We report on transmission electron microscopy and optical characterization of GaN dots embedded in an AlN matrix. GaN dots were grown by molecular beam epitaxy on top of an AlN layer deposited on (0001) sapphire. We found that the GaN dots (average diameter of 16 nm, average height of 4 nm) are coherently grown on AlN, but nucleate next to threading edge dislocations that were propagating in AlN. The presence of these adjacent dislocations does not inhibit the optical emission of the GaN dots: contrary to the photoluminescence of thick GaN layers or GaN/AlGaN quantum wells, the photoluminescence of these GaN dots is found to be temperature independent.

We experimentally investigate the influence of AlN buffer growth on the nucleation and the polarity of a self-organized assembly of GaN nanowires (NWs) grown on Si. Two complementary growth mechanisms for AlN buffer deposited on Si are demonstrated. Both emphasize the aggregation of Si on the AlN surface and the growth of large cubic crystallites, namely, AlN pedestals. Further growths of GaN NWs assembly reveal that the GaN 2D layer found at the bottom of the NW assembly is the result of the coalescence of Ga-polar pyramids, whereas AlN pedestals are observed as preferential but not exclusive NW nucleation sites. NWs are N-polar or exhibit inversion domains with a Ga-polar core/N-polar shell structure. This suggests that N-polarity is a necessary condition to trigger NW self-organized nucleation due to a different facets energy hierarchy between the Ga- and the N-polar sides.

The effects of AlN overgrowth on the structural properties of GaN nanostructures (quantum wells and quantum dots) grown by plasma-assisted molecular beam epitaxy have been investigated using Rutherford backscattering spectroscopy, transmission electron microscopy, and reflection high-energy electron diffraction. The capping process induces a remarkable change in the dimensions of the nanostructures. The overgrowth process implies a thinning of the GaN quantum well and an isotropic reduction of the GaN island size. We demonstrate that this thickness/size reduction affects only the top GaN/AlN interface. The phenomenon is attributed to an exchange mechanism between Al atoms from the cap layer and Ga atoms in the nanostructures. We also demonstrate that this exchange is thermally activated and depends on the strain state of the nanostructures.

It is shown that a two-dimensional GaN layer grown on (0001) AlN under Ga-rich conditions remains two-dimensional while annealing under a Ga flux due to a surfactant effect of Ga. In contrast, further annealing under vacuum without the Ga flux leads to evaporation of excess Ga and to spontaneous transformation of the GaN layer into islands if the initial layer is thicker than about 2.5 monolayers. The resulting morphology is studied by atomic force microscopy and transmission electron microscopy. The latter reveals that these islands sit on top of a continuous 2.5 monolayer thick wetting layer, i.e., they represent a Stranski–Krastanow structure.

Growth of catalyst-free AlN nanowires has been achieved by plasma-assisted molecular beam epitaxy on ${\text{SiO}}_2/\text{Si}$ (100), by taking advantage of Volmer–Weber growth mode of AlN on amorphous ${\text{SiO}}_2$ . Using a combination of high resolution transmission electron microscopy and Raman spectroscopy, it is found that AlN nanowires are completely relaxed, which has been assigned to the compliant character of ${\text{SiO}}_2$ . Elastic strain relaxation of AlN nanowires has been further confirmed by photoluminescence experiments, showing in addition that spectra are dominated by near-band edge emission.

Nitride quantum dots (QD’s) grown in the wurtzite phase present a strong vertical ordering along the (0001) direction when they are stacked in multilayers. This alignment results from a minimum of the elastic energy density at the surface of the AlN capping layer induced by the buried GaN dot underneath. The aim of this work is to investigate the strain distribution in a quantum dot multilayer using high-resolution transmission electron microscopy and anomalous grazing incidence x-ray diffraction. This x-ray method is based on the strong sensitivity of the elastic scattering cross section to the Ga compounds for energies in the vicinity of the Ga absorption edge. It is observed that uncapped GaN dots are almost completely relaxed, while embedded quantum dots are compressively strained. In addition, a modulation of the in-plane lattice parameter in the AlN spacer layer is clearly identified, induced by the QD’s on the surrounding matrix which causes the vertical alignment.