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.
展开▼
