An Atomistic Study of Surface Vacancy Diffusion
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AN ATOMISTIC STUDY OF SURFACE VACANCY DIFFUSION
L. ZHAO*, R. NAJAFABADIt, and D. J. SROLOVTZt* * University of Michigan, Applied Physics Program, Ann Arbor, MI 48109 t University of Michigan, Dept. of Materials Science and Engineering, Ann Arbor, MI 48109 ABSTRACT
Diffusion of atoms and molecules on surfaces plays an important role in the growth of thin films. In the present study, the surface vacancy diffusion on Cu and Ni (100) and (111) planes is investigated via atomistic simulations. This investigation is performed using the Embedded Atom Method (EAM) interatomic potentials and the finite temperature properties are determined within the local harmonic and quasiharmonic frameworks. This study helps reveal fundamentals of surface vacancy diffusion in the thin film growth. Our results show that the vacancy diffusion is important on (100) surface but it is not the dominant diffusion mechanism on (111) plane. INTRODUCTION
gurface diffusion plays an important role in a wide variety of physical processes, such as the growth of thin-films, catalysis, morphology evolution, etc. To date, there have been a large number of experimental and theoretical studies that have attempted to elucidate the fundamental nature of surface diffusion. As in diffusion in the interior of crystalline solids, both vacancy and interstitial diffusion mechanisms may also operate on a surface. In addition, surface diffusion via adatom migration also occurs. Since the crystallographic constraints on the free surface are not as severe as in the interior of a crystal, diffusion mechanism which are a combination of these are also known to occur. The relative importance of these different diffusion mechanisms has not yet been generally established due to experimental difficulties. For example, one of the commonly used tools for studying surface diffusion - field ion microscopy (FIM), cannot be easily applied to study surface vacancy diffusion. Another approach for studying surface diffusion is through the use of computer simulations. Molecular dynamics, in conjunction with the simple Lennard-Jones interatomic potential, has been applied to both vacancy and adatom diffusion on otherwise perfect surfaces [1]. The overlayer dynamics method and embedded atom method type potentials have been applied to the classical dynamics of clusters of Ag adatoms and vacancies on the Ag(100) surface [2]. The Embedded Atom Method (EAM) was used to study diffusion mechanisms on Ni surfaces by molecular statics and molecular dynamics [3]. Comparisons between simulation and experiment often show excellent agreement for activation energies, the preexponential factors in the diffusivity can show significant differences. This point is briefly reviewed below. Since surface diffusion via the vacancy mechanism has received relatively little attention compared with adatom migration, the focus of the present work is on the application of the free energy minimization [4] method to determine the migration energy and the diffusivity for surface vacancy controlled diffusion on the
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