Size Distribution Of Quantum Dots Research Articles

Optical properties of Si-doped InAs/InP quantum dots

Abstract InAs quantum dots (QDs) were grown on InP substrates by low pressure-metalorganic chemical vapor deposition. Disilane (Si2H6) was used as an n-type dopant. The positions of Si doping were varied: buffer layer, capping layer, modulation doping, and QD itself. Surface treatment of InP by Si2H6 was also performed to see the effect of Si on InAs QD. Photoluminescence (PL) and atomic force microscopy (AFM) were used to characterize optical and structural properties of QDs, respectively. It was found that the PL peak positions varied from 0.73 to 0.88 eV with the position of Si doping. PL peak blue shift in modulation doped sample was explained in terms of state filling effect. It was found that Si doping at QD itself was the most effective way to obtain the strongest integrated PL intensity without degrading the QD size distribution.

Critical layer thickness enhancement of InAs overgrowth on porous GaAs

In the present work we have investigated the initial stage of InAs layer grown on porous GaAs (π-GaAs) substrate by using reflection high-energy electron diffraction (RHEED) and low temperature photoluminescence (PL). RHEED measurements show that the 2D–3D growth mode transition appears after a deposition of 4.2 atomic monolayer (ML) of InAs, which is higher than that deposited on nominal GaAs (1.7 ML). PL investigations show two luminescence bands at 1.24 and 1.38 eV. The 1.24 eV PL peak emission is associated to the radiative transitions in InAs quantum dots (QDs), whereas the 1.38 eV PL peak emission is attributed to the InAs wetting layer (WL). The results show that π-GaAs is a promising candidate to obtain a reduced QDs size distribution, and to grow pseudomorphic epitaxial layer on GaAs substrate with higher indium concentration.

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.

Infrared Lateral Photoconductivity of InGaAs Quantum Dots: the Temperature Dependence

We report the temperature dependence of lateral infrared photoconductivity in multilayer InGaAs/GaAs heterostructures with selectively doped quantum dots fabricated by metalorganic chemical vapor deposition. Two spectral lines of normal-incidence intersubband photoconductivity (90 meV and 230 meV) and a line originating from interband transitions (930 meV) were observed. The photoconductivity line 230 meV is revealed up to the temperature 140 K. The long-wavelength photoconductivity line 90 meV is quenched rapidly at the temperature 30 + 40 K owing to redistribution of photoexcited carriers between small and large dots. The obtained results confirm the hypothesis about bimodal distribution of quantum dot sizes.

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.

Understanding the growth mode transition in InAs/GaAs(0 0 1) quantum dot formation

Scanning tunneling microscopy has been used to monitor the growth by molecular beam epitaxy of InAs quantum dots (QDs) on GaAs(0 0 1) at and near the critical coverage ( θ crit). Direct evidence is obtained for the existence of small irregular 3D islands of height 6–12 Å (2–4 ML) which contain ∼150 atoms or more. These features develop rapidly (within 0.05 ML of θ crit) into regular mature QDs with an average volume >1 × 10 4 atoms. Scaling analysis of the QD size distributions suggests that strain may have a significant influence during QD nucleation and the initial stages of growth, but is unimportant during the later stages of QD development.

Applications of lead-salt microcavities for mid-infrared devices

Optically pumped lasers and light detectors based on microcavities with highly efficient Pb1 - x Eux Te/EuTe Bragg interference mirrors are described for the 3–6 µm wavelength range. The lasers operate up to and above room temperature with a minimum pump intensity threshold of about 15kW/cm2 The responses of the photodetectors show well pronounced cavity resonances. Furthermore, a microcavity is used to determine the absorption spectrum of a superlattice containing three-dimensionally ordered, self-organised PbSe quantum dots. The quantum dot absorption peak, with a width of only 9 meV, confirms the exceptionally high homogeneity of the quantum dot size distribution, observed so far only for these lead-salt quantum dots.

