Structure and morphology of ultrathinCo/Ru(0001) films

Abstract

We follow the layer by layer growth of cobalt on ruthenium by means of low-energy electron microscopy (LEEM) and diffraction (LEED). Around 500 K each layer forms through the nucleation and growth of triangular islands. Fully dynamical calculations of the diffraction intensities establish that the first monolayer (ML) grows pseudomorphically, i.e. following the Ru hexagonal close packed (hcp) stacking sequence. In films thicker than a ML, the in-plane lattice spacing is relaxed and the resulting superstructures produce satellite spots in the diffraction patterns. Our LEED analysis indicates that in two ML films the first Co layer is stacked in two ways on Ru, but there are no stacking faults between the Co layers themselves. In three ML thick regions, additional stacking faults are located at the topmost Co layer. These stacking faults are associated with distinct island shapes and unique selected-area diffraction spectra. Our study supports the simple picture that island orientation reflects the stacking type.