Browsing by Author "Sun, KW"

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Calculations of the inter-subband scattering rates of electrons in GaAs/AlGaAs quantum wells
(2003-05-01)
Lee, HC
;
Sun, KW
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
In this presentation, we report the calculated results on hot electron relaxation through inter- and intra-subband scatterings with LO phonons in quantum well structures. The scattering rates were calculated for electrons excited both in the gamma valley and L valleys. The types of optical phonons adapted in our model are determined based on the dielectric continuum model. We have also studied the dependence of the scatterings on the structure parameters of the quantum well. (C) 2003 Elsevier Science Ltd. All rights reserved.
Carrier capture and relaxation in InAs quantum dots
(2005-09-01)
Sun, KW
;
Chen, JW
;
Lee, BC
;
Lee, CP
;
Kechiantz, AM
We have investigated the carrier capture and relaxation processes in InAs/GaAs self-assembled quantum dots at room temperature by time-resolved photoluminescence techniques with a high time resolution of similar to 200 fs. Following the initial fast relaxation in GaAs barriers, we have observed rising processes in time-resolved PL intensity at the energies of quantum dot confined states and the wetting layer. The rising processes are assigned to the carrier capture from the barriers into the wetting layer and confined states in InAs dots and subsequent relaxation in each detected energy level. We found that the carrier capture rate is faster than the intra-dot relaxation within the range of excitation densities that we investigated. Under high excitation intensity, the electronic states in the dots were populated mainly by carriers directly captured from the barrier. However, at low excitation densities, the PL rise times were influenced by the carrier diffusion.
Carrier-carrier scattering in GaAs/AlxGa1-xAs quantum wells
(2000-06-15)
Sun, KW
;
Song, TS
;
Sun, CK
;
Wang, JC
;
Kane, MG
We have studied carrier dynamics in highly nonequilibrium two-dimensional (2D) carrier distributions generated with femtosecond laser pulses in p-doped GaAs/Al0.32Ga0.68As quantum wells at photoexcited carrier densities between 10(9) and 10(11) cm(-2). The initially nonthermal carrier distribution is quickly broadened due to inelastic carrier-carrier scattering, with the broadening rate increasing as carrier density is increased. Measurements of the unrelaxed peak height in the hot electron-neutral acceptor luminescence spectra are compared with calculations of the carrier distribution using integration of the 2D dynamically screened Boltzmann equation. Our results indicate that carrier-carrier scattering becomes as significant a scattering mechanism as LO-phonon emission at density of about 10(10) cm(-2).
Carrier-carrier scattering: an experimental comparison of 5 and 3 nm AlxGa1-xAs/GaAs quantum wells
(2000)
Sun, KW
;
Sun, CK
;
Wang, JC
;
Wang, SY
;
Lee, CP
We have studied carrier dynamics among highly nonequilibrium carrier distributions generated with femtosecond laser pulses in Be:AlxGa1-xAs/GaAs quantum wells with different well width at hot carrier densities between 10(9) and 10(11) cm(-2). The spectra, at moderately high densities (greater than or equal to 10(10) cm(-2)) indicate that the initially narrow electron distribution is altered in a time less than or equal to the LO-phonon emission time, as a result of rapid carrier-carrier scattering. At an excitation density of about 10(10) cm(-2), the carrier-carrier scattering rate is faster in narrower quantum wells. By measuring the intensity of the unrelaxed peak, it is possible to determine the relative rates of carrier-carrier scattering and LO-phonon emission. Our results indicate that carrier-carrier scattering becomes as significant as LO-phonon emission at a density of about 1 x 10(10) cm(-2) in the 5 nm quantum wells. (C) 2000 Elsevier Science Ltd. All rights reserved.
Guided three-dimensional molecular self-assembly on silicon substrates
(2006-06-26)
Chang, CC
;
Sun, KW
;
Kan, LS
;
Kuan, CH
;
生物科技學系
We demonstrate three-dimensional (3D) self-assemble growth of the metallothionein (Mn,Cd-MT-2) molecules on patterned semiconductor substrates. The MT molecules deposited on the patterned substrates were found to grow into 3D rod or ring-type nanostructures, depending on the shape of patterned nanostructures on the substrates. Dense arrays of 3D molecular nanorods or rings with an area density close to 10(10) cm(-2) were demonstrated with a pore size of 20 nm and a pitch size of 100 nm. Those engineered molecular nanostructures provide an excellent opportunity for biological applications, sensing sources of nanodevices, biochemical reactions on surfaces, and even single molecule studies. (c) 2006 American Institute of Physics.
