O. Stier

Excited states in self‐organized InAs/GaAs quantum dots: Theory and experiment

In photoluminescence spectra of nanometer‐scale pyramidal‐shaped InAs/GaAs quantum dots allowed optical transitions involving excited hole states are revealed in addition to the ground state transition. Detailed theoretical calculations of the electronic structure, including strain, piezoelectric and excitonic effects, agree with the experimental data and lead to unambiguous assignment of the transitions.

Many-particle effects in type II quantum dots

Many-particle effects are investigated in the photoluminescence of type II GaSb/GaAs quantum dots (QDs). With increasing excitation density, i.e., exciton occupation, the photoluminescence shows first a blueshift and then saturates developing a plateau region. The peculiar behavior is attributed to Coulomb charging and state filling of the localized holes to dominate the many-particle regime. A high temperature stability makes the GaSb/GaAs QDs suitable for room-temperature devices.

Maximum modal gain of a self-assembled InAs/GaAs quantum-dot laser

Gain and threshold current of a self-assembled InAs/GaAs quantum-dot (QD) laser are simulated. A small overlap integral of the electron and hole wave functions in pyramidal QDs is shown to be a possible reason for the low single-layer modal gain, which limits lasing via the ground-state transition at short (under a millimeter) cavity lengths.

Effect of excited-state transitions on the threshold characteristics of a quantum dot laser

The general relationship between the gain and spontaneous emission spectra of a quantum dot (QD) laser is shown to hold for an arbitrary number of radiative transitions and an arbitrary QD-size distribution. The effect of microscopic parameters (the degeneracy factor and the overlap integral for a transition) on the gain is discussed. We calculate the threshold current density and lasing wavelength as a function of losses. The conditions for a smooth or step-like change in the lasing wavelength are described. We have simulated the threshold characteristics of a laser based on self-assembled pyramidal InAs QDs in the GaAs matrix and obtained; a small overlap integral for transitions in the QDs and a large spontaneous radiative lifetime. These are shown to be a possible reason for the low single-layer modal gain, which limits lasing via the ground-state transition for short (several hundreds of micrometers) cavity lengths.

Doping dependent ZnCdSe/ZnSe-superlattice disordering

The doping dependent intermixing of ZnCdSe/ZnSe superlattices was studied by secondary ion mass spectroscopy. The chlorine or nitrogen doped and undoped structures were grown by molecular beam epitaxy. Heat treatment was performed in the temperature range of 300 to 550 °C under different conditions, namely Zn or N atmosphere, vacuum and protected by a Si3N4 cap. The diffusion of Cd was found to be Fickian for all kinds of doping. While identical Cd diffusion coefficients were observed for the undoped and the chlorine-doped superlattice, a distinct enhancement by three orders of magnitude was found for nitrogen-doped structures. The p-type conductivity, and not the nitrogen itself, was identified to be responsible for the Cd diffusion enhancement by additional implantation studies.

Recombination kinetics and intersubband relaxation in semiconductor quantum wires

We present a detailed and systematic investigation of carrier capture, relaxation, cooling and radiative recombination in a one-dimensional semiconductor quantum wire of high structural perfection and optical quality over a large range of excitation (carrier) densities. Experimental evidence for a complete lack of 1D bandgap renormalization is found. Even up to high carrier densities, >106 cm-1, where strong band filling is already present and directly visible in the luminescence, no shift of bandgap to low energy is found. The carrier cooling in 1D is appreciably slower than in comparable 2D structures, thus leading to high carrier temperatures. This confirms theoretical predictions of reduced phonon scattering probability in one-dimensional structures. The temperature dependence of the radiative lifetime of the 1D carriers is investigated. The theoretically predicted T dependence is not found. On the contrary an empirical tau rad=0.02 T ns K-1 law is fulfilled.

