生长方向对量子点应变与应变弛豫的影响

由于材料弹性的各向异性与表面能的各向异性, 不同的生长方向或生长面, 量子点有不同的力学性能与行为. 本文基于各向异性弹性理论的有限元方法, 以金字塔型自组织InAs/GaAs半导体量子点为研究对象, 分别在7个常见的生长方向或生长面上, 对其应变能和应变弛豫能、自由能等进行了分析计算, 得到了这些能量随生长方向的变化规律. 结果表明(211)量子点应变弛豫能最大, 而(100)量子点应变弛豫能最小. 这些结果可为可控制备量子点提供理论参考. 关键词： 量子点 生长方向 平衡形态 应变弛豫     

有限元法分析不同形状量子点的应变能及弛豫度变化

利用有限元方法研究了不同形状量子点的应变能量分布和弛豫度随着高宽比变化的规律.分析了量子点间距和量子点形状对量子点应变弛豫的影响,定量地讨论了量子点的弛豫度与量子点形状之间的关系.计算结果表明,在不考虑表面能的情况下,当量子点高宽比增加时,弛豫度上升,并且发现平顶金字塔形量子点最先达到稳定;岛间距增大时,量子点内应变能下降,其中立方体形量子点应变能下降最快.研究表明,量子点的弛豫度可以成为控制量子点成岛形状的重要依据. 关键词： 量子点 弛豫度     

埋置量子点应力分布的有限元分析

通过衬底材料和外延材料的交替生长方式制备出多层排列的自组装量子点超晶格结构.这些埋置量子点的应力/应变场影响着它们的光电性能、压电性能以及力学稳定性.基于各向异性弹性理论的有限元方法,研究了埋置金字塔形应变自组织Ge/Si半导体量子点的应力/应变分布以及流体静应变和双轴应变分布,并与非埋置量子点的应力/应变分布做了比较,指出了它们之间的异同以及覆盖层对量子点应力/应变分布的影响. 关键词： 量子点 应力分布 应变分布     

低维半导体材料应变分布

在各向同性弹性理论的假设下，探讨了理想简单化的二维、一维与零维半导体材料量子阱、量子线与量子点的应力和应变分布规律，并讨论了它们应力、应变与应变能密度分布之间的差异.结果有助于定性理解更复杂形状结构的低维半导体材料的应力、应变及应变能分布. 关键词： 低维材料 应变分布 量子阱 量子线 量子点     

InAs/GaAs透镜形量子点超晶格材料的纵向和横向周期对应变场分布的影响

对量子点超晶格材料中量子点纵向周期和同层量子点的横向周期间距对量子点及其周围应变场分布的影响进行了系统的研究.结果表明，横向和纵向周期通过衬底材料之间的长程相互作用对量子点沿中心轴路径应变分布的影响效果正好相反，在适当条件下，两者对量子点应变场分布的影响可以部分抵消.同时也论证了在单层量子点和超晶格量子点材料中，计算量子点的电子结构时，应综合考虑量子点空间周期分布对载流子限制势的影响，不能简单的利用孤立量子点模型来代替. 关键词： 应变 半导体量子点 自组织     

用格林函数法计算量子点中的应变分布

自组装量子点材料作为一种新型的光电材料无论在理论和实际应用都成为当今物理学界的研 究热点.由GaAs包围的InAs小岛，由于较大的晶格失配（≈-0.067），应变效应在量子点 的 形成过程中起主导作用.大部分计算量子点结构应变分布的方法都是基于数值解法，需要大 量的计算工作.给出用格林函数法推导各种常见形状量子点应变分布的解析表达式详细过程，讨论了弹性各向异性和形状各向异性对量子点应变分布的影响程度.结果表明对于不 同形状量子点结构中主要部分的应变分布都是相似的，流体静压变部分的特征值随量子点形状的变化不 关键词： 自组装量子点 格林函数 应变分布     

