Since their prediction in 1996, non-equilibrium polariton condensates have attracted a lot of interest, as they should allow for the realization of ultralow threshold coherent light-emitters [1]. However, the steep dispersion and the short radiative lifetime of lower polaritons (LP) within the light cone hinder their relaxation toward the center of the Brillouin zone, limiting the polariton lasing threshold. Experimental studies carried out on GaAs and CdTe-based microcavities however revealed that one available channel to bypass efficiently the relaxation bottleneck was the relaxation of LPs from the excitonic reservoir directly to the ground state of the LP branch (LPB) through the emission of one LO-phonon [2]. Here, we study by non-resonant angle- and time-resolved photoluminescence experiments the LP relaxation dynamics in a GaN-based multi-quantum well microcavity, for which polariton condensation has been reported between 10 and 340 K [3]. Regarding the strong Fröhlich interaction in GaN, we particularly investigate the role of LO-phonon on the overall LP relaxation mechanism. We first show that LPs efficiently bind into cavity biexcitons and we demonstrate that the latter complexes are in thermal equilibrium with the excitonic reservoir. We then not only demonstrate the existence of a LO-phonon enhanced relaxation channel from the excitonic reservoir toward the ground state, but also the direct feeding of the LPB by the radiative dissociation of cavity biexcitons mediated by one LO-phonon. In other words, when the energy of the bottom of the LPB corresponds to that of the first LO-phonon replica of the cavity biexciton, we observe an enhanced scattering of polaritons toward the k// = 0 state, as well as a decrease in the condensation threshold. This peculiar observation indicates that the LO-phonon assisted dissociation of excitonic molecules constitutes an additional and efficient relaxation channel driving the condensation threshold of polaritons [4]. A. Imamoglŭ et al., Phys. Rev. A 53, 4250 (1996). see for instance F. Boeuf et al., Phys. Rev. B 62, R2279 (2000). J. Levrat et al., Phys. Rev. B 81, 125305 (2010). P. Corfdir et al., submitted to Phys. Rev. B (2012).

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).

Zn1-XCdXO alloys offer the possibility to cover a wide spectral range by controlling the Cd concentration. However, the growth of this alloy, especially with high Cd contents, can lead to phase separation and to segregation of Cd due to its high vapour pressure. This problem may be circumvented by using nanocolumnar structures, where high Cd incorporation may be achieved [1]. However, in order to have electrical access to the ZnCdO nanowires necessary for any device, ohmic contacts must be deposited on the nanocolumns. This requires the use of a post-growth thermal cycle which may be higher than the nanowire growth temperature, and thus can also affect its optical, electrical and structural properties. In this study, we have performed an optical (cw-PL), structural (HRTEM, XRD, and micro-EDX), and electrical analysis of a series of ZnCdO nanowires grown by MOCVD on sapphire as a function of rapid thermal annealing (RTA), spanning from below (200ºC) to above (550ºC) the growth temperature (300ºC). The nanowires were typically 1-3 µm-long and 100-200 nm-wide, and contain different Cd contents (x=0, 0.14, 0.27 and 0.45). Through HRTEM and micro-EDX of single as-grown nanowires the crystal structure has been established to be wurtzite, with measured Cd contents equivalent to the nominal values, and with no indication of phase segregation or formation of cubic clusters, contrary to what has been reported by other groups at such high contents. Annealing at temperatures higher than the growth temperature produces a large increase in the PL intensity and a narrowing of the linewidth of the nanowires, especially at the highest temperatures. This effect is found to be the result of two mechanisms. First, annealing induces an increase of the free-electron concentration, especially in the reference ZnO nanowires where the as-grown electron carrier concentration is low. This in turn causes a decrease of the surface-related depletion region and thus the total nanowire volume contributing to PL is increased. Second, a decrease in the non-radiative recombination through defects is also observed, effect especially large in the Cd-containing nanowires, where the change in the surface-related depletion region is found to be negligible. However, the PL improvement also carries a blue-shift of the peak energy, which is as large as 130 meV for 550ºC annealings. HRTEM and micro-EDX of annealed single nanowires indicate that the wurtzite crystal structure is preserved even at the highest annealing cycles, and again no evidence of Cd-rich cluster formation is observed. Rather, the overall Cd content is found to decrease over the entire nanowire, between 3-5 % for the highest annealed temperature, in agreement with the change in lattice parameter observed by XRD in the ensemble of annealed nanowires. The optimum RTA cycle producing a large enhancement of the PL and minimum blue-shift., i.e. loss of Cd, is therefore found to be 450ºC, which is sufficiently high to allow the formation of ohmic contacts on the nanowires. References: [1] K. Yamamoto, T. Tsuboi, T. Ohashi, T. Tawara, H. Gotoh, A. Nakamura, and J. Temmyo, J. Cryst. Growth 312, 10 (2010).

