Diffusion of hydrogen atoms in silicon layers deposited from molecular beams on dielectric substrates

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.