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The Study on Strain Distributions and Equilibrium Morphologies of Anisotropic Quantum Dots

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Tutor: ZhouWangMin
School: Zhejiang University of Technology
Course: Solid Mechanics
Keywords: quantum dots,anisotropy,equilibrium morphology,strain distribution,elastic relax
CLC: O471.1
Type: Master's thesis
Year:  2011
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Semiconductor quantum dots have attracted considerable theoretical and experimental researches due to its excellent special optical properties and great potential prospect in today¡¯s material fields. Because of the strong 3-demensional confinement, these nano-scale quantum dots show some excellent characteristics over bulk materials. In order to obtain quantum dots devices with excellent optical properties, the researchers conduct a theoretical and experimental study on internal and external factors that are in connection with the characteristics of quantum dots. In the process of self-organized growth, some physical parameters such as anisotropy ratio, thermal expansion coefficients, lattice constants and crystal symmetry are essential for the analyses of nucleation sites, distribution arrangement, equilibrium morphologies and strain distributions of quantum dots. Consequently, investigation of relationship between various physical parameters and the heteroepitaxial quantum dots system has become a scientific problem which is needed to be solved imperatively.Currently, the theory about which the anisotropy are relevant to heteroepitaxial equilibrium morphologies and strain distributions is not yet a complete one, in this article we study systematically the heteroepitaxial quantum dots based on finite element method of anisotropic theory of elasticity. The results are summarized as follows:1. Based on elasticity theory and finite element method, distribution curves of the stress and strain respect to different anisotropies are investigated in isolated pyramidal quantum dots. The results show that the strain and stress mainly occurred at or near the interface, within the quantum dots from bottom to top, gradually become smaller. The stress and strain with different anisotropies are similar in distribution feature, and are different. All of these strain values will increase correspondingly with increasing of anisotropy.2. The free energy of quantum dots with different anisotropies are calculated, and the functional relationship between anisotropy and equilibrium morphologies are obtained. The strain relaxation will decline with increasing of anisotropy, equilibrium aspect ratio will increase with increasing of anisotropy, and then decreases. While A =1, isotropic, equilibrium aspect ratio reaches a maximum. 3. Elastic relaxation, critical volume and energy barrier with the transition of growth mode under the influence of different anisotropies are investigated based on the thermodynamic equilibrium theory and energetics. With the same aspect ratio, the greater the anisotropy, the smaller the strain energy relaxation. Besides, the relaxation factors will decline with increasing of aspect ratio, however, the strain energy relaxation of quantum dot can be greater than the wetting layer. The critical volume of nucleation, critical energy barrier and critical transition volume will increase with increasing of anisotropy.
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