Shape and Composition Effects on Excitons and Biexcitons in Quantum Dots

Exciton and biexciton states in realistic quantum dots are investigated theoretically. The few-particle interactions are shown to depend sensitively on the structural properties of the dots. The state energies are obtained by a configuration interaction method added on eight-band k . p calculations of single-particle states. By comparing the calculations to empirical pseudopotential ones, the accuracy of the method is established.

Few-Particle Effects in Self-Organized Quantum Dots

Few-particle effects are investigated both experimentally and theoretically for self-organized quantum dots (QDs). The actual confinement potential, reflecting the structure-dependent low-symmetry and inhomogeneous strain, is demonstrated to have a strong impact on the few-particle states. Eight-band k·p calculations for the InAs/GaAs model system indicate both binding and anti-binding biexciton states depending on the separation of the electron and hole wave functions. Single-dot experiments on CdSe/ZnSSe QDs enable the identification of the neutral and charged exciton and biexciton states and demonstrate a large variation of the corresponding binding energies attributed to the variation of the structural properties of the probed QDs. First eight-band k·p model calculations for epitaxial CdSe QDs demonstrate a pronounced influence of the Cd-concentration. Finally, lateral energy transfer processes originating at weakly localizing Cd-fluctuations are identified in time-resolved experiments. The unique properties of three-dimensionally confined excitons in semiconductor quantum dots (QDs) have been studied intensively in recent years, see e.g. [1] and references therein. The last three years saw an increasing interest in the physics of multi-exciton and charged-exciton states in selforganized QDs [2,3,4,5,6,7]. Such multi-particle states are of interest both for basic physics and device applications. Understanding the properties of manyparticle states improves insight into the complex interplay of the Coulomb interaction and the external confinement. At the same time the detailed knowledge of many-particle effects is essential for the development of semiconductor devices operating at high excitation densities like, e. g., lasers. Single-QD spectroscopy has proven to be a powerful tool in the study of few-particle effects, see e.g. [8] and references therein. It allows to investigate emission spectra without the obstructing influence of inhomogeneous broadening resulting from statistical variations of the structural properties. In spite of the enormous progress in the single-QD spectroscopy in recent years, which has been stimulated by defect-free self-organized QDs allowing to investigate the intrinsic optical properties, the interpretation of experimental data as well as theoretical predictions of few-particle states generally neglect the intricate structural properties of real mesoscopic QD structures. In particular interband transitions, involving both electron and hole states, are sensitive to the structural properties of self-organized QDs leading to asymmetric electron and hole wave functions [9]. The related effects of the B. Kramer (Ed.): Adv. in Solid State Phys. 41, 39–50 (2001) c

Line broadening and localization mechanisms in CdSe/ZnSe quantum dots

Abstract We analyze the line-broadening mechanisms of single CdSe quantum dot (QD) emission lines. A jitter in the emission energy of individual CdSe QDs is reported for the first time. The jitter is caused by the quantum-confined Stark effect induced by the randomly fluctuating charges of defects in the vicinity of the QDs. These random processes lead to a broadening of the emission lines and usually inhibit the determination of a true homogeneous line width. On the other hand, identical jitter allows the unambiguous assignment of groups of emission lines to single QDs. A strong thermal broadening of the QD emission lines is observed. From our observations, parameters of the phase relaxation due to acoustic and LO phonon scattering, which is the main line broadening mechanism, are derived.

Exciton Level Crossing in Coupled InAs/GaAs Quantum Dot Pairs

The electronic and optical properties of vertical pairs of capped, structurally identical InAs pyramidal quantum dots with {101} facets in GaAs are investigated theoretically, For distances smaller than 17 ML a strong distance dependent quantum coupling between the two dots is predicted for electrons, leading to a term splitting between the ground and the first excited state, Holes are asymmetrically affected by the strain and the piezoelectric potential which, together, prevent a term splitting like in the electron case. The excited hole states vary in their character, dependent on the spacer thickness. Consequently, exciton absorption spectra significantly depend on the vertical dot distance, regarding the energetic range, order, and shape of peaks, and the polarization anisotropies.