Updated on 2022/09/25

写真a

 
TAKEUCHI, Atsushi
 
Affiliation
Faculty of Science and Engineering, School of Advanced Science and Engineering
Job title
Professor

Concurrent Post

  • Faculty of Commerce   School of Commerce

  • Faculty of Science and Engineering   Graduate School of Advanced Science and Engineering

  • Affiliated organization   Global Education Center

Research Institute

  • 2020
    -
    2022

    理工学術院総合研究所   兼任研究員

Education

  •  
    -
    1992

    Osaka University   Doctor of Science  

  • 1983.04
    -
    1985.03

    Osaka University   Graduate School, Division of Engineering Science  

  • 1979.04
    -
    1983.03

    Osaka University   Faculty of Engineering Science  

Degree

  • 大阪大学   博士(理学)

  • (BLANK)

Research Experience

  • 2020.09
    -
    Now

    早稲田大学 研究院長

  • 2016.09
    -
    2020.09

    早稲田大学 理工学術院長

  • 2014.09
    -
    2016.09

    早稲田大学 先進理工学部長兼研究科長

  • 2007
    -
     

    北海道大学 非常勤講師

  • 2001.12
    -
    2005.03

    科学技術振興事業団 さきがけ研究員

  • 2004
    -
     

    北海道大学大学院工学研究科 非常勤講師

  • 2002.04
    -
     

    早稲田大学理工学部 教授

  • 1997.04
    -
    2002.03

    Waseda University   School of Science and Engineering

  • 2002
    -
     

    静岡大学工学部 非常勤講師

  • 1998
    -
     

    東北大学電気通信研究所 非常勤講師

  • 1997.03
    -
     

    富士通研究所退職

  • 1996.04
    -
    1997.03

    フェムト秒テクノロジー研究機構研究員(つくば, 出向)

  • 1997
    -
     

    東北大学電気通信研究所 非常勤講師

  • 1997
    -
     

    大阪大学基礎工学部 非常勤講師

  • 1992.09
    -
    1993.09

    マックスプランク固体研究所 (独, stuttgart) 客員研究員

  • 1985.12
    -
    1986.11

    電子技術総合研究所に滞在

  • 1985.04
    -
     

    富士通研究所入社. 主に、冷水佐壽室長(後:阪大教授・基礎工学研究科長)−武藤俊一主任研(現:北大名誉教授)グループで半導体量子構造の超高速現象の評価に従事

▼display all

Professional Memberships

  •  
     
     

    物理学会

  •  
     
     

    電子情報通信学会

  •  
     
     

    応用物理学会

 

Research Areas

  • Applied physical properties

Research Interests

  • Semiconductor Physics

Papers

  • Prolonged spin relaxation time in Zn-doped GaAs/GaAsP strain-compensated superlattice

    Shunsuke Ohki, Xiuguang Jin, Tomoki Ishikawa, Takuya Kamezaki, Kizuku Yamada, Shunichi Muto, Atsushi Tackeuchi

    APPLIED PHYSICS LETTERS   111 ( 2 ) 022405  2017.07  [Refereed]

     View Summary

    A GaAs/GaAsP strain-compensated superlattice (SL) is a highly promising spin-polarized electron source. To realize higher quantum efficiency, it is necessary to consider spin relaxation mechanisms. We have investigated the electron spin relaxation time in a Zn-doped GaAs/GaAsP strain-compensated SL by time-resolved spin-dependent pump and probe reflection measurements. The long spin relaxation time of 104 ps was observed at room temperature (RT), which is about three times longer than that of conventional undoped GaAs multiple quantum wells. Even when the excitation power increases from 30 to 110 mW, the change in the spin relaxation time at RT was small. This relationship implies that the intensity of the electron beam can be increased without affecting the spin relaxation time. These results indicate that a Zn-doped GaAs/GaAsP strain-compensated SL has the great advantage for use as a spin-polarized electron source. Published by AIP Publishing.

    DOI

  • The striking influence of rapid thermal annealing on InGaAsP grown by MBE: material and photovoltaic device

    Lian Ji, Ming Tan, Chao Ding, Kazuki Honda, Ryo Harasawa, Yuya Yasue, Yuanyuan Wu, Pan Dai, Atsushi Tackeuchi, Lifeng Bian, Shulong Lu, Hui Yang

    JOURNAL OF CRYSTAL GROWTH   458   110 - 114  2017.01  [Refereed]

     View Summary

    Rapid thermal annealing (RTA) has been performed on InGaAsP solar cells with the bandgap energy of 1 eV grown by molecular beam epitaxy. With the employment of RTA under an optimized condition, the open voltage was increased from 0.45 to 0.5 V and the photoelectric conversion efficiency was increased from 11.87-13.2%, respectively, which was attributed to the crystal quality improvement of p-type InGaAsP and therefore a reduced recombination current inside depletion region. The integral photoluminescence (PL) intensity of p-type InGaAsP increased to 166 times after annealing at 800 degrees C and its PL decay time increased by one order of magnitude. While the changes of nominally undoped and n-doped InGaAsP were negligible. The different behaviors of the effect of RTA on InGaAsP of different doping types were attributed to the highly mobile "activator" - beryllium (Be) atom in p-type InGaAsP.

    DOI

  • Analysis of quantum efficiency improvement in spin-polarized photocathode

    Xiuguang Jin, Shunsuke Ohki, Tomoki Ishikawa, Atsushi Tackeuchi, Yosuke Honda

    JOURNAL OF APPLIED PHYSICS   120 ( 16 ) 164501  2016.10  [Refereed]

     View Summary

    GaAs/GaAsP strain-compensated superlattices (SLs) were developed for spin-polarized photocathode applications. High crystal quality was maintained with SL thicknesses up to 720 nm (90-pairs); however, the quantum efficiency (QE) did not increase linearly with the SL thickness but became saturated starting from an SL thickness of 192 nm (24-pairs). Time-resolved photoluminescence measurements revealed that the carrier lifetime in the GaAs/GaAsP strain-compensated SL was as short as 20.5 ps at room temperature, which causes the elimination of photoexcited electrons before emission. A simulation based on a diffusion model was implemented to quantitatively evaluate the effect of the carrier lifetime on the QE. The simulation results were in good agreement with the experimental results and demonstrate that a carrier lifetime of over 120 ps is required for a two-fold improvement of the QE. Published by AIP Publishing.

    DOI

  • Effect of thermal annealing on electron spin relaxation of beryllium-doped In0.8Ga0.2As0.45P0.55 bulk

    Hao Wu, Lian Ji, Ryo Harasawa, Yuya Yasue, Takanori Aritake, Canyu Jiang, Shulong Lu, Atsushi Tackeuchi

    AIP ADVANCES   6 ( 8 ) 085119  2016.08  [Refereed]

     View Summary

    The effect of thermal annealing on the electron spin relaxation of beryllium-doped In0.8Ga0.2As0.45P0.55 bulk was investigated by time-resolved spin-dependent pump and probe reflection measurement with a high time resolution of 200 fs. Three similar InGaAsP samples were examined one of which was annealed at 800 degrees C for 1 s, one was annealed at 700 degrees C for 1 s and the other was not annealed after crystal growth by molecular beam epitaxy. Although the carrier lifetimes of the 700 degrees C-annealed sample and the unannealed sample were similar, that of the 800 degrees C-annealed sample was extended to 11.6 (10.4) ns at 10 (300) K, which was more than two (four) times those of the other samples. However, interestingly the spin relaxation time of the 800 degrees C-annealed sample was found to be similar to those of the other two samples. Particularly at room temperature, the spin relaxation times are 143 ps, 147 ps, and 111 ps for the 800 degrees C-annealed sample, 700 degrees C-annealed sample, and the unannealed sample, respectively. (C) 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

    DOI

  • Picoseconds carrier spin relaxation in In0.8Ga0.2As/Al0.5Ga0.5As/AlAs0.56Sb0.44 coupled double quantum wells

    Tomoki Ishikawa, Shin-ichiro Gozu, Teruo Mozume, Masaki Asakawa, Shunsuke Ohki, Atsushi Tackeuchi

    2016 COMPOUND SEMICONDUCTOR WEEK (CSW) INCLUDES 28TH INTERNATIONAL CONFERENCE ON INDIUM PHOSPHIDE & RELATED MATERIALS (IPRM) & 43RD INTERNATIONAL SYMPOSIUM ON COMPOUND SEMICONDUCTORS (ISCS)   52   04CM05  2016  [Refereed]

    DOI

  • Study of single crystal CuInSe2 thin films and CuGaSe2/CuInSe2 single quantum well grown by molecular beam epitaxy

    Sathiabama Thiru, Masaki Asakawa, Kazuki Honda, Atsushi Kawaharazuka, Atsushi Tackeuchi, Toshiki Makimoto, Yoshiji Horikoshi

    JOURNAL OF CRYSTAL GROWTH   425   203 - 206  2015.09  [Refereed]

     View Summary

    High quality CuGaSe2 and CuInSe2 single crystalline layers are grown on GaAs (001) by employing the deposition sequence of migration enhanced epitaxy using a solid source molecular beam epitaxy system. When CuGaSe2 is grown on CuInSe2 at moderate temperatures, severe interdiffusion takes place at the heterojunction of CuGaSe2/CuInSe2. This problem has been solved by optimizing the growth temperature and deposition rates of the constituent elements. Thus, we have successfully grown CuGaSe2/CuInSe2 single quantum well with sharp interfaces on GaAs (001) for the first time. Intense photoluminescence from the single quantum well with 10 nm well width is demonstrated. (C) 2015 Elsevier B.V. All rights reserved.

    DOI

  • Observation of picosecond electron spin relaxation in InGaAsP by time-resolved spin-dependent pump and probe reflection measurement

    Ryo Harasawa, Naoki Yamamoto, Hao Wu, Takanori Aritake, Shulong Lu, Lian Ji, Atsushi Tackeuchi

    PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS   252 ( 6 ) 1244 - 1247  2015.06  [Refereed]

     View Summary

    We have investigated the electron spin relaxation in undoped InGaAsP grown on an InP substrate. Time-resolved spin-dependent pump and probe reflection measurements revealed a spin relaxation behavior between 10 and 300K. We have observed the electron spin relaxation times of 980ps at 10K and 95ps at room temperature. The observed presence of the carrier density dependence and the weak temperature dependence of spin relaxation time imply that the Bir-Aronov-Pikus (BAP) process is effective at 10-30K. At 100-300K, the observed presence of the temperature dependence and the absence of the carrier density dependence of spin relaxation time indicate that the D'yakonov-Perel' (DP) and Elliott-Yafet (EY) processes are effective. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

    DOI

  • Investigation of InGaAsP solar cells grown by solid-state molecular beam epitaxy

    Lian Ji, Ming Tan, Kazuki Honda, Ryo Harasawa, Yuya Yasue, Yuanyuan Wu, Pan Dai, Atsushi Tackeuchi, Lifeng Bian, Shulong Lu, Hui Yang

    SOLAR ENERGY MATERIALS AND SOLAR CELLS   137   68 - 72  2015.06  [Refereed]

     View Summary

    We report on the study of InGaAsP solar cells grown by solid-state molecular beam epitaxy (MBE) on InP. The effect of growth temperature on device performance is investigated. Under standard one-sun air-mass 1.5 global (AM1.5) illumination, an efficiency of 18.8% has been obtained for In0.78Ga0.22As0.45P0.52 single-junction solar cells grown at high temperature. Time-resolved photoluminescence (PL) results demonstrate that the In0.78Ga0.22As0.48P0.52 optical quality is greatly improved in the case of a high growth temperature. A longer PL decay time of In0.78Ga0.22As0.48P0.52/InP in contrast to In0.78Ga0.22As0.48P0.52/In0.52Al0.42As indicates that InP is more promising as the back surface field for future solar cell performance improvements. (C) 2015 Elsevier B.V. All rights reserved.

    DOI

  • Investigation of CuGaSe2/CuInSe2 double heterojunction interfaces grown by molecular beam epitaxy

    Sathiabama Thiru, Masaki Asakawa, Kazuki Honda, Atsushi Kawaharazuka, Atsushi Tackeuchi, Toshiki Makimoto, Yoshiji Horikoshi

    AIP ADVANCES   5 ( 2 ) 027120  2015.02  [Refereed]

     View Summary

    In-situ reflection high-energy electron diffraction (RHEED) observation and Xray diffraction measurements were performed on heterojunction interfaces of CuGaSe2/CnInSe(2)/CuGaSe2 grown on GaAs (001) using migration-enhanced epitaxy. The streaky RHEED pattern and persistent RHEED intensity oscillations caused by the alternate deposition of migration-enhanced epitaxy sequence are observed and the growths of smooth surfaces are confirmed. RHEED observation results also confirmed constituent material interdiffusion at the heterointerface. Cross-sectional transmission electron microscopy showed a flat and abrupt heterointerface when the substrate temperature is as low as 400 degrees C. These have been confirmed even by X-ray diffraction and photoluminescence measurements. (C) 2015 Author(s).

    DOI

  • Carrier recombination dynamics of MBE grown InGaAsP layers with 1 eV bandgap for quadruple-junction solar cells

    Lian Ji, Shulong Lu, Yuanyuan Wu, Pai Dai, Lifeng Bian, Masayuki Arimochi, Tomomasa Watanabe, Naohiro Asaka, Mitsunori Uemura, Atsushi Tackeuchi, Shiro Uchida, Hui Yang

    SOLAR ENERGY MATERIALS AND SOLAR CELLS   127   1 - 5  2014.08  [Refereed]

     View Summary

    The carrier recombination dynamics of InGaAsP material with a bandgap energy of 1 eV for quadruple-junction solar cells grown by solid-source molecular beam epitaxy have been investigated by the employment of time-resolved photoluminescence (PL) measurement. For the nominally undoped material, the PL decay time increases with increasing temperature, which indicates that radiative recombination dominates the recombination process. The radiative and the nonradiative recombination time constants were calculated on the basis of the temperature-dependent PL decay time and the integrated PL intensity. With the incorporation of Be (as the p-type dopant) into the material, the PL decay time decreases with increasing temperature, and a double-exponential PL decay curve is observed in the case of the material with a higher doping density. An InGaAsP-based single-junction photovoltaic device with a bandgap of 1 eV was fabricated, and an efficiency of 16.4% was obtained under the AM1.5G solar spectra. (C) 2014 Elsevier B.V. All rights reserved.

    DOI

  • Observations of exciton and carrier spin relaxation in Be doped p-type GaAs

    Naohiro Asaka, Ryo Harasawa, Shulong Lu, Pan Dai, Atsushi Tackeuchi

    APPLIED PHYSICS LETTERS   104 ( 11 ) 112404  2014.03  [Refereed]

     View Summary

    We have investigated the exciton and carrier spin relaxation in Be-doped p-type GaAs. Time-resolved spin-dependent photoluminescence (PL) measurements revealed spin relaxation behaviors between 10 and 100K. Two PL peaks were observed at 1.511 eV (peak 1) and 1.497 eV (peak 2) at 10K, and are attributed to the recombination of excitons bound to neutral Be acceptors (peak 1) and the band-to-acceptor transition (peak 2). The spin relaxation times of both PL peaks were measured to be 1.3-3.1 ns at 10-100K, and found to originate from common electron spin relaxation. The observed existence of a carrier density dependence of the spin relaxation time at 10-77K indicates that the Bir-Aronov-Pikus process is the dominant spin relaxation mechanism. (C) 2014 AIP Publishing LLC.

    DOI

  • Picosecond spin relaxation in low-temperature-grown GaAs

    M. Uemura, K. Honda, Y. Yasue, S. L. Lu, P. Dai, A. Tackeuchi

    APPLIED PHYSICS LETTERS   104 ( 12 ) 122403  2014.03  [Refereed]

     View Summary

    The spin relaxation process of low-temperature-grown GaAs is investigated by spin-dependent pump and probe reflectance measurements with a sub-picosecond time resolution. Two very short carrier lifetimes of 2.0 ps and 28 ps, which can be attributed to nonradiative recombinations related to defects, are observed at 10 K. The observed spin polarization shows double exponential decay with spin relaxation times of 46.2 ps (8.0 ps) and 509 ps (60 ps) at 10K (200 K). The observed picosecond spin relaxation, which is considerably shorter than that of conventional GaAs, indicates the strong relevance of the Elliott-Yafet process as the spin relaxation mechanism. For the first (second) spin relaxation component, the temperature and carrier density dependences of the spin relaxation time indicate that the Bir-Aronov-Pikus process is also effective at temperatures between 10K and 77 K, and that the D'yakonov-Perel' process is effective between 125K (77 K) and 200 K. (C) 2014 AIP Publishing LLC.

