GROWTH AND MORPHOLOGY OF PB ON SI(111)

Tunneling microscopy, thermal desorption, Rutherford backscattering, and low-energy electron diffraction are used to study the structures and coverages of the phases of Pb on the Si(111) 7 x 7 surface. For room-temperature deposition at low coverage on the 7 x 7 surface, Pb atoms occupy sites above the rest atoms and between the Si adatoms (with a preference for the faulted half of the unit cell). At 0.6 ML, the Pb forms an ordered overlayer based on the 7 x 7 unit cell. The Pb grows epitaxially up to 3 ML at which point Pb crystals start to form. On annealed samples at low coverage, Pb atoms occupy Si(111) 7 x 7 adatom sites. At 1/6 ML, a new square-root 3 x square-root 3 mosaic phase consisting of alternating chains of Pb and Si adatoms with a high melting point is produced. At 1/3 ML the standard square-root 3 x square-root 3 phase is observed. Between 1/3 and 1 ML, a 1 x 1 Pb overlayer is found on annealed samples, while above 1 ML, a rotated incommensurate phase is observed. As more Pb is added above 1 ML, the 2D Pb density is increased, and the incommensuration is reduced, until island formation begins. We link our discussion of the atomic structure of the interface to the variations in Schottky barrier heights observed for Pb/Si(111) diodes.