Large Quantum Dots Research Articles

Features of the formation of Mn doped InAs/GaAs quantum dots by vapor phase epitaxy

A self-organized InAs/GaAs quantum dot (QD) array is doped with Mn. The effect of the Mn concentration on the morphology and QD luminescence properties is investigated. It is found that Mn deltadoping of the GaAs buffer layer before QD growth with a layer concentration of 1014 cm−2 leads to the formation of an array of large QDs with variable composition In x Ga1 − x As. The effect is explained within a model of In and Ga atom interdiffusion.

Correlated electron—hole transitions in wurtzite GaN quantum dots: the effects of strain and hydrostatic pressure

Within the effective-mass and finite-height potential barrier approximation, a theoretical study of the effects of strain and hydrostatic pressure on the exciton emission wavelength and electron—hole recombination rate in wurtzite cylindrical GaN/AlxGa1−xN quantum dots (QDs) is performed using a variational approach. Numerical results show that the emission wavelength with strain effect is higher than that without strain effect when the QD height is large (> 3.8 nm), but the status is opposite when the QD height is small (< 3.8 nm). The height of GaN QDs must be less than 5.5 nm for an efficient electron—hole recombination process due to the strain effect. The emission wavelength decreases linearly and the electron—hole recombination rate increases almost linearly with applied hydrostatic pressure. The hydrostatic pressure has a remarkable influence on the emission wavelength for large QDs, and has a significant influence on the electron—hole recombination rate for small QDs. Furthermore, the present numerical outcomes are in qualitative agreement with previous experimental findings under zero pressure.

ZnS overlayer on in situ chemical bath deposited CdS quantum dot-assembled TiO2 films for quantum dot-sensitized solar cells

ZnS overlayers were deposited on the CdS quantum dot (QD)-assembled TiO2 films, where the CdS QDs were grown on the TiO2 by repeated cycles of the in situ chemical bath deposition (CBD). With increasing the CdS CBD cycles, the CdS QD-assembled TiO2 films were transformed from the TiO2 film partially covered by small CdS QDs (Type I) to that fully covered by large CdS QDs (Type II). The ZnS overlayers significantly improved the overall energy conversion efficiency of both Types I and II. The ZnS overlayers can act as the intermediate layer and energy barrier at the interfaces. However, the dominant effects of the ZnS overlayers were different for the Types I and II. For Type I, ZnS overlayer dominantly acted as the intermediate layer between the exposed TiO2 surface and the electrolyte, leading to the suppressed recombination rate for the TiO2/electrolyte and the significantly enhanced charge-collection efficiency. On the contrary, for Type II, it dominantly acted as the efficient energy barrier at the interface between the CdS QDs and the electrolyte, leading to the hindered recombination rate from the large CdS QDs to the electrolyte and thus enhanced electron injection efficiency.

Fast iterative interior eigensolver for millions of atoms

We show that a combination of the Generalized Davidson method and harmonic Ritz values (called harmonic Davidson) is well-suited for solving large interior eigenvalue problems using a plane wave basis. The algorithm enables us to calculate impurity and band edge states for systems of 100,000 atoms in about one day on 32 cores. We demonstrate the capabilities of the method by calculating the electronic states of a large GaAs quantum dot embedded in an AlAs matrix.

Large array of single, site-controlled InAs quantum dots fabricated by UV-nanoimprint lithography and molecular beam epitaxy

We present the growth of single, site-controlled InAs quantum dots on GaAs templates using UV-nanoimprint lithography and molecular beam epitaxy. A large quantum dot array with a period of 1.5 µm was achieved. Single quantum dots were studied by steady-state and time-resolved micro-photoluminescence experiments. We obtained single exciton emission with a linewidth of 45 µeV. In time-resolved experiments, we observed decay times of about 670 ps. Our results underline the potential of nanoimprint lithography and molecular beam epitaxy to create large-scale, single quantum dot arrays.

