Contrary to c-plane heterostructures, the absence of built-in electric fields in non-polar nitride-based heterostructures allows for the growth of thick quantum wells (QWs) without any decrease in the overlap between electron and hole wave functions [1]. However, lattice mismatched substrates are generally used to grow non-polar GaN, inducing strain in the heteroepitaxial layers that relax through the generation of non-radiative recombination centers. Here, we investigate by time-resolved photoluminescence the temperature (T) dependence of charge carriers recombination mechanisms in non-polar (Al,Ga)N/GaN quantum wells (QW) grown by molecular beam epitaxy on the a-facet of bulk GaN crystals. We study the influence of both QW width and barrier Al-content on the dynamics of excitons in the 10 - 320 K range.

Microcavity polaritons attract nowadays a lot of interest from the scientific community as they allow for the realization of low-threshold coherent light emitters that operate up to 300 K. It is usually admitted that, at low-temperature and negative cavity detunings, the mechanism limiting the polariton lasing threshold is the relaxation of polaritons towards the bottom of the lower-polariton branch, which is mediated by scattering with acoustic phonons. However, it has been theoretically predicted that in multi quantum well (QW) microcavities, dark exciton and biexciton states should play an important role in the overall relaxation mechanisms of cavity polaritons. Here, we address this issue through a time-resolved photoluminescence (TR-PL) study of the relaxation dynamics at 10 K of microcavity polaritons in a 3!-cavity containing 67 Al0.2Ga0.8N/GaN QWs. The vacuum Rabi splitting of our microcavity is 60 meV and cavity detunings (!) ranging between -100 and 0 meV are investigated. At 10 K, the microcavity PL at small angle is dominated by a transition at 3.63 eV attributed to lower polaritons with a small in-plane wave vector. Two additional non-dispersive lines at 3.673 and 3.693 eV are also observed. From excitation/power-dependent experiments, we attribute them to the recombination of cavity biexcitons and localized dark excitons, respectively. We show by TR-PL that the lower polariton PL at zero-angle exhibits at all times the same PL decay as biexcitons. From this dynamical behavior, together with the !-dependence of the biexciton emission energy, we can describe the interplay between polaritons and biexcitons. In particular, we demonstrate that in nitride-based multi QW microcavities, the mechanism limiting the relaxation of polaritons towards the center of the Brillouin zone is the radiative dissociation of cavity biexcitons into lower polaritons with high in-plane wave vector.

Ordered GaN-based nanocolumnar structures are very appealing for optoelectronic applications like LEDs due to their homogenous size and length distribution which facilitates the fabrication of devices. One very important issue for LED applications is the incorporation of InGaN structures in the ordered GaN nanocolumns (NCs) to produce emission in the visible optical region. Due to the lower growth temperature of InN, caused by the much weaker bonding of indium and nitrogen, the growth of InGaN structures has to be performed at a much lower temperature (700ºC) than the one "traditionally" used for the growth of ordered GaN nanocolumns (around 900ºC). This fact poses a big problem at the time of combining these two materials while maintaining the nanocolumnar growth morphology. This work reports on the selective area growth of GaN on a Ti-mask using lower substrate temperatures than normally used for SAG growth of GaN on a Ti-mask and nominally nitrogen rich conditions. For this purpose, a two step growth procedure is followed: (1) growth of ordered GaN NCs under "traditional" conditions, namely at 900 ºC and nominally under Ga-rich conditions and (2) change of the growth conditions to a substrate temperature of 840 ºC and nominally N-rich conditions. It will be shown that when growing at lower temperature it is essential to adjust the III/V ratio in order to preserve the nanocolumnar structure. Additionally we will show that this method presents a way of selective area growth of GaN NCs with negligible lateral growth, which is difficult to obtain using "traditional" ordered growth conditions, while enhancing the vertical growth rate.

Selective area growth (SAG) of GaN nanocolumns (NCs), making use of patterned or masked (nanoholes) substrates, yields a periodic, homogeneous distribution of nanostructures, that makes their processing much easier compared with self-assembled ones. In addition, the control on the diameter and density of NCs avoids dispersion in the electrooptical characteristics of the heterostructures based on this type of material (embedded InGaN/GaN quantum disks for example). Selective area growth using a mask with nanohole arrays has been demonstrated by rf-plasma-assisted MBE [1, 2]. Although some works have recently been published regarding SAG heterostructures [3], the control of the morphology of the NC is still an open question (i.e. lateral growth rate, topmost shape, geometry of the quantum disk, etc.). In the present work we report on the control of the topmost shape of the NC by adjusting the substrate temperature (and therefore the GaN decomposition and Ga desorption rates) leading to different types of geometries: from pyramidal islands, to nanocolumns with different topmost shapes (semi-polar planes, like those shown in Figures 1.b, or c-planes like the ones in Figures 1.c & 1.d). These different geometries will give quite different results when growing NCs with InGaN QDiscs embedded.

Self-organised nanocolumnar systems based on group-III nitride semiconductors present numerous original characteristics, of which the absence of extended defects is certainly the most promising for optoelectronic applications. Nevertheless, another feature, namely the large surface/volume ratio of nanocolumns with typical diameters in the 20-60 nm range, deserves further investigation. We focus for this talk on luminescence properties of GaN and InGaN nanocolumns grown by plasma-assisted molecular beam epitaxy on silicon (111) substrates. Two cases of surface-related properties will be addressed: 1) The origin of the photoluminescence doublet at 3.450-3.456 eV will be discussed in terms of the particular physics of donor-bound excitons, when donor nuclei sit within a layer of ~10 nm from the side surface of a GaN nanocolumn. Detailed studies of power- and temperature-dependent PL spectroscopy will be analyzed. 2) Changes in photoluminescence properties of nitride nanocolumnar systems placed in various environments (vacuum, air...) will be presented and discussed in terms of photochemical effects and their consequences on the dynamics of excitonic recombination.