Browsing by Author "Ling, H. S."

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Carrier Dynamics in InAs/GaAs Quantum Rings
(2009)
Lin, C. H.
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Ling, H. S.
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Sun, K. W.
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Lee, C. P.
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Lin, S. D.
We have investigated the temperature dependence of the carrier relaxation dynamics in InAs/GaAs self-assembled quantum rings by using the time-resolved photoluminescence technique with a time resolution of similar to 250 ps. The carrier relaxation in the QRs not only shows strong dependence on the lattice temperature, it is also significant slower than the carrier relaxation in the quantum dots at temperature above 150 K.
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.
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.
Shape dependent carrier dynamics in InAs/GaAs nanostructures
(2009-12-01)
Lin, C. H.
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Ling, H. S.
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Su, S. K.
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Lin, S. D.
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Lee, C. P.
We present systematic experimental studies of the temperature dependence of continuous wave and time-resolved photoluminescence spectroscopy in self-assembled InAs/GaAs nanostructures as the shape of quantum structures evolved from dot to ring. The carrier dynamics show strong dependence on the geometrical shape of the nanostructures under investigation. An increase in photoluminescence decay time of the excited and ground states is observed as the shape of nanostructures changed from dot, volcano, to ring. It is attributed to the carrier thermalization between the dark and ground states. The photoluminescence excitation spectra of the quantum rings reveal resonances related to the dark states. A rate equation model is proposed to interpret the observed carrier dynamics.
Spectral response and device performance tuning of long-wavelength InAs QDIPs
(2011-05-01)
Ling, H. S.
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Wang, S. Y.
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Lee, C. P.
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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
Long-wavelength InAs QDIPs with different thicknesses of InGaAs cap layer in the confinement-enhanced dots-in-a-well (CE-DWELL) structure were investigated. The sample with 3 nm cap layer shows primarily single band detection at 7 mu m that stems from the transition between QD states, but the 5 nm and 7 nm samples reveal voltage-tunable dual band detection in the region of 6-11 mu m from transitions to the states in the dots as well as the states in the well. The wavefunction coupling of transition states is effectively modified by the change of cap-layer thickness and the bias polarity, which leads to the observed spectral change and demonstrates the flexibility of the CE-DWELL QDIPs. For higher peak quantum efficiency (QE), thin InGaAs cap layers are used to obtain focused absorption strength. With 3 nm cap layer in the CE-DWELL, a respectable QE of 7.23% is reached for 10-stack QDIPs with 7 mu m detection wavelength at 77K. and a high detectivity of 3.4 x 10(10) Jones is also obtained. (C) 2010 Elsevier B.V. All rights reserved.
Temperature dependence of quantum efficiency in Quantum Dot Infrared Photodetectors
(2011-05-01)
Wang, S. Y.
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Ling, H. S.
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Lee, C. P.
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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
The behavior of quantum efficiency in QDIPs was studied in details with simple InAs/GaAs QDs and DWELL QDs structures. Despite of the large difference of the excited state energy between the two samples, the QE shows similar trends with temperature and bias voltage. The voltage to reach the QE plateau decreases with temperature and the maximum QE decreases with temperature. Considering the repulsive potential from the charge inside the QDs, the effective barrier height and thickness for the photoexcited carrier is much reduced and the QE variation with voltage follows the calculated tunneling probability. Furthermore, the multi-phonon interaction which leads to the relaxation of the excited carrier is shown to be important to the decrease of QE with temperature. The enhanced relaxation rate decreases the maximum QE value at higher temperature. (C) 2010 Elsevier B.V. All rights reserved.
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.