Structure and morphology of the Ag/MgO(001) interface during in situ growth at room temperature

The structure and morphology of Ag deposits grown at room temperature on hi gh-quality MgO(001) surfaces have been investigated in situ, from 0.2 to 30 0 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 island s are found from 0.2 ML. The average in-plane width, height, and in-plane s eparation of growing islands are deduced and are found to reproduce well th e Mg Is x-ray photoemission spectroscopy spectrum previously reported by ot her authors. The height over width ratio of the islands is similar to 0.37 +/- 0.05 at all stages of the deposit. Ag grows in cube-on-cube epitaxy wit h 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 Angstrom), the small Ag islands are coh erent with the MgO. Below 1 ML (island width smaller than 35 Angstrom), the y have their bulk lattice parameter, and between 1 and 4 ML they become mor e 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 re ach a critical size and misfit dislocations are introduced at the edges. Ab ove 30 ML, the film is almost continuous, and the interfacial misfit disloc ations reorder to form a square network, oriented along [110] directions. S tacking faults appear in Ag at this stage. A small amount of twinned Ag als o starts to grow around 4 ML. This unusual evolution of the strain in the A g islands and the following introduction of misfit dislocations are interpr eted on the basis of a one-dimensional Frenkel-Kontorova model involving a very weak Ag-MgO interaction and a weak corrugation of the interatomic pote ntial. Quantitative measurements and analysis of the MgO crystal truncation rods (CTR's) during growth were shown to provide different structural para meters of the interface that are important for theoretical calculation, esp ecially the epitaxial site, above oxygen atoms of the substrate, and the in terfacial distance (2.52 +/- 0.1 Angstrom). The origin of the interference along the CTR's is discussed according to the strain state of the epitaxial Ag. [S0163-1829(99)14131-6].