gamma transfer to Gamma-Gamma sequential tunneling., JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS, JAPAN J APPLIED PHYSICS, 35(2B)1302 - 1305, Feb. 1996, English, 研究論文(学術雑誌)
  • Anomalously delayed carrier transport in GaAs/AlAs thin-barrier superlattices
    N Ohtani, H Mimura, K Tominaga, M Hosoda, T Watanabe, G Tanaka, K Fujiwara
    We have observed an anomalously delayed photocurrent component related to Gamma-X transfer in a type-I GaAs/AlAs thin-barrier superlattice, for the first time to our knowledge, by electrical time-of-flight experiments. The appearance of the delayed photocurrent was strongly influenced by the reverse bias voltage, but independent of the excitation intensity or temperature. The delayed photocurrent originates from carriers trapped in the X-valley states in the remote AlAs barriers. When the delayed photocurrent is observed, the energy level of the Gamma-valley ground state in the GaAs well is resonantly located closer to the energy levels of the X-valley states in the remote AlAs barriers. In particular, when the X-valley state is in resonance with the adjacent Gamma-valley second state, the amount of delayed carriers strongly increases and establishes a second peak in the photocurrent temporal response., SOLID-STATE ELECTRONICS, PERGAMON-ELSEVIER SCIENCE LTD, 40(1-8)759 - 762, 1996, English, 研究論文(学術雑誌)
  • EVIDENCE FOR GAMMA-CHI TRANSPORT IN TYPE-I GAAS/ALAS SEMICONDUCTOR SUPERLATTICES
    M HOSODA, N OHTANI, H MIMURA, K TOMINAGA, P DAVIS, T WATANABE, G TANAKA, K FUJIWARA
    We report the first evidence that the Gamma-X transfer mechanism plays a significant role in carrier transport in type-I semiconductor superlattices under an electric field. An anomalously delayed photocurrent was observed in a GaAs/AlAs type-I superlattice under ultrashort optical pulse excitation. This phenomenon can be explained by a switch of the electron transport path from Gamma-Gamma to Gamma-X-Gamma-X, caused by an electric-field induced change of the subband alignment of the second Gamma state (Gamma 2) in the well and the X1 state in the adjacent barrier., PHYSICAL REVIEW LETTERS, AMERICAN PHYSICAL SOC, 75(24)4500 - 4503, Dec. 1995, English, 研究論文(学術雑誌)
  • DELAYED PHOTOCURRENT AFFECTED BY GAMMA-X RESONANCE IN GAAS/ALAS TYPE-I SHORT-PERIOD SUPERLATTICES
    H MIMURA, N OHTANI, M HOSODA, K TOMINAGA, T WATANABE, G TANAKA, K FUJIWARA
    Delayed photocurrents were observed in GaAs/AlAs type-I short-period superlattices by measuring time-resolved photoresponses under ultrashort optical pulse excitation. According to the envelope function calculations, the X(1) state in AlAs barriers resonates with the Gamma(2) state in the adjacent GaAs wells at a bias voltage where the delayed photocurrents were conspicuous. These results strongly suggest that the dynamic carrier transport process is significantly influenced by X(1)-Gamma(2) resonance effects in the superlattices. (C) 1995 American Institute of Physics., APPLIED PHYSICS LETTERS, AMER INST PHYSICS, 67(22)3292 - 3294, Nov. 1995, English, 研究論文(学術雑誌)

