Thesis presented October 25, 2013

Abstract:
This work focuses on structural and optical properties of III-nitrides wide-band gap semiconductors (Al_xGa_1-xN et h-BN), emitting in the ultraviolet range (4-6 eV). Nano-objects properties being modified by dimensional reduction, this work was mostly focused on the study of nanostructures of these materials (AlN and Al_xGa_1-xN nanowires, BN nanotubes and nanosheets). Careful search for correlation between their structure and luminescence has also been carried out. Concerning Al_xGa_1-xN materials, nanowires have been grown by plasma-assisted molecular beam epitaxy. The use of GaN nanowires bases has allowed us to promote the growth of non-coalesced 1D Al_xGa_1-xN nanostructures. We have shown that the incorporation of gallium is very temperature-dependent, giving rise to nanowires made of a highly inhomogeneous alloy at several scales (from nanometer to a hundred nanometers). These inhomogeneities strongly influence the optical properties, dominated by localized states. Altogether these results allow us to propose a growth mechanism of these nanowires. Concerning BN materials, comparison of the properties of nanostructures with those of the bulk material (hexagonal BN) has been carried out. After that h-BN bulk has been further investigated, we have revealed that nanosheets with more than 6 monolayers present a luminescence similar to h-BN. This indicates a low influence of dimensional reduction in h-BN, contrary to the case of nanowires made of other nitrides. Finally we have shown that the main nanotubes investigated in this work, which are multiwall, have a complex structure that is micro-faceted, and that the defects are likely responsible of the observed luminescence.

Keywords:
Nanowires, Nanotubes, Nitride semiconductors, Algan, BN, Molecular beam epitaxy, Photoluminescene, Cathodoluminescence, Electron microscopy

On-line thesis.