High-density and narrow size-distribution InAs quantum dots formed by a modified two-step growth

We develop a modified two-step method of growing high-density and narrow size-distribution InAs/GaAs quantum dots (QDs) by molecular beam epitaxy. In the first step, high-density small InAs QDs are formed by optimizing the continuous deposition amount. In the second step, deposition is carried out with a long growth interruption for every 0.1 InAs monolayer. Atomic force microscope images show that the high-density ($\sim $5.9$\times $10$^{10}$\,cm$^{ - 2})$ good size-uniformity InAs QDs are achieved. The strong intensity and narrow linewidth (27.7\,meV) of the photoluminescence spectrum show that the QDs grown in this two-step method have a good optical quality.     

High-density and narrow size-distribution InAs quantum dots formed by a modified two-step growth

We develop a modified two-step method of growing high-density and narrow size-distribution InAs/GaAs quantum dots (QDs) by molecular beam epitaxy. In the first step, high-density small InAs QDs are formed by optimizing the continuous deposition amount. In the second step, deposition is carried out with a long growth interruption for every 0.1 InAs monolayer. Atomic force microscope images show that the high-density ($\sim $5.9$\times $10$^{10}$\,cm$^{ - 2})$ good size-uniformity InAs QDs are achieved. The strong intensity and narrow linewidth (27.7\,meV) of the photoluminescence spectrum show that the QDs grown in this two-step method have a good optical quality.     

Self-assembly of InAs quantum dots on GaAs(001)by molecular beam epitaxy

Currently, the nature of self-assembly of three-dimensional epitaxial islands or quantum dots (QDs) in a lattice-mismatched heteroepitaxial growth system, such as InAs/GaAs(001) and Ge/Si(001) as fabricated by molecular beam epitaxy (MBE), is still puzzling. The purpose of this article is to discuss how the self-assembly of InAs QDs in MBE InAs/GaAs(001) should be properly understood in atomic scale. First, the conventional kinetic theories that have traditionally been used to interpret QD self-assembly in heteroepitaxial growth with a significant lattice mismatch are reviewed briefly by examining the literature of the past two decades. Second, based on their own experimental data, the authors point out that InAs QD self-assembly can proceed in distinctly different kinetic ways depending on the growth conditions and so cannot be framed within a universal kinetic theory, and, furthermore, that the process may be transient, or the time required for a QD to grow to maturity may be significantly short, which is obviously inconsistent with conventional kinetic theories. Third, the authors point out that, in all of these conventional theories, two well-established experimental observations have been overlooked: i) A large number of “floating” indium atoms are present on the growing surface in MBE InAs/GaAs(001); ii) an elastically strained InAs film on the GaAs(001) substrate should be mechanically unstable. These two well-established experimental facts may be highly relevant and should be taken into account in interpreting InAs QD formation. Finally, the authors speculate that the formation of an InAs QD is more likely to be a collective event involving a large number of both indium and arsenic atoms simultaneously or, alternatively, a morphological/structural transformation in which a single atomic InAs sheet is transformed into a three-dimensional InAs island, accompanied by the rehybridization from the sp²-bonded to sp³- bonded atomic configuration of both indium and arsenic elements in the heteroepitaxial growth system.     

Photoluminescence and lasing properties of InAs/GaAs quantum dots grown by metal-organic chemical vapour deposition

Photoluminescence （PL） and lasing properties of InAs/GaAs quantum dots （QDs） with different growth procedures prepared by metalorganic chemical vapour deposition are studied. PL measurements show that the low growth rate QD sample has a larger PL intensity and a narrower PL line width than the high growth rate sample. During rapid thermal annealing, however, the low growth rate sample shows a greater blueshift of PL peak wavelength. This is caused by the larger InAs layer thickness which results from the larger 2-3 dimensional transition critical layer thickness for the QDs in the low-growth-rate sample. A growth technique including growth interruption and in-situ annealing, named indium flush method, is used during the growth of GaAs cap layer, which can flatten the GaAs surface effectively. Though the method results in a blueshift of PL peak wavelength and a broadening of PL line width, it is essential for the fabrication of room temperature working QD lasers.     

