Formation Of Self-assembled Quantum Dots Research Articles

(Invited) Nanowires from Dilute Nitride and Dilute Bismide Alloys for Nanophotonics

GaAs-based nanowires (NWs) have a great potential for applications in photonics and optoelectronics, partly owing to flexibility in band structure engineering via alloying, strain and structural polymorphism. Among promising GaAs-based materials for device applications are highly mismatched GaNAs and GaAsBi alloys. The giant bowing in the bandgap energy characteristic for these materials allows easy tuning of emission wavelengths, while other modifications of their electronic structure promise to improve performance of infrared lasers and solar cells. In this talk we will review our recent results related to the electronic properties of novel GaNAs and GaAsBi NWs and provide several examples highlighting their advantages.First of all, we will show that a GaNAs-based mutishell NW can be used as an efficient coherent photon source, where the NW acts both as a miniaturized optical resonator and as a photonic gain medium. Its wavelength can be efficiently tuned by minor changes in N composition, e.g. down to 1 µm with only 2.5 % of nitrogen [1]. The lasing mode is found to arise from the fundamental HE11a mode of the Fabry-Perot cavity from a single NW, exhibiting optical polarization along the NW axis. Moreover, due to superior nonlinear properties of GaNAs, self-frequency conversion of the stimulated emission through second harmonic generation and sum-frequency generation is observed, providing coherent light emission in the cyan-green range.Secondly, we will demonstrate that alloying of GaAs with N and/or Bi leads to the formation of self-assembled quantum dots (QDs) embedded in NWs and, therefore, is an efficient way to fabricate hybrid QD-NW structures with high optical quality. In the case of dilute nitrides, optically active and highly localized QD states are formed due to local fluctuations in N composition [2], whereas in dilute bismides they are caused by Bi segregation at twin planes [3]. Such QDs act as single photon emitters, promising for application in quantum communication technologies.And finally, we will show that favorable band alignment at the interface between a dilute nitride alloy and parental N-free material leads to very efficient energy upconversion due to two-step two-photon absorption in the related NW heterostructures, with the efficiency of this process being among the highest reported in semiconductor nanostructures. Specifically, we show [4] that, in radial GaAs(P)/GaNAs(P) core/shell NW heterostructures, the upconversion efficiency increases by 500 times as compared with that of the constituent materials, even under an excitation power as low as 100 mW/cm2 that is comparable to the one-sun illumination. The upconversion efficiency can be further improved by 8 times through engineering the electric-field distribution of the excitation light inside the NWs so that light absorption is maximized within the desired region of the heterostructure.

Kinetic Monte Carlo simulations of self-organization of Ge islands on Si(001)

Kinetic Monte Carlo simulations of the growth of Ge on Si(001) indicate self-assembled quantum dot formation via nucleation at 500 K and instability at 600 K.

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.

Self Assembly of semiconductor nanostructures

Abstract In this work we present the growth and characterization of GaAs self-assembled quantum wires (SAQWRs), and InAs self-assembled quantum dots (SAQDs) by molecular beam epitaxy on (631)-oriented GaAs substrates. Adatoms on the (631) crystal plane present a strong surface diffusion anisotropy which we use to induce preferential growth along one direction to produce SAQWRs. On the other hand, InAs SAQDs were obtained on GaAs(631) with SAQWRs by the Stransky–Krastanov (S-K) growth method. SAQDs grown directly on (631) substrates presented considerable fluctuations in size. We study the effects of growing a stressor layer before the SAQDs formation to reduce these fluctuations.Keywords : Quantum wires, quantum dots; selfassembly; molecular beam epitaxy.

The Strain Distribution and Equilibrium Morphology for Dome-Shaped Ge/Si Semiconductor Quantum Dot

The stress and strain in a growing sample are driving forces leading to the formation of self-assembled quantum dots in lattice-mismatched heteroepitaxy.The stress and strain distributions, as well as the dependence of the strain energy on the aspect ratio, of a dome self-assembled Ge/Si semiconductor quantum dot are investigated based on finite element method of anisotropic theory of elasticity. The free energy consisting of the strain energy and surface energy is defined, and used to study the equilibrium shape of the systems. The results are consistent with experiment observations.

