Diffusion Monte Carlo Formation Energies of Silicon Self-Interstitial Defects

Silicon self-interstitial defects can hinder the fabrication of semiconductor devices. Several stable single-, di-, and tri-interstitial clusters found with \emph{ab initio} and tight-binding simulations are believed to form in silicon\footnote{D. A. Richie et al. \emph{Phys. Rev. Lett}. \textbf{92}, 45501 (2004).}. Since experimental detection of self-interstitials remains a challenge, accurate theoretical methods are needed to study their properties. The first Diffusion Monte Carlo (DMC) calculations found single-interstitial defect formation energies to be about 1 eV higher than predicted by density functional theory (DFT)\footnote{W.-K. Leung et al. \emph{Phys. Rev. Lett.} \textbf{83}, 2351 (1999).}. This indicates that DFT may be insufficient for the study of silicon self-intersitials. We confirm the discrepancy between DMC and DFT formation energies for three single-interstitial structures (X, H, and T) and extend the comparison to several di- and tri-interstitial clusters.