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

We present a spectroscopic study of nanocavities obtained by small modifications of a W1 waveguide in an AlN photonic crystal membrane. The AlN film containing GaN quantum dots is grown on silicon. The photonic crystal structure is defined by e-beam lithography and etched by inductively coupled plasma reactive ion etching, while the membrane is released by selective etching of the silicon substrate. The room temperature photoluminescence of the embedded quantum dots reveals the existence of even-symmetry and odd-symmetry confined cavity modes and guided modes. Cavity mode quality factors up to 4400 at 395 nm and 2300 at 358 nm are obtained.

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).

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. We have recently reported the condensation of polaritons in a bulk ZnO microcavity over an unprecedented range of exciton-photon compositions and of temperatures, up to room temperature [1]. The complete phase diagram of the ZnO polariton laser has been measured (cf figure 1a), 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 (figure 1b). Strongly excitonic (96% exciton fraction) as well as strongly photonic condensates (83% photon fraction) are realized (figure 1c). Moreover imaging of the condensate allows to evidence in-plane free propagation of the polariton condensate starting from the excitation spot. This tunability is obtained on a fully-hybrid microcavity with a high quality factor (Q>2000) and a large Rabi splitting (250 meV); it represents an important progress compared to our previous demonstration of a ZnO polariton laser in a Q=450 microcavity [2], 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. Figure 1. (a) Detuning dependence of the polariton condensation threshold for a 2.5 polariton branch, under quasi-continuous excitation; (b) Polariton density at the condensation threshold deduced from the kinetic model; (c) Angle-resolved emission below and above threshold (T=300K, almost zero detuning). The authors acknowledge financial support from FP7 program through ITN networks CLERMONT4 (235114) and SPIN-OPTRONICS (237252). References [1] F. Li et al., Phys. Rev. Lett. 110, 196406 (2013) [2] T. Guillet et al., Appl. Phys. Lett. 98, 211105 (2011) [3] H. Franke et al., New J. Phys. 14, 013037 (2012); T.C. Lu et al., Optics Express 20, 5530 (2012) [4] J. Levrat et al., Phys. Rev. B 81, 125305 (2010)

Semiconductor-based microcavities appear as a prolific system for studying light-matter interaction between a spatially-confined photonic mode and an excitonic resonance. The quasiparticles arising from this coupling (microcavity-polaritons) have enabled in the last years the observation of new lasing regimes as well as polariton Bose-Einstein condensates, vortices and lately solitons. In this panorama ZnO appears as an alternative material to more mature ones, such as GaAs or CdTe, with larger oscillator strengths and enhanced exciton stability. These two properties render ZnO very interesting for studies and applications where large particle densities and/or high temperatures are required. However, the fabrication of ZnO-based microcavities is still challenging and it often requires the use of either nitrides or dielectric materials for the DBRs. Indeed, polariton lasing was demonstrated for the first time in a ZnO-based microcavity only in 2011 [1, 2]. In this work we report on the optical study of a fully-hybrid ZnO-based microcavity in which we combine a high quality active region made up of bulk ZnO and a high cavity quality factor, thanks to the use of two dielectric DBRs. Indeed, the Q-factor of the cavity measured by micro-reflectivity exceeds systematically 1000. Furthermore, as the ZnO active region has a wedge shape a large range of detuning between the cavity and the excitonic mode is accessible. Polariton lasing, as characterized by a strong (typically a factor 5) linewidth reduction, a small blueshift compared to the Rabi splitting (typically 30 times smaller), and an increased intensity emission (four orders of magnitude increase) will be reported. The study of the lasing regime as a function of temperature and detuning will be presented and the differences with respect to other materials, arising due to ZnO peculiarities, will be emphasized. [1] T. Guillet et al., Appl. Phys. Lett. 99 (2011) 161104. [2] L. Orosz et al., Phys. Rev. B.(2012) accepted for publication.