基于连续弹性理论分析量子线线宽对应变分布和带隙的影响

基于连续弹性理论分别采用数值方法和格林函数法讨论了量子线的应变分布.格林函数法可以得到应变分布的解析表示式,对规则形状的量子线的应变分布计算比较方便;连续弹性理论采取的是数值解法,结果精度不如格林函数法,但是能方便计算任意形状量子线的应变分布情况, 并可以考虑不同材料的弹性常数的影响.文章还具体讨论了量子线线宽对应变分布和带隙的影响,结果表明：沿线宽方向,应变的绝对值逐渐减小,并随线宽的增加而变大;带隙则随线宽的减小而增大. 关键词： 连续弹性理论 格林函数法 应变 带隙     

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     

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     

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     

基于有限元法的光子并矢格林函数重整化及其在自发辐射率和能级移动研究中的应用

任意微纳结构中量子点的自发辐射率和能级移动均可用并矢格林函数表达.当源点和场点在同一位置时,格林函数的实部是发散的.为解决这一发散问题,可采用重整化格林函数方法.本文提出一种计算重整化格林函数和散射格林函数的方法.该方法利用有限元,计算点电偶极子的辐射场,将其在量子点体积内做平均得到重整化的并矢格林函数,减去均匀空间中解析的重整化格林函数,得到重整化的散射格林函数.在均匀空间情况下,本方法所得数值结果与解析解一致.将该方法应用到银纳米球系统,以解析的散射格林函数作为参考,结果表明该方法能准确处理散射格林函数的重整化问题.将该方法应用到表面等离激元纳米腔中,发现有极大的自发辐射增强和能级移动,且该结果不依赖于量子点的体积.这些研究在光与物质相互作用领域具有积极的意义.     

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     

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     

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     

镍晶格中的晶格格林函数

在含缺陷体系的模拟中，格林函数作为一种重要的多尺度耦合方法，可以将缺陷产生的局域应变场和长程应力场耦合起来.在镍基高温合金蠕变的初期阶段，位错主要分布在基体相中，通过格林函数的多尺度桥接模式，可得到基体相中位错芯的平衡构型，为位错-杂质复合体间相互作用提供计算基础.基于晶格动力学理论和声子谱计算中的力常数矩阵，通过第一布里渊区特殊k点取样以及傅里叶变换，计算完整镍晶格中晶格格林函数，可用于镍基材料的多尺度模拟计算.     

Electronic and optical properties of InAs/InP self-assembled quantum dots on patterned substrates

We present atomistic theory of electronic and optical properties of a single InAs quantum dot grown on a pyramidal InP nanotemplate. The shape and size of the dot is assumed to follow the nanotemplate shape and size. The electron and valence hole single particle states are calculated using atomistic effective–bond–orbital model with second nearest-neighbor interactions. The electronic calculations are coupled to separately calculated strain distribution via Bir–Pikus Hamiltonian. The optical properties of InAs dots embedded in InP pyramids are calculated by solving the many-exciton Hamiltonian for interacting electron and hole complexes using the configuration–interaction method. The effect of quantum-dot geometry on the optical spectra is investigated by a comparison between dots of different shapes.     

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.     

Self-organized GaN/AlN hexagonal quantum-dots:strain distribution and electronic structure

This paper presents a finite element method of calculating strain distributions in and around the self-organized GaN/AlN hexagonal quantum dots. The model is based on the continuum elastic theory, which is capable of treating a quantum dot with an arbitrary shape. A truncated hexagonal pyramid shaped quantum dot is adopted in this paper. The electronic energy levels of the GaN/AlN system are calculated by solving a three-dimension effective mass Shrodinger equation including a strain modified confinement potential and polarization effects. The calculations support the previous results published in the literature.     

Strain fields in arbitrary shaped quantum wires

The intent of this work is to develop a more generalized approach towards strain field calculations in embedded quantum wires (QWRs). Higher degree polynomials are used to achieve better discretization of QWR in arbitrary shapes and to avoid some of the singular points in the strain field calculations. Calculations are performed for simpler geometries such as triangular and square shaped QWRs to verify the validity of the approach. The same approach is tested for more complicated shapes such as crescent shaped QWRs with and without lateral quantum wells. The strain field distributions, are observed to be similar to those obtained from the analytical expressions. However, in the case of crescent shaped QWRs, the strain distribution is different in the region above the QWR. The difference is the result of the better discretization and of the removed singular points. The use of higher degree polynomials provides better discretization for shapes of interest.     

Electron energy level statistics in graphene quantum dots

Motivated by recent experimental observations of size quantization of electron energy levels in graphene quantum dots [7] we investigate the level statistics in the simplest tight-binding model for different dot shapes by computer simulation. The results are in a reasonable agreement with the experiment which confirms qualitatively interpretation of observed level statistics in terms of “Dirac billiards” without taking into account many-body effects. It is shown that edge effects are in general sufficient to produce the observed level distribution and that even strong bulk disorder does not change the results drastically.

     

光致发光谱研究自组织InAs双模量子点态填充

采用固态源分子束外延技术在GaAs(100)衬底上,制备了InAs量子点,对样品进行原子力显微镜测试,统计结果表明量子点尺寸呈双模分布。光致发光谱研究表明,在室温和77 K下,小量子点的发光峰均占主导地位,原因可能是：(1)大量子点的态密度小于小量子点;(2)捕获载流子速率,大量子点小于小量子点;(3)大量子点与盖层存在较大的应变势垒和可能出现的位错和缺陷,导致温度变化引起载流子从小尺寸量子点转移到大尺寸的量子点中概率很小。     

Intersubband,interband transitions,and optical properties of two vertically coupled hemispherical quantum dots with wetting layers

The electron and heavy hole energy levels of two vertically coupled In As hemispherical quantum dots/wetting layers embedded in a Ga As barrier are calculated numerically. As the radius increases, the electronic energies increase for the small base radii and decrease for the larger ones. The energies decrease as the dot height increases. The intersubband and interband transitions of the system are also studied. For both, a spectral peak position shift to lower energies is seen due to the vertical coupling of dots. The interband transition energy decreases as the dot size increases, decreases for the dot shapes with larger heights, and reaches a minimum for coupled semisphere dots.