Modelling surface growth in IBAD with rate equations
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Modelling surface growth in IBAD with rate equations Jussi K. Sillanp¨aa¨ *, Ismo T. Koponen** and Niels Grønbech-Jensen* * Department of Applied Science, UC Davis, Davis, CA 95616, USA ** Department of Physics, University of Helsinki, P.O.Box 9, SF-00014 University of Helsinki, Finland
ABSTRACT We develop a rate-equation model for surface growth in ion beam assisted deposition. In terms of the rate coefficients the model describes how island detachment and breakup, adatom diffusion and interlayer transitions of adatoms affect the growth. We identify parameter values corresponding to different modes of growth and discuss our results and ways to improve the model. 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]. The effects of IBAD on growth have been studied experimentally for over twenty years (see e.g. [3] and references therein), but only recently has there been growing interest towards the theoretical understanding of the microscopic phenomena affecting the growth in IBAD [4–6]. A particularly interesting study of the microscopic processes in IBAD, based on molecular dynamics (MD) simulations, has been carried out by Jacobsen et al. [5]. It is the first report where a detailed description of microscopic processes due to ion bombardment is combined with kinetic Monte Carlo (KMC) simulation of macroscopically observed growth phenomena, providing insight into rate equation models which have so far suffered from a lack of microscopic evidence to assess the form of the phenomenological rate coefficients [7, 8]. We have recently studied the effects of different surface processes in IBAD on submonolayer growth by using rate equation models [7, 8], and modelled layer-by-layer growth with a much simplified model which utilizes only two variables per layer [9]. Here we develop a rate-equation model for layer-by-layer growth. The microscopic processes affecting the growth can be divided into processes related to the ion bombardment and to thermally activated rate processes, which are essentially the same as in MBE growth. Ion bombardment causes adatom and defect production (omitted here because of their relative unimportance [7]) and island detachment and breakup. All these processes occur on much shorter timescale than thermally activated rate processes. The effects of all these processes on growth are described with reaction rates and the growth process itself is modeled by rate equations governed by the reaction rates. This kind of modeling lacks the quantitative accuracy of the more detailed KMC-MD models, but it has the advantage of being capable of providing rapidly the qualitative information of the growth. Combined with the
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