MESOSCOPIC SCALE OBSERVATIONS OF SURFACE ALLOYING, SURFACE PHASE TRANSITIONS, DOMAIN COARSENING, AND 3D ISLAND GROWTH: PbonCu(100)

Low energy electron microscopy (LEEM) is used to investigate the dynamics of Pb overlayer growth on Cu(100). By following changes in surface morphology during Pb deposition, we measure the amount of Cu transported to the surface as the Pb first alloys into the surface during formation of thec(4×4)phase and subsequently dealloys during conversion to thec(2×2)phase. We find that the added coverage of Cu during alloying is consistent with the proposed model for thec(4×4)alloy phase, but the added coverage during dealloying is not consistent with the accepted model for thec(2×2)phase. To account for the discrepancy, we propose that Cu atoms are incorporated in thec(2×2)structure. Island growth and step advancement during the transition from thec(2×2)to$c(5\sqrt{2}\times\sqrt{2}) R 45^\circ$structure agrees with this model. We also use LEEM to identify the order and temperature of the two-dimensional melting phase transitions for the three Pb/Cu(100) surface structures. Phase transitions for the$c(5\sqrt{2}\times\sqrt{2}) R 45^\circ$andc(4×4)structures are first-order, but thec(2×2)transition is second-order. We determine that rotational domains of the$c(5\sqrt{2}\times\sqrt{2}) R 45^\circ$structure coarsen from nanometer- to micron-sized dimensions with relatively mild heating (~ 120°C), whereas coarsening ofc(4×4)domain requires considerably higher temperatures (~ 400°C). In studies of three-dimensional island formation, we find that the islands grow asymmetrically with an orientational dependence that is directly correlated with the domain structure of the underlying$c(5\sqrt{2}\times\sqrt{2}) R 45^\circ$phase.