Growth, structure, and morphology of the Pd/MgO(001) interface: Epitaxial site and interfacial distance

Abstract

The growth at room temperature of Pd on an atomically flat MgO(001) surface has been investigated by grazing incidence x-ray scattering. The structure and morphology of deposits in the range 0.2 to 183 monolayers (ML) were analyzed in situ, in ultrahigh vacuum (UHV). The growth proceeds by nucleation, growth, and coalescence of islands. Channeling between islands starts around 18 ML, and above 35 ML, the film is continuous and fully covers the substrate. Pd is in cube-on-cube epitaxy on MgO(001), with an average lattice parameter between those of bulk Pd and MgO. Neither stacking faults nor twins were found. As growth proceeds, Pd is increasingly relaxed, but most of the Pd in the first one or two atomic planes is fully lattice matched with the substrate. The islands are coherent with the substrate up to \ensuremath{\sim}4\char21{}5 ML of Pd deposited. Above 5 ML, interfacial misfit dislocations are introduced at their edges. These dislocations reorder to form a square network above 35 ML. We have determined the epitaxial site, above oxygen ions of the substrate, and the interfacial distance $(2.22\ifmmode\pm\else\textpm\fi{}0.03\AA{}).$ The evolution of the interfacial distance as a function of the amount of Pd deposited is deduced by a quantitative analysis of the substrate crystal truncation rods. Similarities and differences with the Ag/MgO and Ni/MgO interfaces are discussed, and general trends on the interfacial structure and morphology are deduced.