Nanostructured Si thin films, also referred to as polymorphous Si, were grown by chemical vapor deposition using modulated radiofrequency (RF) plasmas of SiH4, highly diluted in Ar. The plasma conditions were adapted to obtain a new type of Si thin films, between the amorphous and the microcrystalline ones, by allowing the formation of nanocrystalline Si particles in the plasma gas phase and their incorporation into the growing film. These films consist of Si-ordered domains (1-5 nm) embedded in an amorphous matrix, as seen by high-resolution transmission electron microscopy. Films with different crystallite size and density were obtained depending on the plasma modulation and discharge conditions. Selected area electron diffraction has highlighted that such Si-ordered domains are crystals with face-centered-cubic structure, clearly different from the Si diamondlike structure.