Etude theorique des proprietes electroniques et optiques des super-reseaux de silicium/bioxyde de silicium (French text)

The observation of intense luminescence in Si/SiO2 superlattices has opened up new vistas in theoretical research, with a view to fabricate Si-based devices suitable for optoelectronic applications. Crystalline silicon has an indirect energy gap that makes this material less competitive compared to other luminescent materials. The fabrication of silicon-based luminescent materials would thus provide multiple applications in the future. Three achievements are presented in this work. (a) Several Si/SiO2 superlattice models have been constructed and studied within a first-principles framework in order to evaluate their structural, electronic and optical properties. The first two models are derived from crystalline phases of silicon and silicon dioxide. From these models, the interfaces are shown to play a significant role on their optical properties. New structurally-relaxed models have thus been constructed in order to satisfy more closely the interface topology and evaluate accurately the confinement effects on their optical properties. (b) Direct bandgaps are obtained in the structurally-relaxed models. Their absorption coefficient has been calculated (by applying the Fermi Golden rule) and compared to that of bulk Si, giving a clear demonstration of the enhanced absorption (and emission) properties of Si confined systems, compared to bulk Si. A blueshift with increased confinement has been confirmed. Furthermore, the precise role of suboxide Si atoms at the interfaces has been investigated. These suboxides are shown to have two main effects: (i) increase slightly the energy gap; (ii) reduce the dispersion in the band structure near the Fermi level. (c) A direct application of the Slater transition state theory applied to crystalline silicon has been performed and compared to X-Ray near-edge absorption spectroscopy measurements. The Slater transition state theory constitutes a subset of the density functional theory for ensembles. Very good agreement between this theory and the experiment is obtained. This type of calculations has been applied to the Si/SiO2 superlattices in order to estimate and characterize the electronic properties of the confined region in the conduction bands.