InN is a very promising material for optoelectronics, high frequency devices and terahertz applications. However, there is no convenient matched substrate and, InN is usually grown onto sapphire. Due to the large attice mismatch and the dissimilarity in atomic arrangement, the epitaxial growth of InN onto sapphire Is a challenge. The initial stages of the growth are of primary importance for the obtention of high quality InN films. Epitaxial InN layers can be improved by nitriding the substrate but the quality of InN strongly depends on the temperature and time of nitridation used. We investigated the effect of these parameters on the crystalline quality, morphology, optical and electrical properties of InN layers. Raising the temperature and the time of nitridation is not beneficial. We also studied the effect of different buffer layers (CaN, AlN). Growth on template seems to be better than on low-temperature buffer (GaN and AlN). After optimising all parameters, we obtained InN layers on lightly nitrided sapphire which correspond, up to know and in our knowledge, to the best results in MOCVD low pressure: the layer growth mode is 2D, with a root mean square roughness of 0.62nm, a reproducible mobility of 800cm(2)/Vs and an electron concentration about 9x10(18) cm(-3). (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.