自组织量子点的优化生长

在不同生长条件下,生长低组分InGaAs/GaAs自组织量子点并且使用接触式AFM进行测量.通过对生长条件的优化,得到高密度、高均匀性的量子点MBE生长条件,这对于自组织量子点在器件方面的应用,比如量子点红外探测器和量子点激光器,是非常重要的.同时,还与优化的InAs/GaAs生长条件进行了比较.     

液滴外延自组织GaAs纳米结构

介绍了利用液滴外延法在晶格匹配体系AlGaAs/GaAs上自组织生长几种GaAs纳米结构.实验证实Ga液滴的形貌随晶化温度和As束流的不同而发生变化,形成一些有趣的GaAs纳米结构,如量子点、量子单环、量子双环、耦合量子双环和中国古币形状等.本文对这些纳米结构的生长机制进行了讨论.     

液滴外延自组织GaAs纳米结构

介绍了利用液滴外延法在晶格匹配体系AlGaAs/GaAs上自组织生长几种GaAs纳米结构.实验证实Ga液滴的形貌随晶化温度和As束流的不同而发生变化,形成一些有趣的GaAs纳米结构,如量子点、量子单环、量子双环、耦合量子双环和中国古币形状等.本文对这些纳米结构的生长机制进行了讨论.     

MBE自组装量子点生长和结构形态研究(续)

3.2 InAs/GaAs(001) 与Ge/Si量子点实验研究结果的普适性和简单性相比,InAs/GaAs的情况要复杂得多,对生长条件的敏感性也要高许多.G.Constantini等人[46]提出对Ge/Si体系的观察结果也同样适用于InAs/GaAs,但是他们的InAs量子点生长是以在500℃、0.008 ML/s的速率进行的.     

InGaAs／GaAs自组织量子点光致发光特性研究

用PL谱测试研究了GaAs和不同In组份InxGa1-xAs(x=0.1,0.2,0.3)覆盖层对分子外延生长的InAs/GaAs自组织量子点发光特性的影响,用InxGa1-xAs外延层覆盖InAs/GaAs量子点,比用GaAs做 其发光峰能量向低有端移动,发光峰半高度变窄,量子点发光峰能量随温度的红移幅度较小,理论计算证实这是由于覆盖层InxGa1-xAs减小了InAs表面应力导致发光峰红移,而In元素有效抑制了InAs/GaAs界面组份的混杂,量子点的均匀性得到改善,PL谱半高宽变窄,用InGaAs覆盖的In0.5Ga0.5As/GaAs自组织量子点实现了1．3μm发光,室温下PL谱半高宽为19．2meV,是目前最好的实验结果。     

In0．2Ga0．8As-GaAs复合应力缓冲层上的1.3μm InAs／GaAs自组织量子点

用光荧光谱和原子力显微镜测试技术系统研究了在2 nm In0.2Ga0.8As和x ML GaAs的复合应力缓冲层上生长的InAs/GaAs自组织量子点的发光特性和表面形貌.采用In0.2Ga0.8As与薄层GaAs复合的应力缓冲层,由于减少了晶格失配度致使量子点密度从约1.7×109 cm-2显著增加到约3.8×109cm-2.同时,复合层也有利于提高量子点中In的组份,使量子点的高宽比增加,促进量子点发光峰红移.对于x=10 ML的样品室温下基态发光峰达到1350 nm.     

空间有序的量子点超晶格的红外吸收

利用分子束外延技术,在高温下(540℃)生长了具有三维空间有序的自组织InGaAs/GaAs量子点超晶格结构,利用傅里叶变换红外光谱仪测量到了明显的垂直人射吸收峰,中心响应波长在11μm.作为对比,在低温下(480℃)生长了相同的结构,傅里叶变换红外光谱几乎没有测量到明显的垂直入射吸收峰.高分辨率X射线双晶衍射测量表明高温生长的量子点超晶格具有更好的晶体质量,原子力显微镜测量表明在高温540℃下生长的量子点具有明显的横向有序;而在低温480℃下生长的量子点并没有显示出横向有序.在进行垂直入射的吸收测量时,为了扣除量子点超晶格的周期结构带来的干涉效应,提出使用生长条件完全相同但量子点区没有掺杂的样品作为背景,提高了测量的准确性及分辨率.结果表明空间有序的量子点超晶格结构比空间无序的量子点超晶格更适宜作红外探测器结构.     

