Browsing by Author "Lo, M. C."

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Characteristics of In(Ga)As quantum ring infrared photodetectors
(2009-02-01)
Ling, H. S.
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Wang, S. Y.
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Lee, C. P.
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Lo, M. C.
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電子工程學系及電子研究所
Characteristics of In(Ga)As quantum ring infrared photodetectors (QRIPs) were investigated under normal incidence configuration. Compared with quantum dot infrared photodetectors (QDIPs), QRIPs showed wider photocurrent spectra, more stable responsivity with temperature change, and lower dark current activation energy. The wide detection band comes from the transitions from the quantum ring (QR) ground states to different excited states. The shallow confinement states generate higher dark current and enhance the carrier flow between the QRs within the same QR layer. This carrier flow averages out the repulsive potential and makes QRIPs behave similarly to the quantum well infrared photodetectors instead of QDIPs. With an Al(0.27)Ga(0.73)As current blocking layer, the performance of QRIPs was greatly enhanced.
Confinement-enhanced dots-in-a-well QDIPs with operating temperature over 200 K
(2009-11-01)
Ling, H. S.
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Wang, S. Y.
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Lee, C. P.
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Lo, M. C.
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電子工程學系及電子研究所
LWIR InAs/Al(0.3)Ga(0.7)As/In(0.15)Ga(0.85)As confinement-enhanced DWELL (CE-DWELL) QDIPs with operation temperatures higher than 200 K are reported. A thin Al(0.3)Ga(0.7)As barrier layer was inserted above the InAs QDs to improve the confinement of QD states in the In(0.15)Ga(0.85)As DWELL structure and the device performance. The better confinement of the electronic states increases the oscillator strength of the infrared absorption. The higher excited state energy also improves the escape probability of the photoelectrons. Compared with the conventional DWELL QDIPs, the quantum efficiency increases for more than 20 times and the detectivity is an order of magnitude higher at 77 K. With better device parameters of CE-DWELL, it is possible to achieve high quantum efficiency, high operating temperature and long wavelength detection at the same time. (C) 2009 Elsevier B.V. All rights reserved.
Detection wavelength and device performance tuning of InAs QDIPs with thin AlGaAs layers
(2009-11-01)
Wang, S. Y.
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Ling, H. S.
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Lo, M. C.
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Lee, C. P.
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電子工程學系及電子研究所
QDIPs with thin inserted AlGaAs layers adjacent to the QDs were investigated for the tailoring of detection wavelength and device performance. Simple InAs/GaAs QDIPs and DWELL QDIPs with different insertion layer structure were studied. The thin AlGaAs layer is shown to effectively modify the electron wavefunction and associated confined state energies which lead to the change of the detection wavelength and the polarization dependent quantum efficiency. Furthermore. the dark current and gain also change with different device structures. The insertion of AlGaAs layers provides an additional freedom of tuning the electronics states involved in the infrared detection and also enables the improvement of the absorption and device performance. (C) 2009 Elsevier B.V. All rights reserved.
Development of a Parallel Explicit Finite-Volume Euler Equation Solver using the Immersed Boundary Method with Hybrid MPI-CUDA Paradigm
(2020-02-01)
Kuo, F. A.
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Chiang, C. H.
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Lo, M. C.
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Wu, J. S.
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機械工程學系
This study proposed the application of a novel immersed boundary method (IBM) for the treatment of irregular geometries using Cartesian computational grids for high speed compressible gas flows modelled using the unsteady Euler equations. Furthermore, the method is accelerated through the use of multiple Graphics Processing Units - specifically using Nvidia's CUDA together with MPI - due to the computationally intensive nature associated with the numerical solution to multi-dimensional continuity equations. Due to the high degree of locality required for efficient multiple GPU computation, the Split Harten-Lax-van-Leer (SHLL) scheme is employed for vector splitting of fluxes across cell interfaces. NVIDIA visual profiler shows that our proposed method having a computational speed of 98.6 GFLOPS and 61% efficiency based on the Roofline analysis that provides the theoretical computing speed of reaching 160 GLOPS with an average 2.225 operations/byte. To demonstrate the validity of the method, results from several benchmark problems covering both subsonic and supersonic flow regimes are presented. Performance testing using 96 GPU devices demonstrates a speed up of 89 times that of a single GPU (i.e. 92% efficiency) for a benchmark problem employing 48 million cells. Discussions regarding communication overhead and parallel efficiency for varying problem sizes are also presented.