II–VI quantum dots grown by MOVPE

II–VI quantum dots (QDs) have been under debate on their ripening properties for some time, and a unified understanding of this phenomenon will be given in this paper. Improvement of QD size uniformity is the main concern for practical applications. Scaling of the dot size distributions on some II–VI QDs will be discussed for the better understanding of the size distributions and for the improvement of the dot size uniformity. QD size distributions also tend to smear out the special features of QDs which are expected to modulate electron–photon interactions in optical microcavities coupled with QDs. Study of energy relaxation processes in QDs reveals the possibility of selectively exciting QDs which are in resonance with longitudinal optical (LO)-phonon emission processes. This scheme will be applied to the study of the strong coupling regime of optical microcavities coupled with QDs. CdS/ZnS QDs embedded in pyramidal three-dimensional microcavities are also examined to study the capability to observe enhanced spontaneous emissions by the coupling with cavity modes in pyramidal microcavities.

Self-Narrowing and Photoetching Effects on the Size Distribution of CdS Quantum Dots Prepared by a Reverse-Micelle Method

We have investigated self-narrowing and photoetching effects on the size distribution of CdS quantum dots (QDs). The QDs with a narrow size distribution are prepared by a reverse-micelle method and their mean radii are controlled by changing the water content in the reverse micelle. The mean radius and size-distribution width estimated from the line-shape analysis of absorption spectra agree well with those obtained from transmission electron microscope images, which indicates that the analysis is a convenient and reasonable method of estimating the size-distribution width. For the QDs whose initial size distribution is larger than ∼10%, the size-distribution reduction due to a “self-narrowing process” is observed: The size-distribution width is spontaneously reduced with the time elapsed after the QD preparation. The further reduction of the size-distribution width is achieved by performing a photoetching treatment. The above results clearly show the possibility of preparing CdS QDs with the narrow size distribution by using the self-narrowing and photoetching effects.

Inhomogeneous broadening of the ground electron level in a quantum dot array

The connection between the size and shape distribution of quantum dots (QDs) and the density of electron states is analyzed. It is shown that, in an array of nonidentical QDs, the density of states in the vicinity of the ground level takes the form of an asymmetric peak whose shape and position are determined by the statistical parameters of the array (equilibrium radius, as well as variance and asymmetry of the size distribution of the QDs). General relationships between these parameters and the shape of the peak are determined.

Designed emitter states in resonant tunneling through quantum dots

Resonant tunneling through a single layer of self-assembled quantum dots (QDs) is compared to tunneling through two layers of vertically aligned (stacked) dots. The difference can be viewed as going from a two-dimensional emitter to a zero-dimensional emitter. The temperature dependence of current peaks originating in tunneling through individual QDs and individual stacks is used to clarify this point. In addition, we show that the statistical size distribution of self-assembled quantum dots causing the inhomogeneous broadening in luminescence experiments can be analyzed in a resonant tunneling experiment.

Dependence of the InAs size distribution on the stacked layer number for vertically stacked InAs/GaAs quantum dots

Abstract Atomic force microscope (AFM), transmission electron microscopy (TEM), and photoluminescence measurements were carried out to investigate the dependence of the InAs quantum dot size distribution on the stacked layer number for vertically stacked InAs/GaAs quantum dots (QDs) grown on (0 0 1) GaAs substrates. AFM and TEM images showed that the size of the QDs increased with increase in the stacked layer number up to the deposition time of 20 s. However, the size distribution uniformity of the QDs was improved with increase in the stacked layer number when the deposition time and the stacking layer of the InAs QDs gradually decreased. These results can help in an improved understanding of the control of sizes of QDs in InAs/GaAs QD arrays.

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.

Phonons in self-assembled Ge/Si structures

We present the results of an investigation dealing with fundamental vibrations in periodical Ge/Si structures with small-size Ge quantum dots (QDs) performed using macro- and micro-Raman spectroscopy under resonant and off-resonant conditions. Samples with different number of repetition of Ge and Si layers contain Ge QDs with an average dot base size of 15 nm and a QD height of 2 nm . Periodic oscillations observed in the low-frequency region of the Raman spectra are assigned to folded LA phonons in the Ge QD superlattices. The measured phonon frequencies are in a good agreement with those calculated using the Rytov model. These oscillations are superimposed with a broad continuous emission originating from the whole acoustic dispersion branch due to a breaking up of translational invariance. The Raman spectra of the structure with single Ge QD layer reveal a series of peaks corresponding to LA phonons localized in the Si layer. Using the measured phonon frequencies and corresponding wave vectors the dispersion of the LA phonons in the Si is obtained. The longitudinal-acoustic wave velocity determined from the dispersion is 8365 ms −1 and in excellent agreement with that derived from the Brillouin study. In the optical phonon range, the LO and TO phonons localized in Ge QDs are observed. The position of the LO Ge phonons shifts downwards with increasing excitation energy (from 2.5 to 2.7 eV) indicating the presence of a QD size distribution in Ge dot superlattices. Raman scattering from Ge QDs is size-selectively enhanced by the resonance of the exciting laser energy and the confined excitonic states.

