Theoretical and experimental study of the effect of InAs growth rate on the properties of QD arrays in InAs/GaAs system

The dependence of properties of quantum dot (QD) arrays in an InAs/GaAs system on the InAs growth rate has been investigated theoretically and experimentally. The derived kinetic model of the formation of coherent nanoislands allows the calculation of the average size, surface density of islands, and wetting layer thickness as functions of the growth time and conditions. Optical properties of InAs/GaAs QDs have been studied for the case of two monolayers (ML) of the material deposited at different growth rates. Predictions of the theoretical model are compared with the experimental data. It is shown that with two ML of the deposited material the characteristic lateral size of QDs decreases and the thickness of the residual wetting layer increases with rising growth rate.     

Effect of the cap-layer composition on the electronic properties of InAs/GaAs quantum dots

The capping of an array of self-assembled InAs/GaAs quantum dots by an InGaAs quantum-well layer leads to an increase in their size due to indium enrichment of the region near the top of the quantum dots, which decreases the energy of the ground-state optical transition in quantum dots by 50 meV and shifts the hole wave function toward the top of the quantum dot.     

Ultra-low density InAs quantum dots

We show that InAs quantum dots (QDs) can be grown by molecular beam epitaxy (MBE) with an ultralow density of sin 10⁷ cm^?2 without any preliminary or post-growth surface treatment. The strain-induced QD formation proceeds via the standard Stranski-Krastanow mechanism, where the InAs coverage is decreased to 1.3–1.5 monolayers (MLs). By using off-cut GaAs (100) substrates, we facilitate the island nucleation in this subcritical coverage range without any growth interruption. The QD density dependences on the InAs coverage are studied by photoluminescence, atomic force microscopy, transmission electron microscopy, and are well reproduced by the universal double exponential shapes. This method enables the fabrication of InAs QDs with controllable density in the range 10⁷–10⁸ cm^?2, exhibiting bright photoluminescence.     

不同淀积厚度InAs量子点的喇曼散射

利用喇曼散射方法在77K温度下对不同淀积厚度的InAs/GaAs量子点材料进行了研究.在高于InAs体材料LO模的频率范围内观察到了量子点的喇曼特征峰,分析表明应变效应是影响QD声子频率的主要因素.实验显示,随着量子点层淀积厚度L的增加,InAs量子点的声子频率由于应变释放发生红移.在加入InAlAs应变缓冲层的样品中,类AlAs声子峰随L增大发生了蓝移,从侧面证实了InAs量子点层的应变释放过程.     

Effects of GaAs-spacer strain on vertical ordering of stacked InAs quantum dots in a GaAs matrix

Vertical ordering in stacked layers of InAs/GaAs quantum dots is currently the focus of scientific research because of its potential for optoelectronics applications. Transmission electron microscopy was applied to study InAs/GaAs stacked layers grown by molecular-beam-epitaxy with various thicknesses of GaAs spacer. Thickness dependencies of quantum dot size and their ordering were observed experimentally and, then, compared with the results of strain calculations based on the finite element method. The vertical ordering did occur when the thickness of the GaAs spacer was comparable with the dot height. The ordering was found to be associated with relatively large InAs dots on the first layer. Quantum dots tend to become larger in size and more regular in plane with increasing numbers of stacks. Our results suggest that the vertical ordering is not only affected by strain from the InAs dots on the lower layer, but by total strain configuration in the multi-stacked structure.     

Self-organized InAs/GaAs quantum dots multilayers with growth interruption emitting at 1.3 μm

We have grown single, 10 and 20 InAs/GaAs quantum dots (QDs) multilayers by molecular beam epitaxy in Stranski-Krastanov growth mode with and without growth interruption. Multilayer structures of InAs QDs have been studied by photoluminescence (PL) and atomic force microscopy (AFM) techniques. Between 1 and 10 layers of QDs, 10 K PL shows a shift energy, and a PL linewidth reduction. Moreover, AFM image of the 10 layers sample shows that the InAs QDs size remains constant and almost uniform when the growth is without interruption. These effects are attributed to electronic coupling between QDs in the the columns. However, we show the possibility of extending the spectral range of luminescence due to InAs QDs up to 1.3 μm. Realisation of such a wavelength emission is related to formation of lateral associations or coupling of QDs (LAQDs or LCQDs) during InAs deposition when growth interruption (20 s) is used after each InAs QDs layer deposition. The growth interruption applied after the deposition of the InAs layer allows the formation of well-developed InAs dots (large dot size).     

