AVS 45th International Symposium
    Surface Science Division Thursday Sessions
       Session SS1-ThA

Paper SS1-ThA3
Ion-Induced Surface Diffusion of Ge on Si(111)

Thursday, November 5, 1998, 2:40 pm, Room 308

Session: Surface Diffusion
Presenter: E.G. Seebauer, University of Illinois, Urbana
Authors: E.G. Seebauer, University of Illinois, Urbana
R. Ditchfield, University of Illinois, Urbana
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In thin film deposition, bombardment of the surface with low-energy (10-100 eV) ions during growth has been shown to improve film quality in a variety of applications. In many cases, the improvement is attributed to enhancements in surface diffusion of the depositing species. However, such effects have never been quantified in a direct way, and the mechanisms for energy transfer from the ions to the mobile atoms remain unclear. Here we report for the first time the direct quantification of ion-influenced surface diffusion, using Ge on Si(111) as the adsorption system and second harmonic microscopy as the measurement probe. The effects are significant and fall into two regimes of temperature. Below about 700 C, the activation energy E@sub diff@ remains unaffected, but the pre-exponential factor D@sub o@ increases. The increase varies as the square root of ion energy and mass (going from He to Ar to Xe), culminating in a factor of 10 increase for Xe near 60 eV. Simulations by molecular dynamics reproduce this effect nearly quantitatively, and point to an increase in the average jump length due to billard-ball-like collisions as the governing mechanism. At higher temperatures, both E@sub diff@ and D@sub o@ decrease dramatically, again according to a square-root energy and mass dependence. E@sub diff@ falls by 75%, while D@sub o@ falls by 8 orders of magnitude. Molecular dynamics simulations again reproduce the effects, and point to ion-mediated changes in the number of mobile adatoms as the governing mechanism. In both temperature regimes, the effects set in only above a threshold energy of about 15 eV.