In this paper, we present the results of the investigation of GaN-on-Si layers grown by the ammonia MBE technique within the technologically acceptable temperature range (775 °C–825 °C) by photoluminescence technique. The lowest value of the concentration of defects was obtained at a growth temperature of 825 °C. The increase in the concentration of defects in the film with decreasing growth temperature can be explained by the deviation from the  optimum growth temperature and consequently by the deterioration of the crystalline perfection of the GaN layers.