Growth of Ag on Cu(100) studied by STM: From surface alloying to Ag superstructures

The nucleation and growth of Ag on Cu(100) and the resulting surface structures have been studied with variable temperature (150–330 K) scanning tunneling microscopy (STM). At temperatures below 250 K, islands of Ag, having a pseudohexagonal, c(10×2) overlayer structure, nucleate and grow on Cu terraces and at step edges at Ag subsaturation coverages. However, at temperatures at or above 300 K, a substitutional Ag-Cu surface alloy forms, showing that the formation of the surface alloy phase is an activated process. It is found that only a limited amount of Ag (∼0.13 ML) can be accommodated within the Cu(100) surface layer. At higher coverages (0.13 ML⩽${\mathrm{\theta }}_{\mathrm{Ag}}$<0.9 ML), the strain energy induced by the alloyed Ag becomes so high that the Ag atoms segregate into small patches of c(10×2) superstructure located within the Cu surface layer. Upon Ag deposition at or above the first monolayer at 425 K, a simple pseudohexagonal overlayer structure is observed, indicating that the Ag-Ag interaction dictates the overall structure. Based on atomically resolved STM images of the c(10×2) superstructure, a structural model is presented, and a mechanism is suggested which explains how the surface alloy phase is converted into the overlayer structure. © 1996 The American Physical Society.