(370c) Investigation of the Growth Mechanism during Plasma-Assisted Deposition of a-C:H

We have employed classical molecular-dynamics (MD) simulations, based on the modified extended Brenner potential, to study the structure and properties of hydrogenated amorphous carbon (a-C:H) thin films. The structure and properties of these films are defined by the sp²-to-sp³ hybridization ratio and the H content. In particular, interaction of H generated in the plasma results in local and overall transformations due to reactions such as, insertion into C-C bonds, surface H abstraction, bulk diffusion, and hydrogen-induced etching. In this presentation, we will discuss the mechanism and energetics of these reactions and show how they affect the film structure.

To study the radical-surface interactions, we have developed a procedure for creating realistic a-C:H thin films starting with diamond(001) slabs. Due to the density difference between a-C:H (1.7-2.2 g/cm³) and diamond (3.52 g/cm³) a certain number of atoms were randomly removed from the diamond(001) slab. The resulting structure was amorphized at 1000 K, hydrogenated, relaxed and then thermalized at 700K to create the a-C:H films. We have also formulated a scheme to characterize the sp²-to-sp³ hybridization ratio of these films based on the coordination number and the dangling bonds for each carbon atom, and the values obtained agree well with the range of values reported in the literature for experimentally deposited a-C:H films. Furthermore, the sp²-to-sp³ hybridization ratio and the H content in these films can be varied by tuning their initial density. H-induced reactions were observed by impinging H atoms on the surface of these realistic a-C:H films at 700 K. The detailed energetics of the various reaction pathways along with the structure and energies of the corresponding equilibrium and transition-state configurations will be presented.