Distribution Of Quantum Dots Research Articles

Near-Field Scanning Optical Microscopy of Phase Separation Effects in Dilute Nitride Alloys.

ABSTRACTThe effect of nitrogen composition on structural parameters of intrinsic quantum dots (QDs) has been studied in GaAs1-yNy and InxGa1-xAs1-yNy alloys (y∼0. 015–0.03) using low-temperature near-field scanning optical microscopy (NSOM) combined with magneto-photoluminescence spectroscopy. We used measurements of the diamagnetic shift (magnetic field strength 0–10T), temperature dependent spectra (temperature range 5–300K) and near-field monochromatic images for the estimation of the size, nitrogen excess and density of QDs. The obtained values (size ∼10–30 nm, nitrogen excess ∼0.005 and density ∼100 /μm-3) suggest spontaneous formation (phase separation) of QDs. Strong lateral inhomogeniety of the QD distribution on a micron length scale was observed.

Self-consistent Coulomb effects and charge distribution of quantum dot arrays

This paper considers the self-consistent Coulomb interaction within arrays of self-assembled InAs quantum dots (QDs) which are embedded in a pn structure. Strong emphasis is being put on the statistical occupation of the electronic QD states which has to be solved self-consistently with the actual three-dimensional potential distribution. A model which is based on a Green's function formalism including screening effects is used to calculate the interaction of QD carriers within an array of QDs, where screening due to the inhomogeneous bulk charge distribution is taken into acount. We apply our model to simulate capacitance-voltage (CV) characteristics of a pn structure with embedded QDs. Different size distributions of QDs and ensembles of spatially perodic and randomly distributed arrays of QDs are investigated.

Grazing incidence small-angle X-ray scattering studies of the synthesis and growth of CdS quantum dots from constituent atoms in SiO2 matrix

Grazing incidence small angle X-ray scattering was applied to study the synthesis and growth of CdS quantum dots (QDs) from Cd and S atoms implanted in SiO2. For a dose of 1017/cm2, the partial synthesis of CdS QDs occurred already during implantation, with only moderate size increase upon subsequent annealing up to T a=1073 K. The dynamics of QD synthesis and growth were considerably different for just two times lower dose, where synthesis started only if the implanted samples were annealed at T a = 773 K or higher, with a strong increase of the size of QDs upon annealing at higher T a. The results suggest that high-dose implantation followed by low-temperature annealing could lead to better defined sizes and narrower size distributions of QDs.

Growth and characterization of InAs quantum dots on GaAs (100) emitting at 1.31μm.

ABSTRACTSelf-organized InAs quantum dots (QDs) were grown in the Stranski-Krastanov regime, by gas-source molecular beam epitaxy (GSMBE), on (100) GaAs substrates. Two important parameters have been optimized in order to grow high quality QDs with a very good reproducibility: InAs growth rate and GaAs cap layer deposition rate. Atomic force microscopy (AFM) analysis shows a unimodal QD distribution and the room temperature photoluminescence (RTPL) spectrum of the optimized sample reveals a 1.3 μm emission with a 19 meV full width at half maximum (FWHM). Photoluminescence (PL) measurements versus excitation power density and photoluminescence excitation (PLE) measurements clearly show multi-component PL emission from transitions associated with fundamental and related excited states of QDs. Furthermore a good growth reproducibility is observed. The results are promising for further work which will lead to laser fabrication.

Growth and characterization of InAs quantum dots on GaAs (100) emitting at 1.31μm.

ABSTRACTSelf-organized InAs quantum dots (QDs) were grown in the Stranski-Krastanov regime, by gas-source molecular beam epitaxy (GSMBE), on (100) GaAs substrates. Two important parameters have been optimized in order to grow high quality QDs with a very good reproducibility: InAs growth rate and GaAs cap layer deposition rate. Atomic force microscopy (AFM) analysis shows a unimodal QD distribution and the room temperature photoluminescence (RTPL) spectrum of the optimized sample reveals a 1.3 μm emission with a 19 meV full width at half maximum (FWHM). Photoluminescence (PL) measurements versus excitation power density and photoluminescence excitation (PLE) measurements clearly show multi-component PL emission from transitions associated with fundamental and related excited states of QDs. Furthermore a good growth reproducibility is observed. The results are promising for further work which will lead to laser fabrication.