Hot-electron relaxation via optical phonon emissions in GaAs/AlxGa1-xAs quantum well structures: dependence upon the alloy composition and barrier width
(2000-12-01)
Sun, KW
;
Chang, HY
;
Wang, CM
;
Wang, SY
;
Lee, CP
We present a systematic investigation of the dependence of the hot-electron-optical-phonon interactions on Al composition and barrier width in GaAs/AlxGa1-xAs MQW structures. Raman scattering measurements at 15 K are presented for samples with different barrier widths and Al composition. The optical phonon energies emitted by the photoexcited electrons in quantum wells were also determined by using hot-electron-neutral acceptor luminescence techniques. It is shown that the relaxation of hot electrons in the quantum wells is dominated by the GaAs LO phonon emission for small x, but by AlAs-like LO phonons for larger Al composition. For samples with larger barriers, the electrons in the GaAs layer relax mostly through the AlAs-like optical phonon emission. However, in samples with smaller barriers, the relaxation of hot electrons is dominated by the GaAs optical phonon emission.
Inter- and intra-subband relaxation of hot electrons in GaAs/AlGaAs quantum wells
(2003-05-01)
Sun, KW
;
Huang, CL
;
Huang, GB
;
Lee, HC
;
電子工程學系及電子研究所
In this work, we have studied the inter- and intra-subband scattering of hot electrons in quantum wells using the hot electron-neutral acceptor luminescence technique. We have observed direct evidence of the emission of confined optical phonons by hot electrons excited slightly above the n = 2 subband in GaAs/Al0.37Ga0.63As quantum wells. Scattering rates of photoexcited electrons via inter- and intra-subband LO phonon emission were calculated based on the dielectric continuum model. We found that, for wide wells with the Al composition of our experiments, both the calculated and experimental results suggest that the scattering of the electrons is dominated by the confined LO phonon mode. In the calculations, scatterings among higher subbands are also dominated by the same type of phonon at well width of 10 nm. (C) 2003 Elsevier Science Ltd. All rights reserved.
Investigation of electron-optical phonon interactions in moderate wide InxGa1-xAs/GaAs strained quantum wells
(2001-12-01)
Lin, SD
;
Lee, HC
;
Sun, KW
;
Lee, CP
;
電子工程學系及電子研究所
In this presentation, we have calculated the electron-optical phonon scattering rate of GaAs/AlAs quantum wells and average electron energy loss rate as a function of well width of the GaAs/AlxGa1-xAs quantum wells. We have also studied the Raman and hot electron-neutral acceptor luminescence in moderate wide InxGa1-xAs/GaAs strained quantum wells (with 10 nm in well width and 30 nm in barrier width) to determine the dominant phonon mode emitted by the hot electrons in the wells at 15 K. The hot electron-neutral acceptor luminescence spectrum of the strained quantum well sample shows an oscillation period of about 22 meV which indicates that the hot electrons relaxed mostly through emissions of the InAs confined phonons. (C) 2001 Elsevier Science B.V. All rights reserved.
Mn,Cd-metallothionein-2: A room temperature magnetic protean
(2006-02-24)
Chang, CC
;
Lee, SF
;
Sun, KW
;
Ho, CC
;
Chen, YT
Naturally occurring metallothionein (MT) is a metal binding protein, which binds to seven Zn2+ through 20 conserved cysteines and forms two metal binding clusters with a Zinc-Blende structure. We demonstrate that the MT, when substituting the Zn2+ ions by Mn2+ and Cd2+, exhibits magnetic hysteresis loop observable by SQUID from 10 to 330 K. The magnetic moment may have originated from the bridging effect of the sulfur atoms between the metal ions that leads to the alignment of the electron spins of the Mn2+ ions inside the clusters. The protein backbone may restrain the net spin moment of Mn2+ ions from thermal fluctuation. The modified magnetic-metallothionein is a novel approach to creating molecular magnets with operating temperatures up to 330 K. (c) 2005 Elsevier Inc. All rights reserved.