Identification of candidate material systems for quantum dot solar cells including the effect of strain

AbstractHeterostructures that include self‐assembled quantum dots (SAQDs) have been suggested as model systems for the realization of novel high efficiency solar cells such as those based on intermediate bands (IBs). The lattice mismatch in the epitaxial growth of these structures, necessary for the formation of SAQDs, introduces strain throughout the structure, making the selection of materials systems with appropriate physical parameters problematic. The model solid theory is used to calculate the energy band edge alignment at Γ point of such quantum dot (QD) heterostructures including the effects of strain. With the modified band gaps due to strain, a materials search was performed for high efficiency QD solar cells among III‐V binaries and ternaries with negligible valence band offsets. This requirement of the valence band offset along with the limited band gap ranges for optimum efficiency results in only a few feasible materials systems being identified. The optimum barrier/dot material system found was Al0.57In0.43As/InP0.87Sb0.13 grown on lattice matched metamorphic buffer layer, but due to miscibility gap concerns it is suggested that the Al0.50In0.50As/InAs0.41P0.59 fully strained system may be preferred. Copyright © 2010 John Wiley & Sons, Ltd.

Effects of elastic heterogeneity and anisotropy on the morphology of self-assembled epitaxial quantum dots

Epitaxial self-assembled quantum dots (SAQDs) are of both technological and fundamental interest, but their reliable manufacture still presents a technical challenge. To better understand the formation, morphology, and ordering of epitaxial SAQDs, it is essential to have an accurate model that can aid further experiments and predict the trends in SAQD formation. SAQDs form because of the destabilizing effect of elastic mismatch strain, but most analytic models and some numerical models of SAQD formation either assume an elastically homogeneous anisotropic film-substrate system or assume an elastically heterogeneous isotropic system. In this work, we perform the full film-substrate elastic calculation and incorporate it into a stochastic linear model of the initial stages of SAQD formation process for the case of fast deposition followed by annealing. We find that using homogeneous elasticity can cause errors in the elastic energy density as large as 26%. The wavelength corresponding to the fastest growing mode in the linear model is used as an estimate for SAQD spacing. We calculate that homogeneous elasticity can lead to an error of about 11% in the estimated value of average spacing established during the initial stages of SAQD formation process. We also quantify the effect of elastic heterogeneity on the order estimates of SAQDs and confirm previous finding on the possibility of order enhancement by growing a film near the critical film height.

Predicting and Understanding Order of Heteroepitaxial Quantum Dots

Heteroepitaxial self-assembled quantum dots (SAQDs) will allow breakthroughs in electronics and optoelectronics. SAQDs are a result of Stranski-Krastanow growth whereby a growing planar film becomes unstable after an initial wetting layer is formed. Common systems are Ge$_{x}$Si$_{1-x}$/Si and In$_{x}$Ga$_{1-x}$As/GaAs. For applications, SAQD arrays need to be ordered. The role of crystal anisotropy, random initial conditions and thermal fluctuations in influencing SAQD order during early stages of SAQD formation is studied through a simple stochastic model of surface diffusion. Surface diffusion is analyzed through a linear and perturbatively nonlinear analysis. The role of crystal anisotropy in enhancing SAQD order is elucidated. It is also found that SAQD order is enhanced when the deposited film is allowed to evolve at heights near the critical wetting surface height that marks the onset of non-planar film growth.

Formation of self-assembled quantum dots on AlInAs and GaInAs matrices using a GaSb sublayer

In this letter, the authors present the formation of InAs quantum dots on GaInAs and AlInAs lattice matched on InP (001) substrates via molecular beam epitaxy by inserting a two monolayer GaSb sublayer below the InAs quantum dot material. They show that the formation of quantum dots is favored on indium-free and antimony-rich surfaces while quantum dashes are preferentially formed on indium-rich and antimony-free surfaces. Using a thin layer of GaSb between the InAs quantum dot material and the AlInAs or GaInAs matrix, single quantum dots were formed even with low quantum dot densities (≈1∕μm2). These quantum dots give rise to photoluminescence between 1100 and 1500nm depending on the matrix material.

Order of epitaxial self-assembled quantum dots: linear analysis

Epitaxial self-assembled quantum dots (SAQDs) are of interest for nanostructured optoelectronic and electronic devices such as lasers, photodetectors and nanoscale logic. Spatial order and size order of SAQDs are important to the development of usable devices. It is likely that these two types of order are strongly linked; thus, a study of spatial order will also have strong implications for size order. Here a study of spatial order is undertaken using a linear analysis of a commonly used model of SAQD formation based on surface diffusion. Analytic formulas for film-height correlation functions are found that characterize quantum dot spatial order and corresponding correlation lengths that quantify order. Initial atomic-scale random fluctuations result in relatively small correlation lengths (about two dots) when the effect of a wetting potential is negligible; however, the correlation lengths diverge when SAQDs are allowed to form at a near-critical film height. The present work reinforces previous findings about anisotropy and SAQD order and presents as explicit and transparent mechanism for ordering with corresponding analytic equations. In addition, SAQD formation is by its nature a stochastic process, and various mathematical aspects regarding statistical analysis of SAQD formation and order are presented.

