Paramagnetic centers of amorphous carbon thin films: Influence of hydrogen content and O₂ permeation

Electron paramagnetic resonance (EPR) studies of sputtered amorphous hydrogenated carbon (a-C) films have been conducted in several gaseous ambients, at three microwave frequencies, and as a function of film H content. Unpaired electron concentrations are found to decrease with increasing H content while EPR linewidths are shown to be a function of H content; unresolved proton hyperfine interaction is the linewidth determining factor for H content greater than 24% while conductivity is suggested to be the most significant linewidth determining factor for H content less than 24%. Furthermore, the EPR linewidth of a-C films broadens in the presence of gaseous O2 and demonstrates that these films are permeable to O2 and other gases.