Molecular beam epitaxial growth and photoluminescence studies of InAs self-organized quantum dots on patterned GaAs (001) substrates
(2001)
Lee, BC
;
Lee, HM
;
Wu, JC
;
Chang, YP
;
Sun, KW
We present experimental results on the growth of InAs self-organized quantum dots on patterned substrates via molecular beam epitaxy. Luminescence spectra of these quantum dots have been studied.
Nano-surgery on cancer cells by protein-nanodiamond complex.
(2006-03-07)
Chang, CC
;
Chao, JI
;
Sun, KW
;
Cheng, CL
;
生物科技學系
Nonthermal carrier dynamics in AlxGa1-xAs/GaAs quantum wells
(2000-05-01)
Sun, KW
;
Song, TS
;
Wang, SY
;
Lee, CP
;
電子工程學系及電子研究所
We present new time-resolved measurements that demonstrate the effect of injected carrier densities on carrier-carrier scattering rates of highly nonequilibrium carrier distributions in the p-doped Al0.32Ga0.68As/GaAs quantum wells with well width of 5 and 3 nm. The spectra at high densities (greater than or equal to 10(10) cm(-2)) demonstrate that the initially narrow electron distribution is altered in a time less than or equal to the LO phonon emission time as the result of rapid carrier-carrier scattering. At about the same excitation density (similar to 10(10) cm(-2)), the carrier-carrier scattering rate is faster in 3 nm quantum wells in comparison to 5 nm wells. Our results also indicate that carrier-carrier scattering becomes as significant as LO phonon emission at density of about 1 X 10(10) cm(-2) in the 5 nm quantum wells. (C) 2000 Elsevier Science B.V. All rights reserved.
Optical phonon emission in GaAs/AlAs and GaAs/Al0.7Ga0.3As multiple quantum well structures
(2000-12-01)
Sun, KW
;
Wang, CM
;
Chang, HY
;
Wang, SY
;
Lee, CP
We have performed Raman scattering measurements and hot electron-neutral acceptor (hot(e, Angstrom)) luminescence experiments on Be-doped GaAs/AlAs and GaAs/Al0.7Ga0.3As multiple quantum well structures, with fixed well width of 50 Angstrom and barrier thickness of 5, 25, 50,120 Angstrom, to determine the optical phonon energy emitted by the hot electrons excited in the quantum wells. It was shown that the relaxation of electrons in the GaAs layer is dominated by the AlAs-like optical phonon emission for samples with larger barriers, but by GaAs optical phonons for smaller barriers. (C) 2001 Elsevier Science B.V. All rights reserved.
Raman and hot electron-neutral acceptor luminescence studies of electron-optical phonon interactions in GaAs/AlxGa1-xAs quantum wells
(2000)
Sun, KW
;
Chang, HY
;
Wang, CM
;
Song, TS
;
Wang, SY
Using two optical techniques, we have studied the hot electron-optical phonon interactions in GaAs/AlxGa1-xAs multiple-quantum wells. Raman scattering measurements at 15 K are presented for the Al composition of x = 0.3, 0.5, 0.7 and 1.0. The GaAs-like and AlAs-like phonon frequencies of the first-order modes are also measured as a function of Al composition. The optical phonon energies emitted by the photoexcited electrons in quantum wells are determined by using hot electron-neutral acceptor luminescence techniques. It is shown that the relaxation of hot electrons in the quantum wells is dominated by the GaAs LO phonon emission for small x, but by AlAs-like LO phonons for larger Al composition. (C) 2000 Elsevier Science Ltd. All rights reserved.
Selective growth and photoluminescence studies of InAs self-organized quantum dot arrays on patterned GaAs(001) substrates
(2002-10-01)
Sun, KW
;
Wu, JC
;
Lee, BC
;
Lee, CP
;
電子工程學系及電子研究所
In this paper, we present experimental results oil the selective growth of InAs self-organized quantum dots on patterned substrates using electron-beam lithography and molecular beam epitaxy. Higher dot densities were found on the patterned substrate with a particular pattern orientation compared to the densities of dots grown on the non-patterned area. Good quality of the quantum dot arrays and long-range ordering was also achieved. We have also studied the luminescence spectra of these quantum dots. Dots grown on patterned substrates do indeed show different luminescence characteristics compared to dots on non-patterned surfaces. (Some figures in this article are in colour only in the electronic version).