Currently, a growing interest is paid to the study of nonpolar nitride-based heterostructures, as they allow for the growth of thick QWs, while keeping an optimal overlap between electron and hole wave functions [1]. The growth of wide QWs is indeed a key issue to produce high-power nitride-based optoelectronic devices, as they allow reducing the carrier density in the QW, lessening the efficiency of Auger-like mechanisms. However, non-lattice matched foreign substrates are generally used to grow nonpolar GaN, inducing strain in the heteroepitaxial layers. Even when processing techniques such as epitaxial lateral overgrowth are used, strain relaxation through the generation of dislocations or basal stacking faults leads to a drastic reduction of exciton lifetime [2]. In this work, we therefore investigate the dynamics of excitons in single (Al,Ga)N/GaN QWs deposited directly on the a-facet of GaN crystals. We first extract by cathodoluminescence experiments dislocation density and exciton diffusion length at 300 K of 2.105 cm-2 and 100 nm, respectively, demonstrating that dislocations should not play any significant role in the recombination of excitons at room-temperature [3]. We then study by time-resolved photoluminescence the dynamics of excitons in the 10-320 K range for QW samples with various width and barrier Al-content. We first show that, for all samples, the effective lifetime of QW excitons increases with temperature, evidencing the absence of nonradiative phenomena in the low-temperature range. The largest temperature range of purely radiative recombination (up to 240 K) has been observed for a 7 nm thick Al0.06Ga0.94N/GaN QW, i.e. a QW with small exciton localization energy (2 meV) [4]. This observation therefore evidences the possibility of achieving nonpolar room-temperature UV emitters combining a rather narrow emission line with a good radiative efficiency at 300 K. In the high temperature range, a drop in the QW photoluminescence lifetime is always accompanied by an increase in the barrier emission lifetime, until both emissions follow the same dynamics. Supported by a model accounting for the thermodynamic equilibrium between excitons and free carriers in the QWs and the (Al,Ga)N barriers, we demonstrate that at high temperatures, the nonradiative recombination of charge carriers in the (Al,Ga)N barriers is the mechanism limiting the photoluminescence lifetime of excitons confined in the QWs [4]. We finally propose to tackle the thermal escape of carriers by the growth of thick QWs rather than increasing the barrier Al-content, which is important from the defect/strain generation point of view. [1] P. Waltereit et al., Nature 406, 865 (2000). [2] P. Corfdir et al., J. Appl. Phys. 107, 043524 (2010); T. J. Badcock et al., Appl. Phys. Lett. 93, 101901 (2008). [3] P. Corfdir et al., Phys. Rev. B 83, 245326 (2011). [4] P. Corfdir et al., J. Appl. Phys. 111, 033517 (2012).

