Thermal stability of diamondlike carbon films

Abstract

IR and UV-VIS optical spectroscopy have been used to study the transformations of the properties of diamondlike carbon films following isothermal anneals from Ta=300 to 650 °C. Several processes due to the annealing have been observed: (1) the increase of the absorption due to vibrations of unsaturated bonds of C(sp2) atoms at ∼1600 cm−1 for Ta≥350 °C, (2) the decrease of the absorption due to C(sp3)–H bonds at Ta≥350 °C, and (3) the reduction of the optical energy gap. Analysis of the kinetics has shown that the dehydrogenation of the alloys and the formation of unsaturated bonds may proceed independently. The reduction of the energy gap is related to the formation of C(sp2) atoms with unsaturated bonding which occurs mostly in hydrogen-free regions. Intensive graphitization of the films occurs above Ta=650 °C. Transformations of C–H bonds are proposed to occur via fast rearrangement in stressed regions leading to formation of new C(sp2)–H bonds and formation of methane molecules as the most important product of the anneals inside the polymeric highly hydrogenated regions in the alloys. It appears that both bond removal and reordering have taken place as a result of annealing.