GaN is a wide gap semiconductor which is now used to produce blue and green light emitting diodes, blue laser diodes and which has numerous other potential applications, like high frequency HEMT transistors, UV sensors, etc. A complicated two step process, using a low temperature buffer layer, subsequently annealed and followed by the deposition of the monocrystalline semiconductor was developed, and due to the excellent results obtained following this method, a rush towards applications resulted. There is now a need to investigate in more detail the growth mechanisms, and the influence of the growth parameters, in order to ensure a better reproducibility of the results. In this paper, we report an investigation of the growth mechanisms and the influence of the growth parameters using in-situ reflectance experiments. The reflectance measurements allow us to follow the growth rates, and the changes in the surface morphology (transitions between islands growth and 2D growth). Additional exsitu measurements (AFM ) were performed at different stages of the growth process to ensure additional information. As a result, we demonstrate that the recrystallization of the low temperature buffer layer is a critical step, which is drastically influenced by the composition of the annealing atmosphere (amount of ammonia present in the gas phase), while the deposition temperature and buffer thickness have a moderate effect. We will discuss here the growth mechanisms which may be involved to explain such a behavior.