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Research Activities > Programs > Nonequilibrium Interface and Surface Dynamics 2007

Chemical potential for nucleation of surface islands using beams of self-ions

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Chemical potential for nucleation of surface islands using beams of self-ions

Professor Peter Flynn

University of Illinois, Urbana-Champaign


On an existing low energy electron microscope (LEEM), built by Tromp, we have installed an accelerator that provides intense beams of negative self-ions at normal incidence to the LEEM sample.  At low ion impact energies the beam yields  epitaxial (hypothermal) growth and drives the chemical potential m* positive.  Above a 'neutral' impact energy at which the beam creates only surface Frenkel pairs, the beam erodes the surface and drives m* negative.  On large terraces, 6-10mm in radius, created by ion beam methods to be described, we have investigated the ion beam intensity needed to nucleate and grow single new adatom and advacancy islands.  From the measured beam intensities, the value of m* needed for nucleation is determined for both adatom and advacancy islands.  Since the surface adatom and advacancy defects are strongly reacting, the interpretation that links intensity to chemical potential employs a recent theory[1] in which a single m* describes the reactivity of both species of antidefect.  We obtain a global prediction that nucleation in the two cases occurs for approximately equal but opposite changes of m* caused by ion beams.  In experiments on Pt(111) using Pt-  ions with impact energies of 50 eV and 500 eV for growth and erosion, respectively, in the temperature range 800K < T < 1000K, the predicted symmetry between the islands formed from the two antidefects is clearly demonstrated.  Moreover, the magnitude of m* required to drive nucleation through ion beam irradiation is within 30% of the value predicted by Pimpinelli and Villain in an earlier treatment of sublimation[2]. 

[1]  C P Flynn, Phys Rev B71, 085422 (2005).

[2]  A Pimpinelli and J Villain, Physica A204, 521 (1994).

*  Research supported by the Department of Energy.

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