Browsing by Author "Lee, J"

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Analysis of size distributions of type IIZnTe/ZnSe quantum dots
(2004-12-01)
Lee, J
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Yang, CS
;
Chang, CT
;
Liu, JR
;
Chou, WC
The photoluminescence (PL) spectrum of the ZnTe/ZnSe quantum dot (QD) structure which has a type II band alignment. was investigated. A broader structure peaking at 2.180 eV together with the features of the ZnSe buffer layer located at 2.820 eV in the PL spectrum at 10 K were observed. The broadness of the PL attributes to the spatial inhomogeneity of the QD sizes. The PL spectrum was a normal distribution with a variance of 0.002 eV(2). However, the dot size distribution was a deformed Gaussian. Using only one set of data of dot size distribution measured by the Atomic Force Microscopy (AFM), the statistical estimates with standard errors were evaluated using the bootstrap methodology. The probability distribution was inferred by calculating the means, percentiles and correlation coefficients of the base diameter and the height of the QDs. We concluded that the dot distributions were indeed a deformed Gaussian and the correlation coefficient of the diameter and the height of the QDs was 0.49 +/- 0.01. The uniformity of the dot size distribution was poor. To infer the dot size distribution from the PL spectrum, we numerically solved the Schrodinger equation by elaborating on the orthogonal periodic functions (OPF) approximation for the type II lens shaped QDs. The convergence of the probability density was discussed in a great detail. We found that the band edge discontinuities inside the dot acts as a barrier with DeltaE(v) = 315 meV for the conduction band and as a well with DeltaE(v) = 735 meV for the valence band. In the AFM measurement, the offset in dot height is 14 Angstrom and the aspect ratio is 0.04. We employed the Schrodinger equation to correlated the PL spectrum (intensity versus optical energy) to the AFM data (number of QDs versus dot size) and the agreement was excellent. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Bayes inference for technological substitution data with data-based transformation
(1997-03-01)
Kuo, L
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Lee, J
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Cheng, P
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Pai, J
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統計學研究所
Bayesian inference via Gibbs sampling is studied for forecasting technological substitutions. The Box-Cox transformation is applied to the time series AR(I) data to enhance the linear model fit. We compute Bayes point and interval estimates for each of the parameters from the Gibbs sampler. The unknown parameters are the regression coefficients, the power in the Box-Cox transformation, the serial correlation coefficient, and the variance of the disturbance terms. In addition, we forecast the future technological substitution rate and its interval. Model validation and model choice issues are also addressed. Two numerical examples with real data sets are given.
Formation of self-assembled ZnTe quantum dots on ZnSe buffer layer grown on GaAs substrate by molecular beam epitaxy
(2002-07-01)
Kuo, MC
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Yang, CS
;
Tseng, PY
;
Lee, J
;
Shen, JL
Self-assembled ZnTe quantum dot structures were grown by molecular beam epitaxy on GaAs substrates with a 200 nm ZnSe buffer layer. Surface morphology was studied by atomic force microscopy. A three-dimensional Volmer-Weber growth mode was identified. Two types of dots were observed. Strong photoluminescence observed at 1.9-2.2 eV was attributed to emission from the large type II ZnTe quantum dots. Emission from the smaller ZnTe quantum dots was observed at an energy of around 2.26 eV. The density of the larger and smaller dots was approximately 10(8)/cm(2) and 10(9)/cm(2), respectively. (C) 2002 Elsevier Science B.V. All rights reserved.
Industrial noise source identification by using an acoustic beamforming system
(1998-04-01)
Bai, MR
;
Lee, J
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機械工程學系
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Department of Mechanical Engineering
A noise source identification technique is proposed for industrial applications by using a microphone array and beamforming algorithms. Both of the directions and the distances of long-range noise sources are calculated. The conventional method the minimum variance (MV) method and the multiple signal classification (MUSIC) method are the main beamforming algorithms employed in this study. The results of numerical simulations and field tests indicate the effectiveness of the acoustic beamformer in identifying noise sources in industrial environments.
