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.     

The Ostwald ripening at nanoengineering of InAsSbP spherical and ellipsoidal quantum dots on InAs (100) surface

We present the results of growth of quasi-ternary InAsSbP spherical and ellipsoidal quantum dots (QDs) on InAs (100) surface by the method of liquid-phase epitaxy. Coarsening of QDs due to coalescence and Ostwald ripening was investigated by atomic-force and scanning electron microscopy. Ellipsoidal QDs prolated in [010] and oblated in [001] directions have been grown. Elongation ratios for the ellipsoidal QDs were measured in all three directions. It is shown that elongation of spherical QDs to ellipsoidal is started at QDs diameter of ～50 nm. Shape transformation of the QDs’ size distribution function from the Gram-Charlier-like to the Gaussian and then to the Lifshits-Slezov-like distribution was revealed at increasing the nucleation time.     

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。     

A comprehensive study of the effect of <Emphasis Type="Italic">in situ</Emphasis> annealing at high growth temperature on the morphological and optical properties of self-assembled InAs/GaAs QDs

We investigate the effect of in situ annealing during growth pause on the morphological and optical properties of self-assembled InAs/GaAs quantum dots (QDs). The islands were grown at different growth rates and having different monolayer coverage. The results were explained on the basis of atomic force microscopy (AFM) and photo-luminescence (PL) measurements. The studies show the occurrence of ripening-like phenomenon, observed in strained semiconductor system. Agglomeration of the self-assembled QDs takes place during dot pause leading to an equilibrium size distribution. The PL properties of the QDs are affected by the Indium desorption from the surface of the QDs during dot pause annealing at high growth temperature (520°C) subsiding the effect of the narrowing of the dot size distribution with growth pause. The samples having high monolayer coverage (3.4 ML) and grown at a slower growth rate (0.032 ML s⁻¹) manifested two different QD families. Among the islands the smaller are coherent defect-free in nature, whereas the larger dots are plastically relaxed and hence optically inactive. Indium desorption from the island surface during the in situ annealing and inhomogeneous morphology as the dots agglomerate during the growth pause, also affects the PL emission from these dot assemblies.     

Ordered InAs Quantum Dots with Controllable Periods Grown on Stripe-Patterned GaAs Substrates

GaAs （001） substrates are patterned by electron beam lithography and wet chemical etching to control the nucleation of lnAs quantum dots （QDs）. InAs dots are grown on the stripe-patterned substrates by solid source molecular beam epitaxy. A thick buffer layer is deposited on the strip pattern before the deposition of InAs. To enhance the surface diffusion length of the In atoms, InAs is deposited with low growth rate and low As pressure. The AFM images show that distinct one-dimensionally ordered InAs QDs with homogeneous size distribution are created, and the QDs preferentiMly nucleate along the trench. With the increasing amount of deposited InAs and the spacing of the trenches, a number of QDs are formed beside the trenches. The distribution of additional QDs is long-range ordered, always along the trenchs rather than across the spacing regions.     

Self-organized formation of shell-like InAs/GaAs quantum dot ensembles

Formation of a multimodal quantum dot (QD) ensemble by strained layer epitaxy of InAs on GaAs near the critical value for the onset of the 2D-3D transition is studied. Reflection anisotropy spectroscopy is employed to confirm that a smooth surface is maintained during strained layer growth prior to QD formation. Instantaneous capping after deposition leads to InAs quantum wells with some thickness flucuations. Multimodal QD InAs ensembles form after an at least short growth interruption prior to cap layer deposition. The QDs consist of pure InAs with heights varying in steps of complete InAs monolayers. Related exciton energies indicate a simultaneous increase of both height and lateral extension, i.e. a shell-like increase of sizes. The formation of the multimodal QD ensemble is described by a kinetic approach. A growth scenario is presented where QDs having initially shorter base length stop vertical growth at a smaller height, accounting for the experimentally observed shell-like sub-ensemble structure.     

