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。     

钝化低温法生长多层InGaN量子点的结构和光学特性

采用一种新方法生长多层InGaN/GaN量子点,研究所生长样品的结构和光学特性。该方法采用了低温生长和钝化工艺,所以称之为钝化低温法。第一层InGaN量子点的尺寸平均宽度40nm,高度15nm,量子点密度为6.3×10¹⁰/cm²。随着层数的增加,量子点的尺寸也逐渐增大。在样品的PL谱测试中,观察到在In(Ga)As材料系中普遍观察到的量子点发光的温度特性---超长红移现象。它们的光学特性表明:采用钝化低温法生长的纳米结构中存在零维量子限制效应。     

GaSb/GaAs复合应力缓冲层上自组装InAs量子点的生长

研究了GaSb/GaAs复合应力缓冲层上自组装生长的InAs量子点.在2ML GaSb/1ML GaAs复合应力缓冲层上获得了高密度的、沿［100］方向择优分布量子点.随着复合应力缓冲层中GaAs层厚度的不同,量子点的密度可以在1.2×10¹⁰cm^-2和8×10¹⁰cm^-2进行调控.适当增加GaAs层的厚度至5ML,量子点的发光波长红移了约25nm,室温下PL光谱波长接近1300nm. 关键词： 自组装量子点 分子束外延 Ⅲ-Ⅴ族化合物半导体     

原子轰击调制离子束溅射沉积Ge量子点的生长演变

采用离子束溅射沉积的方法在Si衬底上生长Ge量子点,观察到量子点的生长随Ge原子层沉积厚度θ的增加经历了两个不同的阶段.当θ在6—10.5个单原子层(ML)范围内时,量子点的平均底宽和平均高度随θ增加同时增大,生长得到高宽比较小的圆顶形Ge量子点,伴随着量子点的生长,二维浸润层的厚度同时增大,量子点的分布密度缓慢增加;当θ在11.5一17 ML范围内时,获得高宽比较大的圆顶形Ge量子点,量子点以纵向生长为主导,二维浸润层的离解促进量子点的成核和长大,量子点的分布密度随θ的增加快速增大;量子点在θ由10.5 ML增加到11.5 ML时由一个生长阶段转变到另一个生长阶段,其分布密度同时发生6.4倍的增加.离子束溅射沉积Ge量子点的生长演变与在热平衡状态下生长的量子点不同,在量子点的不同生长阶段,其表面形貌和分布密度的变化特点是在热力学条件限制下表面原子动态演变的结果,θ的变化是引起系统自由能改变的主要因素.携带一定动能的溅射原子对生长表面的轰击促进表面原子的扩散迁移,同时压制量子点的成核,在浸润层中形成超应变状态,因而,改变体系的能量和表面原子的动力学行为,对量子点的生长起重要作用.     

ZnSe:Cu/CdS核壳量子点的合成及光学性能研究

以CdS为壳层材料对核水溶性ZnSe:Cu量子点进行包覆,得到ZnSe:Cu/CdS核壳结构的量子点。研究了壳层厚度对ZnSe:Cu量子点光学性能的影响,采用TEM、XRD、PL和UV-Vis手段对所得样品进行表征。实验结果表明:量子点为立方闪锌矿结构,分散性好,形状为球形,经壳层修饰后量子点的粒径由2.7 nm增大到4.0 nm。随着包覆CdS壳层数的增加,量子点的发射和紫外吸收谱红移,说明量子点在长大,证明CdS壳层生长在ZnSe:Cu量子点的表面,形成了核壳结构的ZnSe:Cu/CdS量子点。包覆CdS壳层后ZnSe:Cu量子点的发光强度减弱,但稳定性得到了提高。     

以S-K和V-W模式生长ZnCdSe和ZnSeS量子点及其特性

用低压金属有机化学气相外延(LP-MOCVD)技术,以Stranski Krastanow(S-K)模式,在GaAs衬底上生长了CdSe和ZnCdSe量子点(QDs)。用原子力显微镜(AFM),观测到了外延层低于临界厚度时,CdSe自组装量子点的形成过程,并把其机理归结为表面扩散效应和应变弛豫效应的联合作用。依据理论计算外延层临界厚度值的指导,用LP-MOCVD技术在GaAs衬底上生长了ZnCdSe量子点,详细观测了ZnCdSe量子点的形成和演变,这些过程可用Ostwald熟化过程和形成过程的联合作用来解释。用LP-MOCVD技术,以Volmer Weber(V-W)模式,在GaAs衬底上生长了ZnSeS量子点,随着生长时间的增加,量子点尺寸增大,而量子点密度减少,这些现象可用表面自由能来解释。     