Discretization of Electronic States in LargeInAsP/InPMultilevel Quantum Dots Probed by Scanning Tunneling Spectroscopy

The topography and the electronic structure of InAsP/InP quantum dots are probed by cross-sectional scanning tunneling microscopy and spectroscopy. The study of the local density of states in such large quantum dots confirms the discrete nature of the electronic levels whose wave functions are measured by differential conductivity mapping. Because of their large dimensions, the energy separation between the discrete electronic levels is low, allowing for quantization in both the lateral and growth directions as well as the observation of the harmonicity of the dot lateral potential.

Size-dependent carrier dynamics in self-assembled CdTe/ZnTe quantum dots

We investigate size-dependent carrier dynamics in self-assembled CdTe/ZnTe quantum dots (QDs) grown using molecular beam epitaxy and atomic layer epitaxy. Photoluminescence (PL) spectra show that the excitonic peak corresponding to transitions from the ground electronic subband to ground heavy-hole band in CdTe/ZnTe QDs shifts to a lower energy with increasing ZnTe buffer thicknesses. This shift of the PL peak can be attributed to size variation of the CdTe QDs. In particular, carrier dynamics in CdTe QDs grown on various ZnTe buffer layer thicknesses is studied using time-resolved PL measurements. As a result, the decay time of CdTe QDs is shown to increase with increasing ZnTe buffer layer thicknesses due to the reduction of the exciton oscillator strength in the larger QDs.

Multiple Exciton Generation and Recombination Dynamics in Small Si and CdSe Quantum Dots: An Ab Initio Time-Domain Study

Multiple exciton generation and recombination (MEG and MER) dynamics in semiconductor quantum dots (QDs) are simulated using ab initio time-dependent density functional theory in combination with nonadiabatic molecular dynamics. The approach differs from other MEG and MER theories because it provides atomistic description, employs time-domain representation, allows for various dynamical regimes, and includes electron-phonon interactions. MEG rapidly accelerates with energy, reflecting strong energy dependence of double exciton (DE) density of states. At early times, MEG is Gaussian rather than exponential. Exponential dynamics, assumed in rate theories, starts at a later time and becomes more important in larger QDs. Phonon-assisted MEG is observed at energies below the purely electronic threshold, particularly in the presence of high-frequency ligand vibrations. Coupling to phonons is essential for MER since higher-energy DEs must relax to recombine into single excitons (SEs), and SEs formed during MERs must lose some of their energy to avoid recreating DEs. MER simulated starting from a DE is significantly slower than MER involving an optical excitation of a SE, followed by MEG and then MER. The latter time scale agrees with experiment, emphasizing the importance of quantum-mechanical superpositions of many DEs for efficient MER. The detailed description of the interplay between MEG and MER coupled to phonons provides important insights into the excited state dynamics of semiconductor QDs and nanoscale materials in general.

The Influence of Interlayer Molecules in Luminescence of Self-Assembled Bilayer PbS Quantum Dot Films

We synthesized organic solvent-soluble and water-soluble PbS quantum dots (QDs) with different sizes. The organic solvent-soluble PbS QDs dispersed in tetrachloroethylene were used to prepare bilayers structures of QDs bound by dithiol linkers on GaAs. The water-soluble PbS QDs were used to prepare bilayer structures of QDs on quartz based on alternating adsorption of polyelectrolyte. For bilayer films on GaAs, it was found that the stacking sequence of QDs affects the quantum yield and emission wavelength of the larger QDs. However, for bilayer films with different stacking sequences on quartz, the larger QDs show similar PL intensities and emission wavelength independent of the sequence. The probable mechanism for this difference observed is discussed in terms of charge transfer between QDs.

