Cu掺杂硅量子点的电子结构和光学性质

采用基于密度泛函理论的第一性原理的方法,对Cu掺杂H钝化硅量子点的形成能、态密度、磁性和光学性质进行了计算,考虑了Cu占据硅量子点替代和间隙的不同位置.结果表明:Cu趋向占据硅量子点表面的六角形间隙位置.Cu掺杂硅量子点引入了杂质能级,带隙变窄.由于Cu的d态p态和Si的p态耦合,导致Cu替代掺杂硅量子点具有铁磁性,且在低能区出现了一个较强的吸收峰.     

半导体单量子点的分子束外延生长及调控

Ⅲ-Ⅴ化合物半导体外延单量子点具有类原子的分立能级结构，能够按需产生单光子和纠缠多光子态，而且可以直接与成熟的集成光子技术结合，因此被认为是制备高品质固态量子光源、构建可扩展性量子网络最有潜力的固态量子体系之一。本综述的重点是介绍高品质单量子点的分子束外延生长及精确调控的方法。首先介绍了晶圆级均匀单量子点的分子束外延生长，并探讨了调控浸润层态和量子点对称性的生长方法；接下来概述了利用应变层调控量子点发射波长的方法，总结了几种常见的电调控单个量子点器件的设计原理；最后讨论了最近为实现优异量子点光源而开发的液滴外延生长技术。     

InGaAs应变减少层对InAs量子点发光波长红移的影响

用有限元法对InAs/GaAs量子点材料的应变分布进行了研究,特别强调了三元化合物In0.2Ga0.8As应变减少层对各个应变分量的影响。在应变减少层作用下沿着平行生长方向和垂直于生长方向的应变分量增强;对电子结构有重要影响的静水应变和双轴应变分量也得到了增强。采用八带k.p理论,研究了在有应变减少层的条件下,应变对带边的影响,计算结果表明,与没有应变减少层相比,应变导致带隙变窄,定性解释了实验观察到的发光波长红移现象。通过调整相关参数,可以采用应变减少层技术实现光纤通信系统用的长波长发射激光器。     

量子点激光器和量子点能态的计算

本文首先从态密度的角度,分析了半导体激光器从三维到零维发展的内因;介绍了实现量子点的工艺和量子点激光器的研究进程;评述了影响量子点激光器发展的"瓶颈"问题.在文章的后半部分,介绍了量子点电子态的计算,其中着重介绍了k·p微扰法.     

C60分子的电子传导和量子流分布研究

利用基于Green's function的tight-binding方法,对由两条原子线电极连接C60分子远端构成的电子传导系统进行了理论计算和数值模拟,得出了入射电子通过C60分子传输到远端点的电子传输谱.其结果揭示了电子传导过程中C60分子的开关特性,并且得出了电子传输能量与分子轨道共振时传输概率峰值的出现及振荡特征.利用Fisher-Lee关系式和量子流密度理论,在传输概率峰值的能量点E=-1.38eV处获得了C60分子内的量子流分布,给出了键量子流的最大值和最小值.对全部分子键上的量子流数值进行了图形模拟,其结果符合量子流动量守恒定律.     

垒层温度对InGaN量子点/量子阱复合结构内量子效率的影响

使用金属有机化学气相沉积(metal organic chemical vapor deposition, MOCVD)方法生长了三个具有不同垒层温度的InGaN/GaN量子阱。由于高密度V型坑的形成,完整的量子阱结构被破坏,转变成了InGaN量子点(quantum dots, QDs)/量子阱(quantum well, QW)复合结构。通过变功率光致发光谱和变温光致发光谱,分析了在不同的垒层温度下量子限制斯塔克效应(quantum confined Stark effect, QCSE)、非辐射复合中心密度和载流子局域化效应的变化。结果表明:在较低的垒层温度下,QCSE较弱,因为在较低的温度下,V型坑的深度较深,应力释放较明显,残余应变较低;非辐射复合中心密度也随着温度的升高而逐渐增大;样品的内量子效率(internal quantum efficiency, IQE)随着垒层生长温度的升高而降低。QCSE的增强和非辐射复合中心密度的增大是垒层生长温度升高时内量子效率下降的主要因素。     

确定性固态量子光源基础材料与器件

量子光源是量子通信和光量子计算的基础模块。光子的单光子性保证了通信的无条件安全,光子的高不可分辨性保证了计算方案的复杂度。在各类固态材料候选体系中,基于半导体量子点体系的单光子源和纠缠光子源保持着量子光源品质的最高纪录,展现了巨大的潜力。分子束外延是目前最适合制备固态半导体量子点的生长方法,超高真空、超纯材料、原位监测和生长过程中参数的高度可控等特点使其优势明显。为了实现同时具备高效率、高单光子纯度、高不可分辨性和高纠缠保真度的量子光源,量子点的材料生长、外部调控、钝化技术和测量技术等都需要系统优化提升。本文将综述基于分子束外延生长实现固态量子点体系量子光源的基础材料与器件的研究进展,讨论两种常见量子点的制备原理以及外延生长中各类参数对量子点品质的影响,包括背景真空、源料纯度、衬底温度、生长速率和束流比等。本文随后简介了外部调控、表面钝化、测量技术等手段优化量子光源器件性能的技术细节和实验进展,最后对量子光源在基础科学研究和量子网络构建中取得的进展进行总结,并对其实际应用与发展前景进行展望。     

