The effects of an amorphous interfacial silicon nitride (SixNy) layer on the morphology, structure and optoelectronic properties of GaN nanowires (NWs), grown on Si (111) substrates by plasma assisted molecular beam epitaxy, have been investigated. The unintentional Si nitridation, during the first stages of direct GaN NW growth on the bare Si surface, has been compared to intentional Si nitridation prior to GaN growth. The intentional nitridation resulted in a uniform similar to 1.5 nm amorphous SixNy interlayer at the GaN/Si interface, while an irregular and non-uninform interface, with partial presence of amorphous SixNy appeared for direct GaN on Si growth. The homogeneity of the interfacial structure enhanced the degree of crystallographic alignment of the GaN NWs, concerning both tilt and twist. It also decreased the dispersion of NW heights that is otherwise triggered by different nucleation times on structurally different sites of the substrate. The average height of the NWs was similar for both cases but their average diameter was increased from 25 nm to 40 nm on the uniform amorphous SixNy interlayer, possibly an effect of weak epitaxial constraints. Reduced overall intensity and increased defect-related emission at 3.417 eV characterized the 20 K photoluminescence spectra for direct GaN growth on Si. The results contribute to a better understanding of how the GaN/Si interfacial structure affects the GaN NW growth and properties. (C) 2016 Elsevier B.V. All rights reserved.