Density and size control of InP/GaInP quantum dots on GaAs substrate grown by gassource molecular beam epitaxy

We demonstrate a method to controllably reduce the density of self-assembledInP quantum dots (QDs) by cyclic deposition with growth interruptions.Varying the number of cycles enabled a reduction of the QD density from7.4 × 1010 cm − 2 to1.8 × 109 cm − 2 for the same total amount of deposited InP. Simultaneously, a systematic increase of the QDsize could be observed. Emission characteristics of different-sized InP QDs were analyzed.Excitation power dependent and time-resolved measurements confirm a transition from type Ito type II band alignment for large InP quantum dots. Photon autocorrelation measurementsof type I QDs performed under pulsed excitation reveal pronounced antibunching (g(2)(τ = 0) = 0.06 ± 0.03) as expected for a single-photon emitter. The described growth routine has great promisefor the exploitation of InP QDs as quantum emitters.

In-plane polarization anisotropy of ground state optical intensity in InAs/GaAs quantum dots

The design of optical devices such as lasers and semiconductor optical amplifiers for telecommunication applications requires polarization insensitive optical emissions in the region of 1500 nm. Recent experimental measurements of the optical properties of stacked quantum dots have demonstrated that this can be achieved via exploitation of inter-dot strain interactions. In particular, the relatively large aspect ratio (AR) of quantum dots in the optically active layers of such stacks provide a two-fold advantage, both by inducing a red shift of the gap wavelength above 1300 nm, and increasing the TM001-mode, thereby decreasing the anisotropy of the polarization response. However, in large aspect ratio quantum dots (AR &amp;gt; 0.25), the hole confinement is significantly modified compared with that in lower AR dots—this modified confinement is manifest in the interfacial confinement of holes in the system. Since the contributions to the ground state optical intensity (GSOI) are dominated by lower-lying valence states, we therefore propose that the room temperature GSOI be a cumulative sum of optical transitions from multiple valence states. This then extends previous theoretical studies of flat (low AR) quantum dots, in which contributions arising only from the highest valence state or optical transitions between individual valence states were considered. The interfacial hole distributions also increases in-plane anisotropy in tall (high AR) quantum dots (TE110 ≠ TE−110), an effect that has not been previously observed in flat quantum dots. Thus, a directional degree of polarization (DOP) should be measured (or calculated) to fully characterize the polarization response of quantum dot stacks. Previous theoretical and experimental studies have considered only a single value of DOP: either [110] or [−110].

Kondo effect and mesoscopic fluctuations

Two important themes in nanoscale physics in the last two decades are correlations between electrons and mesoscopic fluctuations. Here we review our recent work on the intersection of these two themes. The setting is the Kondo effect, a paradigmatic example of correlated electron physics, in a nanoscale system with mesoscopic fluctuations; in particular, we consider a small quan- tum dot coupled to a finite reservoir (which itself may be a large quantum dot). We discuss three aspects of this problem. First, in the high-temperature regime, we argue that a Kondo temperature TK which takes into account the mesoscopic fluctuations is a relevant concept: for instance, physical properties are universal functions of T /TK. Secondly, when the temperature is much less than the mean level spacing due to confinement, we characterize a natural cross-over from weak to strong coupling. This strong coupling regime is itself characterized by well-defined single-particle levels, as one can see from a Nozieres Fermi-liquid theory argument. Finally, using a mean-field technique, we connect the mesoscopic fluctuations of the quasiparticles in the weak coupling regime to those at strong coupling.

Measurement of the separation dependence of resonant energy transfer between CdSe/ZnS core/shell nanocrystallite quantum dots

