T. Bretagnon - Academia.edu (original) (raw)

Papers by T. Bretagnon

Physical Review Applied, 2016

We report on the exciton propagation in polar (Al,Ga)N/GaN quantum wells over several micrometers... more We report on the exciton propagation in polar (Al,Ga)N/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 (QWs) grown on GaN template substrate. By comparing microphotoluminescence images of two identical QWs grown on sapphire and on GaN, we reveal the twofold role played by GaN substrate in the transport of excitons. First, the lower threading dislocation densities in such structures yield higher exciton radiative efficiency, thus limiting nonradiative losses of propagating excitons. Second, the absence of the dielectric mismatch between the substrate and the epilayer strongly limits the photon guiding effect in the plane of the structure, making exciton transport easier to distinguish from photon propagation. Our results pave the way towards room-temperature gate-controlled exciton transport in wide-bandgap polar heterostructures.

Materials Science Forum, 1986

TURKISH JOURNAL OF PHYSICS, 2014

This paper reviews the optical properties of ZnO/(Zn, Mg)O single quantum wells grown by molecula... more This paper reviews the optical properties of ZnO/(Zn, Mg)O single quantum wells grown by molecular beam epitaxy. Both heteroepitaxial quantum well growth along the polar c-direction and homoepitaxial quantum well growth on the nonpolar M plane cases are considered. The optical properties of these quantum wells are investigated by using reflectance, continuous wave photoluminescence, and time-resolved photoluminescence spectroscopies. The quantum-confined Stark effect dominates the properties of the excitons for polar quantum wells. The magnitude of the internal electric field that is induced by both spontaneous and piezoelectric polarizations is determined by comparing the experimental results with a variational calculation of excitonic energies and lifetimes. For nonpolar quantum wells, the optical spectra reveal strong in-plane optical anisotropies, as predicted by the group theory. Moreover, the radiative recombination of free excitons is dominating the quantum well photoluminescence even at room temperature.

Journal of Applied Physics, 2014

ABSTRACT Optical properties of GaN/Al0.2Ga0.8N multiple quantum wells grown with semi-polar (10-1... more ABSTRACT Optical properties of GaN/Al0.2Ga0.8N multiple quantum wells grown with semi-polar (10-11) orientation on patterned 7°-off Si (001) substrates have been investigated. Studies performed at 8 K reveal the in-plane anisotropic behavior of the QW photoluminescence (PL) intensity for this semi-polar orientation. The time resolved PL measurements were carried out in the temperature range from 8 to 295 K to deduce the effective recombination decay times, with respective radiative and non-radiative contributions. The non-radiative component remains relatively weak with increasing temperature, indicative of high crystalline quality. The radiative decay time is a consequence of contribution from both localized and free excitons. We report an effective density of interfacial defects of 2.3 × 1012 cm−2 and a radiative recombination time of τloc = 355 ps for the localized excitons. This latter value is significantly larger than those reported for the non-polar structures, which we attribute to the presence of a weak residual electric field in the semi-polar QW layers.

Physical Review B, 2015

We investigate the transport of dipolar indirect excitons along the growth plane of polar (Al,Ga)... more We 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 measure a continuous red shift of the emission, as excitons propagate away from the excitation spot. This shift corresponds to a steady-state gradient of exciton density, measured over several tens of micrometers. Time-resolved micro-photoluminescence 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 microns 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 exciton emission pattern on the scale up to 100 microns. The exciton propagation length is strongly temperature dependent, the emission being quenched beyond a critical distance governed by nonradiative recombination.