    DOI

  • Time-resolved ultraviolet photoluminescence of ZnO/ZnGa2O4 composite layer

    Qing Yang, Xiaohong Zhou, Takao Nukui, Yu Saeki, Sotaro Izumi, Atsushi Tackeuchi, Hirokazu Tatsuoka, Shuhua Liang

    AIP ADVANCES   4 ( 2 ) 4864750  2014.02  [Refereed]

     View Summary

    The ultraviolet photoluminescence of ZnO/ZnGa2O4 composite layer grown by the thermal oxidation of ZnS with gallium was investigated by the time-resolved photoluminescence as a function of measuring temperature and excitation power. With increase of excitation power, the (DX)-X-0 emission is easily saturated than the DAP emission from ZnO/ZnGa2O4 composite layer, and which is dramatically enhanced as compared with that from pure ZnO layer grown without gallium. The radiative recombination process with ultra-long lifetime controlled the carrier recombination of ZnO/ZnGa2O4 composite layer. (C) 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

    DOI

  • Observation of optical anisotropy of highly uniform InAs quantum dots

    M. Uemura, J. Ohta, R. Yamaguchi, K. Yamaguchi, A. Tackeuchi

    JOURNAL OF CRYSTAL GROWTH   378   463 - 465  2013.09  [Refereed]

     View Summary

    We have investigated the optical anisotropy of single-layer highly uniform InAs quantum dots (QDs) by photoluminescence (PL) measurements. The high uniformity of the QDs, whose PL spectrum was clearly separated into each energy level, enabled us to observe the optical anisotropy of the ground state, the first excited state and the second excited state separately. The degree of linear polarization (DLP) was obtained to be 15.8%, 14.3% and 8.3% for the ground state, the first excited state and the second excited state, respectively. This result shows that the DLP is smaller for higher-energy states. We also measured the time evolutions of the PL polarization components using a streak camera. The DLP was found to remain constant during the measured time range. (c) 2013 Elsevier B.V. All rights reserved.

    DOI

  • Photoluminescence and Raman studies on Ge-based complexes in Si-doped GaInP epilayers grown on Germanium

    W. He, S. L. Lu, D. S. Jiang, J. R. Dong, A. Tackeuchi, H. Yang

    JOURNAL OF APPLIED PHYSICS   112 ( 2 ) 023509  2012.07  [Refereed]

     View Summary

    Photoluminescence and Raman scattering measurements were performed on Si-doped GaInP grown on Ge. The deep broadband photoluminescence (PL) emission centered around 1.4 eV exhibits a strong dependence of strength on Si dopants, which is suggested to be due to [Ge-(Ga,Ge-In)-V-(Ga,V-In)] complexes. A strong evidence to support the existence of [Ge-(Ga,Ge-In)-Si-(Ga,Si-In)] complexes in Si-doped GaInP is shown by Raman spectra. The blue shift of broad PL emission and the increased recombination life time with increased temperatures were explained by the competition between [Ge-(Ga,Ge-In)-V-(Ga,V-In)] and [Ge-(Ga,Ge-In)-Si-(Ga,Si-In)] complexes. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4737611]

    DOI

  • Exciton and carrier spin relaxations in InGaAs lattice-matched to off-cut Ge substrates

    Takenori Ushimi, Hiromi Nakata, Toshihiro Ishizuka, Kazutoshi Sasayama, Shulong Lu, Jianrong Dong, Atsushi Tackeuchi

    APPLIED PHYSICS LETTERS   100 ( 25 ) 252414  2012.06  [Refereed]

     View Summary

    We have investigated the exciton and carrier spin relaxations in InGaAs lattice-matched to Ge substrates. Time-resolved spin-dependent pump and probe reflectance measurements revealed a spin relaxation behavior between 10 and 300 K. The presence of the carrier density dependence of spin relaxation time at 10-200K implies that the Bir-Aronov-Pikus process is effective. At 250-300 K, the strong temperature and weak carrier density dependences of spin relaxation time show that the D'yakonov-Perel' process is dominant. The longest observed spin relaxation time of 2.6 ns at 77K is explained by the decrease in the spatial overlap of electrons and holes. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4730386]

    DOI

  • Temperature Dependence of Spin Relaxation Time in InAs Columnar Quantum Dots at 10 to 150K

    Sota Nakanishi, Kazutoshi Sasayama, Yoshitsugu Oyanagi, Ryo Yamaguchi, Shulong Lu, Lianhe Li, Andrea Fiore, Atsushi Tackeuchi

    JAPANESE JOURNAL OF APPLIED PHYSICS   51 ( 4 ) 04DM05  2012.04  [Refereed]

     View Summary

    We have investigated carrier spin relaxation in InAs columnar quantum dots (CQDs) using time-resolved photoluminescence measurement. The CQDs were formed by depositing a 1.8 monolayer InAs seed dot layer and a short-period GaAs/InAs superlattice (SL). The spin relaxations of the 3- and 35-period SL CQDs show double exponential decay up to 50 and 130 K, respectively. The spin relaxation times of the fast component, whose amplitudes are 4-11 times larger than that of the slow component, are around 100 ps for the two samples. For the 3-period SL CQDs, the fast spin relaxation time shows no temperature dependence up to around 50 K, indicating the relevance of the Bir-Aronov-Pikus process. The slow spin relaxation time of the 35-period SL CQDs was found to decrease from 3.42 ns at 10 K to 0.849 ns at 130 K. This large change may be explained by the Elliott-Yafet process considering acoustic phonon scattering. (C) 2012 The Japan Society of Applied Physics

    DOI

  • Exciton spin relaxation in In0.53Ga0.47As/AlAs0.56Sb0.44 quantum wells

    K. Sasayama, S. Nakanishi, R. Yamaguchi, Y. Oyanagi, T. Ushimi, S. Gozu, T. Mozume, A. Tackeuchi

    APPLIED PHYSICS LETTERS   100 ( 9 ) 092401  2012.02  [Refereed]

     View Summary

    The spin relaxation process of In0.53Ga0.47As/AlAs0.56Sb0.44 quantum wells is investigated by spin-dependent pump and probe reflectance measurements with a high time resolution of 200 fs. The observed spin relaxation time of 17.5 ps at 20K indicates high potential for applications to high-speed optical devices. A positive temperature dependence of the spin relaxation time due to the unique band structure is observed at 30-100 K. The spin relaxation is found to be mainly governed by the Bir-Aronov-Pikus process [Sov. Phys. JETP 42, 705 (1976)] below 100K and by the D'yakonov-Perel' process above 100 K. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3690833]

    DOI

  • Carrier spin relaxation in InGaAs/AlAsSb quantum wells

    T. Nukui, S. Gozu, T. Mozume, S. Izumi, Y. Saeki, A. Tackeuchi

    PHYSICS OF SEMICONDUCTORS: 30TH INTERNATIONAL CONFERENCE ON THE PHYSICS OF SEMICONDUCTORS   1399   659 - 660  2011  [Refereed]

     View Summary

    We have investigated the exciton spin relaxation in InGaAs/AlAsSb quantum wells by time-resolved spin-dependent pump and probe reflectance measurements. The spin relaxation time of 1.38 mu m-electron-heavy-hole excitons at 150 K is obtained to be 34 43 ps at an excitation power of 50 80 mW. The observed carrier density dependence and temperature independence of the spin relaxation time indicate that the spin relaxation mechanism is dominated by the Bir-Aronov-Pikus process.

    DOI

  • Carrier spin relaxation in GaInNAsSb/GaNAsSb/GaAs quantum well

    T. Asami, H. Nosho, L. H. Li, J. C. Harmand, S. L. Lu, A. Tackeuchi

    PHYSICS OF SEMICONDUCTORS: 30TH INTERNATIONAL CONFERENCE ON THE PHYSICS OF SEMICONDUCTORS   1399   665 - 666  2011  [Refereed]

     View Summary

    We have investigated the carrier spin relaxation in GaInNAsSb/GaNAsSb/GaAs quantum well (QW) by time-resolved photoluminescence (PL) measurement. The sample consists of an 8-nm-thick GaIn0.36N0.006AsSb0.015 well, 5-nm-thick GaN0.01AsSb0.11 intermediate barriers and 100-nm-thick GaAs barriers grown by molecular beam epitaxy on a GaAs(100) substrate. The spin relaxation time and recombination lifetime at 10 K are measured to be 228 ps and 151 ps, respectively. As a reference, we have also obtained a spin relaxation time of 125 ps and a recombination lifetime of 63 ps for GaInNAs/GaNAs/GaAs QW. This result shows that crystal quality is slightly improved by adding Sb, although these short carrier lifetimes mainly originate from a nonradiative recombination. These spin relaxation times are longer than the 36 ps spin relaxation time of InGaAs/InP QWs and shorter than the 2 ns spin relaxation time of GaInNAs/GaAs QW.

    DOI

  • Novel ultraviolet photoluminescence of ZnO/ZnGa2O4 composite layers

    Qing Yang, Yu Saeki, Sotaro Izumi, Takao Nukui, Atsushi Tackeuchi, Akihiro Ishida, Hirokazu Tatsuoka

    APPLIED SURFACE SCIENCE   256 ( 22 ) 6928 - 6931  2010.09  [Refereed]

     View Summary

    ZnO/ZnGa2O4 composite layers were synthesized by simple thermal oxidation of ZnS substrates with gallium in the air. The continuous-wave and time-resolved photoluminescence measurements for the composites were performed at room temperature. It is found that the visible deep level emission from ZnO in ZnO/ZnGa2O4 composite layer was almost suppressed. In addition, the UV emission with long lifetime was also observed in comparison with that of pure ZnO layer without ZnGa2O4. (C) 2010 Elsevier B. V. All rights reserved.

    DOI

  • Observation of Spin Relaxation in InGaAs/AlAsSb Quantum Wells

    Sotaro Izumi, Shin-Ichiro Gozu, Teruo Mozume, Yu Saeki, Takao Nukui, Atsushi Tackeuchi

    JAPANESE JOURNAL OF APPLIED PHYSICS   49 ( 4 ) 04DM03  2010  [Refereed]

     View Summary

    We have investigated the exciton spin relaxation in InGaAs/AlAsSb quantum wells using time-resolved spin-dependent pump and probe reflectance measurements. The spin relaxation time of 1.46 mu m electron-heavy hole excitons at room temperature is obtained to be 27-54 ps for an excitation power of 20-100 mW. The carrier density dependence of the exciton spin relaxation time was clearly observed, suggesting that the spin relaxation mechanism is strongly related to the Bir-Aronov-Pikus process at room temperature. (C) 2010 The Japan Society of Applied Physics

    DOI

  • Picosecond Carrier Recombination of Single-Crystalline GaN Nanorods Grown on Si(111) Substrates

    Atsushi Tackeuchi, Chan Ho Yoo, Tae Whan Kim, Young Hae Kwon, Tae Won Kang, Takao Nukui, Taisuke Fujita, Yoshiaki Nakazato, Yu Saeki, Sotaro Izumi

    JAPANESE JOURNAL OF APPLIED PHYSICS   49 ( 7 ) 070201  2010  [Refereed]

     View Summary

    Time-resolved photoluminescence measurement was performed on one-dimensional GaN nanorods with c-axis-oriented single-crystalline wurtzite structures. The GaN nanorods were grown on Si(111) substrates by an improved hydride vapor phase epitaxy without a catalyst. Fast carrier recombinations of less than 10 ps were observed. The short recombination times of the GaN nanorods with few defects show the presence of nonradiative fast recombinations at the surface and interface. (C) 2010 The Japan Society of Applied Physics

    DOI

  • Spin-Polarized Localized Exciton Photoluminescence Dynamics in GaInNAs Quantum Wells

    Shulong Lu, Hidetaka Nosho, Atsushi Tackeuchi, Lifeng Bian, Jianrong Dong, Zhichuan Niu

    JAPANESE JOURNAL OF APPLIED PHYSICS   48 ( 10 ) 100206  2009.10  [Refereed]

     View Summary

    We have investigated spin polarization-related localized exciton photoluminescence (PL) dynamics in GaInNAs quantum wells by time-resolved PL spectroscopy. The emission energy dependence of PL polarization decay time as well as polarization-independent PL decay time suggests that the acoustic phonon scattering in the process of localized exciton transfer from the high-energy localized states to the low-energy ones dominates the PL polarization relaxation. By increasing the excitation power from 1 to 10 mW, the PL polarization decay time is changed from 0.17 to more than 1 ns, which indicates the significant effect of the trapping of localized electrons by nonradiative recombination centers. These experimental findings indicate that the spin-related PL polarization in diluted nitride semiconductors can be manipulated through carrier scattering and recombination process. (C) 2009 The Japan Society of Applied Physics

    DOI

  • Time-resolved Studies of Carrier and Spin Dynamics in Quantum Dots and Wide Band-gap Materials

    Atsushi Tackeuchi, Tae Whan Kim, Joo Hyung You, Hong Seok Lee, Hong Lee Park, Takako Chinone, Ji-Hao Liang

    JOURNAL OF THE KOREAN PHYSICAL SOCIETY   55 ( 1 ) 107 - 111  2009.07  [Refereed]

     View Summary

    Time-resolved studies using ultra-short laser pulses unveil interesting aspects of carrier and spin dynamics in compound semiconductors. Here, thermally-activated carrier transfers between CdTe/ZnTe quantum clots (QDs) and ultrafast spin-relaxations in bulk GaN are reported. Carrier transfer among CdTe/ZnTe self-organized QDs was studied using time-resolved photoluminescence measurements. The carriers in the high-energy ground states of small QDs are confirmed to transfer to the lower-energy ground states of larger QDs, even at 10 K. The energy dependence of the PL decay time changes uniquely With increasing temperature. The change in the energy dependence of the PL decay time call be explained by thermally-activated carrier transfer. Excitonic spin-relaxations in hexagonal GaN and cubic GaN are observed. The A-band free exciton in hexagonal GaN shows a sub-picosecond spin-relaxation of 0.47 ps at 1.50 K. The acceptor-bound exciton in hexagonal GaN shows spin-relaxation times of 1.40 - 1.14 ps at 15 - 50 K. Meanwhile, the spin-relaxation times in cubic GaN at 1.5 - 75 K are found to be longer than 5 ns. The long nanosecond spin-relaxation time in cubic GaN is consistent with the observation that spin-relaxation time becomes longer for wider-band-gap zincblende semiconductors.

  • Spin Relaxation in InAs Columnar Quantum Dots

    Takehiko Umi, Hidetaka Nosho, Shulong Lu, Lianhe Li, Andrea Fiore, Atsushi Tackeuchi

    JAPANESE JOURNAL OF APPLIED PHYSICS   48 ( 4 ) 04199  2009.04  [Refereed]

     View Summary

    We have investigated carrier spin dynamics in InAs columnar quantum dots (CQDs) by time-resolved photoluminescence (PL) measurement. The CQDs were formed by depositing a 1.8 monolayer InAs seed dot layer and a short-period GaAs/InAs superlattice (SL). The spin relaxation time was found to be prolonged from 1.6 to 5.3 ns by increasing the number of SL periods from 3 to 35. The PL decay time also increased from 0.93 to 1.9 ns, indicating a decrease in the spatial overlap of electron and hole wave functions. The changes in both the spin relaxation time and the PL decay time suggest that the Bir-Aronov-Pikus process is the main spin relaxation mechanism. (C) 2009 The Japan Society of Applied Physics

    DOI

  • Localized and free exciton spin relaxation dynamics in GaInNAs/GaAs quantum well

    S. L. Lu, L. F. Bian, M. Uesugi, H. Nosho, A. Tackeuchi, Z. C. Niu

    APPLIED PHYSICS LETTERS   92 ( 5 ) 051908  2008.02  [Refereed]

     View Summary

    We have investigated the exciton spin relaxation in a GaInNAs/GaAs quantum well. The recombination from free and localized excitons is resolved on the basis of an analysis of the photoluminescence characteristics. The free exciton spin relaxation time is measured to be 192 ps at 10 K, while the localized exciton spin relaxation time is one order of magnitude longer than that of the free exciton. The dependence of the free exciton spin relaxation time on the temperature above 50 K suggests that both the D'yakonov-Perel' and the Elliot-Yafet effects dominate the spin relaxation process. The temperature independence below 50 K is considered to be due to the spin exchange interaction. The ultralong spin relaxation time of the localized excitons is explained to be due to the influence of nonradiative deep centers. (c) 2008 American Institute of Physics.

    DOI

  • Short exciton radiative lifetime in submonolayer InGaAs/GaAs quantum dots

    Zhangcheng Xu, Yating Zhang, Atsushi Tackeuchi, Yoshiji Horikoshi, Jorn M. Hvam

    APPLIED PHYSICS LETTERS   92 ( 6 ) 063103  2008.02

     View Summary

    The exciton radiative lifetime in submonolayer (SML) InGaAs/GaAs quantum dots (QDs) grown at 500 degrees C was measured by using time-resolved photoluminescence from 10 to 260 K. The radiative lifetime is around 90 ps and is independent of temperature below 50 K. The observed short radiative lifetime is a key reason for the high performance of SML QD devices and can be explained by the theory of Andreani [Phys. Rev. B 60, 13276 (1999)] calculating the radiative lifetime of QDs formed at the interface fluctuations of a quantum well, as the SML QDs are 20-30 nm in diameter and embedded within the lateral InGaAs QW. (C) 2008 American Institute of Physics.

