Effect of topological disorder on structural, mechanical, and electronic properties of amorphous silicon nitride: An atomistic study

Abstract

We present a first-principles study of the effect of atomic variability on the structural, mechanical, and electronic properties of amorphous silicon nitride. Using a combination of molecular dynamics and density functional theory calculations, we predict an ensemble of statistically independent, well-relaxed, and stress-free amorphous silicon nitride structures. We analyze the short-, intermediate-, and long-range orders of the structures generated using radial distribution functions, ring statistics, bond angle distributions, and translational invariance parameters. Though energetically very similar, these structures span a wide range of densities (2.75--3.25 g/cm${}^{3}$) and bulk moduli (115--220 GPa) in good agreement with the fabrication-dependent experimental range. Chemical bonds and atomic defects are identified via a combination of bond distance cutoff and maximally localized Wannier function analysis. A significant number of the amorphous structures generated ($\ensuremath{\sim}$30$%$) are defect free providing an ideal reference to characterize the formation energy of the various point defects and their defect energy levels. An analysis of the Kohn-Sham density of states and energetics of the structures reveals that defects in amorphous dielectrics have a distribution of associated properties (e.g., formation energies and electronic energy levels) due to variations in local atomic structure; this should be taken into consideration in physics-based continuum models of these materials.