Energy dependent carrier relaxation in self-assembled InAs/GaAs quantum dots
(2008)
Ling, H. S.
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Lee, C. P.
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Wang, S. Y.
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Lo, M. C.
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電子工程學系及電子研究所
We performed the selective excitation photoluminescence (SEPL) spectroscopy studies on InAs/GaAs self-assembled quantum dots (QDs). Under different excitation energies, different groups of QDs are selected and emit light. The excited carriers relax to the ground state through different mechanisms when excited at different energies. Three distinct regions with different mechanisms in carrier excitations and relaxation are identified in the emission spectra. These three regions can be categorized, from high energy to low energy, as continuum absorption, electronic state excitation, and multi-phonon resonance. The QDs special joint density-of-state tail extending from the wetting layer peak facilitates the carrier relaxation and was suggested to explain these spectral results. (c) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Energy-dependent carrier relaxation in self-assembled InAs quantum dots
(2008-06-15)
Ling, H. S.
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Lee, C. P.
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Lo, M. C.
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電子工程學系及電子研究所
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Department of Electronics Engineering and Institute of Electronics
Selective excitation photoluminescence spectroscopy was employed to study InAs/GaAs self-assembled quantum dots (QDs). Under different excitation energies, different groups of QDs are selected and then emit light. The excited carriers relax to the ground state through different mechanisms when excited at different energies. Three distinct regions with different mechanisms in carrier excitation and relaxation are identified in the emission spectra. These three regions can be categorized from high energy to low energy, as continuum absorption, electronic state excitation, and multiphonon resonance. The special joint density of state tail of the QD that extends from the wetting layer band edge facilitates carrier relaxation and is posited to explain these spectral results. (C) 2008 American Institute of Physics.
High quantum efficiency dots-in-a-well quantum dot infrared photodetectors with AlGaAs confinement enhancing layer
(2008-05-12)
Ling, H. S.
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Wang, S. Y.
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Lee, C. P.
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Lo, M. C.
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電子工程學系及電子研究所
We demonstrate the high quantum efficiency InAs/In(0.15)Ga(0.85)As dots-in-a-well (DWELL) quantum dot infrared photodetectors (QDIPs). A thin Al(0.3)Ga(0.7)As layer was inserted on top of the InAs quantum dots (QDs) to enhance the confinement of QD states in the DWELL structure. The better confinement of the electronic states increases the oscillation strength of the infrared absorption. The higher excited state energy also improves the escape probability of the photoelectrons. Compared with the conventional DWELL QDIPs, the quantum efficiency increases more than 20 times and the detectivity is about an order of magnitude higher at 77 K. (c) 2008 American Institute of Physics.
Temperature dependent responsivity of quantum dot infrared photodetectors
(2007-04-01)
Wang, S. Y.
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Lo, M. C.
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Hsiao, H. Y.
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Ling, H. S.
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Lee, C. P.
Temperature dependent behavior of the responsivity of InAs/GaAs quantum dot infrared photoderectors was investigated with detailed measurement of the current gain. The current gain varied about two orders of magnitude with 100 K temperature change. Meanwhile, the change in quantum efficiency is within a factor of 10. The dramatic change of the current gain is explained by the repulsive coulomb potential of the extra carriers in the QDs. With the measured current gain, the extra carrier number in QDs was calculated. More than one electron per QD could be captured as the dark current increases at 150 K. The extra electrons in the QDs elevated the Fermi level and changed the quantum efficiency of the QDIPs. The temperature dependence of the responsivity was qualitatively explained with the extra electrons. (c) 2006 Elsevier B.V. All rights reserved.