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Mass transport during step motion on the Si(111) (1x1) surface studied by low energy electron microscopy

Authors Pang, Angbo
Issue Date 2009
Summary Atomic steps are common defects at surfaces that can play an important role in many physical phenomena. Step morphology will be affected, or even dictated, by the kinetic processes that mediate growth and its inverse, sublimation. At the same time, competing coarsening processes will occur that depend crucially upon the step line tension through the Gibbs-Thomson relation. A proper description of step morphological phenomena therefore requires accurate knowledge of step line tension, as well as step kinetic parameters. The complex interplay between step kinetic and coarsening effects was investigated on the Si(111) (1x1) surface by examining step motion during island decay using low energy electron microscopy. These investigations provide quantitative information on the step line tension, kinetic length and step permeability. It is shown that the line tension decreases linearly with increasing temperature between 1145 K and 1233 K with a temperature coefficient of −0.14 meV/Å K. The kinetic length is determined to be 75a at 1163K, where a is the lattice constant. This locates step motion firmly in the diffusion-limited regime. Steps are also determined to be impermeable in the context of diffusion limited step kinetics. We also find that the role of desorption in island decay increases dramatically in the temperature range (1145−1380 K) that island decay is studied. Consequently, we generalize the current model of island decay to take account of desorption. Evaluation of the island decay time with this model referenced to the temperature-dependent line tension accurately determines activation energies that are central to mass transport and sublimation. Similar investigations of vacancy island decay were also carried out. Surprisingly, island decay and vacancy island decay behavior cannot be explained consistently using any form of model that treats mass transport exclusively in terms of the diffusion of adatoms that are generated at steps. An adatom-vacancy decay model is proposed to address this problem. The foregoing experimental investigations are possible because of the capability of LEEM to image the motion of atomic height surface steps in real-time. In order to better understand this unique capability, a Fourier optics calculation of image formation in LEEM is carried out. The adaptation of the existing Fourier optics theory for transmission electron microscopy to the treatment of LEEM is explained. The model calculation incorporates imaging errors that are caused by the objective lens, contrast aperture, imperfect source characteristics, and voltage and current instabilities. It is used to evaluate the appearance of image features that arise from pure phase objects such as surface steps as well as amplitude objects. Resolution and the predicted experimental observation is also discussed. This formalism can also be used after appropriate modification to treat image formation in other emission microscopies.
Note Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2009
Language English
Format Thesis
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