Metastability from Photoluminescence of n-type GaN
  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. 
C. S. Park and T. W. Kang