Self-assembled quantum dot formation during the growth of In 0.4Ga 0.6As on GaAs(0 0 1) by metal-organic vapor phase epitaxy: The role of In segregation

The stabilization of planar In 0.4Ga 0.6As on GaAs, above thickness typically reported to undergo spontaneous island formation, has been observed. The effect of growth temperature, AsH 3 partial pressure, and In 0.4Ga 0.6As thickness on island size and density was evaluated. Planar films are stabilized by low growth temperatures and high AsH 3 partial pressures. Spontaneous island formation is enhanced by high growth temperatures and low AsH 3 partial pressures. The results presented are interpreted within the phenomenology of In segregation to the surface, which has been identified previously for the InGaAs/GaAs system. Based on equilibrium models for the critical thickness for island formation, the critical segregation layer thickness for island formation is predicted to be 0.6 nm.

Anisotropy and order of epitaxial self-assembled quantum dots

Epitaxial self-assembled quantum dots (SAQDs) represent an important step in the advancement of semiconductor fabrication at the nanoscale that will allow breakthroughs in electronics and optoelectronics. In these applications, order is a key factor. Here, the role of crystal anisotropy in promoting order during early stages of SAQD formation is studied through a linear analysis of a commonly used surface evolution model. Elastic anisotropy is used as a specific example. It is found that there are two relevant and predictable correlation lengths. One of them is related to crystal anisotropy and is crucial for determining SAQD order. Furthermore, if a wetting potential is included in the model, it is found that SAQD order is enhanced when the deposited film is allowed to evolve at heights near the critical surface height for three-dimensional film growth.

Two-dimensional phase-field simulation of self-assembled quantum dot formation

The morphological evolution of a strained heteroepitaxial thin film of Si 1− x Ge x /Si during deposition has been investigated by two-dimensional (2D) phase-field simulations. The phase-field model developed can form facet morphologies that are modeled using the generalized gradient correction coefficient for a crystal with a high anisotropy of surface energy, and can simulate the sequential shape transitions well known in the Si 1− x Ge x /Si(0 0 1) system: a 2D wetting layer, faceted three-dimensional pyramids, and multifaceted domes. The effects of Ge composition and mobility on island formation and island evolution have also been studied. Some typical and important phenomena observed in previous experimental studies during Si 1− x Ge x /Si(0 0 1) heteroepitaxy have been simulated using the phase-field model.

Formation of self-assembled quantum dots of iron oxide thin films by spray pyrolysis from non-aqueous medium

Quantum dots (QDs) of iron oxide have been deposited onto ITO coated glass substrates by spray pyrolysis technique, using ferric chloride (FeCl 3·7H 2O) in non-aqueous medium as a starting material. The non-aqueous solvents namely methanol, ethanol, propanol, butanol and pentanol were used as solvents. The effect of solvents on the film structure and morphology was studied. The structural, morphological, compositional and optical properties were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive analysis of X-rays (EDAX), and optical absorption measurement techniques.

Formation of self-assembled quantum dots induced by the Stranski–Krastanow transition: a comparison of various semiconductor systems

To account for the occurrence (or not) of the Stranski–Krastanow (SK) transition (two-dimensional to 3D change of surface morphology) during the epitaxial growth of various lattice-mismatched semiconductor systems, we present a simple equilibrium model taking into account not only the lattice mismatch, but also the dislocation formation energy and the surface energy. It demonstrates the importance of these parameters especially for II–VI systems such as CdTe/ZnTe and CdSe/ZnSe. For II–VIs indeed, as misfit dislocations are easier to form than in III–Vs (such as InAs/GaAs) or IV systems (Ge/Si), the 3D elastic transition is short-circuited by the plastic transition. Nevertheless, by lowering the surface energy cost, telluride and selenide quantum dots can also be grown as predicted by our model and as shown experimentally by reflection high-energy electron diffraction (RHEED), atomic force microscopy and optical measurements. This model is also applied to the case of GaN/AlN, before discussing its limits. To cite this article: H. Mariette, C. R. Physique 6 (2005).

Growth of highly strained InGaAs quantum wells on GaAs substrates—effect of growth rate

Highly strained In x Ga 1− x As ( x∼0.5) quantum wells were grown on GaAs substrates at low temperature by metal organic vapor-phase epitaxy. By depositing this material in the kinetically limited growth regime, high-quality pseudomorphic layers were obtained. It was found that by using an extremely slow growth rate, the Stranski–Krastanov transition for self-assembled quantum dots formation could be suppressed and the resulting layer had very smooth surface morphology. It is suggested that step flow motion of adatoms dominates at such slow growth rates. By using this technique, a double quantum well structure with room temperature luminescence at 1215 nm and spectral linewidth of 48 meV was obtained.

