Upward mass transport and alloying during the growth of Co on Cu(111)

Co growth on Cu(111) was investigated at several temperatures between 120 K and 300 K by variable-temperature fast-scanning tunneling microscopy at submonolayer coverage. Islands nucleate heterogeneously at step edges and homogeneously on terraces. The height and area distribution difference between these two types of differently nucleated islands is attributed to a step edge alloy. Furthermore, the transformation from one-monolayer high islands to two-monolayer high islands is followed in time-lapsed sequences between 145 and 165 K. A surprising low-energy barrier for upward mass transport of ${E}_{\mathrm{upward}}\ensuremath{\approx}(0.15\ifmmode\pm\else\textpm\fi{}0.04)$ eV is determined for islands on terraces. At 120 and 150 K, the terrace islands are pure Cu; in contrast, at room temperature, terrace islands larger than $\ensuremath{\approx}120$ ${\mathrm{nm}}^{2}$ alloy at their border.