Size-shrinkage effect of InAs quantum dots during a GaAs capping growth

The modification of the InAs quantum dots (QDs) by the GaAs capping growth was studied by using cross-sectional STEM and atomic force microscopy. In case of the GaAs capping growth at 450 °C, it was found that the lateral size of the InAs QDs significantly decreases rather than the height and that this size-shrinkage effect is enhanced for the large QDs. The shrinkage behavior is mainly attributed to the indium surface segregation, strongly depending on the surface strain of the QDs. The growth process of the GaAs capping layer plays an important role for achieving the size ordering of the embedded QDs.     

Resonant tunneling of electrons through single self-assembled InAs quantum dot studied by conductive atomic force microscopy

Resonant tunneling of electrons through a quantum level in single self-assembled InAs quantum dot (QD) embedded in thin AlAs barriers has been studied. The embedded InAs QDs are sandwiched by 1.7-nm-thick AlAs barriers, and surface InAs QDs, which are deposited on 8.3 nm-thick GaAs cap layer, are used as nano-scale electrodes. Since the surface InAs QD should be vertically aligned with a buried one, a current flowing via the buried QD can be measured with a conductive tip of an atomic force microscope (AFM) brought in contact with the surface QD-electrode. Negative differential resistance attributed to electron resonant tunneling through a quantized energy level in the buried QD is observed in the current–voltage characteristics at room temperature. The effect of Fermi level pinning around nano-scale QD-electrode on resonance voltage and the dependence of resonance voltage on the size of QD-electrodes are investigated, and it has been demonstrated that the distribution of the resonance voltages reflects the size variation of the embedded QDs.     

近红外波段InAs量子点结构与光学特性

采用分子束外延技术,分别在480,520℃的生长温度下,制备了淀积厚度2.7ML的InAs/GaAs量子点。用原子力显微镜对样品进行形貌测试和统计分布。结果表明,在相应的生长温度下,量子点密度分别为8.0×10¹⁰,5.0×10⁹cm^-2,提高生长温度有利于获得大尺寸的量子点,并且量子点按高度呈双模分布。结合光致发光谱的分析,在480℃的生长条件下,最近邻量子点之间的合并导致了量子点尺寸的双模分布;而在525℃的生长温度下,In偏析和InAs解析是形成双模分布的主要原因。     

Theoretical study of quantum dots distribution function and the process of nucleation during Ostwald ripening in InAsSbP system

Experimental results on distribution of quantum dots versus sizes in InAsSbP system at different growth times are analyzed theoretically. It is shown that depending on growth time the process of nucleation and ripening of QDs is controlled either by transition kinetics in the grain-matrix boundary (Wagner distribution) or by the volume diffusion (Lifshitz-Slyozov distribution). Comparing theoretically calculated results with experimental data, numerical value of the reaction rate on the grain surface and the volume diffusion coefficient at nucleation temperature T = 550°C were estimated.     

Optimum Quantum Yield of the Light Emission from 2 to 10 nm Hydrosilylated Silicon Quantum Dots

Optimizing the light‐emitting efficiency of silicon quantum dots (Si QDs) has been recently intensified by the demand of the practical use of Si QDs in a variety of fields such as optoelectronics, photovoltaics, and bioimaging. It is imperative that an understanding of the optimum light‐emitting efficiency of Si QDs should be obtained to guide the design of the synthesis and processing of Si QDs. Here an investigation is presented on the characteristics of the photoluminescence (PL) from hydrosilylated Si QDs in a rather broad size region (≈2–10 nm), which enables an effective mass approximation model to be developed, which can very well describe the dependence of the PL energy on the QD size for Si QDs in the whole quantum‐confinement regime, and demonstrates that an optimum PL quantum yield (QY) appears at a specific QD size for Si QDs. The optimum PL QY results from the interplay between quantum‐confinement effect and surface effect. The current work has important implications for the surface engineering of Si QDs. To optimize the light‐emission efficiency of Si QDs, the surface of Si QDs must be engineered to minimize the formation of defects such as dangling bonds at the QD surface and build an energy barrier that can effectively prevent carriers in Si QDs from tunneling out.     

