Dynamics of trion formation in GaAs quantum wells

We show a double path mechanism for the formation of charged excitons (trions); they are formed through bi- and trimolecular processes. This directly implies that both negatively and positively charged excitons coexist in a quantum well, even in the absence of excess carriers. The model is substantiated by time-resolved photoluminescence experiments performed on a very high quality InxGa1-xAs quantum well sample, in which the photoluminescence contributions at the energy of the trion and exciton and at the band edge can be clearly separated and traced over a broad range of times and densities. The unresolved discrepancy between the theoretical and experimental radiative decay time of the exciton in a doped semiconductor quantum well is explained by the same model.

Dynamics and coherent effects on high density plasmas in semiconductor nanostructures

Lars Kappei

This thesis work contains an experimental study of many-body and cooperative effects in quantum wells. Many-body effects prove important for an understanding of the physical and specifically optical properties of semiconductors. The Coulomb interaction between electrons profoundly modifies the electronic states and the light-matter interaction, leading to a number of density-dependent effects that can not be accounted for in the one-electron picture which is the starting point of most textbooks on the subject. The simplest of these modifications is the exciton, the bound state of one electron and one hole which leads to the emergence of two-particle states energetically below the continuous band electron-hole states. Optical excitation in the band continuum is possibly followed by the formation of excitons upon release of the excess energy to the lattice vibrations. Chapter 5 contains an experimental analysis of the dynamics of this process. A high quality single In0.05Ga0.95As/GaAs quantum well sample allows us to separately study the temporal evolution of the continuum photoluminescence on the one hand and the excitonic photoluminescence on the other. The results indicate a rather fast formation process, leading to the establishment of a species interconversion equilibrium between free pairs and bound pairs at higher density. However, at the lowest excitation density, we found evidence for a non-equilibrium species repartition. At high electron-hole pair density, the two-particle bound exciton state disappears due to the screening of the Coulomb interaction by the surrounding carriers as well as the blocking of the constituting states according to the Pauli exclusion principle. The transition from an excitonic population to a free carrier plasma is referred to as excitonic Mott transition. Chapter 6 reports an experimental investigation of the repercussions of this transition on the optical spectra, performed on the In0.05Ga0.95As/GaAs quantum well of chapter 5. The time-resolved spectra do not show an abrupt change of photoluminescence intensity or spectral form when transforming gradually from an exciton dominated spectrum to free plasma photoluminescence, indicating a rather gradual transition. The separate visibility of continuum and excitonic PL contributions below the transition density reveals the total absence of exciton binding energy variations, indicating a very low exciton ionization degree in contrast to theoretical work on the subject. Another high-density effect arising from the particle interactions is the energetic lowering of the electronic states due to screening and correlation, denoted as band gap renormalization (BGR). Chapter 7 presents measurements of BGR using a single GaAs/Al0.25Ga0.75As quantum well of 60 Å width. We obtain good agreement with calculations of the carrier self energy within the framework of the random phase approximation. The last part of this thesis (chapter 8) reports an experimental test of the possibility of cooperative spontaneous emission, often denoted as superradiance, from a high-density electron-hole plasma. This effect has been extensively studied in optically excited atomic gases, where it leads under certain conditions to the reemission of the stored energy in a single coherent radiation burst instead of the slow exponential decay according to the spontaneous radiation of a single atom. We find no evidence for the occurrence of this effect in our samples, probably due to the fast incoherent polarization decay in the carrier plasma.

EPFL2005

Resonant femtosecond emission from quantum well excitons: The role of Rayleigh scattering and luminescence

Benoît Marie Joseph Deveaud

We study the ultrafast properties of secondary radiation of semiconductor quantum wells under resonant excitation. We show that the exciton density dependence allows one to identify the origin of secondary radiation. At high exciton densities, the emission is due to incoherent luminescence with a rise time determined by exciton-exciton scattering. For low densities, when the distance between excitons is much larger than their diameter, the temporal shape is independent of density and rises quadratically, in excellent agreement with recent theories for resonant Rayleigh scattering.

1997

Tailored-Potential Pyramidal Quantum Dot Heterostructures

Valentina Troncale

Semiconductor quantum dots are usually compared to artificial atoms, because their electronic structure consists of discrete energy levels as for natural atoms. These artificial systems are integrated in solid materials and can be localized with a spatial precision of the order of nanometers. Besides, they conserve their quantum properties even at quite high temperatures (∼ 10 K). These properties make quantum dots one of the most suitable systems for the realization of quantum devices and computers. However, the energy states and the optical properties of quantum dots are much more complicated than for atoms, because a quantum dot is never an isolated potential well. Instead, its electronic structure depends on the crystallin structure of the semiconductor material and on the Coulomb ion-electron and electron-electron correlations. In particular, the presence of several valence bands and their mixing, induced by quantum confinement, gives rise to novel properties which are still not completely understood and exploited in applications. To get a major advance in this field, a full deterministic control of the spatial shape of the quantum confinement is needed, combined with a deeper understanding of the connections between electronic and optical properties. This thesis work has these two main objectives. We realized and experimentally studied different quantum dot systems, in pyramidal hetero-structures grown with MOCVD techniques. These systems allowed the realization of several different geometries for the carrier confining potential, with a precision in the order of nanometers. The optical characterization has been obtained in particular by means of polarization-resolved microphotoluminescence, magneto-photoluminescence, excitation photoluminescence (PLE), and interferometry techniques. For single quantum dots, we have observed and characterized for the first time new excitonic complexes, arising from excited hole states. This allowed a full caracterizatioon of the valence band hole states in our peculiar system. By means of photon correlation measurements, we have also experimentally demonstrated that, even in presence of a large family of exciton states, these quantum dot systems can emit single photons. We have then realized much more complex quantum dot structures, double dot systems (quantum dot molecules) and a completely new system called Dot-in-Dot (DiD). This latter is composed by a small inner dot surrounded by an electrostatic potential well (which can be considered as an outer elongated dot). Such a composite system is characterized by a strong valence band mixing. This state superposition is however very sensitive to small variations of the confining potential. Therefore the degree of valence band mixing can be easily switched by the introduction of a week external field. Since the valence band mixing determines the polarization properties of the emitted light, the DiD changes the polarization properties of its emission spectrum under the action of an external field. In particular, we have experimentally demonstrated this effect for an external static magnetic field, while we have numerically predicted a very similar effect for a static electric field. In the latter case, the polarization switching is a direct consequence of the quantum confined Stark effect induced in the DiD. Hence the DiD appears to be an ideal candidate for realizing emitters of single photons with tunable and controllable polarization.