HIGH COVERAGE PHASES OF PB ON THE SI(111) SURFACE - STRUCTURES AND PHASE-TRANSITIONS

We have studied the high coverage phases of Pb on a Si(lll) surface us ing a scanning tunneling microscope (STM). For annealed samples, 1 ML Pb forms an incommensurate (IC) phase composed of alternating domains of two types of trimer regions and a quasi-1 x 1 region. The detailed morphology of the domains depends sensitively on the stress fields res ulting from imperfections on the surface. We observe that the stress f ields originating from nearby reconstructions transform the IC phase i nto a 1 X 1 structure. The transformation involves only small displace ments of Pb atoms. We observe that the 1/3 ML root 3 phase suppresses the formation of trimers in the 1 X 1 phase, but Pb islands and Pb clu sters enhance their formation. The same structural transformation from IC to 1 X 1 can occur at high temperatures. The local transformation temperature depends strongly on surface imperfections nearby. The high coverage phase for room temperature (RT) deposition, Pb/Si-7 X 7, is also resolved. Pb deposition on this phase causes interesting changes in its morphology. The 1 X 1 adatoms in Pb/Si-7 X 7 appear to relax no rmal to the surface relative to the surrounding unreconstructed region s. The formation of the IC phase for Pb/Si(111) offers a novel system for understanding incommensurate surface reconstructions in a chemisor ption system.