InAs/GaAs Quantum Dots with Multimodal Size Distribution

Fabrication of self-assembled InGaAs, GaAs, and InAs quantum dots by chemical beam epitaxy

Self-organized growth of InAs, InGaAs, and GaSb quantum dots in GaAs matrix is investigated to establish a basis for a targeted engineering. All studied dots form in the Stranski–Krastanow mode on a two-dimensional wetting layer in a regime which is predominantly controlled by kinetics. Equilibrium-near conditions are found on a local scale. The material transfer during dot formation shows distinct differences for the three studied kind of dots. For InGaAs and GaSb dots a pronounced transfer from an intermixed wetting layer to the dots is found. Such dots have an inhomogenous composition, and dot ensembles with a bimodal size distribution may be obtained in both cases. For InAs dots material transfer is observed solely between the purely binary dots, while the wetting layer remains unaffected and is not intermixed. Dots with a multimodal size distribution are demonstrated, referring to well-defined self-similar dot shapes and size variations in steps of integral InAs monolayers.

Atomic-scale probing of metallic and semiconductor nanostructures

Improvement of performance of GaAs solar cells by inserting self-organized InAs/InGaAs quantum dot superlattices

We describe the optical emission and the carrier dynamics of an ensemble of self-assembled GaAs quantum dots embedded in GaP(001). The QD formation is driven by the 3.6% lattice mismatch between GaAs and GaP in the Stranski-Krastanow mode after deposition of more than 1.2 monolayers of GaAs. The quantum dots have an areal density between 6 and 7.6 × 1010 per cm−2 and multimodal size distribution. The luminescence spectra show two peaks in the range of 1.7 and 2.1 eV. The samples with larger quantum dots have red emission and show less thermal quenching compared with the samples with smaller QDs. The large QDs luminescence up to room temperature. We attribute the high energy emission to indirect carrier recombination in the thin quantum wells or small strained quantum dots, whereas the low energy red emission is due to the direct electron-hole recombination in the relaxed quantum dots.

Scintillation and optical properties of an integrated InAs/GaAs Quantum Dot (QD) Scintillation Detector were investigated to improve efficiency and reduce the decay time of luminescence. The photoluminescent properties of InAs QDs embedded in a GaAs matrix were studied as-grown by molecular beam epitaxy (MBE) on GaAs substrates, and on foreign substrates fabricated as thick (10-25 μm) waveguides. The luminescent efficiency of QDs as-grown on GaAs substrates were examined using photoluminescence (PL) measurements at room temperature and the thermal quenching of PL was analyzed by comparing the integrated PL intensities at 77K-400K. PL measurements were used to optimize QD AlAs capping, QD density (by varying spacing between QD layers), p-type modulation doping between QD layers, and the relative red-shift in QD luminescence from the GaAs band-edge. Room temperature PL revealed improved efficiency using QDs with >350 meV redshifted PL from the addition of a ~2 monolayer (ML) AlAs capping layer on top of the QDs and increased QD layer spacing (~800 nm). PL across a wide range of temperatures showed reduced PL thermal quenching with three distributed p-type modulation doping layers as compared to a single modulation doping layer.

Colloidal quantum dots (QDs) emitting in the infrared (IR) are promising building blocks for numerous photonic, optoelectronic and biomedical applications owing to their low-cost solution-processability and tunable emission. Among them, lead- and mercury-based QDs are currently the most developed materials. Yet, due to toxicity issues, the scientific community is focusing on safer alternatives. In this regard, indium arsenide (InAs) QDs are one of the best candidates as they can absorb and emit light in the whole near infrared spectral range and they are RoHS-compliant, with recent trends suggesting that there is a renewed interest in this class of materials. This review focuses on colloidal InAs QDs and aims to provide an up-to-date overview spanning from their synthesis and surface chemistry to post-synthesis modifications. We provide a comprehensive overview from initial synthetic methods to the most recent developments on the ability to control the size, size distribution, electronic properties and carrier dynamics. Then, we describe doping and alloying strategies applied to InAs QDs as well as InAs based heterostructures. Furthermore, we present the state-of-the-art applications of InAs QDs, with a particular focus on bioimaging and field effect transistors. Finally, we discuss open challenges and future perspectives.

