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     

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     

Ge/Si半导体量子点的应变分布与平衡形态

基于各向异性弹性理论的有限元方法，研究了金字塔形自组织Ge/Si半导体量子点应变能随高宽比变化的规律：系统的应变能随着高宽比的增大而逐渐减小.并通过自由能(应变能与表面能之和)讨论了量子点的平衡形态.结果表明，对于固定体积的量子点，存在一个高宽比值，称之为平衡高宽比，使得系统的自由能最低.同时，还给出了量子点的应力、应变、流体静应变及双轴应变分布.这些可以作为阐明应变自组织量子点实验的理论基础. 关键词： 量子点 应变分布 自由能 平衡形态     

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     

离子束溅射Ge量子点的应变调制生长

采用离子束溅射技术制备了单层和双层Ge量子点, 通过原子力显微镜对比了不同Si隔离层厚度和不同掩埋量子点密度情况下表层量子点的尺寸和形貌差异, 系统研究了掩埋Ge量子点产生的应变对表层量子点的浸润层及形核的影响, 并用埋置应变模型对其进行解释. 实验结果表明, 覆盖Ge量子点的Si隔离层中分布着的应变场, 导致表层量子点浸润层厚度的降低, 从而增大点的体积; 应变强度随隔离层厚度的减小而增加, 造成表层量子点形状和尺寸的变化; 此外, 应变还调控了表层量子点的空间分布. 关键词： Ge量子点 埋层应变 离子束溅射     

分子束外延生长InGaN/AlN量子点的组分研究

报道了分子束外延生长的绿光波段InGaN/AlN量子点材料,并综合考虑InGaN量子点的应变弛豫,以及应力和量子限制斯塔克效应对量子点发光波长的影响,提出了一种结合反射式高能电子衍射原位测量与光致荧光测量确定InGaN量子点组分的方法.     

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     

温度对GaSb/GaAs量子点尺寸分布的影响

采用低压金属有机物化学气相沉积 (LP-MOCVD) 法制备GaSb/GaAs量子点。通过对不同生长温度的样品进行分析发现温度的变化对GaSb/GaAs量子点的相位角无明显影响,量子点的形状是透镜型。由于量子点特殊的应力分布,可实现量子点的"自限制"生长。量子点的化学势不连续性以及Ostwald熟化机制的影响使得量子点尺寸分布在一定范围内不连续,会出现两种尺寸模式的量子点生长。Sb原子的表面迁移率对GaSb/GaAs量子点生长有较大的影响。升高温度可有效改善量子点的分立性,在升温过程中量子点体现出其熟化过程,高温时表面原子的解析附作用对量子点尺寸和密度的影响较大。     

异质结构的应变和应力分布模型研究

基于组合杆的平衡条件，分别建立了晶格失配、热失配以及由两者共同导致的异质结构应变 和应力分布模型，并获得了异质结构的晶格失配应变、热失配应变、弯曲应变以及曲率半径 的分析解. 同时，运用所建的模型，计算了HgCdTe/CdZnTe异质结构的应变和应力分布.结果 表明：应力最大值均在界面处，而中性面仅是材料厚度和弹性参数的函数，与晶格失配、晶 格弛豫、热失配等参数无关，且该异质结构的曲率半径是衬底厚度的函数，随衬底厚度的减 小而减小，而要保证HgCdTe/CdZnTe器件在液氮温度下不发生断裂，衬底的厚度必须大于临界值. 关键词： 异质结构 应变分布模型 应力分布模型 晶格失配     

分子束外延生长InGaN／A1N量子点的组分研究

报道了分子束外延生长的绿光波段InGaN／A1N量子点材料,并综合考虑InGaN量子点的应变弛豫,以及应力和量子限制斯塔克效应对量子点发光波长的影响,提出了一种结合反射式高能电子衍射原位测量与光致荧光测量确定InGaN量子点组分的方法．     

Small and high-density GeSiC dots stacked on buried Ge hut-clusters in Si

Self-assembled GeSiC dots stacked on a Ge hut-cluster layer buried in Si have been investigated. The critical thickness for formation of GeSiC dots is reduced owing to the strain fields from the buried hut-clusters. By utilizing the stacked structure, the dot size is decreased and the uniformity is improved. The highest density of the GeSiC dots with stacked structures is 7.4×10¹⁰ cm⁻², which is six times larger than that of single GeSiC dots. The formation of the self-assembled GeSiC dots is strongly influenced by being stacked with buried Ge dots as well as C incorporation.     