On the formation mechanism of epitaxial Ge islands on partially relaxed SiGe buffer layers

We describe the morphological evolution of Ge epitaxial films grown on partially relaxed SiGe buffer layers. Three different types of surface sites exist on the buffer layer surface when the spacing between the buried dislocations is significantly larger than the surface diffusion length of Ge adatoms. The three types of nucleation sites include those over dislocation intersections, those over single dislocation lines, and those far away from dislocations. Epitaxial growth of Ge on such a relaxed buffer under near equilibrium condition exhibits three-stage nucleation and growth of Ge islands, also known as self-assembled quantum dots (SAQD). Denuded zones exist next to the buried dislocation lines. The temperature dependence of the denuded zone width together with the average spacing between SAQDs provides sufficient information for determining both the activation energy and the pre-exponential factor of surface diffusion of Ge adatoms. SAQDs located at the three sites are strained differently as indicated by the distinct difference in the critical dot size for pyramid-to-dome transition. The experimental observation can be explained by assuming the energy barrier for the surface diffusion of Ge adatoms increases with decreasing misfit strain as predicted by theory [van de Walle et al., Phys Rev. B 67, R41308 (2003)]. Calculations of Ge adatom concentration profile based on Fick’s second law of diffusion does predict the presence of denuded zones with their widths being dependent on the diffusion coefficient; i.e., on the substrate temperature. The observed morphological evolution of Ge epitaxial films supports Ge island nucleation and growth as the SAQD formation mechanism as opposed to strain-induced surface instability.

Growth and Overgrowth of Ge/Si Quantum Dots: An Observation by Atomic Resolution HAADF-STEM Imaging

ABSTRACTThe formation of self-assembled quantum dots (QD) is of increasing interest for applications in optical, nanoelectronic, biological and quantum computing systems. From the perspective of fabrication technology, there are great advantages if the whole device can be made using a single Si substrate. Furthermore, GeSi is a model semiconductor system for fundamental studies of growth and material properties. In practice, as the MBE growth of heterostructures is inherently a non-equilibrium process, the formation of self-assembled nanostructures is both complex and sensitive to growth and overgrowth conditions. The morphology, structure and composition of QDs can all change during growth. It is therefore crucial to understand their structures at different stages of growth at the atomic scale. Here, the characterization of QD growth using high-resolution high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) imaging is presented. Both the formation of uncapped QDs and the effect of the encapsulation are investigated, and the morphological and compositional evolution of the QDs and wetting layers are observed directly at the atomic scale for the first time. During encapsulation, the Ge content in the centres of the QD remains unchanged, despite significant intermixing, lateral spreading and a laterally inhomogeneous Ge distribution inside the Ge QD. The initial non-uniform wetting layer for the uncapped Ge QD becomes uniform after encapsulation, and a 3-monolayer-thick core with ∼ 60% Ge content is formed in the 2 nm-thick wetting layer with an average Ge content of ∼ 30%. The results were obtained by direct analysis of the Z-contrast STEM imaging without involving complex image simulations.

Study of the GaAs growth on pseudomorphic Si layers for the formation of self-assembled quantum dots

Si layers were pseudomorphically grown by MBE on AlGaAs layers, the subsequent growth mode of GaAs was studied as a function of growth temperature, and of the Si layer thickness. At high temperatures (∼580°C), GaAs grows on Si/AlGaAs in a two-dimensional (2D) mode. The use of moderate temperatures (∼500°C) is effective to induce a change to a three-dimensional (3D) growth of GaAs. By employing a Si layer with a thickness of one monolayer, we succeeded to obtain GaAs self-assembled islands with an average lateral size of 20 nm and a height of 1.3 nm, these dimensions are appropriate for the synthesis of quantum dots.

Wetting layer evolution in InAs/GaAs( [formula omitted]) heteroepitaxy: effects of surface reconstruction and strain

InAs heteroepitaxy on the (2×4) and c(4×4) reconstructed surfaces of GaAs(0 0 1) has been studied using scanning tunnelling microscopy, with particular emphasis on the formation and evolution of the two-dimensional (2D) In x Ga 1− x As wetting layer. The influence of the initial GaAs reconstruction on the growth behaviour is significant for InAs coverages ( θ)<0.8 ML, but both qualitative (surface morphology and reconstruction) and quantitative (step density) surface characteristics converge for θ>1 ML. The effects of surface reconstruction are only important for sub-ML InAs coverages, with strain effects dominating at higher coverages. STM topographs indicate a high and rapid disordering of the WL for θ>0.5 ML, implying that strain-driven alloyed WL formation is important even below θ=1 ML. The 2D→3D growth mode transition and the formation of self-assembled quantum dots (QDs) is delayed at low As fluxes. In this case strain relief occurs first by a reconstruction change from (1×3) to a more In-rich (4×2) structure, with QD formation taking place only after the deposition of an additional amount of InAs (∼0.2 ML).