Height Of Quantum Dots Research Articles

Exciton states and interband optical transitions in wurtzite InGaN/GaN quantum dot nanowire heterostructures: Effects of hydrostatic pressure

Considering the hydrostatic pressure, the strong built-in electric field as well as the three-dimensional confinements of the electrons and holes, we solve the confined exciton states and investigate the interband optical transitions in wurtzite In x Ga 1− x N/GaN strained quantum dot (QD) nanowire heterostructures (NWHETs) using a variational method under the effective mass approximation and the simplified coherent potential approximation. In our calculations, the effective masses of the electron and hole, dielectric constants, phonon frequencies, energy gaps, radius and height of QDs and piezoelectic polarizations are taken into account as a function of hydrostatic pressure. Our results show that the hydrostatic pressure has a significant influence on the exciton states and interband optical transitions. The exciton binding energy increases with the hydrostatic pressure. The emission wavelength has a blue-shift (red-shift) if the hydrostatic pressure (QD height) increases. For the small radius or the large height QDs, the hydrostatic pressure dependence of the exciton binding energy is more obvious. The hydrostatic pressure can effectively enhance the exciton oscillator strength and improve the light emission efficiency of In x Ga 1− x N/GaN QD NWHETs. The physical reason has been analyzed in depth.

Molecular beam epitaxy based growth of cubic GaN quantum dots

AbstractZinc‐blende GaN/AlN quantum dots were grown in a molecular beam epitaxy system by two alternative methods. In method A the quantum dots were formed by the Stranski‐Krastanov process, whereas in method B the quantum dots were created by a vapor‐liquid‐solid process, respectively. The density of the Stranski‐Krastanov quantum dots was adjustable in a range of 5 x 109 cm‐2 to 5 x 1012 cm‐2. The density of the quantum dots grown by method B was controllable in the range of 5 x 108 cm‐2 to 5 x 1012 cm‐2. For both methods the samples with a high density of quantum dots have shown strong optical activity in photoluminescence spectroscopy. The emission energy of the quantum dots was tuneable in a range of 3.5 eV to 3.9 eV by alteration of the quantum dot height. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Second‐order polaron resonances in self‐assembled quantum dots

AbstractWe theoretically study the optical properties of an InAs/GaAs quantum dot (QD) near the area of the second‐order resonance between an electron confined in the QD and two longitudinal optical phonons. We present the absorption spectra of an inhomogeneously broadened QD ensemble and show that the minimal model needed for an accurate description of such a system needs to account for 3‐phonon states.We study also the influence of the QD height to width ratio on the optical properties of the polaron system. The dependence of the width of the resonance and the position of the second‐orderresonant feature on the height to width ratio is presented (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Insight into optical properties of strain-free quantum dot pairs

Self-assembled GaAs/AlGaAs quantum dot pairs (QDPs) are grown by molecular beam epitaxy using high temperature droplet epitaxy technique. A typical QDP consists of dual-size quantum dots as observed based on atomic force microscopy image. The average height of quantum dot is 5.7 nm for the large quantum dots and 4.6 nm for the small ones. The average peak-to-peak distance of the two dots is about 75 nm. The optical properties of GaAs QDPs are studied by measuring excitation power-dependent and temperature-dependent photoluminescence. Unique photoluminescence properties have been observed from both excitation power-dependent and temperature-dependent measurements. Excitation power-dependent as well as temperature-dependent PL measurements have suggested lateral exciton transfer in the QDPs.

纤锌矿GaN/AlxGa1-xN应变柱形量子点中杂质态结合能及其压力效应

In the framework of effective mass approximation,a variational method is adopted to discuss the binding energies of hydrogenic impurity in a strained wurtzite cylindrical quantum dot by considering the hydrostatic pressure.The results indicate that the binding energies of hydrogenic impurity with strain effect are higher than that without strain effect when the quantum dot height is small,but the binding energies with strain effect become lower than that without strain effect as the quantum dot height increases.It is also found that the binding energies of hydrogenic impurity decrease when the mole fraction of Al increase.In addition,the bin-ding energies of impurity increase obviously with hydrostatic pressure,and the hydrostatic pressure has a remarkable influence on the donor binding energy for small quantum dot.