Misc

  • Phase Diagram of Static and Dynamic Electric Field Domain Formation in Semiconductor Superlattices
    N. Ohtani, N. Egami, H. T. Grahn, K. H. Ploog
    International Conference on Nonequilibrium Carrier Dynamics in Semiconductors (HCIS), (MoP-38)74 , Jul. 1999, English, 研究発表ペーパー・要旨(国際会議)
  • Electric-field effects on photoluminescence properties in a GaAs/AlAs marginal type-Ⅰsuperlattice
    N. Ohtani, C. Domoto, N. Egami, H. Mimura, M. Ando, M. Nakayama, M. Hosoda
    International Conference on Modulated Semiconductor Structures (MSS), (19)291 - 292, Jul. 1999, English, 研究発表ペーパー・要旨(国際会議)
  • Carrier Density Dependence and Phase Diagram of Static and Dynamic Domain Formation in Semiconductor Superlattices
    N. Ohtani, N. Egami, H. T. Grahn, K, H. Ploog
    18th Electronic Materials Symposium, (D5)35 - 36, 1999, English, 研究発表ペーパー・要旨(全国大会,その他学術会議)
  • Electroluminescence in Undoped GaAs/AlAs Superlattices due to Avalanche Breakdown
    C. Domoto, N. Ohtani, K. Kuroyanagi, P. Vaccaro, N. Egami
    International Conference on Solid State Devices and Materials, (C-6-5)342 - 343, Sep. 1998, English, 研究発表ペーパー・要旨(国際会議)
  • X-X Electron Resonance Detected by Type-ⅡPhotoluminescence in a GaAs/AlAs Superlattice
    M. Nakayama, M. Ando, H. Nishimura, N. Ohtani, N. Egami, K. Fujiwara, M. Hosoda
    International Conference on Physics of Semiconductors (ICPS), (Th-P46), Aug. 1998, English, 研究発表ペーパー・要旨(国際会議)
  • Carrier Density and Temperature Dependence of Photocurrent Self-oscillation in GaAs/AlAs Superlattices
    N. Ohtani, N. Egami, H. T. Grahn, K. H. Ploog
    International Conference on Physics of Semiconductors (ICPS), (Th-P73), Aug. 1998, English, 研究発表ペーパー・要旨(国際会議)
  • Photocurrent Self-Oscillations in Undoped GaAs/AlAs Superlattices Modulated by an External AC Voltage
    N. Ohtani, N. Egami, K. Fujiwara, H. T. Grahn
    International Workshop on Nano Physics and Electronics (NEP), (L-04)95 - 96, Sep. 1997, English, 研究発表ペーパー・要旨(国際会議)
  • Influence of Γ-X Resonance on Current-Voltage Characteristics in GaAs/InAlAs Strained Superlattices
    K. Kuroyanagi, N. Ohtani, N. Egami, K. Tominaga, M. Ando, M. Nakayama
    International Conference on Solid State Devices and Materials (SSDM), (D-5-3)236 - 237, Sep. 1997, English, 研究発表ペーパー・要旨(国際会議)
  • Over 1.3 mu m continuous-wave laser emission from InGaSb quantum-dot laser diode fabricated on GaAs substrates
    N Yamamoto, K Akahane, S Gozu, N Ohtani
    A silicon atom irradiation technique is proposed to create high-density InGaSb quantum dots (QDs) on a GaAs substrate. This technique irradiates Si atoms immediately before InGaSb QD growth, and the density of QDs with Si atoms is about 100 times higher than that without Si atoms. The high-density InGaSb QDs show long-wavelength photo-and electroluminescence emissions around 1.3 and 1.5 mu m in the optical communications waveband. An Sb-based QD laser diode (LD) containing an InGaSb QD active layer was fabricated on a GaAs substrate. This Sb-based QD-LD shows a 1.37 mu m continuous-wave laser emission at room temperature. (c) 2005 American Institute of Physics., APPLIED PHYSICS LETTERS, AMER INST PHYSICS, 86(20)203118 , May 2005, English
  • Photoluminescence from high Gamma-electron subbands and intersubband electroluminescence using X-Gamma carrier injection in a simple GaAs/AlAs superlattice
    C Domoto, T Nishimura, N Ohtani, K Kuroyanagi, PO Vaccaro, T Aida, H Takeuchi, M Nakayama
    We report the interband photoluminescence from high Gamma-electron subbands and mid-infrared electroluminescence originating from an intersubband transition in a simple GaAs (15.3 nm)/AlAs (4.5 nm) superlattice embedded in a p-i-n structure. Interband photoluminescence properties under applied bias voltages provide conclusive evidence that electrons populate the fourth Gamma (Gamma4) electron subband in the GaAs layer. This electron population results from the carrier injection into the Gamma4 subband from the adjacent X1-subband in the AlAs layer, which is initiated by the X1-Gamma4 resonance. We calculate the overlap integral of the envelope functions for Gamma-electron and heavy-hole subbands in order to discuss the carrier population in high Gamma subbands based on the photoluminescence intensities. The results of analysis suggest that a population inversion can be obtained between the Gamma4 and Gamma4 subbands under the X1-Gamma4 resonant condition. The energy of the,intersubband electroluminescence, 100 meV, agrees with the energy spacing between the Gamma4 and Gamma3 subbands. This demonstrates that the carrier injection into the higher F subband using X-Gamma scattering will be useful for designing of intersubband-emission devices., JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS, INST PURE APPLIED PHYSICS, 41(8)5073 - 5077, Aug. 2002, English
  • Observation of Gamma-X resonances in type-I GaAs/AlAs semiconductor superlattices: Anomaly in photoluminescence
    M Hosoda, H Mimura, N Ohtani, K Tominaga, K Fujita, T Watanabe, H Inomata, M Nakayama
    We report the direct observation of Gamma-X resonances in type-I GaAs/AlAs semiconductor superlattices under an electric field. Photoluminescence (PL) associated with the first X-z state in the AlAs and the first Gamma heavy-hole state in the GaAs was found in type-I superlattices. The PL strongly increased its intensity due to Gamma-X mixing, by tuning the resonance between the X, state and the second Gamma-electron state in the adjacent GaAs layer. This observation gives clear evidence that Gamma-X scattering and mixing often occur even in type-I superlattices under an electric field., PHYSICAL REVIEW B, AMERICAN PHYSICAL SOC, 55(20)13689 - 13696, May 1997, English

