Asymmetric surface intermixing during thin-film growth in the Co-Al system: Role of local acceleration of the deposited atoms

Surface intermixing behavior during thin-film deposition in the Co-Al system was investigated on the atomic scale by three-dimensional classical molecular dynamics simulation. Asymmetry of the surface intermixing was observed: Al deposition on a Co substrate resulted in an Al thin-film with an atomically sharp interface, while a Co thin-film deposited on an Al substrate had an interfacial intermixing layer of B2 structure. This phenomenon is discussed in terms of the kinetics of atomic intermixing on the surface. A kinetic criterion for the atomic intermixing is whether the increased kinetic energy of the deposited atom near the surface is larger than the energy barrier to atomic intermixing on the surface. Local acceleration of the deposited atoms near the surface provides an explanation of the puzzling phenomenon of the significant intermixing under low-energy deposition conditions such as thermal evaporation or molecular beam epitaxy.