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 using 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 resulting from imperfections on the surface. We observe that the stress fields originating from nearby reconstructions transform the IC phase into a 1 X 1 structure. The transformation involves only small displacements 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 clusters 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 normal to the surface relative to the surrounding unreconstructed regions. The formation of the IC phase for Pb/Si(111) offers a novel system for understanding incommensurate surface reconstructions in a chemisorption system.