InAs quantum dots in a GaAs matrix are studied. Those quantum dots are used in applications to enhance the overall efficiency of multi-junction solar cells beyond 40%. Photoluminescence measurements at 77 K using a 532 nm laser have been performed on an epitaxially grown structure of self-assembled InAs quantum dots in a GaAs matrix upon a Ge substrate, where three energy levels are determined at E_n=0=1.01 eV, E_n=1=1.07 eV and E_n=2=1.13 eV. Theoretical calculations of the energy levels determine the quantum dots to be 7 nm high and have a 37 nm base diameter, which is close to atomic force microscopy measurements performed on the samples. Intensity dependant photoluminescence measurements reveal the saturation of the first excited energy level at 5×10⁶ W/m². A general model for the saturation of the first quantum dot excited energy level is then developed. This saturation model is applied to the AM1.5D solar spectrum at 297 K to determine the concentration of solar energy needed to saturate the first excited energy level within a multi-junction solar cell. Saturation was determined to be at ~1.56×10⁴ suns (where 1 sun = 1000 W/m²). Since current solar concentrations are between 500-1000 suns concentration, the saturation of such quantum dots will not occur.

The structural and optical properties of self-assembled InAs quantum dots (QDs) with various InGaAs structures were investigated by transmission electron microscopy (TEM) and photoluminescence (PL). The emission peak position of InAs QDs covered by a 6 nm In0.15Ga0.85As layer was 1.26 μm with PL linewidth of 31 meV, which is narrower than that of QDs in a GaAs matrix. By inserting a 1 nm In0.15Ga0.85As layer below the InAs QD layer with a 6 nm In0.15Ga0.85As overgrowth layer, the emission peak position was redshifted with larger energy-level spacing between the ground states and the first excited states compared to that of QDs with an In0.15Ga0.85As overgrowth layer only. By covering the InAs QDs on a 1 nm In0.15Ga0.85As layer with an 8 nm InxGa1−xAs layer having graded In composition, the emission peak position was 1.32 μm with relatively larger energy-level spacing and narrower PL linewidth compared to QDs covered by an In0.15Ga0.85As layer. The longer emission wavelength with relatively larger energy-level spacing was largely related to the change in the QD shape and size, especially the aspect ratio (height/width), which was confirmed by cross-sectional TEM images.

Direct detection of electronic states for individual indium arsenide (InAs) quantum dots grown by molecular beam epitaxy

We present anisotropic nonparabolic position-dependent effective-mass calculations of the bound energy levels of electrons confined in lens-shaped InxGa1−xAs quantum dots embedded in a GaAs matrix. The strain and In gradient inside the quantum dots and their wetting layer (due to the strong In segregation effect present in the InxGa1−xAs/GaAs system) were taken into account. The bound eigenstates and eigenenergies of electrons in a finite 3D confinement potential were determined by the full numerical diagonalization of the Hamiltonian. The quantum dots and their wetting layer were sliced into a finite number of monolayers parallel to the substrate surface, each one with a specific In concentration, in order to be able to reproduce any composition profile along the growth direction. A comparison between the eigenenergies of the “pure” InAs quantum dots and the quantum dots with an inhomogeneous In content indicates that In segregation dramatically affects their electronic structure and must be taken into account if one wishes to accurately simulate the real optoelectronic properties of such nanostructures.

Low‐dimensional III–V semiconductors have many advantages over other semiconductors; however, they are not particularly stable under physiological conditions. Hybridizing biocompatible organic molecules with advanced optical and electronic semiconductor devices based on quantum dots (QDs) and quantum wires could provide an efficient solution to realize stress‐free and nontoxic interfaces to attach larger functional biomolecules. Monitoring the modifications of the optical properties of the hybrid molecule–QD systems by grafting various types of air‐stable diazonium salts onto the QD structures surfaces provides a direct approach to prove the above concepts. The InAs/GaAs QD structures used in this work consist of a layer of surface InAs QDs and a layer of buried InAs QDs embedded in a wider‐bandgap GaAs matrix. An enhancement in photoluminescence intensity by a factor of 3.3 from the buried QDs is achieved owing to the efficient elimination of the dangling bonds on the surface of the structures and to the decrease in non‐radiative recombination caused by their surface states. Furthermore, a narrow photoluminescence band peaking at 1620 nm with a linewidth of 49 meV corresponding to the eigenstates interband transition of the surface InAs QDs is for the first time clearly observed at room temperature, which is something that has rarely been achieved without the use of such engineered surfaces. The experimental results demonstrate that the hybrid molecule–QD systems possess a high stability, and both the surface and buried QDs are very sensitive to changes in their surficial conditions, indicating that they are excellent candidates as basic sensing elements for novel biosensor applications.

