Strain-induced surface segregation and ordering in pseudomorphic metal-alloy overlayers

The stability of pseudomorphic metal films on a substrate composed of a different and lattice-mismatched metal against strain relief through intermixing is studied via atomistic simulations. Interdiffusion and lattice relaxation of the initial pseudomorphic structure is treated using a Monte Carlo procedure in which the structural energy of the bimetallic system is obtained using the embedded-atom method. Pseudomorphic overlayers of Ag were placed on [100] substrates of Pt (4.3% mismatch) and of Au (0.25% mismatch). Upon annealing at 1000 K, the Ag alloys with both substrates through interdiffusion. The Ag forms a uniform solid solution with the Au substrate. However, the Ag is trapped near the Pt surface by formation of slightly disordered ${\mathrm{Pt}}_3$Ag layers. This trapping of Ag near the Pt surface in the ordered ${\mathrm{Pt}}_3$Ag phase is induced by the misfit strain of the interdiffusing overlayer.