Properties Of Self-assembled Quantum Dots Research Articles

Altering the luminescence properties of self-assembled quantum dots in GaAs by focused ion beam implantation

Using quantum dots (QDs) as single-photon sources draws the attention in many quantum communication technologies. One pathway towards manufacturing single-photon sources is focused ion beam (FIB) implantation in molecular beam epitaxy-grown QD samples to disable all QDs around an intentional one for single photoluminescence (PL) emission. In this paper, we investigate the lattice disorders in the vicinity of InAs/GaAs QDs introduced by FIB implantation of gallium and indium ions. For high fluences, we achieve total elimination of the QDs photoluminescence. The impact of the different ion species and fluences is studied by low-temperature PL measurements. Furthermore, we deduce a simple model based on the trap-assisted recombination for the description of the degradation of the PL emission. It allows the determination of the fluences at which the PL emission is suppressed. Moreover, we identify the implantation-induced non-radiative defects by temperature-dependent PL measurements.

Energy-tunable sources of entangled photons: a viable concept for solid-state-based quantum relays.

We propose a new method of generating triggered entangled photon pairs with wavelength on demand. The method uses a microstructured semiconductor-piezoelectric device capable of dynamically reshaping the electronic properties of self-assembled quantum dots (QDs) via anisotropic strain engineering. Theoretical models based on k·p theory in combination with finite-element calculations show that the energy of the polarization-entangled photons emitted by QDs can be tuned in a range larger than 100meV without affecting the degree of entanglement of the quantum source. These results pave the way towards the deterministic implementation of QD entanglement resources in all-electrically-controlled solid-state-based quantum relays.

Multimillion-atom modeling of InAs/GaAs quantum dots: interplay of geometry, quantization, atomicity, strain, and linear and quadratic polarization fields

Electronic structure and optical properties of self-assembled quantum dots strongly depend on an intricate interplay of the quantum mechanical size quantization and the atomistic built-in/internal electrostatic fields in the underlying material system. Specifically, built-in fields in zincblende quantum dots originate mainly from: (1) fundamental crystal atomicity and the interfaces between two dissimilar materials, (2) microscopic distribution of strain, and (3) the piezoelectric polarization. In this paper, we first study the origin and nature of these internal fields in InAs/GaAs quantum dots having three different geometries, namely, box, dome, and pyramid. We then quantify and delineate the impact of these internal fields on the one-particle electronic states in terms of symmetry-lowering and localization in the wavefunctions, shift in the energy states and bandgap, anisotropy and non-degeneracy in the $$P$$P level, and formation of mixed excited bound states near the Brillouin zone center. Finally, we study the geometry and size-dependence of interband optical transitions in the XY and XZ planes treating the quantum size quantization and the internal fields as parameters. The computational framework employs a combination of fully atomistic valence force-field molecular mechanics, 20-band nearest-neighbor $$sp^{3} d^{5} s^{*}$$sp3d5s? tight-binding electronic bandstructure models, and appropriate post-processing tools to obtain the interband optical transition rates. In particular, to model piezoelectricity, four different polarization models (based on the experimental and ab initio coefficients accounting for both linear and non-linear effects) have been considered in increased order of accuracy. With the non-linear piezoelectricity generally opposing the linear counterpart, the net piezoelectric field is found to be negligible in smaller dots but exhibits non-vanishing effects as the dot height is increased.

Optical properties of charged quantum dots doped with a single magnetic impurity

We present a microscopic theory of the optical properties of self-assembled quantum dots doped with a single magnetic manganese (Mn) impurity and containing a controlled number of electrons. The single-particle electron and heavy-hole electronic shells are described by two-dimensional harmonic oscillators. The electron-electron, electron-hole Coulomb as well as the short-range electron spin-Mn spin and hole spin-Mn spin contact exchange interactions are included. The electronic states of the photoexcited electron-hole-Mn complex and of the final electron-Mn complex are expanded in a finite number of configurations and the full interacting Hamiltonian is diagonalized numerically. The emission spectrum is predicted as a function of photon energy for a given number of electrons and different number of confined electronic quantum dot shells. We show how emission spectra allow to identify the number of electronic shells, the number of electrons populating these shells and, most importantly, their spin. We show that electrons not interacting directly with the spin of Mn ion do so via electron-electron interactions. This indirect interaction is a strong effect even when Mn impurity is away from the quantum dot center.