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     

The refractive nonlinearities of InAs/GaAs quantum dots above-bandgap energy

The third-order optical nonlinear refractive properties of InAs/GaAs quantum dots grown by molecular beam epitaxy have been measured using the reflection Z-scan technique at above-bandgap energy. The nonlinear refractive index and nonlinear absorption index of the InAs/GaAs quantum dots were determined for wavelengths from 740 to 777 nm. The measured results are compared with the nonlinear refractive response of several typical III-V group semiconductor materials. The corresponding mechanisms responsible for the large nonlinear response are discussed.     

Studying the mechanism of ordered growth of InAs quantum dots on GaAs/InP

We study the mechanism of ordered growth of InAs quantum dots (islands) on a GaAs/InP substrate in theory and point out that the tensile strain can be used to control InAs/InP self-assembled quantum dots arrangement. Photoluminescence spectrum, and atomic force microscopy images have been investigated. In the experiment, ordered InAs islands have been obtained and the maximum density of quantum dots is 1.6×10¹⁰ cm⁻² at 4 monolayers InAs layer.     

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.     

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.     

The effect of post‐growth thermal annealing on the emission spectra of GaAs/AlGaAs quantum dots grown by droplet epitaxy

We fabricated GaAs/AlGaAs quantum dots by droplet epitaxy, and obtained the geometries of the dots by scanning transmission electron microscopy. Post‐growth thermal annealing is essential for the optical activation of quantum dots grown by droplet epitaxy. We measured the emission energy shifts of the dots and the underlying superlattice by post‐ growth thermal annealing, and specified the emission from dots by selectively etching the structure down to a low layer of quantum dots. We studied the influence of the degree of annealing on the optical properties of the dots from the peak shifts of the superlattice, since the superlattice has a uniform and well‐defined geometry. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The preferential formation site of dislocations in InAs/GaAs quantum dots

In this paper, taking elastic anisotropy into consideration, we use a dislocation position dependent model to calculate the preferential formation site of the pure edge and 60° mixed dislocation segment in different shaped InAs/GaAs quantum dots (QDs). From the result, it is clear that for the pure edge dislocations the most energy favorable position is always the base center of the quantum dots. While as to the 60° mixed dislocations, the positions near to the edge of the quantum dot base are the energy favorable area and the exact position is changed with different aspect ratio of the quantum dot.     

Raman study of InAs quantum dots on GaAs/InP grown by low pressure metal-organic chemical vapor deposition

Raman spectra of InAs quantum dots (QDs) on InP substrate were investigated. Both longitudinal-optic (LO) and transverse-optic (TO) frequency of InAs QDs showed a large blue-shift comparing to its bulk due to the compressive strain in InAs QDs. Raman scattering of InAs QDs with a thin GaAs interlayer was studied. We obtained that the peak position of LO and TO mode of InAs QDs became larger blue-shifted when we inserted the GaAs layer. At the same time, we found a red-shift of the frequency of GaAs LO mode because of tensile strain. Theoretical calculation was performed and its prediction coincided with our experiment results well. They both showed that strain played an important role in formation of InAs QDs.     

Size dependence of biexciton binding energy in single InAs/GaAs quantum dots

This paper studies the size dependence of biexciton binding energy in single quantum dots (QDs) by using atomic force microscopy and micro-photoluminescence measurements. It finds that the biexciton binding energies in the QDs show ``binding' and ``antibinding' properties which correspond to the large and small sizes of QDs, respectively. The experimental results can be well interpreted by the biexciton potential curve, calculated from the exciton molecular model and the Heitler--London method.     

Optical probe of InAs/GaAs self-assembled quantum dots grown using low growth rate and growth interruptions

We have investigated the optical properties of InAs/GaAs self-assembled quantum dots (QDs), grown at 500 °C using a low growth rate (0.014 ML/s), growth interruptions and a two-stage capping process. The samples exhibited large-size dots with densities in the range (3-4.5) × 10⁹ cm⁻². Macro-photoluminescence (macro-PL) measurements revealed the presence of five electronic sub-bands in the dots, with the ground state (GS) emission exhibiting a linewidth of ∼70 meV. Because of the dots large size and composition dispersions, associated with the growth method, it was possible to resolve single dots emissions using micro-PL (μ-PL) excitation in the barrier layers of the as-grown samples. The sharp PL lines were detected 60-140 meV above the GS peak energy. High-resolution resonant optical excitation of the dots PL evidenced that these fine lines originate from exciton complexes confined to the GS of individual dots. Non-resonant power dependence μ-PL spectroscopy results further confirmed the occurrence of both single exciton (X) and biexciton (XX) radiative recombinations. Finally, with increasing lattice temperature up to 95 K, PL emissions from most of these nanostructures suffered the usual thermal quenching, with activation energies (E_a) ranging between 12 and 41 meV. The relatively small values of E_a suggest that the growth technique implemented here favors the formation of defects centers in the vicinity of the QDs.     

