Modeling of the sp3/sp2 ratio in ion beam and plasma-deposited carbon films

Abstract

Model calculations are described which predict the fraction of tetrahedral (sp3) versus graphitic (sp2) bonding in ion beam or plasma-deposited amorphous carbon films on the basis of the preferential displacement of sp2 atoms. Displacement yields obtained from static trim simulations are used as input data for a simple analytical growing layer model of ion beam deposition. The sp3/sp2 ratio is found to increase with increasing carbon ion energy between 30 eV and 1 keV. Assuming equal probabilities for a free (implanted or displaced) atom to become trapped at either sp2 or sp3 sites results in sp3/sp2 ratios between 1 and 3.5. More refined dynamic simulations with tridyn confirm the trends with slightly lower sp3/sp2 ratios for ion beam or plasma deposition, also involving hydrogen. A decrease of the sp3/sp2 ratio towards high energies cannot be explained by preferential displacement only, in contrast to proposals in recent literature.