jats:titleAbstract</jats:title> jats:pThe suitability of Aljats:italic jats:subx</jats:sub> </jats:italic>Gajats:sub1−jats:italicx</jats:italic> </jats:sub>N nucleation layers (NLs) with varying Al fraction jats:italicx</jats:italic> for the metal organic vapour phase epitaxy of zincblende GaN on (001) 3C-SiC was investigated, using x-ray photoelectron spectroscopy, atomic force microscopy, and x-ray diffraction. The as-grown NLs exhibited elongated island structures on their surface, which reduce laterally into smaller, more equiaxed islands with increasing AlN composition. During high-temperature annealing in a mixture of NHjats:sub3</jats:sub> and Hjats:sub2</jats:sub> the nucleation islands with low Al fraction ripened and increased in size, whereas this effect was less pronounced in samples with higher Al fraction. The compressive biaxial in-plane strain of the NLs increases with increasing AlN composition up to jats:italicx</jats:italic> = 0.29. GaN epilayers grown over NLs that have low Al fraction have high cubic zincblende phase purity and are slightly compressively strained relative to 3C-SiC. However, those samples with a measured Al fraction in the NL higher than 0.29 were predominantly of the hexagonal wurtzite phase, due to formation of wurtzite inclusions on various {111} facets of zb-GaN, thus indicating the optimal Al composition for phase-pure zb-GaN epilayer growth.</jats:p>