甲烷浓度对CVD自支撑金刚石薄膜残余应力的影响

根据唯像理论研究了化学气相沉积自支撑金刚石薄膜中织构和弹性性能之间的关系,揭示了晶体取向和织构对残余应变和残余应力的影响.织构使CVD金刚石薄膜杨氏模量和宏观残余应变呈现各向异性,而且两者呈现相反的分布规律.织构和杂质都能影响金刚石薄膜的弹性模量和残余应变,但是织构的影响是主要的.实验中测量的残余应变很好的符合了根据薄膜织构计算得到的弹性模量值.因此可以借助改变薄膜织构来调节薄膜的残余应力.     

偏压对Ge/Si单量子点电流分布的影响

采用扫描探针显微镜(SPM),对离子束溅射自组装生长的Ge/Si量子点的电学性能进行分析.实验结果表明,施加正向偏压于圆顶形量子点上时,由于表面氧化物的生成,扫描区域的电流信号是不可恢复的.施加负向偏压时,单量子点区域的电流分布特征保持环状,且电流剖面图呈双峰结构,随着负向偏压的增大,各个电流峰顶部形状由尖锐变为截平,呈现出饱和的趋势.通过对电流值大小分布的统计,证实了Ge/Si量子点的电流传导主导机制为热电子发射.     

1.3微米发光自组织InAs/GaAs量子点的电致发光研究

用优化的MBE参数生长了1.3μm发光的InAs/GaAs量子点材料,并制成发光二极管,对不同温度和有源区长度下样品的电致发光谱进行了细致的研究.观察到两个明显的电致发光峰,分别对应于量子点基态和激发态的辐射复合发光.实验表明,由于能态填充效应的影响,适当增大量子点发光器件有源区长度,更有利于获得基态的光发射.这个结果提供了一种控制和调节InAs/GaAs量子点发光二极管和激光器的工作波长的方法.     

Influence of the strain on the formation of GaInAs/GaAs quantum structures

The influence of the strain on the dot morphology of GaInAs quantum dots has been investigated. The strain was varied by the In content in GaInAs/GaAs quantum dots from 60% down to 30% by keeping the emission wavelength at about 900 nm at 10 K. Spectral properties are compared with morphological results determined by scanning electron and scanning transmission electron microscopy confirming a change of the dot geometry from circular to elongated shapes during an overgrowth process. These lowly strained quantum dot layers with enlarged dot sizes exhibit a reduced dot density of 6–9×10⁹ cm⁻² and a strongly enhanced oscillator strength, which make them very interesting for single quantum dot and cavity quantum electrodynamic experiments as well as for applications like single photon emitters.     

Spin-flip relaxation mechanism in self-assembled quantum dots via phonon emission

Abstract

The spin-flip relaxation time for electrons in self-assembled coupled quantum dots (SACQDs) due to emission of acoustic phonons is presented. We focus on arbitrary shaped quantum dots made with InAs embedded in a wetting layer InAs and surrounded by GaAs. The electron states are calculated using the 8-band strain dependent theory. The deformation potential and the piezo- electric acoustic phonon modes are employed to estimate the electron-phonon coupling. Spin-flip time is calculated by Fermi’s Golden Rule and its dependence on the shape of the QDs, the size and the lattice temperature is investigated.     

Energetics of CdSxSe1−x quantum dots in borosilicate glasses

The thermodynamics of CdSe quantum dots embedded in a glass matrix is of great interest because of the numerous applications as optical materials. In this study, the energetics and stability of CdSe quantum dots in a borosilicate glass matrix is investigated as a function of size using high-temperature oxide melt solution calorimetry. CdS_0.1Se_0.9 nanoparticles (1-40 nm) embedded in glass were analyzed by photoluminescence spectroscopy, electron microprobe, X-ray fluorescence, high-energy synchrotron X-ray diffraction, and (scanning) transmission electron microscopy using both electron energy loss and energy dispersive X-ray spectroscopy. As CdSe particles coarsen, their heat of formation becomes more exothermic. The interfacial energy of CdSe QDs embedded in a borosilicate glass, determined from the slope of enthalpy of drop solution versus calculated surface area, is 0.56 ± 0.01 J/m².     

