Impact of GaNAs strain compensation layer on the electronic structure of InAs/GaAs quantum dots

The strain and electron energy levels of InAs/GaAs(001) quantum dots (QDs) with a GaNAs strain compensation layer (SCL) are investigated. The results show that both the hydrostatic and biaxial strain inside the QDs with a GaNAs SCL are reduced compared with those with GaAs capping layers. Moreover, most of the compressive strain in the growth surface is compensated by the tensile strain of the GaNAs SCL, which implies that the influence of the strain environment of underlying QDs upon the next-layer QDs’ growth surface is weak and suggests that the homogeneity and density of QDs can be improved. Our results are consistent with the published experimental literature. A GaNAs SCL is shown to influence the strain and band edge. As is known, the strain and the band offset affect the electronic structure, which shows that the SCL is proved to be useful to tailor the emission wavelength of QDs. Our research helps to better understand how the strain compensation technology can be applied to the growth of stacked QDs, which are useful in solar cells and laser devices.     

Influence of nanomechanical force on the electronic structure of InAs/GaAs quantum dots

We show nanomechanical force is useful to dynamically control the optical response of self-assembled quantum dots, giving a method to shift electron and heavy hole levels, interval of electron and heavy hole energy levels, and the emission wavelength of quantum dots (QDs). The strain, the electron energy levels, and heavy hole energy levels of InAs/GaAs(001) quantum dots with vertical nanomechanical force are investigated. Both the lattice mismatch and nanomechanical force are considered at the same time. The results show that the hydrostatic and the biaxial strains inside the QDs subjected to nanomechanical force vary with nanomechanical force. That gives the control for tailoring band gaps and optical response. Moreover, due to strain-modified energy, the band edge is also influenced by nanomechanical force. The nanomechanical force is shown to influence the band edge. As is well known, the band offset affects the electronic structure, which shows that the nanomechanical force is proven to be useful to tailor the emission wavelength of QDs. Our research helps to better understand how the nanomechanical force can be used to dynamically control the optics of quantum dots.     

Finite element analysis of stress and strain distributions in InAs/GaAs quantum dots

In this paper, we perform systematic calculations of the stress and strain distributions in InAs/GaAs truncated pyramidal quantum dots (QDs) with different wetting layer (WL) thickness, using the finite element method (FEM). The stresses and strains are concentrated at the boundaries of the WL and QDs, are reduced gradually from the boundaries to the interior, and tend to a uniform state for the positions away from the boundaries. The maximal strain energy density occurs at the vicinity of the interface between the WL and the substrate. The stresses, strains and released strain energy are reduced gradually with increasing WL thickness. The above results show that a critical WL thickness may exist, and the stress and strain distributions can make the growth of QDs a growth of strained three-dimensional island when the WL thickness is above the critical value, and FEM can be applied to investigate such nanosystems, QDs, and the relevant results are supported by the experiments.     

Approximate calculation of electronic energy levels of axially symmetric quantum dot and quantum ring by using energy dependent effective mass

Calculations of electronic structures about the semiconductor quantum dot and the semiconductor quantum ring are presented in this paper. To reduce the calculation costs, for the quantum dot and the quantum ring, their simplified axially symmetric shapes are utilized in our analysis. The energy dependent effective mass is taken into account in solving the Schr?dinger equations in the single band effective mass approximation. The calculated results show that the energy dependent effective mass should be considered only for relatively small volume quantum dots or small quantum rings. For large size quantum materials, both the energy dependent effective mass and the parabolic effective mass can give the same results. The energy states and the effective masses of the quantum dot and the quantum ring as a function of geometric parameters are also discussed in detail.     

The influences of thickness of spacing layer and the elastic anisotropy on the strain fields and band edges of InAs/GaAs conical shaped quantum dots

Based on the continuum elastic theory, this paper presents a finite element analysis to investigate the influences of elastic anisotropy and thickness of spacing layer on the strain field distribution and band edges (both conduction band and valence band) of the InAs/GaAs conical shaped quantum dots. To illustrate these effects, we give detailed comparisons with the circumstances of isolated and stacking quantum dot for both anisotropic and isotropic elastic characteristics. The results show that, in realistic materials design and theoretical predication performances of the optoelectronic devices, both the elastic anisotropy and thickness of the spacing layer of stacked quantum dot should be taken into consideration.     

Piezoelectric effects and electronic structures of InAs/GaAs quantum dots grown along (111) and (011) directions

Piezoelectric effects and electronic structures of InAs/GaAs quantum dots grown along (111) and (011) directions are investigated in this paper. The finite element method is used. Electronic energy levels are calculated by solving the three-dimensional effective mass Schr?dinger equation including a strain modified confinement potential and piezoelectric effects. The difference in electronic structure between quantum dots grown along the (111) direction and the (011) direction are compared. The cubic and truncated pyramidal shaped quantum dots are adopted.     

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.     

Electronic states of InAs/GaAs tyre-shape quantum ring

In the framework of the effective mass theory, this paper calculates the electron energy levels of an InAs/GaAs tyre-shape quantum ring （TSQR） by using the plane wave basis. The results show that the electron energy levels axe sensitively dependent on the TSQR＇s section thickness d, and insensitively dependent on TSQR＇s section inner radius R1 and TSQR＇s inner radius R2. The model and results provide useful information for the design and fabrication of InAs/GaAs TSQRs.     

