Transport of indirect excitons in ZnO quantum wells

We report on spatially- and time-resolved emission measurements and observation of transport of indirect excitons in ZnO/MgZnO wide single quantum wells

Indirect excitons in wide bandgap semiconductor quantum wells

Cette thèse est consacrée à l'étude expérimentale des excitons dans des puitsquantiques polaires fabriqués à partir de semi-conducteurs à large bande interdite. En raison de la structure de ces matériaux à cristaux wurtzite, les électrons et les trous sont séparés le long de l'axe de croissance du puits quantique, de sorte que les excitons peuvent être considérés comme des excitons indirects (IX) : ils forment une famille de quasi-particules bosoniques à longue durée de vie, dont le moment dipolaire est orienté selon l'axe de croissance du puits. Les IX sont considérés comme un système modéle pour l'étude des états collectifs dans les gaz quantiques bosoniques. Ils sont aussi prometteurs pour le développement de dispositifs excitoniques. Leur longue durée de vie, leur répulsion dipolaire, permettent aux IXs de se déplacer sur de grandes distances avant de se recombiner, ce qui offre la possibilité d'étudier le transport d'exciton par imagerie optique. Dans cette thèse, nous abordons le transport des IXs dans des puits quantiques de GaN/(Al,Ga)N et de ZnO/(Mg,Zn)O. Ce choix de matériau est motivé par l'énergie de liaison élevée des IXs ainsi obtenue. Elle est suffisamment élevée pour, en thèorie, stabiliser les IXs jusqu'à la température ambiante. Mais ce choix poseaussi un certain nombre de défis expérimentaux, car (i) le temps de vie radiatifdépend fortement de la densité d'excitons, ce qui rend la mesure de la densitéexcitonique très complexe ; (ii) la recombinaison non radiative activée thermiquement supprime le signal de photoluminescence excitonique à température ambiante ; (iii) la propagation excitonique coexiste avec une propagation photonique le long du plan du puit quantique, ce qui complique l'analyse ; (iv) il existe un fort champ électrique le long de l'axe de croissance, et aussi desuctuations dans l'épaisseur du puits quantique, ce qui crée un fort élargissement inhomogène de l'émission excitonique. Nous avons abordé toutes ces questions et nous démontrons dans ce travail que les excitons se propagent effectivement dans le plan du puits quantique. Nous arrivons à cette conclusion en combinant des expériences de micro-photoluminescence en régime continu avec des mesures de spectroscopie résolues en temps, et en comparant nos données expérimentales avec divers modèles numériques basés sur les équations dedérive et de diffusion. Dans du matériau de qualité, des puits GaN/(Al,Ga)N obtenus sur substrats GaN, nous avons observé une propagation à temprature ambiante sur plus de 10 µm, et sur plus de 20 µm à 4 K. Nos résultats suggérent que la propagation des excitons sous excitation à onde continue est facilitée par l'écrantage du désordre par les excitons. Néanmoins, la propagation excitonique est encore limitée par la diffusion des excitons sur les défautsiii plutôt que par la diffusion exciton-exciton. Ainsi, l'amélioration de la qualité des interfaces du puits quantique pourrait encore permettre une propagation excitonique sur de plus grandes distances.This thesis is devoted to experimental study of excitons in polar quantum wells(QWs) based on wide band-gap semiconductors. Due to wurtzite crystal structureof these materials, electron and hole are separated in the QW growth axis, sothat excitons can be considered as indirect excitons (IX), a family of long-living bosonic quasi-particles with dipole moment oriented along the QW growth axis. IX are considered as a model system for studies of collective states in quantum gases of bosons, and are also promising for the development of excitonic circuit devices. Long lifetimes and dipole repulsion allow IXs to travel over large distances before recombination providing the opportunity to study exciton transport by optical imaging. In this thesis we address IX transport in a set of GaN/(Al,Ga)N and ZnO/(Mg,Zn)O QWs. This choice of IX is motivated by high binding energy, and potential stability up to room temperature, but present a number of experimental challenges, including (i) dramatic dependence of the exciton radiative lifetime on the exciton density that makes exciton density measurement very complex, (ii) thermally activated nonradiative recombination that quenches exciton PL at room temperature,(iii) coexistence of photon propagation with exciton propagation along the QW plane, and strong inhomogeneous broadening of the exciton emission due to strong built-in electric field and the presence of both monolayeructuations of the QW thickness and the fluctuations of alloy composition in the barriers. We have addressed all these issues and demonstrated exciton propagation by combining continuous wave µ-photoluminescence and time-resolved spectroscopy measurements, supplemented by modelling of the exciton transport within drift-diffusion formalism. In the best quality GaN/(Al,Ga)N QWs grown on free-standing GaN substrates we achieved room-temperature propagation over ~10 µm and up to 20 µm at 4 K. Our results suggest that propagation of excitons under continuous-wave excitation is assisted by effcient screening of the in-plane disorder. Nevertheless, exciton propagation is still limited by the exciton scattering on defects rather than by exciton-exciton scatteringso that improving interface quality can boost exciton transport further

Transport of Indirect Excitons in Polar GaN/AlGaN Quantum Well Structures Grown on Sapphire and GaN Substrates.

