Role of nitrogen on the atomistic structure of the intergranular film in silicon nitride: A molecular dynamics study

Molecular dynamics simulations of intergranular films (IGFs) containing Si, O, N, and Ca in contact with three different types of surface terminations of Si,N, were performed using a multi-body interatomic potential. IGFs with the same Ca concentration (12 mol% CaO) but different nitrogen concentrations [N/(N + O) = 0, 15, 30, and 50%] were studied. In all 12 IGFs, Ca ions did not compete with the first adsorbed layer of Si at the IGF/basal crystal interface, but did so at the IGF/prism crystal interface. The simulations show the epitaxial adsorption of Si, O, and N from the IGF onto the basal and prism crystal surfaces. While more adsorbed N was expected as nitrogen concentration increases, there was a significantly larger amount of N adsorbed to the basal surface than to the prism surface. It was found that Ca ions sit closer to the prism surface than the basal surface but move closer to the crystal at both surfaces with increasing nitrogen concentration, although the effect was more pronounced at the basal interface. With the increase of nitrogen concentration, the percentage of two-coordinated oxygen remained about the same, but there was a change in the type of defect oxygen present. In all the simulations, the central position of the first peak in the Si-O pair distribution fixation (PDF) ranges from 1.63 to 1.65 Angstrom, and that of Si-N PDF ranges from 1.71 to 1.73 Angstrom, both consistent with experimental findings. Furthermore, the first peak of both the Si-O and Si-N PDF shifts to larger values as the nitrogen concentration increases, indicating the elongation of the Si-O and Si-N bond in the IGF with the increase of nitrogen concentration. Elongation of both the Si-N and Si-O bonds could lead to weakening of the IGF as nitrogen concentration increases, although competing changes in bonding of the O complicate the effect of N addition on the strength of the IGF.