Si组分对SiGe量子点形状演化的影响

研究了自组织生长SiGe岛（量子点）中Si组分对形状演化的影响.采用UHV/CVD方法生长了 不同Si组分的SiGe岛，用AFM对其形状和尺寸分布进行了分析，实验结果表明SiGe岛从金字 塔形向圆顶形转变的临界体积随Si组分的增大而增大.通过对量子点能量的应变能项进行修正，解释了量子点中Si组分对形状演化的影响.在特定的工艺条件下得到了单模尺寸分布的 金字塔和圆顶形量子点.结果表明，通过调节SiGe岛中的Si组分，可以实现对SiGe岛形状和 尺寸的控制. 关键词： 自组织生长SiGe岛 Si组分 临界体积     

p型掺杂1.3μm InAs/GaAs量子点激光器的最大模式增益特性的研究

对p型掺杂13 μm InAs/GaAs量子点激光器的最大模式增益进行了实验和理论分析.实验上,测量了不同腔长激光器阈值电流密度与总损耗的对应关系,拟合出的最大模式增益为175 cm^-1,与相同结构非掺杂量子点激光器的最大模式增益一致.同时理论分析表明,p型掺杂对InAs/GaAs量子点激光器的最大模式增益并无影响,并且最大模式增益的计算结果与实验值相符.具有较小高度或高宽比的量子点能达到更高的最大模式增益,而较高的最大模式增益对p型掺杂13 μm InAs/GaAs自组织量子点激光器在光通信系统中的应用具有重要意义. 关键词： 最大模式增益 p型掺杂 InAs/GaAs量子点激光器     

InAs/GaAs量子点的静压光谱压力系数研究

采用有效质量模型和非线性弹性理论计算了不同尺寸InAs/GaAs量子点的静压光谱发光峰的 压力系数(PC).量子点峰位随压力的变化主要来自禁带宽度和电子束缚能随压力变化两方面 的贡献.由于InAs/GaAs量子点是一个应变体系，体系的晶格常数，失配应变和弹性系数均随 外加压力变化，使得加压后量子点的禁带宽度相对于非应变体系略有减小，同时势垒高度增 加，电子束缚程度增加.两者共同作用引起的InAs应变层的禁带宽度压力系数减小是导致量 子点的压力系数小于InAs体材料的主要原因.同时计算结果表明，电子束缚能随压力变化对 不同尺寸量子点的压力系数的影响不同，量子点尺寸越小，受其影响越大，压力系数也越大 . 关键词： 量子点 压力系数 应变     

隔离层厚度和盖层厚度对InAs/GaAs量子点应变分布和发射波长的影响

分析了量子点盖层生长过程中隔层厚度对应变分布的影响,指出隔层材料的纵向晶格常数与量子点材料的纵向晶格常数对应变分布具有重要意义.定性说明了应变因素在隔离层生长过程中对量子点高度塌陷产生的影响.讨论了当隔离层顶面与量子点高度持平后,增加盖层厚度对应变分布的影响.基于变形势理论,讨论了上述几何参数的变化对发光波长的影响,并与实验结果进行了对比.结果表明,在量子点加盖过程中,应变因素对其形貌和发光特性具有重要作用,以应变工程为基础的发射波长调控是拓展量子点波长发射范围的有效途径. 关键词： 应变工程 半导体量子点 隔离层 盖层     

The equilibrium composition in GexSi1-x/Si self-assembled alloy quantum dot

<正>The equilibrium composition in strained quantum dot is the result of both elastic relaxation and chemical mixing effects,which have a direct relationship to the optical and electronic properties of the quantum-dot-based device.Using the method of moving asymptotes and finite element tools,an efficient technique has been developed to compute the composition profile by minimising the Gibbs free energy in self-assembled alloy quantum dot.In this paper,the composition of dome-shaped Ge_xSi_(1-x)/Si quantum dot is optimized,and the contribution of the different energy to equilibrium composition is discussed.The effect of composition on the critical size for shape transition of pyramid-shaped GeSi quantum dot is also studied.     

The strain relaxation of InAs/GaAs self-organized quantum dot

This paper presents a detailed analysis of the dependence of degree of strain relaxation of the self-organized InAs/GaAs quantum dot on the geometrical parameters. Differently shaped quantum dots arranged with different transverse periods are simulated in this analysis. It investigates the total residual strain energy that stored in the quantum dot and the substrate for all kinds of quantum dots with the same volume, as well as the dependence on both the aspect ratio and transverse period. The calculated results show that when the transverse period is larger than two times the base of the quantum dots, the influence of transverse periods can be ignored. The larger aspect ratio will lead more efficient strain relaxation. The larger angle between the faces and the substrate will lead more efficient strain relaxation. The obtained results can help to understand the shape transition mechanism during the epitaxial growth from the viewpoint of energy, because the strain relaxation is the main driving force of the quantum dot's self-organization.     

