Studies of the growth and local atomic bonding of a-C_xN_yH_z films

摘要

A systematic study of the growth and local atomic bonding (LAB) of a series of a-C_xN_yH_z alloy films prepared via plasma enhanced chemical vapor deposition (PECVD) from mixtures of NH_3/C_2H_2 and N_2/C_2H_2 is presented. The effects of the reactants and their ratio on the film composition and bonding of hydrogen, the optical constants, and the optical energy gap have been determined. It is found that the presence of nitrogen and hydrogen in the plasma decreases the net film deposition rate due to enhanced etching and that incorporation of nitrogen into the film is limited to N/Capprox approx 0.1. For the films deposited from N_2/C_2H_2 as the concentration of N in the film increases, the concentration of C also increases while that of H decreases. No evidence for the type of bonding expected in the theoretically-predicted C_3N_4 compound has been found. It is observed that hydrogen is preferentially bonded to nitrogen and there is evidence for N-H...N hydrogen bonding in the films. The predictions of the free energy model (FEM) previously developed for the bonding in a-C_xN_yH_z alloys are compared with these experimental results. The importance of entropy in determining the most stable state of these alloys is demonstrated. The FEM successfully predicts and explains some of the experimentally-observed properties of the alloys.