Disclosed is a method for forming a buffer layer for growing gallium nitride single crystals on a sapphire substrate using hydride vapor phase epitaxy (HVPE), wherein the buffer layer is formed in the form of a doped vertical gallium nitride (GaN) single crystal film with a nanoporosity of 0.10 to 0.15 μm on the sapphire substrate by reacting HCl and NH3 as a Group III/V mix gas.The nanoporous buffer layer interposed on the interface between the sapphire substrate and gallium nitride reduces tensile stress generated by the difference in thermal expansion coefficient between gallium nitride and the sapphire substrate, enables growth of the gallium nitride layer to a thickness of 1 micrometer (μm) to several millimeters (mm) without causing cracks, and reduces the lattice constant difference to improve crystallinity.