Separation of the sp3 and sp2 components in the C1s photoemission spectra of amorphous carbon films

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 sp2 and sp3 hybrids forms of carbon. The sp3/sp2 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 sp3/sp2 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 ? plasmon in the harder films below 900 K, despite the presence of sp2 atoms and ? bonds at those temperatures.