Theoretical study of quantum confined Stark shift in InAs／GaAs quantum dots

The quantum confined Stark effect (QCSE) of the self-assembled InAs/GaAs quantum dots has been investigated theoretically. The ground-state transition energies for quantum dots in the shape of a cube, pyramid or “truncated pyramid” are calculated and analysed. We use a method based on the Green function technique for calculating thestrain in quantum dots and an efficient plane-wave envelope-function technique to determine the ground-state electronic structure of them with different shapes. The symmetry of quantum dots is broken by the effect of strain. So the properties of carriers show different behaviours from the traditional quantum device. Based on these results, we also calculate permanent built-in dipole moments and compare them with recent experimental data. Our results demonstrate that the measured Stark effect in self-assembled InAs/GaAs quantum dot structures can be explained by including linear grading.     

Influences of lattice mismatches on equilibrium morphologies and strain distributions of quantum dots

The morphologies of quantum dots and distributions of stresses in and around quantum dots structures have a significant effect on photoelectric properties and electronic structures. Optical and electronic devices of different efficiencies based on quantum dots can be manufactured by choosing self-assembly of different materials to control epitaxial growth. In this article we investigate the equilibrium morphologies and the strain distributions of self-assembled pyramidal semiconductor quantum dots in Stranski-Krastanov growth mode based on the finite element method of the anisotropic theory of elasticity. We also give the equilibrium morphologies and the distribution of the stress and the strain, the hydrostatic strain and the biaxial strain for different lattice mismatched quantum dots. The results can serve as a basis for interpretation of experiments.     

Inverted electron-hole alignment in InAs-GaAs self-assembled quantum dots

New information on the electron-hole wave functions in InAs-GaAs self-assembled quantum dots is deduced from Stark effect spectroscopy. Most unexpectedly it is shown that the hole is localized towards the top of the dot, above the electron, an alignment that is inverted relative to the predictions of all recent calculations. We are able to obtain new information on the structure and composition of buried quantum dots from modeling of the data. We also demonstrate that the excited state transitions arise from lateral quantization and that tuning through the inhomogeneous distribution of dot energies can be achieved by variation of electric field.     

Exciton polarization, fine-structure splitting, and the asymmetry of quantum dots under uniaxial stress

We derive a general relation between the fine-structure splitting (FSS) and the exciton polarization angle of self-assembled quantum dots under uniaxial stress. We show that the FSS lower bound under external stress can be predicted by the exciton polarization angle and FSS under zero stress. The critical stress can also be determined by monitoring the change in exciton polarization angle. We confirm the theory by performing atomistic pseudopotential calculations for the InAs/GaAs quantum dots. The work provides deep insight into the dot asymmetry and their optical properties and a useful guide in selecting quantum dots with the smallest FSS, which are crucial in entangled photon source applications.     

Effect of elastic anisotropy on the strain fields and band edges in stacked InAs/GaAs quantum dot nanostructures

The effect of elastic anisotropy on the strain fields and confinement potentials in InAs/GaAs quantum dot (QD) nanostructures was investigated for an isolated dot and a stacked multi-layer dots using finite element analysis and model solid theory. The assumption of isotropy tends to underestimate especially hydrostatic strain that is known to modify confinement potentials in conduction band. Consideration of anisotropy results in a wider band gap and shallower potential well as compared with the isotropic model. Since the band gap and potential well depth would be related to opto-electronic properties of quantum dot systems via quantum mechanical effects, it is suggested that consideration of elastic anisotropy in the calculation of strains and band structures is necessary for the design of QD-based opto-electronic devices.     

The influence of strain-reducing layer on strain distribution and ground state energy levels of GaN/AlN quantum dot

This article deals with the strain distributions around GaN/AlN quantum dots by using the finite element method. Special attention is paid to the influence of Al_0.2Ga_0.8N strain-reducing layer on strain distribution and electronic structure. The numerical results show that the horizontal and the vertical strain components are reinforced in the GaN quantum dot due to the presence of the strain-reducing layer, but the hydrostatic strain in the quantum dot is not influenced. According to the deformation potential theory, we study the band edge modifications and the piezoelectric effects. The result demonstrates that with the increase of the strain reducing layer, the transition energy between the ground state electron and the heavy hole increases. This result is consistent with the emission wavelength blue shift phenomenon observed in the experiment and confirms that the wavelength shifts toward the short wavelength range is realizable by adjusting the structure-dependent parameters of GaN/AlN quantum dot.     

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.