Contrib. Plasma Phys. 46, No. 7-9, 757 – 762 (2006) / DOI 10.1002/ctpp.200610074 Monte Carlo Modelling of Structure and Porosity of Co- Deposited Layers P. N. Maya ∗ , S. P. Deshpande, and M. Warrier Institute for Plasma Research, BHAT, Gandhinagar, Gujarat, India - 382428 Received 19 October 2005, accepted 9 February 2006 Published online 22 August 2006 Key words Monte Carlo, film growth, co-deposited layer, porosity, structure. PACS 66.30.-h,66.30.Ny A 3-dimensional Monte Carlo model for growth of hydrocarbon films on surfaces having dangling bonds is presented. The model is microscopic in the sense that it incorporates the stereoscopic orientations of the de- posited hydrocarbons and macroscopic in that it considers shadowing effects and porosity. The parameters like surface roughness, porosity, H/C ratio and sp 2 /sp 3 ratio are related to (a) steric repulsion, (b) probability for hydrogen abstraction, (c) sp 2 /sp 3 configuration preference and (d) radical site densities. It is seen that the steric repulsion between the co-deposited molecules is an important parameter in deciding the micro–porosity and surface roughness of the films. The surface roughness of the grown films appears to show a consistent scaling exponent when compared to the well known ballistic model for nonlinear evolution of interfaces. Initial results on porosity and structure of the grown films for square samples ranging from 7-56 ˚ A linear dimensions with periodic boundary conditions are presented. c  2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Introduction and Motivation Carbon based materials like graphite or carbon fibrous composites (CFC) are extensively used for making plasma facing components (PFC) in magnetic confinement based fusion devices. However, in recent times certain limi- tations of such materials have come to the fore, posing major questions on the type of plasma facing material to be used [1]. This is because of the strong interaction of hydrogen (H), deuterium (D) or tritium (T) with carbon. Erosion of carbon [2,3] and its co-deposition with H-isotopes results in an undesirable buildup of fuel inventory in the form of a thin layer on the inner surface of fusion devices. There is a clear evidence of the formation of co-deposited layers at surfaces shielded from direct interaction with the plasma and the radiation emanating from it, at remote locations in the fusion devices [4–6]. The deposits are highly porous [7] and can form both hard [5] and soft films [4,6]. It is expected that deposits at such remote locations will be one of the major channels of fuel (H/D/T) retention in fusion reactors [1]. Therefore, there is a need to understand their formation and structure from the above point of view. Some of the existing models for co-deposited layer growth in fusion devices have studied the growth rate considering the interactions like hydrogen abstraction, surface coverage of radical sites and incoming fluxes of various species using a zero-dimensional particle balance [8,9]. Micro-structural information cannot be obtained from such models. In extreme contrast to such macro-scale models, there are the Molecular Dynamic (MD) models, where, information on each and every movement of atoms on a femto-second timescale is retained. This leads to prohibitive computing times for realistic sample sizes (several tens of thousand ˚ A 3 , at least). The Monte Carlo (MC) techniques used in the past, in the context of diamond-like-carbon (DLC) formation, appear to be most suited for our purpose [10–13], where, a probabilistic outcome of an atomic interaction is considered, rather than the detailed dynamics. These studies, however, were for the cases where there was a continuous presence of plasma in contact with the film, with H atoms and energetic ions incident on the film actively taking part in its ∗ Corresponding author: e–mail: mayapadi@ipr.res.in, Phone: +91-79 23969001 Ext: 301, Fax: +91-79 23969017 c  2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim