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Being a flexible wide band gap semiconductor, hexagonal boron nitride (h-BN) has great potential for technological applications like efficient deep ultraviolet light sources, building block for two-dimensional heterostructures and room temperature single photon emitters in the ultraviolet and visible spectral range. To enable such applications, it is mandatory to reach a better understanding of the electronic and optical properties of h-BN and the impact of various structural defects. Despite the large efforts in the last years, aspects such as the electronic band gap value, the exciton binding energy and the effect of point defects remained elusive, particularly when considering a single monolayer. Here, we directly measured the density of states of a single monolayer of h-BN epitaxially grown on highly oriented pyrolytic graphite, by performing low temperature scanning tunneling microscopy (STM) and spectroscopy (STS). The observed h-BN electronic band gap on defect-free regions is (6.8 ± 0.2) eV. Using optical spectroscopy to obtain the h-BN optical band gap, the exciton binding energy is determined as being of (0.7 ± 0.2) eV. In addition, the locally excited cathodoluminescence and photoluminescence show complex spectra that are typically associated to intragap states related to carbon defects. Moreover, in some regions of the monolayer h-BN we identify, using STM, point defects which have intragap electronic levels around 2.0 eV below the Fermi level.

International audience; Hexagonal boron nitride (hBN) is an important insulator that is incorporated into numerous 2D electronic, optoelectronic, and photonic devices, whereas natural hBN is a mixture of 20% 10B and 80% 11B isotopes, and monoisotopic hBN is a variant with just a single boron isotope, either 10B or 11B. Consequently, monoisotopic hBN has a higher thermal conductivity and a stronger neutron absorption (in the case of h10BN), making it superior for neutron detectors, heat management materials in nano flexible electronic devices, and phonon polariton-based nanophotonics. Here we synthesized approximately monoisotopic hBN using boron powder containing a single boron isotope and nitrogen, and grew single crystals from a Fe–Cr metal flux at atmospheric pressure. Narrow Raman peaks from the shear (r1.3 cm

International audience; Excitons in wide nitride Quantum Wells (QWs) are naturally indirect due to the strong internal electric field: within such excitons the electron and the hole are spatially separated, resulting in strong dipole moments and long radiative lifetimes. These properties offer the possibility to explore the collective exitonic phases with complex phase diagrams predicted theoretically in the temperature-density-dipole moment parameter space. Here we study 8 nm-wide GaN/(AlGa)N QWs presenting 1MV/cm built-in electric field in the growth direction, microsecond-long radiative lifetimes, 8 meV exciton emission linewidth and 100 µm transport lengths at cryogenic temperatures. To address the quasi-equilibrium properties of the strongly dipolar excitons (i.e. whose electric dipoles are twice as long as their Bohr radii d = 2aB), we confine them in the QW’s plane using electrostatic traps with a semi-transparent Schottly electrode. These traps consist in semitransparent electrodes deposited on the sample’s surface. We make micro-photoluminescence (µPL) measurements (less than 1µm- resolution) upon point-like optical excitation. We thus obtain the spectrally resolved information on the emission of the excitons as a function of power and density. Our experiments demonstrate an efficient trapping of excitons in the plane of the QW, with a trapping potential of an order of 50 meV. We demonstrate that it is possible to uniformly fill areas up to hundreds of microns in length with excitons and/or electron hole pairs in the QW’s plane. At T=4K, the emission from these areas exhibits a peculiar behavior. At low densities, such that there is no measurable screening of the built-in electric field, the emission from the trap is weak and spatially homogeneous. However, for weak built-in electric field screenings (as low as 10 meV, ~1011 cm-2 pair density), the PL brightens up and becomes spatially inhomogeneous – droplet like. We tentatively interpret this fragmentation as the coexistence of the gas and liquid phases of the excitons. As expected, the fragmented character of the emission disappears when either power and/or temperature is further increased, due to either exciton heating (gas phase), ionization or dissociation (Mott transition). Our results suggest that excitons in GaN QWs are promising candidates for the study of collective phenomena in cold dipolar bosonic systems. The advantages of the nitrides over arsenides include both higher critical temperatures for the onset of collective phenomena, and the ability to increase the dipolarity of systems based on their small exciton Bohr radius.

International audience; Excitons in nitride quantum wells (QWs) are naturally indirect due to the strong internal electric field: electron and hole within such excitons are spatially separated, leading to strong dipole moments and long radiative lifetimes. The physics of indirect excitons (IXs) has been extensively studied in GaAs-based heterostructures: they can propagate over large distances, be trapped in gate-controlled electrostatic traps, and form a cold and dense gas of interacting bosons. Compared to traditional IXs in arsenide heterostructures, IXs in GaN QWs have much larger binding energies and smaller Bohr radii. This allows exploring IX propagation up to room temperature, and over a much larger density range. The length scale of the excitonic transport is an important figure of merit of the indirect excitons. The excitonic transport was first observed at low temperature in the case of hetero-epitaxy of the GaN QWs on sapphire substrates [1]. The improvement of the sample quality through homo-epitaxy on GaN substrates allowed observing an efficient transport up to room temperature [2]. Under a focused non-resonant excitation of the quantum well, we observe a spatial decay of the exciton density (deduced from the local blue shift of the QW transition), with a length scale of 100µm at 10K and 30µm at T=300K in our most recent homo-epitaxial samples. Such long propagation distances are a pre-requisite to the development of IX-based optoelectronic devices operating on a large temperature range, and to the exploration of exciton collective states. The realization of an excitonic trap is a first step forward, and we recently investigated the exciton transport in electrostatic traps formed when a semi-transparent gate is deposited on top of the QW sample. Trapping of the excitons is observed, with a typical trap depth of 60-90 meV. The excitons can also be generated outside the trap and relax into the trap, so to spatially separate the trap from the generation spot, which is expected to be a source of heat and decoherence for the excitonic cloud. These results are promising for the investigation of trapped collective states of IXs, and their cooling into exotic phases.

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International audience; Light emission in polar group-III nitride quantum wells (QWs) optically pumped at densities significantly below the lasing threshold is usually interpreted in terms of excitons. Whereas thin GaN QWs with fast radiative lifetimes are employed in light-emitting devices, the excitons in thick QWs (typ. 7nm) are characterized by a non-zero dipole moment and long radiative lifetimes, due to the built-in electric field. These features result in many interesting properties of dipolar excitons: they can propagate over large distances and thus cool down to the lattice temperature before recombination, offering the possibility for studies of cold and dense gas of interacting bosons. In this context, two important issues must be addressed: the strength of the exciton-exciton interaction, and the excitonic Mott transition, that sets the maximum density of excitons. Combining spatially and time-resolved photoluminescence in different excitation geometries, we provide a set of experimental data showing that the emission linewidth is proportional to the emission energy in a wide range of carrier densities and temperatures. The proportionality coefficient is almost temperature independent. These results are compared to the existing data in various material systems and discussed in terms of the Mott transition and its criteria, exciton-exciton interaction and its possible contribution to the linewidth, as well as the role of disorder in the density-dependent emission spectra.

International audience; We investigate the opto-electronic properties of hexagonal boron nitride grown by high temperature plasma-assisted molecular beam epitaxy. We combine atomic force microscopy, spectroscopic ellipsometry, and photoluminescence spectroscopy in the deep ultraviolet to compare the quality of hexagonal boron nitride grown either on sapphire or highly oriented pyrolytic graphite. For both substrates, the emission spectra peak at 235 nm, indicating the high optical quality of hexagonal boron nitride grown by molecular beam epitaxy. The epilayers on highly oriented pyrolytic graphite demonstrate superior performance in the deep ultraviolet (down to 210 nm) compared to those on sapphire. These results reveal the potential of molecular beam epitaxy for the growth of hexagonal boron nitride on graphene, and more generally, for fabricating van der Waals heterostructures and devices by means of a scalable technology.

International audience; This communication will display the use of novel organosilica materials embedding π-conjugated moieties1 as semiconductors into various applications. For field effect transistors, the π-conjugated core chosen is [1]benzothieno[3,2-b][1]benzothiophene (BTBT)2, first functionalized with hydroxyl groups3 and then modified with hydrolysable and cross-linkable triethoxysilyl moieties. After polycondensation, this compound forms a hybrid material found to be a semiconductor and can be incorporated into field effect transistors. Taking advantage of the sol-gel chemistry involved here, we built Hybrid Field Effect Transistors that are fully cross-linked with covalent bonds. Molecules are cross-linked to each other, covalently bonded to the silicon oxide dielectric and also covalently bonded to the gold electrode thanks to the use of an appropriate surface modifier in between. This is the first report of fully covalent transistors. Those devices with modest mobilities show impressive resilience against polar, aliphatic and aromatics solvents (even under sonication). A solgel processable derivative of fullerene will also be described as promising interlayer for Perovskite n-i-p photovoltaic solar cells4.

International audience; In this paper, I will review our recent results demonstrating that hBN has an indirect bandgap at 5.9 eV. I will show that the optical properties of hBN are profoundly determined by phonon-assisted transitions with a mirror symmetry between emission and absorption around the indirect exciton at 5.9 eV (Figure 1). I will provide a comprehensive analysis of the emission spectrum in the deep ultraviolet in terms of phonon-assisted transitions involving either virtual or real excitonic states, the latter being provided by structural defects. I will finally point out the complex relaxation dynamics of the quantum gas formed by the reservoir of indirect excitons.

National audience; ZnO est un matériau semiconducteur présentant des excitons forts, en terme de force d’oscillateur et d’énergie de liaison. Il est donc particulièrement intéressant pour la génération et le contrôle de condensats de polaritons, jusqu’à température ambiante.La réalisation de microcavités ZnO planaires de grand facteur de qualité a longtemps constitué un défi important. Nous avons réalisé dès 2011 le premier laser à polaritons ZnO (à T=120K et un désaccord exciton-photon nul) [1], dans une microcavité hybride (DBR AlN/AlGaN, ZnO réalisé par épitaxie MBE, DBR SiO2/SiN). Son facteur de qualité, Q=450, était cependant trop faible pour permettre un fonctionnement du laser à polaritons à température ambiante. Dans une approche radicalement différente, une microcavité a ensuite été réalisée en insérant une couche active ZnO massive de grande qualité cristalline entre deux miroirs diélectriques SiO2/HfO2 ; le facteur de qualité a alors atteint Q>2000, pour un dédoublement de Rabi de 250 meV. Cette cavité a permis d’observer la condensation des polaritons sur une plage de composition exciton-photon et de température sans précédent, jusqu’à T=300K [2]. Le diagramme de phase complet du laser à polaritons a été mesuré, montrant que le seuil de condensation à 300K est seulement 6 fois supérieur à celui observé à 8K. Ce résultat est en bon accord qualitatif avec des simulations de la relaxation des excitons et des polaritons dans un modèle cinétique. L’accordabilité de cette microcavité constitue un progrès important en comparaison des travaux antérieurs [3,4].La formation et la propagation du condensat de polaritons sont étudiées dans la microcavité de Q>2000 par imagerie 2D de l’émission via un spectromètre imageur, dans l’espace réel comme dans l’espace de Fourier des impulsions. Le laser d’excitation est focalisé sur quelques microns, et les contributions du réservoir d’excitons et des différentes branches polaritoniques sont mesurées (figure 1). Les excitons diffusent sur quelques microns, alors que le condensat de polaritons se propage sur des distances atteignant 10 à 20 µm. Les profils spatiaux du condensat sont comparés aux résultats d’un modèle de formation et de propagation en symétrie cylindrique, permettant d’identifier les différents régimes de fonctionnement : génération, répulsion du condensat par interaction avec le réservoir d’excitons, amplification du condensat lors de la propagation. Ces résultats seront comparés avec les démonstrations récentes dans les microcavités GaAs.La microcavité précédente présente un gradient important d’épaisseur, qui rend difficile la réalisation de condensats de forme variée. Des cavités équivalentes ont alors été réalisées sur des DBR AlN/AlGaN sur mesas Si, dans lesquels le laser à polaritons fonctionne aussi jusqu’à température ambiante [5]. Elles ont permis de travailler sous faisceau laser d’excitation défocalisé ou façonné. Le rôle de la localisation des polaritons sur la distribution spatiale du condensat sera discuté.

National audience; Dans le domaine de la nano-photonique, les matériaux semiconducteurs nitrures (GaN, AlN) présentent un intérêt grandissant en raison de leurs spécificités, et de leur complémentarité avec les filières déjà établies. Ces matériaux présentent en effet une plage de transparence large, du moyen infra-rouge à l’ultra-violet ; ils présentent aussi des susceptibilités non-linéaires d’ordre 2 et 3 intéressantes ; des émetteurs efficaces jusqu’à température ambiante peuvent être intégrés dans les couches actives. Enfin, la filière nitrure de l’industrie des semiconducteurs se développe actuellement très rapidement.Nous avons développé au cours des dernières années plusieurs facettes exploitant ces avantages de la nano-photonique GaN. La maîtrise des procédés spécifique à GaN/AlN a permis de réaliser des micro-résonateurs de haut facteur de qualité, atteignant dans l’ultra-violet Q=7000 dans des microdisques [1] et Q=4400 dans des cavités à cristaux photoniques membranaires [2], et Q=34000 dans l’infra-rouge [3]. Ces résonateurs sont désormais intégrés dans des circuits photoniques complexes [3]. Ces structures sont de plus réalisées à partir de couches GaN/AlN sur substrat de silicium, de faible coût, ce qui ouvre des perspectives riches de couplage avec la photonique silicium. Nous présentons ici deux démonstrations récentes et originales. D’une part, nous avons exploité le confinement de la lumière dans une cavité photonique membranaire pour exalter la génération de seconde harmonique (SHG). La structure est constituée d’un guide d’accès membranaire GaN/AlN de 345nm d’épaisseur, se terminant de manière effilée pour optimiser le couplage de la lumière via une fibre lentillée. Un cristal photonique est inséré au centre du guide d’onde, avec un mode de cavité à 1577nm (Q=25000). Lorsque le laser continu d’injection est résonant avec ce mode, une émission de seconde harmonique est observée depuis la surface par un objectif de microscope couplé à un capteur CCD (figure 1). Il est intéressant d’observer que cette image possède une résolution comparable à la distance entre trous du cristal photonique. En particulier, la forme en X de l’image de second harmonique a permis d’identifier le processus non-linéaire d’ordre 2 le plus efficace pour générer la SHG, par comparaison avec des simulations FDTD. La perspective de ce travail est d’étendre vers le bleu/l’UV la plage spectrale de SHG, et de mieux profiter de la géométrie membranaire pour récolter la SHG.D’autre part, nous avons réalisé un laser à microdisques fonctionnant sous pompage optique impulsionnel dans une plage spectrale jusqu’ici inexplorée pour des micro-résonateurs : il émet à 275nm, dans l’UV-C, grâce à une couche active à base de puits quantiques GaN/AlN de 2.8 monocouches d’épaisseur nominale sur substrat silicium (figure 2). Seules des couches actives beaucoup plus complexes avaient à ce jour permis d’atteindre une émission laser à ces longueurs d’ondes dans des lasers-rubans ; celles-ci étaient de plus incompatibles avec une intégration en circuit photonique ou en microlaser. Le choix de puits quantiques sans alliages ternaires permet ici d’atteindre une bonne efficacité quantique à 300K, malgré une forte densité de dislocations (7 1010 cm-2). Le facteur beta de ce microlaser a été estimé à 3,5.10-4, toujours à 300K, ce qui est dans la plage basse des valeurs atteintes pour des microlasers émettant dans l’infra-rouge.

International audience; 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 generation and control of polariton condensates up to room temperature.The realization of planar ZnO microcavities with good quality factors has long been a strong challenge. We first demonstrated in 2011 a ZnO polariton laser operating at zero exciton-photon detuning and a temperature of 120 K [1], based on a hybrid microcavity (AlN/AlGaN DBR, MBE-grown ZnO, SiO2/SiN DBR) with a quality factor Q=450. This value was still too small to allow the observation of polariton lasing up to room temperature. A different approach was implemented based on a ZnO active layer of high crystalline quality, embedded between two dielectric DBR; such a fully-hybrid microcavity exhibits a high quality factor (Q>2000) and a large Rabi splitting (250 meV). We have observed the condensation of polaritons in this bulk ZnO microcavity over an unprecedented range of exciton-photon compositions and of temperatures, up to room temperature [2]. The complete phase diagram of the ZnO polariton laser has been measured, showing that its threshold is only 6 times larger at 300 K than at 8 K. It is in a good qualitative agreement with the simulations of exciton and polariton relaxation in a kinetic model. This tunability represents an important progress compared to our previous demonstration [1], as well as to other recent reports [3]. It also confers a strong advantage to ZnO microcavities compared to GaN [4], since strong excitonic condensates can here be investigated.The cavity with Q>2000 however presents a strong gradient of the cavity thickness hindering the generation of extended condensates. Equivalent cavities without such a gradient have therefore been developed in the last years, based on high quality AlN/AlGaN DBRs on mesas. Our most recent investigations are dedicated to the role of the shape of the exciting laser spot on the condensate generation. Condensate propagation is observed under tightly focused excitation, driven by the interaction with the reservoir as in GaAs microcavities, whereas localization of the polariton condensates is observed for larger excitation areas.

International audience; We create polariton clouds and polariton condensates to characterize their propagation at different temperatures. Our experimental results show that in a thickness gradient microcavity, polariton propagation is mainly ballistic and does not depend on the microcavity gradient. Above threshold, we compare the experimental results to a theoretical model [1] to understand the peculiar k-space images of the polariton condensate. Our results indicate that these images are due to the repulsion of the condensate from the center of excitation by the exciton reservoir created by a small excitation spot (2~3 µm).

International audience; Properties of self-assembled quantum dots (QDs), often referred to as artificial atoms in solid state, are very interesting in order to study physical effects specific to single quantum systems such as the single-photon emission. Thanks to their huge vertical quantum confinement, GaN QDs are emitting even at room temperature and on a large UV range. This peculiarity makes GaN QDs good candidates for fundamental physics as well as conception of devices such as UV-LEDs. In so-called thick QDs (height larger than 2.3 nm), the energies of the confined electronic states are determined by the quantum confined Stark effect. On the contrary, in thin QDs (height smaller than 2.3 nm) the quantum confinement plays the major role, thus leading to a significantly smaller homogeneous linewidth and to the resolution of the exciton fine-structure splitting [Kindel, PRB81, 241309]. As far as time-resolved measurements are concerned, several papers reported experiments in thick QDs emitting at 350nm or more, either in polar [Bretagnon, PRB73, 113304] or non-polar nanostructures [Founta, APL86, 171901]. We report photoluminescence decay time measurements in thin GaN QDs emitting between 280nm and 360nm. For QDs emitting at 290nm we observe a monoexponential decay and the abscence of any red shift for the luminescence spectrum shows that we are in the regime of a single electronhole pair per dot. In this case, we measure the intrinsic radiative recombination time, without any perturbation due to the Stark effect. We find a radiative lifetime of about 350 ps in good agreement with our theoritical model.

