AVS 52nd International Symposium
    Surface Science Wednesday Sessions
       Session SS1-WeM

Paper SS1-WeM4
Real-Time Microscopy of Phase Transitions on Pb/Ge(111)

Wednesday, November 2, 2005, 9:20 am, Room 200

Session: Growth and Alloying of Surfaces
Presenter: S. Chiang, University of California, Davis
Authors: S. Chiang, University of California, Davis
Y. Sato, University of California, Davis
Correspondent: Click to Email
Using low energy electron microscopy (LEEM), we have elucidated the phase diagram for the growth of Pb on Ge(111). The Pb layer can form two stable @sr@3x@sr@)R30° phases on the surface, less dense @alpha@ and denser @beta@ phases. As Pb is deposited on Ge, the Pb atoms substitute into the top layer, causing released Ge atoms to form into c(2x8) adatom islands, with the size and density of these Ge islands controllable by the substrate temperature. With 11.6 eV electrons at 300C, we see the growth of the white (1x1) phase with respect to the darker @alpha@ phase in a coexistence region. During the reversible @beta@ to (1x1) phase transition, we discovered a novel phase separation mechanism. Above the 1.33ML @beta@ saturation coverage, a sharp first order phase transition is observed near 295C. For Pb coverage just <1.33ML, the @beta@ to (1x1) phase transition is no longer sharp, with the @beta@ and (1x1) phases coexisting and the transformation occurring from 232C to 181C. Reducing coverage by ~0.01ML causes a dramatic change. When the transition starts, small domains of the new phase appear and disappear, due to fluctuations between the two phases. As the initial domains stabilize in the new phase, additional domains appear and fluctuate until the whole surface is completely transformed. By using statistical mechanics analysis,@footnote 1@ we attribute the fluctuating domains to thermal fluctuations of the density of Pb atoms within a domain. By comparing LEEM images of the @beta@ and (1x1) phases during the phase transition between [@alpha@+(1x1)] and (@alpha@+@beta@), the Pb coverage of the (1x1) phase at the eutectic point was determined to be ~1.29 ML. @FootnoteText@ @footnote 1@ N. Bartelt, Sandia National Laboratories, Livermore, CA.