When a substrate layer (desired semiconductor crystal) made of a group III
nitride compound is grown on a base substrate comprising a lot of
projection parts, a cavity in which a semiconductor crystal is not
deposited may be formed between each projection part although it depends
on conditions such as the size of each projection part, arranging
interval between each projection part and crystal growth. So when the
thickness of the substrate layer is sufficiently larger compared with the
height of the projection part, inner stress or outer stress become easier
to act intensively to the projection part. As a result, such stress
especially functions as shearing stress toward the projection part. When
the shearing stress becomes larger, the projection part is ruptured. So
utilizing the shearing stress enables to separate the base substrate and
the substrate layer easily. The larger the cavities are formed, the more
stress tends to concentrate to the projection parts, to thereby enable to
separate the base substrate and the substrate layer more securely.