Influence of the quantum-well shape on the light emission characteristics of InGaN/GaN quantum-well structures and light-emitting diodes

Structural and optical properties of various shapes of quantum wells (QWs), including rectangular, triangular, trapezoidal, and polygonal ones are investigated. Photoluminescence (PL) measurements show that the highest light emission efficiency and the best reproducibility in the intensity and wavelength are obtained from trapezoidal QWs. The temperature dependence of PL spectra indicates the more localized nature of excitons in the trapezoidal QWs. A plan-view transmission electron microscopy shows that quantum dots (QDs) are formed inside the dislocation loop in trapezoidal QWs. The distribution of QDs in size and composition becomes more uniform with trapezoidal QWs than with rectangular QWs, leading to superior light-emission characteristics. It is suggested that QD engineering and dislocation control are possible, to some extent, by the modulation of the QW shape in InGaN/GaN-based light-emitting devices.

Formation of InAs Dots on AlGaAs Ridge Wire Structures by Selective Area MOVPE Growth

We investigate the formation of InAs quantum dots (QDs) on the top of AlGaAs ridge wire structures along the [110] direction by selective-area metalorganic vapor phase epitaxy (SA-MOVPE) and study their size distribution. It is found that the size distribution of QDs depends on the top width W of AlGaAs wires and growth amount θ of InAs. InAs dot size across the AlGaAs wires are limited by W and it becomes uniform for narrower W. In contrast, the size along the wire exhibits non-uniformity, particularly for narrower W and large θ. In addition, the shapes of InAs dot change depending on W. The results are discussed based on the diffusion of In from facets and masked area and the coalescence of QDs.

Growth and characterisation of InAs/InGaAs quantum dots like structure on GaAs/Si substrate by AP-MOCVD

Atmospheric pressure-metal organic chemical vapour deposition (AP-MOCVD) was used to grow InAs and InGaAs quantum dots (QDs) on GaAs/Si substrate. The influence of growth temperature on the QD density and distribution was investigated. Scanning probe microscopy (AFM and DFM) was used to investigate the growth behaviour of the QD structure. The growth properties of InAs/InGaAs QDs on the highly stressed GaAs/Si substrate under S–K growth mode at different deposition temperatures have been analysed. It was identified that the growth temperature plays major role in determining the growth and distribution of both InAs and InGaAs QDs due to the temperature-dependent dislocation propagation from the GaAs/Si substrate. The growth of InGaAs QDs was found “In” composition dependent. A high QD density of ∼5×1010–6×1010cm−2 was obtained on GaAs/Si at a growth temperature of 500°C.

Photo-luminescence Properties of CuCl Quantum Dots and the Dependence of Biexciton Formation Rates on Quantum Dot Sizes

We fabricated CuCl quantum dots (QDs) in a glass matrix. The blue shift of the exciton spectrum caused by quantum confinement effects was observed in the linear absorption and the photo-luminescence (PL) spectrum. Inhomogeneous line broadening was mainly induced by the size distribution of CuCl QDs and we found that the size distribution has Gaussian profiles. By solving the steady state rate equation, we also found that the intensities of biexciton PL were proportional to the square of exciton PL intensities, and the results were well accorded with the experimental results. The formation rates of biexciton were proportional to the volumes of CuCl QDs.

Photoluminescence of InAs self-organized quantum dots on (0 0 1)InP substrate with GaAs interlayer

In this paper, InAs self-organized quantum dots (QDs) were deposited on (0 0 1)InP substrate with and without a GaAs interlayer by low pressure metalorganic chemical vapor deposition. An atomic force microscope image showed that InAs QDs arranged regularly on the GaAs interlayer. Comparison between photoluminescence (PL) spectra of InAs QDs with and without GaAs interlayer indicated that larger area density and much narrower size distribution of InAs QDs were obtained by inserting the GaAs interlayer. The area density of InAs QDs increased while the size distribution hardly changed when the thickness of InAs increased from 2.5 to 4 ML (monolayer). However, when the thickness of InAs increased to 6 ML, the mean size of InAs QDs increased while the area density of InAs QDs decreased because of the coalescence of QDs. PL spectra with different thickness of GaAs interlayer indicated that the largest density of InAs QDs could be obtained when the thickness of GaAs interlayer was 3 nm.

