Annealing-Induced Modification of Superhard Conductive Carbon Film

Carbon deposited by electron cyclotron resonance plasma sputtering (ECR-C) is known to have physical properties different from those of amorphous carbon (a-C), i.e., an unexpected high degree of hardness, despite the large proportion of sp² bonding. We studied the effects of annealing (up to 700 °C) ECR-C films on the microstructure and electrical conductive property by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and conducting atomic force microscopy (AFM). Annealing causes the evaporation of argon atoms (7.4 at. %) in the film and decreases the sp³/sp² ratio from 0.31 to 0.24, the full width half maximums (FWHMs) of G and D peaks and the I(D)/I(G) ratio. The electrical conductivity and linearity of I–V curves measured by conducting AFM increase with annealing temperature. This is explained by the ordering of the sp² phase, i.e., the decrease in the numbers of argon atoms and defects in aromatic rings, which act as ionizable sites for Poole–Frenkel conduction. The variations in FWHM, I(D)/I(G) ratio and electrical conductivity are much smaller than those caused by annealing a-C with an sp³ fraction lower than approximately 70%. This indicates that ECR-C is thermally stable compared with a-C, i.e., the basic structure of ECR-C is maintained during annealing up to 700 °C with a small increase in the size of sp² nanocrystallites and a lowering of disorder.