Morphologies of epitaxial islands on a lattice-misfitted substrate

Under certain growth conditions for systems with a film/substrate lattice misfit, the deposited material is known to aggregate into island-like shapes. We have obtained an analytical expression of the total free energy, which consists of strain energy, surface energy and interfacial energy of a coherent island/substrate system, and the change of equilibrium aspect ratio versus the volume of the island and the misfit of lattices in the system, which provides a broad perspective on island behaviour. These then were used to study the equilibrium shapes of the system. The results show that in order to minimize the total free energy, a coherent island will have a particular height-to-width aspect ratio, called equilibrium aspect ratio, that is a function of the island volume and misfit. The aspect ratio is increased with increasing island volume at a fixed misfit, and with increasing misfit strain between the island and substrate at a fixed island volume. Moreover, the effect of misfit dislocation on the equilibrium shape of the island is also examined. The results obtained are in good agreement with experiment of observations and thus can serve as a basis for interpreting the experiments.     

Equilibrium morphologies of epitaxially strained islands

An analytical expression of the free energy consisting of the strain energy, surface energy and interfacial energy for the coherent island/substrate system, as well as the evolving relations of aspect ratio against volume of the island, and misfit of the system, which provides a broad perspective on island behavior, is obtained, and used to study the equilibrium shapes of the systems. A two-dimensional model assuming linear elastic behavior is used to analyze an isolated island with elastic properties similar to those of the substrate. The results show that in order to minimize the total free energy, a coherent island will adopt a particular shape and height-to-width aspect ratio that are a function of only the island volume. The effect of a misfit dislocation on the equilibrium shape of an island is in passing examined. These can serve as a basis for interpretation of experiments.     

Fabrication of quantum dot transistors incorporating a single self-assembled quantum dot

We report on the fabrication and the characterization of quantum dot transistors incorporating a single self-assembled quantum dot. The current–voltage characteristics exhibit clear staircase structures at room temperature. They are attributed to electron tunneling through the quantized energy levels of a single quantum dot.     

Influence of strain on the magneto-exciton in single and coupled InP/GaInP quantum disks

We investigated theoretically the influence of strain on the exciton in both single and three vertically coupled self-assembled quantum dot systems in the presence of a perpendicular magnetic field. For the single disk, we find that the heavy-hole exciton is the ground state, while for the system of three stacked disks, the light hole state was found to be lower in energy. Results for the diamagnetic shift were compared with experimental results.     

Integral and local density of states of InAs quantum dots in GaAs/AlGaAs heterostructure observed by ballistic electron emission spectroscopy near one-electron ground state

Density of states is studied by a ballistic electron emission microscopy/spectroscopy on self-assembled InAs quantum dots embedded in GaAs/AlGaAs heterostructure prepared by metal–organic vapor phase epitaxy. An example of integral quantum dot density of states which is proportional to superposition of a derivative of ballistic current–voltage characteristics measured at every pixel (1.05 nm×1.05 nm) of quantum dot is presented. For the two lowest observed energy levels of quantum dot (the maxima in density of states) the density of states is mapped and correlated with the shape of quantum dot. It was found that prepared quantum dots have a few peaks on their flatter top and a split of the lowest energy level can be observed. This effect can be explained by inhomogeneous (nonuniform) stress distribution in the examined quantum dot.     

Strain effects on optical properties of pyramidal InAs/GaAs quantum dots

Strain distribution and optical properties in a self-assembled pyramidal InAs/GaAs quantum dot grown by epitaxy are investigated. A model, based on the theory of linear elasticity, is developed to analyze three-dimensional induced strain field. In the model, the capping material in the heterostructure is omitted during the strain analysis to take into account the sequence of the fabrication process. The mismatch of lattice constants is the driving source of the induced strain and is treated as initial strain in the analysis. Once the strain analysis is completed, the capping material is added back to the heterostructure for electronic band calculation. The strain-induced potential is incorporated into the three-dimensional steady-state Schrödinger equation with the aid of Pikus–Bir Hamiltonian with modified Luttinger–Kohn formalism for the electronic band structure calculation. The strain field, the energy levels and wave functions are found numerically by using of a finite element package FEMLAB. The energy levels as well as the wave functions of both conduction and valence bands of quantum dot are calculated. Finally, the transition energy of ground state is also computed. Numerical results reveal that not only the strain field but also all other optical properties from current model show significant difference from the counterparts of the conventional model.