Scalable Formation of Concentrated Monodisperse Lignin Nanoparticles by Recirculation-Enhanced Flash Nanoprecipitation

A highly controllable and scalable process for fabrication of large amounts of concentrated lignin nanoparticles (LNPs) is reported. These lignin core nanoparticles are formed through flash nanoprecipitation, however, scaling up of the fabrication process requires fundamental understanding of their operational formation mechanism and surface properties. It is shown how a semicontinuous synthesis system with a recirculation loop makes it possible to produce flash precipitated lignin nanoparticles in large amounts for practical applications. The roles of the process parameters, including flow rates and lignin concentration, are investigated and analyzed. The results indicate that the LNPs are formed by a process of continuous burst nucleation at the point of mixing without diffusive growth, which yields nanoparticles of highly uniform size following a modified LaMer nucleation and growth mechanism. This mechanism makes possible facile process control and scale-up. Effective control of the resulting nanoparticle size is achieved through the initial concentration of lignin in the injected solution. The impressive capability to produce suspensions of any predesigned multimodal distribution is demonstrated. The resulting nanofabrication technique can produce large volumes of concentrated LNP suspensions of high stability and tightly controlled size distributions for biological or agricultural applications.     

Formation of InAs quantum dots and wetting layers in GaAs and AlAs analyzed by cross-sectional scanning tunneling microscopy

We have used cross-sectional scanning-tunneling microscopy (X-STM) to compare the formation of self-assembled InAs quantum dots (QDs) and wetting layers on AlAs (1 0 0) and GaAs (1 0 0) surfaces. On AlAs we find a larger QD density and smaller QD size than for QDs grown on GaAs under the same growth conditions (500 °C substrate temperature and 1.9 ML indium deposition). The QDs grown on GaAs show both a normal and a lateral gradient in the indium distribution whereas the QDs grown on AlAs show only a normal gradient. The wetting layers on GaAs and AlAs do not show significant differences in their composition profiles. We suggest that the segregation of the wetting layer is mainly strain-driven, whereas the formation of the QDs is also determined by growth kinetics. We have determined the indium composition of the QDs by fitting it to the measured outward relaxation and lattice constant profile of the cleaved surface using a three-dimensional finite element calculation based on elasticity theory.     

Composition of the “GaAs” quantum dot,grown by droplet epitaxy

Self-assembled strain-free quantum dot (QD) structures were grown on AlGaAs surface by the droplet epitaxal method. The QDs were developed from pure Ga droplets under As pressure. The QDs were investigated by atomic force microscopy (AFM) and transmission electron microscopy (TEM). Both techniques show that the QDs are very uniform in size and their distribution on the surface is also homogeneous. The high resolution cross-sectional TEM investigation shows perfect lattice matching between the QD and the substrate, and also the faceting of the side walls of QD can be identified exactly by lattice planes. Analytical TEM (elemental mapping by EELS) unambiguously identifies the presence of Al in the QD.     

Tuning vertically stacked InAs/GaAs quantum dot properties under spacer thickness effects for 1.3 μm emission

Coherent InAs islands separated by GaAs spacer (d) layers are shown to exhibit self-organized growth along the vertical direction. A vertically stacked layer structure is useful for controlling the size distribution of quantum dots. The thickness of the GaAs spacer has been varied to study its influence on the structural and optical properties. The structural and optical properties of multilayer InAs/GaAs quantum dots (QDs) have been investigated by atomic force microscopy (AFM), transmission electron microscopy (TEM), and photoluminescence (PL) measurements. The PL full width at half maximum (FWHM), reflecting the size distribution of the QDs, was found to reach a minimum for an inter-dots GaAs spacer layer thickness of 30 monolayers (ML). For the optimized structure, the TEM image shows that multilayer QDs align vertically in stacks with no observation of apparent structural defects. Furthermore, AFM images showed an improvement of the size uniformity of the QDs in the last layer of QDs with respect to the first one. The effect of growth interruption on the optical properties of the optimized sample (E30) was investigated by PL. The observed red shift is attributed to the evolution of the InAs islands during the growth interruption. We show the possibility of increasing the size of the QDs approaching the strategically important 1.3 m wavelength range (at room temperature) with growth interruption after InAs QD deposition.     

