Diffusion behavior of single adatoms near and at steps during growth of metallic thin films on Ni surfaces

Diffusion of single adatoms approaching both descending and ascending steps on Ni(111), (110) and (100) has been investigated with the embedded atom method (EAM) and molecular statics (MS). (1) It was found that there exists a "forbidden" region near both descending and ascending steps on Ni(111). Adatoms have to overcome a slightly higher energy barrier to get into the "forbidden" region, which extends 2-3 nearest-neighboring spacings from the steps. This is consistent with FIM experiments for incorporation of Ir adatoms into ascending steps of Ir clusters. (2) Detailed calculations for incorporation of adatoms over descending steps of type B on Ni(111) have revealed that exchange diffusion, in which an adatom replaces the position of an atom in the step and exchange their roles, is energetically favored over ordinary jumps and is the dominant diffusion mechanism at the descending steps. This is consistent with recent FIM experiments of W adatom diffusion at Ir cluster edges of type B on Ir(111). (3) Exchange diffusion was also found to be favored over direct jumps at the descending steps on Ni(110) and Ni(100).