Strain relief through stair-rod dislocations in ultrathin epitaxial metal films: Defect geometry and energetics

Abstract

Epitaxial Ni films deposited on Ir(100) were investigated by tunneling microscopy (STM), quantitative low-energy electron diffraction (LEED), and density-functional theory (DFT). For film thicknesses beyond 3 monolayers the large tensile strain ({approx_equal}9%) is relieved by the formation of stair-rod-like dislocations. Their favorable energetics is revealed by DFT calculations which also determine the defects' structural parameters. On the unstructured Ir(100)-(1x1) surface they develop in an irregular way, i.e., without long-range order. In STM they are visible as shallow depressions or by decoration with further adsorbed adatoms. In contrast to this case of missing long-range order, the dislocations are ordered in films on the Ir(100)-(5x1)-H surface, whereby (5x1)-periodic Ir wires at the interface act as pinning centers. So, their detailed atomic structure is accessible experimentally by quantitative LEED with crystallographic precision. Features similar to Ni are also observed for Co films.