Low temperature photoluminescence of InAs self-organized quantum dots on (0 0 1) InP substrate with GaAs interlayer

In this paper, InAs self-organized quantum dots (QDs) were deposited on (0 0 1) InP substrate with and without a GaAs interlayer by low pressure metalorganic chemical vapor deposition (LP-MOCVD). An atomic force microscope (AFM) image showed that InAs QDs arranged regularly on the GaAs interlayer. Comparison between photoluminescence (PL) spectra of InAs QDs with and without GaAs interlayer indicated that larger area density and much narrower size distribution of InAs QDs were obtained by inserting the GaAs interlayer. The area density of InAs QDs increased while the size distribution hardly changed when the thickness of InAs increased from 2.5 monolayer (ML) to 4 ML. However, when the thickness of InAs increased to 6 ML, the mean size of InAs QDs increased while the area density of InAs QDs decreased because of the coalescence of QDs. PL spectra with different thickness of GaAs interlayer indicated that the largest density of InAs QDs could be obtained when the thickness of GaAs interlayer was 3 nm.

Structural characterization of self-assembled InAs quantum dots grown by MBE

Molecular-beam epitaxy grown self-assembled InAs quantum dots (QDs) have been investigated by means of grazing incidence X-ray techniques. We reveal that the lateral distribution of InAs QDs is anisotropic and the most pronounced ordering of dot distribution is in [1 1 0] direction. Moreover, we determine the dot shape to be an octagonal-based truncated pyramid with {1 1 1} and {1 0 1} facet families. We also find that the strain is elastically relaxed with different components when dots are formed. The volume distribution of partially strained InAs inside QDs is peaked at intermediate strain values. In addition, a small volume fraction of relaxed In x Ga 1− x As is found.

Grazing incidence structural characterization of InAs quantum dots on GaAs(0 0 1)

Grazing incidence small angle X-ray scattering (GISAXS) and X-ray diffraction (GIXRD) experiments have been performed on self-assembled InAs quantum dots (QDs) grown by molecular beam epitaxy (MBE). In GISAXS we find pronounced diffuse scattering peaks with high diffraction orders along [110], [1−10] and [100] orientation. From the intensity observation of QD facet truncation rods, we determine an octagonal-based dot shape with {111} and {101} facet families. By analyzing structural parameters such as mean dot–dot distance and correlation length, we propose an anisotropic ordering of QD distribution. From GIXRD of the (202) Bragg peak, we reveal different strain components with elastic deformation in lateral and vertical directions of samples when the InAs QDs are formed. Moreover, we also resolve additional diffraction intensity from unstrained InxGa1−xAs alloy.

Atomically resolved structure of InAs quantum dots

InAs was grown by molecular-beam epitaxy onto GaAs(001) until quantum dots (QDs) formed. At this point, the growth was interrupted and the uncovered QDs were investigated in situ by scanning tunneling microscopy (STM). Atomically resolved STM images of the QDs revealed that four dominating bounding facets occur, whose Miller indices were identified to be {137}. The assignment of the facet orientation was based on experiments on planar high Miller index GaAs surfaces. In addition, the latter experiments indicated that {137} facets are thermodynamically stable only up to a certain size. This conclusion is assumed to explain the sharp size distribution of InAs QDs.

Substrate dependence of InGaAs quantum dots grown by molecular beam epitaxy

A systematic study of self-organized In0.5Ga0.5As quantum dots (QDs) and islands grown by molecular beam epitaxy on (100) and (n11) A/B GaAs substrates is given, where n varies from 1 to 5. Low-temperature photoluminescence results show that the properties of the dots have a strong dependence on the substrate orientation as revealed by atomic force microscopy, consistent with the differences in size, shape, and distribution of QDs on different substrates. From (100) to (111) surface, the photoluminescence peak position of dots on B surfaces is found to blueshift more than that on A surfaces. QDs are also formed on (511) A surface. The positional distribution of these dots exhibits a wavy shape, which is related to the corrugated structure of this surface. Two kinds of islands are formed on (111) A surface, but further work is needed to explain the mechanism of these islands.

