Method for separating group 13 element nitride layer, and composite substrate

A method of separating a layer of a nitride of a group 13 element:

• the method comprising the steps of;
• preparing a composite substrate comprising a sapphire substrate and said layer of said nitride of said group 13 element provided on said sapphire substrate, said layer of said nitride of said group 13 element comprising gallium nitride, aluminum nitride or gallium aluminum nitride, and said composite substrate satisfying the following formulas (1), (2) and (3);
• and
• irradiating a laser light to said composite substrate from the side of said sapphire substrate to decompose crystal lattice bonds at an interface between said sapphire substrate and said layer of said nitride of said group 13 element,
• wherein the formula (1) is 5.0≤(an average thickness (μm) of said layer of said nitride of said group 13 element/ a diameter (mm) of said sapphire substrate)≤10.0,
• wherein the formula (2) is 0.1≤a warpage (mm) of said composite substrate×(50/a diameter (mm) of said composite substrate)2≤0.6,
• wherein the formula (3) is 1.10≤a maximum value (μm) of a thickness of said layer of said nitride of said group 13 element / a minimum value (μm) of said thickness of said layer of said nitride of said group 13 element.