Shell-like formation of self-organized InAs∕GaAs quantum dots (original) (raw)

Quantum size effect in self-organized InAs/GaAs quantum dots

Physical Review B, 2000

The quantum size effect of exciton transitions is investigated experimentally and theoretically for selforganized InAs/GaAs quantum dots ͑QD's͒. Photoluminescence excitation ͑PLE͒ experiments are reported for a series of samples with QD's varying in average size, revealing size-dependent excitation resonances. Temperature-dependent measurements show that the PLE spectra mirror the absorption spectra of QD's with a certain ground state transition energy. The observed PLE resonances are identified based on their energy, relative intensity, and sensitivity to size variations in comparison to results of eight-band k•p calculations for pyramidal InAs/GaAs QD's with ͕101͖ side facets. Band mixing, strain, and the particular geometry of the three-dimensional confinement lead to a rich fine structure with a variety of ''forbidden'' excitonic transitions. A good agreement between experiment and theory is found for large QD's (E det Շ1.1 eV), whereas the agreement becomes worse for smaller QD's. The discrepancies arise, most likely, from the uncertainties in the size-and growth-dependent variations of the QD shape and composition as well as Coulomb-induced localized wetting layer states.

Ripening of self-organized InAs quantum dots

Physica E: Low-dimensional Systems and Nanostructures, 2004

The temporal evolution of the size and the shape of self-organized InAs/GaAs quantum dots (QDs) grown using MOCVD is investigated. During a growth interruption after the deposition of the QD material a ripening process is observed, where some QDs grow at the expense of other QDs. A multimodal distribution of the QD ground-state transition energies is observed and attributed to QDs di ering in height by entire numbers of atomic monolayers. This distribution is used to track the evolution of the QD ensemble during the growth interruption more detailed. A shape transition from very at, truncated-pyramid-like QDs to higher, more pyramidal QDs is suggested. An additional antimony ux at the end of the growth interruption leads to an accelerated ripening resulting in a signiÿcant red shift of the QD luminescence, which is explained by the surfactant properties of antimony on InAs.

Control of structural and excitonic properties of self-organized InAs/GaAs quantum dots

Physica E: Low-dimensional Systems and Nanostructures, 2006

A systematic dependence of excitonic properties on the size of self-organized InAs/GaAs quantum dots is presented. The bright exciton fine-structure splitting changes from negative values to more than 0.5 meV, and the biexciton binding energy varies from antibinding to binding, as the height of truncated pyramidal dots increases from 2 to above 9 InAs monolayers. A novel mode of metalorganic vapor phase epitaxy was developed for growing such quantum dots with precise shape control. The dots consist of pure InAs and feature heights varying in steps of complete InAs monolayers. Such dot ensembles evolve from a strained, rough twodimensional layer with a thickness close to the critical value for the onset of the 2D-3D transition. Dots with a common height represent subensembles with small inhomogeneous broadening. Tuning of subensemble emission energy is achieved by varying the mean lateral extension of the respective QDs. Detailed knowledge of the structural properties of individual dots enable realistic k. p calculations to analyze the origin of the observed excitonic properties. The binding energies of charged and neutral excitons increase due to correlation by the gradually increasing number of bound states for increasing dot size. The monotonously increasing magnitude of the fine-structure splitting with dot size is shown to be caused by piezoelectricity. The identification of key parameters allows to tailor exciton properties, providing a major step towards the development of novel applications.

Electronic Properties of Self-Organized Quantum Dots

2007

Contents Part 1. Electronic Structure Calculations Chapter 1. Introduction Chapter 2. Method of calculation 2.1. Calculation of strain 2.2. Piezoelectricity and the reduction of lateral symmetry 2.3. Single Particle States 2.4. Many-Particle States 2.5. Optical Properties Part 2. InGaAs/GaAs Quantum Dots Chapter 3. Impact of Size, Shape and Composition on Piezoelectric Effects and Single-Particle States 3.1. The Investigated structures: Variation of size, shape and compostion 3.2. The Impact of the piezoelectric field 3.3. The vertical and lateral aspect ratio 3.4. Varying composition profiles 3.5. Conclusions Chapter 4. Few-particle Energies versus Geometry and Composition 4.1. Interrelation of QD-structure, strain and piezoelectricity, and Coulomb interaction 4.2. The Impact of QD size (series A and H) 4.3. The aspect ratio 4.4. Different composition profiles 4.5. Correlation vs. QD size, shape and particle type 4.6. Conclusions Chapter 5. Multimodal QD-size distribution: Theory and Experiment 5.1. Sample growth 5.2. Determination of QD-morphology and the spectrum of excited states 5.3. Predicted absorption spectra of truncated pyramidal InAs/GaAs QDs 5.4. Single-dot spectra obtained from cathodoluminescence spectroscopy 5.5. Results and Interpretation 5.6. Conclusion Chapter 6. Stacked quantum dots 6.1. Energetics of QD stacks 6.2. Role of strain and piezoelectricity 6.3. Strength of electronic coupling in pairs of identical QDs 6.4. Small perturbations of the size homogeneity 6.5. Asymmetric QD molecules: Coupling of different electronic shells 6.6. Tailoring the TE-TM ratio in semiconductor optical amplifiers 6.7. Conclusions 6 CONTENTS Part 3. Other Material Systems Chapter 7. Electronic and optical properties of InAs/InP quantum dots on InP(100) and InP(311)B substrates 7.1. Choice of model QDs 7.2. Absorption spectra for InAs/InP QDs 7.3. Impact of substrate orientation on the QDs optical properties 7.4. Conclusions Chapter 8. Inverted GaAs/Al x Ga 1−x As Quantum Dots 8.1. Choice of the model QDs 8.2. Influence of interface intermixing on the optical properties 8.3. External magnetic fields 8.4. Discussion 8.

