Diffusion-reaction mechanisms of nitriding species in SiO₂

We study using first-principles total-energy calculations, diffusion-reaction processes involved in the thermal nitridation of SiO ₂. We consider NO, NH, N₂, and atomic N in different charge states as the nitriding species in α-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 agrees well with the finding of higher N concentrations 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.