The surface morphology and the room temperature 1.54 µm photoluminescence (PL) intensity from GaN:Er grown by gas source molecular beam epitaxy have been investigated as a function of C concentration as introduced by CBr4. Similar to previous results with increasing Er level, increasing the C concentration initially improved the surface smoothness as measured by atomic force microscopy (AFM) and scanning electron microscopy (SEM), with RMS roughness improving by a factor of seven over undoped GaN. The PL also improved dramatically. However, the highest amounts of C investigated produced a decrease in the PL as well as a roughening of the film surface. These effects indicate that the GaN:Er had reached its C solubility limit, producing an increased amount of defect induced nonradiative recombination.