采用AlSb缓冲层生长2.3μm InGaAsSb/AlGaAsSb多量子阱结构

针对常见的GaSb衬底的InGaAsSb/AlGaAsSb多量子阱结构进行了设计和外延生长。样品通过X-射线测试,有多级衍射卫星峰出现,表明量子阱结构的均匀性和界面质量较好,引入AlSb缓冲层可以降低衬底与外延层之间的界面自由能,使AlSb起到了一个滤板的作用,抑制了位错的扩散。光荧光谱测试表明,室温下量子阱结构中心发光波长在2.3μm附近。     

MBE自组织生长多层竖直自对准InAs量子点结构的研究

利用MBE方法在(001)GaAs衬底上生长了多层竖直自对准InAs量子点结构。透射电子显微镜的观察表明,多层量子点成一系列柱状分布。同单层量子点相比,多层量子点的光荧光谱线发生红移。这表明由于量子点中载流子波函数的扩展和交迭,柱中量子点之间有耦合现象发生。光荧光谱线半高宽随温度的反常变化说明载流子还会在邻近柱中隧穿.     

核壳型ZnS∶Cu/ZnS量子点的制备及发光性质

本文采用水相合成方法制备了ZnS∶Cu量子点并进行了ZnS壳层修饰,研究了壳层厚度对ZnS∶Cu量子点光学性质的影响,采用TEM、XRD、PL、PLE和UV-Vis等测试方法对其进行了表征。实验结果表明,合成的ZnS∶Cu/ZnS量子点为立方闪锌矿,尺寸分布均匀呈球形,分散性良好,经过壳层修饰平均粒径由2 nm增加到3.2 nm。随着ZnS壳与ZnS核量的比的增加,量子点的PLE激发峰位置和UV-Vis吸收谱线出现红移,也说明了量子点的尺寸增大,证明ZnS在ZnS∶Cu量子点的表面生长,形成了核壳结构的ZnS∶Cu/ZnS量子点。随着壳层增厚,量子点与铜离子发光中心相关的发射峰强度先增大后减小,当壳核比n_s/n_c=2.5时,发光强度达到最大。     

影响GaN基蓝光LED能带结构与光谱特性关系的仿真研究

本文采用Silvaco TCAD软件对GaN基InGaN/GaN量子阱蓝光发光二极管(LED)的光谱特性进行了仿真研究。研究结果表明:光谱会随着注入电压的增加而产生蓝移现象,并出现0.365μm处的紫外光发光峰;发光效率在正向电流较小时增长很快,随着正向电流进一步增加而逐渐趋于饱和;随着量子阱中In组分和量子阱阱层厚度的增加,发光光谱出现红移现象,并且发光效率下降。仿真结果对GaN基InGaN/GaN量子阱结构蓝光LED的设计和优化提供一定的依据。     

Magneto-optical single dot spectroscopy of GaSb/GaAs type II quantum dots

We have investigated magneto-optical properties of GaSb/GaAs self-assemble type II quantum dots by single dot spectroscopy in magnetic field. We have observed clear Zeeman splitting and diamagnetic shift of GaSb/GaAs quantum dots. The diamagnetic coefficient ranges from 5 to 30 μeV/T². The large coefficient and their large distribution are attributed to the size inhomogeneity and electron localization outside the dot. The g-factor of GaSb/GaAs quantum dots is slightly larger than that of similar type I InGaAs/GaAs quantum dots. In addition, we find almost linear relationship between the diamagnetic coefficient and the g-factor. The linear increase of g-factor with diamagnetic coefficient is due to an increase of spin-orbit interaction with dot size.     

Coulomb binding of electrons to multiply charged GaSb/GaAs self-assembled quantum dots

We explore the Coulomb binding of electrons to holes confined to type-II GaSb self-assembled quantum dots. We demonstrate that at low laser power electrons are more weakly bound to holes trapped by the dots than to holes in the wetting layer. On the other hand, at high laser power the hydrogenic binding energy of dot excitons increases by more than a factor of two, and so exceeds that of wetting layer excitons. We attribute this to the strong binding of ‘core’ electrons to dots that are highly charged with holes by optical pumping.     

