Structure and morphology of the Ag/MgO(001) interface duringin situgrowth at room temperature

Abstract

The structure and morphology of Ag deposits grown at room temperature on high-quality MgO(001) surfaces have been investigated in situ, from 0.2 to 300 equivalent monolayers (ML) of Ag deposited. Surface x-ray diffraction and grazing incidence small angle x-ray scattering parallel and perpendicular to the surface were combined. Nucleation, growth, and coalescence of islands are found from 0.2 ML. The average in-plane width, height, and in-plane separation of growing islands are deduced and are found to reproduce well the Mg $1s$ x-ray photoemission spectroscopy spectrum previously reported by other authors. The height over width ratio of the islands is $\ensuremath{\sim}0.37\ifmmode\pm\else\textpm\fi{}0.05$ at all stages of the deposit. Ag grows in cube-on-cube epitaxy with respect to the MgO(001) substrate. A very unusual evolution of the state of strain in Ag with increasing amount of Ag deposited is observed. Below 4--6 ML (island width smaller than 90 \AA{}), the small Ag islands are coherent with the MgO. Below 1 ML (island width smaller than 35 \AA{}), they have their bulk lattice parameter, and between 1 and 4 ML they become more and more strained by the MgO substrate, with an average lattice parameter intermediate between those of Ag and of MgO. Around 4--6 ML, the islands reach a critical size and misfit dislocations are introduced at the edges. Above 30 ML, the film is almost continuous, and the interfacial misfit dislocations reorder to form a square network, oriented along 〈110〉 directions. Stacking faults appear in Ag at this stage. A small amount of twinned Ag also starts to grow around 4 ML. This unusual evolution of the strain in the Ag islands and the following introduction of misfit dislocations are interpreted on the basis of a one-dimensional Frenkel-Kontorova model involving a very weak Ag-MgO interaction and a weak corrugation of the interatomic potential. Quantitative measurements and analysis of the MgO crystal truncation rods (CTR's) during growth were shown to provide different structural parameters of the interface that are important for theoretical calculation, especially the epitaxial site, above oxygen atoms of the substrate, and the interfacial distance $(2.52\ifmmode\pm\else\textpm\fi{}0.1\AA{}).$ The origin of the interference along the CTR's is discussed according to the strain state of the epitaxial Ag.