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 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.

GaN/AlGaN polar single quantum wells have attracted particular attention due to their large binding energies and the presence of strong internal electric fields that naturally induce long-lived excitons. In this case, the exciton transport is driven by the potential landscape caused by the local alteration of the built-in electric field, which results from the exciton accumulation at the same point. We study by spatially- and time-resolved PL experiments AlGaN/GaN coupled quantum wells (QW). The active zone of studied sample consists of a 30 nm thick Al0.20Ga0.80N barrier, a 4.4 nm (17 atomic monolayers) GaN QW followed by 1 monolayer of Al0.20Ga0.80N and a 1.6 nm (6 atomic monolayers) GaN QW and 30 nm thick Al0.20Ga0.80N cap layer. By solving Schroedinger-Poisson equation, we illustrate the two regimes of strongly screened electric field and the totally un-screened QW system. We show that the screening of the build-in electric field enhances the electron-hole wave-function overlap and increases the optical recombination energy. In other words, dipoledipole repulsion between excitons raises their potential energy and drastically increases their radiative recombination rate. A blue-shift of 0.3 eV under continues wave, localized excitation and a quasi-continuous red-shift of the emission as excitons are drifted away from the excitation spot up to distances of 90 μm are observed experimentally. Strong LO-phonon replica are also noticed, as a result of the strongly dipolar character of excitons [1]. These replicas follow the same position dependence as the zerophonon line. Using our Schroedinger-Poisson equation solver, we can extract from the measured blue-shift the position-dependent exciton density. Last, we present simulations of the observed transport features, namely the exciton densities and luminescence intensities by solving the nonlinear drift-diffusion equation. We establish that the long-range excitonic transport is mainly dominated by drift caused by repulsive dipole-dipole interaction between excitons in combination with their long lifetimes. [1] S. Kalliakos, X. B. Zhang, T. Taliercio, P. Lefebvre, B. Gil, N. Grandjean, B. Damilano, and J. Massies. Applied Physics Letters 80, 428 (2002).

This paper reviews the optical properties of ZnO/(Zn, Mg)O single quantum wells grown by molecularbeam epitaxy. Both heteroepitaxial quantum well growth along the polar c-direction and homoepitaxial quantum wellgrowth on the nonpolar M plane cases are considered. The optical properties of these quantum wells are investigatedby using re ectance, continuous wave photoluminescence, and time-resolved photoluminescence spectroscopies. Thequantum-con ned Stark e ect dominates the properties of the excitons for polar quantum wells. The magnitude of theinternal electric eld that is induced by both spontaneous and piezoelectric polarizations is determined by comparingthe 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 radiativerecombination of free excitons is dominating the quantum well photoluminescence even at room temperature.

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.

Time-resolved photoluminescence spectra of orange light emitting (Ga,In)N-based devices have been grown by metal-organic vapour phase epitaxy on C plane sapphire for indium compositions ranging up to 23 percents. The temperature dependent time resolved photoluminescence spectra collected through the 8K-300K range are found to exhibit behaviours that are found to be very similar to what is reported in literature in samples with different designs (well width, indium composition in the well layer). However, our quantum devices always exhibit a two-mode exponential decay with a long decay time about 4 to five times longer than the short one, which was not reported so far.The photoluminescence decay times are wavelength-dependent as always found for indium rich quantum well, a behaviour that is interpreted in terms of carrier localization in indium-rich regions of the alloy active layers. The spectral dependence of the decay time, which were fitted using sigmoidal function, give access to an average decay time, centred at a given energy, with a phenomenological broadening constant. The average splitting between characteristic energies for long and short decay times, the broadening constants both increase with the indium composition. Radiative and non-radiative recombination times are deduced in the whole temperature range. The average decay time increase exponentially with the product of the well width to indium composition. This quantity is found to be an excellent indicator of the device performance.