Method for forming identification marks on refractory material single crystal substrate, and refractory material single crystal substrate

A method for forming an identification mark on a refractory material single crystal substrate, the refractory material single crystal substrate being made of a single crystal which is formed by one selected from the group consisting of sapphire, gallium nitride, aluminum nitride, diamond, boron nitride, zinc oxide, gallium oxide, and titanium dioxide, the method comprising:

• (a) scanning a principal surface of the refractory material single crystal substrate with a laser beam at a first energy density such that a groove is formed in the principal surface of the refractory material single crystal substrate, thereby forming an identification mark which is constituted of one or more grooves in the principal surface of the refractory material single crystal substrate;
• and
• (b) scanning an inside of the groove formed in the principal surface of the refractory material single crystal substrate with a laser beam at a second energy density that is lower than the first energy density.