A DFT study of the compressibility of amorphous silicon oxynitride

Studies on network models of amorphous silicon oxynitride, a-SiNO, show a continuous change of physical and structural properties close to what can be expected from Vegard’s rule. The addition of N into SiO2 increases the bulk modulus, B0, thus stiffens the material. We find values of B0 for amorphous models of SiO2, Si2N2O, and Si3N4, of 37, 115, and 192 GPa, respectively. At two points in the phase diagram we observe a change in the character of the dependence of B0 on the nitrogen content: Firstly, a drop of B0 for low N content, at about N / ( O + N ) ≈ 1 3 , corresponding to a a softening of the network. Secondly, an abrupt stiffening accompanied with a modulation of the tetrahedral network close to the Si3N4 limit.