Influence of bimodal distribution and excited state emission on photoluminescence spectra of InAs self-assembled quantum dots

Optical properties of self-assembled quantum dots (SAQDs) samples were investigated through photoluminescence (PL) with variation of temperature and excitation density. Two types of InAs SAQDs were studied: dots grown directly on GaAs, and dots grown inside an InGaAs quantum well, heterostructure known as dot-in-a-well (DWELL). The QDs structures were produced through conventional growth (with several deposition rates) as well as through pulsed growth mode, which consists in deposition cycles with growth interruption intervals. In the samples of QDs grown on GaAs, two PL peaks were identified and their behavior as a function of temperature can be explained by taking into account a bimodal distribution of dot sizes. For the DWELL sample only one PL peak was identified, which was assigned to the emission of the ground state of QDs belonging to a larger Gaussian distribution of dot sizes. SAQDs samples showed an anomalous behavior of the peak energy, integrated intensity and linewidth (FWHM) of the PL signal, as a function of temperature. To explain the observed results, a model was proposed that takes into account the size distribution of dots and the excited state emission of the family of larger dots. The QDs can be distributed in a bimodal size distribution, and the emission from the excited state can be observed, or not, depending on the growth conditions and on the potential profile of the dot. Such aspects can be combined to explain several peculiarities of the PL spectra observed in this and other published papers on SAQDs optical properties.

Temperature dependent empirical pseudopotential theory for self-assembled quantum dots

We develop a temperature dependent empirical pseudopotential theory to study the electronic and optical properties of self-assembled quantum dots (QDs) at finite temperature. The theory takes the effects of both lattice expansion and lattice vibration into account. We apply the theory to InAs/GaAs QDs. For the unstrained InAs/GaAs heterostructure, the conduction band offset increases whereas the valence band offset decreases with increasing temperature, and there is a type-I to type-II transition at approximately 135 K. Yet, for InAs/GaAs QDs, the holes are still localized in the QDs even at room temperature, because the large lattice mismatch between InAs and GaAs greatly enhances the valence band offset. The single-particle energy levels in the QDs show a strong temperature dependence due to the change of confinement potentials. Because of the changes of the band offsets, the electron wavefunctions confined in QDs increase by about 1–5%, whereas the hole wavefunctions decrease by about 30–40% when the temperature increases from 0 to 300 K. The calculated recombination energies of excitons, biexcitons and charged excitons show red shifts with increasing temperature which are in excellent agreement with available experimental data.

Effects of charged defects on the electronic and optical properties of self-assembled quantum dots

We investigate the effects of point charge defects on the single particle electronic structure, emission energies, fine structure splitting and oscillator strengths of excitonic transitions in strained In$_{0.6}$Ga$_{0.4}$As/GaAs and strain-free GaAs/Al$_{0.3}$Ga$_{0.7}$As quantum dots. We find that the charged defects significantly modify the single particle electronic structure and excitonic spectra in both strained and strain-free structures. However, the excitonic fine structure splitting, polarization anisotropy and polarization direction in strained quantum dots remain nearly unaffected, while significant changes are observed for strain-free quantum dots.

The polarization response in InAs quantum dots: theoretical correlation between composition and electronic properties

III–V growth and surface conditions strongly influence the physical structure and resulting optical properties of self-assembled quantum dots (QDs). Beyond the design of a desired active optical wavelength, the polarization response of QDs is of particular interest for optical communications and quantum information science. Previous theoretical studies based on a pure InAs QD model failed to reproduce experimentally observed polarization properties. In this work, multi-million atom simulations are performed in an effort to understand the correlation between chemical composition and polarization properties of QDs. A systematic analysis of QD structural parameters leads us to propose a two-layer composition model, mimicking In segregation and In–Ga intermixing effects. This model, consistent with mostly accepted compositional findings, allows us to accurately fit the experimental PL spectra. The detailed study of QD morphology parameters presented here serves as a tool for using growth dynamics to engineer the strain field inside and around the QD structures, allowing tuning of the polarization response.