    DOI

  • Carrier density dependence of nonresonant carrier tunneling in GaAs double quantum wells - effect of exciton and free carrier thermodynamic

    S. L. Lu, T. Ushiyama, A. Tackeuchi, S. Muto

    PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 5, NO 1   5 ( 1 ) 78 - +  2008  [Refereed]

     View Summary

    Nonresonant carrier tunneling dynamics in GaAs/AlGaAs double quantum wells as a function of carrier density was investigated by using time-resolved pump and probe measurements. At low temperatures, tunneling time increases with increasing carrier density. At high temperatures, no dependence of tunneling time on carrier density was shown. This unique behavior is well explained by the thermodynamic effect between excitons and free carriers as a function of carrier density on the tunneling process. At low temperatures, a larger fraction of excitons at a higher carrier density, which have a relatively slow tunneling time in contrast to free electrons, leads to a slow tunneling time. The independence of the tunneling time at high temperatures is attributed to the insensitivity of the exciton population to the increased carrier density. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

    DOI

  • Energy relaxation time of hot carriers photoexcited in InGaN

    T. Ushiyama, T. Toizumi, Y. Nakazato, A. Tackeuchi

    PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 5, NO 1   5 ( 1 ) 143 - 145  2008  [Refereed]

     View Summary

    The energy relaxation time of hot carriers photoexcited in bulk InGaN is measured. The time-resolved pump and probe transmission measurements with subpicosecond time resolution show that the hot-carrier relaxation time is 0.92 ps at 15 K. The hot-carrier relaxation time becomes significantly shorter at higher temperatures. At temperatures higher than 150 K, there are no meaningful differences between rise times. This strong temperature dependence indicates that electron-phonon scattering dominates the carrier relaxation process. (c) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

  • Competition between quantum-confined Stark effect and free-carrier screening effect in AlGaN/GaN multiple quantum wells

    T. Fujita, T. Toizumi, Y. Nakazato, A. Tackeuchi, T. Chinone, J. H. Liang, M. Kajikawa

    PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 5, NO 1   5 ( 1 ) 356 - +  2008

     View Summary

    The competition between the quantum-confined Stark effect (QCSE) and the free-carrier screening effect in AlGaN/GaN multiple quantum wells (MQWs) has been investigated by time-resolved photoluminescence (PL) measurement. AlGaN/GaN MQWs is a promising material for the next-generation ultraviolet light-emitting diodes and laser devices. The large charges in the PL energy and the decay time are observed with changing carrier density. We show that the energy shift and the change in decay time are explained well by the free-carrier screening effect that compensates for the internal electric field. (c) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  • Carrier spin relaxation current topics in solid state physics in undoped GaAs double quantum wells

    S. L. Lu, T. Ushiyama, A. Tackeuchi, S. Muto

    PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 5, NO 1   5 ( 1 ) 326 - +  2008  [Refereed]

     View Summary

    Carrier spin relaxation in undoped GaAs/AlGaAs asymmetric double quantum wells (QWs) at room temperature was studied by spin-dependent pump and probe reflection measurements. The electron tunneling, which occurs between the two lowest conduction bands in the narrow and wide QWs, has a negligible effect on the electron spin relaxation. The most striking feature is that a long hole spin relaxation time was directly observed in the case of fast electron tunneling while it was not observed in the case of relatively slow electron tunneling. The hole that remains in the QWs due to the escape of the electron tunneling out makes the direct observation of hole spin relaxation possible. The measured hole spin relaxation time varies from 107 ps to 80 ps when the excitation power changes from 10 mW to 40 mW. This study provides direct experimental evidence of the important effect of the interaction between electrons and holes on the hole spin relaxation. (c) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

    DOI

  • Generation of highly circularly polarized light from uniform InAs/GaAs quantum dots

    K. Kusunoki, N. Tsukiji, T. Umi, A. Tackeuchi, K. Yamaguchi

    PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 5, NO 1   5 ( 1 ) 378 - +  2008  [Refereed]

     View Summary

    Highly circularly polarized light is generated from highly uniform InAs/GaAs quantum dots. These quantum dots allowed us to observe the spin dynamics at each energy state selectively without disturbing the inhomogeneous broadening. Time-resolved spin-dependent pbotoluminescence measurements show that the spin polarization at the second excited state reaches a maximum of 45% at 10 K, and that the spin polarization is greater at higher-energy states, This large spin polarization is explained by the spin Pauli blocking effect. (c) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

    DOI

  • Transition from excitonic tunneling to free carrier tunneling in GaAs/AlGaAs double quantum wells

    Shulong Lu, Takafumi Ushiyama, Taisuke Fujita, Koji Kusunoki, Atsushi Tackeuchi, Shunichi Muto

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS   46 ( 6A ) 3305 - 3308  2007.06  [Refereed]

     View Summary

    Nonresonant carrier tunneling has been studied as a function of temperature in GaAs/AlGaAs double quantum wells (DQWs). Time-resolved pump and probe reflectance measurements allow the direct observation of tunneling at any temperature between 15 K and room temperature. We found that for two DQWs with different barrier thicknesses, the tunneling time abruptly decreases above a critical temperature while it remains almost constant below the critical temperature. This critical temperature is shown to correspond to the exciton binding energy. Rate equation analysis explains this behavior as the thermalization of excitons into free electrons that have a faster tunneling time than excitons.

    DOI

  • Picosecond spin relaxation of acceptor-bound exciton in wurtzite GaN

    H. Otake, T. Kuroda, T. Fujita, T. Ushiyama, A. Tackeuchi, T. Chinone, J. -H. Liang, M. Kajikawa

    APPLIED PHYSICS LETTERS   89 ( 18 ) 182110  2006.10  [Refereed]

     View Summary

    The spin relaxation process of acceptor-bound excitons in wurtzite GaN is observed by spin-dependent pump and probe reflectance measurement with subpicosecond time resolution. The time evolutions measured at 15-50 K have a single exponential component corresponding to the electron spin relaxation time of 1.40-1.14 ps. The spin relaxation time is found to be proportional to T-0.175, where T is the temperature. This weak temperature dependence indicates that the main spin relaxation mechanism is the Bir-Aronov-Pikus process [Sov. Phys. JETP 42, 705 (1976)]. (c) 2006 American Institute of Physics.

    DOI

  • Thermally activated carrier transfer among CdTe/ZnTe self-organized quantum dots

    Atsushi Tackeuchi, Shogo Miyata, Seiji Sugawa, Koji Kusunoki, Tae Whan Kim, Jae-Ho Kim

    APPLIED PHYSICS LETTERS   89 ( 11 ) 112125  2006.09  [Refereed]

     View Summary

    Carrier transfer among CdTe/ZnTe self-organized quantum dots (QDs) was studied using time-resolved photoluminescence (PL) measurements. The authors have confirmed that carriers in the high energy ground states of small QDs transfer to the lower-energy ground states of larger QDs even at 10 K. The energy dependence of PL decay time changes uniquely with increasing temperature. They have found that the change in the energy dependence of PL decay time can be explained by thermally activated carrier transfer. (c) 2006 American Institute of Physics.

    DOI

  • Spin relaxation and antiferromagnetic coupling in semiconductor quantum dots

    A Tackeuchi, T Kuroda, K Yamaguchi, Y Nakata, N Yokoyama, T Takagahara

    PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES   32 ( 1-2 ) 354 - 358  2006.05  [Refereed]

     View Summary

    We report carrier spin dynamics in highly uniform self-assembled InAs quantum dots and the observation of antiferromagnetic coupling between semiconductor quantum dots. The spin relaxation times in the ground state and the first excited state were measured to be 1.0 and 0.6 ns, respectively, without the disturbance of inhomogeneous broadening. The measured spin relaxation time decreases rapidly from 1.1 ns at 10 K to 200ps at 130 K. This large change in the spin relaxation time is well-explained in terms of the mechanism of acoustic phonon emission. In coupled quantum dots, the formation of anti ferromagnetic coupling is directly observed. Electron spins are found to flip at 80 ps after photoexcitation via the interdot exchange interaction. The antiferromagnetic coupling exists at temperatures lower than 50-80K. A model calculation based on the Heitler-London approximation supports the finding that the antiferromagnetic coupling is observable at low temperature. These carrier spin features in quantum dots are suitable for the future quantum computation. (c) 2006 Elsevier B.V. All rights reserved.

    DOI

  • Nanosecond excitonic spin relaxation in cubic GaN

    A Tackeuchi, H Otake, Y Ogawa, T Ushiyama, T Fujita, F Takano, H Akinaga

    APPLIED PHYSICS LETTERS   88 ( 16 ) 162114  2006.04  [Refereed]

     View Summary

    The excitonic spin relaxation process in cubic GaN is observed by spin-dependent pump and probe reflectance measurements with subpicosecond time resolution. The spin polarization presents at temperatures lower than 100 K. The spin relaxation times at 15-75 K are found to be longer than 5 ns and short spin relaxation times on the picosecond order are not present. Although these long spin relaxation times are in striking contrast to the subpicosecond spin relaxation of A-band free excitons in hexagonal GaN, they are consistent with the dependence that spin relaxation time becomes longer for wider-band gap zinc blende semiconductors. (c) 2006 American Institute of Physics.

    DOI

  • Picosecond spin relaxations of acceptor-bound exciton and A-band free exciton in wurtzite GaN

    A. Tackeuchi, H. Otake, T. Fujita, T. Kuroda, T. Chinone, J. -H. Liang, M. Kajikawa

    PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 3, NO 12   3 ( 12 ) 4303 - 4306  2006  [Refereed]

     View Summary

    The spin relaxation process of acceptor-bound excitons in wurtzite GaN is observed by spin-dependent pump and probe reflectance measurement with subpicosecond time resolution. The time evolutions measured at 15-50 K have a single exponential component corresponding to the electron spin relaxation time of 1.40-1.14 ps. These spin relaxation times are slightly longer than those of the A-band free excitons of 0.47-0.25 ps in GaN at 150-225 K. The spin relaxation time is found to be proportional to T-0.175, where T is the temperature. This weak temperature dependence indicates that the main spin relaxation mechanism is the Bir-Aronov-Pikus process. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

  • Size dependence of carrier recombination efficiency in GaN quantum dots

    A Neogi, H Everitt, H Morkoc, T Kuroda, A Tackeuchi

    IEEE TRANSACTIONS ON NANOTECHNOLOGY   4 ( 2 ) 297 - 299  2005.03  [Refereed]

     View Summary

    The dependence of radiative recombination rate and efficiency on GaN quantum-dot (QD) size and temperature is studied by time-resolved photoluminescence (PL) spectroscopy. The emission is dominated. by radiative recombination at low temperatures (< 125 K) and exhibits high PL efficiency at room temperature. The radiative lifetime and the relative quantum efficiency decrease with the decreasing QD size.

    DOI

  • Exciton localization in vertically and laterally coupled GaN/AlN quantum dots

    A Neogi, H Morkoc, T Kuroda, A Tackeuchi, T Kawazoe, M Ohtsu

    NANO LETTERS   5 ( 2 ) 213 - 217  2005.02  [Refereed]

     View Summary

    Near-field and time-resolved photoluminescence measurements show evidence of exciton localization in vertically and laterally coupled GaN quantum dots (QDs). The binding energies in multiple period QDs (MQDs) are observed to be stronger by more than six times compared to single period QDs (SQDs). Excitons in MQDs have a short (450 ps) lifetime and persist at room temperature, while SQDs exhibit extraordinarily long (> 5 ns) lifetime at 10 K due to reduced spatial overlap of electron and hole wave functions in strained QDs.

    DOI

  • Coupling of spontaneous emission from GaN-AlN quantum dots into silver surface plasmons

    A Neogi, H Morkoc, T Kuroda, A Tackeuchi

    OPTICS LETTERS   30 ( 1 ) 93 - 95  2005.01  [Refereed]

     View Summary

    We have demonstrated the decay of spontaneous emission (SE) from AlN-GaN quantum dots (QDs) into silver surface plasmon (SP) modes in the ultraviolet at approximately 375-380 nm. Using time-resolved photoluminescence (PL), we show that the electron-hole recombination rate in AlN-GaN QDs is enhanced when SE is resonantly coupled to a metal SP mode, corresponding to the dip in the continuous-wave PL spectrum. Exciton recombination by means of silver SP modes is as much as 3-7 times faster than in normal QD SE and depends strongly on emission wavelength and thickness of the silver. (C) 2005 Optical Society of America.

  • Subpicosecond exciton spin relaxation in GaN

    T Kuroda, T Yabushita, T Kosuge, A Tackeuchi, K Taniguchi, T Chinone, N Horio

    APPLIED PHYSICS LETTERS   85 ( 15 ) 3116 - 3118  2004.10  [Refereed]

     View Summary

    The spin-relaxation process of A-band exciton in GaN is observed by spin-dependent pump and probe reflectance measurement with subpicosecond time resolution. The spin-relaxation times at 150-225 K are 0.47-0.25 ps. These are at least one order of magnitude, shorter than those of the other Ill-V compound semiconductors. The spin-relaxation time tau(s) is found to be proportional to T-1.4, where T is the temperature. (C) 2004 American Institute of Physics.

    DOI

  • Exciton spin relaxation dynamics in InGaAs/InP quantum wells

    S Akasaka, S Miyata, T Kuroda, A Tackeuchi

    APPLIED PHYSICS LETTERS   85 ( 11 ) 2083 - 2085  2004.09  [Refereed]

     View Summary

    We have investigated the exciton spin relaxation mechanism between 13 and 300 K in InGaAs/InP quantum wells using time-resolved spin-dependent pump and probe absorption measurements. The exciton spin relaxation time, tau(s) above 40 K was found to depend on temperature, T, according to tau(s)proportional toT(-1.1), although the spin relaxation time is constant below 40 K. The clear carrier density dependence of the exciton spin relaxation time was observed below 40 K, although the carrier density dependence is weak above 40 K. These results imply that the main spin relaxation mechanism above and below 40 K are the D'yakonov-Perel' process and the Bir-Aronov-Pikus process, respectively. (C) 2004 American Institute of Physics.

    DOI

  • Spin relaxation dynamics in highly uniform InAs quantum dots

    A Tackeuchi, R Ohtsubo, K Yamaguchi, M Murayama, T Kitamura, T Kuroda, T Takagahara

    APPLIED PHYSICS LETTERS   84 ( 18 ) 3576 - 3578  2004.05  [Refereed]

     View Summary

    We have investigated carrier spin dynamics in highly uniform self-assembled InAs quantum dots. The highly uniform quantum dots allowed us to observe the spin dynamics in the ground state and that in the second state separately, without the disturbance of inhomogeneous broadening. The spin relaxation times in the ground state and the second state were measured to be 1.0 and 0.6 ns, respectively. Our measurements reveal the absence of the carrier density dependence of the spin relaxation time. The measured spin relaxation time decreases rapidly from 1.1 ns at 10 K to 200 ps at 130 K. This large change in the spin relaxation time is well explained in terms of the mechanism of acoustic phonon emission. (C) 2004 American Institute of Physics.

    DOI

  • Electron spin flip by antiferromagnetic coupling between semiconductor quantum dots

    A Tackeuchi, T Kuroda, Y Nakata, M Murayama, T Kitamura, N Yokoyama

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS   42 ( 7A ) 4278 - 4281  2003.07  [Refereed]

     View Summary

    We have investigated antiferromagnetic coupling between semiconductor quantum dots. Electron spin is observed to flip at 80 ps after photoexcitation via the interdot-exchange interaction. The spin relaxation time under the antiferromagnetic order is extended to 10-12 ns, one order of magnitude longer than that in isolated quantum dots. The antiferromagnetic order exists at temperatures lower than 50-80 K. The photoluminescence experiments for various carrier densities show that antiferromagnetic coupling disappears when the electron pairing probability is low. A model calculation based on the Heitler-London approximation supports the finding that the antiferromagnetic order is observable at low temperature.

    DOI

  • Enhanced radiative efficiency in GaN quantum dots grown by molecular beam epitaxy

    A Neogi, H Everitt, H Morkoc, T Kuroda, A Tackeuchi

    IEEE TRANSACTIONS ON NANOTECHNOLOGY   2 ( 1 ) 10 - 14  2003.03  [Refereed]

     View Summary

    Self-assembled GaN quantum dots (QDs), grown on AIN by molecular beam epitaxy, were investigated by time-resolved photoluminescence spectroscopy. We investigate the emission mechanism in GaN QDs by comparing the carrier recombination dynamics in single and multiple period QDs. At 100 K, the PL decay time in single period QD structures is considerably shorter than in stacked QDs: Compared to single period QDs, the room temperature PL efficiency is considerably enhanced in 20 period QDs due to the reduction in nonradiative recombination processes.

    DOI

  • Direct observation of phonon relaxation bottleneck in InAs quantum dots of high-uniformity

    T Kitamura, R Ohtsubo, M Murayama, T Kuroda, K Yamaguchi, A Tackeuchi

    2ND INTERNATIONAL CONFERENCE ON SEMICONDUCTOR QUANTUM DOTS   0, No. 4   1165 - 1168  2003  [Refereed]

     View Summary

    The intersubband carrier relaxation dynamics in InAs quantum dots of high-uniformity has been investigated using time-resolved photoluminescence measurement. The clearly separated photoluminescence spectra of the ground state and the second state were observed. The rate equation analysis gives the intersubband relaxation time from the second state to the ground state of 360 ps at 10 K. The slow intersubband relaxation can be attributed to the phonon relaxation bottleneck. The intersubband relaxation time at room temperature is evaluated to be 320 ps by extrapolation of 10-200 K data.

    DOI

  • Observation of spin Pauli blocking in InAs high-uniform quantum dots

    M Murayama, R Ohtsubo, T Kitamura, T Kuroda, K Yamaguchi, A Tackeuchi

    2ND INTERNATIONAL CONFERENCE ON SEMICONDUCTOR QUANTUM DOTS   0, No. 4   1145 - 1148  2003  [Refereed]

     View Summary

    We have investigated carrier spin dynamics in self-assembled InAs/GaAs high-uniform quantum dots. The high-unifonn quantum dots allowed us to observe the spin dynamics in the ground state and that in the second state separately, without the disturbance of the inhomogeneous broadening. Spin Pauli blocking due to which the spin polarization in the second state is greater than that in the ground state has been clearly observed by time-resolved spin-dependent photoluminescence measurements. The spin relaxation times in the ground state and the second state were measured to be 1 ns and 0.8 ns, respectively.