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.     

Anomalous temperature dependence of photoluminescence spectra from InAs/GaAs quantum dots grown by formation–dissolution–regrowth method

Two kinds of InAs/GaAs quantum dot(QD) structures are grown by molecular beam epitaxy in formation–dissolution–regrowth method with different in-situ annealing and regrowth processes. The densities and sizes of quantum dots are different for the two samples. The variation tendencies of PL peak energy, integrated intensity, and full width at half maximum versus temperature for the two samples are analyzed, respectively. We find the anomalous temperature dependence of the InAs/GaAs quantum dots and compare it with other previous reports. We propose a new energy band model to explain the phenomenon. We obtain the activation energy of the carrier through the linear fitting of the Arrhenius curve in a high temperature range. It is found that the Ga As barrier layer is the major quenching channel if there is no defect in the material. Otherwise, the defects become the major quenching channel when some defects exist around the QDs.     

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.     

Numerical investigation on threading dislocation bending with InAs/GaAs quantum dots

The threading dislocations (TDs) in GaAs/Si epitaxial layers due to the lattice mismatch seriously degrade the performance of the lasers grown on silicon. The insertion of InAs quantum dots (QDs) acting as dislocation filters is a pretty good alternative to solving this problem. In this paper, a finite element method (FEM) is proposed to calculate the critical condition for InAs/GaAs QDs bending TDs into interfacial misfit dislocations (MDs). Making a comparison of elastic strain energy between the two isolated systems, a reasonable result is obtained. The effect of the cap layer thickness and the base width of QDs on TD bending are studied, and the results show that the bending area ratio of single QD (the bending area divided by the area of the QD base) is evidently affected by the two factors. Moreover, we present a method to evaluate the bending capability of single-layer QDs and multi-layer QDs. For the QD with 24-nm base width and 5-nm cap layer thickness, taking the QD density of 10¹¹ cm^-2 into account, the bending area ratio of single-layer QDs (the area of bending TD divided by the area of QD layer) is about 38.71%. With inserting five-layer InAs QDs, the TD density decreases by 91.35%. The results offer the guidelines for designing the QD dislocation filters and provide an important step towards realizing the photonic integration circuits on silicon.     

InAs/GaAs量子点1.3 μm单光子发射特性

采用双层耦合量子点的分子束外延生长技术生长了InAs/GaAs量子点样品,把量子点的发光波长成功地拓展到1.3 μm.采用光刻的工艺制备了直径为3 μm的柱状微腔,提高了量子点荧光的提取效率.在低温5 K下,测量得到量子点激子的荧光寿命约为1 ns;单量子点荧光二阶关联函数为0.015,显示单量子点荧光具有非常好的单光子特性;利用迈克耳孙干涉装置测量得到单光子的相干时间为22 ps,对应的谱线半高全宽度为30 μeV,且荧光谱线的线型为非均匀展宽的高斯线型.     

Investigation of populated InAs/GaAs quantum dots by photoluminescence and photoreflectance

We studied self-assembled InAs/GaAs quantum dots by contrasting photoluminescence and photoreflectance spectra from 10 K to room temperature. The photoluminescence spectral profiles comprise contributions from four equally separated energy levels of InAs quantum dots. The emission profiles involving ground state and excited states have different temperature evolution. Abnormal spectral narrowing occurred above 200 K. In the photoreflectance spectra, major features corresponding to the InAs wetting layer and GaAs layers were observed. Temperature dependences of spectral intensities of these spectral features indicate that they originate from different photon-induced modulation mechanisms. Considering interband transitions of quantum dots were observed in photoluminescence spectra and those of wetting layer were observed in photoreflectance profiles, we propose that quantum dot states of the system are occupied up to the fourth energy level which is below the wetting layer quantum state.