Effects of deposition temperature on the properties of hydrogenated tetrahedral amorphous carbon

Abstract

The properties of hydrogenated carbon films deposited from a highly ionized hydrocarbon plasma beam are studied as a function of deposition temperature. At low temperatures, the films have high sp3 bonding, density, and compressive stress and are very smooth. Two transition temperatures are observed, a lower transition T1 around 250 °C, dependent on ion energy, due to graphitization of C–C bonds, and a higher one T2 at about 450 °C due to the loss of hydrogen. The roughness rises at T1 and falls above T2. These transitions are used to understand the subplantation deposition mechanism. The optical gap varies differently, decreasing gradually across T1 due to ordering of sp2 sites. We also report the temperature dependence of the x-ray diffraction, Raman spectrum, elastic modulus, hardness, substrate adhesion, friction coefficient, refractive index, and paramagnetic defect density. The friction coefficient of ta-C:H is low (0.05–0.1), and is maintained at ambient humidities, unlike for a-C:H. The friction mechanism is attributed to shear-induced graphitization. The spin density is found to decline with deposition temperature and the spin resonance line is argued to be exchange narrowed.