    DOI

  • Enhancement of spontaneous recombination rate in a quantum well by resonant surface plasmon coupling

    A Neogi, CW Lee, HO Everitt, T Kuroda, A Tackeuchi, E Yablonovitch

    PHYSICAL REVIEW B   66 ( 15 ) 153305  2002.10  [Refereed]

     View Summary

    Using time-resolved photoluminescence measurements, the recombination rate in an In0.18Ga0.82N/GaN quantum well (QW) is shown to be greatly enhanced when spontaneous emission is resonantly coupled to a silver surface plasmon. The rate of enhanced spontaneous emission into the surface plasmon was as much as 92 times faster than QW spontaneous emission into free space. A calculation, based on Fermi's golden rule, reveals that the enhancement is very sensitive to silver thickness and indicates even greater enhancements are possible for QW's placed closer to the surface metal coating.

    DOI

  • Influence of free carrier screening on the luminescence energy shift and carrier lifetime of InGaN quantum wells

    T Kuroda, A Tackeuchi

    JOURNAL OF APPLIED PHYSICS   92 ( 6 ) 3071 - 3074  2002.09  [Refereed]

     View Summary

    We studied the influence of free carrier screening on the luminescence energy shift and carrier lifetime of InGaN multiple quantum wells (MQWs) mainly in relation to a quantum-confined Stark effect. We performed a systematic time-resolved photoluminescence measurement of MQWs for various carrier densities and three different well widths (2.5, 4.0, and 5.5 nm). We show that the energy shift and the change in carrier lifetime are explained well by the free carrier screening effect which compensates for the internal electric field. (C) 2002 American Institute of Physics.

    DOI

  • 緑色発光LEDの時間分解エレクトロルミネッセンス測定

    黒田剛正, 藪下智仁, 竹内淳, 堀尾直史, 谷口和与至, 山下陽滋, 千野根崇子, 船岡千洋

    信学技報   ED2002-77, LQE2002-52   17 - 20  2002.06

  • Localized exciton dynamics in InGaN quantum well structures

    SF Chichibu, T Azuhata, H Okumura, A Tackeuchi, T Sota, T Mukai

    APPLIED SURFACE SCIENCE   190 ( 1-4 ) 330 - 338  2002.05  [Refereed]

     View Summary

    InGaN multiple quantum well laser diode (LD) wafer that lased at 400 nm. was shown to have the InN mole fraction, x, of only 6% in the wells. Nanometer-probe compositional analysis showed that the fluctuation of x was as small as 1% or less, which is the resolution limit, However, the wells exhibited a Stokes-like shift (SS) of 49 meV and an effective localization depth E-0 was estimated by time-resolved photoluminescence (TRPL) measurement to be 35 meV at 300 K. Since the effective electric field due to polarization in the wells is estimated to be as small as 286 kV/cm, SS is considered to originate from an effective bandgap inhomogeneity. Because the well thickness fluctuation was insufficient to produce SS or E-0, the exciton localization is considered to be an intrinsic phenomenon in InGaN material. Indeed, bulk cubic In0.1Ga0.9N, which does not suffer any polarization field or thickness fluctuation effect, exhibited an SS of 140 meV at 77 K and similar TRPL results. The origin of the localization is considered to be due to the large bandgap bowing and In clustering in InGaN material. Such shallow and low density localized states are leveled by injecting high density carriers under the lasing conditions for the 400 nm LDS. (C) 2002 Elsevier Science B.V. All rights reserved.

  • Antiferromagnetic coupling between semiconductor quantum dots

    A Tackeuchi, T Kuroda, R Sasou, Y Nakata, N Yokoyama

    PHYSICA B-CONDENSED MATTER   314 ( 1-4 ) 25 - 29  2002.03  [Refereed]

     View Summary

    We have investigated magnetic coupling between semiconductor quantum dots. The antiferromagnetization process based on the interdot exchange interaction has been directly observed using time-resolved spin-dependent photoluminescence measurements. The antiferromagnetic order that proves the interdot exchange interaction energy to be negative is found to exist at temperatures lower than 50-80K. The spin relaxation time under the antiferromagnetic order is extended to 10-12ns, an order of magnitude longer than that in isolated quantum dots. (C) 2002 Elsevier Science B.V. All rights reserved.

  • 半導体量子ドット間の反強磁性結合による電子スピンの反転

    竹内 淳, 黒田剛正, 中田義昭, 横山直樹

    日本物理学会誌   第57巻 ( 12号 ) 904  2002

  • Time-resolved electroluminescence study of green light-emitting diode

    T Kuroda, N Horio, K Taniguchi, H Sato, C Funaoka, A Tackeuchi

    INTERNATIONAL WORKSHOP ON NITRIDE SEMICONDUCTORS, PROCEEDINGS   0, 25   25 - 28  2002  [Refereed]

     View Summary

    We have performed time-resolved photoluminescence and electroluminescence measurements for an InGaN quantum well at 300 K to investigate the carrier dynamics. Electroluminescence measurement yields more direct information of the carrier dynamics of device operation, because carriers are generated by current injection in optoelectronic devices. The time evolution of EL spectra indicates that spatial indium fluctuations become significant with increasing In content in nitride-based optoelectronic applications.

  • Localized exciton dynamics in strained cubic In0.1Ga0.9N/GaN multiple quantum wells

    SF Chichibu, M Sugiyama, T Onuma, T Kitamura, H Nakanishi, T Kuroda, A Tackeuchi, T Sota, Y Ishida, H Okumura

    APPLIED PHYSICS LETTERS   79 ( 26 ) 4319 - 4321  2001.12  [Refereed]

     View Summary

    Radiative and nonradiative recombination dynamics in strained cubic (c-) In0.1Ga0.9N/c-GaN multiple quantum wells were studied using temperature-dependent time-resolved photoluminescence (TRPL) spectroscopy. In contrast to hexagonal InGaN quantum wells, low-excitation photoluminescence peak energy increased moderately with decreasing well thickness L and the PL lifetime did not strongly depend on L. The results clearly indicated that the piezoelectric field was not acting on the transition process. The TRPL signal was well fitted as a stretched exponential decay from 10 to 300 K, showing that the spontaneous emission is due to the radiative recombination of excitons localized in disordered quantum nanostructures such as In clusters. The localized states were considered to have two-dimensional density of states at 300 K (quantum disk size), since the radiative lifetime increased with increasing temperature above 150 K.(C) 2001 American Institute of Physics.

  • Time-resolved photoluminescence study of InGaN MQW with a p-contact layer

    T Kuroda, R Sasou, A Tackeuchi, H Sato, N Horio, C Funaoka

    PHYSICA STATUS SOLIDI B-BASIC RESEARCH   228 ( 1 ) 125 - 128  2001.11  [Refereed]

     View Summary

    To clarify the influence of diffused Mg impurities on the carrier recombination in InGaN multiple quantum wells (MQWs), we have performed a systematic study using time-resolved photoluminescence (PL) measurements. It was found that the MQWs with a p-contact layer and the MQWs with a nondoped GaN layer had almost the same carrier lifetime and PL intensity below 200 K. However, the MQWs with the p-contact layer had shorter carrier lifetime and lower PL intensity than the MQWs with the nondoped GaN laver above 200 K. This degradation of the PL for the MQWs with the p-contact laver can be attributed to nonradiative recombination caused by the diffusion of Mg impurities from the p-contact laver into MQWs.

  • Band gap bowing and exciton localization in strained cubic InxGa1-xN films grown on 3C-SiC (001) by rf molecular-beam epitaxy

    SF Chichibu, M Sugiyama, T Kuroda, A Tackeuchi, T Kitamura, H Nakanishi, T Sota, SP DenBaars, S Nakamura, Y Ishida, H Okumura

    APPLIED PHYSICS LETTERS   79 ( 22 ) 3600 - 3602  2001.11  [Refereed]

     View Summary

    Spontaneous emission mechanisms in InGaN alloys were studied by determining the effective band gap energies using photoluminescence excitation spectroscopy and static and time-resolved photoluminescence (PL) measurements on fully strained cubic (c-) InxGa1-xN films on c-GaN templates, which were grown by rf molecular-beam epitaxy on smaller lattice-mismatched 3C-SiC (001) substrates prepared on Si (001). The c-InxGa1-xN alloys exhibited large band gap bowing. The PL decay dynamics showed that the emission is due to recombination of localized excitons, the same as in the case of hexagonal InGaN. The c-InxGa1-xN exhibited a larger Stokes-like shift and a larger localization depth, showing that the material's inhomogeneity is much enhanced compared to that of the hexagonal polytype. (C) 2001 American Institute of Physics.

  • Ultrafast all-optical modulation by near-infrared intersubband transition in n-doped InGaAs/AlAsSb quantum wells

    A Neogi, H Yoshida, T Mozume, N Georgiev, T Akiyama, A Tackeuchi, O Wada

    OPTICAL AND QUANTUM ELECTRONICS   33 ( 7-10 ) 975 - 983  2001.07  [Refereed]

     View Summary

    We demonstrate the ultrafast modulation of interband (IB)-resonant light (1.0-1.1 mum) by near-infrared intersubband (ISB)-resonant light (1.55-1.9 mum) in n-doped InGaAs/AlAsSb multiple quantum wells (QWs) using non-degenerate pump-probe spectroscopy. A modulation with an absorption recovery time of 1.0-2.0 ps has been observed in a planer-type modulation device due to ultrafast ISB relaxation of the carriers. The IB carrier relaxation process in the absence of an ISB-resonant light has also been investigated by time-resolved photoluminescence (PL) measurement. The modulation speed is determined by the inter- and intra-subband relaxation of the carriers in the conduction subband. The modulation speed at 1.1 mum due to an ISB-resonant pump light at 1.95 mum has been observed to be 1.4 ps at excitation energy of 500 fJ/mum(2).

  • Observation of interdot tunneling process of spin-polarized electrons

    A Tackeuchi, Y Nakata, R Sasou, K Mase, T Kuroda, N Yokoyama

    PHYSICA E   10 ( 1-3 ) 32 - 35  2001.05  [Refereed]

     View Summary

    We have directly observed the spin-polarized electron tunneling process between double quantum dots (QDs) using spin-dependent time-resolved photoluminescence measurements. The spin relaxation times in isolated In0.9Al0.1As Stranski-Krastanow (SK)-mode QDs and InAs SK-mode QDs were measured as 1.3 and 1.2 ns, respectively. The fact that these times are longer than those of quantum wells indicates that the spin relaxation rate is suppressed in QDs by the increased zero dimensionality. The measurement of tunneling between double QDs which consist of In0.9Al0.1As QDs and InAs QDs indicates that the spin relaxation time is not affected by the tunneling process. The spin-polarized carriers were found to tunnel from one dot to another dot while retaining spin polarization. (C) 2001 Elsevier Science B.V. All rights reserved.

  • 半導体量子構造のスピン物性と電子スピンを用いた量子演算の可能性

    武藤俊一, 竹内 淳

    応用物理   70巻 ( 3 ) 291 - 295  2001

    DOI CiNii

  • Dynamics of carrier tunneling between vertically aligned double quantum dots

    A Tackeuchi, T Kuroda, K Mase, Y Nakata, N Yokoyama

    PHYSICAL REVIEW B   62 ( 3 ) 1568 - 1571  2000.07  [Refereed]

     View Summary

    We have directly measured carrier tunneling times between vertically aligned double quantum dots (QD's) using time-resolved photoluminescence measurement. The vertically aligned double QD structure consists of In0.9Al0.1As QD's, a GaAs barrier layer, and InAs QD's. The tunneling times were measured for the three different barrier thicknesses. The dependence of the tunneling time on the barrier thickness is in agreement with the Wentzel-Kramers-Brillouin approximation. The nonresonant tunneling rate between QD's is found to be suppressed to one-tenth of the tunneling rate between quantum wells.

  • No spin polarization of carriers in InGaN

    A Tackeuchi, T Kuroda, A Shikanai, T Sota, A Kuramata, K Domen

    PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES   7 ( 3-4 ) 1011 - 1014  2000.05  [Refereed]

     View Summary

    We report the spin polarization of carriers photoexcited in bulk InGaN by circularly polarized femtosecond optical pulses. No spin polarization is observed in the picosecond time region using spin-dependent pump and probe absorption measurements with a time resolution of 0.35 ps. This is in contrast to the existence of spin polarization in GaAs quantum wells or in InGaAs quantum wells which have a spin relaxation time in the picosecond time region. The unique band structure of InGaN, which has weak spin-orbit interaction, and an in-plane potential fluctuation due to the compositional inhomogeneity of In explains the lack of spill polarization. (C) 2000 Elsevier Science B.V. All rights reserved.

  • A pump and probe study of photoinduced internal field screening dynamics in an AlGaN/GaN single-quantum-well structure

    A Shikanai, T Deguchi, T Sota, T Kuroda, A Tackeuchi, S Chichibu, S Nakamura

    APPLIED PHYSICS LETTERS   76 ( 4 ) 454 - 456  2000.01  [Refereed]

     View Summary

    Photogenerated carrier dynamics in an AlGaN/GaN single quantum well has been studied using a conventional degenerate pump and probe technique at room temperature. Photoinduced absorption at the exciton resonance has been observed. It is explained by the absorption coefficient change, through the quantum-confined Stark effect and the quantum-confined Franz-Keldish effect, caused by the photoinduced internal electric-field screening. In comparison with biased GaAs multiple quantum wells, a slower time evolution of differential transmission signals has been also found. Its origin is attributed to the longer carrier sweep-out time due to the potential profile of the sample in conjunction with the longer carrier recombination time. (C) 2000 American Institute of Physics. [S0003-6951(00)02404-9].

  • No Spin Polarization of Carriers in InGaN

    A. Tackeuchi, T. Kuroda, A. Shikanai, T. Sota, A. Kuramata, K. Domen

    Physica E   7   1011  2000

  • Luminescence Energy Shift and Carrier Lifetime Change Dependence on Carrier Density in InGaN/InGaN Quantum Wells

    T. Kuroda, A. Tackeuchi, T. Sota

    Applied Physics Letters   76/25   3753  2000

  • Picosecond electron-spin relaxation in GaAs/AlGaAs quantum wells and InGaAs/InP quantum wells

    A Tackeuchi, T Kuroda, S Muto, O Wada

    PHYSICA B-CONDENSED MATTER   272 ( 1-4 ) 318 - 323  1999.12  [Refereed]

     View Summary

    The spin-relaxation process of electrons at room temperature is investigated for GaAs/AlGaAs multiple-quantum wells (MQWs) and InGaAs/InP MQWs. The spin-relaxation times are measured for various well thicknesses using time-resolved spin-dependent pump and probe absorption measurements. The spin-relaxation time, tau(s), for GaAs MQWs was found to depend on the electron confinement energy, E-1e, according to tau(s) proportional to E-le(-2.2), demonstrating that the main spin-relaxation mechanism at room temperature is the D'yakonov-Perel' process. The measured tau(s) of InGaAs MQWs vary depending on the quantum confinement energy, E-le, according to tau(s) proportional to E-le(-1.0). tau(s) for QWs by the Elliott-Yafet process is calculated and shown to vary according to tau(s) proportional to E-le(-1). The spin-relaxation mechanism and possible applications using this fast spin-relaxation process are discussed. (C) 1999 Elsevier Science B.V. All rights reserved.

  • Electron spin relaxation in GaAs/AlGaAs quantum wires analyzed by transient photoluminescence

    T Nishimura, XL Wang, M Ogura, A Tackeuchi, O Wada

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS   38 ( 8B ) L941 - L944  1999.08  [Refereed]

     View Summary

    We report on the spin relaxation in GaAs-based quantum wires and wells studied at 50 K < T < 180 K by analyzing the circularly polarized photoluminescence transients. A 3.5-times shortening of the electron spill relaxation time is observed in GaAs/AlGaAs quantum wires compared with that in quantum wells at 180 K. The observed results suggest that a spin Rip in the quantum wires is caused predominantly by the D'yakonov-Perel' effect due to spin-orbit interaction. The strong quantum confinement of electrons in the reduced dimensionality of the quantum structure leads to a larger spin splitting energy and a stronger temperature dependence of spin relaxation.

  • Electron spin-relaxation dynamics in GaAs/AlGaAs quantum wells and InGaAs/InP quantum wells

    A Tackeuchi, T Kuroda, S Muto, Y Nishikawa, O Wada

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS   38 ( 8 ) 4680 - 4687  1999.08  [Refereed]

     View Summary

    We have investigated the spin-relaxation process of electrons at room temperature in undoped GaAs/AlGaAs multiple-quantum wells (MQWs) and InGaAs/lnP MQWs. The spin-relaxation times are measured for different well thicknesses using time-resolved spin-dependent pump and probe absorption measurement. The spin-relaxation time, tau(s), for GaAs MQWs was found to depend on the electron confinement energy, E-1e, according to tau(s) proportional to E-ie(-2.2), demonstrating that the main spin-relaxation mechanism at room temperature is the D'yakonov-Perel' process. The measured spin-relaxation times of InGaAs MQWs whose band-gap is about half that of GaAs MQWs are about 5 ps and vary depending on the quantum confinement energy EI,, according to tau(s) proportional to E-ie(-1.0). The spin-relaxation time by Elliott-Yafet process, which becomes stronger for narrower band-gap materials, is calculated for quantum wells and shown to vary according to tau(s) proportional to E-ie(-1). The spin-relaxation mechanism and possible applications using this fast spin-relaxation process are discussed.

