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.     

Electron microscopy of GaAs-based structures with InAs and As quantum dots separated by an AlAs barrier

Electron microscopy studies of GaAs-based structures grown by molecular beam epitaxy and containing arrays of semiconductor InAs quantum dots and metal As quantum dots are performed. The array of InAs quantum dots is formed by the Stranski-Krastanov mechanism and consists of vertically coupled pairs of quantum dots separated by a GaAs spacer 10 nm thick. To separate the arrays of semiconductor and metal quantum dots and to prevent diffusion-induced mixing, the array of InAs quantum dots is overgrown with an AlAs barrier layer 5 or 10 nm thick, after which a GaAs layer is grown at a comparatively low temperature (180°C). The array of As quantum dots is formed in an As-enriched layer of the low-temperature GaAs by means of post-growth annealing at 400–760°C for 15 min. It is established that the AlAs barrier layer has a surface profile corresponding to that of a subbarrier layer with InAs quantum dots. The presence of such a profile causes the formation of V-shaped structural defects upon subsequent overgrowth with the GaAs layer. Besides, it was obtained that AlAs layer is thinned over the InAs quantum dots tops. It is shown that the AlAs barrier layer in the regions between the InAs quantum dots effectively prevents the starting diffusion of excess As at annealing temperatures up to 600°C. However, the concentration of mechanical stresses and the reduced thickness of the AlAs barrier layer near the tops of the InAs quantum dots lead to local barrier breakthroughs and the diffusion of As quantum dots into the region of coupled pairs of InAs quantum dots at higher annealing temperatures.     

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.     

Coupling of electron states in the InAs/GaAs quantum dot molecule

Deep level transient spectroscopy (DLTS) is used to study electron emission from the states in the system of vertically correlated InAs quantum dots in the p-n InAs/GaAs heterostructures, in relation to the thickness of the GaAs spacer between the two layers of InAs quantum dots and to the reverse-bias voltage. It is established that, with the 100 Å GaAs spacer, the InAs/GaAs heterostructure manifests itself as a system of uncoupled quantum dots. The DLTS spectra of such structures exhibit two peaks that are defined by the ground state and the excited state of an individual quantum dot, with energy levels slightly shifted (by 1–2 eV), due to the Stark effect. For the InAs/GaAs heterostructure with two layers of InAs quantum dots separated by the 40 Å GaAs spacer, it is found that the quantum dots are in the molecule-type phase. Hybridization of the electron states of two closely located quantum dots results in the splitting of the levels into bonding and antibonding levels corresponding to the electron ground states and excited states of the 1s ⁺, 1s ^?, 2p ⁺, 2p ^?, and 3d ⁺ types. These states manifest themselves as five peaks in the DLTS spectra. For these quantum states, a large Stark shift of energy levels (10–40 meV) and crossing of the dependences of the energy on the electric field are observed. The structures with vertically correlated quantum dots are grown by molecular beam epitaxy, with self-assembling effects.     

GaAs基上的InAs量子环制备

在分子束外延系统中,利用3nmGaAs薄盖层将InAs自组装量子点部分覆盖,然后在500°C以及As2气氛中退火一分钟,制成纳米尺度的InAs量子环。这一形成敏感地依赖于退火时的生长条件和生长InAs自组装量子点时的淀积量。InAs在GaAs表面的扩散以及同时发生的In-Ga互混控制着InAs量子环的形成。     

Effect of a low-temperature-grown GaAs layer on InAs quantum-dot photoluminescence

The photoluminescence of InAs semiconductor quantum dots overgrown by GaAs in the low-temperature mode (LT-GaAs) using various spacer layers or without them is studied. Spacer layers are thin GaAs or AlAs layers grown at temperatures normal for molecular-beam epitaxy (MBE). Direct overgrowth leads to photoluminescence disappearance. When using a thin GaAs spacer layer, the photoluminescence from InAs quantum dots is partially recovered; however, its intensity appears lower by two orders of magnitude than in the reference sample in which the quantum-dot array is overgrown at normal temperature. The use of wider-gap AlAs as a spacer-layer material leads to the enhancement of photoluminescence from InAs quantum dots, but it is still more than ten times lower than that of reference-sample emission. A model taking into account carrier generation by light, diffusion and tunneling from quantum dots to the LT-GaAs layer is constructed.     

