SURFACE-MORPHOLOGY OF PB OVERLAYERS GROWN ON SI(100)-(2X1)

The behavior of Pb overlayers deposited on Si(100)-(2 X 1) at room temperature was investigated by scanning-tunneling microscopy and low-energy electron microscopy. Surface reconstructions of (2 X 2), c (4 X 8), (2 X 1), and c(4 X 4) were observed as Pb coverage increased from 0.5 to 1.5 monolayers. After the appearance of the c(4 X 4) phase, Pb islands with threefold-symmetric (111) orientation were observed on the twofold- or fourfold-symmetric Si(100) surface. The transformation of the island shape during growth can be broadly categorized as hexagonal-->triangular-->hexagonal. An internal structure of stepped-down depression was formed on all islands. As deposition continued, the depression was gradually filled by a step-flow growth mechanism. Our observations suggest that the initial growth of the islands is dominated by diffusion of Pb atoms on the substrate to the island edges; later, when the islands become sufficiently large, growth is determined mainly by atom diffusion on the Pb(111) islands.