  • Carrier mobility dependence of electron spin relaxation in GaAs quantum wells

    R Terauchi, Y Ohno, T Adachi, A Sato, F Matsukura, A Tackeuchi, H Ohno

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS   38 ( 4B ) 2549 - 2551  1999.04  [Refereed]

     View Summary

    We have investigated the electron mobility (mu) dependence and the electron quantized energy dependence of the electron spin relaxation time (tau(s)) in n-type and undoped GaAs/AlGaAs multiple quantum wells at room temperature, tau(s) proportional to mu(-1) obtained from the experimental results is consistent with the theoretical prediction based on the D'yakonov-Perel' theory.

  • Carrier dynamics of quantum confined structures

    S Muto, A Tackeuchi

    MATERIALS SCIENCE & ENGINEERING R-REPORTS   22 ( 3 ) 79 - 111  1998.04  [Refereed]

     View Summary

    This article reviews aspects of carrier dynamics concerning semiconductor quantum wells in picosecond and femtosecond regime with emphasis on tunneling and spin relaxation which we were experimentally involved in. We refer to recent significant developments in the field. We also pick up intriguing aspects of quantum dots on possible applications to both electronic and photonic devices related to the carrier dynamics. (C) 1998 Elsevier Science S.A. All rights reserved.

  • Electron spin relaxation dynamics in InGaAs/InP multiple-quantum wells

    A Tackeuchi, O Wada

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS   37 ( 1 ) 98 - 99  1998.01  [Refereed]

     View Summary

    The electron spin relaxation of InGaAs/InP multiple-quantum wells is investigated at mom temperature using time-resolved polarization-dependent absorption measurements. The spin relaxation time is dependent on the quantum confined energy, E-1e, according to tau(s) proportional to E-1e(-1). The dependence differs from that of the D'yakonov-Perel' interaction, which governs the spin relaxation of GaAs quantum wells at room temperature, and suggests the possibility of the existence of an additional spin relaxation mechanism.

  • Femtosecond saturable absorption recovery in a type-II tunneling bi-quantum well for long-wavelength operation

    Y Matsui, K Ogawa, A Tackeuchi, Y Ogawa, A Suzuki

    PHYSICA STATUS SOLIDI B-BASIC RESEARCH   204 ( 1 ) 416 - 419  1997.11  [Refereed]

     View Summary

    We demonstrate femtosecond bleached absorption recovery using a type-Il tunneling biquantum well (TBQ) structure tailored for long-wavelength operation. The type-IT TBQ structure consisted of strained-layer InGaAlAs narrow wells and InGaAsP wide wells with staggered band lineup, separated by thin InAlAs barriers. In the structures, the photocreated electrons in InGaAlAs layers energetically relax to InGaAsP layers via LO-phonon assisted tunneling through InAlAs barrier! resulting in the bleaching recovery due to the ultrafast spatial separation of electrons and holes. The bleached absorption recovery time was as fast as 350 fs in the excitation wavelength range of 50 nm near the InGaAlAs bandgap.

  • 半導体量子井戸におけるスピン緩和のダイナミクス

    竹内淳, 和田修

    応用物理     960  1997.09

  • Exciton spin dynamics in GaAs quantum wells

    S Adachi, T Miyashita, S Takeyama, Y Takagi, A Tackeuchi

    JOURNAL OF LUMINESCENCE   72-4   307 - 308  1997.06  [Refereed]

     View Summary

    We have investigated the temperature dependence of the exciton spin relaxation between 10 and 300 K in undoped GaAs quantum wells by using the nonlinear transmission pump-probe and the four-wave-mixing techniques. The electron spin relaxation rate above 40 K is found to be proportional to the product of the temperature and the momentum relaxation time, which indicates that the D'yakonov-Perel' interaction governs the spin-flip dynamics. The temperature independence below 30 K is considered to be due to the band-mixing effect that determines the hole spin relaxation, and the spin exchange is effective to the exciton spin relaxation in these temperatures.

  • Electron spin relaxation in InGaAs/InP multiple-quantum wells

    A Tackeuchi, O Wada, Y Nishikawa

    APPLIED PHYSICS LETTERS   70 ( 9 ) 1131 - 1133  1997.03  [Refereed]

     View Summary

    The electron spin relaxation of InGaAs/InP multiple-quantum wells (MQW) is investigated using time-resolved polarization-dependent absorption measurement. The MQW has an excitonic absorption at 1.54 mu m which is suitable for application in optical communications. A theoretical prediction assuming the D'yakonov-Perel' interaction as the main relaxation mechanism gives a spin relaxation rate for the InGaAs quantum well over twice as high as that for the GaAs quantum well. The spin relaxation time measured at room temperature is 5.2 ps and found to be an order of magnitude faster than that of a GaAs quantum well. (C) 1997 American Institute of Physics.

  • Polarization choices in exciton-biexciton system of GaAs quantum wells

    S Adachi, T Miyashita, S Takeyama, Y Takagi, A Tackeuchi, M Nakayama

    PHYSICAL REVIEW B   55 ( 3 ) 1654 - 1660  1997.01  [Refereed]

     View Summary

    We study the induced absorption due to biexciton formation in absorption bleaching pump-probe signals and the quantum beats between exciton and biexciton in the four-wave-mixing signals. The origin is confirmed by the polarization dependence of the signals in both experiments. The definite discrimination between exciton-biexciton quantum beats and the others by the polarization choices is demonstrated clearly. We also show the well-width dependence of the biexciton binding energy that is determined by the period of the quantum beats in the range of well width 45-150 Angstrom of GaAs/AlxGa1-xAs (x=0.3-1.0) multiple quantum wells.

  • Ultrafast all-optical spin polarization switch using quantum-well etalon

    Y Nishikawa, A Tackeuchi, M Yamaguchi, S Muto, O Wada

    IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS   2 ( 3 ) 661 - 667  1996.09  [Refereed]

     View Summary

    Ultrafast all-optical switch is proposed and demonstrated using picoseconds spin-polarization relaxation in a multiple-quantum-well (MQW) etalon structure, The decay time of the conventional all-optical switching in MQW etalon is restricted by the carrier lifetime, typically in nanoseconds, Using carrier spin relaxation, the polarization change of the probe beam has been demonstrated to be switched with a pulse width of 4 ps and a contrast of 4:1 at a pump pulse energy of 50 fJ/mu m(2). In the present device, the contrast is determined by the polarization rotation angle of the probe beam, and the polarization-rotation angle has been shown to be proportional to the total well thickness, It is predicted that a contrast can be improved over 13 dB by optimizing the MQW etalon structure, indicating potential applicability to ultrafast optical communication systems, The optimization would improve the transmission from a present value of about 1%.

  • Room-temperature electron spin dynamics in GaAs/AlGaAs quantum wells

    A Tackeuchi, Y Nishikawa, O Wada

    APPLIED PHYSICS LETTERS   68 ( 6 ) 797 - 799  1996.02  [Refereed]

     View Summary

    We have investigated the spin relaxation process of electron at room temperature in nominally undoped GaAs/A1GaAs multiple-quantum wells. The spin relaxation times are measured for different well thicknesses using time-resolved polarization absorption measurement. The spin relaxation time tau(s), is found to depend on the electron confined energy E(le), according to alpha E(le)(-2.2), showing that the main spin relaxation mechanism at room temperature is the D'yakonov-Perel' Perel' interaction. (C) 1996 American Institute of Physics.

  • Ultrafast optical sampling pump-probe measurement of exciton spin relaxation in GaAs/AlGaAs quantum wells

    S Adachi, S Takeyama, Y Takagi, A Tackeuchi, S Muto

    APPLIED PHYSICS LETTERS   68 ( 7 ) 964 - 966  1996.02  [Refereed]

     View Summary

    Spin dynamics in GaAs/AlGaAs multiple quantum wells at room temperature are studied in subpicosecond to nanosecond time region by using the optical sampling nondegenerate pump-probe technique. Polarization-dependent absorption signals resonant to the light-hole (lh) exciton revealed the subpicosecond spin relaxation time which was several tens of times shorter than that of the heavy-hole (hh) exciton. Resonant pumping to lh and probing at hh exhibited no spin dependence within the time resolution, showing the spin memory being washed out completely during the energy relaxation from lh to hh. (C) 1996 American Institute of Physics.

  • Picosecond all-optical switching using tunneling and spin-relaxation in quantum well structures

    A Tackeuchi, Y Nishikawa, S Muto, O Wada

    OPTOELECTRONICS-DEVICES AND TECHNOLOGIES   10 ( 4 ) 561 - 574  1995.12  [Refereed]

     View Summary

    We propose and demonstrate two approaches for controlling the recovery time from excitonic absorption bleaching, Each approach has a new feature that the recovery time of optical nonlinearity is controlled by the tunneling or the spin-relaxation effect in quantum well structures. In the tunneling bi-quantum well structure utilizing the tunneling effect, the recovery time is reduced to the picosecond region with the reduction of tunneling barrier thickness. The recovery time is three orders of magnitude shorter than the nanosecond recovery of conventional multiple quantum wells (MQW). The spin relaxation of MQW has been shown to be as fast as several tens of picoseconds. The all-optical gate operations of a Fabry-Perot etalon are demonstrated with a full signal recovery of 17 ps using tunneling and 7-ps recovery using spin relaxation.

  • TIME-RESOLVED STUDY OF CARRIER TRANSFER AMONG INAS/GAAS MULTI-COUPLED QUANTUM DOTS

    A TACKEUCHI, Y NAKATA, S MUTO, Y SUGIYAMA, T USUKI, Y NISHIKAWA, N YOKOYAMA, O WADA

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS & EXPRESS LETTERS   34 ( 11A ) L1439 - L1441  1995.11  [Refereed]

     View Summary

    Carrier transfer among InAs/GaAs self-organized multi-coupled quantum dots was studied using time-resolved photoluminescence. In the multi-coupled quantum dots, since quantum dots couple with the other dots laterally, the photoexcited carriers tunnel into the relatively larger quantum dots which have lower energy levels. The photoluminescence decay time of multi-coupled quantum dots strongly depends on the energy in contrast with conventional quantum dots. The energy dependence can be explained with a cascade-like tunneling model assuming a tunneling time between quantum dots of 1300 ps.

  • PERFORMANCE OF ALL-OPTICAL SWITCH UTILIZING THE SPIN-DEPENDENT TRANSIENT ROTATION IN A MULTIPLE-QUANTUM-WELL ETALON

    Y NISHIKAWA, A TACKEUCHI, M YAMAGUCHI, S MUTO, O WADA

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS & EXPRESS LETTERS   34 ( 10A ) L1283 - L1285  1995.10  [Refereed]

     View Summary

    The performance of the spin-dependent transient rotation (STR) for a multiple-quantum-well (MQW) etalon is studied. An on/off contrast of 4:1 obtained for a pump pulse energy of 50 fJ/mu m(2) at room temperature is larger than that obtained by other picosecond switching methods using etalons. The experimentally observed polarization-rotation angle of the probe beam has been found to be proportional to the total well thickness, which is consistent with the theoretical prediction. It is predicted that the contrast can be improved to over 20:1 for a pump pulse energy of 50 fJ/mu m(2) by using a MQW etalon with a total well thickness of 1 mu m.

  • LARGE LATERAL MODULATION IN INAS/GAAS INPLANE STRAINED SUPERLATTICE ON SLIGHTLY MISORIENTED (110)INP SUBSTRATE

    Y NISHIKAWA, Y NAKATA, A TACKEUCHI, S MUTO, O WADA

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS   34 ( 7B ) L915 - L917  1995.07  [Refereed]

     View Summary

    An InAs/GaAs in-plane strained superlattice (IFS SL) grown by molecular beam epitaxy (MBE) is investigated using photoluminescence (PL) measurements. The polarization-dependent PL at 0.77 eV (77 K) with a full width at half maximum of 25 meV indicates that a laterally modulated structure is grown. With the use of the empirical nearest-neighbor sp(3)s(double dagger) tight-binding energy band calculation for the IPSSL, we deduced that an In0.74Ga0.26As/In0.26Ga0.74As IPSSL was actually grown. This reveals that an indium composition modulation Delta x of about 0.48 was achieved, which is more than twice as large as that in in-plane superlattices using other material systems.

  • BAND-GAP RENORMALIZATION AND EXCITONIC EFFECTS IN TUNNELING IN ASYMMETRIC DOUBLE-QUANTUM WELLS

    A TACKEUCHI, AP HEBERLE, WW RUHLE, K KOHLER, S MUTO

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS   34 ( 5A ) L543 - L546  1995.05  [Refereed]

     View Summary

    Tunneling in asymmetric double quantum wells is studied using time-resolved photoluminescence. The photoluminescence lineshape and peak position of the narrow quantum well are strongly influenced by band-gap renormalization caused by the tunneling carriers. Tunneling is quenched in a field regime of +/- 10 kV/cm around the ground state resonance due to excitonic effects.

  • INGAAS/INALAS INPLANE SUPERLATTICES GROWN ON SLIGHTLY MISORIENTED (110) INP SUBSTRATES BY MOLECULAR-BEAM EPITAXY

    Y NAKATA, O UEDA, A TACKEUCHI, S NAKAMURA, S MUTO

    JOURNAL OF CRYSTAL GROWTH   150 ( 1-4 ) 341 - 345  1995.05  [Refereed]

     View Summary

    InGaAs/InAlAs in-plane superlattices (IPSLs) composed of InAs/GaAs and InAs/AlAs monolayer superlattices were grown using molecular beam epitaxy. The substrates were misoriented (110) InP tilting 3 degrees toward the [00 $$($) over bar 1] direction. We grew half monolayers of AlAs and GaAs and single monolayers of InAs alternately, keeping regular arrays of single monolayer steps. The structures were evaluated by transmission electron microscopy (TEM). In a transmission electron diffraction pattern from the ($($) over bar$$ 110) cross-section, we observed two types of superstructure spot pairs double-positioned in the [001] direction, indicating the formation of the intended IPSL structures. In a cross-sectional TEM dark-field image, we observed the InGaAs/InAlAs superlattice structures formed almost in the [001] direction. The mean period of the superlattices was approximately 4 nm, which was comparable to the terrace width expected from the substrate tilt angle. However, IPSL structures were not completely formed, i.e., the lateral interfaces meandered along the growth direction, and partial disorderings were often observed. The photoluminescence spectrum from the IPSL had a peak corresponding to the InGaAs (2 nm thick)/InAlAs (2 nm thick)superlattice in addition to a peak corresponding to the In0.5Al0.25Ga0.25As alloy.

  • NEAR-1.3-MU-M HIGH-INTENSITY PHOTOLUMINESCENCE AT ROOM-TEMPERATURE BY INAS/GAAS MULTI-COUPLED QUANTUM DOTS

    A TACKEUCHI, Y NAKATA, S MUTO, Y SUGIYAMA, T INATA, N YOKOYAMA

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS & EXPRESS LETTERS   34 ( 4A ) L405 - L407  1995.04  [Refereed]

     View Summary

    We propose a new quantum dot system called multi-coupled quantum dots. In this system, since quantum dots couple with adjacent dots, the photoexcited carriers tunnel into the larger quantum dots which have lower energy states. This energy relaxation results in narrower and stronger photoluminescence than with conventional quantum dots. InAs/GaAs self-organized multi-coupled quantum dots show strong photoluminescence near 1.3 mu m at room temperature, whose intensity is as large as in the well-known highly efficient InGaAs/GaAs quantum wells.

  • ALL-OPTICAL PICOSECOND SWITCHING OF A QUANTUM-WELL ETALON USING SPIN-POLARIZATION RELAXATION

    Y NISHIKAWA, A TACKEUCHI, S NAKAMURA, S MUTO, N YOKOYAMA

    APPLIED PHYSICS LETTERS   66 ( 7 ) 839 - 841  1995.02  [Refereed]

  • DEPENDENCE OF RESONANT ELECTRON AND HOLE TUNNELING TIMES BETWEEN QUANTUM-WELLS ON BARRIER THICKNESS

    AP HEBERLE, XQ ZHOU, A TACKEUCHI, WW RUHLE, K KOHLER

    SEMICONDUCTOR SCIENCE AND TECHNOLOGY   9 ( 5 ) 519 - 522  1994.05  [Refereed]

     View Summary

    We investigated resonant tunnelling of electrons and holes between coupled quantum wells using time-resolved luminescence spectroscopy. Exponential dependence of tunnelling times on barrier width is observed for electrons but not for holes. The tunnelling times of electrons are correctly described by a model which takes into account inhomogeneous broadening.

  • GAMMA-X ELECTRON-TRANSFER IN TYPE-II TUNNELING BI-QUANTUM WELLS

    A TAKEUCHI, U STRAUSS, WW RUHLE, T INATA

    SOLID-STATE ELECTRONICS   37 ( 4-6 ) 809 - 812  1994.04  [Refereed]

     View Summary

    We have studied the energy band structure and the GAMMA-X carrier transfer mechanisms for type II tunneling bi-quantum wells consisting of GaAs wells, barriers of different thicknesses, and AlAs layers by cw and time-resolved photoluminescence measurements. The cw photoluminescence spectra of the direct recombination of X electrons in the 7.1 nm thick AlAs layers with GAMMA holes in the 2.85 nm thick GaAs wells show weak zero-phonon lines indicating that the AlAs confined states at X(xy) are lower than those X(z). Time-resolved photoluminescence reveals that the carrier transfer time depends stronger on temperature for thicker AlGaAs barriers. Two scattering mechanisms, temperature-dependent phonon scattering and the temperature-independent interface scattering, are probably involved in the carrier transfer, the latter becoming smaller with increasing AlGaAs barrier thickness. Our results are compared with those obtained for smaller type II GaAs/AlAs superlattices.

