Les boîtes quantiques GaN présentent des propriétés de confinement très attrayantes, jusqu'à 300K. De nombreux développements récents ont montré qu'elles peuvent se comparer favorablement aux boîtes quantiques antérieures et mieux maîtrisées, InAs et CdTe en particulier. Mais l'étude de la structure fine des complexes excitoniques dans les boîtes GaN n'a jusqu'à présent pas été abordée. Nous présentons les propriétés de polarisation, à basse température, de boîtes isolées GaN/AlN. La photoluminescence est fortement polarisée linéairement, jusqu'à 90%, suivant des directions non corrélées aux axes cristallins. Nous n'observons pas de dédoublement du doublet radiatif. Nous interprétons ces résultats dans le cadre d'un modèle matriciel, incluant toutes les symétries pertinentes, qui permet une comparaison entre les systèmes GaN, InAs et CdTe. Nous montrons que la forte polarisation est liée à la proximité des bandes de valence A et B de GaN, et nous proposons que l'absence de doublet soit la signature d'un fort splitting de structure fine, ou bien de l'émission d'excitons chargés.

We present numerical simulations of angle-resolved reflectivity of bulk ZnO microcavities with SiN/SiO2 distributed Bragg reflectors. The goal of these studies is to determine the conditions for observation of strong exciton-photon coupling at room temperature in such systems, which has been reported very recently. Indeed ZnO microcavities present a very specific hierarchy of interactions compared to GaAs or II-VI microcavities : the awaited Rabi splitting is larger than the Rydberg energy for excitons, which is larger than the splitting between the A,B,C valence bands. In our model the coupling of both fundamental and excited states of the optically active free excitons (XA, XB, XC) with the cavity mode is taken into account. The specific values of oscillator strength and excitonic binding energy in ZnO require that we include the above band gap absorption continuum. We do so by using Elliott's model of the dielectric function. We neglect disorder and inhomogeneous broadening in our calculations. The reflectivity spectra of the microcavity is calculated by a transfer matrix model, where either the ZnO thickness or the angle is varied in order to calculate the dispersion of the polaritonic branches. Those are compared to the dispersions obtained from a quasi-particle model which includes the excitonic continuum. The expansion coefficients of the polaritonic modes upon the photon-exciton basis are determined, as well as their coherence time. We show that the lower polariton branch is a well-defined and well-mixed exciton-photon state for low angles and/or negative detunings. The energy splitting induced by the strong oscillator strength in ZnO can be larger than the exciton binding energy, thus pushing the upper polariton branch into the absorption continuum and leading to a large intrinsic broadening. Therefore experimental evidence of strong coupling regime in bulk ZnO microcavities and its interpretation are difficult. Finally, as a criterion of strength of the strong coupling, we compute the ratio of the splitting between polariton branches to the polaritonic mode homogeneous broadening, for different detunings.

We have investigated a series of samples embedding ZnO/(Zn,Mg)O quantum wells of different sizes, in wurtzite phase, by using timeresolved photoluminescence. The samples were grown by molecular beam epitaxy on ZnO templates, themselves deposited on sapphire substrates. The presence of large internal electric fields in these quantum wells manifests itself not only through the energies of the optical recombinations, but also through the size dependence of the recombination times. An envelope-function model that includes the variational calculation of the exciton binding energy allows us to determine a value of 0.9 MV/cm for the internal electric field.

Wide bandgap semiconductors are attractive candidates for polariton-based devices operating at room temperature. We present numerical simulations of reflectivity, transmission and absorption spectra of bulk GaAs, GaN and ZnO microcavities, in order to compare the particularities of the strong coupling regime in each system. Indeed the intrinsic properties of the excitons in these materials result in a different hierarchy of energies between the valence-band splitting, the effective Rydberg and the Rabi energy, defining the characteristics of the exciton-polariton states independently of the quality factor of the cavity. The knowledge of the composition of the polariton eigenstates is central to optimize such systems. We demonstrate that, in ZnO bulk microcavities, only the lower polaritons are good eigenstates and all other resonances are damped, whereas upper polaritons can be properly defined in GaAs and GaN microcavities.

We report micro-photoluminescence studies of single GaN/AlN quantum dots grown along the (0001) crystal axis by molecular beam epitaxy on Si(111) substrates. The emission lines exhibit a linear polarization along the growth plane, but with varying magnitudes of the polarization degree and with principal polarization axes that do not necessarily correspond to crystallographic directions. Moreover, we could not observe any splitting of polarized emission lines, at least within the spectral resolution of our setup (1 meV). We propose a model based on the joint effects of electron-hole exchange interaction and in-plane anisotropy of strain and/or quantum dot shape, in order to explain the quantitative differences between our observations and those previously reported on, e.g. CdTe- or InAs-based quantum dots.

We report on an investigation of superconductivity in n-type InN. There is an optimum carrier density for the occurrence of the superconductivity. The lowest carrier density is limited by the Mott transition of ne~2×1017cm−3 and the highest density is limited by the superconductor to metal transition of ne~5×1020cm−3. As in the layered cuprate high Tc superconductors, the vortex solid of InN is melted by thermal fluctuation and by the external magnetic field. The field dependence of the critical current Jc exhibits an exponential decay over a wide range of magnetic field, which is ascribed to the current tunneling through micro Josephson-junctions. We propose a mechanism where the anisotropy of Jc is related to the presence of In-In chains of finite length in the ab plane.