空间有序的量子点超晶格的红外吸收

利用分子束外延技术,在高温下(540℃)生长了具有三维空间有序的自组织InGaAs/GaAs量子点超晶格结构,利用傅里叶变换红外光谱仪测量到了明显的垂直人射吸收峰,中心响应波长在11μm.作为对比,在低温下(480℃)生长了相同的结构,傅里叶变换红外光谱几乎没有测量到明显的垂直入射吸收峰.高分辨率X射线双晶衍射测量表明高温生长的量子点超晶格具有更好的晶体质量,原子力显微镜测量表明在高温540℃下生长的量子点具有明显的横向有序;而在低温480℃下生长的量子点并没有显示出横向有序.在进行垂直入射的吸收测量时,为了扣除量子点超晶格的周期结构带来的干涉效应,提出使用生长条件完全相同但量子点区没有掺杂的样品作为背景,提高了测量的准确性及分辨率.结果表明空间有序的量子点超晶格结构比空间无序的量子点超晶格更适宜作红外探测器结构.     

GaSb量子点液相外延生长

采用改进的快速推舟液相外延技术在GaAs衬底上成功地生长了GaSb量子点材料.通过原子力显微镜观测了不同生长参数下GaSb量子点材料的形貌(形状、尺寸、密度、尺寸分布均匀性等).分析了不同衬底、不同生长源配比、生长源与衬底的不同接触时间等生长条件参数对GaSb量子点生长的影响.研究表明在GaAs衬底上、富镓生长源配比以及较短的生长源和衬底接触时间下更易获得高质量的GaSb量子点.上述生长条件的摸索和研究对于GaSb量子点器件应用具有重要意义.     

半导体量子点的发光性质研究

用PL谱测试研究了半导体量子点的光致发光性能,分析了不同In组分覆盖层对分子束外延生长的量子点发光特性的影响,及导致发光峰红移的原因.结果显示,In元素有效抑制界面组分的混杂,使得量子点的均匀性得到改善,PL谱半高宽变窄.用InAs覆盖的In0.5Ga0.5As/GaAs自组织量子点实现了1.3um发光.     

The sandwich InGaAs/GaAs quantum dot structure for IR photoelectric detectors

A new possibility for growing InAs/GaAs quantum dot heterostructures for infrared photoelectric detectors by metal-organic vapor-phase epitaxy is discussed. The specific features of the technological process are the prolonged time of growth of quantum dots and the alternation of the low-and high-temperature modes of overgrowing the quantum dots with GaAs barrier layers. During overgrowth, large-sized quantum dots are partially dissolved, and the secondary InGaAs quantum well is formed of the material of the dissolved large islands. In this case, a sandwich structure is formed. In this structure, quantum dots are arranged between two thin layers with an increased content of indium, namely, between the wetting InAs layer and the secondary InGaAs layer. The height of the quantum dots depends on the thickness of the GaAs layer grown at a comparatively low temperature. The structures exhibit intraband photoconductivity at a wavelength around 4.5 μm at temperatures up to 200 K. At 90 K, the photosensitivity is 0.5 A/W, and the detectivity is 3 × 10⁹ cm Hz^1/2W^?1.     

Effect of local structural defects on the precipitation of as in the vicinity of InAs quantum dots in a GaAs matrix

Electron-microscopy studies of GaAs structures grown by the method of molecular-beam epitaxy and containing arrays of semiconductor InAs quantum dots and metallic As quantum dots are performed. An array of InAs quantum dots is formed using the Stranski-Krastanow mechanism and consists of five layers of vertically conjugated quantum dots divided by a 5-nm-thick GaAs spacer layer. The array of As quantum dots is formed in an As-enriched GaAs layer grown at a low temperature above an array of InAs quantum dots using postgrowth annealing at temperatures of 400–600°C for 15 min. It is found that, during the course of structure growth near the InAs quantum dots, misfit defects are formed; these defects are represented by 60° or edge dislocations located in the heterointerface plane of the semiconductor quantum dots and penetrating to the surface through a layer of “low-temperature” GaAs. The presence of such structural defects leads to the formation of As quantum dots in the vicinity of the middle of the InAs conjugated quantum dots beyond the layer of “low-temperature” GaAs.     

Molecular beam epitaxy of AlGaAs/Zn(Mn)Se hybrid nanostructures with InAs/AlGaAs quantum dots near the heterovalent interface

Features of the growth of InAs quantum dots in an Al_0.35Ga_0.65As matrix by molecular beam epitaxy at different substrate temperatures, deposition rates, and amounts of deposited InAs are studied. The optimum conditions for growing an array of low-density (≤2 × 10¹⁰ cm^?2) small (height of no more than 4 nm) self-organized quantum dots are determined. The possibility of the formation of optically active InAs quantum dots emitting in the energy range 1.3–1.4 eV at a distance of no more than 10 nm from the coherent heterovalent GaAs/ZnSe interface is demonstrated. It is established that inserting an optically inactive 5-nm GaAs quantum well resonantly coupled with InAs quantum dots into the upper AlGaAs barrier layer enhances the photoluminescence efficiency of the quantum-dot array in hybrid heterostructures.     