  • STUDY OF TUNNELING TIME IN TUNNELING BI-QUANTUM-WELL AND RESONANT-TUNNELING BARRIER STRUCTURES

    Y SUGIYAMA, T INATA, A TACKEUCHI, Y NAKATA, S NAKAMURA, S MUTO

    MICROWAVE AND OPTICAL TECHNOLOGY LETTERS   7 ( 3 ) 85 - 88  1994.02  [Refereed]

     View Summary

    We studied the electron tunneling time in tunneling bi-quantum-well and resonant tunneling barrier structures with a pump-probe photo-luminescence system using optical mixing. The tunneling lime decreased as the temperature increased, which is attributed to electrons in an exciton stale thermalizing into free electrons having a faster tunneling lime. (C) 1994 John Wiley and Sons, Inc.

  • PICOSECOND OPTICAL SIGNAL RECOVERY IN TUNNELING BI-QUANTUM WELL STRUCTURE

    A TACKEUCHI, S MUTO, Y NISHIKAWA

    FUJITSU SCIENTIFIC & TECHNICAL JOURNAL   30 ( 2 ) 188 - 194  1994  [Refereed]

     View Summary

    Fast recovery from the excitonic absorption bleaching in tunneling bi-quantum well (TBQ) structures is proposed and demonstrated. The TBQ has a new feature that conventional superlattices will never have; that is, control of the recovery time of optical nonlinearity using tunneling. By reducing the tunneling barrier thickness, the recovery time can be reduced to the picosecond region, which is three orders of magnitude faster than the nanosecond recovery of conventional MQWs. By using type-II TBQs, we have demonstrated all-optical gate operation of a Fabry-Perot etalon. The type-II TBQ etalon with 1.7-nm barriers exhibits a full signal recovery of 17 ps.

  • ULTRAFAST EXCITON SPIN RELAXATION IN GAAS/ALGAAS AND CDMNTE MULTIPLE-QUANTUM WELLS

    Y TAKAGI, S ADACHI, S TAKEYAMA, A TACKEUCHI, S MUTO, JJ DUBOWSKI

    JOURNAL OF LUMINESCENCE   58 ( 1-6 ) 202 - 205  1994.01  [Refereed]

     View Summary

    We report on the direct observation of ultrafast spin relaxation of both the heavy-hole and the light-hole excitons in GaAs/AlGaAs multiple quantum wells (MQW) at room temperature using the femtosecond absorption bleaching detection. Also, we report on the time-resolved reflection measurements in CdMnTe MQW. A variety of polarization-dependent and wavelength-dependent time profiles are observed in the picosecond time region.

  • EFFECT OF REACTIVE ION-BEAM ETCHING DAMAGE ON EXCITON ABSORPTION RECOVERY-TIME OF MULTIPLE QUANTUM-WELL WIRES

    A ENDOH, H ARIMOTO, H KITADA, A TACKEUCHI, S MUTO

    JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B   11 ( 2 ) 183 - 186  1993.03  [Refereed]

     View Summary

    We studied the effect of process damage on exciton absorption recovery time of multiple quantum well narrow wires. The wires were fabricated to the order of 100 nm by focused ion beam lithography and subsequent reactive ion beam etching. The recovery time decreases with increased damage. The recovery times of the damaged samples slightly increase after thermal annealing. The effect of etching damage on the recovery time was discussed by the numerical calculation assuming nonradiative recombination by defects.

  • DIFFERENCES IN TUNNELING TIME BETWEEN 77-K AND ROOM-TEMPERATURE FOR TUNNELING BIQUANTUM WELLS

    A TACKEUCHI, Y SUGIYAMA, T INATA, S MUTO

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS   31 ( 12A ) 3823 - 3824  1992.12  [Refereed]

     View Summary

    We measured the recovery time from excitonic absorption bleaching in tunneling biquantum wells at 77 K and room temperature. The absorption recovery time corresponds to the tunneling time of electrons from narrow to wide wells. We have found that tunneling at 77 K is about 2.5 times slower than at room temperature.

  • PICOSECOND SIGNAL RECOVERY IN TYPE-II TUNNELING BI-QUANTUM-WELL ETALON

    A TACKEUCHI, T INATA, Y NAKATA, S NAKAMURA, Y SUGIYAMA, S MUTO

    APPLIED PHYSICS LETTERS   61 ( 16 ) 1892 - 1894  1992.10  [Refereed]

     View Summary

    We demonstrate picosecond signal recovery in all optical gate operation using a type II tunneling bi-quantum well (TBQ) etalon. The type II TBQ consists of a series of GaAs wells, AlGaAs barriers, and AlAs layers. In this structure, photoexcited electrons in the GaAs wells escape by tunneling through the AlGaAs barriers toward X states in the AlAs layers. Therefore, the time for recovery from excitonic absorption bleaching in GaAs wells is controlled directly by the AlGaAs barrier thickness. The type II TBQ etalon with 1.7 nm barriers showed a fast signal recovery of 17 ps by carrier tunneling.

  • FABRICATION OF SUB-100-NM WIRES AND DOTS IN GAAS/ALGAAS MULTIQUANTUM WELL USING FOCUSED ION-BEAM LITHOGRAPHY

    H KITADA, H ARIMOTO, A TACKEUCHI, Y YAMAGUCHI, Y NAKATA, A ENDOH, S MUTO

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS   31 ( 7B ) L990 - L991  1992.07  [Refereed]

     View Summary

    We describe a new lateral patterning technique using focused ion beam (FIB) lithography and also two subsequent pattern transfer processes consisting of reactive ion etching and reactive ion beam etching. In the FIB lithography we used a trilevel resist structure consisting of a CMS resist, an Al film, and an SiO2 film. This technique provided 60-nm-wide wires and 100 nm dots in a GaAs/AlGaAs multiquantum well (MQW). Twenty-five-nm-wide wires are also fabricated in GaAs by narrowing the SiO2 mask laterally by wet etching.

  • FAST RECOVERY FROM EXCITONIC ABSORPTION BLEACHING IN TYPE-II GAAS/ALGAAS/ALAS TUNNELING BIQUANTUM WELL

    A TACKEUCHI, T INATA, Y SUGIYAMA, Y NAKATA, S NAKAMURA, S MUTO

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS   31 ( 6A ) L669 - L672  1992.06  [Refereed]

     View Summary

    We propose the type-II tunneling biquantum-well (TBQ) structure which consists of a series of GaAs wells, AlGaAs barriers and AlAs layers. The type-II TBQ has no significant optical absorption except for the GaAs wells and shows fast recovery from excitonic absorption bleaching. In this structure, photoexcited electrons in the GaAs wells escape by tunneling through the AlGaAs barriers toward X states in the AlAs layers. The recovery time of excitonic absorption bleaching in the GaAs wells was reduced to 8 ps as the AlGaAs barrier thickness was decreased to 1.1 nm.

  • PICOSECOND ABSORPTION RECOVERY OF 100 NM GAAS/ALGAAS MQW WIRES

    A TACKEUCHI, H KITADA, H ARIMOTO, Y SUGIYAMA, T INATA, Y YAMAGUCHI, Y NAKATA, S NAKAMURA, S MUTO

    SURFACE SCIENCE   267 ( 1-3 ) 267 - 269  1992.04  [Refereed]

     View Summary

    We have studied narrow multiple quantum-well (MQW) wires using conventional absorption spectroscopy and time-resolved absorption measurements. Wires down to 130 nm were fabricated from MQW's using focused ion-beam lithography and electron cyclotron-resonance chlorine-plasma etching. In this structure, the photoexcited carriers diffuse toward sidewalls and recombine on the sidewall surface. We confirmed from the absorption spectrum of MQW wires that the exciton peak is preserved even in wires 130 nm wide. We show that the excitonic absorption recovery of MQW wires is reduced to 11 ps, preserving excitonic optical nonlinearity.

  • TEMPERATURE-DEPENDENCE OF PHOTOLUMINESCENCE DECAY TIME IN TUNNELING BI-QUANTUM-WELL STRUCTURES

    Y SUGIYAMA, A TACKEUCHI, T INATA, S MUTO

    INSTITUTE OF PHYSICS CONFERENCE SERIES   30 ( 120 ) 235 - 238  1992  [Refereed]

     View Summary

    We studied the nonresonant electron tunneling time in tunneling bi-quantum-well structures with a pump-probe photoluminescence (PL) system using optical mixing. The PL decay time of narrow wells for an AlxGa1-xAs (x = 0.51) barrier of 4.0 nm, determined by tunneling, decreases from 40 ps to 24 ps as the temperature increases from 6K to 132K. We attribute this to electrons in an exciton state thermalizing into free electrons having a faster tunneling rate.

  • Fast recovery of excitonic absorption bleaching in tunneling Bi‐quantum well

    Atsushi Tackeuchi, Tsuguo Inata, Shunichi Muto, Yoshihiro Sugiyama, Toshio Fujii

    Electronics and Communications in Japan (Part II: Electronics)   75 ( 5 ) 25 - 33  1992

     View Summary

    Excitons in a quantum well of a semiconductor exhibit large optical nonlinearity. Therefore, a quantum well of a semiconductor is regarded as a key to realizing a device which can control light using light. However, the lifetime of excitons, which are generated optically, is several nanoseconds to several tens nanoseconds. Therefore, the nonlinear optical effect including absorption saturation continues for the lifetime of the excitons. To shorten the lifetime of the excitons without disturbing the optical nonlinearity, a new superlattice structure is proposed called the “tunneling bi‐quantum well.” This structure uses the tunneling effect and measures the absorption recovery time. As a result, it was found that in a tunneling bi‐quantum well in a GaAs/AlGaAs system with a barrier width of 1.7 nm, the absorption recovery time was as short as 1 ps. Copyright © 1992 Wiley Periodicals, Inc., A Wiley Company

    DOI

  • TIME EVOLUTION OF EXCITONIC ABSORPTION BLEACHING OF RESONANT TUNNELING BI-QUANTUM-WELL STRUCTURES

    A TACKEUCHI, S MUTO, T INATA, T FUJII

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS   30 ( 11A ) 2730 - 2733  1991.11  [Refereed]

     View Summary

    We report the time evolution of excitonic absorption bleaching in resonant tunneling bi-quantum-well (TBQ) structures, in which the ground electron level in a narrow well has the same energy as the second electron level in a wide well. In resonant TBQ structure, we observed a reduction in the absorption recovery time and an increase in the tail-to-peak ratio of the absorption change. By comparing the absorption change of resonant TBQ with the e2-hh2 excitonic absorption change of conventional multiple quantum wells (MQW), we show that the increase in the tail-to-peak ratio of a resonant TBQ can be attributed to the thermally remaining holes in the first excited subband of heavy holes.

  • PICOSECOND EXCITONIC ABSORPTION RECOVERY OF 100 NM GAAS/ALGAAS NARROW MULTIPLE QUANTUM-WELL WIRES

    A TACKEUCHI, H KITADA, H ARIMOTO, Y SUGIYAMA, A ENDOH, Y NAKATA, T INATA, S MUTO

    APPLIED PHYSICS LETTERS   59 ( 9 ) 1114 - 1116  1991.08  [Refereed]

     View Summary

    We report the time-resolved absorption measurement of narrow multiple quantum-well (MQW) wires to investigate their fast recoveries from excitonic absorption bleaching. Wires down to 130 nm were fabricated from MQWs using focused ion beam lithography and electron cyclotron-resonance chlorine-plasma etching. In this structure, the photoexcited carriers diffuse toward the sidewalls and recombine on the surface of the sidewalls. We show that the strong optical nonlinearity of excitons is preserved, even in wires of 130 nm width, and having a fast recovery time in the picosecond region. We also briefly discuss the possibility of making quantum wires which have a faster recovery time and larger optical nonlinearity.

  • LONGITUDINAL-OPTICAL-PHONON ASSISTED TUNNELING IN TUNNELING BI-QUANTUM WELL STRUCTURES

    S MUTO, T INATA, A TACKEUCHI, Y SUGIYAMA, T FUJII

    APPLIED PHYSICS LETTERS   58 ( 21 ) 2393 - 2395  1991.05  [Refereed]

     View Summary

    We discuss the electron tunneling time observed in a new AlGaAs/GaAs superlattice structure, the tunneling bi-quantum well (TBQ). To calculate the nonresonant tunneling time, we made experiments on resonant tunneling to confirm that the 60% rule of conduction-band discontinuity accurately evaluates the free tunneling probability of electrons. We found that the observed recovery time agrees quite well with the calculated longitudinal optical phonon emission tunneling time for thin (less-than-or-equal-to 10 monolayers) barriers.

  • FAST RECOVERY OF EXCITONIC ABSORPTION BLEACHING IN TUNNELING BIQUANTUM WELL STRUCTURES

    A TACKEUCHI, S MUTO, T INATA, T FUJII

    APPLIED PHYSICS LETTERS   58 ( 15 ) 1670 - 1672  1991.04  [Refereed]

     View Summary

    This letter demonstrates the fast recovery from excitonic absorption bleaching in tunneling biquantum well structures. A tunneling biquantum well consists of a series of narrow and wide wells. Recovery time was reduced to 1 ps using 1.7-nm-thick tunneling barriers, three orders of magnitude shorter than the few nanoseconds recovery due to radiative recombination. This is the fastest tunneling process observed to date.

  • TBQにおける励起子吸収の超高速回復

    竹内淳, 稲田嗣夫, 武藤俊一, 杉山芳弘, 藤井俊夫

    電子情報通信学会論文誌   C-I, J74-C-I ( 11 ) 458 - 464  1991

  • DIRECT OBSERVATION OF PICOSECOND SPIN RELAXATION OF EXCITONS IN GAAS/ALGAAS QUANTUM-WELLS USING SPIN-DEPENDENT OPTICAL NONLINEARITY

    A TACKEUCHI, S MUTO, T INATA, T FUJII

    APPLIED PHYSICS LETTERS   56 ( 22 ) 2213 - 2215  1990.05  [Refereed]

  • PICOSECOND CHARACTERIZATION OF INGAAS INALAS RESONANT TUNNELING BARRIERS GROWN BY MBE

    S MUTO, A TACKEUCHI, T INATA, E MIYAUCHI, T FUJII

    SURFACE SCIENCE   228 ( 1-3 ) 370 - 372  1990.04  [Refereed]

  • FAST RECOVERY OF EXCITONIC ABSORPTION PEAKS IN TUNNELING BI-QUANTUM-WELL STRUCTURES

    A TACKEUCHI, S MUTO, T INATA, T FUJII

    ULTRAFAST PHENOMENA VII   53   265 - 267  1990  [Refereed]

  • PICOSECOND CHARACTERIZATION OF INGAAS/INALAS RESONANT TUNNELING BARRIER DIODE BY ELECTRO-OPTIC SAMPLING

    A TACKEUCHI, T INATA, S MUTO, E MIYAUCHI

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS   28 ( 5 ) L750 - L753  1989.05  [Refereed]

  • VERY HIGH-SPEED GAINAS METAL-SEMICONDUCTOR-METAL PHOTODIODE INCORPORATING AN ALLNAS/GALNAS GRADED SUPERLATTICE

    O WADA, H NOBUHARA, H HAMAGUCHI, T MIKAWA, A TACKEUCHI, T FUJII

    APPLIED PHYSICS LETTERS   54 ( 1 ) 16 - 17  1989.01  [Refereed]

  • トンネル双量子井戸とMQW微細線の超高速応答

    竹内淳, 稲田嗣夫, 有本宏, 北田秀樹, 杉山芳弘, 中田義昭, 仲村智, 遠藤聡, 山口正臣, 武藤俊一

    電気学会 電子材料研究会資料   EFM-92-11

▼display all

Misc

  • InAs/GaAs Multi-Coupled Quantum Dots Structure Enabling High-Intensity, Near-1.3-μm Emission due to Cascade Carrier Tunneling

    TACKEUCHI Atsushi, NAKATA Yoshiaki, MUTO Shunichi, INATA Tsuguo, USUKI Tatsuya, SUGIYAMA Yoshihiro, YOKOYAMA Naoki, WADA Osamu

      1995   719 - 721  1995.08

    CiNii

  • 28p-ZH-2 Spin relaxation of excitons in GaAs/AlGaAs quantum wells

    Adachi S, Takeyama S, Takagi Y, Takeuchi A, Muto S

      1992 ( 2 ) 200 - 200  1992.09

    CiNii

Awards

  • JSAP Fellow

    2017.09   The Japan Society of Applied Physics  

  • JSAP Paper Award

    2004.09  

  • IEICE Excellent Paper Award

    1993  

  • IEEJ Excellent Paper Presentation Award

    1993  

Research Projects

  • 半導体量子構造による円偏光の高偏極長スピン寿命電子への変換

    Project Year :

