Wetting layers effect on InAs/GaAs quantum dots

FEM combining with the K·P theory is adopted to systematically investigate the effect of wetting layers on the strain-stress profiles and electronic structures of self-organized InAs quantum dot. Four different kinds of quantum dots are introduced at the same height and aspect ratio. We found that 0.5 nm wetting layer is an appropriate thickness for InAs/GaAs quantum dots. Strain shift down about 3%∼4.5% for the cases with WL (0.5 nm) and without WL in four shapes of quantum dots. For band edge energy, wetting layers expand the potential energy gap width. When WL thickness is more than 0.8 nm, the band edge energy profiles cannot vary regularly. The electron energy is affected while for heavy hole this impact on the energy is limited. Wetting layers for the influence of the electronic structure is obviously than the heavy hole. Consequently, the electron probability density function spread from buffer to wetting layer while the center of hole's function moves from QDs internal to wetting layer when introduce WLs. When WLs thickness is larger than 0.8 nm, the electronic structures of quantum dots have changed obviously. This will affect the instrument's performance which relies on the quantum dots' optical properties.     

Direct interband light absorption in a cylindrical quantum dot in quantizing magnetic field

In this paper the direct interband transitions in cylindrical quantum dot (QD) made of GaAs are studied in the presence of a magnetic field. Two models of QD confinement potential are discussed. For both models the expressions for absorption coefficients and dependencies of effective threshold frequencies of absorption on the value of applied magnetic field and on geometrical sizes of QD are obtained. The selection rules corresponding to different transitions between quantum levels are found.     

Disorder-induced natural quantum dots in InAs/GaAs nanostructures

Properties of excitons confined to potential fluctuations due to indium distribution in the wetting layer which accompany self-assembled InAs/GaAs quantum dots are reviewed. Spectroscopic studies are summarized including time-resolved photoluminescence and corresponding single-photon emission correlation measurements. The identification of charge states of excitons is presented which is based on results of a theoretical analysis of interactions between the involved carriers. The effect of the dots’ environment on their optical spectra is also shown.     

Charged excitons in quantum dots: novel magnetic behavior and Auger processes

We describe theoretically multiply-charged excitons interacting with a continuum of delocalized states. Such excitons exist in relatively shallow quantum dots and have been observed in recent optical experiments on InAs self-assembled dots. The interaction of an exciton and delocalized states occurs via Auger-like processes. To describe the optical spectra, we employ the Anderson-like Hamiltonian by including the interaction between the localized exciton and delocalized states of the wetting layer. In the absence of a magnetic field, the photoluminescence line shapes exhibit interference effects. When a magnetic field is applied, the photoluminescence spectrum demonstrates anticrossings with the Landau levels of the extended states. We show that the magnetic-field behavior of charged excitons is very different to that of diamagnetic excitons in three and two-dimensional systems.     

Single InAs/GaAs quantum dots: Photocurrent and cross-sectional AFM analysis

Photocurrent (PC) spectroscopy is employed to study the carrier escape from self-assembled InAs/GaAs quantum dots (QDs) embedded in a Schottky photodiode structure. As a function of the applied field, we detect a shift of the exciton ground-state transition due to the quantum-confined Stark effect (). The tunneling time, which is directly related to the observed photocurrent linewidth due to τ/(2Γ), changes by a factor of five in the photocurrent regime. The measured linewidth dependency on the electric field is modeled by a simple 1D WKB approximation for the tunneling process, which shows that the energetic position of the wetting layer is important for the measured tunneling time out of the dot. In addition to that we present cross-sectional atomic force measurements (AFM) of the investigated photodiode structure. The method needs a minimum of time and sample preparation (cleaving and etching) to obtain the dot density, dot distribution, and give an estimate of the dot dimensions. Etching only the cleaved surface of the sample opens up the opportunity to determine the properties of a buried dot layer before or even after device fabrication.     

Observation of monolayer-splitting for InAs/GaAs quantum dots

A pronounced modulation is observed in the photoluminescence (PL) spectrum of self-organized InAs/GaAs quantum dots (QDs), recorded at low excitation densities. The clearly distinguishable peaks are identified as a multimodal distribution of the ground state transition energy, originating from a discrete, stepwise variation of the structural properties of the QDs, which is associated with an increase of the QD height in monolayer (ML) steps. The observation of a ML splitting implies a flat QD shape with well-defined upper and lower interfaces as well as negligible indium segregation. The electronic properties of the InAs/GaAs QDs were investigated by PL and PL-excitation spectroscopy and are discussed based on realistic calculations for flat InAs/GaAs QDs with a truncated pyramidal shape based on an extended 8-band k·p model. The calculations predict a red shift of the ground state transition with each additional ML, which saturates for heights above 9 ML, is in good agreement with experiment.     

