Browsing by Author "Lee, M. C."

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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.
Experimental investigation of the effect of nozzle throat diameter on the performance of a hybrid rocket motor with swirling injection of high-concentration hydrogen peroxide
(2019-11-01)
Wei, S. S.
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Lee, M. C.
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Chien, Y. H.
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Chou, T. H.
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Wu, J. S.
One of the issues of using a normal graphite nozzle for the hybrid rocket propulsion is the serious throat erosion due to combustion with a higher O/F ratio. This may undermine the throttling capability of the hybrid rocket engine. In this study, we would like to address how the propulsion performance changes under the conditions of different nozzle throat diameters and O/F ratios. We designed and tested a 40-kgf class single-port hybrid rocket motor with the swirling injection of oxidizer, which utilizes the 90 wt% hydrogen peroxide and polypropylene (PP) as oxidizer and fuel, respectively. Three different diameters of graphite nozzle throat (10, 11, and 12 mm) were used to approximate various conditions of nozzle erosion, while the mass flow rates of the injected oxidizer were kept the same. The hot-fire test results indicated that the thrust was nearly the same even the nozzle throat diameters and O/F ratios were different. Especially, the discrepancies among the measured thrusts and oxidizer ISP under various test conditions were found to be less than 1%, which is highly beneficial for the purpose of the thrust control using mass flow rate control of the oxidizer for a hybrid rocket engine with swirling injection of high-concentration hydrogen peroxide.
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.
Impacts of ammonia background flows on structural and photoluminescence properties of InN dots grown on GaN by flow-rate modulation epitaxy
(2006-12-25)
Ke, W. C.
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Lee, L.
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Chen, C. Y.
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Tsai, W. C.
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Chang, W. -H.
Structural and photoluminescence (PL) properties of InN dots grown on GaN by metal organic vapor phase epitaxy using the flow-rate modulation technique, and their dependence on growth conditions, were investigated. An ammonia (NH3) background flow was intentionally supplied during indium deposition periods to control the kinetics of adatoms and hence the morphology of InN dots. Samples prepared under lower NH3 background flows generally exhibit narrower and more intense PL signals peaked at lower emission energies. The authors point out that the NH3 background flow is an important parameter that controls not only the nucleation process but also the emission property of InN dots. (c) 2006 American Institute of Physics.
Optical investigations of InN nanodots capped by GaN at different temperatures
(2007-03-26)
Ku, C. S.
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Chou, W. C.
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Lee, M. C.
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電子物理學系
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Department of Electrophysics
InN nanodots capped with GaN for temperatures from 600 to 730 degrees C were investigated. While the dot emission intensity at 0.77 eV decreased with increasing capping temperature, two extra visible emission bands appeared around 2.37 eV (green band) and 2.96 eV (violet band). Furthermore, x ray diffraction shows that the 71.7 degrees and 70.2 degrees peaks were tentatively attributed to InGaN alloy with In fractions of 14.8% and 34.2%, respectively. Moreover, the near-field measurements helped reveal the regions of different emissions. The violet-band mapping showed a spatial distribution in contrast to nanodot distribution but the green band showed a uniform distribution that apparently reflects the capping induced InGaN alloy. (c) 2007 American Institute of Physics.
Optimal design of Ge-dot photoMOSFETs for highly-integrated monolithic Si Photonics
(2016)
Kuo, Ming-Hao
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Lee, M. C.
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Tien, J. W.
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Lai, Wei-Ting
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Li, Pei-Wen
Ge-dot/SiO2/SiGe-channel photoMOSFETs are demonstrated on a Si-photonics platform. The photoMOSFET is fabricated in standard CMOS processes with a self-organized gate stack of Ge-dot/SiO2/SiGe-channel using thermal oxidation of SiGe nanopillars over Si3N4 layers on Si substrates. The Ge-dot photoMOSFET, with 3 mu m channel length and 70 mu m channel width, features responsivity of over 2000A/W and 100 A/W with 6pW and 0.2 mu W, respectively, under illumination at 850nm. The responsivity is further improved by reducing the dot size and increasing the spatial density and most importantly, is insensitive to the gate oxide thickness, which is a great benefit for reducing the input capacitance and thus, improving speed and power consumption.
Pressure-induced metallization and resonant Raman scattering in Zn(1-x)Mn(x)Te
(2008-07-01)
Lin, Y. C.
