High Al content (Al,G)aN is the most promising material system to produce deep UV light-emitting diodes and lasers and has prospects to realize high power electronics. Growth of these layers on native AlN substrates with low dislocation densities holds the promise to realize devices with high external quantum efficiency. Recent results, however, show despite high structural perfection large hexagonal hillocks, with a lateral extent of several µm form, depending on the film thickness. In this paper we combine AFM, defect selective etching, CL, TEM and nano-beam scanning X-ray diffraction to experimentally disentangle the mutual influence of surface steps, dislocations, strain, and composition on the forming surface morphology. To rationalize our experimental findings, we develop a theoretical model that accounts for the adatom kinetics of Al and Ga on the growth surface in the presence of a-type dislocations.
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