Optical spectroscopy of a single InAs/GaAs quantum dot in high magnetic fields

We report on the measurements of the photoluminescence from the s-shell of a single InAs/GaAs quantum dot in magnetic fields up to 23 T. The observed multiline emission is attributed to different charge states of a single dot. Characteristic anticrossing of emission lines is explained in terms of hybridization of final states of a triply charged exciton (X⁻³).     

Hydrostatic pressure effects on exciton states in InAs/GaAs quantum dots

Based on the effective-mass approximation, the hydrostatic pressure effects on exciton states in InAs/GaAs self-assembled quantum dots (QDs) are studied by means of a variational method. Numerical results show that the exciton binding energy has a minimum with increasing dot height for any hydrostatic pressure. The interband emission energy increases when the hydrostatic pressure increases. In particular, we find that hydrostatic pressure has a remarkable effect on exciton states for small QD size. Our results are in agreement with experiment measurements.     

Structure of InAs/GaAs quantum dots grown with Sb surfactant

InAs quantum dots in GaAs, grown under the presence of Sb by metalorganic chemical vapor deposition, were studied with cross-sectional scanning tunneling microscopy. Large flat quantum dots with a truncated pyramidal shape, base lengths between 15 and 30 nm, heights of 1–3 nm, and a rather pure InAs stoichiometry were found for the case of an Sb supply during the InAs deposition. If Sb is already supplied during GaAs stabilization prior to InAs deposition, the dots become even larger and tend to get intermixed with Ga, but remain coherently strained with a reversed cone-like In distribution. Regarding the quantum dot growth Sb acts as surfactant, whereas an incorporation of individual Sb atoms was observed in the wetting layer.     

Photocurrent spectroscopy of InAs/GaAs self-assembled quantum dots: observation of a permanent dipole moment

The nature of the confined electronic states in InAs/GaAs self-assembled quantum dots is studied using photocurrent spectroscopy measured as a function of applied electric field. A field asymmetry of the quantum confined Stark effect is observed, consistent with the dots possessing a permanent dipole moment. The sign of this dipole indicates that for zero field the hole wave function lies above that of the electron, in disagreement with the predictions of all recent calculations. Comparison with a theoretical model demonstrates that the experimentally determined alignment of the electron and hole can only be explained if the dots contain a non-zero and non-uniform Ga content.     

A theoretical model for quantum nanostructures electronic wave functions, magnetic field effects

Analytical solutions of electronic wave functions in symmetric quantum ring (QR), quantum wire (QWR) and quantum dots (QD) structures are given using a parabolic coordinates system. The solutions for low-energy states are combinations of Bessel functions. The density of states of perfect 1D QR and QWR are shown to be equivalent. The continuous evolution from a 0D QD to a perfect 1D QR can be precisely described. The sharp variation of electronic properties, related to the build up of a potential energy barrier at the early stage of the QR formation, is studied analytically. Paramagnetic and diamagnetic couplings to a magnetic field are computed for QR and QD. It is shown theoretically that magnetic field induces an oscillation of the magnetization in QR.     

Selective growth of GaAs quantum dots and vertical quantum wires in two-dimensional V-grooves

We propose and demonstrate a novel technique for the fabrication of quantum dot (QD) structures using metal organic chemical vapor deposition (MOCVD). The GaAs quantum dots are grown at the bottom of the two-dimensional V-groove (2DVG) structures which are composed of (1 1 1)A and (1 1 1)B-facets on GaAs(1 0 0). The 2DVG is formed by MOCVD selective growth on a SiO₂ patterned substrate. It should be noted that the 2DVGs cannot be formed by a chemical wet etching technique because the facet's anisotropy of etching ratios are different. By changing the growth condition, we can obtain GaAs QD structures which have a size of less than 10 nm, and vertical GaAs quantum wires (V-QWRs) in 2DVGs. We have observed photoluminescence from each structure. We have also demonstrated stacking of GaAs QDs in the 2DVG on GaAs (1 0 0).     

Optical and transport studies in coupled InAs quantum dots embedded in GaAs

We studied optical and electron transport properties of coupled InAs quantum dots (QDs) embedded in GaAs. Photoluminescence (PL) from the high dot density samples indicated asymmetry in the PL spectra when the ambient temperature is lower than about 50 K. Comparing this result with theoretical calculations, it is shown that this phenomenon is explained by the inter-dot electronic coupling effect. In the photo-conductance measurement, resonance peaks in the current–voltage characteristics were observed in the low-temperature region. The dependence of the resonance voltage on the magnetic field intensity was studied to extract the g-factor. It is also shown that the resonances are attributed to the current corresponding to the electron transport through QDs. According to these results, it is concluded that the inter-dot electronic coupling in the self-assembled InAs/GaAs QD systems occurs when the inter-dot spacing is as low as several nanometers and the ambient temperature is less than about 50 K.     