The separation dependence of the interaction between two resonant groups of CdSe$/$ZnS nanocrystallite quantum dots is studied at room temperature. A near-field scanning optical microscope is used to bring a group of monodisperse $\ensuremath{\sim}$6.5-nm-diam nanocrystallite quantum dots, which are attached to the microscope probe, into close proximity of an $\ensuremath{\sim}$8.5-nm-diam group of nanocrystallite quantum dots which are deposited on a solid immersion lens. Information extracted from photoluminescence, photoluminescence excitation, and absorption curves as well as numerical calculations of the energy levels show that the third excited excitonic energy level of the large quantum dots nearly matches the ground excitonic energy level for the small quantum dots. Quenching of the small quantum dots' photoluminescence signal is observed as they approach the large quantum dots. On average, the separation between microscope probe and solid immersion lens changed in the 15--50-nm range. The transition probability between these two groups of quantum dots is calculated to be $(2.60\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}47}\phantom{\rule{4.pt}{0ex}}{\text{m}}^{6})/{R}^{6}$, within the experimentally obtained range of transition probabilities $(0.70\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}47}\phantom{\rule{4.pt}{0ex}}{\text{m}}^{6})/{R}^{6}$--$(11.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}47}\phantom{\rule{4.pt}{0ex}}{\text{m}}^{6})/{R}^{6}$ . The F\"orster radius, as a signature of energy transfer efficiency, is experimentally found to be in the 14--22-nm range.

Effects of electric field and shape on the bound polaron in a wurtzite ellipsoidal finite-potential quantum dot

The ground state of bound polaron in a weakly prolate ellipsoidal quantum dot in the presence of an external electric field has been calculated within the effective-mass approximation by using a variational method in the framework of perturbation theory. A Fröhlich-like electron-phonon interaction Hamiltonian which accounts for the longitudinal optical (LO) and transverse optical (TO) polarizations mixing due to the anisotropy is used in the calculation. The binding energy of the bound polaron is calculated by taking the electron couples with both branches of LO-like and TO-like phonons. The interaction between impurity and phonons has also been considered. The results show that the influence of phonons is dependent strongly upon the dot shape and the anisotropy effects of wurtzite crystals, and the influence of electric field is obvious only for the large quantum dot size. The TO-like phonon contribution to the binding energy is positive, but the contribution of LO-like phonon is negative and more important than TO-like phonon.

Capacitance Transient Analysis of Different-Sized InAs/GaAs Quantum Dot Structures

The energy states of InAs/GaAs self-assembled quantum dots (QDs) were analyzed by comparing between two QD systems with different QD sizes. The electrical properties of the QD systems were investigated via capacitance-voltage measurements and capacitance transient spectroscopy (also known as deep-level transient spectroscopy) with selective carrier injection and extraction which can be achieved with very small pulse amplitude under bias variation. For the large QDs, several energy states were found with the use of selective carrier injection and extraction. The thermal-activation energies obtained from the capacitance transient spectra of the large QDs were distributed from 70 to 600 meV. This energy distribution was originated from the quantized states of the individual QDs and the size distribution of the QDs. The spectra of the small QDs showed a well-defined energy state of E(c) - 132 meV. From these results, it was estimated that two to four electrons fill a single QD under the proper measurement bias of 0.2 V pulse.

Effects of Varying MBE Growth Conditions on Layered Zn-Se-Te Structures

We report the effects of varying growth parameters on the photoluminescence (PL) and time-resolved photoluminescence (TRPL) emission from layered Zn-Se:Te structures with submonolayer quantities of ZnTe grown by a combination of migration-enhanced epitaxy (MEE) and molecular beam epitaxy (MBE). Dependences on Te effusion cell temperature (T Te source) and “wait time” (duration for which all shutters are closed, t all-off) are investigated to show that these parameters contribute towards changing the size and/or composition of the quantum dots (QDs). Optical studies showed that increasing T Te source resulted in the formation of larger QDs. Increasing t all-off from 0 s to 3 s contributed towards the formation of larger QDs, while further increasing t all-off to 9 s led to desorption of Te adatoms which resulted in smaller QDs. These parameters are easy to control, and thus understanding their effects should help us to gain better control towards obtaining high-quality QDs.