    2019.04
    -
    2022.03
     

     View Summary

    半導体中の電子スピンの自由度を利用すれば従来のエレクトロニクスでは実現できなかった新しい機能をデバイスに付加できます。特に円偏光からスピン偏極電子への高効率の変換の実現は新たな光スピントロニクスデバイスの開発に重要です。また、この変換の実現によってスピントランジスタ等の動作実証に不可欠の高いスピン偏極率を持つスピン偏極電子の注入も可能となります。本研究は、半導体量子構造を用いて「円偏光」を「高いスピン偏極率を有するスピン偏極電子」に変換し半導体中に注入することを目的としており、本提案の半導体量子構造が実現されれば、スピントロニクスデバイス研究の飛躍的進展が期待できます。半導体中のスピンの自由度を利用すれば従来のエレクトロニクスでは実現できなかった新しい機能をデバイスに付加できる。特に円偏光からスピン偏極電子への高効率の変換の実現は新たな光スピントロニクスデバイスの開発に重要である。また、この変換の実現によってスピントランジスタ等の動作実証に不可欠の高いスピン偏極率を持つスピン偏極電子の注入も可能となる。本研究は、半導体量子構造を用いて「円偏光」を「高いスピン偏極率を有するスピン偏極電子」に変換し半導体中に注入することを目的とする。今年度は、スピン偏極電子源に用いる半導体量子構造の候補として、GaSb/AlSb多重量子井戸のスピン緩和時間を測定した。二種類のGaSb量子井戸の井戸幅は、13.4nmと48nmである。チタンサファイアレーザーとオプティカルパラメトリック発振器を光源とする時間幅サブピコ秒の光パルスを用いたポンプ・プローブ測定の結果、10Kでのスピン緩和時間は47ps(井戸幅:13.4nm)と164ps(井戸幅:48nm)であり、300Kではそれぞれ6.7 psと7.6psまで高速化することが分かった。この量子井戸のバンドギャップエネルギーは波長換算で1.5マイクロメーターになるが、この波長域でのスピン緩和時間の測定は、我々が測定したInGaAs/InP量子井戸以外には測定例が少なく新しい知見が得られたと言える。また、光通信用発光デバイス等への応用も期待できる。円偏光の照射によってホール準位から電子を光励起する場合、ヘビーホール準位からの励起とライトホール準位からの励起で電子スピンの向きが逆になる。したがって、円偏光を高いスピン偏極率を有する電子に変換するためには、ヘビーホールとライトホールの量子準位をエネルギー的に分離し、ヘビーホール準位のみを選択的に光励起するのが望ましい。従来のスピン偏極電子源ではこの両エネルギー準位の分離のために歪構造が用いられているが、本研究では井戸幅の狭い量子井戸を用いることに特徴がある。スピン偏極電子源に用いる半導体量子構造の候補として、GaSb/AlSb多重量子井戸のスピン緩和時間を測定した。二種類のGaSb量子井戸の井戸幅は、13.4nmと48nmである。この量子井戸の低温でのバンドギャップエネルギーは波長換算で1.5マイクロメーターを超えるので光通信用発光デバイスへの応用も期待できる。オプティカルパラメトリック発振器を光源とするポンプ・プローブ測定の結果、10Kでのスピン緩和時間は47ps(井戸幅:13.4nm)と164ps(井戸幅:48nm)であり、300Kではそれぞれ6.7 psと7.6psまで高速化することが分かった。この量子構造の応用上のポテンシャルについてはさらに調べる予定である。井戸幅が狭いとスピン緩和時間が短くなるので、これを解決するために狭い量子井戸で作ったスピン偏極電子を幅の広い量子井戸にトンネル効果を利用して移して長寿命化を図る。この目的のために、Type-IIトンネル双量子井戸のトンネル時間とスピン緩和時間を調べる予定である。Type-IIトンネル双量子井戸では、タイプI型量子井戸(GaAs)の隣にバリア層(AlGaAs)を介してタイプII型量子井戸(AlAs)を積層している。タイプI型量子井戸に光励起されたスピン偏極電子はタイプII型量子井戸のX点にトンネルする。その結果、電子とホールが空間的に分離されることによって交換相互作用が減少するので、長いスピン緩和時間が期待される。また、高いスピン偏極率と長いスピン緩和時間を実現する半導体量子構造として、GaSb/AlSb多重量子井戸を含む新たな量子構造の応用可能性の検討も進める

  • Highly spin polarized quantum dots with long spin relaxation time for future circularly polarized light emitting diodes

    Project Year :

    2010.04
    -
    2013.03
     

     View Summary

    For the realization of the circularly polarized light emitting diode, spin polarizations and spin relaxation times of some III-V compound semiconductors were investigated. InAs columnar quantum dots showed 3.42 ns-spin relaxation at 1.06 micron meter. The highly uniform InAs quantum dots exhibited the stable eliptical polarization caused by the in-plane anisostropies of the dot shape and strain distribution. Highly Si doped GaInP showed 210 ns-spin relaxation which is one of the longest spin relaxations ever observed

  • 新機能性物質における自己組織化ナノ領域の計測と制御

    文部科学省 

    Project Year :

    2005
    -
    2009
     

  • 高機能性新半導体デバイス開発

    文部科学省 

    Project Year :

    1998
    -
    2002
     

  • 過渡的磁性半導体の機能の研究

     View Summary

    過渡的磁性半導体のスピン物性解明のために、量子井戸内のスピン緩和機構と量子ドット間の交換相互作用の検討を行った。まず、量子井戸内のスピン緩和機構として、Elliott-Yafet効果のスピン緩和時間を理論的に求め、スピン緩和時間が量子化エネルギーに反比例することを明らかにした。これは、さきに実験的に求めたInGaAs/InP量子井戸のスピン緩和時間の量子化エネルギー依存性と同じ依存性であり、E-Y効果が支配的なスピン緩和機構であることを示唆している。InGaAs/InP量子井戸で量子化エネルギーによってスピン緩和時間が制御できることを明らかにしたことにより、光通信に対応する波長1.55ミクロンで、スピンをデバイス応用しうる可能性に大きく道を開いた。また、量子ドット間の交換相互作用の検討では、ハイトラー・ロンドン近似を用いて交換相互作用を計算した。現在、量子ドットをメモリーやコンピューティングに応用する提案がなされているが、とくにドット中に生成されたキャリアのスピンが近接するドットのキャリアスピンと持つどのような相互作用を持つかは、今後、量子ドットのスピンをデバイス応用する際の最重要ファクターの一つになる。このため、今回は、一対の近接した量子ドット(結合量子ドット)に光励起された電子の交換相互作用エネルギーを計算し、さらにドットを立方晶に配置した場合のネール温度を求めた。その結果、結合量子ドットが反強磁性になること、また準現実的なドット構造ではそのネール温度が150Kに達することを明らかにした

  • 過渡的磁性半導体の機能の研究

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    過渡的磁性半導体のスピン物性解明のために、量子井戸内のスピン緩和機構と量子ドット間の交換相互作用の検討を行った。まず、量子井戸内のスピン緩和機構として、Elliott-Yafet効果のスピン緩和時間を理論的に求め、スピン緩和時間が量子化エネルギーに反比例することを明らかにした。これは、さきに実験的に求めたInGaAs/InP量子井戸のスピン緩和時間の量子化エネルギー依存性と同じ依存性であり、E-Y効果が支配的なスピン緩和機構であることを示唆している。InGaAs/InP量子井戸で量子化エネルギーによってスピン緩和時間が制御できることを明らかにしたことにより、光通信に対応する波長1.55ミクロンで、スピンをデバイス応用しうる可能性に大きく道を開いた。また、量子ドット間の交換相互作用の検討では、ハイトラー・ロンドン近似を用いて交換相互作用を計算した。現在、量子ドットをメモリーやコンピューティングに応用する提案がなされているが、とくにドット中に生成されたキャリアのスピンが近接するドットのキャリアスピンと持つどのような相互作用を持つかは、今後、量子ドットのスピンをデバイス応用する際の最重要ファクターの一つになる。このため、今回は、一対の近接した量子ドット(結合量子ドット)に光励起された電子の交換相互作用エネルギーを計算し、さらにドットを立方晶に配置した場合のネール温度を求めた。その結果、結合量子ドットが反強磁性になること、また準現実的なドット構造ではそのネール温度が150Kに達することを明らかにした

  • 過渡的磁性半導体の機能の研究

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    過渡的磁性半導体のスピン物性の解明と応用を目指して、InGaNのキャリアのスピン分極を調べるとともに、半導体量子ドットの電子スピンを用いた量子演算の検討を行った。InGaNは、近年の青色レーザなどで注目されるワイドギャップ半導体であるが、キャリアのスピン分極が生成されないことが明らかになった。これは、InGaN固有のバンド構造や結晶構造に起因していると考えられる、InGaNではエピ上でIn組成のゆらぎがあり、この組成ゆらぎのために重い正孔と軽い正孔の準位がエネルギー的に混じっている可能性が高い。半導体量子ドットの電子スピンを用いた量子演算の検討では、3重結合量子ドットにおける2電子状態を用いたゲートを提案した。系を貫く静磁場と量子ドットのサイズに依存した回転磁場、およびフォトン支援トンネリングを用いて量子コンピューティングに必要な1ビットの回転と制御NOT演算を実現できる。スピンの回転とトンネル現象のエネルギー分解能のみを用いても、少なくとも5つの量子ビットまで拡張できることが分かった。また、量子演算を行う際に重要な量子ドット間のトンネル時間を実測した。トンネル時間のバリア層幅依存性を測定し、トンネル時間がWKB近似にのることを明らかにするとともに、量子井戸間のトンネル時間より一桁遅いことを明らかにした

  • 超高速スピン応答光デバイス

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    0次元構造である量子ドットは状態密度がデルタ関数的であり、また、振動子強度が大きいためレーザーの閾値電流を下げると期待されている。しかし、量子ドットレーザーなどの光デバイスにおいてはフォノンボトルネックが閾値電流の低減や高速動作の障害となる可能性がある。そこで今年度は量子ドット内のフォノンボトルネック現象を高時間分解計測を用いて調べた。量子ドット中での高エネルギー側の量子準位から下の準位へのキャリアのエネルギー緩和では、エネルギー保存則と運動量保存則を満たす必要がある。通常は、LOフォノン放出をともなうエネルギー緩和が支配的であるが、離散的なエネルギー準位をもつ量子ドットでは、その準位間のエネルギー差と一致するフォノンがないため、LOフォノンを介したエネルギー緩和が抑制され、準位間の緩和時間が遅くなると予想されている。本研究では、極めて高均一な量子ドットを用いることによって各準位の発光の観測を可能にした。明瞭に分離された各準位のPLスペクトルの時間分解フォトルミネッセンス測定を行い、レート方程式を用いて直接的に準位間緩和時間を導出した。その結果、再結合時間は、高温になるにつれ短くなったが、準位間緩和時間は、温度に依存せずほぼ一定であった。この長い準位間緩和時間は、フォノンボトルネックによることが明らかになった。10Kから200Kの結果から300Kの準位間緩和時間を外挿すると、320psとなる。この遅い準位間緩和時間は量子ドットの基底準位にキャリアを供給するのを妨げ、室温での発光デバイスの動作に影響を及ぼすと考えられる。また、一方では高エネルギー準位でのキャリアの滞在時間を引き延ばすため、高エネルギー準位でのキャリアスピンの振る舞いの観測を可能にすると予測される

  • Research on the coherent control of electron spins in quantum dots

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    Self-assembled InAlAs/AlGaAs quantum dots are grown by the Stranski-Krastanow mode of Molecular Beam Epitaxy (MBE). We measured the spin relaxation time of the structure under the non-resonant optical excitation by using 80 fs pulses by the titanium sapphire laser at wavelength of 780 nm. We used the right circular polarized light for the excitation of carrires to obtain fixed electron spins. The luminescence from the quantum dots at 10 K was divided into the right circular and the left circular components and was temporary resolved by the streak-scope. While the lifetime of the exciton was about 1 18 ns, the spin relaxation time determined by the decay of the polarization of luminescence was 1.31 us, which is comparable to the lifetime. The result indicates that the major relaxation mechanisms in the quantum well are not effective in the quantum dots and that the quantum dot is a good material for the coherent control of electron spins.As the spin-electronisc material for the future generations, wide-gap semiconductors and a polymer was studied with emphasis on the magnetic or spintronic properties

  • Optical Control And Application Of Spin Generation And Relaxation

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    The exciton spin relaxation mechanism was investigated between 13 and 300 K in InGaAs/InP quantum wells using time-resolved spin-dependent pump and probe absorption measurements. The clear carrier density dependence of the exciton spin relaxation time was observed below 40 K, although the carrier density dependence is weak above 40 K. These results imply that the main spin relaxation mechanism above and below 40 K are the D'yakonov-Perel' process and the Bir-Aronov-Pikus process, respectively.Carrier spin dynamics was investigated in highly uniform self-assembled InAs quantum dots. The measured spin relaxation time decreases rapidly from 1.1 ns at 10 K to 200 ps at 130 K. This large change in the spin relaxation time is well explained in terms of the mechanism of acoustic phonon emission.The spin relaxation process of A-band exciton in GaN is observed with sub-picosecond's time resolution. The spin relaxation times at 150-225 K are 0.47-0.25 ps. These are at least one order of magnitude shorter than those of the other III-V compound semiconductors. The excitonic spin relaxation process in cubic GaN is observed. The spin relaxation times at 15 K -75 K are found to be longer than 5 ns. Although these long spin relaxation times are in striking contrast to the sub-picosecond spin relaxation of A-band free excitons in hexagonal GaN, they are consistent with the dependence that spin relaxation time becomes longer for wider-band-gap zincblende semiconductors

  • 光スピントロニクスデバイス研究調整班

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    1.強磁性スピン系の光操作と光磁化およびその応用(宗片)(1)(Ga,Mn)Asへの窒素添加効果を実験的に調べ、窒素添加によってホール濃度とキュリー温度が低下すると結論した。(2)光誘起才差運動の起源を実験的に検討し、光照射により磁気異方性が変化した結果発生する、持続時間の長い(1-10ns)有効磁場によるものであることを結論した。(3)開発を進めてきたレーザ走査型磁気光学顕微鏡がとうとう立ち上がり、(Ga,Mn)As中で光誘起磁化の直接観測に世界に先駆けて成功した。さらに、(4)作製法に工夫をこらしたIII-V族p-nヘテロダイオードにおいて、再現性の良い円偏光依存光起電力を得ることに成功するとともに、ヘテロ接合に対するスピン拡散過程の理論的解析を世界に先駆けて開始した。2.スピン生成と緩和の光制御と応用(竹内)ワイドギャップ半導体の代表格であるGaN中のキャリア電子のスピン緩和には、六方晶GaN中の1psの超高速過程と立方晶GaN中の10nsの遅い過程の2通りが存在することを実験的に明らかにした。後者については、スピン軌道相互作用の減少によるものであると説明されるが、前者についてはこの物質固有の新しいスピン緩和現象であり、その解明は今後の課題であると結論した。3.近接場光学技術によるナノ光スピントロニクス(斎木)近接場光学顕微鏡(NSOM)の磁気光学効果の高感度化につながる手法として、ホモダイン検出とロックイン検出とを組み合わせる測定技術を引き続き研究し、原子ステップを持つサファイア基板上のNiO薄膜中に形成された段差2nmのナノチャンネルにおいて、上部と下部で1:4という極めて大きなコントラストを有する表面像を得ることができた。本手法の有用性を示した。また、低温・磁場下で動作するNSOMにおいて、ドリフトや振動などの細かい問題点をほぼ解決し、30nmの分解能による単一量子ドットの発光イメージングに成功した。磁気発光分光も可能となり、量子ドット発光のゼーマン分裂や反磁性シフトなどが観測できるようになった

  • 超高速スピン応答光デバイス

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    0次元構造である量子ドットは状態密度がデルタ関数的であり、また、振動子強度が大きいためレーザーの閾値電流を下げると期待されている。しかし、量子ドットレーザーなどの光デバイスにおいてはフォノンボトルネックが閾値電流の低減や高速動作の障害となる可能性がある。そこで今年度は量子ドット内のフォノンボトルネック現象を高時間分解計測を用いて調べた。量子ドット中での高エネルギー側の量子準位から下の準位へのキャリアのエネルギー緩和では、エネルギー保存則と運動量保存則を満たす必要がある。通常は、LOフォノン放出をともなうエネルギー緩和が支配的であるが、離散的なエネルギー準位をもつ量子ドットでは、その準位間のエネルギー差と一致するフォノンがないため、LOフォノンを介したエネルギー緩和が抑制され、準位間の緩和時間が遅くなると予想されている。本研究では、極めて高均一な量子ドットを用いることによって各準位の発光の観測を可能にした。明瞭に分離された各準位のPLスペクトルの時間分解フォトルミネッセンス測定を行い、レート方程式を用いて直接的に準位間緩和時間を導出した。その結果、再結合時間は、高温になるにつれ短くなったが、準位間緩和時間は、温度に依存せずほぼ一定であった。この長い準位間緩和時間は、フォノンボトルネックによることが明らかになった。10Kから200Kの結果から300Kの準位間緩和時間を外挿すると、320psとなる。この遅い準位間緩和時間は量子ドットの基底準位にキャリアを供給するのを妨げ、室温での発光デバイスの動作に影響を及ぼすと考えられる。また、一方では高エネルギー準位でのキャリアの滞在時間を引き延ばすため、高エネルギー準位でのキャリアスピンの振る舞いの観測を可能にすると予測される

  • Optical spin manipulation of coupled quantum dots

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Specific Research

  • 半導体量子構造による円偏光の高偏極長スピン寿命電子への変換

    2020  

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     本研究は、円偏光励起によって高いスピン偏極率を実現する半導体量子構造と、光励起された高スピン偏極電子を長いスピン寿命を維持したまま流す半導体量子構造を組み合わせることによって、これまで不可能だった“高スピン偏極で長スピン寿命のスピン偏極電子”への変換の実現を目的とする。この実現には各種の化合物半導体でのスピン緩和時間の情報が極めて重要である。本研究ではGaAs/AlGaAs/AlAs type-II トンネル双量子井戸において電子のスピン緩和時間を調べた。

