Thermal stability and relaxation in diamond-like-carbon. A Raman study of films with different sp3 fractions (ta-C to a-C)

The thermal stability and relaxation processes in hydrogen free diamond-like-carbon films with different ${\mathrm{sp}}^3$ fractions (40%, 60%, and 80%) are comparatively studied for the first time by visible Raman spectroscopy. The 80% ${\mathrm{sp}}^3$ film is very stable under annealing in the entire temperature region investigated (300–1270 K) and shows only a minor change of the optical transmission, most likely due to a mild ${\mathrm{sp}}^2$ clustering, but no graphitization. This is very important for practical applications requiring a high thermal stability. The films with lower ${\mathrm{sp}}^3$ fraction show a thermal stability which decreases with the decreasing ${\mathrm{sp}}^3.$ Graphitization starts at 700 K for the 40% ${\mathrm{sp}}^3$ film.