Thierry Bretagnon

Barrier composition dependence of the internal electric field in ZnO∕Zn1−xMgxO quantum wells

Time-integrated photoluminescence experiments are used to study the excitonic optical recombinations in wurtzite ZnO∕Zn1−xMgxO single quantum wells of varying widths and magnesium compositions. By comparing experimental results with a variational calculation of excitonic energies, the authors determine the magnitude of the built-in electric field that is induced by both spontaneous and piezoelectric polarizations. It is found that the electric field varies linearly with magnesium composition. By taking into consideration the well-known distribution of electric field among the barrier and the well layers in multiquantum wells, the authors show that their results are fully consistent with previously reported data.

Polariton lasing in a hybrid bulk ZnO microcavity

We demonstrate polariton lasing in a bulk ZnO planar microcavity under non-resonant optical pumping at a small negative detuning (δ ∼ −1/6 the 130 meV vacuum Rabi splitting) and a temperature of 120 K. The strong coupling regime is maintained at lasing threshold since the coherent nonlinear emission from the lower polariton branch occurs at zero in-plane wavevector well below the uncoupled cavity mode. The contribution of multiple localized polariton modes above threshold and the non-thermal polariton statistics show that the system is in a far-from-equilibrium regime, likely related to the moderate photon lifetime and in-plane photonic disorder in the cavity.

Confined Excitons in GaN–AlGaN Quantum Wells

We calculate the original properties of excitons in GaN–AlGaN quantum wells by a variational approach in the envelope function formalism. The separation of electrons and holes by huge internal electric fields induces an enhanced dependence of exciton binding energy and oscillator strength on the well width. We demonstrate the necessity to perform excitonic calculations for obtaining, in particular, reliable values of oscillator strengths (thus radiative lifetimes), which are extremely sensitive to the well width.

High quality factor nitride-based optical cavities: microdisks with embedded GaN/Al(Ga)N quantum dots

We compare the quality factor values of the whispering gallery modes of microdisks (μ-disks) incorporating GaN quantum dots (QDs) grown on AlN and AlGaN barriers by performing room temperature photoluminescence (PL) spectroscopy. The PL measurements show a large number of high Q factor resonant modes on the whole spectrum, which allows us to identify the different radial mode families and to compare them with simulations. We report a considerable improvement of the Q factor, which reflects the etching quality and the relatively low cavity loss by inserting QDs into the cavity. GaN/AlN QDs-based μ-disks show very high Q values (Q>7000) whereas the Q factor is only up to 2000 in μ-disks embedding QDs grown on the AlGaN barrier layer. We attribute this difference to the lower absorption below bandgap for AlN barrier layers at the energies of our experimental investigation.

Photoluminescence energy and linewidth in GaN/AlN stackings of quantum dot planes

We analyze the room temperature photoluminescence properties of several multilayer stackings of GaN/AlN quantum dots. We report drastic differences of emission energies and linewidths between continuous wave and time-resolvedphotoluminescence experiments. In continuous wave experiments, the screening of internal electric fields by accumulation of e-h pairs in quantum dot planes induces a blue-shift as well as an unexpected narrowing of the emission line, when the laser intensity is increased. Under intense, pulsed excitation, in time-resolvedphotoluminescence, a substantial blue-shift is induced, due to the partial cancelation of the quantum confined Stark effect. When the system is again free to relax, we observe a time-dependent red-shift of the line, which maintains a fairly constant width. We attribute the observed behavior of energies and linewidths to the intricate contributions of the in-plane distribution of dot sizes and of the depth-dependent decrease of the degree of excitation of the different planes. We support our interpretations by the use of a model based on a self-consistent solution of the Schrodinger and Poisson equations within the envelope function approximation.

Low temperature reflectivity study of nonpolar ZnO/(Zn,Mg)O quantum wells grown on M-plane ZnO substrates

We report growth of high quality ZnO/Zn0.8Mg0.2O quantum wells on M-plane oriented ZnO substrates. The optical properties are studied by reflectance spectroscopy. The optical spectra reveal strong in-plane optical anisotropies, as predicted by group theory, and clear reflectance structures, as an evidence of good interface morphologies. Signatures of confined excitons built from spin-orbit split-off valence band, analogous to C-exciton in bulk ZnO, are detected using a light polarized along the c-axis. Experiments performed in orthogonal polarization, show confined states analogous to A and B bulk excitons. Envelope function calculations including excitonic interaction nicely match the experimental results.

AlN photonic crystal nanocavities realized by epitaxial conformal growth on nanopatterned silicon substrate

An original method to fabricate III-nitride photonic crystal membranes without etching of III-N materials is reported. A photonic crystal pattern is first realized in a silicon substrate. GaN quantum dots embedded in a thin AlN layer are then grown by conformal epitaxy using ammonia-based molecular beam epitaxy on the top of the patterned silicon substrate and a free-standing membrane is achieved by selective etching of the silicon substrate through the holes of the photonic crystal. Room temperature microphotoluminescence measurements show a quality factor as high as 1800 at 425 nm on a modified L3 cavity. Possibility to achieve lasing with this system is discussed.

Spin-Exchange Interaction in ZnO-based Quantum Wells

Wurtzitic ZnO/(Zn,Mg)O quantum wells grown along the (0001) direction permit unprecedented tunability of the short-range spin exchange interaction. In the context of large exciton binding energies and electron-hole exchange interaction in ZnO, this tunability results from the competition between quantum confinement and giant quantum confined Stark effect. By using time-resolved photoluminescence we identify, for well widths under 3 nm, the redistribution of oscillator strengths between the A and B excitonic transitions, due to the enhancement of the exchange interaction. Conversely, for wider wells, the redistribution is cancelled by the dominant effect of internal electric fields, which dramatically reduce the exchange energy.

Laser emission with excitonic gain in a ZnO planar microcavity

The lasing operation of a ZnO planar microcavity under optical pumping is demonstrated from T=80 to 300 K. At the laser threshold, the cavity switches from the strong coupling to the weak coupling regime. A gain-related transition, which appears while still observing polariton branches and, thus, with stable excitons, is observed below 240 K. This shows that exciton scattering processes, typical of II-VI semiconductors, are involved in the gain process.

Polar and semipolar GaN/Al0.5Ga0.5N nanostructures for UV light emitters

AlxGa1−xN-based ultra-violet (UV) light emitting diodes (LEDs) are seen as the best solution for the replacement of traditional mercury lamp technology. By adjusting the Al concentration, a large emission spectrum range from 360 nm (GaN) down to 200 nm (AlN) can be covered. Owing to the large density of defects typically present in AlxGa1−xN materials usually grown on sapphire substrates, LED efficiencies still need to be improved. Taking advantage of the 3D carrier confinement, quantum dots (QDs) are among the solutions currently under investigation to improve the performances of UV LEDs. The objectives of this work are to present and discuss the morphological and optical properties of GaN nanostructures grown by molecular beam epitaxy on the (0 0 0 1) and the (11–22) orientations of Al0.5Ga0.5N. In particular, the dependence of the morphological properties of the nanostructures on the growth conditions and the surface orientation will be presented. The optical characteristics as a function of the nanostructure design (size, shape and dimensionality) will also be shown and discussed. The electroluminescence characteristics of a first series of QD-based GaN/Al0.5Ga0.5N LEDs grown on the polar (0 0 0 1) plane will be investigated.