Structural and optical changes induced by incorporation of antimony into InAs/GaAs(001) quantum dots

We present experimental evidence of Sb incorporation inside InAs/GaAs001 quantum dots exposed to an antimony flux immediately before capping with GaAs. The Sb composition profile inside the nanostructures as measured by cross-sectional scanning tunneling and electron transmission microscopies show two differentiated regions within the quantum dots, with an Sb rich alloy at the tip of the quantum dots. Atomic force microscopy and transmission electron microscopy micrographs show increased quantum-dot height with Sb flux exposure. The evolution of the reflection high-energy electron-diffraction pattern suggests that the increased height is due to changes in the quantum-dot capping process related to the presence of segregated Sb atoms. These structural and compositional changes result in a shift of the room-temperature photoluminescence emission from 1.26 to 1.36 m accompanied by an order of magnitude increase in the room-temperature quantum-dot luminescence intensity.

Annealing behavior on luminescence properties of self‐assembled InGaAsN/GaP quantum dots

AbstractSelf assembled In0.6Ga0.4AsN/GaP quantum dots (QDs) were grown by solid source molecular beam epitaxy (SSMBE). Structural and optical properties of as‐grown and annealed In0.6Ga0.4AsN/GaP QDs were investigated. Cross sectional high‐resolution transmission electron microscopy (HR‐TEM) study did not show any structural defects and N‐related chain structures. The integrated PL intensity at 725 °C rapid thermal annealing (RTA) for sample prepared at RF power of 390 W was obtained 3.7 times higher than the as‐grown sample measured at 18 K. The blueshifts of ∼ 0.35 and ∼ 0.55 meV/°C were observed annealed up to 725 and 800 °C, respectively as compare to as‐grown samples. Average QD height was reduced from 5 to 3 nm, which may be due to interdiffusion of In and Ga atoms. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of growth parameters on the surface morphology of InAs quantum dots grown on GaAs/Ge/Si1−xGex/Si substrate

We present the heterogeneous growth of InAs quantum dots (QDs) on Si platform using GaAs/Ge/Si1−xGex/Si substrate. Self-assembled InAs QDs were grown on GaAs/Ge/Si1−xGex/Si substrate by employing molecular beam epitaxy (MBE). The areal density, width and height of QDs were characterized using atomic force microscopy (AFM). The undulation originating from the graded Si1−xGex/Si substrate causes no noticeable change in dot densities and dot dimensions across the undulated surface. The effects of V/III ratio and InAs coverage on structural properties of the QDs were investigated. The dot density increases with increasing V/III ratio and QDs with high density of 1011cm−2 were obtained, attributed to the reduced diffusion length of the adatoms. Lateral dimension of the QDs increases as the InAs coverage increases. The QDs coalesce at 3.0 monolayer (ML) InAs coverage. This work is beneficial to those working on III–V on Si integration.

Growth of Uniform and Self-Aligned InAs Quantum Dots on Vicinal (100) GaAs Substrate by Metal Organic Chemical Vapor Deposition Technique for Laser Applications

Self-assembled InAs quantum dots (QDs) were grown on vicinal (100) GaAs substrate. Effect of InAs coverage and growth temperature was studied to optimize QDs for laser applications. The density and size distribution of QDs varied with the change in InAs coverage and growth temperature. Growth temperature was varied from 400 to 450°C and InAs coverage was varied between 1.40 and 2.35 monolayers (MLs). Optimum growth temperature was found to be 420°C. Density of QDs was first increased and then decreased with increase in InAs coverage. Linear increase in size and height of QDs was observed with increase in both growth temperature and InAs Coverage. It was observed that these two parameters play crucial role in optimization of uniform QDs.

Growth and Characterization of InP Ringlike Quantum-Dot Molecules Grown by Solid-Source Molecular Beam Epitaxy

In this paper, we have studied the fabrication of InP ringlike quantum-dot molecules on GaAs(001) substrate grown by solid-source molecular beam epitaxy using droplet epitaxy technique and the effect of In deposition rate on the physical and optical properties of InP ringlike quantum-dot molecules. The In deposition rate is varied from 0.2 ML/s to 0.4, 0.8 and 1.6 ML/s. The surface morphology and cross-section were examined by ex-situ atomic force microscope and transmission electron microscope, respectively. The increasing of In deposition rate results in the decreasing of outer and inner diameters of InP ringlike quantum-dot molecules and height of InP quantum dots but increases the InP quantum dot and ringlike quantum-dot molecule densities. The photoluminescence peaks of InP ringlike quantum-dot molecules are blue-shifted and FWHM is narrower when In deposition rate is bigger.