快速率生长MBE InAs/GaAs(001)量子点

用快速率(1.0ML/s)生长MBE InAs/GaAs(001)量子点。原子力显微镜观察结果表明,在量子点体系形成的较早阶段,量子点密度N(θ)随InAs沉积量θ的变化符合自然指数形式N(θ)∝ek(θ-θc),这与以前在慢速生长(≤0.1ML/s)条件下出现的标度规律N(θ)∝(θ-θc)α明显不同。另外,在N(θ)随θ增加的过程中,快速率生长量子点的高度分布没有经历量子点平均高度随沉积量θ逐渐增加的过程。这些实验观察说明,以原子在生长表面作扩散运动为基础的生长动力学理论至少是不全面的,不适用于解释InAs量子点的形成。这些观察和讨论说明,即使在1.0ML/s的快速率生长条件下,量子点密度也可以通过InAs沉积量有效地控制在1.0×108cm-2以下,实现低密度InAs量子点体系的制备。     

Growth and characterizations of InP self-assembled quantum dots embedded in InAIP grown on GaAs substrates

We report the characteristics of InP self-assembled quantum dots embedded in In_0.5Al_0.5P on GaAs substrates grown by metalorganic chemical vapor deposition. The InP quantum dots show increased average dot sizes and decreased dot densities, as the growth temperature increases from 475°C to 600°C with constant growth time. Above the growth temperature of 600°C, however, dramatically smaller and densely distributed self-assembled InP quantum dots are formed. The small InP quantum dots grown at 650°C are dislocation-free “coherent” regions with an average size of ∼20 nm (height) and a density of ∼1.5 × 10⁸ mm⁻². These InP quantum dots have a broad range of luminescence corresponding to red or organge in the visible spectrum.     

Low threshold current operation of self-assembled InAs/GaAs quantum dot lasers by metal organic chemical vapour deposition

Low threshold current operation of self-assembled InAs/GaAs quantum dot lasers grown by metal organic chemical vapour deposition is reported. Continuous-wave lasing at room temperature with low threshold current (6.7 mA) was achieved at the wavelength of 1.18 /spl mu/m. The threshold current of 6.7 mA is the lowest value so far achieved in quantum dot lasers grown by metal organic chemical vapour deposition. Comparison with photoluminescence spectra indicates that the observed lasing originates from the ground state of InAs quantum dots.     

Nano-patterning and growth of self-assembled quantum dots

Different techniques for the preparation of patterned GaAs substrates for subsequent overgrowth are presented, including focused ion beam direct writing and laser holography followed by wet chemical or dry etching. GaAs-based buffer layers were grown on the patterns and consequently covered with self-assembled quantum dots (QDs). The effect of a strained InGaAs layer grown directly on the patterned substrates and its influence on QD formation and ordering is shown. The dot density, lateral distribution and size distribution of the dots are measured using atomic force microscopy. A comparison of the growth of QDs on patterned and unpatterned substrates indicates that on patterned substrates a higher QD density at the same InAs deposition can be achieved.     

Emission of electrons from the ground and first excited states of self-organized InAs/GaAs quantum dot structures

Capacitance- and conductance-voltage studies have been carried out on Schottky barrier structures containing a sheet of self-organized InAs quantum dots. The dots are formed in GaAs n-type matrices after the deposition of four monolayers of InAs. Quasi-static analysis of capacitance-voltage measurements indicates that there are at least two filled electron levels in the quantum dots, located 60 and 140 meV below the GaAs conduction band edge. The conductance of the structure depends on the balance between measurement frequency and the thermionic emission rate of carriers from the quantum dots. An investigation of the temperature-dependent conductance at different frequencies as a function of the reverse bias allows us to study separately the electron emission rates from the ground and first excited levels in the quantum dots. We estimate that the electron escape times from both levels of the quantum dots become comparable at room temperature and equal to about 100 ps.     

