EFFECTS OF ATOM ENERGY ON METAL-ON-METAL FILM NUCLEATION AND GROWTH

Molecular dynamics simulations of the deposition of f.c.c. metal films on f.c.c. metal substrates have been conducted using embedded-atom potentials. The energies of atoms arriving at the substrate were varied over the range 0.1-40 eV. Substrate-film atom combinations have been chosen to investigate the relationship of relative atomic sizes, atomic masses, and heats of mixing with the effects of atom arrival energy. The results were also compared with experimental data in the literature regarding the epitaxy of f.c.c. metal films on Ag and Cu substrates. In the simulations, the deposition of Pd, Ag, Pt, and Au was studied on Cu substrates, and deposition of Ni, Cu and Ag was examined on Ag substrates. As had been noted previously for simulations of Ag deposition on Ag substrates, atom arrival energies of 10 eV and greater resulted in some film-substrate interface mixing. The present results demonstrated that this interface mixing effect could also be strongly influenced by the chemical thermodynamics of the system. For 10 eV depositions of 4 monolayers on Cu(100) substrates, 1 Ag atom (0.005 ML), 4 Au atoms (0.02 ML), and 30 Pt atoms (0. 1 5 ML) were found mixed into the top substrate layer. Both experimental measurements and calculations using the potentials employed in this study show that, for mixing into a Cu host, the heat of solution is positive for Ag, slightly negative for Au, and significantly negative for Pt (3 times the magnitude for Au). In the investigation of heteroepitaxy, Ni and Cu deposited on Ag(100) substrates grew as (100)-oriented films, with their lattice parameters expanded in the plane of the substrate. For oversize atoms deposited on Cu(100) substrates, it was found that increasing misfit was accommodated by shear along close-packed directions in the plane of the substrate surface, producing stacking faults in the growing film. At sufficiently high misfit, these shear lines merge, producing a (111)-oriented film. It is proposed that the experimental observation that Pd grows on Cu(100) with a (100) orientation while Pt grows on the same substrate with a (111) orientation results from the higher initial density of these faults, which makes it more energetically favorable for dislocations to shear the entire film to the (111) orientation as the thickness increases.