International audience; We present spatially- and time-resolved photoluminescence experiments on polar GaN/AlGaN quantum well structures,showing exciton propagation distance of tens of micrometers from generation position and exciton radiative lifetimes oftens of microseconds. We study in detail the dynamics and range of their relaxation/diffusion along the growth plane, asa function of temperature.

International audience; Spatial observation and optical manipulation of polariton quantum liquids are raising a large interest in the studies of polariton condensates in microcavities [1]. In this work, we report the study of polariton condensates in a semi-hybrid ZnO microcavity where the condensation of polaritons up to room temperature has been achieved. This cavity is based on high quality AlN/AlGaN DBRs on 200µm ∗ 200µm mesas. The obtention of a low condensation threshold in a microcavity is related to its local quality factor Q. However the generation of the condensate also strongly depends on the photonic disorderof the cavity i.e. on polariton localization. The purpose of this work is to demonstrate the influence of this last parameter using different large excitation shapes. Figure 1.a shows a cross sectional energy mapping below threshold of polaritons at negative detuning. It is done by scanning the sample with a focused CW excitation spot. The energy fluctuation is less than 4 meV and the local quality factor is above 1900. We then excite the sample with large excitation areas. The photoluminescence profile created by an elongated excitation (60µm ∗ 5µm) is shown on figure 1.b. The intensity profile indicates that the polariton condensate is not uniform. In fact we can clearly see the localization of polaritons on sitesthat can be separated by more than 10 µm . We assume that this effect is related to the photonic disorder. The impact of size and shape of the excitation area on the condensate intensity profile will be discussed.

International audience; Light-emitting devices based on high-Al content (Al,Ga)N suffer from high densities of dislocations or other point defects, counterbalanced by the carrier localization arising from disorder in the ternary alloy. One way to improve the internal quantum efficiency is the controlled Si-doping of AlxGa1-xN/AlyGa1-yN multi-quantum wells, which was assigned to the reduction of point defects by reduction of internal strains, for some ideal concentration of Si. Time-resolved photoluminescence (TR-PL) can be used [S. F. Chichibu et al., Appl. Phys. Lett. 99, 051902 (2011)] to try and correlate the observed PL time-decays with the density of defects, in relation with the nonradiative recombination probability of photoexcited carriers. We present TR-PL studies of MOVPE-grown Si-doped AlxGa1-xN/AlyGa1-yN multi-quantum wells with typical values of x=0.6 and y=0.7 and of AlxGa1-xN epilayers with x up to 0.86. High-Al content MQWs and epilayers exhibit similar bi-exponential PL decay dynamics, with the slower component rapidly quenched when T is increased. The fast decay component remains in the nanosecond range at all temperatures and the PL intensity loss is limited by carrier localization.

International audience

International audience; Time-resolved photoluminescence of high-aluminum-content (Al,Ga)N quantum wells and epilayers emitting in the 215-255 nm range.

International audience; While increasing the Cd content in ZnCdO can lead to a very large nominal change in bandgap, incorporating high Cd concentrations has shown to be extremely difficult and can enhance the formation of different types of defects. Indeed, the tendency that Cd has to segregate and form clusters can yield alloy disorder which in turn can impact the carrier recombination paths, thus limiting the total radiative efficiency. In order to analyze the different recombination mechanisms and their dynamics we have used time-resolved photoluminescence (TRPL) as a function of temperature in dense ensembles of ZnCdO nanowires grown by MOCVD [1]. The nanowires were 1-3 µm-long and 100-200 nm-wide and contain different Cd contents, ranging from 0 to 45%, and were analyzed as-grown and after exposure to different rapid thermal annealing (RTA) cycles [2]. The comparison between the decay time of as-grown and thermally annealed ZnO nanowires revealed a higher lifetime after annealing, providing clear evidence of a reduction of non-radiative channels. In the case of the ternary ZnCdO nanowires, the carrier lifetime is larger than in the Cd-free case. This larger lifetime may be explained to arise from the localization of the photogenerated excitons at potential fluctuations produced by alloy disorder. As in the case of ZnO, annealing also yields a higher carrier lifetime in ZnCdO, indicative of the removal of non-radiative recombination paths. Temperature-dependent TRPL confirmed this exciton localization behaviour in the ternary nanowires. Indeed radiative emission was observed at higher temperatures in the ternaries, especially after thermal annealing, due to the enhanced carrier lifetime. In the case of the ZnO, even after thermal annealing where the non-radiative paths have been largely removed, increasing the temperature produces a rapid collapse of the PL likely due to the absence of exciton localization, as in the case of the ZnCdO nanowires. References: [1] K. Yamamoto, T. Tsuboi, T. Ohashi, T. Tawara, H. Gotoh, A. Nakamura, and J. Temmyo, J. Cryst. Growth 312, 10 (2010). [2] T. Ohasi, K. Yamamoto, A. Nakamura and J. Temmyo, J. J. of Applied Physics, vol.47, pp.2961 (2008).

