Atomic mechanisms of the formation of an ordered surface alloy: An STM investigation of Mn/Cu(100)

Atomic processes important in surface alloy formation for Mn/Cu(100) were determined using STM (scanning tunneling microscopy). By exploiting the pronounced corrugation, incorporated Mn atoms were distinguished from Cu atoms in the STM images. Mn atoms are preferentially incorporated in the vicinity of island edges and substrate steps. In the proximity of substrate steps, the rate of incorporation into the lower terrace is correlated with the kink density of the steps. This indicates that the interlayer mass transport necessary for Mn incorporation into the lower terrace is particularly efficient close to kinks. Atomic exchange processes and vacancy mechanisms which proceed in the vicinity of kinks can explain this observation. Vacancy annihilation by Mn adatoms is the only process which at least qualitatively can account for both the observed coverage dependence of Mn incorporation and the importance of kinks. The alloying of the upper terrace of substrate steps is driven by an attachment of Mn adatoms. This step is essentially independent of the step orientation, which explains why the concentration of incorporated Mn in the vicinity of steps does not depend upon the step direction. Subsequent step fluctuations and more importantly, vacancy-mediated diffusion of incorporated Mn atoms, lead to a more homogeneous Mn distribution with time.