Thermally grownSi3N4thin films on Si(100): Surface and interfacial composition

Synchrotron photoemission measurements of the Si(2p) and N(1s) levels have been made on ${\mathrm{Si}}_3$ ${\mathrm{N}}_4$ thin films grown in situ by high-temperature reaction of Si(100) with ${\mathrm{NH}}_3$. Surface sensitivity is enhanced in comparison to laboratory photoemission experiments by selecting photon energies that minimize the photoelectron mean free path in the nitride film. From the results, we are able to determine not only the types of chemical species present, but their approximate location within the film as well. Careful analysis of the Si(2p) photoemission spectra reveals the presence of a unique silicon species with a Si(2p) binding energy intermediate between elemental silicon and silicon in ${\mathrm{Si}}_3$ ${\mathrm{N}}_4$. Furthermore, the persistence of this species with increasing nitride-film thickness supports its assignment to a monolayer of silicon at the outermost surface layer, on top of the growing stoichiometric ${\mathrm{Si}}_3$ ${\mathrm{N}}_4$ film. These surface silicon atoms can be distinguished from silicon atoms in intermediate oxidation states at the ${\mathrm{Si}}_3$ ${\mathrm{N}}_4$/Si interface. The spectroscopic evidence for chemically distinct surface, nitride, and interfacial silicon is discussed in terms of the silicon nitridation mechanism.