Successful growth of two different quantum dots on one substrate

We report the successful growth of ZnSe and ZnTe quantum dots (QDs) embedded in ZnS on GaAs substrate. These QDs have good optical properties and show quantum confinement effect. High-resolution electron scanning microscope studies show that these QDs are grown in Volmer–Weber mode. It is found that the size of the QDs is controlled by the growth duration. When the growth time is short, high density of QDs could be fabricated, but with a long growth time the small QDs get together to form a large cluster. We also show that with this growth method it is possible to grow both ZnSe quantum and ZnTe QDs on one substrate at the same time. For this dual QDs system, two peaks corresponding to the emission from the ZnSe dots (3.0 eV, blue–violet) and ZnTe dots (2.6 eV, green–blue) could be observed at the same time in the photoluminescence measurement.     

CdSe/ZnSe/ZnS多壳层结构量子点的制备与表征

展示了一种简捷的多壳层量子点合成路线。在含有过量Se源的CdSe体系中直接注入Zn源,"一步法"合成了CdSe/ZnSe量子点;进一步以CdSe/ZnSe为"核",表面外延生长ZnS壳层制备了核/壳/壳结构CdSe/ZnSe/ZnS量子点。相对于以往报道的多壳层结构量子点的制备方法,该方法通过减少壳层的生长步骤有效地简化了实验操作,缩短了实验周期,同时减少对原料的损耗。对量子点进行高温退火处理,能够大幅提高CdSe/ZnSe/ZnS量子点的发光量子产率。透射电镜、XRD以及光谱研究表明:所制备的量子点接近球形,核与壳层纳米晶均为闪锌矿结构,最终获得的CdSe/ZnSe/ZnS量子点的光致发光量子产率达到53％。为了实现量子点的表面生物功能化,通过巯基酸进行了表面配体交换修饰,使量子点表面具有水溶性的羧基功能团,并且能够维持较高的光致发光量子产率。     

Evolution of surface morphology and photoluminescence characteristics of 1.3-μm In0.5Gao.5As/GaAs quantum dots grown by molecular beam epitaxy

Evolution of surface morphology and optical characteristics of 1.3-μm In0.5Gao.5As/GaAs quantum dots (QDs) grown by molecular beam epitaxy (MBE) are investigated by atomic force microscopy (AFM) and photoluminescence (PL). After deposition of 16 monolayers (ML) of In0.5Ga0.5As, QDs are formed and elongated along the [110] direction when using sub-ML depositions, while large size InGaAs QDs with better uniformity are formed when using ML or super-ML depositions. It is also found that the larger size QDs show enhanced PL efficiency without optical nonlinearity, which is in contrast to the elongated QDs.     

Kinetics of self-assembled InN quantum dots grown on Si (111) by plasma-assisted MBE

One of the scientific challenges of growing InN quantum dots (QDs), using Molecular beam epitaxy (MBE), is to understand the fundamental processes that control the morphology and distribution of QDs. A systematic manipulation of the morphology, optical emission, and structural properties of InN/Si (111) QDs is demonstrated by changing the growth kinetics parameters such as flux rate and growth time. Due to the large lattice mismatch, between InN and Si (~8%), the dots formed from the Strannski–Krastanow (S–K) growth mode are dislocated. Despite the variations in strain (residual) and the shape, both the dot size and pair separation distribution show the scaling behavior. We observed that the distribution of dot sizes, for samples grown under varying conditions, follow the scaling function.     

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。