Books etc

  • Introduction to semiconductor electric devices
    OHTANI Naoki
    Morikita Publishing Co., Ltd., Oct. 2019, 単著, Japanese, 学術書

Conference Activities & Talks

  • Preparation of carbon nitride thin films alginate gel for application to water decomposition by photocatalytic effect
    K. Mizuno, N. Ohtani
    The 3rd International Symposium on Recent Progress of Energy and Environmental Photocatalysis, Nov. 2019, English, 口頭発表(一般)
  • Influence of strong acid on structural and photoluminescence properties of nano-amorphous graphitic carbon nitride dispersed in nitric acid
    T. Watanabe, M. Hirai, K. Takarabe, N. Ohtani
    The 9th Asia-Pacific Workshop on Widegap Semiconductors, Nov. 2019, English, 口頭発表(一般)
  • Fabrication of organic-inorganic hybrid thin films by sol-gel processusing metal-alkoxide
    R. Kasuga, N. Ohtani
    The 7th International Symposium on Organic and Inorganic Electronic Materials and Related Nanotechnologies (EM-NANO 2019), 20 Jun. 2019, English, 口頭発表(一般)
  • Study of isolation method of green fluorescent pigments contained in cherry tomatoes using column chromatography
    A. Noma, N. Ohtani
    The 7th International Symposium on Organic and Inorganic Electronic Materials and Related Nanotechnologies (EM-NANO 2019), 20 Jun. 2019, English, 口頭発表(一般)
  • Separation of blue fluorescent pigments contained in Fraxinus lanuginosa f. serrata using reverse-phase column chromatography and investigation of luminescent properties
    T. Ohtsu, Y. Kinoe, N. Ohtani
    The 7th International Symposium on Organic and Inorganic Electronic Materials and Related Nanotechnologies (EM-NANO 2019), 20 Jun. 2019, English, 口頭発表(一般)
  • Preparation of organic-inorganic hybrid emissive thin films by sol-gel reaction using photo-curing binary crosslinking
    Y. Jitsui, N. Ohtani
    XX International Sol-Gel Conference (Sol-gel 20), Jun. 2019, English, 口頭発表(一般)
  • Fabrication of nano-amorphous graphitic carbon nitride dispersion and preparation of thin films by electrostatic-spraying method
    T. Watanabe, M. Hirai, K. Takarabe, N. Ohtani
    The 7th International Symposium on Organic and Inorganic Electronic Materials and Related Nanotechnologies} (EM-NANO 2019), Jun. 2019, English, 口頭発表(一般)
  • Improvement of resistance of solution-processed ZnO Film by controlling annealing condition
    T. Nojiri, N. Ohtani
    The 7th International Symposium on Organic and Inorganic Electronic Materials and Related Nanotechnologies} (EM-NANO 2019), Jun. 2019, English, 口頭発表(一般)
  • Preparation of colloidal fluorescent quantum dot thin films by electrostatic spraying deposition
    R. Yamada, D. Kim, N. Ohtani
    Symposium on Organic and Inorganic Electronic Materials and Related Nanotechnologies (EM-NANO 2019), Jun. 2019, English, 口頭発表(一般)
  • Photoluminescence properties of emissive polymer MEH-PPV affected by antioxidant effect of natural beta-carotene extracted from spinach
    Y. Magata, S. Imada, N. Ohtani
    Symposium on Organic and Inorganic Electronic Materials and Related Nanotechnologies (EM-NANO 2019), Jun. 2019, English, 口頭発表(一般)
  • Fabrication of nano-amorphous graphitic carbon nitride thin film by wet process and its optical properties
    T. Watanabe, M. Hirai, K. Takarabe, N. Ohtani
    the International Workshop on Nitride Semiconductors (IWN2018), Nov. 2018, English, ポスター発表
  • Preparation of Graphitic Carbon Nitride Thin Films by Evaporating Melamine
    S. Minato, N. Ohtani