Strain tuning of quantum dot optical transitions via laser-induced surface defects

We discuss the fine-tuning of the optical properties of self-assembled quantum dots by the strain perturbation introduced by laser-induced surface defects. We show experimentally that the quantum dot transition red-shifts, independently of the actual position of the defect, and that such frequency shift is about a factor five larger than the corresponding shift of a micropillar cavity mode resonance. We present a simple model that accounts for these experimental findings.

Optical Properties of Self-Assembled Quantum Dots in Single and Double-Layer Configurations

Optical Properties of Self-Assembled Quantum Dots in Single and Double-Layer Configurations

An introduction to self-assembled quantum dots

We give an overview of semiconductor structures that confine charge carriers on a length scale comparable to their de Broglie wavelength, comment on the production and the device applications of such structures, and then describe the properties of self-assembled quantum dots. An idealized model is discussed that reduces the motion of electronic charge carriers confined to a quantum dot to familiar classroom problems. The model allows us to calculate the energy levels in a quantum dot and compare them to the emission spectra of real samples. We calculate the emission spectrum of an InAs self-assembled quantum dot imbedded in GaAs and discuss the limitations of the model.

Electronic properties of self-assembled quantum dots of sodium on Cu(1 1 1) and their interaction with water

The electronic properties of thin films of Na on Cu(1 1 1) and their interaction with water have been investigated at room temperature by high resolution electron energy loss spectroscopy. The first Na layer is characterized by two features tentatively assigned to charge density waves. The second Na layer grows as small islands. The loss spectrum of this layer shows a feature at 3.0 eV identified as a Mie resonance. Increasing alkali coverage, Na islands form a continuous film, as indicated by the appearance of a Na surface plasmon and by the disappearance of the Mie resonance. Water vapour strongly interacts with Na layers as shown by the OH–Na vibration whose frequency shifts from 36 meV to 53 meV as a function of alkali coverage.

Effects of linear and nonlinear piezoelectricity on the electronic properties ofInAs∕GaAsquantum dots

The existence of enormous strain fields in self-assembled quantum dots has led to the expectation of dramatic effects of piezoelectricity. However, only linear piezoelectric tensors were used in all previous calculations. We calculate the piezoelectric properties of self-assembled quantum dots using the linear and quadratic piezoelectric tensors derived from first-principles density functional theory. We find that the previously ignored quadratic term has similar magnitude as the linear term and the two terms tend to cancel each other. We show the effect of piezoelectricity on electron and hole energy levels and wave functions as well as on correlated absorption spectra.

Electron transport through individual Ge self-assembled quantum dots on Si

Electrical properties of self-assembled quantum dots have been the subject of intensive research due to quantum confinement. Here the authors report on the fabrication of Ge quantum dots (QDs) onto Si (100) by ultrahigh-vacuum ion beam sputtering and the electrical properties of individual QDs. Transmission electron microscopy images show that samples with completely incoherent or coherent semispherical islands can be produced under different ion energies. The current-voltage (I-V) characteristics with conductive atomic force microscopy at room temperature. exhibit linear behavior at low bias and nonlinear behavior at large bias from coherent islands, whereas the staircase structures are clearly observed in the I-V curve from incoherent islands, which are attributed to electron tunneling through the quantized energy levels of a single Ge QD.

Tight-binding model for semiconductor quantum dots with a wurtzite crystal structure: From one-particle properties to Coulomb correlations and optical spectra

In this work we investigate the electronic and optical properties of self-assembled quantum dots by means of a tight-binding model. Coulomb and dipole matrix elements are calculated from the one-particle wave functions which fully include the atomistic wurtzite structure of the low-dimensional heterostructures and serve as an input for the calculation of optical spectra. For the investigated $\mathrm{In}\mathrm{N}∕\mathrm{Ga}\mathrm{N}$ material system, the optical selection rules are found to be strongly affected by band-mixing effects for the localized valence band states. Within this framework, excitonic absorption and emission spectra are analyzed for different sizes of the investigated lens-shaped quantum dots, including the influence of the intrinsic and strain-induced electrostatic field of the wurtzite structure. A dark exciton ground state for small quantum dots is found.

