Rate-equation modelling of ion beam assisted homoepitaxy on low-index surfaces of Ag and Cu
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Rate-equation modelling of ion beam assisted homoepitaxy on low-index surfaces of Ag and Cu Jussi K. Sillanp¨aa¨ *, Ismo T. Koponen** and Niels Grønbech-Jensen* * Department of Applied Science, UC Davis, Davis, CA 95616, USA ** Department of Physical Sciences, University of Helsinki, P.O.Box 9, SF-00014 University of Helsinki, Finland ABSTRACT We develop a rate-equation model for ion beam assisted homoepitaxy. The model describes how island diffusion, detachment and breakup, generation of surface damage and interlayer transitions of adatoms and monovacancies affect the growth. We simulate the (100) and (110) surfaces of silver and the (100) surface of copper using potential energy barriers calculated using either surface embedded atom method (SEAM) or the effective medium theory (EMT). We study how the choice of the potential barriers affects the growth and comment on the suitability of SEAM and EMT for calculating barriers for surface simulation. We demonstrate how different processes affect the microstructure of the film, compare the growth on different surfaces and study the scaling properties of the results. INTRODUCTION Low energy (typically 20 - 600 eV) ion beams can be applied for improving the properties and growth of thin film surfaces in deposition [1–4]. The objective of ion beam assisted deposition (IBAD) is to obtain smooth layer-by-layer growth in low temperatures, either directly by destruction of growing 3D structures or by making the 2D growth the kinetically more favorable growth mode through promoting interlayer transitions [1–4]. Although the effects of IBAD on growth have been studied experimentally for over twenty years (see e.g. [3] and references therein), only recently has there been growing interest towards the theoretical understanding of the microscopic phenomena affecting the growth in IBAD [4–7]. Because of the low deposition speeds (typically several hundred seconds to grow a monolayer) and the relative large number of atoms required to capture all relevant phenomena, atomistic simulations of IBAD are extremely timeconsuming and several unrealistic assumptions concerning, e.g., the interatomic potentials and the time between impacting ions, have to be made. Atomistic simulations, such as the combined molecular dynamics (MD) / kinetic Monte Carlo (KMC) study by Jacobsen et al.[5], do, however, provide important insights into rate equation models which have so far suffered from a lack of microscopic evidence to assess the form of the phenomenological rate coefficients [8–10]. We have recently studied the effects of different surface processes in IBAD on submonolayer growth by using rate equation models [8, 9], and modelled layer-by-layer growth with both a much simplified model utilizing only two variables per layer [11] and a more detailed model including islands of all sizes [10]. Here we study submonolayer growth with a rate-equation model which incorporating also monovacancies and vacancy islands. The microscopic processes affecting the growth can be divided into processes related to
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