Epitaxial growth modes and interlayer relaxation of thin Cu films grown on Ru(0001) and oxygen-precovered Ru(0001)

We have studied the growth of thin Cu films grown on Ru(0001) and oxygen-precovered Ru(0001), using X-ray photoelectron diffraction and measuring experimental Cu 2 p 3 2 ( E kin = 556 eV) and Ru 3d ( E kin = 1206 eV) intensities in the full 2π solid angle above the surface for Cu coverages from submonolayer up to several monolayers (ML). Our data indicate that the first Cu layer grows pseudomorphically on Ru(0001), in agreement with prior studies. At higher coverages, comparison of our data with single scattering and multiple scattering diffraction calculations on various atomic clusters indicates that the short-range structure is fcc Cu(111)-like, consistent with prior STM work, but with significant interlayer contraction that persists up to ≥ 5 ML coverage. In addition, the Cu overlayer grows in two possible orientations on the Ru(0001) surface, with a preference towards one of the two possible orientations at certain coverages. Finally, we have investigated the effect of oxygen preadsorbed on the Ru(0001) surface on the growth of the Cu overlayer. All of the oxygen is found to float on top of the Cu in a highly disordered configuration, and the oxygen is not found to act as a surfactant for coverages ≤ 3 ML, but rather promotes multilayer or island growth relative to growth on clean Ru(0001).