Comparison between the reaction mechanisms of nitridation of Si(100) by a NH₃ molecular beam and by a N₂⁺ ion beam

Nitridation of a Si(100) surface takes place with NH₃ at temperatures higher than 600°C and with a low energy N₂⁺ ion beam at room temperature, forming a thin film of nitride. The nitridation processes have been studied in situ under ultrahigh vacuum by X-ray photoelectron spectroscopy (XPS). The reaction with NH₃ occurs on the surface with Si atoms supplied by out-diffusion from the substrate, producing a nitride film. On the other hand, the N atoms of the low energy N₂⁺ ion beam (100-1000 eV) penetrate into the bulk by their kinetic energy and form the nitride layer near the surface. The difference of these mechanisms is clearly observed in the evolution of the N1s and Si2p XPS spectra during the reaction.