Provided is a method for fabricating a wafer product including an active layer grown on a gallium oxide substrate and allowing an improvement in emission intensity. In step S105, a buffer layer 13 comprised of a Group III nitride such as GaN, AlGaN, or AlN is grown at 600 Celsius degrees on a primary surface 11a of a gallium oxide substrate 11. After the growth of the buffer layer 13, while supplying a gas G2, which contains hydrogen and nitrogen, into a growth reactor 10, the gallium oxide substrate 11 and the buffer layer 13 are exposed to an atmosphere in the growth reactor 11 at 1050 Celsius degrees. A Group III nitride semiconductor layer 15 is grown on the modified buffer layer. The modified buffer layer includes, for example, voids. The Group III nitride semiconductor layer 15 can be comprised of GaN and AlGaN. When the Group III nitride semiconductor layer 15 is formed of these materials, excellent crystal quality is obtained on the modified buffer layer 14.