During molecular beam epitaxy of GaN:Tm films, substrate temperature strongly influences the rare earth incorporation, surface morphology and luminescence spectrum. The Tm incorporation into films grown between 730 and 830 degrees C was estimated by wavelength-dispersive X-ray (WDX) spectroscopy. Comparative WDX, atomic force microscopy (AFM) and cathodoluminescence (CL) mappings reveal that at an optimal growth temperature between 775 and 780 degrees C, a high Tm content (similar to 2.2 at%) and a smooth surface morphology can be obtained, leading to an intense sharp TM3+ emission. For lower substrate temperatures, Ga droplets and large (similar to 8-15 mu m) circular pits mar the sample surface; for higher temperatures, the sharp CL lines disappear due to low Tm content (<= 0.8 at%). (C) 2008 Elsevier B.V. All rights reserved.