Effects of strain and hydrostatic pressure on the binding energy of excitons in wurtzite cylindrical quantum dots

AbstractA variational method is used to discuss the binding energies of excitons in a strained wurtzite cylindrical quantum dot by considering the hydrostatic pressure effect. Besides the built‐in electric field due to the spontaneous and piezoelectric polarizations, the strain dependence of parameters is also considered in our calculation. The results indicate that the binding energies of excitons with strain effect are higher than that without strain effect when the quantum dot height is small, but the binding energy with strain effect becomes lower than that without strain effect as the quantum dot height increases. The contribution from the built‐in electric field to the strain effect is dominant, but the contribution from the strain dependence of parameters is quite small (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Quantum size effects on excitons in strained InAs/InP quantum dots

Exciton states confined in a strained InAs/InP quantum dot are investigated using variational technique within the single band effective mass approximation. The strain contribution to the potential is determined via deformation potential theory, modified by the strain tensor. Ground state positively charged donor exciton binding energy and the interband emission energy are studied with the height and radius of cylindrical quantum dot. The valence band anisotropy is included in our theoretical model by using different hole masses in different spatial directions. The results show that both donor exciton binding energy and the interband emission energy are decreased when the dot radius and the height are decreased and the heavy-hole exciton in a cylindrical quantum wire is more strongly bound than the light-hole exciton.

On strain state and pseudo‐moiré TEM contrast of InSb quantum dots coherently grown on InAs surface

AbstractIn this article, we report on the theoretical analysis of transmission electron microscopy (TEM) images of surface InSb quantum dots (QDs) coherently grown on InAs substrate. A finite element method (FEM) is used to calculate elastic fields and total displacements in a QD and an adjusted region of the substrate. The effects of QD form factor and QD aspect ratio δ on displacements and TEM images are analyzed. A quasilinear dependence of radial displacements on radial coordinate for spherical, elliptical, and truncated spherical QDs is demonstrated. It has been found that the displacement field does not depend on the shape and aspect ratio for QDs with δ > δc1, and the upper part of a QD remains practically undistorted for QDs with δ ≥ δc2. For InSb/InAs heterosystem these critical values are δc1 ≈ 0.13 and δc2 ≈ 0.33. The total displacements are used for computation of TEM diffraction contrast associated with QDs. To achieve this the Howie–Whelan dynamic approach is utilized. Calculated TEM images of heavily strained QDs demonstrate the picture of pseudo‐moiré with a strong dependence of moiré‐like fringe distance Δ on aspect ratio δ. This dependence gives the possibility to determine the aspect ratio and height of QDs from the results of TEM experiments.

Presentation and experimental validation of a model for the effect of thermal annealing on the photoluminescence of self-assembled InAs/GaAs quantum dots

We present a model for the effect of thermal annealing on a single-layer InAs/GaAs quantum dot (QD) heterostructure and study the corresponding variation in full photoluminescence (PL) spectrum. In/Ga interdiffusion due to annealing is modeled by Fickian diffusion and the Schrödinger equation is solved separately for electrons and holes to obtain ground state PL peaks of the heterostructure at different annealing temperatures. We theoretically examine the decrease in strain effects and carrier confinement potentials with annealing. PL spectra of the entire ensemble of QDs, annealed at different temperatures, are calculated from a lognormal distribution of QD heights derived from experimental atomic force microscopy (AFM) data. Results from our calculations, which illustrate the blueshift in emission wavelength and linewidth variation in PL with annealing, are in excellent agreement with experimental PL observations on the same samples. This highlights the potential of the model to assist in precisely engineering the optical properties of QD materials for specific device applications. Moreover, the simplicity of the model and its multiple useful features including computation of material interdiffusion, band profiles and full PL spectra make it a valuable tool to study annealing effects on QD heterostructures.

GaAsSb-capped InAs quantum dots: From enlarged quantum dot height to alloy fluctuations

The Sb-induced changes in the optical properties of GaAsSb-capped InAs/GaAs quantum dots (QDs) are shown to be strongly correlated with structural changes. The observed redshift of the photoluminescence emission is shown to follow two different regimes. In the first regime, with Sb concentrations up to $\ensuremath{\sim}12\mathrm{%}$, the emission wavelength shifts up to $\ensuremath{\sim}1280\text{ }\text{nm}$ with a large enhancement of the luminescence characteristics. A structural analysis at the atomic scale by cross-sectional scanning tunneling microscopy shows that this enhancement arises from a gradual increase in QD height, which improves carrier confinement and reduces the sensitivity of the excitonic band gap to QD size fluctuations within the ensemble. The increased QD height results from the progressive suppression of QD decomposition during the capping process due to the presence of Sb atoms on the growth surface. In the second regime, with Sb concentrations above $\ensuremath{\sim}12\mathrm{%}$, the emission wavelength shifts up to $\ensuremath{\sim}1500\text{ }\text{nm}$, but the luminescence characteristics progressively degrade with the Sb content. This degradation at high Sb contents occurs as a result of composition modulation in the capping layer and strain-induced Sb migration to the top of the QDs, together with a transition to a type-II band alignment.

