Browsing by Author "Chang, W. H."

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Anomalous diamagnetic shift for negative trions in single semiconductor quantum dots
(2010-03-01)
Fu, Y. J.
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Lin, S. D.
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Tsai, M. F.
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Lin, H.
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Lin, C. H.
We report on the magnetic response of negative trions X(-) in single self-assembled InAs/GaAs quantum dots. Unlike the conventional quadratic diamagnetic shift for neutral excitons, the observed X(-) diamagnetic shifts are small and nonquadratic. In particular, we also observed a reversal in sign of the conventional diamagnetic shift. A theoretical analysis indicates that such anomalous behaviors for X(-) arise from an apparent change in the electron wave function extent after photon emission due to the strong Coulomb attraction induced by the hole in its initial state. This effect can be very pronounced in small quantum dots, where the electron wave function becomes weakly confined and extended much into the barrier region. When the electrons gradually lose confinement, the magnetic response of X(-) will transit gradually from the usual quadratic diamagnetic shift to a quartic dependence, and finally into a special paramagnetic regime with an overall negative energy shift.
Carrier dynamics in isoelectronic ZnSe(1-x)O(x) semiconductors
(2010-07-26)
Lin, Y. C.
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Chung, H. L.
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Chou, W. C.
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Chen, W. K.
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Chang, W. H.
This study explores the effects of both Oxygen and temperature on the carrier dynamics of isoelectronic ZnSe(1-x)O(x) (x=0.027 and 0.053) semiconductors using photoluminescence (PL) and time-resolved PL spectroscopy. We find that the Kohlrausch law is highly consistent with the complex decay traces induced by isoelectronic O traps, and the mechanism of carrier recombination undergoes a complicated change from trapped to free excitons with the increase in temperature. Complex recombination processes are clarified using the relaxation model based on various decay channels. These findings are consistent with the initial fall in the stretching exponent beta followed by its monotonic increase with increasing temperature. (C) 2010 American Institute of Physics. [doi:10.1063/1.3473776]
Carrier dynamics in isoelectronic ZnSe1-xOx semiconductors
(2010-07-26)
Lin, Y. C.
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Chung, H. L.
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Chou, W. C.
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Chen, W. K.
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Chang, W. H.
This study explores the effects of both Oxygen and temperature on the carrier dynamics of isoelectronic ZnSe1-xOx (x=0.027 and 0.053) semiconductors using photoluminescence (PL) and time-resolved PL spectroscopy. We find that the Kohlrausch law is highly consistent with the complex decay traces induced by isoelectronic O traps, and the mechanism of carrier recombination undergoes a complicated change from trapped to free excitons with the increase in temperature. Complex recombination processes are clarified using the relaxation model based on various decay channels. These findings are consistent with the initial fall in the stretching exponent beta followed by its monotonic increase with increasing temperature. (C) 2010 American Institute of Physics. [doi:10.1063/1.3473776]
Designer germanium quantum dot phototransistor for near infrared optical detection and amplification
(2015-02-06)
Kuo, M. H.
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Lai, W. T.
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Hsu, T. M.
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Chen, Y. C.
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Chang, C. W.
We demonstrated a unique CMOS approach for the production of a high-performance germanium (Ge) quantum dot (QD) metal-oxide-semiconductor phototransistor. In the darkness, low off-state leakage (I-off similar to 0.27 pA mu m(-2)), a high on-off current ratio (I-on/I-off similar to 10(6)), and good switching behaviors (subthreshold swing of 175 mV/dec) were measured on our Ge-QD phototransistor at 300 K, indicating good hetero-interfacial quality of the Ge-on-Si. Illumination makes a significant enhancement in the drain current of Ge QD phototransistors when biased at both the on-and off-states, which is a great benefit from Ge QD-mediated photoconductive and photovoltaic effects. The measured photocurrent-to-dark-current ratio (I-photo/I-dark) and the photoresponsivities from the Ge QD phototransistor are as high as 4.1 x 10(6) and 1.7 AW(-1), respectively, under an incident power of 0.9mW at 850 nm illumination. A superior external quantum efficiency of 240% and a very fast temporal response time of 1.4 ns suggest that our Ge QD MOS phototransistor offers great promise as optical switches and transducers for Si-based optical interconnects.
Distance dependence of energy transfer from InGaN quantum wells to graphene oxide
(2013-08-01)
Lin, T. N.
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Huang, L. T.
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Shu, G. W.
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Yuan, C. T.
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Shen, J. L.
