Figure 1: Evolution of PL spectrum in Mg doped (sample I) and In-Mg codoped (sample II) GaN with increasing temperature.

Two sets of defect-related PL bands can be clearly resolved. One set is the ultraviolet luminescence (UVL) band (from 3.1 to 3.35 eV). Another set is the blue luminescence (BL) band (from 2.2 to 3.1 eV) peaking at 2.9 eV which should be attributed to V-N related deep donors. The characteristic temperature T0, at which the abrupt quenching begins, seem to decreases with increasing In concentration. Furthermore, the obviously enhanced blue luminescence indicates the increasing of hole concentration in sample II.