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