Optical spectroscopy of self-organized nanoscale hetero-structures involving high-index surfaces

Similar effects are responsible for self-organization of periodically corrugated surface structures and ordered dot arrays on crystal surfaces. Strain relaxation on facet edges may result in the appearance of periodically corrugated surfaces for lattice-matched growth. Strain relaxation on facet edges and island interaction via the strained substrate act as driving forces for the formation of ordered arrays of uniform, strained lattice-mismatched islands on a crystal surface. A pseudoperiodic square lattice is manifested for the InAs-GaAs(100) system. Less ordered dots are formed on the GaAs(100) surface with a 4 monolayer GaSb deposition. New experimental methods are applied for the characterization of faceted nanoscale structures. For GaAs-AlAs multilayer structures grown on (311)A substrates, interface corrugation results in optical anisotropy of the same sign as expected from the low symmetry growth direction, making the main origin of the anisotropy unclear. Our quantitative optical reflectance and reflectance anisotropy studies show that the interface corrugation plays an important role for thin (less than 4 nm) GaAs layers. Mesa arrays from samples with InAs quantum dots grown on (100) surface are fabricated. The photoluminescence intensity is found to depend only weakly on the mesa size (1000 nm to 250 nm). The estimated electron-hole pair capture time into the InAs dot at room temperature is less than 1 ps. We also found a weak dependence of the threshold current density on the deep mesa stripe width (down to 3 μm) in the case of room temperature operated quantum dot injection lasers.     

Mechanisms of InGaAlAs solid solution decomposition stimulated by InAs quantum dots

Mechanisms of InGaAlAs solid solution decomposition stimulated by a purposely deposited layer of InAs quantum dots are studied. Decomposition of the solid solution results in an increase in the effective quantum dot size and the shift of the photoluminescence line to as far as 1.3 μm. When aluminum atoms are added to the solid solution, the effect of In atom “conservation” within the dots is observed, which also causes an increase in the effective dot size. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 34, No. 3, 2000, pp. 330–333. Original Russian Text Copyright ? 2000 by Tsatsul’nikov, Volovik, Bedarev, Zhukov, Kovsh, Ledentsov, Maksimov, Maleev, Musikhin, Ustinov, Bert, Kop’ev, Bimberg, Alferov.     

Effect of multicomponent InAsSbP matrix surface on formation of InSb quantum dots at MOVPE growth

Indium-antimonide quantum dots (7–9 × 10⁹ cm²) are produced on an InAs(001) substrate by metal-organic vapor-phase epitaxy at a temperature of T = 440°C. Epitaxial deposition occurred simultaneously onto an InAs binary matrix and an InAsSbP quaternary alloy matrix layer lattice-matched to the InAs substrate in terms of the lattice parameter. Transformation of the quantum-dot shape and size is studied in relation to the chemical composition of the working matrix surface, onto which the quantum dots are deposited. The use of a multicomponent layer makes it possible to control the lattice parameter of the matrix and the strains produced in the system during the formation of self-assembled quantum dots.     

The effects of quantum dot coverage in InAs/(In)GaAs nanostructures for long wavelength emission

We present a study on the effects of quantum dot coverage on the properties of InAs dots embedded in GaAs and in metamorphic In_0.15Ga_0.85As confining layers grown by molecular beam epitaxy on GaAs substrates. We show that redshifted emission wavelengths exceeding 1.3 μm at room temperature were obtained by the combined use of InGaAs confining layers and high quantum dot coverage. The use of high InAs coverage, however, leads to detrimental effects on the optical and electrical properties of the structures. We relate such behaviour to the formation of extended structural defects originating from relaxed large-sized quantum dots that nucleate in accordance to thermodynamic equilibrium theories predicting the quantum dot ripening. The effect of the reduced lattice-mismatch of InGaAs metamorphic layers on quantum dot ripening is discussed in comparison with the InAs/GaAs system.     

