Rare-earth (RE)-doped GaN has been shown to be an extremely versatile optoelectronic material, with light emission throughout the visible spectrum as well as at important near-infrared wavelengths. RE-doping of GaN has resulted in the successful fabrication of electroluminescent devices (ELD) with red, green and blue (RGB) color emissions using Eu, Er and Tm, respectively. Throughout studies with GaN:REs we have observed that RE optical emission from GaN films is a strong function of the parameters such as RE concentration, Ga flux, growth temperature and so on. Therefore, it was necessary to study the effect of those parameters on optical and structural properties of RE-doped GaN films by using Er-doped GaN in order to maximize ELD brightness and efficiency as well as to apply the results to real devices. As a result of optimization based on various optical and morphological investigations, we reached a conclusion: (1) the optimum Er concentration is ~1 at. %, (2) the growth temperature is ~600 °C, (3) the optimum growth condition of Er optical activity is under slightly N-rich flux near the stoichiometric region.