摘要
半导体量子点以其特有的光电特性和极具潜力的应用前景,成为半导体领域的热点课题。纳米尺度的量子点结构对内部载流子具有较强的三维限制作用,表现出明显优于大块体材料的光学特性。为了得到具有优异光电特性的量子点器件,研究人员对影响量子点特性的内外因素进行了深入研究,分别从理论和实验两个方面进行探讨。在自组织生长过程中,各向异性值、热膨胀系数、晶格常数以及晶体对称性等物理参数都会对量子点的成核位置、分布排列、平衡形态以及应变分布产生重大影响。因此,研究各物理参数对异质外延量子点体系的影响便成为迫切需要解决的科学问题。
目前,关于各向异性与自组织量子点的平衡形态及应变分布等的关系还没有系统性的定量研究。本文基于各向异性弹性理论的有限元方法对异质外延量子点进行了系统的研究,主要工作和成果如下:
1.基于弹性理论和有限单元法研究了不同各向异性值孤立金字塔形量子点的应力、应变分布。研究表明,应变与应力主要集中在界面附近,在量子点内部由下往上,逐渐变小。量子点各向异性系数不同时,量子点内的应力、应变分布特征大体相同,数值略有差异。所有这些应变都会随着各向异性的增大而相应的增大。
2.计算了不同各向异性值量子点的相关能量,得到了各向异性与平衡形态的函数关系。结果表明,应变弛豫能随着各向异性系数的增大而减小,量子点平衡高宽比随着各向异性数值的增大呈现先增大后减小的分布特征。当A =1时,即各向同性材料时,平衡高宽比数值最大。
3.基于异质外延体系的热力学平衡理论和能量学理论研究了不同各向异性系数时量子点及外延体系的弹性弛豫、生长模式转变过程中临界体积和临界成核功。在相同高宽比下,各向异性系数愈大,应变弛豫能愈小,即愈不利于岛状生长。另外,量子点和外延体系的弛豫因子都随着高宽比的增加都不断降低,但量子点弛豫的应变能大于浸润层弛豫的应变能。临界成核体积、成核功以及临界转变体积都随着各向异性系数的增大而增大。
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.
引文
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