(294a) Atomic Hydrogen Interactions with Amorphous Carbon Thin Films

Hydrogenated amorphous carbon (a-C:H) is a technologically useful material with a wide variety of applications due to its high hardness and chemically inert nature. a-C:H films are generally deposited using plasma enhanced chemical vapor deposition from hydrocarbon feed gases. The structure and properties of these films are defined by the sp²-to-sp³ hybridization ratio and the H content. It is well known that hydrocarbons diluted in hydrogen are a precursor for the growth of microcrystalline and nanocrystalline diamond. Interaction of H generated in the plasma results in local and overall transformations to a diamond-like structure due to reactions such as hydrogenation, insertion into C-C bonds, surface H abstraction, and hydrogen-induced etching. We have employed classical molecular-dynamics (MD) simulations based on the modified extended Brenner potential and experiments to study atomic H interactions with a-C:H thin films. Using MD, we first developed a procedure for creating realistic a-C:H thin films and formulated a scheme to characterize the sp²-to-sp³ hybridization ratio. These films were then impinged with H atoms at random locations and the specific chemical reactions of the H atoms with the a-C:H surface were identified through a detailed analysis of the MD trajectories. The MD simulations showed that hydrogenation occurs primarily at the sp² sites and converts them to sp³-hybridized C atoms. Depending on the hybridization of the next-nearest neighbor, a dangling bond may or may not be created. The hydrogenation reaction is highly exothermic, > 2.5 eV, and proceeds via a direct Eley-Rideal mechanism with a negligible activation energy barrier. In certain cases hydrogenation may also cleave a C-C bond. Further hydrogenation eventually leads to etching of the film through desorption of stable hydrocarbon species. In addition, barrierless surface hydrogen abstraction from both sp³ and sp²-hybridized C atoms is also observed. The reaction events observed through MD simulations are consistent with the surface characterization of D-exposed a-C:H films using Raman spectroscopy, spectroscopic ellipsometry, and in situ attenuated total reflection Fourier-transform infrared spectroscopy.