Annealing of In 0.45Ga 0.55As/GaAs quantum dots overgrown with large monolayer (11 ML) coverage for applications in thermally stable optoelectronic devices

The effects of thermal annealing on the large monolayer (11 ML) coverage of In 0.45Ga 0.55As/GaAs quantum dots (QDs) is being investigated in this study. Low temperature (8 K) photoluminescence (PL) spectra exhibits suppressed blueshift of the strongest PL emission peak even at high temperature annealing (800 °C). TEM and DCXRD characterizations showed the existence of the dots with good crystalline quality at annealing temperatures of 800–850 °C. The physics of annealing induced compositional modification of the InGaAs QDs with various monolayer coverage has been studied by a theoretical model and simulation. All our studies establish the thermal stability of large ML coverage InGaAs QDs, which is desirable for optoelectronic devices required for selective wavelength tuning in specific applications.

Temperature dependence of the photoluminescence spectra from InAs(P)/InP multilayers containing thick quantum dots: Dot-size-dependent carrier dynamics

We have studied the temperature dependence (\ensuremath{\sim}10--293 K) of the photoluminescence (PL) from InAs/InP(001) quantum dot (QD) multilayers with thin spacer layers (\ensuremath{\sim}5 nm) emitting in the 0.6--0.8 eV spectral region. The QD emission redshifts less than the InAs bulk material band gap with increasing temperature. This behavior is accompanied by an important rise of the relative PL intensity of the higher-energy contributions to the spectra. The room-temperature emission is rather strong---over 20% of the low-temperature value---since deep confinement effects prevent the thermal escape of the carriers out of these relatively large QD's. In addition, the carrier transfer from the wetting layers to the QD's increases with the number of layers at low temperatures. A dot-size-dependent analysis of the carrier dynamics using a rate-equation model leads to the following interpretation of our experimental results: (i) the radiative emission intensity from thicker dots quenches at lower temperature through thermalization to excited nearly dark states, and (ii) carriers initially captured by the wetting layer are preferentially transferred to thinner QD's whose emission energy is higher than \ensuremath{\sim}0.7 eV. In multilayers, the experimental observations can be explained without involving electronic coupling between QD's of different layers, even though the distance between the vertically aligned nanostructures is small.

Electronic structures of alloy quantum dots with nonuniform composition

In this study, the significant effect of the nonuniform composition in alloy quantum dots (QDs) on electronic structure is analyzed in depth. The equilibrium composition profiles in experimentally observed dome and barn shaped GeSi/Si QDs are determined by combining the finite element method and the method of moving asymptotes. Due to the composition variation, the total band edge of heavy hole is dominated by the band offset and spin-orbit coupling rather than the strain effect. The numerical results reveal that the wave function of heavy hole trends to be localized in the Ge-rich region at the top of the large QD. Moreover, the size effect gradually compensates the composition effect as the size of QD decreases.

Effect of Eu-implantation and annealing on the GaN quantum dots excitonic recombination

Undoped self-assembled GaN quantum dots (QD) stacked in superlattices (SL) with AlN spacer layers were submitted to thermal annealing treatments. Changes in the balance between the quantum confinement, strain state of the stacked heterostructures and quantum confined Stark effect lead to the observation of GaN QD excitonic recombination above and below the bulk GaN bandgap. In Eu-implanted SL structures, the GaN QD recombination was found to be dependent on the implantation fluence. For samples implanted with high fluence, a broad emission band at 2.7 eV was tentatively assigned to the emission of large blurred GaN QD present in the damage region of the implanted SL. This emission band is absent in the SL structures implanted with lower fluence and hence lower defect level. In both cases, high energy emission bands at approx. 3.9 eV suggest the presence of smaller dots for which the photoluminescence intensity was seen to be constant with increasing temperatures. Despite the fact that different deexcitation processes occur in undoped and Eu-implanted SL structures, the excitation population mechanisms were seen to be sample-independent. Two main absorption bands with maxima at approx. 4.1 and 4.7 to 4.9 eV are responsible for the population of the optically active centres in the SL samples.