We report the distance-dependent energy transfer from an InGaN quantum well to graphene oxide (GO) by time-resolved photoluminescence (PL). A pronounced shortening of the PL decay time in the InGaN quantum well was observed when interacting with GO. The nature of the energy-transfer process has been analyzed, and we find the energy-transfer efficiency depends on the 1/d(2) separation distance, which is dominated by the layer-to-layer dipole coupling. (C) 2013 Optical Society of America
Effect of donor-acceptor concentration ratios on nonradiative energy transfer in closely packed CdTe quantum dots
(2009-09-28)
Lin, Y. C.
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Wang, W. J.
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Chung, H. L.
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Chou, W. C.
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Chen, W. K.
This study investigates the dependence of Forster resonance energy transfer (FRET) on donor-acceptor (D/A) concentration ratios in mixed-size CdTe quantum dots (QDs) films by using photoluminescence (PL) and time-resolved PL spectroscopy. Experimental results indicate that an increasing donor concentration significantly quenches the emission intensity and lifetime in donor QDs and enhances that in acceptor QDs, providing clear evidence of increased FRET efficiency. When D/A ratio exceeds 6, however, the emission intensity and the lifetime of acceptors start to decline, reflecting a decreasing FRET efficiency because of a markedly declining availability of acceptor QDs. (C) 2009 American Institute of Physics. [doi:10.1063/1.3242371]
Effect of ZnSe partial capping on the ripening dynamics of CdSe quantum dots
(2007-02-19)
Lai, Y. J.
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Yang, C. S.
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Chen, W. K.
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Lee, M. C.
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Chang, W. H.
The ripening dynamics of CdSe quantum dots (QDs) partially capped with ZnSe layer are investigated. Atomic force microscopy (AFM) images show that the ripening of QDs is dramatically accelerated by depositing a ZnSe partial capping layer. The driving force of ripening enhancement is attributed to the increasing strain energy with capping thickness. For a ZnSe partial capping layer of below 3 ML, photoluminescence exhibits a clear redshift with increasing ZnSe monolayers. It is attributed to the size of the CdSe QD increases with ZnSe partial capping, in a manner that is consistent with the results of the AFM study. (c) 2007 American Institute of Physics.
Efficient energy transfer from InGaN quantum wells to Ag nanoparticles
(2013)
Shu, G. W.
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Chiu, C. H.
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Huang, L. T.
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Lin, T. N.
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Yang, C. C.
Nonradiative energy transfer from an InGaN quantum well to Ag nanoparticles is unambiguously demonstrated by the time-resolved photoluminescence. The distance dependence of the energy transfer rate is found to be proportional to 1/d(3), in good agreement with the prediction of the dipole interaction calculated from the Joule losses in acceptors. The maximum energy-transfer efficiency of this energy transfer system can be as high as 83%.
Energy transfer from InGaN quantum wells to Au nanoclusters via optical waveguiding
(2011-03-14)
Shu, G. W.
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Lin, C. C.
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Lin, H. T.
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Lin, T. N.
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Shen, J. L.
We present the first observation of resonance energy transfer from InGaN quantum wells to Au nanoclusters via optical waveguiding. Steady-state and time-resolved photoluminescence measurements provide conclusive evidence of resonance energy transfer and obtain an optimum transfer efficiency of similar to 72%. A set of rate equations is successfully used to model the kinetics of resonance energy transfer. (C) 2011 Optical Society of America
Epitaxial stabilization of a monoclinic phase in Y(2)O(3) films on c-plane GaN
(2011-05-15)
Chang, W. H.
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Chang, P.
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Lee, W. C.
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Lai, T. Y.
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Kwo, J.
Nanometer-thick Y(2)O(3) films were grown epitaxially on GaN (0 0 0 1) using molecular beam epitaxy (MBE). The structures of the oxide films were studied in situ by reflection high energy electron diffraction (RHEED) during the growth and ex situ by high resolution X-ray diffraction using synchrotron radiation. At atmospheric pressure. Y(2)O(3) exists in either cubic or hexagonal structure. For the first time, the high-pressure monoclinic phase of Y(2)O(3), stabilized by epitaxy, was prepared and preserved under atmospheric pressure. The electrical characterization carried out on the Y(2)O(3)/GaN metal-oxide-semiconductor (MOS) capacitors showed a leakage current density of similar to 3.3 x 10(-6) A/cm(2) at 1.5 MV/cm, which remained almost the same after 800 degrees C annealing. A dielectric constant of similar to 20 and a hysteresis of similar to 250 mV were deduced from the capacitance-voltage (C-V) curves for the epitaxial monoclinic Y(2)O(3). (C) 2010 Elsevier B.V. All rights reserved.
