Enhancement of Exciton Emission from Zno Nanocrystalline Films by Pulsed Laser Annealing

Pulsed ArF laser annealing in air and in hydrogen atmosphere improves the optical properties of ZnO nanostructured films. Independently on the ambient atmosphere, laser annealing produces two major effects on the photoluminescence (PL) spectra: first, the efficiency of the exciton PL increases due to decrease of the number of non-radiative recombination centers; second, the intensity of the defect-related orange band decreases because of the removing of excessive oxygen trapped into the films during deposition. However, annealing in the ambient air also increases the intensity of the green band related to oxygen vacancies. We show that the combination of laser annealing and passivation of oxygen vacancies by hydrogen results in films free of defect-related emission and keeps intact their nanostructural character

Optical pumping in semiconductors

Optical Pumping in Semiconductors (OPS) arises from the transfer of angular momentum from light to the localized states of a semiconductor. Spin polarized electrons are thus excited in the conduction band; their polarization is convenient measured through the circular polarization of photoluminescence. This review gives an insight of the various studies based on OPS. After describing the first OPS experiment, we show that this technique allows the determination of band structure properties, and the optical detection of conduction electron spin resonance. The nuclei are polarized by hyperfine interaction, which permits the optical detection of nuclear resonance. A magnetic field transverse to the direction of light propagation produces an electronic depolarization analogous to the Hanle effect. The electron lifetime and spin relaxation time are measured under steady-state conditions by comparison to their Larmor frequency in this transverse field. By activation to Negative Electron Affinity of a GaAs surface, electrons oriented by OPS can be photoemitted into vacuum, leading to a highly spin-polarized beam : we describe a collision experiment in which such a beam transfers angular momentum to atoms

Ablation laser par laser UV impulsionnel : principale approche pour la synthèse controlée des nanostructures de silicium

Nous montrons que l'utilisation d'un laser UV impulsionnel (ArF*, λ = 193 nm) est très bénéfique pour la synthèse de nano-agrégats de Si dans le régime d'ablation. L'éjection ionique de monomères Si* et de petits agrégats Six+ (x = 2-11) augmente fortement le taux de condensation dans la plume induite par laser UV. De plus, l'importante production d'ions non thermiques assure la stabilisation de la température de la surface irradiée permettant de contrôler la taille des nano-agrégats (1-5 nm) à l'aide d'un seul paramètre : la fluence laser. On peut alors changer la position spectrale de la bande de photoluminescence des nano-agrégats de Si du proche IR au proche UV par la sélection de la taille des nano-agrégats. Ces résultats sont en accord avec l'effet du confinement quantique

Theory of the Anomalous Tunnel Hall Effect at Ferromagnet-Semiconductor Junctions

International audienceWe report on theoretical investigations of carrier scattering asymmetry at ferromagnet-semiconductor junctions. By an analytical 2 × 2 spin model, we show that, when Dresselhaus interactions is included in the conduction band of III-V T d symmetry group semiconductors, the electrons may undergo a difference of transmission vs. the sign of their incident parallel wavevector normal to the in-plane magnetization. This asymmetry is universally scaled by a unique function independent of the spin-orbit strength. This particular feature is reproduced by a multiband k • p tunneling transport model. Astonishingly, the asymmetry of transmission persists in the valence band of semiconductors owing to the inner atomic spin-orbit strength and free of asymmetric potentials. We present multiband 14 × 14 and 30 × 30 k • p tunneling models together with tunneling transport perturbation calculations corroborating these results. Those demonstrate that a tunnel spin-current normal to the interface can generate a surface transverse charge current, the so-called Anomalous Tunnel Hall Effect