Correlation energies of many-body complexes in biased InAs/GaAs quantum dots

The aim of this work is to analyze theoretically the correlation energies for neutral, positively, negatively charged exciton and bi-exciton. So, we propose a model consistent with experimental observations that is small InAs truncated pyramids with circular base lying on wetting layer, both buried into GaAs matrix.In a first step and in contrast to other works, we are able to evaluate coulombic interactions between electron and hole, two electrons and two holes by perturbative method at the second order. In a second step, the correlation energies of many-body complexes X, X^-, X⁺ and XX are investigated as a function of quantum dots basis radius r_c and the applied electric field.Our main goal is to provide realistic estimation for the correlation energies of excitons, charged excitons and bi-excitons while retaining at the same time a transparent formalism, which could easily be transposed to structures of actual interest.The present work provides evidence of the stability of excitons, charged excitons and bi-excitons in InAs/GaAs quantum dots. Calculated correlation energies of many-body complexes are consistent with those reported by recent photoluminescence measurements.     

Nano-probe-assisted technology of indium-nano-dot formation for site-controlled InAs/GaAs quantum dots

We have successfully and reproducibly fabricated uniform indium (In) nano-dots at a selected point. Nano-dot formation was realized using an atomic force microscope (AFM) probe with a specially designed cantilever, which was equipped with a hollow pyramidal tip with a sub-micron size aperture on the apex and an In-reservoir tank within the stylus. The In nano-dots formed in this study can be directly converted to InAs quantum dots by subsequent irradiation of arsenic flux in the molecular beam epitaxy chamber, which is connected to the AFM chamber through an ultra-high-vacuum tunnel.     

Emission and strain in InGaAs/GaAs quantum wells with InAs quantum dots obtained at different temperatures

The photoluminescence (PL), its temperature dependence and X ray diffraction (XRD) have been studied in the symmetric In_0.15Ga_0.85As/GaAs quantum wells (QWs) with embedded InAs quantum dots (QDs), obtained with the variation of QD growth temperatures (470–535 °C). The increase of QD growth temperatures is accompanied by the enlargement of QD lateral sizes (from 12 up to 28 nm) and by the shift non monotonously of PL peak positions. The fitting procedure has been applied for the analysis of the temperature dependence of PL peaks. The obtained fitting parameters testify that in studied QD structures the process of In/Ga interdiffusion between QDs and capping/buffer layers takes place partially. However this process cannot explain the difference in PL peak positions.     

Second-harmonic generation in asymmetric quantum dots in the presence of a static magnetic field

The second-harmonic generation(SHG) coefficient in an asymmetric quantum dot(QD) with a static magnetic field is theoretically investigated.Within the framework of the effective-mass approximation,we obtain the confined wave functions and energies of electrons in the QD.We also obtain the SHG coefficient by the compact-density-matrix approach and the iterative method.The numerical results for the typical GaAs/AlGaAs QD show that the SHG coefficient depends strongly on the magnitude of magnetic field,parameters of the asymmetric potential and the radius of the QD.The resonant peak shifts with the magnetic field or the radius of the QD changing.     

Phonon-assisted polar exciton–transitions in self-organized InAs/GaAs quantum dots

Phonon-assisted exciton transitions are investigated for self-organized InAs/GaAs quantum dots (QDs) using selectively excited photoluminescence (PL) and PL excitation spectroscopy. The results unambiguously demonstrate intrinsic recombination in the coherent InAs/GaAs QDs and the absence of a Stokes shift between ground state absorption and emission. Phonon-sidebands corresponding to a phonon energy of 34 meV are resolved and Huang–Rhys parameters of 0.015 and 0.08 are found for phonon-assisted emission and absorption, respectively, which are about one order of magnitude larger than in bulk InAs. Calculations of the exciton–LO–phonon interaction based on an adiabatic approximation and realistic wave functions for ideal pyramidal InAs/GaAs QDs show this enhanced polar coupling to result from the particular confinement and the strain-induced piezoelectric potential in such strained low-symmetry QDs.     

Ultrafast carrier dynamics of resonantly excited InAs/GaAs self-assembled quantum dots

We study theoretically the time development of electronic relaxation in quantum dots. We consider the process of relaxation of the state with an electron prepared at the beginning of relaxation in the electronic ground state. We obtain a fast (in picoseconds) increase of electronic population in the excited state. Also, we consider the process of relaxation of an electron from an excited state in the dot. Here we obtain an incomplete depopulation of the electron from the excited state. We compare these results to experiments in which a fast decrease of luminescence is reported during the first period of relaxation after resonant excitation of the ground state. We estimate numerically the role of electron–LO–phonon (Fröhlich's coupling) mechanism in these processes. We show that this effect may be attributed to the influence of multiple scattering of quantum dot electrons on LO phonons. A single-electron two-energy-level quantum dot model is used to demonstrate this effect in an isolated semiconductor quantum dot.     

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.