Methods of controlling the emission wavelength in InAs/GaAsN/InGaAsN heterostructures on GaAs substrates

Studies of the properties of InGaAsN compounds and methods of controlling the emission wavelength in InAs/GaAsN/InGaAsN heterostructures grown by molecular beam epitaxy on GaAs substrates are reviewed. The results for different types of heterostructures with quantum-size InGaAsN layers are presented. Among those are (1) traditional InGaAsN quantum wells in a GaAs matrix, (2) InAs quantum dots embedded in an (In)GaAsN layer, and (3) strain-compensated superlattices InAs/GaAsN/InGaAsN with quantum wells and quantum dots. The methods used in the study allow controllable variations in the emission wavelength over the telecommunication range from 1.3 to 1.76 μm at room temperature.     

Electron microscopy of GaAs Structures with InAs and as quantum dots

An electron-microscopy study of GaAs structures, grown by molecular-beam epitaxy, containing two coupled layers of InAs semiconductor quantum dots (QDs) overgrown with a thin buffer GaAs layer and a layer of low-temperature-grown gallium arsenide has been performed. In subsequent annealing, an array of As nanoinclusions (metallic QDs) was formed in the low-temperature-grown GaAs layer. The variation in the microstructure of the samples during temperature and annealing conditions was examined. It was found that, at comparatively low annealing temperatures (400–500°C), the formation of the As metallic QDs array weakly depends on whether InAs semiconductor QDs are present in the preceding layers or not. In this case, the As metallic QDs have a characteristic size of about 2–3 nm upon annealing at 400°C and 4–5 nm upon annealing at 500°C for 15 min. Annealing at 600°C for 15 min in the growth setup leads to a coarsening of the As metallic QDs to 8–9 nm and to the formation of groups of such QDs in the area of the low-temperature-grown GaAs which is directly adjacent to the buffer layer separating the InAs semiconductor QDs. A more prolonged annealing at an elevated temperature (760°C) in an atmosphere of hydrogen causes a further increase in the As metallic QDs’ size to 20–25 nm and their spatial displacement into the region between the coupled InAs semiconductor QDs.     

High-power singlemode CW operation of 1.5 /spl mu/m-range quantum dot GaAs-based laser

Narrow ridge stripe lasers with metamorphic InAs/InGaAs quantum dots grown using molecular beam epitaxy on GaAs substrates emit in the 1.5 /spl mu/m wavelength range demonstrating a differential quantum efficiency of about 50%, singlemode operation, and maximum continuous-wave power of 220 mW limited by thermal roll-over. Absence of beam filamentation is demonstrated up to the highest power levels studied.     

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.     

Spatially Localized Formation of InAs Quantum Dots on Shallow Patterns Regardless of Crystallographic Directions

We report on the ability to grow InAs quantum dots into patterns of any shape. We specifically demonstrate the spatial localization of InAs quantum dots on mesa and trench patterns varying from line, square and triangle patterns on GaAs (100) substrates by molecular beam epitaxy. Based on the underlying science, this growth approach enables the localization of InAs QDs on GaAs (100) by controlling the sidewall facets and InAs monolayer coverage.     

