The growth of nanocrystalline silicon on an aluminum underlayer of 4–32 nm thickness on silica substrates by plasma-enhanced chemical vapor deposition (PECVD) or sputter deposition is observed at standard conditions for the preparation of device quality hydrogenated amorphous silicon (substrate temperature of 500 K, deposition rate of 0.5 &mgr;m/h). The crystallite size determined by wide angle x-ray scattering ranges from 10 to 30 nm, and the crystallite fraction reaches 25&percent;. The efficiency of aluminum mediated crystallization is about one order of magnitude higher for PECVD films than for sputtered films. Variations of the incident angle of the x rays show that the formation of silicon crystallites takes place at the Al/Si interface. Diffusion of Al into the silicon is enhanced for the PECVD films, whereas it plays a comparatively minor role for sputter deposition. The effect of the aluminum mediated crystallite growth is related to the existence of a metastable aluminum silicide and diffusion processes. ©1998 American Institute of Physics.