Growth of Ag on Cu(001) studied by full-hemispherical X-ray photoelectron diffraction

Ag films, from the submonolayer range up to 14 monolayers (ML), have been deposited in situ on two differently cut Cu(001) single crystals. Their geometrical structure has been investigated by full-hemispherical X-ray photoelectron diffraction and by low-energy electron diffraction. Structural models developed on the basis of these data have been compared to single-scattering cluster calculations. Our results indicate the formation of a surface alloy in the initial step of the Ag growth. At coverages below 0.3 ML, we find that the local environment of the Ag atoms remains fourfold. A first indication for small quasi-hexagonal c(10 × 2) clusters, which are known to appear at 1 ML, is already observed at 0.3 ML. Ag films above 1 ML are compact and exhibit a threefold fcc (111) structure. Furthermore, we noticed that on one of the crystals one of the two possible c(10 × 2) orientations is clearly dominant, and mainly one of the four expected fcc (111) domains grows. On the other crystal the two c(10 × 2) orientations are equally present. Moreover, two fcc (111) domains rotated by 90° with respect to each other develop. We have clear evidence that, from the two possibilities of fcc (111) growth on each c(10 × 2) domain, only one occurs. The comparison of the results from the two differently cut crystals indicates that the domain population is related to the absolute miscut of the substrate and hence by the course of the steps.