Strain relief and island shape evolution in heteroepitaxial metal growth

Abstract

Atomistic scale calculations reveal that cobalt islands on Cu~001!, as they grow in size, undergo unusual shape evolution. The strain relief in the Co islands is predicted to have a strong effect on the shape of the islands and the morphology of the substrate in the early stage of the film growth. We show that strain and stress at the interface vary strongly on an atomic scale. Our results demonstrate that the strain relief in the early stage of heteroepitaxy is more complicated than suggested by simple considerations based on the lattice mismatch of bulk materials. The classical rule used to predict heteroepitaxial growth is based on the lattice mismatch between film and substrate. 1 However, several recent experiments 2‐4 have shown that the scenario of the strain relaxation at metal interfaces is more complicated than expected from the lattice misfit. In contradiction to lattice mismatch consideration, a tremendous compressive stress in Ni on W~110! below 0.5 monolayer ~ML! was measured. 2 Also, for Fe, Co, and Cu on W~110! compressive stress was found, while tensile stress is expected from mismatch arguments. 3 A giant compressive surface stress for the first few monolayers of silver on Pt~111! was reported, which is far beyond the stress induced by the misfit. 4 The results of stress measurements in the submonolayer range give clear evidence that continuum elasticity relying on bulk reference data is of questionable relevance for films thinner than 10 A. 2 Ab initio studies have shown that for very thin cobalt films the comparison of the bulk lattice parameters of the two materials is inappropriate to study strained Co layers on Cu~001!. 5