MRS Proceedings, 2005

Highly crystalline zinc oxide (ZnO) nanomaterials are synthesized using a seeded growth sol-gel m... more Highly crystalline zinc oxide (ZnO) nanomaterials are synthesized using a seeded growth sol-gel method. In order to control the morphology and the organization of the ZnO nanomaterials, a double hydrophilic block copolymer has been introduced during the seeded growth synthesis: the Polyacrylic acid-Polyacrylamide (PAA-PAM). Depending on the amount of PAA-PAM copolymers, different morphologies were obtained, such as ZnO nanostructured spheres or flat hexagonal crystals. Thus, systematic studies have been done to investigate the influence of the copolymer addition on ZnO nanomaterial morphologies and explain the mechanisms of the morphological modifications.

Physical review. B, Condensed matter, Jan 15, 1989

ABSTRACT

Superlattices and Microstructures, 2014

ABSTRACT The time-resolved photoluminescence properties of yellow emitting (Ga,In)N/GaN multiple ... more ABSTRACT The time-resolved photoluminescence properties of yellow emitting (Ga,In)N/GaN multiple quantum wells are evaluated.•Samples with different In compositions and quantum well thicknesses, all emitting in the yellow, are measured.•A two-mode exponential decay with a long decay time about four to five times longer than the short one is obtained.•The decay time increases exponentially as a function of the product of the In composition and the quantum well thickness.•This behaviour can be generalized for a large set of data from the literature.

Nonpolar (11-20) (Ga,In)N multi-quantum wells have been studied by polarized, temperature depende... more Nonpolar (11-20) (Ga,In)N multi-quantum wells have been studied by polarized, temperature dependent photoluminescence (PL). A non monotonic temperature dependence of the difference in PL peak energies recorded for E⎢⎢c or E⊥c, and of the polarization ratio is observed. We interpret these effects by first an orthorhombic distortion of the crystal and an increased electron-hole exchange energy relative to GaN. This increase is in agreement with calculations on the effect of confinement on the exciton exchange splitting.

Physical Review B, 2003

ABSTRACT We propose a model for the radiative recombination of electron-hole pairs in (Ga,In)N/Ga... more ABSTRACT We propose a model for the radiative recombination of electron-hole pairs in (Ga,In)N/GaN quantum objects, including huge internal electric fields and strong carrier localization. This model explains why the time decay of the photoluminescence keeps a constant nonexponential shape, while its time scale can be varied over several orders of magnitude. Instead of localized excitons, we consider an electron and a hole independently localized at sharp potential fluctuations, along two parallel sheets, forming a two-dimensional pseudo-donor-acceptor pair.

Superlattices and Microstructures, 2006

ABSTRACT We report continuous-wave and time-resolved optical spectroscopy of ZnO nanorods and nan... more ABSTRACT We report continuous-wave and time-resolved optical spectroscopy of ZnO nanorods and nanowires of varying diameters grown by MOVPE, under varying thermo-dynamical conditions. We discuss the influence of these conditions on the different spectral components. Two families of photoluminescence lines are identified. The first one is related to defect-bound excitons and to their two-electron replica. The second one is correlated to near surface states. The relative intensities of various contributions appear to be strongly dependent on the growth conditions and of the diameter of the nanostructures. Photoluminescence decay time of the different lines has been measured on one of the samples.

Revue De Physique Appliquee, 1987

2014 Au cours du processus d'élaboration de photodiodes InP n+ /p pour la conversion d'énergie so... more 2014 Au cours du processus d'élaboration de photodiodes InP n+ /p pour la conversion d'énergie solaire par diffusion de soufre à 700 C dans des substrats d'InP dopés Zn, les propriétés électriques et photoélectriques des substrats sont fortement modifiées. Les mêmes effets sont observés après un simple recuit des substrats à la même température. La modification la plus notable est une forte augmentation de la densité de trous (plus d'un ordre de grandeur) uniforme en profondeur. Une dégradation superficielle des paramètres photoélectriques, longueur de diffusion L et durée de vie 03C4 des porteurs a aussi été observée. Les pièges profonds détectés dans les substrats après traitement thermique ne sont pas typiques de ce traitement. Abstract. 2014 During the process of elaboration of n+ /p InP photodiodes for solar energy conversion, we have