[en] First-principles calculations relating to the atomic structure and electronic properties of {1 0 1-bar 3} GaN surfaces reveal significant differentiations between the two polarity orientations. The (1 0 1-bar 3) surface exhibits a remarkable morphological stability, stabilizing a metallic structure (Ga adlayer) over the entire range of the Ga chemical potential. In contrast, the semiconducting, cleaved surface is favoured on (111-bar 3-bar) under extremely and moderately N-rich conditions, a Ga bilayer is stabilized under corresponding Ga-rich conditions and various transitions between metallic reconstructions take place in intermediate growth stoichiometries. Efficient growth schemes for smooth, two-dimensional GaN layers and the isolation of {1 0 1-bar 3} material from parasitic orientations are identified.

[en] Highlights: ► Identification of misfit dislocations (MD) in-plane configuration in InN/GaN interfaces. ► Energetic mapping designates that MD arrays adopt 〈1 1 −2 0〉 line directions with b = 1/3〈2 −1 −1 0〉. ► Local arrangement of the Moiré fringes depends strongly on the thickness of the TEM foil as revealed by HRTEM image simulations. ► Geometric Phase Analysis on simulated images justifies results obtained by energetic mapping. - Abstract: The enhanced structural mismatch of InN and GaN binary alloys leads in almost spontaneous formation of misfit dislocations (MDs) at the corresponding interfaces, thereby accommodating plastic relaxation. The open issue of the MD array in-plane configuration is addressed through a combination of high resolution transmission electron microscopy (HRTEM) observations with energetic mapping and HRTEM image simulation of InN/GaN interfaces extracted by atomistic modeling. Energetic mapping on the interfacial area of InN/GaN supercells relaxed by the Tersoff interatomic potential, designates that the MD arrays adopt 〈112^¯0〉 line directions and their Burgers vectors are b=1/3〈21^¯1^¯0〉. HRTEM image simulations further reveal that the local arrangement of Moiré fringes observed in these interfaces depends strongly on the thickness of the TEM foil, thus resolving contradictory experimental reports. Geometric Phase Analysis on the simulated images justifies the results obtained by energetic mapping.

[en] GaN quantum dots (QDs) grown in semipolar (1122) AlN by plasma-assisted molecular-beam epitaxy were studied by transmission electron microscopy (TEM) and scanning transmission electron microscopy techniques. The embedded (1122)-grown QDs exhibited pyramidal or truncated-pyramidal morphology consistent with the symmetry of the nucleating plane, and were delimited by nonpolar and semipolar nanofacets. It was also found that, in addition to the (1122) surface, QDs nucleated at depressions comprising (1011) facets. This was justified by ab initio density functional theory calculations showing that such GaN/AlN facets are of lower energy compared to (1122). Based on quantitative high-resolution TEM strain measurements, the three-dimensional QD strain state was analyzed using finite-element simulations. The internal electrostatic field was then estimated, showing small potential drop along the growth direction, and limited localization at most QD interfaces.