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The ability of in situ SiN_x interlayers to lower the density of threading dislocations (TDs) has been studied for the growth of c-plane (0001) GaN epilayers on sapphire by organometallic vapour-phase epitaxy (OMVPE). The TD density in the films may be reduced by up to a factor of 50 to 9×10⁷cm⁻² and depends on the SiN_x coverage and the conditions of the overgrowth. The TD reduction method relies on the formation of facetted islands on the SiN_x-treated GaN surface and the formation of dislocation half-loops between bent-over TDs during the lateral overgrowth. Dislocations that are not annihilated at the interfacial region during the interlayer overgrowth may bend over at the islands’ inclined side facets and annihilate at their coalescence boundaries. Thus, the TD density was reduced at the expense of greater film thickness by increasing the SiN_x coverage and delaying intentionally the coalescence of the GaN …