Using a thermal annealing treatment, amorphous Ni-doped silicon thin films were crystallized into a heterogenous microcrystalline structure. Structural investigations show that NiSi2 precipitates formed at the beginning of the annealing process behave as nucleation centres for the heterogeneous nucleation in the amorphous matrix, and initiate the crystallization of the amorphous silicon thin films. For concentrations below 0.5 at.% Ni the phase transition consists of an epitaxial growth process of crystalline silicon at the NiSi2 precipitates forming a net of needle-like grown silicon crystallites and a second process forming microcrystalline silicon in the space between the needles. Crystallites with a grain diameter of about 10 nm were observed if the distance between the needles was less than 50 nm. The determination of the crystal growth kinetics depending on Ni concentration was carried out by means of the widening of the Tauc gap due to crystallization.