International audienceAn indirect exciton (IX) is a quasiparticle consisting of an electron and a hole spatially separated in two different planes of a quantum nanostructure, thus exhibiting a strongly dipolar character. Current research on transport properties of IXs is opening a pathway to the development of novel optoelectronic devices, which have already been demonstrated in GaAs-based heterostructures [1-3]. Applying the same ideas to IXs in wide-band gap polar quantum wells (QWs) is particularly promising because of much larger exciton binding energies and natural dipoles induced by strong built-in electric fields. We have recently studied the transport of IXs in GaN/AlGaN QWs grown on sapphire substrates, at temperatures up to 80 K [4], by mapping the micro-photoluminescence (µ-PL) signal obtained under intense, point excitation. The low-temperature PL recorded at long distances from the excitation spot (30 < r < 100µm) turned out to be a secondary PL, excited by the light emitted at the central spot, guided along the plane, due to the refractive index contrast between the layer and the substrate. At higher temperatures, this signal is rapidly quenched and the distance reached by the measurable PL is limited by recombination of excitons at non-radiative defects. Using GaN substrates instead of sapphire should both suppress the secondary emission and the nonradiative recombination, by reducing dislocation densities by 3-4 orders of magnitude. In this work, we compare exciton propagations in two GaN/Al0.19Ga0.81N QWs of identical structures, except for the substrates, respectively of GaN and sapphire. For the GaN substrate, we indeed observe the mere propagation of excitons up to 35 µm away from the excitation spot and up to 250 K (see below). We propose a drift/diffusion modelling of exciton transport, accounting for dipole-dipole repulsion in high-density regions and for disorder along the sample plane.[1]Y. Y. Kuznetsova, M. Remeika, A. A. High, A. T. Hammack, L. V. Butov, M. Hanson, and A. C. Gossard. Optics Letters 35 (10), 1587 (2010).[2]A.A. High, E.E. Novitskaya, L.V. Butov, and A.C. Gossard. Science 321, 229 (2008).[3]A.A. High, A.T. Hammack, L.V. Butov, and A.C. Gossard. Optics Letters 32, 2466 (2007).[4]F. Fedichkin, P. Andreakou, B. Jouault, M. Vladimirova, T. Guillet, C. Brimont, P. Valvin, T. Bretagnon, A. Dussaigne, N. Grandjean, P. Lefebvre. Phys. Rev. B 91, 205424 (2015)

Room-Temperature Transport of Indirect Excitons in (Al,Ga)N/GaN Quantum Wells

We report on the exciton propagation in polar ðAl; GaÞN=GaN quantum wells over several micrometers and up to room temperature. The key ingredient to achieve this result is the crystalline quality of GaN quantum wells grown on GaN substrate that limits nonradiative recombination. From the comparison of the spatial and temporal dynamics of photoluminescence, we conclude that the propagation of excitons under continuous-wave excitation is assisted by efficient screening of the in-plane disorder. Modeling within drift-diffusion formalism corroborates this conclusion and suggests that exciton propagation is still limited by the exciton scattering on defects rather than by exciton-exciton scattering so that improving interface quality can boost exciton transport further. Our results pave the way towards room-temperature excitonic devices based on gate-controlled exciton transport in wide-band-gap polar heterostructures

Transport of Indirect Excitons in Polar GaN/AlGaN Quantum Wells.

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Transport of dipolar excitons in (Al,Ga)N/GaN quantum wells.

International audienceWe investigate the transport of dipolar indirect excitons along the growth plane of polar (Al,Ga)N/GaN quantum well structures by means of spatially and time-resolved photoluminescence spectroscopy. The transport in these strongly disordered quantum wells is activated by dipole-dipole repulsion. The latter induces an emission blue shift that increases linearly with exciton density, whereas the radiative recombination rate increases exponentially. Under continuous, localized excitation, we observe continuously decreasing emission energy, as excitons propagate away from the excitation spot. This corresponds to a steady-state gradient of exciton density,measured over several tens of micrometers. Time-resolved microphotoluminescence experiments provide information on the dynamics of recombination and transport of dipolar excitons.We account for the ensemble of experimental results by solving the nonlinear drift-diffusion equation. Quantitative analysis suggests that in such structures, exciton propagation on the scale of 10 to 20 μm is mainly driven by diffusion, rather than by drift, due to the strong disorder and the presence of nonradiative defects. Secondary exciton creation, most probably by the intense higher-energy luminescence, guided along the sample plane, is shown to contribute to the excitonemission pattern on the scale up to 100 μm. The exciton propagation length is strongly temperature dependent, the emission being quenched beyond a critical distance governed by nonradiative recombination