Selective growth of single InAs quantum dots using strain engineering
(2002-01-14)
Lee, BC
;
Lin, SD
;
Lee, CP
;
Lee, HM
;
Wu, JC
A method to achieve ordering and selective positioning of single InAs self-assembled quantum dots (QDs) has been developed. The selective growth was achieved by manipulating the strain distribution on the sample surface. The QDs are formed on predesigned mesas with added strain. Single dots were obtained on small mesas. Using this technique, two-dimensional single QD arrays have been achieved. (C) 2002 American Institute of Physics.
Significance of dimensionality and dynamical screening on hot carrier relaxation in bulk GaAs and quantum wells
(2003-11-01)
Lee, HC
;
Sun, KW
;
Lee, CP
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
We have found theoretically that the reduced dimensionality from bulk GaAs to quantum wells has a strong effect on hot carrier relaxations at highly excited carrier densities. The distinct density of state and the dynamical screening cause hot carriers in quantum wells to relax significantly slower than in the bulk when the carrier density is above a critical value of 2 x 10(18) cm(-3). With the random phase approximation, the dynamical screening in quantum wells appears to be much stronger than that in the bulk and as important as the hot phonon effect at high carrier densities. We also found that the dependence of the average energy-loss rates on the well width in quantum wells becomes more appreciable when Al compositions are high. (C) 2003 Elsevier Ltd. All rights reserved.
Structure effects on electron-optical phonon interaction in GaAs/AlxGa1-xAs quantum wells
(2002-07-01)
Lee, HC
;
Sun, KW
;
Lee, CP
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
Based on the dielectric continuum model, we have studied the electron-optical phonon scattering rates in GaAs/AlxGa1-xAs quantum wells with different structure parameters. It was found that the scattering rate of the symmetric interface phonon mode has a stronger dependence on the Al composition in the barriers than that of the confined mode. The effective phonon energy emitted by hot electrons in GaAs/AlxGa1-xAs quantum wells with various Al compositions was estimated and the calculated value agrees with the experimental results qualitatively. For the dependence on the well width, scattering rates of the S+ mode drop considerably as the well width is increased. The dependence of the electron-optical phonon interaction on structure parameters can be clearly explained by the H and G factors defined in the article. (C) 2002 American Institute of Physics.
Structure effects on inter- and intra-band scattering of electrons in GaAs/AlxGa1-xAs and strained InxGa1-xAs/GaAs quantum wells
(2003)
Lee, HC
;
Sun, KW
;
Lee, CP
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
Based on the dielectric continuum model, we have studied the dependence of electron-optical phonon scattering rates in GaAs/AlxGa1-xAs quantum wells with different structure parameters. It was found that the dependence of scattering rates of symmetric interface mode on Al composition in the barrier was stronger than that of the confined mode. The average phonon energy emitted by hot electrons in GaAs/AlxGa1-xAs quantum wells with various Al composition was estimated and the calculated value agrees with the experimental results qualitatively. For the dependence on the well width, scattering rates of the S+ mode dropped considerably as the well width is increased. The hot electron-neutral acceptor luminescence spectrum of the strained InxGa1-xAs/GaAs quantum well sample shows an oscillation period of about 22 meV which indicates that the hot electrons relaxed mostly through emissions of the InAs confined phonons.
Subwavelength gratings fabricated on semiconductor substrates via E-beam lithography and lift-off method
(2005-03-01)
Sun, KW
;
Huang, SC
;
Kechiantz, A
;
Lee, CP
;
電子工程學系及電子研究所
We present results of the fabrication and measurements on reflective polarizers consisting of stacked bi-layer subwavelength metal gratings prepared on GaAs (100) substrates. These linear gratings were fabricated using electron-beam direct-writing lithography and the lift-off method with periods less than the wavelength of light used for measurements. At normal incidence, the polarizer reflects the light polarized perpendicular to the grating lines (transverse magnetic polarization, TM polarized) but absorbs parallel-polarized light (transverse electric polarization, TE polarized). By optimizing structural parameters, the polarization extinction ratio close to 20 has been experimentally achieved at wavelength of 650 run.