Low Energy Ar Ion Bombardment of (001) Si: Defects and Surface Morphology
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LOW ENERGY Ar ION BOMBARDMENT OF (001) Si: DEFECTS AND SURFACE MORPHOLOGY
M.V.R. Murty and Harry A. Atwater Thomas J. Watson Laboratory of Applied Physics California Institute of Technology, Pasadena, CA 91125
ABSTRACT The interaction of low energy Ar+ ions with several surface defect structures on (001) Si has been investigated using molecular dynamics simulation. The simulations suggest that ions with energy less than 20 eV selectively displace surface atoms without causing bulk damage, and that the displacement energies for different defect structures is different. The most important effect of ion bombardment on surface morphology is the increased formation rate of single adatoms, which may lead to smoother surfaces by enhanced coarsening at typical epitaxial temperatures. Simulations results also imply that the migration component of adatom diffusion is not significantly enhanced by ion bombardment at typical epitaxial temperatures (600 - 800 K). INTRODUCTION Ion bombardment during thin film growth has been employed in various processes such as bias sputtering, reactive sputter deposition and plasma enhanced chemical vapor deposition. Several different phenomena may occur as a result of low energy ion bombardment during film growth, including enhanced adatom diffusion, sputtering, dissociation of small islands and generation of new nucleation sites. There is currently considerable interest in the application of low energy (E < 1 keV) ion beams for both direct ion beam deposition and ion beam-assisted epitaxy. Noble gas ions have been used to modify surface morphology [1] and strain [2] in epitaxial growth of Ge and SiGel-. , suppress island formation in growth of InAs on Si [3] and change the growth mode from 3-D to 2-D in epitaxy of GaAs on Si [4]. Although there is great interest in modification of surface kinetics, in order to grow device-quality films, it is essential to limit the bulk damage caused by the impinging ion beam. Since the ion-surface interactions of interest occur during time intervals of less than 10-12 sec, molecular dynamics (MD) can be used to delineate the essential kinetic processes in the prompt regime during ion-assisted epitaxy. Previous MD studies of ion bombardment on silicon include sputtering by keV range Ar+ ions [5], direct ion beam deposition using 10 eV Si ions [6] and observation of surface channeling on (111) Si [7]. These studies have employed semiempirical classical interatomic potentials and idealized smooth surfaces. A more realistic surface morphology consists of various defect structures such as vacancies, isolated atoms, dimer strings, ledges and kink sites. Some or all of the possible effects of ion bombardment may occur preferentially at certain defect structures. A better understanding of the principal effects of ion bombardment can be gained by studying the interactions of the incident ion beam with a surface consisting of the above-mentioned defect structures. MD calculations are generally limited at present to small ensembles of atoms (typically 100 -1000) and
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