Photoluminescence of InAs quantum dots grown on disoriented GaAs substrates

The photoluminescence spectra in an external magnetic field of an ensemble of InAs quantum dots grown by molecular beam epitaxy on a (001) GaAs substrate with a disorientation in the [010] direction are studied. A redistribution of the photoexcited carriers among different groups of dots under the influence of the magnetic field is observed. The concentration of quantum dots is determined by analyzing the data. Fiz. Tekh. Poluprovodn. 33, 1084–1087 (September 1999)     

Preliminary comparison of ballistic electron emission spectroscopy measurements on InAs quantum dots in a GaAs/AlGaAs heterostructure grown by MBE and MOVPE

Self-assembled InAs quantum dots (SAQDs) in GaAs/GaAlAs structures grown by molecular beam epitaxy (MBE) and metal-organic vapour phase epitaxy (MOVPE) of similar size was examined by ballistic electron emission spectroscopy. Ballistic current-voltage characteristics through the QD in the voltage range from 0.55 to 0.9 V (range where the presence of resonance states of QD is expected) with its derivative (the derivation of the spectroscopic characteristics represents quantum levels in the QD) are given. Differences in the intensities and sharpnesses of the QD levels for MBE and MOVPE grown QDs are observed.     

Comparative Analysis of the Optical and Physical Properties of InAs and In0.8Ga0.2As Quantum Dots and Solar Cells Based on them

Semiconductors - InAs and In0.8Ga0.2As quantum dots in a GaAs matrix as well as GaAs solar cells with quantum dots of both types in the i-region are obtained by metalorganic vapor-phase epitaxy. As...     

自组织生长单层InAs量子点结构中浸润层与量子点发光的时间分辨谱研究

在GaAs(100)的衬底上，采用MBE自组织方法生长了单层层厚分别为2和2．5ML的InAs层。通过原子力显微镜(AFM)观察，证实已在InAs层中形成量子点。采用光致发光谱及时间分辨谱对InAs量子点及浸润层开展研究和对比，分析了单层InAs量子点和浸润层中的载流子迁移过程，较好地解释了实验结果。     

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.     

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.     

Infrared reflection spectra of multilayer epitaxial heterostructures with embedded InAs and GaAs layers

The effect of the thickness of embedded InAs and GaAs layers on the infrared reflection spectra of lattice vibrations for AlInAs/InAs/AlInAs, InGaAs/GaAs/InGaAs, and AlInAs/InGaAs/GaAs/InGaAs/AlInAs multilayer epitaxial heterostructures grown by MOC hydride epitaxy on InP (100) substrates is studied. Relative stresses emerging in the layers surrounding the embedded layers with variation in the number of monolayers from which the quantum dots are formed and with variation the thickness of the layers themselves surrounding the embedded layers are evaluated.     

Spectroscopy of exciton states of InAs quantum molecules

Tunnel-coupled pairs of InAs quantum dots (quantum molecules) were formed by molecular beam epitaxy in a GaAs matrix. Optical and structural properties of the obtained quantum molecules were studied. Four molecular exciton states forming a photoluminescence spectrum were revealed. The photoluminescence decay times indicate the possibility of interlevel radiative recombination from the second excited state, which is of particular importance for designing mid-infrared devices.     

Localization of holes in an InAs/GaAs quantum-dot molecule

Deep-level transient spectroscopy is used to study the emission of holes from the states of a vertically coupled system of InAs quantum dots in p-n InAs/GaAs heterostructures. This emission was considered in relation to the thickness of a GaAs interlayer between two layers of InAs quantum dots and to the reversebias voltage U_r. It is established that hole localization at one of the quantum dots is observed for a quantum-dot molecule composed of two vertically coupled self-organized quantum dots in an InAS/GaAs heterostructure that has a 20-Å-thick or 40-Å-thick GaAs interlayer between two layers of InAs quantum dots. For a thickness of the GaAs interlayer equal to 100 Å, it is found that the two layers of quantum dots are incompletely coupled, which results in a redistribution of the hole localization between the upper and lower quantum dots as the voltage U_r applied to the structure is varied. The studied structures with vertically coupled quantum dots were grown by molecular-beam epitaxy using self-organization effects.