While III-V semiconductor nanowires crystallize into a wurtzite geometry, instead of the zinc-blende crystal structure more common in the bulk materials, they usually exhibit along their length large densities of cubic material insertions [1]. Although usually regarded as detrimental to the carrier transport properties, the polytypism of III-V nanowires has recently attracted a huge interest from the scientific community. Planar ZB inclusions in the WZ phase of a III-V semiconductor should behave as shallow type-II quantum wells (QWs), where electrons are confined in the ZB layers. This prediction thus allows for a new kind of bandgap engineering, where the bandgap along a structure depends only on the crystal phase of a single material. Despite the huge progresses in the growth of these so-called crystal phase quantum wells, there have been, so far, only a few attempts at modeling the emission properties of these structures. In addition, up until now, excitonic effects have been systematically neglected, based on the assumption that due to the type-II band alignment between the wurtzite and zinc-blende and phases, the binding energy of the exciton should be small compared to the bulk case [2]. In this work, we therefore compute by envelope function calculations the emission properties of excitons confined in wurtzite / zinc-blende crystal phase quantum wells. We account not only for the Coulomb interaction between electron-hole pairs, but also for the polarization discontinuities at the wurtzite / zinc-blende interfaces. Although our modeling procedure can be applied to any III-V material, we address specifically the case of GaN, as there is, for this material, a general agreement between the zinc-blende / wurtzite valence and conduction band offsets determined experimentally and those obtained theoretically [3]. Our calculations first reveal that contrary to the assumptions made so far, excitons bound to basal stacking faults, three monolayers thick cubic inclusions in wurtzite material, show an increased binding energy compared to the bulk case. We then describe the coupling between adjacent quantum wells and discuss the consequences on the quantum well emission properties of the large spatial extent of the exciton wave function. Finally, comparing the result of our calculations with available experimental data [4] suggests the absence of built-in electric fields in the narrowest quantum wells, which we relate to the overlap between interface charge density regions. [1] R. E. Algra et al., Nature 456, 369 (2008); P. Caroff et al., Nature Nanotechnology 4, 50 (2009). [2] L. Zhang et al., Nano Lett., 10, 4055 (2010); G. Jacopin et al., J. Appl. Phys. 110, 064313 (2011). [3] C. Stampfl et al., Phys. Rev. B 57, R15051 (1998). [4] P. Corfdir et al., J. Appl. Phys. 105, 043102 (2009); P. P. Paskov et al., J. Appl. Phys. 98, 093519 (2005).

While III-V semiconductor nanowires crystallize into a wurtzite geometry, instead of the zinc-blende crystal structure more common in the bulk materials, they usually exhibit along their length large densities of cubic material insertions [1]. Although usually regarded as detrimental to the carrier transport properties, the polytypism of III-V nanowires has recently attracted a huge interest from the scientific community. Planar ZB inclusions in the WZ phase of a III-V semiconductor should behave as shallow type-II quantum wells (QWs), where electrons are confined in the ZB layers. This prediction thus allows for a new kind of bandgap engineering, where the bandgap along a structure depends only on the crystal phase of a single material. Despite the huge progresses in the growth of these so-called crystal phase quantum wells, there have been, so far, only a few attempts at modeling the emission properties of these structures. In addition, up until now, excitonic effects have been systematically neglected, based on the assumption that due to the type-II band alignment between the wurtzite and zinc-blende and phases, the binding energy of the exciton should be small compared to the bulk case [2]. In this work, we therefore compute by envelope function calculations the emission properties of excitons confined in wurtzite / zinc-blende crystal phase quantum wells. We account not only for the Coulomb interaction between electron-hole pairs, but also for the polarization discontinuities at the wurtzite / zinc-blende interfaces. Although our modeling procedure can be applied to any III-V material, we address specifically the case of GaN, as there is, for this material, a general agreement between the zinc-blende / wurtzite valence and conduction band offsets determined experimentally and those obtained theoretically [3]. Our calculations first reveal that contrary to the assumptions made so far, excitons bound to basal stacking faults, three monolayers thick cubic inclusions in wurtzite material, show an increased binding energy compared to the bulk case. We then describe the coupling between adjacent quantum wells and discuss the consequences on the quantum well emission properties of the large spatial extent of the exciton wave function. Finally, comparing the result of our calculations with available experimental data [4] suggests the absence of built-in electric fields in the narrowest quantum wells, which we relate to the overlap between interface charge density regions. [1] R. E. Algra et al., Nature 456, 369 (2008); P. Caroff et al., Nature Nanotechnology 4, 50 (2009). [2] L. Zhang et al., Nano Lett., 10, 4055 (2010); G. Jacopin et al., J. Appl. Phys. 110, 064313 (2011). [3] C. Stampfl et al., Phys. Rev. B 57, R15051 (1998). [4] P. Corfdir et al., J. Appl. Phys. 105, 043102 (2009); P. P. Paskov et al., J. Appl. Phys. 98, 093519 (2005).