Surface morphology changes during Pb deposition on Cu(1 0 0): evidence for surface alloyed Cu(1 0 0)–c(2×2) Pb

Abstract

Using low energy electron microscopy (LEEM), we have followed Cu(1 0 0) surface morphology changes during Pb deposition at different temperatures. Surface steps advance and two-dimensional (2-D) islands nucleate and grow as deposited Pb first alloys, and then dealloys, on a 125°C Cu(1 0 0) surface. From LEEM images, we determine how much Cu is being displaced at each stage and find that the amount of material added to the top layer for a complete Pb/Cu(1 0 0) c(4×4) reconstruction (a surface alloy) is consistent with the expected c(4×4) Cu content of 0.5 monolayer. However, as the surface changes to the Pb/Cu(1 0 0) c(2×2) overlayer, we find that the displaced material from surface dealloying, 0.22 ML, is more than a factor of two lower than expected based on a pure Pb c(2×2) overlayer. Further, we find that in the 70–130°C range the amount of Cu remaining in c(2×2) increases with increasing substrate temperature during the deposition, showing that surface Cu is alloyed with Pb in the c(2×2) structure at these temperatures. When holding the sample at 125°C, the transformation from the c(2×2) structure to the higher coverage c(5√2×√2) R45° overlayer structure displaces more Cu, confirming the c(2×2) surface alloy model. We also find the c(2×2) surface has characteristically square 2-D islands with step edges parallel to the (1 0 0) type crystallographic directions, whereas the c(5√2×√2) R45° structure has line-like features which run parallel to the dislocation double rows of this surface’s atomic structure and which expand into 2-D islands upon coarsening.