Département de physique

Exciton dynamics in semiconductor confined systems

Piermarocchi, Carlo ; Quattropani, Antonio (Dir.)

Thèse Ecole polytechnique fédérale de Lausanne EPFL : 1998 ; no 1908.

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    Summary
    The purpose of my thesis is to provide a theoretical analysis of the dynamics of optically excited carriers in semiconductor confined systems. In particular, I will focus the investigations on the effects due to the presence of a strong electron-hole Coulomb correlation. The Coulomb interaction leads to the formation of hydrogen-like bound states called excitons. The dynamics of these states is investigated in bare and cavity embedded quantum wells, and in quantum wires. I have investigated the dynamics of the relaxation of excitons in quantum wells due to the interaction with acoustic phonons and I have reproduced the temporal evolution of the photoluminescence emission. I have explained why the decay times observed in non resonant photoluminescence experiments are much slower than the radiative recombination time of a single exciton. Furthermore, deviation from the thermal equilibrium gives a characteristic dependence of these decay times on the temperature. The build-up of the photoluminescence is related to the relaxation by phonon emission of the excited electron-hole pairs. Initially, the pairs created at high energy are not bound, and the formation of bound excitons occurs during the relaxation. I have described the exciton formation due to the emission of acoustic and optical phonons, and I have calculated the characteristic times for this process in GaAs quantum wells. The formation can be geminate or bimolecular. In the geminate formation, the exciton is directly created by the photon of the external pump by simultaneous emission of an optical phonon, while in the bimolecular formation the exciton is created from thermalized electron-hole pairs. In the first case the formation occurs only during the laser pump, while in the second case the formation depends on the total density of carriers available in the crystal. The effects of the phonon assisted formation on the overall dynamics of free carriers, excitons and of the photoluminescence are discussed. Excitons confined in a quantum well coupled with the photons modes of a semiconductor microcavity gives mixed exciton-photon states called microcavity polaritons. The dynamics of the population of polaritons, which present an energy dispersion with an upper and lower branch, shows peculiar characteristics. In particular, a bottleneck region above the minimum of the lower polariton dispersion exists. In this region the population of polaritons is accumulated, and strong deviations from the thermal equilibrium are thus produced. Moreover, for the lower polariton states, a suppression of the scattering by acoustic phonons produces a strong inhibition of the thermal broadening. Finally, the dynamics of the photoluminescence spectra in semiconductor quantum wires is investigated. The time resolved photoluminescence experiments, the radiative recombination modifies adiabatically the total density, and the hypothesis quasi-thermal equilibrium can be applied. The optical spectra probes a system which ranges from a regime of ionized electron hole plasma to a gas of weakly interacting bound excitons. In order to describe such a complex system, I have used a Green's functions technique, and I have modeled the Coulomb interaction using a contact potential. This approach allows to observe in the absorption spectra a gain region close to the excitonic resonance. Moreover, the effects of the electron-hole correlations, calculated in a numerical self consistent way, explain the absence of the shift of the photoluminescence emission due to Coulomb induced band gap renormalization.