  The results of RHEED, FTIR and Raman spectroscopy study of silicon and
germanium films with the thickness up to 200 nm grown from molecular beams on
dielectric Si$_3$N$_4$/SiO$_2$/Si(001) substrates are presented. Noticeable
changes of the intensity of the N$-$H and Si$-$N absorption bands have been
observed in the IR absorbance spectra as a result of the deposition of the
silicon and germanium films. The thicker was the deposited film, the more
considerable were the decrease of N$-$H absorption band intensity and the
increase in that of the Si$-$N band. This tendency has been observed during the
growth of both amorphous and polycrystalline Si or Ge films. The reduction of
IR absorption at the band assigned to the N$-$H bond vibration is explained by
breaking of these bonds followed by the diffusion of the hydrogen atoms from
the Si$_3$N$_4$ layer into the growing film of silicon or germanium. The effect
of the deposited film thickness on the diffusion of hydrogen is discussed
within a model of the diffusion of hydrogen atoms controlled by the difference
in chemical potentials of hydrogen atoms in the dielectric Si$_3$N$_4$ layer
and the growing silicon or germanium film. Hydrogen atoms escape from the
Si$_3$N$_4$ layer only during the deposition of a Si or Ge film when its
thickness gradually grows. The interruption of the film growth stops the
migration of hydrogen atoms into the film because of the decline in the
chemical potential difference.
