A method for preparing a thin layer of GaN from a starting substrate in which at least one thick surface area extending along a free face of the starting substrate includes GaN, where the method includes bombarding the free face of the substrate with helium and hydrogen atoms, the helium being implanted first into the thickness of the thick surface area and the hydrogen being implanted thereafter, and where the helium and hydrogen doses each vary between 1.1017 atoms/cm2 and 4.1017 atoms/cm2. The starting substrate is subjected to a rupture process in order to induce the separation, relative to a residue of the starting substrate, of the entire portion of the thick area located between the free face and the helium and hydrogen implantation depth. The helium is advantageously implanted in a dose at least equal to that of hydrogen, and can also be implanted alone.