  • 半導体量子構造による円偏光の高偏極長スピン寿命電子への変換

    2019  

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    半導体スピントロニクスデバイスとしてスピントランジスタやアンプが提案され世界中で研究が進展している。本研究は、円偏光励起によって高いスピン偏極率を実現する半導体量子構造と、光励起された高スピン偏極電子を長いスピン寿命を維持したまま流す半導体量子構造を組み合わせることによって、これまで不可能だった“高スピン偏極で長スピン寿命のスピン偏極電子”への変換の実現を目的とする。この実現には各種の化合物半導体でのスピン緩和時間の情報が極めて重要である。本研究では同一のGaAs基板上に成長させた3種類の井戸幅のGaAs/AlGaAs量子井戸のスピン緩和時間を調べた。

  • 円偏光励起による高偏極長スピン寿命半導体量子構造電子注入源

    2018  

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    半導体スピントロニクスデバイスとしてスピントランジスタやアンプが提案され世界中で研究が進展している。本研究は、円偏光励起によって高いスピン偏極率を実現する半導体量子構造と、光励起された高スピン偏極電子を長いスピン寿命を維持したまま流す半導体量子構造を組み合わせることによって、これまで存在しなかった“高スピン偏極で長スピン寿命のスピン偏極電子注入源”の実現を目的とする。この実現には各種の化合物半導体でのスピン緩和時間の情報が極めて重要である。InPは小さな有効質量と高い電子移動度を持つ代表的な化合物半導体であることから、本研究では、ZnドープInPバルクのスピン緩和時間を調べた。

  • 円偏光励起による高偏極長スピン寿命半導体ナノ量子構造電子注入源

    2017  

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    III-V族化合物半導体を用いたスピントロニクス応用としてスピン偏極電子源に半導体ナノ量子構造を用いる研究が進んでいる。本研究は、円偏光励起によって高いスピン偏極率の電子を生成する「高スピン偏極量子構造」と、生成された高スピン偏極電子を高いスピン偏極率を維持したまま電界下で移動させる「超長スピン寿命量子構造」からなる高スピン偏極電子注入源を実現することが目的である。この応用では各種の構造でのスピン緩和時間の情報が極めて重要だが、本研究では、GaInNAs量子井戸とInGaAs量子井戸、それにBeドープのInGaAsPバルクやFeドープのInPバルクのスピン緩和時間などを調べた。

  • 円偏光励起による半導体量子構造スピン偏極電子注入源

    2016  

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     III-V族化合物半導体を用いたスピントロニクス応用ではスピン偏極電子源に超格子構造を用いる研究が進んでいる。本研究は、円偏光励起によって高いスピン偏極率の電子を生成する「高スピン偏極量子構造」と、生成された高スピン偏極電子を高いスピン偏極率を維持したまま電界下で移動させる「超長スピン寿命量子構造」からなる高スピン偏極電子注入源を実現することが目的である。この応用では各種の構造でのスピン緩和時間の情報が極めて重要だが、本研究では、トンネル効果を用いて光吸収による応答からの超高速の回復を図るトンネル双量子井戸構造やGaAs/GaAsP歪補償超格子、それにInGaAs/AlGaAs/AlAsSb結合量子井戸、さらにCuInSe2のスピン緩和時間などを調べた。

  • III-V族化合物半導体のキャリアスピンの振舞の解明

    2015  

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    III-V族化合物半導体を用いるスピントロニクス研究では、新デバイスやスピン偏極電子源への応用が期待されている。しかし、スピン緩和時間が調べられた半導体は限られている。本研究では、様々なIII-V族化合物半導体のスピン緩和時間を調べた。その結果、トンネル双量子井戸では、タイプI型とII型ともにスピン緩和時間がトンネル効果の影響を受けることが分かった。また、InGaAs/AlGaAs/AlAsSb結合量子井戸では、高温領域で2種類のスピン緩和過程が働いていることが明らかになった。スピン偏極電子源への応用を目指すGaAs/GaAsP歪補償超格子では、室温で励起光強度を変えても、スピン緩和時間の変化は観測されないことが明らかになった。

  • 機能性円偏光光源のための高スピン偏極長寿命In系化合物半導体

    2013  

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     機能性円偏光光源のための高偏極長寿命In系化合物半導体の研究においては、InP基板上に成長したInGaAsPバルク、低温成長GaAsバルク、Beドープしたp型GaAsバルクのスピン緩和時間を、時間分解ポンプ・プローブ測定や時間分解フォトルミネッセンス測定で調べた。 InP基板上に成長したInGaAsPバルクは、10Kで波長1160nm付近に発光ピークがあるが、円偏光ポンププローブ時間分解計測では、この発光ピーク近傍の波長でスピン緩和過程を観測した。その結果、励起光強度10mWで980psのスピン緩和時間が得られた。また、このスピン緩和時間は励起光強度を変えると変化することが分かった。さらに測定温度を室温まで上げると、スピン緩和時間は95psまで速くなった。これらのスピン緩和時間の温度依存性と励起光強度依存性から、10-30Kでは、Bir-Aronov-Pikus効果が支配的であり、100-300KではD'yakonov-Perel'効果が支配的であることが明らかになった。 In系化合物半導体との比較のために、Inを含まないGaAsバルクのスピン緩和時間もあわせて調べている。Beドープしたp型GaAsバルクでは、10Kで1.511 eVと1.497 eVの二つのフォトルミネッセンスピークが観測され、それぞれ10-100Kで1-3nsの比較的遅いスピン緩和時間が観測された。前者はBeアクセプターに束縛された励起子のスピン緩和であり、後者はバンドからアクセプター準位への遷移に関与するスピン緩和であると考えられる。両者のスピン緩和時間かほとんど同じであることから、ともに伝導電子のスピン緩和を観測しているものと考えられる。また、低温成長GaAsバルクでは、10Kで2ps、28ps、158psの3成分の高速のスピン緩和が観測された。この結果からキャリア散乱によりスピンが反転するElliott-Yafet効果が効いている可能性が高いことがわかった。

  • 円偏光光源のための高偏極長寿命スピン緩和量子ドットの研究

    2009  

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    円偏光光源のための高偏極長寿命スピン緩和量子ドットの研究においては、科研費申請では、量子ドットのスピン緩和研究における未踏領域である波長400-700 nmのフェムト秒光源を導入する予定であった。特定課題研究においては、金額的制約からこの装置の導入は見送り、代替手段によるこの波長域のスピン緩和研究の遂行を図った。このために、今年度は光パラメトリック発振器による波長1.1ミクロンの光を第二高調波発生器を用いて、波長550 nmに変換する実験に注力した。その結果、波長500nm領域で1 mW程度の強度の光が得られるようになった。安定に時間分解フォトルミネッセンス測定を行うためには、この数倍の光強度を得ることが望ましいため、現在、第二高調波発生器の改良を行っている。まもなく波長500nm領域での量子ドット計測の条件がそろう予定であり、CdTe量子ドットなどの測定にとりかかる予定である。また、それ以外の波長領域で円偏光光源となりうる可能性のある材料系として、GaNナノロッド、ZnOナノ粒子、InGaAs/AlAsSb量子井戸の時間分解フォトルミネッセンス測定を行っている。このうちのいくつかについては、発光再結合時間やスピン緩和時間の測定データが得られたので学術誌に投稿中か投稿予定であり、学会発表としては、2010年の秋期の応用物理学会などで発表を行う予定である。

  • 窒化物半導体のフェムト秒超高速スピン緩和の解明

    2007  

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    ワイドギャップ半導体の超高速スピン緩和の解明として、GaInNAs/GaAs量子井戸のスピン緩和時間を測定した。GaInNAs量子井戸は波長1.2-1.3ミクロン付近で発光するため、光通信用の発光デバイスへの応用が期待されている。この応用上魅力的なGaInNAs量子井戸のスピン緩和時間の実測例としては、これまでに77-133psという報告と、2 nsという2件の報告があり、スピン緩和時間に1桁もの大きなひらきがあり混乱を生じていた。 我々は、時間分解フォトルミネッセンス分光によって、GaInNAs/GaAs量子井戸のスピン緩和過程を調べた。その結果、光励起後、200psまでは観測できる寿命の短い1.07eVのPLピークと、それとは独立に1.035eVにある長寿命のPLピークがあることを発見した。この二つのピークのスピン緩和時間を測定したところ、非局在励起子によると思われる1.06eVのピークは192psのスピン緩和時間を持つのに対して、局在励起子によると思われる1.035eVのピークは2nsという遅い緩和時間を持つことが明らかになった。 これまで、GaInNAs量子井戸のスピン緩和時間には二説あり混乱を生じていたが、本研究によって、PLピークが二つあり、それぞれのスピン緩和時間が大きく異なることが初めて明らかになった。この問題が解決したことは、GaInNAs量子井戸の今後のスピン緩和メカニズムの解明と応用において大きな意味があると考えられる。 なお、この研究結果は、早速 Applied Physics Letters に投稿し、2008年早々にVol.92, p.051908 に掲載された。

  • ワイドギャップ半導体超高速スピン緩和の解明

    2006  

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    ワイドギャップ半導体超高速スピン緩和の解明のために、AlGaN/GaN MQW発光のシュタルク効果とキャリアスクリーニング効果の関わりと、InGaNに光励起されたホットキャリアの緩和過程を調べた。 前者については、AlGaN/GaN MQWの時間分解フォトルミネッセンス測定の結果、AlGaN/GaN MQWの緩和時間とPLエネルギーがキャリア濃度に強く依存していることが実験的に明らかになった。これは窒化物半導体特有の大きな内部電場とキャリアによるスクリーニング効果を考えることで説明できる。この効果をシュレディンガー方程式と、ポアソン方程式を自己無撞着に解くことによって算出した。その結果、内部電場の大きさをフィッティングパラメーター(190 kV/cm)としてキャリア密度に対してPLエネルギーの変化量を計算すると、実験で得られた各励起光強度におけるPLエネルギーの変化量と比較的良い一致を得られることがわかった。上記のモデルがこの依存性をよく説明することが明らかとなった。また、励起光強度8 mW以上では、緩和時間は330 ps程度に収束しフラットバンドが実現されていると考えられる。 後者については、InGaNバルクのバンド端よりも高エネルギーに光励起されたホットキャリアの緩和を調べた。サンプルはサファイア基板上に成長させた厚さ0.15 ミクロンのバルクIn0.05Ga0.95Nである。計測にはポンプ・プローブ時間分解吸収計測を用い、光源には極短パルスチタン・サファイアレーザの第二高調波または第三高調波を励起光として用い、プローブ光には第二高調波を用いた。測定系の時間分解能は光源に用いる光パルスの時間幅のみで決まりサブピコ秒である。測定の結果、立ち上がり時間は第三高調波励起の場合に0.9 psほど第二高調波励起より遅いことがわかった。これは、ホットキャリアの緩和に要した時間と考えられる。

  • 半導体極微粒量子構造のスピンダイナミクスの研究

    2002  

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    ・量子ドット間の反強磁性結合の観測 半導体中の電子スピンを自由に操ることが可能になれば、我々は新しい自由度を手に入れることになる。この自由度を応用すれば、新しい機能を持ったデバイスを実現する可能性が生まれるだろう。半導体の中に3次元的に電子を閉じ込めるナノ構造を量子ドットと呼ぶが、この量子ドットの中の電子スピンの向きをデジタル情報の0と1に対応させれば、情報処理の可能性が生まれる。ただし、隣接するドット間のスピン同士で交換相互作用を働かせる必要がある。我々は、半導体量子ドット間の交換相互作用によって反強磁性結合が形成され電子スピンが反転する過程を、時間分解フォトルミネセンス測定によって直接的に観測した。電子スピンの反転が70-200psで起こること、また、反強磁性秩序が、50-80 K以下の温度で存在することが明らかになった。・高均一量子ドットのスピン緩和時間とスピンパウリブロッキングの観測 高均一量子ドットのスピン緩和時間を、時間分解フォトルミネッセンスを用いて測定した。その結果、10Kでのスピン緩和時間が1nsであることが明らかになった。これは、不均一な量子ドットのスピン緩和時間の測定値1.3nsとよく対応する。 一つの量子ドットに二個の電子が入った場合、両者のスピンが同じであれば、パウリの排他原理により、同じエネルギー準位には入れない。このため、基底準位に1個、第二準位に1個、電子が入ることになる。この効果をスピンパウリブロッキングと呼ぶが、このため第二準位のスピン偏極率が基底準位より大きくなると予測される。実験では、第二準位が基底準位の約2倍のスピン偏極率であることが明らかになった。

  • 多重結合量子ドットの集合的ドット間トンネルの研究

    2000  

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     量子ドット間のキャリアのトンネル過程を時間分解フォトルミネッセンス法を用いて調べた。結合量子ドットのサンプル構造は、In0.9Al0.1As量子ドットとInAs量子ドットがGaAsバリア層を挟んだ構造であり、バリア幅を6nm、8nm、10nmと変化させた3種類からなる。この構造では、In0.9Al0.1As量子ドット内の電子は、よりエネルギーの低いInAsドットの基底準位に非共鳴トンネルする。測定の結果、量子井戸間のトンネルと同様に、ドット間のトンネル時間がWKB近似で記述できることが明らかになった。ただし、このトンネル時間は量子井戸間のトンネル時間よりも約一桁遅い。これは、非共鳴トンネルの主要な散乱過程であるフォノン散乱が抑制されているためと考えられる。次に、スピン偏極させた電子のドット間トンネルを調べた。その結果、結合量子ドットのInAlAsドット内でのスピン緩和時間は、バリア幅6nm、8nm、10nmのサンプルに対して1.6ns、1.6ns、1.8nsであり、トンネル時間(130ps、360ps、850ps)との間に強い相関はないことが明らかになった。スピン緩和時間がトンネル過程に強くは影響されないことから、トンネル時間が短ければ、スピン偏極を維持したまま電子の移動が可能であると考えられる。

  • 半導体光スピニクスの超高速応答特性の制御に関する研究

    1998  

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    これまでのエレクトロニクスにおいて、スピンという量子状態はデバイスの機能としては利用されてこなかった。しかし、近年スピンを利用することによって、従来にない新しいデバイス機能が実現できるのではないかという期待が高まっている。本研究では、InGaAs量子井戸でのスピン緩和過程の測定とその機構の解明に取り組み、量子井戸の設計パラメータを変えることによってどの程度スピンの緩和過程を制御できるのかを調べた。その結果、InGaAs量子井戸での電子のスピン緩和時間は量子化エネルギーに反比例する(マイナス一乗に比例する)ことが明らかになった。InGaAs量子井戸のバンドギャップはGaAs量子井戸のバンドギャップの半分程度と小さいが、バンドギャップの縮小によって大きくなる効果として、伝導帯と価電子帯とのバンドミキシングによって生じるスピン緩和の機構(Elliot-Yafet効果)がある。そこで、量子井戸内のスピン緩和機構として、Elliott-Yafet効果のスピン緩和時間を理論的に求め、スピン緩和時間が量子化エネルギーに反比例することを明らかにした。これは、実験的に求めたInGaAs/InP量子井戸のスピン緩和時間の量子化エネルギー依存性と同じ依存性であり、Elliott-Yafet効果が支配的なスピン緩和機構であることを示唆している。InGaAs/InP量子井戸で量子化エネルギーによってスピン緩和時間が制御できることを明らかにしたことにより、光通信に対応する波長1.55ミクロンで、スピンをデバイス応用しうる可能性に大きく道を開いた。

  • 半導体光スピニクスの超高速応答特性の制御に関する研究

    1997  

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    これまでのエレクトロニクスにおいて、スピンという量子状態はデバイスの機能としては利用されてこなかった。しかし、近年スピンを利用することによって、従来にない新しいデバイス機能が実現できるのではないかという期待が高まっている。本研究では、InGaAs量子井戸でのスピン緩和過程の測定とその機構の解明に取り組み、量子井戸の設計パラメータを変えることによってどの程度スピンの緩和過程を制御できるのかを調べた。その結果、InGaAs量子井戸での電子のスピン緩和時間は量子化エネルギーに反比例する(マイナス一乗に比例する)ことが明らかになった。GaAs量子井戸では、スピン緩和時間が量子化エネルギーのマイナス二乗に比例し、スピン軌道相互作用に基づくスピン緩和機構(Dyakonov-Perel効果)が支配的であることを既に明らかにしたが、この依存性とは異なる。このことから、InGaAs量子井戸でのスピン緩和の機構としては他の要素を考える必要があることが明らかになった。InGaAs量子井戸のバンドギャップはGaAs量子井戸のバンドギャップの半分程度と小さいが、バンドギャップの縮小によって大きくなる効果として、伝導帯と価電子帯とのバンドミキシングによって生じるスピン緩和の機構(Elliot-Yafet効果)などを今後考慮する必要がある。また、応用面を考えると、InGaAs量子井戸は光通信での中心波長である1.55ミクロンの波長に対応するが、スピン緩和時間は同じ量子化エネルギーのGaAs量子井戸より一桁以上早く、その時定数は5ps程度と極めて高速であることが明らかになった。今後、全光ゲートスイッチなどの超高速デバイスへの応用の可能性も考えられる。研究成果の発表1998年1月  応用物理学会欧文誌刊行会 A. Tackeuchi and O. Wada, Jpn. J. Appl. Phys. 37 (1998) 98., “Electron Spin Relaxation Dynamics in InGaAs/InP Multiple-quantum Wells”

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