In this paper we introduce experimentally the phenomenon of speckle and its interferometric applications. With the popularization of CCD sensors and webcams, it is now easy to acquire speckles patterns, to reduce them and exploit them. The material used here is what we could find easily in high schools. All Image processing mentioned in this article could be done using the free software called IRIS. Keywords are essentially diffraction, interference phenomena and Fourier optics. After presenting the characteristics of speckles we discuss the phenomenon of speckles interferences by analogy with the conventional 2 and N waves interferences. Finally, we apply interferometry to measure the angular separation between the components of a double star in drawing heavily on the historical experience of Antoine Labeyrie.

International audience

We study wurtzite GaN/AIN quantum dots (QDs) by time-resolved photoluminescence. The properties of nitride based nano-objects are significantly affected by strong built-in electric fields existing in this crystalline phase. These fields induce a spatial separation of electrons and holes in the quantum dot, a Stark shift of the transition energy, and they slow their radiative recombination down to tens of microseconds of decay time for the largest quantum dots. Such long decay times are responsible for the screening of internal fields by using moderate excitation intensities, thus modifying drastically both the energy and the rate of photon emission. Consequently it is difficult to perform spectroscopic studies of these QDs in the regime where a single electron-hole (e-h) pair is present at the most in each dot. Using a pulsed laser excitation, we study the time-resolved dynamics of carrier recombinations, waiting for complete recombination of all electron-hole pairs in the dots. We clearly exhibit the conditions where no screening of the internal electric field occurs. We especially obtain the transition energy and the mono-exponential decay, i.e the radiative recombination time, of the last e-h pair present in the dot, for a wide range of QD sizes. These results are analyzed within an envelope function model for the electron and hole confinement. The effective electric field, averaged over the strain distribution in the dots, is determined to be of 9.0 +/- 0.5 MV/cm.

International audience; Wurtzite GaN∕AlN quantum dots (QDs) are studied by time-resolved photoluminescence. Careful measurements allow us to reach the regime of a single electron-hole pair per dot, evidenced by a monoexponential decay of the luminescence and a stop of the time-dependent shift of the emission energy. The transition energy and the radiative lifetime of the QD ground state are measured in the absence of any electric field screening effect, for a wide range of QD heights. These results are analyzed within an envelope function model for the electron and hole confinement. The effective electric field, averaged over the strain distribution in the dots, is determined to be of 9.0±0.5MV∕cm.

International audience; Continuous-wave, time-integrated, and time-resolved photoluminescence experiments are used to study the excitonic optical recombinations in wurtzite ZnO/Zn0.78Mg0.22O quantum wells of varying widths. By comparing experimental results with a variational calculation of excitonic energies and oscillator strengths, we determine the magnitude (0.9MV∕cm) of the longitudinal electric field that is induced by both spontaneous and piezoelectric polarizations. The quantum-confined Stark effect counteracts quantum confinement effects for well widths larger than 3nm, leading to emission energies that can lie 0.5eV below the ZnO excitonic gap and to radiative lifetimes that can be larger than milliseconds.

International audience

International audience