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.