Integration of electronic effects into molecular dynamics simulations of collision cascades in silicon from first-principles calculations

Abstract

The inclusion of sophisticated density-dependent electronic stopping and electron-phonon coupling calculated with first-principles methods into molecular dynamics simulations of collision cascades has recently become possible thanks to the development of the so-called EPH (for Electron-PHonon) model. This work aims at employing the EPH model in molecular dynamics simulations of collision cascades in Si. In this context, the electronic stopping power is investigated in Si at low energies with Ehrenfest Dynamics calculations. Also, the parametrization of the EPH model for Si, from firstprinciples Ehrenfest Dynamics simulations to actual molecular dynamics simulations of collision cascades, is performed and detailed. We demonstrate that the EPH model is able to reproduce very closely the density-dependent features of the energy lost to electrons obtained with ab initio calculations. Molecular dynamics collision cascade simulations results obtained in Si using the EPH model and the simpler but widely employed Two Temperature Model (TTM) are compared, showing important discrepancies in the collision cascades results obtained depending on the model employed.

Effect of electronic stopping in molecular dynamics simulations of collision cascades in gallium arsenide

“…respectively. As shown by Jarrin et al [43], the EPH-MD results do not strongly depend on these electronic parameters, while for the TTM-MD results, only changing C e had a large impact. In the TTM model, the critical velocity was set to 96.28 Å/ps.…”

“…Caro et al [39] constructed a β function by using a cubic spline with six knots and afterwards added an exponential decay to prevent unbounded stopping at high electron densities. Jarrin et al [43] used a function of the form β(ρ) = c 1 ρ×e c 2 (ρ−c 3 ) where the c's are optimization parameters.…”

“…The use of a constant β function is also used in Ref. [43] and [38]. (The trajectories 001 7 Ga and 001 7 As were not used during the EPH model training as the high-density regions sampled along these trajectories are rarely encountered during the collisional cascades and the dissipation functions will be constant at these densities.)…”

“…In contrast with the TTM, the friction term now depends on the local structural environment allowing for better agreement with ab initio electronic stopping data. Recent work on cascade simulations in Si showed that the EPH model correctly reproduces the density-dependent ESP and that the final radiation damage differs considerably from the TTM predictions [43].…”

“…Real-time time-dependent density functional theory (RT-TDDFT) represents a promising parameter-free approach to obtain the electronic stopping in any target for any projectile, at energies where the assumptions of SRIM can no longer be justified. Recent studies on monoelemental systems [38][39][40][41][42][43] showed that the inclusion of electronic effects obtained via RT-TDDFT in MD cascade simulations affected both the number of defects and the cascade morphology.…”