The formation of a self-limited metallic bilayer is reported during the growth of GaN by plasma-assisted molecular beam epitaxy on graphene on (0001) SiC. Depending on growth conditions, this layer may consist of either Ga or In, which gets intercalated between graphene and the SiC surface. Diffusion of metal atoms is eased by steps at SiC surface and N plasma induced defects in the graphene layer. Energetically favorable wetting of the (0001) SiC surface by Ga or In is tentatively assigned to the breaking of covalent bonds between (0001) SiC surface and carbon buffer layer. As a consequence, graphene doping and local strain/doping fluctuations decrease. Furthermore, the presence of a metallic layer below GaN opens the way to the development of devices with a spontaneously formed metallic electrode on their back side.

We have shown that both the morphology and elongation mechanism of GaN nanowires homoepitaxially grown by plasma-assisted molecular beam epitaxy (PA-MBE) on a [0001]-oriented GaN nanowire template are strongly affected by the nominal gallium/nitrogen flux ratio as well as by additional Ga flux diffusing from the side walls. Nitrogen-rich growth conditions are found to be associated with a surface energy-driven morphology and reduced Ga diffusion on the (0001) plane. This leads to random nucleation on the (0001) top surface and preferential material accumulation at the periphery. By contrast, gallium-rich growth conditions are characterized by enhanced Ga surface diffusion promoting a kinetically driven morphology. This regime is governed by a potential barrier that limits diffusion from the top surface toward nanowire side walls, leading to a concave nanowire top surface morphology. Switching from one regime to the other can be achieved using the surfactant effect of an additional In flux. The optical properties are found to be strongly affected by growth mode, with point defect incorporation and stacking fault formation depending on gallium/nitrogen flux ratio.

The spontaneous growth of GaN nanowires in absence of catalyst is controlled by the Ga flux impinging both directly on the top and on the side walls and diffusing to the top. The presence of diffusion barriers on the top surface and at the frontier between the top and the sidewalls, however, causes an inhomogeneous distribution of Ga adatoms at the nanowire top surface resulting in a GaN accumulation in its periphery. The increased nucleation rate in the periphery promotes the spontaneous formation of superlattices in InGaN and AlGaN 2 nanowires. In the case of AlN nanowires, the presence of Mg can enhance the otherwise short Al diffusion length along the sidewalls inducing the formation of AlN nanotubes. I.

The growth of GaN on graphene by molecular beam epitaxy was investigated. The most stable epitaxial relationship, i.e. [00.1]-oriented grains, is obtained at high temperature and Nrich conditions, which match those for nanowire growth. Alternatively, at moderate temperature and Ga-rich conditions, several metastable orientations are observed at the nucleation stage, which evolve preferentially towards [00.1]-oriented grains. The dependence of the nucleation regime on growth conditions was assigned to Ga adatom kinetics. This statement is consistent with the calculated graphene/GaN in-plane lattice coincidence and supported by a combination of transmission electron microscopy, X-ray diffraction, photoluminescence, and Raman spectroscopy experiments.