Atomic assembly of metal surfaces and interfaces

Inert gas ion impacts can be used to manipulate atomic assembly processes during the growth of metallic superlattices but the detailed mechanisms are not well understood. Molecular dynamics simulations are used to investigate the effects of ion incident angle and fluence upon the reassembly and structure of a copper surface partially covered with cobalt asperities. In the low ion energy regime, increasing the ion fluence decreases the cobalt layer surface roughness while gradually leading to an increase in the degree of interfacial mixing. The flattening of asperities occurs by direct (athermal) ion activation of an Ehrlich–Schwoebel mechanism of atom jumping. Intermixing of cobalt surface atoms in an underlying copper layer is found to occur by a knock-on process and the lowest energy barriers for this occur in low-index 〈1 1 0〉 and 〈1 1 2〉 crystal (channeling) directions. The mechanistic insights gained from the study are used to simulate the ion assisted growth of a Cu/Co/Cu multilayer system. Using ion parameters chosen to selectively activate atomic assembly mechanisms that promote flat, unmixed interfacial structures, it is shown that Cu/Co/Cu multilayer structures with high quality, smooth and chemically sharp interfaces can be obtained by using oblique, low energy, moderate fluence ion assistance with an ion mass that is similar to the atomic mass of the metals.