  We study using first-principles total-energy calculations, diffusion-reaction
processes involved in the thermal nitridation of SiO2. We consider NO, NH, N2
and atomic N in different charge states as the nitriding species in
alpha-quartz. Our results show that none of neutral species react with the SiO2
network remaining at interstitial sites. Therefore, they are likely to diffuse
through the oxide, incorporating nitrogen at near-interface (Si/SiO2) regions.
Whereas, charged species are trapped by the network, nitriding bulk SiO2. For
the diffusing species, we find that NH and atomic N show increasing
diffusivities with temperatures, whereas for NO and N2 they are relatively
constant. This result agree well with the finding of higher N concentration at
the Si/SiO2 interface obtained by thermal processing of SiO2 in NH3 as compared
with those obtained in N2O. Finally, we discuss spin-dependent incorporation
reaction mechanisms of NH and atomic N with the SiO2 network.

Materials Science