Self-organized anisotropic strain engineering: a new concept for quantum dot ordering

We have established a new concept for creating ordered arrays of quantum dots by self-organized epitaxy. The concept is based on self-organized anisotropic strain engineering of strained layer templates and is demonstrated for (In,Ga)As/GaAs superlattice structures on GaAs (100) and strain-induced (In,Ga)As growth instability on GaAs (311)B. The well-defined one- and two-dimensional networks of InAs quantum dots grown on top of these templates are of excellent structural and optical quality. This breakthrough, thus, allows for novel fundamental studies and device operation principles based on single and multiple carrier- and photon-, and coherent quantum interference effects.     

In<Subscript>0.8</Subscript>Ga<Subscript>0.2</Subscript>As Quantum Dots for GaAs Solar Cells: Metal-Organic Vapor-Phase Epitaxy Growth Peculiarities and Properties

The growth peculiarities of In_0.8Ga_0.2As quantum dots and their arrays on GaAs surface by metalorganic vapor-phase epitaxy are investigated. The bimodal size distribution of In_0.8Ga_0.2As quantum dots is established from the photoluminescence spectra recorded at different temperatures. The growth parameters were determined at which the stacking of 20 In_0.8Ga_0.2As quantum-dot layers in the active area of a GaAs solar cell makes it possible to enhance the photogenerated current by 0.97 and 0.77 mA/cm² for space and terrestrial solar spectra, respectively, with the high quality of the p–n junction retained. The photogenerated current in a solar cell with quantum dots is higher than in the reference GaAs structure by ~1% with regard to nonradiative-recombination loss originating from stresses induced by the quantum-dot array.     

In0.4Ga0.6As/GaAs自组织量子点的光伏谱温度特性

自组织生长方法作为一种有效而直接的制备半导体量子点的方法受到重视。本文采用无需在样品上制备电极的电容耦合的光伏谱方法,实验测量了Ｉｎ０．４Ｇａ０．６Ａｓ／ＧａＡｓ自组织量子点在不同的温度下的光伏谱,对测量谱峰进行了指认,研究了量子点谱峰能量位置随温度的依赖关系,实验结果表明,量子点具有与体材料及二维体系不同的温度特性,对实验所测样品,其激子峰能量随温度增加而红移的速率约为ＧａＡｓ体材料带隙变化的１     

Optoelectronic properties of heteronanostructures with combined layers of In(Ga)As/GaAs quantum wells and dots

The effect of the incorporation of an InGaAs quantum well into structures with InAs/GaAs quantum dots grown by gas-phase epitaxy on their optoelectronic properties is analyzed in the mode with increased growth-interruption time. It is established that the quantum-dot energy spectrum is weakly sensitive to variations in the thickness and composition of the double InGaAs/GaAs coating layer. The deposition of a quantum well onto a layer of quantum dots decreases the emission-barrier effective height in them. The conditions under which the quantum well can be used for protecting the quantum-dot active layer against penetration by defects generated during structure-surface anodic oxidation are determined.     

利用大周期光子晶体结构增强InAs/GaAs量子点的激发态发光

在该研究中,通过激光全息和湿法腐蚀的方法在InAs/GaAs量子点材料上制备光子晶体,研究了由激光二极管激发制备了光子晶体的InAs / GaAs量子点材料的光致发光光谱.发现具有光子晶体的量子点材料的光谱显示出多峰结构,光子晶体对短波长部分的发光增强和调制比对长波长部分的增强和调制更明显.InAs / GaAs量子点的光致发光光谱通过刻蚀形成的光子晶体结构得到了调控,并且量子点的激发态发光得到了明显增强.     

Indirect band gaps in quantum dots made from direct-gap bulk materials

The conditions under which the band gaps of free standing and embedded semiconductor quantum dots are direct or indirect are discussed. Semiconductor quantum dots are classified into three categories; (i) free standing dots, (ii) dots embedded in a direct gap matrix, and (iii) dots embedded in an indirect gap matrix. For each category, qualitative predictions are first discussed, followed by the results of both recent experiments and state of the art pseudopotential calculations. We show that: