Low temperature reflectivity study of ZnO/(Zn,Mg)O quantum wells grown on M-plane ZnO substrates

We report growth of high quality ZnO/Zn0.8Mg0.2O quantum well on M-plane oriented ZnO substrates. The optical properties of these quantum wells are studied by using reflectance spectroscopy. The optical spectra reveal strong in-plane optical anisotropies, as predicted by group theory, and marked reflectance structures, as an evidence of good interface morphologies. Signatures ofc onfined excitons built from the spin-orbit split-off valence band, the analog of exciton C in bulk ZnO are detected in normal incidence reflectivity experiments using a photon polarized along the c axis of the wurtzite lattice. Experiments performed in the context of an orthogonal photon polarization, at 90^{\circ}; of this axis, reveal confined states analogs of A and B bulk excitons. Envelope function calculations which include excitonic interaction nicely account for the experimental report

Propriétés excitoniques de puits quantiques ZnO/(Zn,Mg)O

Zinc oxide is a wide gap semiconductor (≈ 3.4 eV) which one of the main advantages is a large exciton binding energy (60 meV). This thesis is dedicated to the study of excitonic properties of ZnO/(Zn,Mg)O single quantum wells (QW). The existence of an internal electric field in polar QW grown along the main axis c is well established for a few years. The effects of this electric field on emission and absorption properties of polar QW grown by molecular beam epitaxy (MBE) on sapphire substrate are discussed. We also demonstrate the direct formation of excitons-phonons complexes in the case of narrow wells by an experience of photoluminescence excitation. Moreover, the presence of this internal electric field combined with the high density of non-radiative defects due to hetero-epitaxial growth leads to a low radiative efficiency. Reflectivity, continuous-wave and time-resolved photoluminescence experiments have been performed on homo-epitaxial QW grown by MBE on non-polar M-plane. First we demonstrate the absence of internal electric field in these QW. We also report the formation of excitons-donors and excitons-excitons complexes. The most important observation is the drastic improvement of radiative efficiency: excitons recombination is essentially radiative up to 325 K. The exciton lifetime increases linearly with temperature, which indicates that excitonic properties are preserved up to room temperature. This outcome is essential for the exploitation of QW based on ZnO in optoelectronics or as a model system.Le ZnO est un semiconducteur à grand gap (≈ 3,4 eV) dont l'un des principaux avantages est une forte énergie de liaison de l'exciton (60 meV). Cette thèse est consacrée à l'étude des propriétés excitoniques des puits quantiques (PQ) ZnO/(Zn,Mg)O. L'existence d'un champ électrique interne dans les PQ polaires crûs selon l'axe principal c est bien établie depuis quelques années. Les propriétés d'émission et d'absorption de puits quantiques polaires élaborés par épitaxie à jets moléculaires (EJM) sur substrat saphir sont discutées. Nous mettons également en évidence par une expérience d'excitation de la photoluminescence la formation directe de complexes excitons-phonons dans le cas de puits étroits. La présence de ce champ électrique interne combinée à la forte densité de défauts non-radiatifs inhérente à l'hétéro-épitaxie conduit à une efficacité radiative médiocre. Des expériences de réflectivité, de photoluminescence continue et résolue en temps ont été effectuées sur des PQ élaborés par EJM sur substrat de ZnO non-polaire (plan M). Ces expériences démontrent l'absence de champ électrique interne, ainsi que la formation de complexes excitons-donneurs et excitons-excitons. L'observation la plus importante est l'amélioration drastique du rendement radiatif : la recombinaison des excitons est essentiellement radiative jusqu'à 325 K. La durée de vie de l'exciton croît linéairement avec la température, ce qui indique que les propriétés excitoniques sont conservées jusqu'à température ambiante. Ce résultat est important en vue de l'exploitation de PQ à base de ZnO en opto-électronique ou comme système modèle

Exciton radiative properties in nonpolar homoepitaxial ZnO/(Zn,Mg)O quantum wells

International audiencePhotoluminescence spectra of nonpolar M-plane ZnO/Zn0.8Mg0.2O quantum wells exhibit strong excitonic peaks from low (10 K) to high (325 K) temperatures. We find that the total integrated intensity remains quasiconstant and that the exciton lifetime increases linearly with the temperature from a value of 750 ps at 100 K until about 2.4 ns at 325 K. This behavior is well described by an original model accounting for the exciton phase space filling. This indicates that radiative recombination of free excitons is dominating the quantum well photoluminescence even at room temperature

Giant Rabi splitting between localized mixed plasmon-exciton states in a two-dimensional array of nanosize metallic disks in an organic semiconductor

International audienceWe study localized surface plasmons in strong coupling with excitons of organic semiconductors. The plasmons are localized in lithographed silver nanodisks organized in square array. A giant Rabi splitting energy of 450 meV is obtained, and typical behaviors of mixed states, i.e., anticrossing of their energies and crossing of their linewidths, are observed. Three-dimensional finite-difference time-domain simulations and coupled oscillator calculations are used to analyze and corroborate the experimental results