Erbium doped GaN (GaN:Er) and low In-content In xGa 1-xN (x∼0.05) epilayers were synthesized by metal organic chemical deposition. The 1.54 μm PL emission intensity was monitored for GaN:Er epilayers grown at different growth temperatures and utilized to establish a value of 1.8 ± 0.2 eV for the formation energy (E F) of the optically active Er 3 centers in GaN. The optically active Er centers are presumably Er and nitrogen vacancy (Er-V N) complexes. The experimentally measured value of the E F of the optically active Er 3 centers is about 0.98 eV larger than the calculated formation energy of Er ions at Ga sites; however, it is 1.1-2.2 eV lower than the formation energy of V N in GaN. Due to the large E F values, relatively high growth temperatures are required to improve the 1.54 μm emission efficiency in GaN:Er.