Realistic heterointerfaces model for excitonic states in growth-interrupted quantum wells

We present a model for the disorder of the heterointerfaces in GaAs quantum wells including long-range components like monolayer island formation induced by the surface diffusion during the epitaxial growth process. Taking into account both interfaces, a disorder potential for the exciton motion in the quantum well plane is derived. The excitonic optical properties are calculated using either a time-propagation of the excitonic polarization with a phenomenological dephasing, or a full exciton eigenstate model including microscopic radiative decay and phonon scattering rates. While the results of the two methods are generally similar, the eigenstate model does predict a distribution of dephasing rates and a somewhat modified spectral response. Comparing the results with measured absorption and resonant Rayleigh scattering in GaAs/AlAs quantum wells subjected to growth interrupts, their specific disorder parameters like correlation lengths and interface flatness are determined. We find that the long-range disorder in the two heterointerfaces is highly correlated, having rather similar average in-plane correlation lengths of about 60 and 90 nm. The distribution of dephasing rates observed in the experiment is in agreement with the results of the eigenstate model. Finally, we simulate highly spatially resolved optical experiments resolving individual exciton states in the deduced interface structure.Comment: To appear in Physical Review

Influence of Coulomb and Phonon Interaction on the Exciton Formation Dynamics in Semiconductor Heterostructures

A microscopic theory is developed to analyze the dynamics of exciton formation out of incoherent carriers in semiconductor heterostructures. The carrier Coulomb and phonon interaction is included consistently. A cluster expansion method is used to systematically truncate the hierarchy problem. By including all correlations up to the four-point (i.e. two-particle) level, the fundamental fermionic substructure of excitons is fully included. The analysis shows that the exciton formation is an intricate process where Coulomb correlations rapidly build up on a picosecond time scale while phonon dynamics leads to true exciton formation on a slow nanosecond time scale.Comment: 18 pages, 7 figure

Spatio-temporal dynamics of quantum-well excitons

We investigate the lateral transport of excitons in ZnSe quantum wells by using time-resolved micro-photoluminescence enhanced by the introduction of a solid immersion lens. The spatial and temporal resolutions are 200 nm and 5 ps, respectively. Strong deviation from classical diffusion is observed up to 400 ps. This feature is attributed to the hot-exciton effects, consistent with previous experiments under cw excitation. The coupled transport-relaxation process of hot excitons is modelled by Monte Carlo simulation. We prove that two basic assumptions typically accepted in photoluminescence investigations on excitonic transport, namely (i) the classical diffusion model as well as (ii) the equivalence between the temporal and spatial evolution of the exciton population and of the measured photoluminescence, are not valid for low-temperature experiments.Comment: 8 pages, 6 figure

Exciton Spin Dynamics in Semiconductor Quantum Wells

In this paper we will review Exciton Spin Dynamics in Semiconductor Quantum Wells. The spin properties of excitons in nanostructures are determined by their fine structure. We will mainly focus in this review on GaAs and InGaAs quantum wells which are model systems.Comment: 55 pages, 27 figure

Ultrafast terahertz probes of transient conducting and insulating phases in an electron–hole gas

Many-body systems in nature exhibit complexity and self-organization arising from seemingly simple laws. The long-range Coulomb interaction between electrical charges generates a plethora of bound states in matter, ranging from the hydrogen atom to complex biochemical structures. Semiconductors form an ideal laboratory for studying many-body interactions of quasi-particles among themselves and with lattice vibrations and light. Oppositely charged electron and hole quasi-particles can coexist in an ionized but correlated plasma, or form bound hydrogen-like pairs called excitons which strongly affect physical properties. The pathways between such states however remain elusive in near-visible optical experiments that detect a subset of excitons with vanishing center-of-mass momenta. In contrast, transitions between internal exciton levels which occur in the far-infrared at terahertz (10 s) frequencies are in dependent of this restriction suggesting their use as a novel pro be of pair dynamics. Here, we employ an ultrafast terahertz probe to directly investigate the dynamical interplay of optically-generated excitons and unbound electron-hole pairs in GaAs quantum wells. Our observations witness an unexpected quasi-instantaneous excitonic enhancement, reveal formation of insulating excitons on a hundred picosecond timescale and manifest conditions under which excitonic populations prevail

Exzitonbildungs- und Relaxationsdynamik nach optischer Anregung an der Halbleiterbandkante

Gegenstand dieser Arbeit ist das detaillierte Studium der Dynamik von korrelierten Elektron-Loch Paaren in der Nähe der Halbleiterbandkante. Es wird eine mikroskopische Dichtematrixtheorie vorgestellt, die die kohärente und inkohärente Dynamik von optisch erzeugten Elektron-Loch Paaren bei bandkantennaher Anregung konsistent beschreibt. In umfangreichen numerischen Untersuchungen wurde der Einfluss verschiedener Streumechanismen auf die Paardynamik und die Auswirkungen von Coulomb Korrelationseffekten auf die einzelnen Wechselwirkungen selber analysiert. Dabei zeigt sich, dass die Coulomb Korrelationen eine sehr interresante und komplizierte Paardynamik in der Nähe der Bandkante induzieren. Dies basiert auf der Tatsache, dass alle hier betrachteten Streuprozesse durch Coulomb Renormierungen von vergleichbarer Stärke sind, und die Absolutwerte der Streuraten und die relativen Gewichte der Mechanismen stark von Anregungsbedingungen und der jeweiligen Temperatur abhängen