  We measured the temperature dependence of photoluminescence involved with the
metastability of unintentionally doped GaN. Reaction energy U of donor atom is
0.1eV and shallow donor is more stable than deep center. The impurity
transition was applied to unintentionally doped GaN at low temperature and
reaction energy U was provided for shallow-deep transition. We propose that the
origin of DX center in unintentionally doped wurtize GaN is considered to be an
oxygen impurity instead of silicon.