GaSb quantum dots (QDs) in a GaAs matrix have attracted much attention for the potential ability to extend the accessible wavelength of QD emitters towards a wavelength of > 1.3 μm owing to their unique electronic and optical properties caused by their staggered (type-II) band alignment. North et al. calculated the electronic structure of self-assembled GaSb/GaAs dots by using the simple one-band model (effective mass model). Therefore it is interesting to carry out the detailed calculations using a more accurate method, in particular in the view of the new experiments. The aim of this work is to study the electronic properties of GaSb/GaAs self-assembled quantum dots (SAD) by using the sp 3 s * empirical tight-binding method. The influence of strain on the electronic structure is determined by minimizing the elastic energy within the Valence Force Field Approach and the piezoelectric potential is calculated by solving the corresponding 3D Poisson equation. Single-particle bound-state energies are computed as a function of the dot sizes. Finally we have found that the theoretical PL spectra are in a good agreement with the experiment and k.p method. empirical models are widely implemented to study a large quantum dots. There are three empirical models, the k.p approximation (9), the pseudopotential model (10) and the tight-binding model (11), (12). The k.p approximation treats a quantum dot as a confined bulk and continuum system, while the pseudopotential and tight-binding models treat the system with the atomistic description. The distinction between the two atomistic models is the degree of atomic detail included in the model. Within the tight-binding model, the atomistic detail is limited to a small basis set, while in the pseudopotential model the feature of wave functions is described with a large basis set. Therefore the tight-binding model is computationally less costly than the pseudopotential model. The tight-binding model is a good candidate for the study of relatively big and complicated systems. At the moment, the theoretical calculations of the type-II quantum dots have not been extensively investigated and there are only few papers that we can mention here. North and co-authors (13) investigated the electronic structure of type-II GaSb/GaAs quantum dots by using the multiband effective-mass calculation. K. Gradkowski and et al. (2) reported the results of the room temperature photoreflectance (PR) and photoluminescence (PL) measurements of molecular beam epitaxy (MBE) grown GaAsSb/GaAs quantum dot structure: one with a capping quantum well and one without it. PL technique is employed to probe the ground-state transition energies. Using the experimental data, they calculated a band structure of these dots by using the 8-band k.p approach. The theoretical analysis accounts well in both GaAsSb/GaAs quantum dot with a capping quantum well and one without it. In this paper, we report the tight-binding calculations of the type-II GaSb/GaAs quantum dots. The strain distribution of the quantum dot is calculated by using the Valence Force Field method. The piezoelectricity is also obtained by solving the 3D Poisson's equation. Then we numerically calculate the electronic structure of the quantum dot as a function of the dot sizes. Finally we compare our calculated results with the experiment and another numerical analysis.

Non-exponential capture of electrons in GaAs with embedded InAs quantum dots

We discuss fabrication of GaAs quantum wires and dots using both selective growth and spontaneous growth technique in MOCVD, including the optical properties of those nano-structures. In the selective growth, triangular-shaped GaAs quantum wires as narrow as 7nm and quantum dots with 25nm lateral width were obtained. The quantum confinement effect is evidenced by photoluminescence (PL) and magneto-PL. On the other hand, self-organizing growth achieved InGaAs quantum dots with a diameter of 15nm. For characterization of the nanostructures, observation of photoluminescence from a single quantum dot was achieved. Finally, as the first step to the ultimate quantum lasers, a vertical microcavity quantum wire lasers was demonstrated.