Effects of Nitridation on the Characteristics of Silicon Dioxide: Dielectric and Structural Properties from ab initio Calculations

By combining ab initio calculations and classical molecular dynamics, we determine how the inclusion of nitrogen in a silica matrix changes its dielectric constant, and elucidate the underlying mechanisms. We find that there is an entire range of nitrogen concentrations (up to $\sim 25\%$) for which the structural pattern of the oxide is preserved in bulk SiON, and the dielectric constant increases mainly because of the variation of the ionic polarizability. This behavior is not sensitive to hydrogen passivation of nitrogen. The few defects, which are associated with electron states near the gap, are mainly centered on undercoordinated nitrogen and undercoordinated silicon, and tend to be removed by hydrogen.

Figure 1

Electron density of states (EDOS) and its atomic projections: $\alpha \mathrm{\text{−}}{\mathrm{SiO}}_2$ (a), ${\mathrm{SiO}}_x{\mathrm{N}}_y$ with $\xi =13.3\%$ (b), $\alpha \mathrm{\text{−}}{\mathrm{SiO}}_{2-y}(\mathrm{NH}{)}_y$ with $\xi =12.5\%$ (c). In each case the zero of energy lies at the middle of the gap. In $\alpha \mathrm{\text{−}}{\mathrm{SiO}}_2$ the value of the gap was fixed equal to the quasi-particle-energy gap calculated in 15; a rigid shift was applied to the whole energy spectrum there and in the two other compounds.

Figure 2

Dielectric constant as a function of nitrogen content $\xi =y/(x+y)$. ${\epsilon }_0(\mathrm{\text{black}})={\epsilon }_{\infty }(\mathrm{\text{blue}})+{\epsilon }_{\mathrm{ion}}(\mathrm{\text{red}})$. Solid circles: ${\mathrm{SiO}}_x{\mathrm{N}}_y$ $(x=2-3y/2)$; asterisks: $\alpha \mathrm{\text{−}}{\mathrm{SiO}}_{2-y}(\mathrm{NH}{)}_y$; crosses: ${\mathrm{SiO}}_x{\mathrm{N}}_{y-z}(\mathrm{NH}{)}_z$ ($x=2-3y/2$; $z<y$). Note that some data points are superposed. Lines represent least-square fittings of the calculated values.