Fast Diffusion Mechanism of Silicon Tri-interstitial Defects

TL;DR

This paper investigates the microscopic self-diffusion process of silicon tri-interstitial defects, revealing a collective screw-like motion along [111] directions and calculating diffusion constants relevant for ion implantation conditions.

Contribution

It combines molecular dynamics, nudged elastic band, and density functional theory to elucidate the diffusion pathway and barriers of silicon tri-interstitials, a novel approach for this defect type.

Findings

Diffusion barrier of 0.49 eV for silicon tri-interstitials

Calculated diffusion constant of 4 x 10^{-5} exp(-0.49 eV / k_B T) cm^2/s

Diffusion may influence interstitial cluster formation during ion implantation

Abstract

We reveal the microscopic self-diffusion process of compact tri-interstitials in silicon using a combination of molecular dynamics and nudged elastic band methods. We find that the compact tri-interstitial moves by a collective displacement, involving both translation and rotation, of five atoms in a screw-like motion along $[111]$ directions. The elucidation of this pathway demonstrates the utility of combining tight-binding molecular dynamics with \textit{ab initio} density functional calculations to probe diffusion mechanisms. Using density functional theory to obtain diffusion barriers and the prefactor, we calculate a diffusion constant of $ 4 \cdot 10^{-5} \exp (- 0.49 {\rm eV} / k_{B} T) {\rm cm^2/s} $. Because of the low diffusion barrier, $I_{3}^{b}$ diffusion may be an important process under conditions such as ion implantation that creates excess interstitials, hence favoring formation of interstitial clusters.