On the Existence Condition for Paired Stationary Bright Solitons Obtained by Using the Paraxial-Ray Approximation Method in Coupled Photovoltaic Photorefractive Crystal Circuit Systems

홍우표 한국물리학회 2008 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.53 No.6

We report the existence of new families of stable stationary paired solitons in the coupled pho- tovoltaic photorefractive crystal circuit system. By applying the paraxial-ray approximation, we obtain the relation betwen the width and the peak amplitude of the stationary paired soliton in terms of the system parameters. We verify the analytical results by direct numerical simulations for the LiNbO3 crystal as a photovoltaic photorefractive medium to show the stability of the paired solitons. The stability of the bright paired solitons for different crystal temperatures has ben investigated. We conclude that the existence condition obtained by using the paraxial-ray approximation method may serve as a more reliable initial condition for understanding coupled photovoltaic photorefractive circuit systems.

Electronic Properties of Pyramidal CdTe/ZnTe Quantum Dots and Comparisons with Experiment

홍우표,Seoung-Hwan Park 한국물리학회 2009 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.55 No.6

The electronic properties of pyramidal CdTe/ZnTe quantum dots were studied using an eightband strain-dependent k·p Hamiltonian. A model based on the finite element method and the theory of elasticity of solids were used to show that the temperature-dependent photoluminescence measurement of the interband transition energies from the ground electronic subband to the ground heavy-hole subband reasonably explains the experimental data. We find that the interband transition energy decreases with increasing wetting layer thickness, regardless of the temperature of the QD, while the interband transition energies decrease with increasing temperature. We conclude that the wetting layer is an important structural element for predicting the electronic properties of CdTe/ZnTe QDs. The electronic properties of pyramidal CdTe/ZnTe quantum dots were studied using an eightband strain-dependent k·p Hamiltonian. A model based on the finite element method and the theory of elasticity of solids were used to show that the temperature-dependent photoluminescence measurement of the interband transition energies from the ground electronic subband to the ground heavy-hole subband reasonably explains the experimental data. We find that the interband transition energy decreases with increasing wetting layer thickness, regardless of the temperature of the QD, while the interband transition energies decrease with increasing temperature. We conclude that the wetting layer is an important structural element for predicting the electronic properties of CdTe/ZnTe QDs.

Finite Element Analysis of Strain Effects on Cuboidal CdTe/ZnTe Quantum Dots by Using the Eight-Band Strained k· p Hamiltonian

홍우표 한국물리학회 2008 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.53 No.6

The strain effects on the conduction and the valence subband energy structures of cuboidal CdTe/ZnTequantumdots(QDs)are analyzed by using a three dimensional finite-elementmethod formulation with the eight-band strained Lutinger-Kohn Hamiltonian. Both the conduction and the valance subband energies of the strained QD are shown to decrease more slowly than those of the unstrained cases with increasing QD size. For both the conduction and the valence subbands, the energy difference between the first and the second excited states diminishes with increasing QD size. We demonstrate that the combined effect of the strain and the size of QD can be sensitive to the transition energy between the conduction and the valence subbands.

Effects of the Capping and the Wetting Layers on the Electronic Properties of Self-assembled Pyramidal CdTe/ZnTe Quantum Dots

홍우표,박승환 한국물리학회 2009 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.55 No.4

The electronic properties of self-assembled pyramidal CdTe/ZnTe quantum dots (QDs) are inves- tigated as functions of the capping and the wetting layer thicknesses. The three-dimensional strain fields induced around the QD in the material matrix are analyzed based on the linear elasticity theory of solids for varying thicknesses of the capping and the wetting layers. The normal strains in the direction of the QD apex is shown to be continuous across the wetting layers for the CdTe/ZnTe QD system, in contrast with those of the InAs/GaAs QDs, which show discontinuities across the wetting layer due to the differences in the material parameters such as Poisson's ratio and Young's modulus of CdTe/ZnTe QD at the interface between the wetting layer and the substrate. The effect of the strain fields on the conduction and the valence subband energies are numerically investigated based on the eight-band kㆍp Hamiltonian. For a fixed capping layer thickness, the transition energy between the ground states in the conduction and in the valance subbands are found to decrease with increasing wetting layer thickness. On other hand, the transition energy shifts upward as the capping layer thickness increases, regardless of the thickness of the wetting layer. We ¯nd the transition energy difference between two capping layer thicknesses above 250 ºA is on the order of 10 meV. We conclude that proper size controll of the capping and the wetting layers for CdTe/ZnTe QDs is required to understand the electronic and optical properties of the QDs. The electronic properties of self-assembled pyramidal CdTe/ZnTe quantum dots (QDs) are inves- tigated as functions of the capping and the wetting layer thicknesses. The three-dimensional strain fields induced around the QD in the material matrix are analyzed based on the linear elasticity theory of solids for varying thicknesses of the capping and the wetting layers. The normal strains in the direction of the QD apex is shown to be continuous across the wetting layers for the CdTe/ZnTe QD system, in contrast with those of the InAs/GaAs QDs, which show discontinuities across the wetting layer due to the differences in the material parameters such as Poisson's ratio and Young's modulus of CdTe/ZnTe QD at the interface between the wetting layer and the substrate. The effect of the strain fields on the conduction and the valence subband energies are numerically investigated based on the eight-band kㆍp Hamiltonian. For a fixed capping layer thickness, the transition energy between the ground states in the conduction and in the valance subbands are found to decrease with increasing wetting layer thickness. On other hand, the transition energy shifts upward as the capping layer thickness increases, regardless of the thickness of the wetting layer. We ¯nd the transition energy difference between two capping layer thicknesses above 250 ºA is on the order of 10 meV. We conclude that proper size controll of the capping and the wetting layers for CdTe/ZnTe QDs is required to understand the electronic and optical properties of the QDs.