Optical absorption coefficient of quantum wires in strain and electric fields with intermixing interfaces
(2005-09-01)
Lee, J
;
Spector, HN
;
Chou, WC
;
Huang, YS
;
電子物理學系
We have theoretically studied the optical absorption coefficient of quantum wires in electric and strain fields with intermixing interfaces. The potential profiles are governed by the intermixing heterojunctions, internal strain due to the lattice mismatch, the external electric field and stress. The second Fick's law describes the wires' intermixing heterojunctions due to the alloy interdiffusion. We adopt the Green's function method to solve the Poisson equation for the displacement to determine the internal strain due to the lattice mismatch. The single-band Schrodinger equation in the effective mass approximation is used to describe the conduction subband structures while the four-band Kohn-Luttinger Hamiltonian is used to describe the valence subband structures. In solving the Schrodinger equation and the Kohn-Luttinger Hamiltonian, we expand the wave functions for the electrons and holes by using linear combinations of the two-dimensional harmonic oscillator wave functions to yield two matrix equations. These matrix equations are numerically solved for their eigen-energies and their corresponding eigenfunctions. We investigate some physical properties of the unstrained quantum wire (GaAs/AlAs) and the strained wire (CdSe/ZnSe) as examples. When the interdiffusion becomes stronger, on one hand, the internal strain is relaxed leading to the band gap shrinkage (if the initial strain epsilon(0)< 0) and, on the other hand, the potential profiles are deformed and the effective band gap becomes wider. Variations of the transition energies with external stress show the anticrossing effect and variations of the transition energies with external electric field show the field emission effect. The hole effective masses can be enhanced or become electronlike by applying stress to the wire. The oscillator strengths of the dipole-allowed intersubband transitions for the y and z polarizations are calculated to infer possible optical transitions. The optical absorption coefficients together with the joint density of states are calculated.
Optical properties of self-assembled ZnTe quantum dots grown by molecular-beam epitaxy
(2005-02-01)
Yang, CS
;
Lai, YJ
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Chou, WC
;
Chen, WK
;
Lee, MC
The morphology and the size-dependent photoluminescence (PL) spectra of the type-II ZnTe quantum dots (QDs) grown in a ZnSe matrix were obtained. The coverage of ZnTe varied from 2.5 to 3.5 monolayers (MLs). The PL peak energy decreased as the dot size increased. Excitation power and temperature-dependent PL spectra are used to characterize the optical properties of the ZnTe quantum dots. For 2.5- and 3.0-ML samples, the PL peak energy decreased monotonically as the temperature increased. However, for the 3.5-ML sample, the PL peak energy was initially blueshifted and then redshifted as the temperature increased above 40 K. Carrier thermalization and carrier transfer between QDs are used to explain the experimental data. A model of temperature-dependent linewidth broadening is employed to fit the high-temperature data. The activation energy, which was found by the simple PL intensity quenching model, of the 2.5, 3.0, and 3.5 MLs were determined to be 6.35, 9.40, and 18.87 meV, respectively. (C) 2005 American Institute of Physics.
Self-consistent calculation for energy band profiles and energy levels of cubic quantum dots
(2005-11-01)
Lee, J
;
Spector, HN
;
Chou, WC
;
電子物理學系
;
Department of Electrophysics
We have performed a self-consistent calculation for the energy band profiles and energy levels of cubic quantum dots, solving both the Schrodinger and Poisson's equations. In particular, we examined the effect of doping on these levels both in the presence and absence of a positive test charge. We found that the number of levels the cubic dot supported depended on the size of the dot, and that the energy of the levels decreased in the presence of a positive test charge. Moreover, we found that the energy levels of both the singlet and triplet states in the dot increased with the doping density. The quasi-Fermi level was found to be lower in the presence of a positive test charge than in its absence, and the quasi-Fermi level increased sharply with increasing doping density both in the absence and presence of the test charge. With very high doping the Fermi level is located in the barrier while with low doping the Fermi level is located down below the ground state energy which implies that the probability of finding an electron in a dot is very small. We also find that at high doping levels the lowest energy states are no longer confined in the dot. The potential energy profile was found to be dramatically affected by both the doping and the presence of a positive test charge. The profile was found to become asymmetric when the test charge was displaced from the center of the dot.