Epitaxial stabilization of a monoclinic phase in Y2O3 films on c-plane GaN
(2011-05-15)
Chang, W. H.
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Chang, P.
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Lee, W. C.
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Lai, T. Y.
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Kwo, J.
Nanometer-thick Y2O3 films were grown epitaxially on GaN (0 0 0 1) using molecular beam epitaxy (MBE). The structures of the oxide films were studied in situ by reflection high energy electron diffraction (RHEED) during the growth and ex situ by high resolution X-ray diffraction using synchrotron radiation. At atmospheric pressure. Y2O3 exists in either cubic or hexagonal structure. For the first time, the high-pressure monoclinic phase of Y2O3, stabilized by epitaxy, was prepared and preserved under atmospheric pressure. The electrical characterization carried out on the Y2O3/GaN metal-oxide-semiconductor (MOS) capacitors showed a leakage current density of similar to 3.3 x 10(-6) A/cm(2) at 1.5 MV/cm, which remained almost the same after 800 degrees C annealing. A dielectric constant of similar to 20 and a hysteresis of similar to 250 mV were deduced from the capacitance-voltage (C-V) curves for the epitaxial monoclinic Y2O3. (C) 2010 Elsevier B.V. All rights reserved.
Growth and optical properties of high-density InN nanodots
(2010-10-15)
Ke, W. C.
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Lee, S. J.
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Kao, C. Y.
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Chen, W. K.
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Chou, W. C.
High density InN/GaN nanodots were grown by pulsed mode (PM) metal-organic chemical vapor deposition (MOCVD). InN nanodots density of up to similar to 5 x 10(10) cm(-2) at a growth temperature of 550 degrees C was achieved. The high diffusion activation energy of 2.65 eV due to high NH(3) flow rate generated more reactive nitrogen adatoms on the growth surface, and is believed to be the main reason for the growth of high density InN nanodots. In addition, an anomalous temperature dependence of the PL peak energy was observed for high density InN nanodots. The high carrier concentration, due to high In vacancy (V(ln)) in the InN nanodots, thermally agitated to the conduction band. As the measurement temperature increased, the increase of Fermi energy resulted in blue-shifted PL peak energy. From the Arrhenius plot of integrated PL intensity, the thermal activation energy for the PM grown InN nanodots was estimated to be E(a)similar to 51 meV, indicating strong localization of carriers in the high density InN nanodots. (C) 2010 Elsevier B.V. All rights reserved.
Growth of optical-quality, uniform In-rich InGaN films using two-heater metal-organic chemical vapor deposition
(2013-11-15)
Fu, S. F.
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Chen, C. Y.
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Li, F. W.
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Hsu, C. H.
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Chou, W. C.
Good-optical-quality, thick InxGa1-xN films with high In content were grown using a homemade two-heater metal-organic chemical vapor deposition system. By varying the growth temperature, it was Found that the In composition of the InGaN epilayer varied from 18 to 59% as the substrate temperature decreased from 750 to 625 degrees C. Our results show that the optical properties in terms of the emission peak wavelength and linewidth are uniformly distributed throughout the entire 2 in. wafer for the x=0.40 InGaN sample. The resultant mean peak wavelength and FWHM are 808 +/- 6 nm and 229 +/- 18 meV, respectively, at 18 K. In addition, for the InGaN film grown at 625 degrees C, a noticeable decrease in the In composition occurred when the ceiling temperature was >800 degrees C, indicative of the occurrence of parasitic reactions in the gas phase. (C) 2013 Elsevier B.V. All rights reserved.
Improving Performance of InGaN/GaN Light-Emitting Diodes and GaAs Solar Cells Using Luminescent Gold Nanoclusters
(2009)
Yang, M. D.
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Wu, S. W.
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Shu, G. W.
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Wang, J. S.
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Shen, J. L.
We studied the optoelectronic properties of the InGaN/GaN multiple-quantum-well light emitting diodes (LEDs) and single-junction GaAs solar cells by introducing the luminescent Au nanoclusters. The electroluminescence intensity for InGaN/GaN LEDs increases after incorporation of the luminescent Au nanoclusters. An increase of 15.4% in energy conversion efficiency is obtained for the GaAs solar cells in which the luminescent Au nanoclusters have been incorporated. We suggest that the increased light coupling due to radiative scattering from nanoclusters is responsible for improving the performance of the LEDs and solar cells. Copyright (C) 2009 M. D. Yang et al.
Interrelation of transport and optical properties in gold nanoclusters
(2010-09-20)
Shu, G. W.
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Chen, T. Y.
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Shen, J. L.
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Lin, C. A. J.