Application of atomic-force-microscope direct patterning to selective positioning of InAs quantum dots on GaAs

The application of atomic-force-microscope (AFM) direct patterning to the selective positioning of InAs quantum dots (QDs) on a (100) GaAs substrate has been proposed and experimentally implemented. The AFM direct patterning was used to generate various patterns of several tens of nanometers in size, and InAs QDs were subsequently grown by a metalorganic chemical vapor deposition technique. A nonuniform distribution of the QDs was observed near the patterns. The detailed shape of the QD distribution and the size of the QDs depended on the geometrical properties such as the sidewall angle, the spacing, and the width of the patterns. We have been able to ascertain, through our work, what growth conditions are necessary for QDs’ alignment along the patterns.

Optical absorption in PbSe spherical quantum dots embedded in glass matrix

A phosphate glass embedded with PbSe semiconductor quantum dots (QDs) has been synthesized and investigated. Optical absorption spectra of PbSe spherical QDs have been studied both theoretically and experimentally. Theoretical analysis of the energy spectra and optical transitions has been carried out in the framework of four-band k⋅p model taking full account of anisotropy effects. It is demonstrated that the strong anisotropy of the energy bands of bulk PbSe gives rise to additional optical transitions in the quantum dots. These anisotropy-induced optical transitions have been clearly observed in the measured absorption spectra of the PbSe quantum dots. The calculated energy positions of the optical transitions are in good agreement with experiment. Five optical transitions were distinguished and interpreted in the absorption spectra measured at T=12 K. For the set of samples with a narrow size distribution of PbSe QDs (diameters in range of 40–110 Å), the energies of three or four optical transitions have been determined from the measured absorption spectra at T=300 K. The calculated dependencies of the energies of the first five optical transitions with QD size are in good agreement with those obtained from the experimental data.

Cathodoluminescence Wavelength Imaging Study of Clustering in InAs/GaAs Self-Assembled Quantum Dots

ABSTRACTCathodoluminescence wavelength imaging (CLWI) of InAs/GaAs self-assembled quantum dots (SAQDs) was performed to study the spatial variation in the spectral lineshape of the broadened quantum dot (QD) ensemble. The lineshape was found to vary on a scale of ∼μm, revealing attendant variations in the size distribution of SAQD clusters on this spatial scale. Energy variations in clusters of SAQDs are found to exhibit a spatial correlation with the efficiency of luminescence and the activation energy for thermal re-emission of carriers. A reduction in the energy variation of the QD clusters occurs when the thickness of the spacer layers in vertically self-organized samples is reduced or the number of stacks is increased. SAQDs were also prepared by punctuated island growth (PIG), in which deposition of the total desired amount is broken into two or more stages each separated by time delays. CLWI reveals a reduced variation in the energy of the dominant CL emission on a ∼μm spatial scale, correlating with a narrower size distribution of larger QDs for PIG, as measured in atomic force microscopy.

Distribution and shape of self-assembled InAs quantum dots grown on GaAs (001)

Abstract Grazing incidence small angle X-ray scattering (GISAXS) and atomic force microscopy (AFM) experiments are employed to study self-assembled InAs quantum dots (QDs) grown by molecular beam epitaxy (MBE) on GaAs (001). The GISAXS spectra show pronounced non-specular diffuse scattering satellite peaks with high diffraction orders up to ±3 along [110], [1-10], and [100] sample azimuthal orientations with respect to the incoming beam, indicating a lateral ordering of the InAs QDs. The correlation lengths of the lateral dot distribution are found to be identical along [110] and [1-10] but smaller along [100] direction. The ratio of the mean dot-dot distances along [100] and [1-10] azimuths is determined to be 1.13, indicating the anisotropic ordering of QD distribution. Broad diffraction peaks are observed at larger scattering angles and associated to dot facet crystal truncation rods (CTR). We determine {111}-like facets along [110] and [1-10] sample azimuths, and {101}-like facets along [100] azimuth.

Structural and infrared absorption properties of self-organized InGaAs/GaAs quantum dots multilayers

Self-organized InGaAs/GaAs quantum dots (QDs) stacked multilayers have been prepared by solid source molecular beam epitaxy. Cross-sectional transmission electron microscopy shows that the InGaAs QDs are nearly perfectly vertically aligned in the growth direction [100]. The filtering effect on the QDs distribution is found to be the dominant mechanism leading to vertical alignment and a highly uniform size distribution. Moreover, we observe a distinct infrared absorption from the sample in the range of 8.6–10.7 µm. This indicates the potential of QDs multilayer structure for use as infrared photodetector.