    the International Workshop on Nitride Semiconductors (IWN2018), Nov. 2018, English, ポスター発表
  • Study on extraction method of blue emissive pigments contained in fraxinus lanuginose
    Y. Kinou, N. Ohtani
    the International Conference on Organic Materials for Electronics and Photonics (KJF-ICOMEP 2018), Sep. 2018, English, ポスター発表
  • Modulation of photoluminescence properties of emissive polymers induced by mixing of beta-carotene
    S. Imada, N. Ohtani
    the International Conference on Organic Materials for Electronics and Photonics (KJF-ICOMEP 2018), Sep. 2018, English, ポスター発表
  • Fabrication and Characterization of Carbon Nitride Fluorescent Material Using Annealed Melamine
    K. Wada, N. Ohtani
    the 34th International Conference on the physics of semiconductors (ICPS34), Jul. 2018, English, ポスター発表
  • Natural dye-sensitized solar cells containing anthocyanin dyes extracted from frozen blueberry using column chromatography method
    A. Mizuno, G. Yamada, N. Ohtani
    the 7th World Conference on Photovoltaic Energy Conversion (WCPEC-7), Jun. 2018, English, ポスター発表
  • Fabrication of emissive thin films using nano-amorphous graphitic carbon nitride powders
    T. Watanabe, M. Hirai, K. Takarabe, N. Ohtani
    The 6th International Symposium on Organic and Inorganic Electronic Materials and Related Nanotechnologies (EM-NANO 2017), Jun. 2017, English, ポスター発表
  • Fabrication of Carbon Nitride-based Emissive Thin Films by Wet Process Using Annealed Melamine
    K. Wada, N. Ohtani
    The 6th International Symposium on Organic and Inorganic Electronic Materials and Related Nanotechnologies (EM-NANO 2017), Jun. 2017, English, ポスター発表
  • Blue-color photoluminescence from natural pigments extracted from Fraxinus lanuginosa
    Y. Kinou, N. Ohtani
    The 6th International Symposium on Organic and Inorganic Electronic Materials and Related Nanotechnologies (EM-NANO 2017), Jun. 2017, English, ポスター発表
  • Antioxidant effect of β-carotene and lutein isolated and purified from spinach on photoluminescence lifetime of organic emissive materials
    S. Imada, T. Ito, N. Ohtani
    The 6th International Symposium on Organic and Inorganic Electronic Materials and Related Nanotechnologies (EM-NANO 2017), Jun. 2017, English, ポスター発表
  • Carrier transport and photoluminescence properties in GaAs/AlAs asymmetric multiple-quantum well superlattices affected by resonances and scatterings between higher subband energies
    N. Ohtani, M. Hosoda
    the XXV International Materials Research Congress (IMRC), Aug. 2016, English, 口頭発表(招待・特別)
  • Photoluminescence properties affected by subband resonances and scatterings in GaAs/AlAs asymmetric sevenfold multiple quantum wells
    R. Wang, K. Hada, K. Yoshida, M. Hosoda, K. Akahane, N. Ohtani
    the 33rd International Conference on the Physics of Semiconductors (ICPS2016), Jun. 2016, English, ポスター発表
  • Fabrication of organic light-emitting diodes and dye-sensitized solar cells using pigments extracted from plants
    OHTANI Naoki
    the 2015 International Electron Devices and Materials Symposium (IEDMS 2015), Nov. 2015, English, ポスター発表
  • Evaluation of Antioxidant Effect of Carotenoids Extracted from Spinach Using Column Chromatography Method for Improving Photoluminescence Lifetime of Organic Emissive Materials
    T. Ito, A. Emoto, N. Ohtani
    the 2015 KJF International Conference on Organic Materials for Electronics and Photonics (KJF-ICOMEP 2015), Sep. 2015, English, ポスター発表
