Amorphous carbon nitride films (a-CNx) were deposited as the electron field emitters by rf magnetron sputtering method at room temperature. The effect of adding Ar to reactive nitrogen gas on the field emission of a-CNx films is investigated. The addition of Ar increases the proportion and size of sp(2)-bonded clusters in the films and an overfull Ar addition decreases the N content in the film, measured by photoluminescence, x-ray photoelectron spectroscopy, and Raman spectra. Field emission testing results show that the Ar addition effectively improves the field emission ability of a-CNx films. Related to the change of chemical bonding structure and components in the films, the authors found that the enhanced field emission of the films was attributable to the increased proportion of sp2-bonded clusters, including sp(2) C-N and sp(2) C-C bondings, and the decreased N content in the film. As a conduction part of the amorphous network in the films, these rich as-formed sp(2)-bonded clusters not only enable the tunneling in the film to make electron emission easier but also lead to degrading the electron emission barriers and raising the Fermi level to enhance electron emission. (c) 2006 American Vacuum Society.