Ultrafast carrier capture in charged InAs quantum dots

We report theoretical and experimental results of our investigation on carrier capture and relaxation processes in undoped and modulation-doped InAs/GaAs self-assembled quantum dots (QDs). We find that carrier capture and relaxation in the ground state is faster in the modulation-doped quantum dots compared to the case in neutral dots at an excitation level as low as one electron–hole pair per dot. The ultrafast photoluminescence (PL) transient rise time observed in the charged dots is attributed to the relaxing of strained field induced by the presence of cold carriers in the dots. The Hamiltonian of electron’s interaction with local vibrating field and carrier capture time are also calculated.     

分子束外延生长过程中GaAs(001)表面铝液滴的扩散成核过程

量子点的性质主要由其密度及尺寸参数控制,而原子在衬底上的成核运动又决定了量子点的密度、直径、高度等参数,因此研究原子的扩散成核过程对自组装制备量子点具有重要意义.本文通过分子束外延生长技术研究了GaAs(001)表面金属铝液滴的成核过程,发现衬底温度和金属铝沉积速率的变化直接影响了液滴的尺寸、密度以及形状等特征.根据经...     

Quantum dot nanostructures and molecular beam epitaxy

In order to fulfil the requirements of the information society there is a growing demand for nanoelectronic devices with new or largely improved performances; these devices are based on low-dimensional carrier systems, and in particular on zero-dimensional ones, that have peculiar properties as compared to the three- and two-dimensional counterparts.

In this paper we review and discuss the basic features of the Molecular Beam Epitaxy growth of quantum dots that are very interesting archetypes of zero-dimensional nanostructures; quantum dots can be obtained by the three-dimensional growth of self-assembled nanoislands that takes place during the preparation of structures based on highly lattice-mismatched materials. Aspects of the morphological, electronic and optical properties of quantum dots will be reviewed and it will be shown how the energy of confined levels for carriers is determined by design and growth parameters of nanostructures and how quantum dot emission wavelengths can be tuned in the windows of optoelectronic and photonic interest, such as that at 0.98, 1.31 and 1.55 μm. An overview of quantum dot devices will be given, with particular attention paid to the quantum dot laser, unarguably the most important application of quantum dots so far.     

Quantification of the In‐distribution in embedded InGaAs quantum dots by transmission electron microscopy

The In‐concentration in InGaAs quantum dots located within a GaAs matrix was determined with the composition evaluation by lattice fringe analysis (CELFA) technique. However, the results obtained with this method cannot account for the three‐dimensional shape of quantum dots and their embedding in GaAs. A correction procedure was developed that takes into consideration the shape of the quantum dots and the TEM sample thickness and quantum‐dot size. After correction, In‐concentration profiles show an increasing In‐content towards the top of the quantum dots which is consistent with the effect of In‐segregation and earlier studies using other experimental techniques. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of a buried misfit dislocation network on the pyramid-to-dome transition size of Ge self-assembled quantum dots on Si(0 0 1)

The critical sizes of the pyramid-to-dome transition of Ge self-assembled quantum dots (SAQDs) grown on relaxed SiGe buffer layers were investigated for the relationship between the misfit strain built in dots and nucleation sites. The strain field of arrays of buried dislocations in a relaxed SiGe buffer layer provided preferential nucleation sites for quantum dots. Burgers vector analysis using plan-view transmission electron microscopy verified that the preferential nucleation sites of Ge SAQDs depended on the Burgers vector direction of corresponding dislocations. The measurement of the lateral distance between SAQDs and dislocations clarified that the location of SAQDs was at the intersection of the dislocation slip plane and the top surface. The samples are fabricated to contain low dislocation densities. The average dislocation spacing is larger than the surface migration length of Ge adatoms, resulting in two groups of SAQDs, those that are located along the dislocations, and those that are not. Atomic force microscopy observations showed a distinctively larger critical size for Ge SAQDs grown over the intersection of the dislocation slip plane and the top surface than those grown in regions between dislocations. These experimental observations indicate that the critical size of the pyramid-to-dome transition is strongly dependent on misfit strain in SAQDs with lower strain being associated with a larger critical size.     

In原子在GaAs(001)表面的成核与扩散研究

近年来,半导体量子点特别是InAs量子点的基本物理性质和潜在应用得到了广泛研究。许多研究者利用InAs量子点结构的改变以调制其光电特性。本文采用液滴外延法在GaAs(001)表面沉积了不同沉积量的In(3 ML、4 ML、5 ML),以研究In的成核机制和表面扩散。实验发现,随着In沉积量的增加,液滴尺寸(包括直径、高度)明显增大。不仅如此,在相同的衬底温度下,沉积量越大,液滴密度越大。利用经典成核理论,计算了GaAs(001)表面In液滴形成的临界厚度为0.57 ML,计算的结果与已报道的实验一致。从In原子在表面的迁移和扩散,以及衬底中Ga和液滴中的In之间的原子互混原理解释了In液滴形成和形貌演化的机理。实验中得到的In液滴临界厚度以及In液滴在GaAs(001)上成核机理,可以为制备InAs量子点提供实验指导。