Three-dimensional stacking of self-assembled quantum dots in multilayer structures

Three-dimensional stacking of semiconductor nano-islands in multilayers or superlattice structures provides a powerful tool for controlling the properties of self-assembled quantum dots. These stackings can be caused by several different mechanisms based on: (i) elastic interactions due to the strain fields of the buried dots; (ii) morphological interactions due to nonplanarized spacer topographies; or (iii) interactions based on chemical composition modulations within the spacer material. All three interactions may give rise to a vertical dot alignment in columns as well as to oblique or staggered dot stackings, depending on the details of the interaction mechanisms. For the interlayer correlations mediated by the elastic strain fields, the elastic anisotropy and surface orientation, but also the dot sizes and spacer layer thicknesses play a crucial role. As a result, transitions between different types of dot stackings can be induced as a function of spacer layer thicknesses and growth parameters. The large range of parameters involved in interlayer correlation formation may allow the controlled synthesis of new types of ordered structures with novel properties. To cite this article: G. Springholz, C. R. Physique 6 (2005).

Optical properties of type-I and II quantum dots

This paper focuses on recent results on the optical properties of self-assembled quantum dots involving type-I InGaAs/GaAs and type-II InP/GaAs interfaces. In the first part, we focus on the InGaAs/GaAs quantum dots, that were used to study the influence of a two-dimensional electron gas on the optical emission of single quantum dots. In the second part, we present the results on type-II InP/GaAs quantum dots. In this system, we observed an experimental evidence indicating that the model used to interpret the blue shift of the type-II quantum well optical emission cannot be applied to type-II quantum dots.

Nanoindentation effect on the optical properties of self-assembled quantum dots

The optical emission spectrum of a quantum dot array is computed for a series of cases in which a nanometer scale microscope tip indenter is used to impose increasing elastic strain on the sample. The system under consideration consists of approximately 30 self-assembled In 0.55Al 0.45As quantum dots buried in a matrix material of Al 0.35Ga 0.65As; the indenter is a tapered optical fiber tip used to collect emitted light from the array while simultaneously imposing strain on the sample. A continuum analysis is used to compute the confined electron and hole energies and wave functions in the presence of the imposed elastic indentation strain field. The analysis includes the consideration of exciton binding energy effects, which are found to be small for quantum dots in the size range of interest. From the computed energy states, the optical conductivity of the system is evaluated using simple scattering rate theory. The blue-shift in light emitted by individual dots predicted by the analysis agrees well with experimental observations by Robinson et al. (Appl. Phys. Lett. 72 (1998) 2081). Finally, a simple dislocation nucleation estimate based on a Rice–Thomson-type analysis is developed for the system. The estimate supports the observation that dislocation activity in the single crystalline sample may be responsible for the experimentally observed optical emission quenching when indentation exceeds a critical depth.

Electrical and optical properties of self-assembled quantum dots

One of the main directions of contemporary semiconductor physics is the production and study of structures with a dimension less than two, i.e. quantum wires (QWi) and quantum dots (QDs), in order to realise novel devices that make use of low-dimensional confinement effects.One of the promising fabrication methods is to use self-organised three-dimensional (3D) structures, such as 3D coherent islands, which are often formed during the initial stage of heteroepitaxial growth in lattice-mismatched systems. Quantum dots, for example, are believed to provide a promising way for a new generation of optical light sources such as injection lasers. While quantum well structures are already widely used in optoelectronic devices, QWi and QDs appear to be much more difficult to fabricate for this purpose. Some of the electrical and optical properties of self-assembled QDs will be reported in this paper.

Influence of high-index GaAs substrates on the growth of highly strained (InGa)As/GaAs heterostructures

A systematic study of the optical and microscopic properties of self-assembled quantum dots in In x Ga 1− x As/GaAs heterostructures has been carried out by varying the In concentration and the (InGa)As layer thickness on both (1 0 0) and (3 1 1)B orientations. Photoluminescence spectroscopy indicates that the dot formation on high-index substrates is delayed at larger amount of (InGa)As with respect to (1 0 0) substrates for both moderate ( x=0.5) and high ( x=1.0) lattice mismatch. Atomic force microscopy shows that consistently smaller and flatter dots are obtained on high-index planes, highlighting the important role played by substrate morphology on the dot self-aggregation process.