The donor impurity distribution in a GaN epitaxial layer was studied using Raman imaging. The A1~LO! Raman line at 735 cm is found to be inversely correlated to the presence of silicon in GaN due to phonon interaction with the free carrier plasma associated with donor impurities in the material. The spatial variation of the A1~LO! signal was imaged directly using newly developed instrumentation. Features with dimension of about 0.5 mm are observed in faceted GaN crystallites. This variation in free carrier concentration is attributed to preferential donor impurity incorporation during growth. © 1996 American Institute of Physics. @S0003-6951~96!03644-3#