The present invention relates to a method for producing a modified surface of a substrate that stimulates the growth of epitaxial layers of group-III nitride semiconductors with substantially improved structural perfection and surface flatness. The modification is conducted outside or inside a growth reactor by exposing the substrate to a gas-product of the reaction between hydrogen chloride (HCl) and aluminum metal (Al). As a single-step or an essential part of the multi-step pretreatment procedure, the modification gains in coherent coordination between the substrate and group-III nitride epitaxial structure to be deposited. Along with epilayer, total epitaxial structure may include buffer inter-layer to accomplish precise substrate-epilayer coordination. While this modification is a powerful tool to make high-quality group-III nitride epitaxial layers attainable even on foreign substrates having polar, semipolar and nonpolar orientation, it remains gentle enough to keep the surface of the epilayer extremely smooth. Various embodiments are disclosed.