Lateral electronic transport in short-period InAs/GaAs superlattices at the threshold of quantum dot formation

Temperature dependences of resistance at 0.7 K<T<300 K, the Hall and Shubnikov-de Haas effects in magnetic fields of up to 40 T, photoluminescence (PL), and morphology of a heterointerface (using an atomicforce microscope) of short-period InAs/GaAs superlattices were investigated. The investigations were carried out for a region of subcritical and critical thickness Q=2.7 monolayers (ML) of InAs. Upon exceeding the critical thickness, the self-organized growth of InAs quantum dots (QDs) set in. The formation of QD layers upon exceeding the critical thickness of InAs Q=2.7 ML is accompanied by a transition of conductivity from metallic to hopping. It is found that at InAs layer thicknesses of Q=0.33 ML and Q=2.0 ML, the PL intensities and electron mobilities in the structures have clearly pronounced maxima. Anisotropy of conductivity, which depends on the thickness of the deposited InAs layers, was observed.     

Growth and photoluminescence study of ZnSe quantum dots

We report detailed photoluminescence (PL) studies of ZnSe quantum dots grown by controlling the flow duration of the precursors in a metal-organic chemical vapor deposition system. The growth time of the quantum dots determines the amount of blue shift observed in the PL measurements. Blue shift as large as 320 meV was observed, and the emission was found to persist up to room temperature. It is found that changing the flow rate and the total number of quantum dot layers also affect the peak PL energy. The temperature dependence of the peak PL energy follows the Varshni relation. From analyzing the temperature-dependent integrated intensity of the photoluminescence spectra, it is found that the activation energy for the quenching of photoluminescence increases with decreasing quantum dot size, and is identified as the binding energy of the exciton in ZnSe quantum dot.     

In situ study of the formation kinetics of InSb quantum dots grown in an InAs(Sb) matrix

Formation of InSb quantum dots grown in an InAs matrix by molecular-beam epitaxy that does not involve forced deposition of InSb is studied. Detection of intensity oscillations in the reflection of high-energy electron diffraction patterns was used to study in situ the kinetics of the formation of InSb quantum dots and an InAsSb wetting layer. The effects of the substrate temperature, the shutter operation sequence, and the introduction of growth interruptions on the properties of the array of InSb quantum dots are examined. Introduction of a growth interruption immediately after completing the exposure of the InAs surface to the antimony flux leads to a reduction in the nominal thickness of InSb and to an enhancement in the uniformity of the quantum-dot array. It is shown that, in the case of deposition of submonolayer-thickness InSb/InAs quantum dots, the segregation layer of InAsSb plays the role of the wetting layer. The Sb segregation length and segregation ratio, as well as their temperature dependences, are determined.     

Density Control of InP/GaInP Quantum Dots Grown by Metal-Organic Vapor-Phase Epitaxy

We investigated structural and emission properties of self-organized InP/GaInP quantum dots (QD) grown by metal organic chemical vapor deposition using an amount of deposited In from 7 to 2 monolayers (ML). In the uncapped samples, using atomic force microscopy (AFM), we observed lateral sizes of 100–200 nm, together with a bimodal height distribution having maxima at ～5 and ～15 nm, which we denoted as QDs of type A and B, respectively; and reduction of the density of the type-B dots from 4.4 to 1.6 μm^–2. The reduction of the density of B-type dots were observed also using transmission electron microscopy of the capped samples. Using single dot low-temperature photoluminescence (PL) spectroscopy we demonstrated effects of Wigner localization for the electrons accumulated in these dots.     

量子点外延生长新模型(英文)

目前在原子尺度上人们对量子点分子束外延生长过程了解很少,所有关于量子点外延生长的理论模型和计算机模拟都是建立在传统的外延生长理论框架内。在传统理论框架内,量子点的生长过程被理解为发生在生长表面上一系列的单一的原子事件,如原子沉积、扩散、聚集等。在这种理论中,外延生长表面原子之间的相互作用被忽略;另外,按照这种理论,量子点生长过程必须是一个相对缓慢的过程。这种理论模型不可能恰当地解释所观察到的大量复杂的量子点外延生长实验现象。作者在两个实验现象基础上,提出了在InAs/GaAs(001)体系中量子点外延生长过程的新模型。这两个实验现象分别是在InAs/GaAs(001)生长表面有大量的"浮游"In原子,一个量子点的生长过程可以在很短的时间内完成(10-4 s)。在提出的新模型中,量子点的自组装过程是一个大数量原子的集体、协调运动过程。