  • Dye sensitized solar cells containing anthocyanin dyes extracted from frozen red cabbages
    D. Isoda, S. Togo, A. Emoto, N. Ohtani
    the 1st International Caparica Conference on Chromogenic and Emissive Materials} (IC3EM), Sep. 2014, English, ポスター発表
  • Optical properties of pigments extracted from vegetables and application to light-emitting diodes
    N. Ohtani, T. Morikawa, Y. Katano, Y. Nishida, A. Emoto
    the 25th Conference of the Condensed Matter Division of the EPS (CMD25), Aug. 2014, English, ポスター発表
  • Inorganic-organic hybrid light-emitting diodes driven by low dc voltage containing CdSe-ZnS core-shell quantum dot emitters
    N. Ohtani, Y. Hagimoto, S. Yoshikawa
    the 32nd International Conference on the Physics of Semiconductors (ICPS-32), Aug. 2014, English, ポスター発表
  • Improved light emission properties and operation lifetime of multi-layered organic light-emitting diodes using dyes extracted from spinach
    Y. Nishida, N. Ohtani
    the 8th International Symposium on Organic Molecular Electronics (ISOME 2014), May 2014, English, ポスター発表
  • High efficient carrier injection into wide-bandgap polymer emissive materials and application to light-emitting diodes operating in the ultraviolet region
    N. Ohtani, M. Takahashi
    the 18th International Conference on Electron Dynamics in Semiconductors, Optoelectronics and Nanostructures (EDISON18), Jul. 2013, English, ポスター発表
  • Evaluation of antioxidant effect of carotenoid extracted from plants on the operating lifetime of organic light-emitting diodes
    Y. Nishida, N. Ohtani
    the 4th International Symposium on Organic and Inorganic Electronic Materials and Related Nanotechnologies (EM-NANO 2013), Jun. 2013, English, ポスター発表
  • Organic light-emitting diodes consisting of SiO2 active layer in which emissive organic materials are dispersed
    Y. Jitsui, S. Kimura, N. Ohtani
    the 31st International Conference on the Physics of Semiconductors (ICPS2012), Jul. 2012, English, ポスター発表
  • Transmission electron microscope observation of organic-inorganic hybrid thin active layers of light-emitting diodes
    Y. Jitsui, N. Ohtani
    the International Conference on Superlattices, Nanostructures, and Nanodevices 2012 (ICSNN2012), Jul. 2012, English, ポスター発表
  • Organic near-infrared photodiodes containing a wide-bandgap polymer and an n-type dopant in the active layer
    N. Ohtani, K. Nakajima, K. Bando
    the 4th International Conference on Optical, Optoelectronic and Photonic Materials and Applications (ICOOPMA10), Aug. 2010, English, ポスター発表
  • Excitonic Rabi oscillations in semiconductor quantum dot observed by photon echo spectroscopy
    K. Asakura, Y. Mitsumori, H. Kosaka, K. Edamatsu, K. Akahane, N. Yamamoto, M. Sasaki, N. Ohtani
    the 30th International Conference on the Physics of Semiconductors (ICPS-30), Jul. 2010, English, ポスター発表
  • Fabrication of organic light-emitting diodes using photosynthetic-pigments extracted from spinach
    N. Ohtani, N. Kitagawa, T. Matsuda
    the 3rd International Symposium on Organic and Inorganic Electronic Materials and Related Nanotechnologies (EM-NANO 2010), Jun. 2010, English, ポスター発表

Research Grants & Projects

  • Compound semiconductor photonic devices, organic electroluminescence, nano-technology

Research Seeds

  • Organic EL element activated in the ultraviolet wavelengh region
    Research Fields: Electric and Electronics, Chemicals, Nanotechnology and Materials
    Affiliation/Name: Naoki Ohtani