The invention discloses a method to achieve a single-crystalline with high quality semi-polar gallium nitride (GaN) hetro-epitaxial layers grown on m-plane sapphire by metalorganic chemical vapour deposition (MOCVD). The method comprises the steps of treating the m-plane sapphire substrate layer by a hydrogen cleaning process to remove contamination; performing a nitridation process on the substrate layer induces the formation of A1N layers with slanted facets on the surface of the sapphire substrate; depositing a GaN nucleation layer on top of the substrate layer; performing a re-crystallisation process on the GaN nucleation layer to change its crystallographic phase of the GaN growth; depositing a GaN buffer layer on top of the GaN nucleation layer; and depositing GaN overgrowth layer on top of the GaN buffer layer.