AVS 51st International Symposium
    Surface Science Thursday Sessions
       Session SS3-ThA

Paper SS3-ThA5
Temperature-Dependent Thresholds for Ion-Stimulated Surface Diffusion: A Comparison of Si and Ge Substrates

Thursday, November 18, 2004, 3:20 pm, Room 213B

Session: Surface Diffusion and Transport
Presenter: E.G. Seebauer, University of Illinois at Urbana-Illinois
Authors: Z. Wang, University of Illinois at Urbana-Illinois
E.G. Seebauer, University of Illinois at Urbana-Illinois
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Ion-surface interactions at low energies (<100 eV) characterize an increasingly diverse array of material processing steps in ion beam assisted deposition (IBAD), plasma enhanced deposition, reactive ion etching (RIE), and other applications. The governing kinetic phenomena are often tacitly considered to lie at one of two poles: physical effects where momentum matching dominates, and chemical effects involving thermal activation of atomic bonds according to Arrhenius expressions. Here we report surface diffusion measurements demonstrating behavior that lies at neither pole. Optical second harmonic microscopy is used to image diffusion of indium adsorbed on Si(111) and Ge(111) under low-energy noble-gas ion bombardment. Both systems exhibit a surprising tradeoff between substrate temperature and the energy threshold at which ion influences become manifest. Molecular dynamics simulations suggest that the effect originates from changes in surface point defect concentrations. Instantaneous nonuniformities in net surface potential induced by thermal vibrations provide a mechanism by which ions can affect these concentrations nonlinearly. The effects generally increase the rate of mass transport across the surface. The simulations and experiments agree in finding that an important factor determining the magnitude of the tradeoff is the strength of the adsorbate-substrate bond, which is lower for In/Ge than for In/Si.