Growth, structural and optical properties of GaN/AlN and GaN/GaInN nanowire heterostructures

After discussing the GaN NW nucleation issue, we will present the structural properties of axial and radial (i.e. core/shell) GaN/AlN NW heterostructures and adress the issue of critical thickness during the growth of such heterostructures. Next, we will present the growth of InGaN NWs on a GaN NW base. It will be shown that the morphology and structural properties of the InGaN NW sections depend on the In content: for high In content a flat top is observed and plastic relaxation is occuring, with mismatch dislocations formed at the InGaN/GaN interface. By contrast, for In content below 25% InGaN NWs exhibit a pencil-like shape assigned to a purely elastic strain relaxation process, without formation of dislocations. Such a strain relaxation process was found to be associated with the spontaneous formation of an InGaN/GaN core-shell heterostructure. As concerns optical properties, it will be shown that the purely elastic relaxation process is associated with a marked In clustering. As a consequence, for samples with a 17% nominal In content, data were found to exhibit a S-shape characteristic of localization, with a minimum around 110K. By contrast, for a 40% In content PL data suggest that localization is much weaker. Finally, a growth model of InGaN/GaN heterostructures will be proposed.