Two different types of amorphous carbon films were deposited on Si substrates, with film hardness of 22 GPa and 40 GPa, by pulsed laser evaporation of graphite targets. The x-ray photoemission spectra (XPS) of the C 1s core level in these films shown two components at 284.3±0.1 eV and 285.2±0.1 eV, which were identified with the ${\mathit{sp}}^2$ and ${\mathit{sp}}^3$ hybrids forms of carbon. The ${\mathit{sp}}^3$/${\mathit{sp}}^2$ concentration ratio deduced from the area of the components had a value of 2/5 for the harder amorphous carbon film and 1/4 for the softer. Upon annealing the harder film at different temperatures, the ${\mathit{sp}}^3$/${\mathit{sp}}^2$ ratio remained nearly constant up to about 900 K and then decreased until reaching a value of zero above 1100 K. The C 1s core level shifted 0.3±0.1 eV toward lower binding energy in the films for annealing temperatures above 900 K. This shift was correlated with an increase in the asymmetry of the C 1s XPS spectra and of the density of states at the Fermi level, as observed by ultraviolet photoemission spectroscopy. There was no detectable π plasmon in the harder films below 900 K, despite the presence of ${\mathit{sp}}^2$ atoms and π bonds at those temperatures. © 1996 The American Physical Society.

The thermal stability of amorphous hard carbon films produced by cathodic arc deposition was studied by near edge x-ray absorption fine structure (NEXAFS) spectroscopy, Raman spectroscopy, and nanoindentation evaluation. Pure carbon films of up to 85% ${\mathrm{sp}}^3$ content were deposited using a pulsed biasing technique and annealed in ultrahigh vacuum up to 850 °C. NEXAFS spectra show no change in the film properties up to 700 °C, and a modification of the spectra for 800 and 850 °C, which indicate graphitization. Raman spectra show only a very slight change up to 850 °C. The nanoindentation data show no change in hardness and elastic modulus with annealing up to 850 °C. The study demonstrates the high thermal stability of the films. The difference in the NEXAFS and the Raman and nanoindentation results can be attributed to the surface sensitivity of NEXAFS in comparison to the more bulk sensitivity of Raman spectroscopy and nanoindentation.

The same method used to determine ${\sigma }^{*}$ excitations in the x-ray-absorption spectra of organic molecules is applied to analyze the structure of the ${\sigma }^{*}$ band in the x-ray absorption spectra of amorphous carbon $(a-\mathrm{C})$ films. The analysis assumes that only a $\sigma$ bond interaction to first neighbors is relevant to explain the structure of the ${\sigma }^{*}$ band in a-C films. This is justified by the local character of the x-ray absorption probe and the short- range order existing in these films. The identification of the different ${\sigma }^{*}$ components is based on the dependence of the ${\sigma }^{*}$ binding energy with bond distance. The ${\sigma }^{*}$ band is built up by summing the components resulting from the possible different types of $\sigma$ bonds in a-C. This method serves to separate the ${\pi }^{*}$ states from the ${\sigma }^{*}$ states and to identify the kind of chemical bonds existing between carbon atoms. This analysis yields a proportion of ${\mathrm{sp}}^3$-bonded atoms of 60% in a film with a density of about 2.9 ${\mathrm{g}/\mathrm{c}\mathrm{m}}^3,$ which is a value closer to what is expected from theoretical calculations. The analysis identifies a component at about 288.5 eV which is associated to strained $\sigma$ bonds of the type of the existing in ${\mathrm{sp}}^3$ bonded rings like cyclopropene. Raman and photoemission spectroscopies help in the interpretation of the x-ray absorption spectra and the assignment of the ${\sigma }^{*}$ components.