Atomic bonding in amorphous alloys based on carbon, nitrogen, and hydrogen: A thermodynamic approach

A thermodynamic model previously outlined for the prediction of the bonding in amorphous hydrogenated carbon-nitrogen alloy films (a-C_xN_yH_z), is extended here to include the effects of enthalpy and entropy. Predictions are presented for the bonding of tetrahedral C(sp³), trigonal C(sp²), pyramidal N(sp³), and trigonal N(sp²) atoms in the alloy as well as the bonding of H to these atoms. When bond energies alone are considered, it is predicted that typical a-C_xN_yH_z films will undergo a phase separation into graphitic regions containing C(sp²) atoms, and C(sp²)-N(sp³)-H groups, and diamond-like or polymeric regions containing only C(sp³) and H atoms. When the effects of entropy are also included, phase separation is eliminated and C(sp³)-C(sp²), C(sp²) = N(sp²), and C(sp²)-N(sp²), bonds are also predicted to be present. The model predictions are compared with experimental results for typical amorphous a-C_xN_yH_z alloys that have been prepared via plasma-enhanced chemical vapor deposition from mixtures of nitrogen and acetylene. In a film with y = 0.07 the carbon atoms sp³/sp² ratio is predicted to be 0.7, while 80% of N atoms are predicted to be trigonal N(sp²) atoms.