Fundamental Concepts of Ion-Solid Interactions: Single Ions, 10−12 Seconds

Abstract

An energetic ion undergoes two major mechanisms of energy loss: (1) screened Coulomb collisions with target atoms (nuclear stopping), and (2) interactions with bound or free electrons in the solid (electronic stopping). This article discusses how these two energy-loss processes—and related quantities such as the ion range, collision cascade behavior, sputtering, and spike effects—depend on the experimental parameters of beam energy, atomic number, and target density. We restrict ourselves to an individual incident ion and a time scale (~10−12 s) sufficient for the cascade to fully develop (Figure 1), but short enough for subsequent quenching or diffusion processes to be ignored. What happens at longer times and higher ion f luences, due to diffusion and cascade overlap phenomena, is treated in the article by Brown and Ourmazd in this issue of the MRS Bulletin.Energy-Loss ConceptsFigure 2 summarizes the velocity (ε1/2) dependence of the two energy-loss processes, nuclear stopping (dε/dρ)n and electronic stopping (dε/dρ)e, in terms of the dimensionless Thomas-Fermi (TF) energy, ε, and length, ρ, units derived by Lindhard. The energy unit ε is defined as the ratio a/b where a is the TF screening length (typically, ~0.01 nm) and b is the distance of closest approach in an unscreened head on collision. The following simple relationship exists between TF and lab energies.where M1, Z1, M2, and Z2 are the masses and atomic numbers of projectile and target atoms.