Silicon layers were deposited by standard PCVD from undiluted silane or sputter-deposition, respectively, onto aluminum underlayers of 4-32 nm thickness. Above a critical thickness of the Al-layer around 8 nm, the growth of nanocrystalline silicon is observed. The intentional introduction of hydrogen into the sputter-deposited bilayers showed a weak effect on crystallite formation. In contrast, the argon pressure during Si-sputtering was found to strongly affect the formation of nc-Si. Effective growth of Si-crystallites is accompanied by a consumption of Al from the underlayer and an increased diffusion of Al into the silicon as detected by X-ray methods and concentration in-depth profiles, respectively. As found by atomic force microscopy, the typical rough surface of the Al-underlayers is preserved during the growth of the Si-overlayers. The effect of the aluminum-mediated growth is explained by a migration of Al-crystallites from the underlayer into the growing film.