AVS 50th International Symposium
    Surface Science Monday Sessions
       Session SS1-MoA

Paper SS1-MoA9
Temperature-Dependent Thresholds for Ion-Stimulated Surface Diffusion: Experiments with Second Harmonic Microscopy

Monday, November 3, 2003, 4:40 pm, Room 326

Session: Stimulated Processes at Surfaces
Presenter: E.G. Seebauer, University of Illinois at Urbana Champaign
Authors: Z. Wang, University of Illinois at Urbana Champaign
E.G. Seebauer, University of Illinois at Urbana Champaign
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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 Ge and In adsorbed on Si(111) under low-energy noble-gas ion bombardment. Both adsorbates exhibit a surprising new form of 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 except for Ge above the Si(111)-(7x7) to (1x1) phase transition, where ion-induced changes in charge-mediated complexation of mobile species with surface defects slows the diffusion.