Mechanistic Insights into the Dry Oxidation of Amorphous Silicon Nitride: A DFT Study

Abstract

Amorphous silicon nitride (Si₃N₄) is vital for modern nanoelectronics, serving as dielectric layers, diffusion barriers, and charge-trapping layers in flash memory devices. From a thermodynamic viewpoint, Si₃N₄ is prone to oxidation, which can compromise the material’s properties. However, experimental studies suggest that kinetic barriers largely prevent the reaction from proceeding at a rapid pace. In this study, we provide new insights based on DFT calculations into the role of oxygen defects in Si₃N₄ and revealing that their perturbation of the local structure can create shallow trap sites. However, these traps compete with native trap precursors and rather stem from structural distortion than from oxygen itself — unlike in the crystalline phase. To elucidate the oxidation resistance, we calculate the barriers connecting interstitial and substitutional defects, which suggests that successive nitrogen replacement necessitates the concerted breaking of Si－N bonds facilitated by a local oxygen excess. The formidable oxidation resistance of Si₃N₄, as well as with the experimentally observed N_{2 (g)}:NO_(g) ratio exhibiting an unexpectedly high abundance of NO_(g), is thereby rationalised.