Both weakly and strongly confined excitons are studied by time-resolved photoluminescence in a nonpolar nitride-based heterostructure grown by molecular beam epitaxy on the a-facet of a bulk GaN crystal, with an ultralow dislocation density of 2 × 105 cm−2. Strong confinement is obtained in a 4 nm thick Al0.06Ga0.94N/GaN quantum well (QW), whereas weakly confined exciton-polaritons are observed in a 200 nm thick GaN epilayer. Thanks to the low dislocation density, the effective lifetime of strongly confined excitons increases between 10 and 150 K, proving the domination of radiative recombination processes. Above 150 K theQWemission lifetime diminishes, whereas the decay time of excitons in the barriers increases, until both barrier and QW exciton populations become fully thermalized at 300 K. We conclude that the radiative efficiency of our GaN QW at 300 K is limited by nonradiative recombinations in the barriers. The increase of exciton-polariton coherence lengths caused by low dislocation densities allows us to observe and model the quantized emission modes in the 200 nm nonpolar GaN layer. Finally, the low-temperature phonon-assisted relaxation mechanisms of such center-of-mass quantized exciton-polaritons are described.

In this study, we present the optical properties of nonpolar GaN/(Al,Ga)N single quantum wells (QWs) grown on either a- or m-plane GaN templates for Al contents set below 15%. In order to reduce the density of extended defects, the templates have been processed using the epitaxial lateral overgrowth technique. As expected for polarization-free heterostructures, the larger the QW width for a given Al content, the narrower the QW emission line. In structures with an Al content set to 5 or 10%, we also observe emission from excitons bound to the intersection of I1-type basal plane stacking faults (BSFs) with the QW. Similarly to what is seen in bulk material, the temperature dependence of BSF-bound QW exciton luminescence reveals intra-BSF localization. A qualitative model evidences the large spatial extension of the wavefunction of these BSF-bound QW excitons, making them extremely sensitive to potential fluctuations located in and away from BSF. Finally, polarization-dependent measurements show a strong emission anisotropy for BSF-bound QW excitons, which is related to their one-dimensional character and that confirms that the intersection between a BSF and a GaN/(Al,Ga)N QW can be described as a quantum wire.

Modern LEDs are based on nanometric semiconductor structures such as quantum wells or quantum dots. Playing with their composition and sizes, we can adjust the light emission wavelength, while improving the radiative efficiency. We will explain how the knowledge of quantum mechanics allows us to tailor the optical properties of these light-emitting systems. We will focus specifically on the case of nanostructures based on group III nitride semiconductors (GaN, InGaN, AlGaN). These materials indeed permit to emit light covering the entire visible spectrum and they therefore emerged in the last decade as systems of choice for applications such as domestic lighting, automotive headlights,... We will discuss the practical limitations of performances encountered with these materials and how these limitations can be suppressed, thanks -again- to an appropriate application of quantum mechanics.

The growth of spontaneous (catalist-free) self-assembled and ordered (localized) III-Nitride nanocolumns by Molecular Beam Epitaxy is addressed. A description of the nucleation process and the nanocolumn growth is given in relation to the growth conditions and the substrate/buffer used. Nucleation proceeds via Volmer-Weber or Stranski-Krastanov mechanisms depending on the initial lattice mismatch between the nanocolumns material and substrate. It is experimentally observed that a high initial mismatch (i.e. GaN on bare Si) leads to isolated, well defined nanocolumns, whereas a smaller one (i.e. GaN on AlN buffered Si) produces a mixture of nanocolumns and a "faceted matrix" of continuous, rough material. Polar wurtzite InN nanocolumns have been grown along the c-plane, on Si(111) and Si(100) substrates. In addition, non-polar InN nanocolumns have also been grown along the a-plane on m-plane GaN/r-plane Al2O3 templates. In this case case, a well defined a-plane orientation is determined by HR-TEM and from the dependence of the E2 Raman mode with the polarization angle. HR-SEM pictures reveal top nanocolumn surfaces showing a- and m- plane facets. PL measurements at low temperature give a emission energy of 0.69 eV, a typical signature of a very high quality material. Though self-assembled nanocolumns are quite useful to understand the basic growth mechanisms and material properties, the growth of ordered arrays of nanocolumns is essential to fabricate real devices, like LEDs and detectors, because of the need of homogeneous geometry and characteristics of individual nanodevices. Details will be given on the use of thin Ti masks with nanoholes of different diameter and pitch and on the specific conditions for localized growth, far from those to grow self-assembled nanocolumns. Different approaches of ordered arrays will be revised with emphasis on the applications to very efficient optoelectronic devices, like phosphor-free white LEDs, and multijunction solar cells.

Since Waltereit et al. demonstrated ten years ago the realization of polarization free (Al,Ga)N/GaN quantum wells (QW) a lot of work has been carried out on non-polar nitride-based heterostructures. The current interest in non-polar GaN is indeed motivated by the absence of built-in electric fields, allowing the growth of wide QWs without any decrease in the overlap between electron and hole wave functions. However, the use of non lattice-matched substrates induces strain that relaxes through the generation of dislocations or stacking faults. The dynamics of QW excitons is then dominated by their trapping on these extended defects. In this work, we have adopted a different approach consisting in the homoepitaxial growth of a 4 nm-thick non-polar Al0.06Ga0.94N/GaN single QW. We first show that the resulting heterostructure is free of stacking fault and presents a low dislocation-density that we estimate to 2.105 cm-2. From 10 to 150 K, the QW effective decay time increases from 110 to 350 ps, evidencing the absence of non-radiative recombination channel in this temperature range. The temperature dependence of exciton decay time is instead ascribed to the combined delocalization of excitons bound to interface roughness and to the thermal distribution of free excitons in k-space. From these results, we deduce an oscillator strength of 2.1013 / cm2 for QW free excitons, in agreement with values reported for thin polar (Al,Ga)N/GaN MQWs where the influence of electric fields is negligible.

Exciton relaxation and recombination mechanisms in a-plane GaN probed by time-resolved cathodoluminescence.