Tetrahedral and Trigonal Carbon Atom Hybridization in Thin Amorphous Carbon Films Synthesized by Radio-Frequency Sputtering

Ultrathin amorphous carbon ( a -C) films were synthesized on Si(100) substrates by low-pressure, radio-frequency sputtering under different plasma discharge conditions. X-ray photoelectron spectroscopy (XPS) revealed different contents of C, Ar, Si, O, and N in the a -C films. The percentages of tetrahedral (sp 3 ) and trigonal (sp 2 ) carbon hybridizations in the films were determined from an analysis of the C1s core level spectra obtained by XPS narrow scanning. It is shown that the concentrations of surface chemisorbed O and N atoms from the ambient depend on the real surface area of the film (roughness effect). The binding energy shifts of the sp 2 and sp 3 hybridizations (observed after the decomposition of the C1s spectra) due to the Ar + ion bombardment are interpreted in terms of the residual compressive stress in the deposited a -C films. The mechanisms of sp 3 carbon hybridization are discussed in the context of XPS results. For negligible Ar + ion bombardment (zero substrate bias), sp 3 carbon hybridization is governed by a thermodynamic process that obeys the principle of minimum surface free energy. However, sp 3 carbon hybridization under plasma discharge conditions resulting in intense Ar + ion bombardment is controlled by collisions of Ar + ions with carbon atoms on the growing film surface and subsequent collisions between excited carbon atoms and other carbon atoms at the a -C film surface.