We present overgrowth of nano-patterned sapphire with different offcut angles by metalorganic vapor phase epitaxy. Hexagonal arrays of nano-pillars were prepared via Displacement Talbot Lithography and dry-etching. 6.6 µm crack-free and fully coalesced AlN was grown on such substrates. Extended defect analysis comparing X-ray diffraction, electron channeling contrast imaging and selective defect etching revealed a threading dislocation density of about 10 9 cm -2 . However, for c-plane sapphire offcut of 0.2° towards m direction the AlN surface shows step bunches with a height of 10 nm. The detrimental impact of these step bunches on subsequently grown AlGaN multi-quantum-wells is investigated by cathodoluminescence and transmission electron microscopy. By reducing the sapphire offcut to 0.1° the formation of step bunches is successfully suppressed. On top of such a sample an AlGaN-based UVC LED heterostructure is realized emitting at 265 nm and showing an emission power of 0.81 mW at 20 mA (corresponds to an external quantum efficiency of 0.86 %).

Si doping of (Al,Ga)N layers grown by metalorganic chemical vapor deposition induces an inclination of threading dislocations (TDs). This inclination leads to a change of the extra half-plane size of edge and mixed type dislocations. Depending on the dislocation density and the doping concentration, these effects are accompanied by the generation of tensile strain, which can also lead to crack formation. Several models have been published in the past in order to explain this process. Different models result in opposite TD inclination directions with respect to the extra half-plane position. Therefore, this work examines the correlation between the extra half-plane position and the inclination direction to clarify the origin of the tensile strain increase using scanning transmission electron microscopy. With this approach, it can be unambiguously experimentally verified that Si doping leads to a shortening of the dislocations half-plane. An analysis of in situ wafer curvature measurement proves that the increase of tensile strain in GaN caused by Si doping can be explained by this process. Aside from the inclination caused by Si doping, a TD inclination in undoped GaN layers has been analyzed. Possible explanations for the inclination process are discussed.

The initial interaction of water with semiconductors determines the electronic structure of the solid-liquid interface. The exact nature of this interaction is, however, often unknown. Here, we study gallium phosphide-based surfaces exposed to H 2 O by means of in situ reflection anisotropy spectroscopy. We show that the introduction of typical imperfections in the form of surface steps or trace contaminants not only changes the dynamics of the interaction, but also its qualitative nature. This emphasises the challenges for the comparability of experiments with (idealised) electronic structure models for electrochemistry.