Dynamics of Superposed Vortex Waves in Strongly Nonlocal Nonlinear Medium

홍우표 한국물리학회 2010 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.57 No.5

We find analytical vortex wave solutions in a strongly nonlocal thermal nonlinear medium with square boundaries. We investigate the dynamics of superposed vortex waves with different topological charges by propagating them for a long distance. We show that the presence of a linear term in the proposed model, which otherwise is the model for the accessible soliton model, results in the formations of various transformation patterns and stable structures during the unstable superposed vortex waves propagation. Some previously found stable solitons, such as the necklace vortex soliton and the Laguerre-Gaussian solitons, were found, and new stable structures, such as the quadrupole soliton and the “petal”-shaped soliton, were shown to form during unstable superposed vortex waves propagation.

절단된 피라미드형 GaN/AlN 반도체 양자점의 크기에 따른 내부 전기 퍼텐셜 분포

홍우표,김종재 한국물리학회 2010 새물리 Vol.60 No.10

The electric potential as functions of the volume of truncated GaN/AlN semiconductor quantum dots(QD) are investigated using the theory of continuum elasticity based on a three-dimensional finite element method. The total dependence of the electric potential contributed by the strain-induced piezoelectric and the spontaneous potentials is found to rapidly decreases with decreasing QD volume. The total electric potential dependence on the QD volume leads to a huge change in the hydrostatic strain component, which is closely related to the energy band gap. We conclude that the effect of the QD volume effects on the electrical and the optical properties of the QD energy band, has to be considered in designing a QD system. 절단된 피라미드형의 GaN/AlN 반도체 양자점에 대해, 탄성체의 연속체이론과 3차원 유한요소 방법을 이용한 수치계산을 통하여, 자발 분극과압전 퍼텐셜 및 내부 퍼텐셜의 크기와 공간 분포의 변화 양상을 양자점의체적 변화에 따른 의존성을 조사하였다. 양자점의 분극 및 압전 퍼텐셜은양자점의 크기에 민감하게 반응하며, 양자점의 체적이 감소함에 따라양자점 내부의 전체 포텐셜은 현저하게 감소함을 확인하였다. 양자점내부 퍼텐셜의 증가는 양자점의 여러 변형 성분들에 다양한 효과를초래하며 양자점의 에너지 밴드갭에 중대한 영향을 미치는 물정역학적변형 성분의 값과 모양에 영향을 미치게 된다. 양자점의 크기 변화에따른 효과는, 양자점 시스템 설계에 있어서 중요한 전기적 및 광학적특성에 많은 영향을 미치기 때문에, 구체적인 양자점의 에너지 밴드구조계산에 필수적으로 고려되어야 할 요소가 될 것이다.

