A DFT study of amorphous silicon oxynitride

We study amorphous silicon oxynitride, a-SiNO, for the entire compositional range between SiO2 and Si3N4. Structural models are constructed using an empirical bond switch algorithm. The continuous random alternating networks (CRAN) generated have well-defined bond topology as to represent the perfect chemical order in such materials. Consequently, the models comprise Si–O and Si–N bonds only and constitute an extension of the classical model of vitreous silica presented by Zachariasen. Models consisting of 108–126 atoms are subsequently optimized using standard density functional methods, including full relaxation of atomic positions and cell geometries. Extensive ab initio (Car–Parrinello) molecular dynamic simulations are carried out for 10–50 ps to test for the chemical and structural stability of the amorphous networks at elevated temperatures. The results show that changes of physical and structural properties within the Si–N–O phase diagram are close to what can be expected from Vegard’s rule. E.g., the addition of N into SiO2 stiffens the network and decreases the insulating character of the material, while elastic properties such as the bulk modulus increase. The elastic properties, namely the bulk modulus, give rise to propose two distinct regions within the ternary phase diagram, with Si2N2O approximately separating both region. At this composition, the dependence of B0 on the N content abruptly changes its slope.