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.

The dynamics of the free-exciton capture by boron acceptors and phosphorus donors in diamond is observed in the picosecond range by time-resolved photoluminescence experiments at low temperature. The formation of boron-bound excitons is observed with a delay of 410 ps after the formation of free excitons. For phosphorus, this delay is 120 ps. This is the result of the free-exciton capture by B0 and P0 impurities. The lifetimes of boron- and phosphorus-bound excitons are measured and found to be equal to 270 and 70 ps, respectively. These values are about four orders of magnitude shorter than for the same impurities in silicon. Ei being the ionization energy of dopants, these results scale well with the E4i dependence of the Auger recombination rate expected for bound excitons in indirect band-gap semiconductors.

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.

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.

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).

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.

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.