Structure and reactivity of amorphous silicon nitride investigated with density-functional methods

Abstract

We present the first results of our investigation of amorphous silicon nitride using ab initio total energy and molecular dynamics methods. The structural models of a-Si 3N 4 and a-Si 3N 4:H are based on continuous random alternating networks. The strong chemical order and the absence of like-atom bonds such as Si–Si and N–N bonds is maintained even after ab initio molecular dynamic simulations at elevated temperatures. The modeling results in two structures for a-Si 3N 4 with densities of 3.2 and 2.6 g/cm 3 , each consisting of 112 atoms. Both structures have very few topological defects, in total only one three-connected Si is present. Although the low-density structure contains a pore, there is no detectable trend of total energy or average connectivity with density. By discarding selected atoms and terminating dangling bonds with hydrogen, two models of a-Si 3N 4:H were constructed free of under-coordinated atoms and with densities of 2.8 and 2.5 g/cm 3 . The hydrogenated models show less distortion of bond lengths and bond angles in comparison to the hydrogen-free models. The density of states (DOS) of the band gap region indicates the presence of gap states that determine the reactivity of the phase.