The microscopic evolution of GaN layers grown by metal-organic chemical vapor deposition using a new Ga treatment method was investigated. With the help of in situ reflectance measurements the coalescence and overgrowth of GaN epilayers was observed. The sample morphology was ex situ characterized by atomic force microscopy, scanning electron microscopy, and optical microscopy. By using orthodox etching in molten KOH-NaOH eutectic the dislocation density and N-polar inclusion density were revealed. Photoluminescence measurements were performed in order to determine the optical properties of the GaN layers. The experimental results demonstrated that by annealing the c-plane sapphire in trimethylgallium it is possible to control the GaN epitaxial layer polarity. The ratio between N and Ga polarity in the deposited GaN layers can be adjusted by means of changing the annealing time in TMG atmosphere of the sapphire substrate. For an annealing time of 20 min the layers are completely N-polar and for a time of 60 min Ga polar epilayer are grown. Published by Elsevier B.V.