Phonon-assisted exciton formation in ZnO/(Zn, Mg)O single quantum wells grown on C-plane oriented substrates.

Les interactions entre les électrons et les photons sont au cœur des recherches liées à l'information quantique, nées en particulier d'échanges croissants entre deux domaines de la physique : l'opto-électronique, qui s'intéresse aux dispositifs d'émission, de détection et de conversion de lumière à base de matériaux semiconducteurs, et l'optique quantique, historiquement centrée sur l'étude des interactions entre atome et rayonnement électromagnétique. Dans ce mémoire, nous explorons les configurations originales de couplage entre les excitations électroniques (les excitons) et les modes photoniques façonnés au sein de nanostructures semiconductrices. Ces travaux ont été développés sur les matériaux semiconducteurs " à grand gap ", GaN et ZnO, dont les excitons sont robustes jusqu'à température ambiante et les propriétés quantiques d'interaction avec la lumière sont les plus fortes parmi les matériaux semiconducteurs. Le premier chapitre est consacrée à l'étude de l'exciton confiné dans une boîte quantique unique GaN. Dans le second chapitre, nous discuterons l'insertion de telles boîtes quantiques dans des cavités photoniques destinées à réaliser des nanolasers à boîtes quantiques. Enfin, dans le troisième chapitre consacré aux microcavités planaires à base de ZnO, nous montrons comment les polaritons, particules hybrides exciton-photon, forment à forte densité des condensats de Bose (des états macroscopiques cohérents) appelés lasers à polaritons, jusqu'à température ambiante. Les perspectives de développement d'optique quantique en cavité dans ces nouveaux systèmes sont enfin présentées.

Time-resolved photuminescence spectroscopy investigations of nonpolar homoepitaxial ZnO/(Zn,Mg)O quantum wells.

Semiconductor-based microcavities appear as a prolific system for studying light-matter interaction between a spatially-confined photonic mode and an excitonic resonance. The quasiparticles arising from this coupling (microcavity-polaritons) have enabled in the last years the observation of new lasing regimes as well as polariton Bose-Einstein condensates, vortices and lately solitons. In this panorama ZnO appears as an alternative material to more mature ones, such as GaAs or CdTe, with larger oscillator strengths and enhanced exciton stability. These two properties render ZnO very interesting for studies and applications where large particle densities and/or high temperatures are required. However, the fabrication of ZnO-based microcavities is still challenging and it often requires the use of either nitrides or dielectric materials for the DBRs. Indeed, polariton lasing was demonstrated for the first time in a ZnO-based microcavity only in 2011 [1]. In this work we report on the optical study of a fully-hybrid ZnO-based microcavity in which we combine a high quality active region made up of bulk ZnO and a high cavity quality factor, thanks to the use of two dielectric DBRs. With the cavity Q-factor measured to be more than 1500, polariton lasing is clearly observed from low to room temperature, characterized by a strong linewidth reduction, a small blueshift compared to the Rabi splitting, and an increased emission intensity (three orders of magnitude increase). Furthermore, the wedged-shape of the ZnO active region allows accessing a large range of detunings between the exciton and cavity modes. Under these conditions, the polariton lasing regime has been systematically studied as a function of temperature and detuning, from low to room-temperature. The detailed phase diagram demonstrates the important role played by LO-phonons in the dynamics of the polariton relaxation in ZnO [2], evidenced by a local threshold minimum as a function of detuning. The different relaxation regimes, i.e. kinetic Vs thermodynamic, are further investigated by analyzing a thick cavity region where several lower polartion branches (LPBs), with very different excitonic/photonic fractions are observed. Condensation is observed to take place on the optimum branch as determined by the actual detuning and excitation power. These observations are promising for realizing future multi-mode tunable lasers and room temperature optical switches. References [1] T. Guillet et al., Polariton lasing in a hybrid bulk ZnO microcavity, Appl. Phys. Lett. 99 161104, 2011. [2] L. Orosz et al., LO-phonon-assited polariton lasing in a ZnO-based microcaivty, Phys. Rev. B, 85, 121201(R), 2012.

The effect of thermal annealing on In0.25Ga0.75N/GaN quantum wells grown by molecular beam epitaxy at 550°C is investigated. A strong increase in the 300 K photoluminescence (PL) intensity is observed for samples annealed at 880°C, while degradation occurs for higher temperatures. The improvement of the optical properties is ascribed to higher internal quantum efficiency (IQE), as indicated by temperature-dependent and time-resolved PL experiments. The effect of carrier localization due to possible quantum dot formation via indium clustering is ruled out based on high-resolution transmission electron microscopy imaging. IQE improvement is thus attributed to a reduction of point defects upon annealing.

ZnO is a wide bandgap semiconductor with strong excitonic properties, in particular a large oscillator strength and a large exciton binding energy. It therefore raises a strong interest for the demonstration of polariton lasing up to room temperature with microcavities in the strong exciton-photon coupling regime. The strong coupling regime has recently been demonstrated for planar ZnO microcavities fabricated by various approaches. In this work, we report the improvement of the quality factor of ZnO microcavities, and the corresponding evolution of lasing mechanisms. For a quality factor Q=450, we have shown that the cavity switches from an exciton laser at large negative detuning, to a polariton laser at smaller negative detuning. The exciton laser appears while still observing polariton branches and, thus, with stable excitons, and is observed below 240K. It emphasizes the role of exciton scattering processes, typical of II-VI semiconductors, that are involved in the gain process [Guillet et al., APL 98, 211105 (2011)]. At smaller negative detuning, and at T=120 K, the cavity persists in the strong coupling regime at threshold with negligible blue shift. Angle-resolved µPL experiments demonstrate the spectral narrowing at threshold, as well as a competition between multiple lasing polariton modes. The far-from-equilibrium behavior of the system is typical of a polariton laser in a cavity presenting photonic disorder [Guillet et al., APL 99, 161104 (2011)]. In a more recent cavity with a quality factor larger than 2000 and a Rabi splitting of 200 meV, polariton lasing is obtained systematically, over a much wider range of exciton-photon detuning and of temperature up to 300K. The large variation of the cavity thickness allows to analyse polariton lasing processes in a monomode (one polariton mode) and in a multi-mode regime (two and more competing polariton modes). The complete phase diagram of the ZnO polariton laser is drawn, showing that the lasing threshold is only 6 times larger at 300 K than at 8 K. Strongly excitonic (92% exciton fraction) as well as strongly photonic condensates (88% photon fraction) are realized, that pave the way to promising advances of the polariton physics in ZnO microcavities.