A correlation between the microstructure and optical properties of hydrogenated amorphous carbon films prepared by RF magnetron sputtering

Abstract Comparative and systematic studies of the effect of the radiofrequency (RF) bias on the microstructure and the optical properties of hydrogenated amorphous carbon (a-C:H) have been carried out on films deposited by RF magnetron sputtering under different RF power varying from 10 to 250 W applied to the graphite target, leading to a negative bias voltage at the target in the range of −60 to −600 V. A combination of infrared (IR) absorption experiments, which give information about the local microstructure (i.e. C–C and C–H bonding), and optical transmission measurements in the UV-visible and near IR, from which we determined the optical gap E 04 and the refractive index n , are applied to fully characterize the samples in their as-deposited state. The results show first that the films deposited at low RF power (i.e. low negative bias) exhibit a more open microstructure (polymeric character) with a lower density than those deposited at high RF power (i.e. high negative bias). They also indicate that the total bonded H content as well as the sp 3 /sp 2 ratio of carbon atoms bonded to H decrease with increasing RF power leading to the formation of higher proportions of C-sp 2 sites. The same tendency is observed for the optical gap E 04 . On the contrary, the refractive index increases with increasing RF power, suggesting the densification of the films in going to a higher RF power.