Dynamique de spin dans des structures semiconductrices à base de ZnO et de GaN

Ce travail de thèse est une contribution à l'étude de la dynamique de spin des porteurs dans les structures semiconductrices de grande énergie de bande interdite à base de GaN et de ZnO. Nous avons mis en œuvre la technique de pompage optique orienté résolu en temps dans le domaine de l'ultra-violet pour mesurer les temps de relaxation de spin dans ces structures.\ud Les propriétés de spin du trou et de l'exciton ont été analysées dans des couches épitaxiées de ZnO à partir des propriétés de polarisation de la photoluminescence issue des complexes d'excitons piégés sur des donneurs neutres. Nous avons mesuré à la fois le temps de relaxation de spin et le temps de cohérence de spin du trou localisé et avons mis en évidence le temps de relaxation de spin rapide de l'exciton libre. \ud Nous avons également réalisé des études de pompage optique orienté sur des structures de GaN en phase cubique (blende de zinc), du matériau massif aux boîtes quantiques. Dans ces dernières, en analysant l'alignement optique de l'exciton dans des conditions d'excitation quasi-résonante, nous avons démontré le blocage de la relaxation de spin de l'exciton jusqu'à température ambiante.------------------------------------------------------------------------------This thesis work is a contribution to the study of the spin dynamics of carriers in ZnO- or GaN-based wide bandgap semiconductor structures. We use time-resolved optical pumping experiments dedicated to the ultra-violet to measure spin relaxation times in those structures.\ud \ud The spin properties of hole and exciton in epitaxial layers of ZnO have been analysed from the polarization properties of the photoluminescence detected from neutral-donor bound exciton complexes. We measure both the localized hole spin relaxation time and spin decoherence time and have evidenced the fast spin relaxation time of the free exciton. \ud \ud We have also performed optical orientation experiments on cubic (zinc blende) GaN structures, from bulk material to quantum dots. In those, by studying the optical alignment of exciton spin under quasi-resonant excitation, we demonstrate the quenching of the exciton spin relaxation up to room temperature.\ud \u

De-Confinement in high multiplicity proton-proton collisions at LHC energies

Recently, the CMS Collaboration has published identified particle transverse momentum spectra in high multiplicity events at LHC energies $\sqrt s$ = 0.9-13 TeV. In the present work the transverse momentum spectra have been analyzed in the framework of the color fields inside the clusters of overlapping strings, which are produced in high energy hadronic collisions. The non-Abelian nature is reflected in the coherence sum of the color fields which as a consequence gives rise to an enhancement of the transverse momentum and a suppression of the multiplicities relative to the non overlapping strings. The initial temperature and shear viscosity to entropy density ratio $\eta/s$ are obtained. For the higher multiplicity events at $\sqrt s$ =7 and 13 TeV the initial temperature is above the universal hadronization temperature and is consistent with the creation of de-confined matter. In these small systems it can be argued that the thermalization is a consequence of the quantum tunneling through the event horizon introduced by the confining color fields, in analogy to the Hawking-Unruh effect. The small shear viscosity to entropy density ratio $\eta/s$ near the critical temperature suggests that the matter is a strongly coupled Quark Gluon Plasma.Comment: 5 pages, 4 figure

Surfactant-free CZTS nanoparticles as building blocks for low-cost solar cell absorbers

A process route for the fabrication of solvent-redispersible, surfactant-free Cu2ZnSnS4 (CZTS) nanoparticles has been designed with the objective to have the benefit of a simple sulfide source which advantageously acts as (i) a complexing agent inhibiting crystallite growth, (ii) a surface additive providing redispersion in low ionic strength polar solvents and (iii) a transient ligand easily replaced by an carbon-free surface additive. This multifunctional use of the sulfide source has been achieved through a fine tuning of((Cu2+)a(Zn2+)b(Sn4+)c(Tu)d(OH?)e)t+, Tu = thiourea) oligomers, leading after temperature polycondensation and S2- exchange to highly concentrated (c > 100 g l-1), stable, ethanolic CZTS dispersions. The good electronic properties and low-defect concentration of the sintered, crack-free CZTSe films resulting from these building blocks was shown by photoluminescence investigation, making these building blocks interesting for low-cost, high-performance CZTSe solar cells

Surfactant-free CZTS nanoparticles as building blocks for low-cost solar cell absorbers

A process route for the fabrication of solvent-redispersible, surfactant-free Cu2ZnSnS4 (CZTS) nanoparticles has been designed with the objective to have the benefit of a simple sulfide source which advantageously acts as (i) a complexing agent inhibiting crystallite growth, (ii) a surface additive providing redispersion in low ionic strength polar solvents and (iii) a transient ligand easily replaced by an carbon-free surface additive. This multifunctional use of the sulfide source has been achieved through a fine tuning of((Cu2+)a(Zn2+)b(Sn4+)c(Tu)d(OH?)e)t+, Tu = thiourea) oligomers, leading after temperature polycondensation and S2- exchange to highly concentrated (c > 100 g l-1), stable, ethanolic CZTS dispersions. The good electronic properties and low-defect concentration of the sintered, crack-free CZTSe films resulting from these building blocks was shown by photoluminescence investigation, making these building blocks interesting for low-cost, high-performance CZTSe solar cells

