What is the real driving force of bilayer ion beam mixing?

Abstract

Molecular dynamics simulations have been used to study the driving force of ion irradiation induced interfacial mixing in metal bilayers in which the relative mass difference of the constituents is considerable. We find no apparent effect of chemical forces, such as heat of mixing or cohesive energy up to 7keV ion energy, although a considerable number of liquid and high energy particles (hot atoms) persist up to even 20ps during the thermal spike. This result is in direct conflict with the widely accepted theory of thermal spike mixing (chemical interdiffusion model). Instead we point out the decisive role of hot (energetic) particles in ion beam mixing of bilayers. The supersaturation of vacancies also occurs, which induces a thermally activated intermixing of the lighter constituent of the bilayer. The delay and the decoupling of the intermixing of the light and heavy constituents is explained as a backscattering effect at the interface: the interface acts as a diffusional barrier for high energy light particles. The heavier atoms are predominantly ejected to the overlayer at the beginning of the thermal spike while the light atoms are injected to the bulk at the beginning of the cooling period (in Ti/Pt) or during the thermal spike with some time delay (Al/Pt). We explain ion induced amorphization by the sufficiently high concentration of energetic (hot) atoms in Al/Pt.