The critical thickness for the relaxation of InGaN layers grown on (0001)GaN on sapphire for an indium content between 10% and 20% has been determined experimentally. The layers were grown by metalorganic vapour phase epitaxy (MOVPE). The indium content was varied by changing growth temperature between 700 and 750 degrees C. In-situ ellipsometry could identify a growth mode transition during layer growth, from relatively smooth InGaN layer to a tougher layer with higher indium content. X-ray diffraction found a completely strained layer with lower indium content and a completely relaxed layer with higher indium content. These findings were consistent with absorption and photoluminescence measurements. (C) 2008 Elsevier B.V. All rights reserved