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

The behavior of Pb overlayers deposited on Si(100)-(2 X 1) at room tem perature was investigated by scanning-tunneling microscopy and low-ene rgy 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 is lands 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 depre ssion was formed on all islands. As deposition continued, the depressi on was gradually filled by a step-flow growth mechanism. Our observati ons suggest that the initial growth of the islands is dominated by dif fusion of Pb atoms on the substrate to the island edges; later, when t he islands become sufficiently large, growth is determined mainly by a tom diffusion on the Pb(111) islands.