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.     

Simulation of electronic structure Hamiltonians using quantum computers

Over the last century, a large number of physical and mathematical developments paired with rapidly advancing technology have allowed the field of quantum chemistry to advance dramatically. However, the lack of computationally efficient methods for the exact simulation of quantum systems on classical computers presents a limitation of current computational approaches. We report, in detail, how a set of pre-computed molecular integrals can be used to explicitly create a quantum circuit, i.e. a sequence of elementary quantum operations, that, when run on a quantum computer, obtains the energy of a molecular system with fixed nuclear geometry using the quantum phase estimation algorithm. We extend several known results related to this idea and discuss the adiabatic state preparation procedure for preparing the input states used in the algorithm. With current and near future quantum devices in mind, we provide a complete example using the hydrogen molecule of how a chemical Hamiltonian can be simulated using a quantum computer.     

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     

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.     

Formation and evolution of multimodal size distributions of InAs/GaAs quantum dots

Self-organized formation and evolution of quantum dot (QD) ensembles with a multimodal size distribution is reported. Such ensembles form after fast deposition near the critical thickness during a growth interruption (GRI) prior to cap layer growth and consist of pure InAs truncated pyramids with heights varying in steps of complete InAs monolayers, thereby creating well-distinguishable sub-ensembles. Ripening during GRI manifests itself by an increase of sub-ensembles of larger QDs at the expense of sub-ensembles of smaller ones, leaving the wetting layer unchanged. The dynamics of the multimodal QD size distribution is theoretically described using a kinetic approach. Starting from a broad distribution of flat QDs, a predominantly vertical growth is found due to strain-induced barriers for nucleation of a next atomic layer on different facets. QDs having initially a shorter base length attain a smaller height, accounting for the experimentally observed sub-ensemble structure. The evolution of the distribution is described by a master equation, which accounts for growth or dissolution of the QDs by mass exchange between the QDs and the adatom sea. The numerical solution is in good agreement with the measured dynamics.     

温度与外磁场对Si均匀掺杂的GaAs量子阱电子态结构的影响

本文在有效质量近似下,通过自洽地求解薛定鄂方程及泊松方程计算了在温度T=273 K,磁感应强度B=25 T,Si均匀掺杂的GaAs/AlGaAs量子阱系统的电子态结构.研究了温度与外磁场对子带能量,本征包络函数,自洽势,电子密度分布,及费米能量的影响.发现在给定磁感应强度B=fi0下,随温度升高子带能量单调增加,费米能量单调递减,自洽势的势阱变深变陡,电子密度分布变宽,峰值降低;在给定温度下,随磁感应强度的增加子带能量及费米能量单调递增,自洽势阱变浅变宽,电子密度分布变窄,峰值升高.     

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.     

Band-mixing and strain effects in InAs/GaAs quantum rings

We analyze for the first time the coupled influence of band mixing, strain, and piezoelectricity on electronic structure, eigenstates, and optical transition strengths for InAs/GaAs quantum-ring structures. It is shown that band mixing and strain alter the level energies and optical absorption coefficients significantly.     

掺杂浓度及掺杂层厚度对Si均匀掺杂的GaAs量子阱中电子态结构的影响

本文通过自洽地求解薛定鄂方程及泊松方程计算了在温度T=0, 有效质量近似下, Si均匀掺杂的GaAs/AlGaAs量子阱系统的电子态结构. 研究了掺杂浓度及掺杂层厚度对子带能量, 本征包络函数, 自洽势, 电子密度分布, 及费米能量的影响. 发现在给定掺杂浓度下, 子带能量随掺杂层厚度的增加单调递减, 自洽势的势阱变宽变浅, 电子密度分布变宽, 峰值变低; 在给定掺杂层厚度下, 随掺杂浓度的增加子带能量及费米能级单调递增, 自洽势阱变深变陡变窄, 电子密度分布的峰值变高, 集中在中心.关键词：掺杂量子阱电子结构半导体GaAs     

Control of structural and excitonic properties of self-organized InAs/GaAs quantum dots

A systematic dependence of excitonic properties on the size of self-organized InAs/GaAs quantum dots is presented. The bright exciton fine-structure splitting changes from negative values to more than 0.5 meV, and the biexciton binding energy varies from antibinding to binding, as the height of truncated pyramidal dots increases from 2 to above 9 InAs monolayers. A novel mode of metalorganic vapor phase epitaxy was developed for growing such quantum dots with precise shape control. The dots consist of pure InAs and feature heights varying in steps of complete InAs monolayers. Such dot ensembles evolve from a strained, rough two-dimensional layer with a thickness close to the critical value for the onset of the 2D–3D transition. Dots with a common height represent subensembles with small inhomogeneous broadening. Tuning of subensemble emission energy is achieved by varying the mean lateral extension of the respective QDs. Detailed knowledge of the structural properties of individual dots enable realistic k·p calculations to analyze the origin of the observed excitonic properties. The binding energies of charged and neutral excitons increase due to correlation by the gradually increasing number of bound states for increasing dot size. The monotonously increasing magnitude of the fine-structure splitting with dot size is shown to be caused by piezoelectricity. The identification of key parameters allows to tailor exciton properties, providing a major step towards the development of novel applications.     

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.     

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