This work reports on the influence of the substrate properties and of the nucleation layer on the growth mode of AlGaN/GaN HEMT (high electron mobility transistor) structures, grown by low pressure metal organic vapour phase epitaxy (LP-MOVPE) on 3 in. 4H-SiC and 6H-SiC semi-insulating substrates. In-situ normal incidence reflectance at 633 nm and 950 nm was used to monitor the evolution of the surface morphology during the growth. The analysis reveals that the growth of the GaN buffer layer on the AlN wetting layer is a critical key step of the process. Reflectance measurements are discussed in term of roughness-related light scattering and absorption. The influence of substrate surface properties has been studied extensively. Physical and electrical properties of the HEMT structures, as evaluated by a complete set of ex-situ characterisation techniques, were found to be strongly influenced by the quality of this interface. A significant improvement of the surface morphology during the growth was obtained by the insertion of an AlGaN/GaN multiquantum well (MQW) between the AlN wetting layer and the GaN layer. Smooth interfaces and an excellent coalescence of AlGaN on AlN and of GaN on the top of the MQW are reported. In addition the technique provides a powerful tool to manage the strain of the structure and to reduce the resulting curvature of the wafer. (C) 2012 Elsevier B.V. All rights reserved.