  In the paper, the processes occurring during low-temperature growth of
non-hydrogenated amorphous Si and polycrystalline Si films on multilayer
Si$_3$N$_4$/SiO$_2$/c-Si substrates from molecular beams under conditions of
ultrahigh vacuum are studied in detail. Diffusion of hydrogen atoms from a
dielectric layer into the growing film is shown to accompany the growth of a
silicon film on a Si$_3$N$_4$ layer deposited by CVD or on a SiO$_2$ layer
obtained by thermal oxidation of a silicon wafer. The process of hydrogen
migration from the dielectric substrates into the silicon film is studied using
FTIR spectroscopy. The reduction of IR absorption at the bands related to the
N$-$H bonds vibrations and the increase of IR absorption at the bands relating
to the Si$-$N bonds vibrations in IR spectra demonstrate that hydrogen atoms
leave Si$_3$N$_4$ layer during Si deposition from a molecular beam. The
absorption band assigned to the valence vibrations of the Si$-$H bond at $\sim
2100$ cm$^{-1}$ emerging in IR spectra obtained at samples deposited both on
Si$_3$N$_4$ and SiO$_2$ layers indicates the accumulation of hydrogen atoms in
silicon films. The difference in chemical potentials of hydrogen atoms in the
dielectric layer and the silicon film explains the transfer of hydrogen atoms
from the Si$_3$N$_4$ or SiO$_2$ layer into the growing silicon film.
