The microscopic evolution of GaN layers grown by metal-organic chemical vapor deposition on sapphire using an Al treatment method replacing traditional techniques was investigated. With the help of in-situ reflectance measurements the coalescence and overgrowth of GaN epilayers were examined and the sample morphology was ex-situ characterized by scanning electron microscopy. For characterization of the GaN layers, energy dispersive X-ray spectroscopy and X-ray diffraction were used for chemical and structural analysis, respectively. The experimental results demonstrated that it is possible to control the GaN epitaxial layer polarity and to induce a stable growth mode by depositing a thin Al-based nucleation layer on the c-plane sapphire. It was found, that during the annealing in trimethylaluminum, at 1170 degrees C some amount of carbon is incorporated into the layer. The X-ray diffraction measurement revealed traces of Al4C3 in the nucleation layer. In GaN epilayers with thickness exceeding 300 nm. inclusions of probably misoriented crystallites or of cubic GaN were observed. A definite proof of the possible capability of this method should arise after careful optimization of this method. (c) 2007 Elsevier B.V. All rights reserved.