Interdot energy transfer in a system of coupled InAs/GaAs quantum dots

We present new results concerning the carrier transfer between quantum dots (QDs) in dense InAs/GaAs QD arrays with a bimodal size distribution explored by means of steady-state and time-resolved photoluminescence (PL). Interdot carrier transfer involves transitions from the ground state of small QDs into lower lying states of larger QDs, a relaxation channel that saturates at high excitation densities. A staircase-like spectral dependence of the PL decay time is observed and analyzed by taking into account the bimodal QD size distribution. We show that monitoring the spectral behavior of the ground-state PL time constants offers a new and independent tool for distinguishing between PL contributions from different dot size families.

Raman study of self-assembled InAs quantum dots embedded in AlAs: influence of growth temperature

Phonon spectra of self-assembled InAs quantum dots (QDs) in an AlAs matrix were studied by Raman spectroscopy. A series of InAs QDs was grown by molecular beam epitaxy at substrate temperatures varied in the range of 420–550°C. The observed asymmetric line shape of LO phonons in InAs QDs and its low-frequency shift with increasing excitation energy are explained by QD size distribution and phonon confinement in small-size dots. Phonons of the InAs wetting layer are also observed in the Raman spectra of QD structures. Two bands of interface phonons in the AlAs frequency region are attributed to phonons associated with two types of interfaces: the planar interface wetting layer/AlAs matrix and the three-dimensional QD/matrix interface. A comparison of the position and the line shape of phonon features of InAs QDs grown at different temperatures reveals that dots grown at low temperatures (∼420°C) have the smallest average size. Increasing the temperature leads to the formation of larger InAs islands. At temperatures higher than 520°C partial re-evaporation of InAs occurs.

Resonant Raman Scattering by Strained and Relaxed Ge Quantum Dots

ABSTRACTFundamental vibrations in Ge/Si structures with strained and relaxed Ge quantum dots (QDs) grown by molecular beam epitaxy were investigated using resonant Raman spectroscopy. Transmission electron microscopy experiments show that the strained Ge QDs are typical “hut clusters” with base size of 15nm and a height of 2nm. A two mode distribution in size (100–200nm and 3–6nm) is found for relaxed QDs. The Raman efficiencies of the Ge optical phonons as a function of excitation energy reveal maxima at 2.35–2.41eV attributed to the E0 resonance in Ge QDs due to electronic confinement. The frequency positions of optical phonons localized in Ge “hut clusters” under non-resonant conditions correspond to fully strained Ge QDs while the frequency position of optical phonons in relaxed Ge QDs corresponds to the value in bulk Ge. With increasing excitation energy (2.5–2.7eV) the position of the Ge optical phonons shifts downwards due to size-confinement effect of optical phonons in strained and relaxed Ge QDs, indicating the presence of a QD size distribution in Ge dot structures.

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.

Growth of Si-doped InAs quantum dots and annealing effects on size distribution

We investigated the Si-doped InAs quantum dots (QDs) grown by molecular beam epitaxy and the annealing effects on the QD size distribution through photoluminescence (PL) spectroscopy. A double-peak feature in PL was observed from as-grown InAs QDs with Si-doping, and excitation intensity dependence of PL indicated that the double-peak feature is related to the ground-state emission from InAs QDs with bimodal size distribution. The PL spectrum from Si-doped InAs QDs subjected to annealing treatment at 800°C in nitrogen ambient showed three additional PL peaks and blue-shift of the double-peak feature observed from as-grown sample. The excitation-intensity-dependent PL and consideration of thermal stability of carriers through temperature-dependent PL measurement demonstrated that three additional peaks come from the InAs QDs with three new branches of QD occurring during annealing process.

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.