SPA-LEED study of the morphology and nucleation of a novel growth mode and the "devil's staircase" on Pb/Si(111)

Yeh, Wang-Chi
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In this thesis we describe two intriguing and unexpected discoveries we made in the Pb/Si(111). A novel growth mode was discovered on Pb thin film grown on Si(111)-7 x 7 reconstructed surface: nano-scale islands of uniform 7-layer height with steep edges and flat tops can form below room temperature. This growth mode is different from the three common modes: Frank-van der Merwe, Volmer-Weber, and Stranski-Krastanov modes. The three common modes could be explained by thermodynamic considerations on the surface free energy and do not lead to the type of self-organization we have discovered. This novel mode is explained by quantum size effects (QSE), which states that the confinement of electrons inside a well requires that the dimensions of the well meet the requirements of energy quantization: the electron wavefunctions have to form standing waves within the confining well. Further study showed that different stable heights that differ by two-layer difference could be selected by changing deposition temperature, total coverage, annealing temperature, or initial interface. Oxygen covered structures are found to retain their selected height up to almost room temperature.;In addition, a novel "devil's staircase" was discovered in Pb/Si(111) between coverage 6/5 ML and 4/3 ML. Experimentally it has been an outstanding challenge to show a "devil's staircase" since physical phenomena are not expected to be described by non-differentiable functions, i.e., the stability curve Deltamu vs. theta of a "devil's staircase". This novel staircase differs from conventional ones in the difference between their two generating phases: their [11[macron]0] dimensions differ by two lattice constants instead of one. It was found unexpectedly that such a staircase can form with macroscopic spatial extent (~0.5 mm) even at low temperature (T ~ 120K) where atoms are not expected to mobile! This unusual result suggests that a high degree of self-organization is possible in the system of Pb/Si(111).

Physics and astronomy, Condensed matter physics