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Fan, W. C.
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Chiu, C. H.
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Ke, F. K.
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Yang, S. L.
Pressure-induced resonant Raman scattering is adopted to analyze the zone-center optical phonon modes and crystal characteristics of Zn(1-x)Mn(x)Te (0 <= x <= 0.26) thin films. The pressure (Pt) at which the semiconducting undergoes a transition to the metallic phase declines as a function of Mn concentration (x) according to the formula Pt(x)=15.7-25.4x+19.0x(2) (GPa). Pressure-dependent longitudinal and transverse optical phonon frequencies and the calculated mode Gruneisen parameters were adopted to investigate the influence of Mn(2+) ions on the iconicity. The experimental results indicate that the manganese ions tend to increase the iconicity of ZnTe under ambient conditions, whereas an external hydrostatic pressure tends to reduce the iconicity and the bond length of Zn(1-x)Mn(x)Te. (c) 2008 American Institute of Physics.
Raman scattering of longitudinal-optical-phonon-plasmon coupling in Cl-doped ZnSe under high pressure
(2007-12-15)
Lin, Y. C.
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Chiu, C. H.
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Fan, W. C.
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Chia, C. H.
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Yang, S. L.
The vibrational, electronic, and crystalline properties of n-type chlorine-doped ZnSe (ZnSe:Cl) layers with a carrier concentration from 8.2x10(15) to 1.8x10(18) cm(-3) are studied by Raman spectroscopy. The spectral line shapes of the longitudinal-optical-phonon and plasmon coupling mode are analyzed using the Raman scattering efficiency and the dielectric function to obtain the electron densities and mobility. The splitting of the transverse-optical (TO) phonon and the redshift of the chlorine-related impurity vibration mode are clearly observed when pressure is applied. The semiconductor-to-metal phase transition pressure of ZnSe:Cl layers declines as the carrier concentration increases, indicating that n-type doping reduces crystal stability. Additionally, the pressure-induced weakening of the longitudinal-optical-phonon-plasmon coupling efficiency suggests that pressure tends to degrade the n-type characteristic of ZnSe:Cl because of the emergence of the new deep donorlike state. (c) 2007 American Institute of Physics.
The structural and optical properties of InN nanodots grown with various V/III ratios by metal-organic chemical vapor deposition
(2009-07-22)
Fu, S. F.
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Wang, S. M.
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Lee, L.
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Chen, C. Y.
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Tsai, W. C.
Self-assembled InN nanodots have been prepared at 650 degrees C with various V/III ratios from 500 to 30 000 by metal-organic chemical vapor deposition (MOCVD). It is found that the dot density and morphological size as well as the optical properties all display drastic changes at V/III = 12000. Generally, denser and smaller InN nanodots with higher emission energy and narrower linewidth were obtained when growth was conducted at V/III ratios slightly lower than 12 000 as compared to those at higher V/III ratios. The physical properties of our MOCVD-grown InN nanodots are sensitive to the surface structure and the morphology is very similar to molecular beam epitaxially grown GaN and InN films, which may be used as a guide to optimize the InN growth.
Vibration and Buckling Analysis of Internally Cracked Square Plates by the MLS-Ritz Approach
(2018-09-01)
Huang, C. S.
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Lee, M. C.
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Chang, M. J.
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土木工程學系
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Department of Civil Engineering
Based on the classical thin plate theory, this paper proposes new sets of enriched basis functions for in-plane and out-of-plane displacements of square plates that can yield admissible functions for the Ritz method using the moving least-squares (MLS) approach. These admissible functions display the discontinuities of displacement and slope across a crack; give the correct singularity order for the stress resultants at the crack tips; and enhance the Ritz method's ability to recognize the existence of an internal crack in a plate. To confirm the validity of the proposed approach, convergence studies were performed on the buckling loads and vibration frequencies of plates with central horizontal cracks, and the results obtained agree closely with the published ones as well as those generated by the commercial finite element software. To demonstrate the importance of including all the in-plane stress resultant components in the analysis, the effects of different in-plane stress resultant components on the buckling loads and vibration frequencies of simply supported center-cracked square plates under uniaxial uniform loading were investigated. The present approach was further employed to study the effects of cracks' lengths, orientations, and locations on the buckling loads and frequencies of cracked square plates under different boundary conditions and in-plane loading conditions.