Grazing Incidence X-ray Diffraction Measurements of Columnar InAs/GaAs Quantum Dot Structures

The lattice constant distribution inside a columnar InAs/GaAs quantum dot (QD) and its crystal orientation dependence were evaluated by grazing incidence X-ray diffraction (GIXD) measurement. The QDs were grown by stacking Stranski–Krastanow (SK)-type InAs QDs directly in the growth direction with very thin GaAs interval layers. We evaluated the dependence of the in-plane lattice constant on QD height by GIXD measurement using equipment available for laboratories. We found that the lattice constants at the top and bottom of the QDs were almost the same when the height and diameter of the QDs were almost equal. As the number of stacks was increased to grow high QDs, the lattice constant at the QD top became larger in the [110] direction than in the [110] direction, but this relationship was reversed at the bottom. We consider that GIXD measurement with compact equipment will contribute to the swift and efficient development of QD devices.

Donor bound excitons confined in a [formula omitted] quantum dot

Exciton states confined in a GaN/Ga 1− x Al x N Quantum dot are investigated within the framework of single band effective mass approximation. Charged donor bound exciton, D + X energy is computed as a function of dot radius for different Al concentration and the height of cylindrical dot. The interband optical transition of GaN/Ga 1− x Al x N dot is calculated with the various structural parameters. The valence band anisotropy is included in our theoretical model by using different hole masses in different spatial directions. Binding energy of the ground state of exciton is calculated, with the inclusion of 2D Hartree dielectric screening function. Our results show that quantum size has a considerable influence on exciton states and interband optical transitions of the narrow dots. The ground state exciton binding energy and the interband emission energy are increased when the cylindrical quantum dot height or radius is decreased and the effect of exciton has influence on the interband emission energy. The donor bound exciton binding energy increases monotonically as Al concentration increases. Our results are compared with the existing available literature.

Effect of different strain reducing layers on InAs quantum dots grown by migration enhanced epitaxy

We investigated the effect of InGaAs and AlGaAs combination strain reducing layers on InAs quantum dots (QDs) grown by migration enhanced epitaxy. The samples were examined by cross-sectional transmission electron microscopy, low-temperature and power dependent photoluminescence (PL). We observed three different size distributions of QDs in the atomic force microscopy image. We found that PL peak 1, 2, and 3 came from three different size distributions, and the energy level of QDs could be modified by changing strain reducing layers irrelevantly to controlling QD height in our samples. Furthermore, we elucidated the role of InGaAs and AlGaAs layer on the energy level modification and related cross-sectional morphology of the QDs.

Influence of crystallization temperature on InP ring-shaped quantum-dot molecules grown by droplet epitaxy

We have studied the influence of crystallization temperature on the structural and optical properties of InP ring-shaped quantum-dot molecules (QDMs) grown by solid-source molecular-beam epitaxy (MBE) using droplet epitaxy technique. The surface morphologies and photoluminescence spectra were examined by ex situ atomic force microscopy (AFM) and micro-photoluminescence (micro-PL) measurement. The increasing of crystallization temperature results in the decreasing of InP quantum-dot (QD) and ring-shaped QDM densities while average outer and inner diameters of InP ring-shaped QDMs become larger. The distributions of the number of InP QDs per InP ring-shaped QDMs, lateral size of InP QDs, height of InP QDs and height and lateral size relative distributions of InP QDs have been statistically investigated. The photoluminescence peaks of InP ring-shaped QDMs are red-shifted when crystallization temperature increases.

InAs/GaAs quantum dot structures emitting in the 1.55 μm band

We investigated different capping layers covering InAs quantum dot structures grown on GaAs substrates by metalorganic vapor phase epitaxy in order to receive strong photoluminescence at 1.55 μm. Atomic force microscopy showed that uncovered InAs quantum dots are 4–5 nm high lenses with a broad luminescence peaking at 1.43 μm. The GaAs capping improves the quantum dot homogeneity but quantum dot dissolution causes blue shift of emitted light. On the other hand, proper engineering of InGaAs strain reducing layer allows to shift the photoluminescence maximum to 1.55 μm. Analysis of photoluminescence and microscopy data supported by calculation of quantum dot electron states shows that this is caused both by the change of the electronic-barrier structure and by the increase of the height of the overgrown quantum dots.