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Nature du désordre et propriétés optiques des excitons dans les fils quantiques semiconducteurs : de la boîte au fil

Abstract : The imaging of local spectroscopic properties, by microphotoluminescence, allowed us to understand and analyze the localization properties of V-shape semiconducting quantum wires, and to identify the structural origins of disorder. We evidenced the differences between two generations of quantum wires. In the first one, the roughness of the hetero-interfaces involved in the confinement of the carriers is important and leads to the localization of excitons; the quantum wire can thus be seen as a collection of quantum boxes and is told to be in the "0D regime". In the second one, the hetero-interface fluctuations are much fewer (2 per µm in the best samples) and the excitons are delocalized over a few hundreds of nanometers. Each island can thus be considered as a portion of real quantum wire, and the wires are told to be in the "1D regime". The electronic properties of the quantum wires have been studied by microphotoluminescence experiments on a single quantum box or island, and have been correlated to their structural properties. This "single nano-object" approach gives us access to the intrinsic properties of these objects, which are free of the inhomogeneous broadening of the results usually obtained on a macroscopic population. In the 0D regime quantum wires, the fine structure of the radiative doublet has been evidenced for the excitons localized in quantum boxes, and it has been related to the exchange Coulomb interaction. The temperature evolution of the microluminescence spectra has been interpreted as the setting up of the strong coupling between excitons and acoustical phonons : the zero phonon peak observed at low temperature is progressively replaced by a broader line corresponding to recombinations assisted by emission and absorption of phonons. In the 1D regime quantum wires, we have shown that the excitons delocalized in the islands feel the residual disorder caused essentially by internal piezoelectric fields. The theory of the exciton has been reworked in the case of quantum wire, in which its singularity needs to pay a carefull attention to the resolution of the hydrogen atom 1D hamiltonian. The radiative lifetime of the excitons has been shown to scale as sqrt(T); this proves that the excitons are locally at thermal equilibrium at the bottom of the band, and that their density of state follows, within kT, the 1/sqrt(E) law expected for a one-dimensional system. We finally evidenced the Mott transition from a diluted gas of excitons to a dense electron-hole plasma as the density of photocreated carriers is increased, and we characterized these density regimes. The formation of biexcitons in the diluted gas of excitons has been confirmed.
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Submitted on : Tuesday, September 21, 2004 - 10:02:56 AM
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Thierry Guillet. Nature du désordre et propriétés optiques des excitons dans les fils quantiques semiconducteurs : de la boîte au fil. Matière Condensée [cond-mat]. Université Pierre et Marie Curie - Paris VI, 2002. Français. ⟨tel-00006929⟩

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