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Theses

Influence of Ultrafast Carrier Dynamics on Semiconductor Absorption Spectra

Abstract : In this thesis, we present a theoretical description of the many-body effects in semiconductor quantum wells and the role they play in the absorption phenomenon. The optical properties of semiconductors and their connections to the thermodynamic properties of the quasi-2D electron-hole plasma are studied in both nonequilibrium and equilibrium regimes. This work was motivated by a series of pump and probe experiments with spin-selective excitation that were performed at Heriot-Watt University. The interpretation of the experimental results is non-trivial given the substantial influence of Coulomb and many-body effects which give rise to a rich variety of broadening and energy renormalizations, and hence places heavy demands on the modeling.

We constructed a simple model to describe the nonequilibrium thermodynamics of the hot electron/hole plasma. We accounted for various dynamical processes such as: relaxation of the carrier distributions, thermalization, plasma cooling, carrier spin-flip, recombination (radiative and nonradiative) and light hole scattering into heavy holes. A full microscopic treatment of the many-body problem being computationally prohibitive, we focused on a phenomenological approach using rate equations and the associated characteristic times for each of the dynamical process included in our analysis. We computed the time dependent energy renormalization and bleaching of the excitonic resonances by solving the semiconductor Bloch equations (screened Hartree-Fock approximation) together with the set of coupled rate equations. We obtained a good qualitative agreement with experiments and further insight into the interplay between the various dynamical processes by varying the phenomenological parameters entering the rate equations. In particular we found that the electron spin-flip occurs on a relatively long time scale (several tens of picoseconds) compared to the carrier distributions relaxation and thermalization (a picosecond or less). We also could monitor the time evolution of the plasma density, energy and temperature. Calculations of the time dependence of the plasma screening were done using the long wavelength and the static plasmon-pole approximations.

In this thesis, we also studied the photoluminescence (PL) in II-VI quantum wells at room temperature. We built a mathematical model to account for Coulomb correlations that are expected to strongly influence the spontaneous emission rate in these materials. We assumed the 1s exciton-free electron scattering to be the main process leading an exciton to the photon line before recombination. The excitonic wavefunctions in a 2D screened Coulomb potential were calculated using the variable phase method. The scattering matrix elements computed, we used Fermi's golden rule to evaluate the contribution of the 1s exciton-free electron scattering to the spontaneous emission rate. PL spectra at room temperature were calculated for various densities, for wide-gap ZnSe and mid-gap GaAs to compare between II-VI and III-V materials. The ZnSe PL spectra exhibit an exciton resonance below the band edge that is not observed in the case of GaAs.
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Contributor : Henni Ouerdane <>
Submitted on : Wednesday, October 1, 2003 - 10:03:13 PM
Last modification on : Monday, October 19, 2020 - 10:53:26 AM
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  • HAL Id : tel-00001905, version 2

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Henni Ouerdane. Influence of Ultrafast Carrier Dynamics on Semiconductor Absorption Spectra. Atomic Physics [physics.atom-ph]. Heriot Watt University, Edinburgh, 2002. English. ⟨tel-00001905v2⟩

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