Sputtering of Si nanospheres

Abstract

Spherical objects, such as clusters, nanoparticles, or aerosol particles, are sputtered when exposed to energetic irradiation. We use Monte Carlo (MC) and molecular dynamics (MD) computer simulation to study this process, with 20 keV Ar impact on a-Si clusters as a prototypical example. The sputter yield is quantified as being influenced by oblique incidence and target curvature. Cluster radii $R$ are scaled to the energy deposition depth, $a$. For large $R$ $(R/a>5)$ sphere sputtering follows closely the sputtering of planar targets, if the variation of the incidence angle on the sphere surface is taken into account. For smaller radii, the yield increases due to the influence of curvature. For radii $R/a\ensuremath{\lesssim}1$ pronounced forward sputtering leads to a maximum in the sputter yield. For smaller $R$, sputter emission becomes isotropic, but decreases in magnitude since not all the projectile energy is deposited in the sphere. However, for all spheres studied $(R\ensuremath{\ge}0.05a)$ the average sputter yield is larger than for infinitely large spheres $(R\ensuremath{\rightarrow}\ensuremath{\infty})$. A simple model based on linear collision cascade theory and assuming that the energy deposition profile is independent of the sphere size predicts sputtering for large spheres well, but fails for small spheres where it strongly underestimates sputtering. The MC data for the smaller spheres are supplemented by MD calculations, which indicate a significant additional contribution caused by spike sputtering.