In this work, solid-state a-GeTe growth is studied during the reactive diffusion of a polycrystalline thin film of hexagonal Te deposited on an amorphous Ge thin film (Te-on-Ge) using in situ X-ray diffraction, in situ transmission electron microscopy, and atom probe tomography. After deposition, an amorphous intermixing layer is observed between the Te and Ge layers. a-GeTe is found to form a 2D layer between the deposited Ge and Te layers during growth, with a thickness increasing linearly with time as predicted by the linear-parabolic model for interfacial reaction limited growth. The activation energy of nucleation and interfacial reactions was determined from different isothermal annealing. The obtained results are compared to the observations previously reported during a-GeTe 3D growth in a sample made of an amorphous Ge layer deposited on a polycrystalline Te layer (Ge-on-Te) in the same magnetron sputtering system under the same conditions.