입방형 GaAs 양자점에 인가된 스트레인 효과 연구

홍우표,박승환,양정민 한국물리학회 2007 새물리 Vol.55 No.5

We investigate the strain effects on the valence energy band structures of cuboidal GaAs quantum dots (QDs) modeled by using a six-band Luttinger-Kohn Hamiltonian and a calculation procedure based on a finite-element method (FEM). We show that the combined effect of the strain and the size of the QD can be very sensitive to the energy structure of the valence band. In the case of the initial tensile misfit strain, the lowest energy of the strained QD is larger than that of the nonstrained QD in the range of relatively small QD size. However, we found that as the QD size increased further, the difference between the two energies gradually decreased. We show that depending on the nature of the strain, i.e., a compressive or tensile strain, the valence band energy can vary from a negative to a positive value. 6-밴드 루틴저-콘 (Luttinger-Kohn) 해밀토니안으로 모델링 될 수 있는입방형 GaAs 양자점의 원자가띠 (valence band)의 에너지 구조를유한요소법을 적용하여 수치적으로 구하고, 스트레인에 의한 양자점원자가띠의 에너지 구조에 대한 효과를 조사하였다. 양자점의 크기와스트레인이 결합된 효과는 에너지띠 구조에 매우 민감하게 작용함을 알수 있었다. 양자점의 초기 인장 스트레인 차가 있는 경우, 가장 낮은원자가띠의 에너지가 스트레인이 없는 양자점 경우보다 크다는 것을밝혔다. 하지만, 양자점의 크기가 더욱 커지는 경우, 스트레인 효과에의한 에너지 차가 점차 줄게 됨을 확인할 수 있었다. 또한, 양자점에가해진 스트레인의 성격에 따라 , 즉 압축력 또는 장력, 원자가 띠의에너지가 음의 값에서 양의 값으로 바뀐다는 것을 알 수 있었다.

Structural Effects on the Electronic Properties of Vertically-coupled CdTe/ZnTe Quantum-dot Molecules

홍우표,박승환,Mark R. A. Shegelski 한국물리학회 2012 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.61 No.12

We study the effects of structure parameters such as the vertical distance between two quantum dots (QDs) and the size of QDs on the electronic properties of vertically separated self-assembled CdTe/ZnTe quantum-dot molecules (QDMs) by using a finite-element method based on the elasticity theory of solids and the effective mass Hamiltonian. A QD-size-dependent switching between the ground and the first excited electron states is shown to occur as the vertical distance increases, even under the assumption of symmetric QDMs, due to a strain induced deformation of the conduction-band edge. A more complex valence-band-edge deformation causes the bonding and the anti-bonding states of the hole wave functions to switch with each other with increasing vertical distance between the QDs. We explicitly identify why the switching can occur in CdTe/ZnTe QDMs by analyzing the strain-induced conduction and valence-band-edge deformations.

Preparation of SiOxNy Films by Facing-Target Sputtering System for Thin-Film Passivation Layers of OLEDs

홍우표,김화민 한국물리학회 2008 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.53 No.3

A facing-target sputtering (FTS) system has been fabricated and its process characteristics have been investigated to search for the possibility of applications to thin-lm passivation systems for organic light emitting (OLEDs). The FTS system is able to prepare a high-quality silicon nitride SiNx lm with a dense microstructure and an excellent uniformity with less than a 5 % variation. In addition, a low-temperature deposition is also possible because the increase in the temperature of the substrate can be restrained due to the bombardment of high-energy particles on the target. Thus, we conclude that the FTS system is a suitable deposition technique for the thin-film passivation layers of OLEDs and the gas barrier layer of a polymer substrate.

실수 및 허수 라만항이 포함된 고차수 비선형 슈뢰딩거 방정식에서의 광솔리톤 존재 조건 및 동역학적 특성 연구

홍우표,김종재 한국물리학회 2007 새물리 Vol.55 No.5

We investigate the dynamics of the bright solitary waves of the higher-order nonlinear Schr\"{o}dinger equation (HONLSE) with both real and imaginary Raman terms, which can model an ultrashort pulse propagating through optical fibers. The analytic bright solitary-wave solutions are found under a constraint on the model coefficients, from which the physical parameters, such as the wavelength needed to launch the pulse, the types of optical fibers, and the required peak power, are obtained. Using the bright solitary waves as the initial profiles for numerical simulations, we show that dynamically stable or marginally stable pulse propagations can be achieved for certain ranges of the wavelengths. 본 논문에서는 광섬유에서 극초단 펄스의 형태로 존재할 수 있는 밝은 광솔리톤 (optical soliton)의 거동을 기술할 수 있는 모델로 실수 및 허수 라만 (Raman)항을 포함하는 고차수 비선형 슈뢰딩거 방정식을 연구하고 광솔리톤의 동역학적 특성을 이해하고자 한다. 모델의 상호 계수들의 관계식을 만족하는 분석적 밝은 광솔리톤 해가 존재할 수 있는 물리적인 조건들인 파장, 필요한 광섬유의 종류, 그리고 광솔리톤을 발생할 수 있는 초기전력 값을 구했다. 이런 초기조건들을 사용하고 수치시늉연구를 통하여 동역학적으로 안정된 솔리톤의 전파가 가능한 파장대를 구했다.