Interlayer exciton mediated second harmonic generation in bilayer MoS2

Second harmonic generation (SHG) is a non-linear optical process, where two photons coherently combine into one photon of twice their energy. Efficient SHG occurs for crystals with broken inversion symmetry, such as transition metal dichalcogenide monolayers. Here we show tuning of non-linear optical processes in an inversion symmetric crystal. This tunability is based on the unique properties of bilayer MoS2, that shows strong optical oscillator strength for the intra- but also inter-layer exciton resonances. As we tune the SHG signal onto these resonances by varying the laser energy, the SHG amplitude is enhanced by several orders of magnitude. In the resonant case the bilayer SHG signal reaches amplitudes comparable to the off-resonant signal from a monolayer. In applied electric fields the interlayer exciton energies can be tuned due to their in-built electric dipole via the Stark effect. As a result the interlayer exciton degeneracy is lifted and the bilayer SHG response is further enhanced by an additional two orders of magnitude, well reproduced by our model calculations.Comment: main paper and supplemen

Kapitza-resistance-like exciton dynamics in atomically flat MoSe2_{2}-WSe2_{2} lateral heterojunction

Being able to control the neutral excitonic flux is a mandatory step for the development of future room-temperature two-dimensional excitonic devices. Semiconducting Monolayer Transition Metal Dichalcogenides (TMD-ML) with extremely robust and mobile excitons are highly attractive in this regard. However, generating an efficient and controlled exciton transport over long distances is a very challenging task. Here we demonstrate that an atomically sharp TMD-ML lateral heterostructure (MoSe$_{2}$-WSe$_{2}$) transforms the isotropic exciton diffusion into a unidirectional excitonic flow through the junction. Using tip-enhanced photoluminescence spectroscopy (TEPL) and a modified exciton transfer model, we show a discontinuity of the exciton density distribution on each side of the interface. We introduce the concept of exciton Kapitza resistance, by analogy with the interfacial thermal resistance referred to as Kapitza resistance. By comparing different heterostructures with or without top hexagonal boron nitride (hBN) layer, we deduce that the transport properties can be controlled, over distances far greater than the junction width, by the exciton density through near-field engineering and/or laser power density. This work provides a new approach for controlling the neutral exciton flow, which is key toward the conception of excitonic devices

A gas-templating strategy to synthesize CZTS nanocrystals for environment-friendly solar inks

A high-temperature gas-templating strategy is proposed to synthesize Cu2ZnSnS4 (CZTS) nanocrystals for all-aqueous solar inks. Our gas templating process route involves the in-situ generation and stabilization of nanosized gas bubbles into a molten KSCN-based reaction mixture at 400 °C. Chemical insights of the templating gas process are provided such as the simultaneous formation of gas bubbles and CZTS nuclei highlighting the crucial role of the nucleation stage on the sponge and resulting nanocrystals properties. The high porosity displayed by the resulting CZTS nanocrystals facilitates their further post-fragmentation, yielding individualized nanocrystals. The advantages of our high temperature gas templating route are illustrated by the following: (i) the low defect concentration displayed by the highly crystalline nanocrystals, (ii) the synthesis of CZTS nanocrystals displaying S2− polar surfaces after ligand exchange. The good photoluminescence properties recorded on the pure CZTS nanocrystals reveal potential for exploration of new complex chalcogenide nanocrystals useful for various applications including photovoltaics and water splitting. Here we demonstrate that using these building blocks, a CZTS solar cell can be successfully fabricated from an environment-friendly all-aqueous ink

Exciton spectroscopy and unidirectional transport in MoSe2-WSe2 lateral heterostructures encapsulated in hexagonal boron nitride

Chemical vapor deposition (CVD) allows lateral edge epitaxy of transition metal dichalcogenide heterostructures. Critical for carrier and exciton transport is the material quality and the nature of the lateral heterojunction. Important details of the optical properties were inaccessible in as-grown heterostructure samples due to large inhomogeneous broadening of the optical transitions. Here we perform optical spectroscopy of CVD grown MoSe$_2$-WSe$_2$ lateral heterostructures, encapsulated in hBN. Photoluminescence (PL), reflectance contrast and Raman spectroscopy reveal optical transition linewidths similar to high quality exfoliated monolayers, while PL imaging experiments uncover the effective excitonic diffusion length of both materials. The typical extent of the covalently bonded MoSe$_2$-WSe$_2$ heterojunctions is 3 nm measured by scanning transmission electron microscopy (STEM). Tip-enhanced, sub-wavelength optical spectroscopy mapping shows the high quality of the heterojunction which acts as an excitonic diode resulting in unidirectional exciton transfer from WSe$_2$ to MoSe$_2$

X-ray Absorption Near-Edge Structure calculations with pseudopotentials. Application to K-edge in diamond and alpha-quartz

We present a reciprocal-space pseudopotential scheme for calculating X-ray absorption near-edge structure (XANES) spectra. The scheme incorporates a recursive method to compute absorption cross section as a continued fraction. The continued fraction formulation of absorption is advantageous in that it permits the treatment of core-hole interaction through large supercells (hundreds of atoms). The method is compared with recently developed Bethe-Salpeter approach. The method is applied to the carbon K-edge in diamond and to the silicon and oxygen K-edges in alpha-quartz for which polarized XANES spectra were measured. Core-hole effects are investigated by varying the size of the supercell, thus leading to information similar to that obtained from cluster size analysis usually performed within multiple scattering calculations.Comment: 11 pages, 4 figure