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Chang, W. H.
Temperature dependence of the electrical conductivity and photoluminescence (PL) in Au nanoclusters (NCs) is investigated. The correlation of the conductivity and PL in Au NCs at different temperatures is evident: (i) for T < 50 K, both the conductivity and PL intensity decrease with temperature, which suggests thermal structural fluctuations; (ii) for 50 K < T < 90 K, conductivity and PL are explained by variable range hopping; (iii) for 90 K < T < 170 K, simple thermal activated hopping dominates in conductivity, with a rate-equation model proposed to analyze the carrier transfer in PL. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3491288]
Interrelation of transport and optical properties in gold nanoclusters
(2010-09-20)
Shu, G. W.
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Chen, T. Y.
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Shen, J. L.
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Lin, C. A. J.
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Chang, W. H.
Temperature dependence of the electrical conductivity and photoluminescence (PL) in Au nanoclusters (NCs) is investigated. The correlation of the conductivity and PL in Au NCs at different temperatures is evident: (i) for T < 50 K, both the conductivity and PL intensity decrease with temperature, which suggests thermal structural fluctuations; (ii) for 50 K < T < 90 K, conductivity and PL are explained by variable range hopping; (iii) for 90 K < T < 170 K, simple thermal activated hopping dominates in conductivity, with a rate-equation model proposed to analyze the carrier transfer in PL. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3491288]
Magneto-optical properties of ZnMnTe/ZnSe quantum dots
(2011-05-15)
Fan, W. C.
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Ku, J. T.
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Chou, W. C.
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Chen, W. K.
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Chang, W. H.
The sigma(+) and sigma(-) circularly polarized time-integrated and time-resolved Photoluminescence (PL) measurements were employed to investigate the carrier spin dynamics of ZnMnTe/ZnSe Quantum Dots (QDs) grown on GaAs substrates by molecular beam epitaxy. The Kohlrausch's stretching exponential function well correlates both the sigma(+) and sigma(-) decay profiles. The measured spin relaxation time is about 23 ns. (C) 2010 Elsevier B.V. All rights reserved.
Optical characterization of isoelectronic ZnSe(1-x)O(x) semiconductors
(2011-05-15)
Lin, Y. C.
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Chung, H. L.
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Ku, J. T.
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Chen, C. Y.
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Chien, K. F.
We have studied the temperature dependence of the band gap and the decay dynamics of isoelectronic ZnSe(1-x)O(x) (x=0-0.053) semiconductors, using photoluminescence (PL) and time-resolved PL spectroscopy. The temperature dependence of the band gap of ZnSe(1-x)O(x) decreases with the increase in 0 concentration. The Kohlrausch law is in good agreement with the O-induced complex decay profiles. As temperature increases, the mechanism of carrier decay undergoes a complicated change from trapped to free excitons. These findings are consistent with the temperature dependence of the stretching exponent beta. (C) 2010 Elsevier B.V. All rights reserved.
Optical characterization of isoelectronic ZnSe1-xOx semiconductors
(2011-05-15)
Lin, Y. C.
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Chung, H. L.
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Ku, J. T.
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Chen, C. Y.
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Chien, K. F.
We have studied the temperature dependence of the band gap and the decay dynamics of isoelectronic ZnSe1-xOx (x=0-0.053) semiconductors, using photoluminescence (PL) and time-resolved PL spectroscopy. The temperature dependence of the band gap of ZnSe1-xOx decreases with the increase in 0 concentration. The Kohlrausch law is in good agreement with the O-induced complex decay profiles. As temperature increases, the mechanism of carrier decay undergoes a complicated change from trapped to free excitons. These findings are consistent with the temperature dependence of the stretching exponent beta. (C) 2010 Elsevier B.V. All rights reserved.
Optical fine structures of highly quantized InGaAs/GaAs self-assembled quantum dots
(2010-06-30)
Ramirez, H. Y.
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Lin, C. H.
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Chao, C. C.
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Hsu, Y.
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You, W. T.
A theoretical model for the electron-hole exchange interaction in three-dimensionally (3D) confining semiconductor nanostructures is presented to explain the observed decreasing tendency of the fine-structure splittings (FSSs) of small InGaAs/GaAs self-assembled quantum dots (QDs) with increasing the emission energies. The experimentally revealed FSS reduction is shown to be highly associated with the significant 3D spreading of electronic orbitals and reduced overlap of electron and hole wave functions in small and/or Ga-diffused QDs. The combination of quantum size and Ga-diffusion effects substantially reduces the averaged e-h exchange interaction and leads to the reduced FSSs in the regime of high emission energy.