Formation and atomic structure of GaSb nanostructures in GaAs studied by cross-sectional scanning tunneling microscopy

GaSb nanostructures in GaAs, grown by metalorganic chemical vapor deposition, were studied with cross-sectional scanning tunneling microscopy. Three different samples were examined, containing a thin quantum well, a quantum well near the critical thickness for dot formation, and finally self-organized quantum dots with base lengths of 5–8 nm and heights of about 2 nm. The dots are intermixed with a GaSb content between 60% and 100%. Also small 3D and 2D islands were observed, possibly representing quantum dots in an early growth stage and quantum dot precursors. All GaSb layers exhibit gaps, which are indications of an island-like growth mode during epitaxy.     

Emission of photoexcited charge carriers from InAs/GaAs quantum dots grown by gas-phase epitaxy

A model describing the emission of photoexcited electrons and holes from an array of InAs quantum dots into the GaAs matrix is suggested. The analytical expression obtained for the emission efficiency takes into account the thermal emission of charge carriers into the GaAs matrix and two-dimensional states of the InAs wetting layer, tunneling and thermally activated tunneling escape, and electron transitions between the quantum-confinement levels in the conduction band of InAs. The temperature dependences of the photosensitivity in the regions of the ground-state and first excited-state optical transitions in InAs/GaAs quantum dots grown by gas-phase epitaxy are investigated experimentally. A number of quantum dot parameters are determined by fitting the results of a theoretical calculation to the experimental data. Good agreement between the theoretical and experimental results is obtained in this way.     

应变补偿层对量子点生长影响的理论研究

量子点的光学特性与量子点的大小均匀性、密度、内部应变以及隔离层的厚度等有密切关系.文中从理论角度定量研究了GaNXAs1-X应变补偿层对InAs/GaAs量子点生长质量的改善作用,分析了应变补偿层对隔离层厚度减小的作用.讨论了应变补偿层的补偿位置和补偿层N组分X对量子点生长时局部应变和体系应变的补偿作用.分析了应变补偿层对体系应变的减少作用,并计算了相邻层量子点的垂直对准概率.研究结果对实验中应变补偿的优化和高质量量子点阵列的生长实现提供了理论依据.     

沉积速率和生长停顿对InAs/GaAs量子点超晶格生长影响的综合分析

采用动力学蒙特卡罗模型模拟了沉积速率和生长停顿对GaAs衬底中垂直耦合InAs 量子点超晶格生长早期阶段的影响.通过对生长表面形态、岛平均尺寸、岛尺寸分布及其标准差等方面的研究,发现综合控制沉积速率和生长停顿时间能够得到大小均匀、排列有序的岛阵列.这对后续量子点超晶格生长过程中量子点的定位有重要影响. 关键词： 动力学蒙特卡罗模拟 量子点超晶格 外延生长     

Formation and crystal structure of GaSb/GaP quantum dots

The atomic structure of GaSb/GaP quantum dots grown via molecular beam epitaxy on a (100) GaP surface at epitaxy temperatures of 420–470°C is investigated. It is established that, depending on morphology of the GaP growth surface, the deposition of 1 ML of GaSb leads to the formation of strained Ga(Sb, P)/GaP or fully relaxed GaSb/GaP quantum dots. The obtained heterostructures exhibit high photoluminescence efficiency.     

Enhancement of the Coulomb correlations in type-II quantum dots

We report on calculation of binding energies of excitons as well as positively and negatively charged excitons and biexcitons in type-II quantum dots. The shape of the GaSb/GaAs quantum dot is assumed lens-like and the energies are calculated within the Hartree–Fock approximation. A large enhancement of the binding energies has been estimated in comparison with the type-I quantum dots (InAs/GaAs) which is in good agreement with the recent experimental findings.     

Energy-selective charging of type-II GaSb/GaAs quantum dots

The hole confinement in type-II self-organized GaSb/GaAs quantum dots (QDs) was investigated by combining optical excitation and time-resolved capacitance spectroscopy. The experimental results indicate energy-selective charging even for type-II QDs. With increasing excitation energy the apparent hole activation energy decreases, which is attributed to light absorption in sub-ensembles of QDs with decreasing hole localization. The large localization energy of about 450 meV and the possibility of optical-multiplexing makes type-II GaSb/GaAs QDs a potential material system for QD memory concepts.