The influence of template type and residual strain of the buffer layer on the structural properties of GaN/AlN superlattices (SLs) was studied using high resolution x-ray diffraction. Using sapphire substrates, an effective thinning of the GaN quantum wells and the corresponding thickening of the AlN barriers were observed in SL structures grown on thin, strained AlN templates as compared with SL structures grown on thick, relaxed GaN templates. Moreover, a bimodal strain relaxation of SL structures in dependence of template type was observed. The SLs grown on AlN templates relax predominantly by the formation of misfit dislocations, while the SLs grown on GaN templates relax predominantly by cracking of the layers. We explain these effects by the influence of residual strain in the buffer/template systems used for the growth processes of SL layers. A correlation is made between the strain state of the system and the cracking processes, the dislocation density, the radius of curvature and the layer thickness.
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