The thermally induced structural evolution of hydrogenated amorphous carbon (a-C:H) films was investigated in situ by X-ray photoelectron spectroscopy for annealing temperatures up to 500 C. A model for the conversion of sp 3-to sp 2-hybridized carbon in a-C:H vs. temperature and time was developed and applied to determine the ranges of activation energies for the thermally activated processes occurring. The energies are consistent with ordering and clustering of sp 2 carbon, scission of sp 3 carbon-hydrogen bonds and formation of sp 2 carbon, and direct transformation of sp 3-to sp 2-hybridized carbon. V

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Silicon- and oxygen-containing hydrogenated amorphous carbon (a-C:H:Si:O) coatings are amorphous thin-film materials composed of hydrogenated amorphous carbon (a-C:H), doped with silicon and oxygen. Compared to a-C:H, a-C:H:Si:O exhibits much lower susceptibility to oxidative degradation and higher thermal stability, making a-C:H:Si:O attractive for many applications. However, the physical mechanisms for this improved behavior are not understood. Here, the thermally induced structural evolution of a-C:H:Si:O was investigated in situ by X-ray photoelectron and absorption spectroscopy, as well as molecular dynamics (MD) simulations. The spectroscopy results indicate that upon high vacuum annealing, two thermally activated processes with a Gaussian distribution of activation energies with mean value E and standard deviation σ take place in a-C:H:Si:O: (a) ordering and clustering of sp carbon ( E ± σ = 0.22 ± 0.08 eV) and (b) conversion of sp- to sp-bonded carbon ( E ± σ = 3.0 ± 1.1 eV). The experimental results are in qualitative agreement with the outcomes of MD simulations performed using a ReaxFF potential. The MD simulations also indicate that the higher thermal stability of a-C:H:Si:O compared to a-C:H (with similar fraction of sp-bonded carbon and hydrogen content) derives from the significantly lower fraction of strained carbon-carbon sp bonds in a-C:H:Si:O compared to a-C:H, which are more likely to break at elevated temperatures.

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