The growth of nanosize islands of iron silicides on Si(100) substrates and epitaxial silicon overgrowth atop them have been studied by low energy electron diffraction and reflectance high energy electron diffraction methods. The near optimal formation conditions of iron silicide islands with high density and minimal sizes have been determined by using of atomic force microscopy. Multilayer (8-10) monolithic structures with buried iron silicide nanocrystallites have been grown after the definition of monocrystalline burying conditions of iron silicides nanocrystallites in silicon lattice. The structure of buried nanocrystallites has been studied in multilayer monolithic heterostructures by high resolution transmission electron microscopy. It was established that in multilayer samples the majority of nanocrystallites have beta-FeSi2 structure, but some of them have gamma-FeSi2 structure. It was observed an influence of additional annealing at 850 degrees C on the morphology and structure of nanocrystallites. By means of deep level transient spectroscopy data one and two trap levels have been observed in multilayer structures (without and with additional annealing, respectively). Photoluminescence spectra have been studied at 4.2 K and the causes of its absence from buried beta-FeSi, NC have been analyzed.