Nature of optical transitions in self-organized InAs/GaAs quantum dots

Physical Review B, 1996

Electronic transitions in nm-scale pyramid-shaped InAs/GaAs quantum dots feature only ground-state electrons. Allowed optical transitions involving excited hole states in addition to the ground-state transition are revealed in absorption and photoluminescence spectra. The experimental data agree with detailed theoretical calculations of the electronic structure, including strain, piezoelectric, and excitonic effects, and lead to unambiguous assignment of the transitions. We find as upper bound for the relative standard deviation of the size fluctuation р0.04. The hole sublevel separation is consistent with a pyramid shape fluctuation between ͕101͖ and ͕203͖ side facets.

Excited states and selection rules in self-assembled InAs/GaAs quantum dots

Physical Review B, 1999

High-pumping-intensity photoluminescence ͑PL͒ studies of InAs self-assembled quantum dots ͑SAQD͒ have been performed under high pressure P up to 70 kbar. The origin of the higher-energy PL lines that appear in the spectra with increasing pumping intensity is determined by using the ⌫-X crossover effect in the conduction band. With increasing P, these lines are sequentially quenched at particular values of P. From this we unambiguously conclude that the lines correspond to the transitions from different excited conduction electron states in the SAQD. This indicates the existence of strong selection rules for the electron-hole recombination in the SAQD. ͓S0163-1829͑99͒51428-8͔ RAPID COMMUNICATIONS

The vertical coupling effect on the electronic states in self-assembled InAs/GaAs quantum dots

Solar Energy Materials and Solar Cells, 2006

A series of self-organized InAs/GaAs quantum dots (QDs) were grown by molecular beam epitaxy to investigate the dependence of transition energy on GaAs spacer layer thickness. The latter was varied of 60, 45, 30, 15, and 10 monolayers (MLs) for the five different samples. The photoluminescence (PL) measurements were carried out. The electronic states in coupled selfassembled InAs/GaAs QDs are investigated through PL properties with the aid of the theoretical calculation. First the energy levels of electrons and holes are calculated by solving the threedimensional Schro¨dinger equation by considering the vertical coupling effect between a finite numbers of QDs. Based on the results the energies transitions between electrons and holes levels are calculated. Modification of PL spectra by increasing number of layers was found and attributed to an increasing vertical coupling. The PL full-width at half-maximum (FWHM), reflecting the size distribution of the QDs, was found to reach a minimum for an inter-dots GaAs spacer layer thickness of 30 MLs. Moreover, the observed behavior PL lines is analyzed theoretically. r

Absorption and Emission Spectroscopy of Self-Assembled Inas/Gaas Quantum Dots

Absorption and emission processes are studied in self-organised InAs/GaAs quantum dots using the spectroscopic techniques of photocurrent (PC) and electroluminescence (EL). Two types of dots are studied, grown using different deposition rates. Clear evidence for state filling and saturation is observed in EL spectra recorded as a function of injection current. The dependence of the transition intensities on current is found to be well described by a recently proposed model. Magneto-optical measurements in fields up to 14T provide information on the nature of the optical transitions, the spatial extent of wavefunctions and allow the determination of effective masses.

Development of continuum states in photoluminescence of self-assembled InGaAs∕GaAs quantum dots

Journal of Applied Physics, 2007

Crossed transitions between the wetting layer valence band and the quantum dot ͑QD͒ electron states are revealed in the photoluminescence from self-assembled In 0.4 Ga 0.6 As/ GaAs QDs. The strength of these transitions becomes comparable with the excitonic transitions for below-GaAs barrier excitation and decreases significantly with below wetting layer excitation. The observed peculiar QD photoluminescence dependences on temperature and excitation density are due partly to interdot carrier transfer through the continuum states related to the wetting layer morphology and to phonon-assisted processes.

Si doping effect on self-organized InAs/GaAs quantum dots

Journal of Crystal Growth, 1999

In situ ultra high vacuum scanning probe microscopy (SPM) and low-temperature photoluminescence (PL) studies have been performed on Si-doped self-organized InAs/GaAs quantum dots samples to investigate the Si doping effects. Remarkably, when Si is doped in the sample, according to the SPM images, more small dots are formed when compared with images from undoped samples. On the PL spectra, high-energy band tail which correspond to the small dots appear, with increasing doping concentration, the integral intensity of the high-energy band tail account for the whole peak increase too. We relate this phenomenon to a model that takes the Si atom as the nucleation center for QDs formation.