Structural Studies of Hydrogenated Amorphous Carbon Infrared Coatings

ABSTRACTHydrogenated amorphous carbon thin films are well known for their mechanical hardness and optical properties which make them useful for applications in infrared device coatings. In this work films have been prepared by plasma. decomposition of methane using an RF diode reactor operating under conditions of high self bias potential (Vb = 1 KeV). The resulting ion bombardment during film growth leads to the formation of hard, insulating carbon coatings which have a band gap of ≃1.1 eV and are transparent in the IR. Nuclear reaction analysis has been used to quantify the atomic concentration of hydrogen incorporated in the films and extended x-ray absorption fine structure (EXAFS) has been used to determine local site geometry. Only first and second nearest neighbor bond lengths are observed with no evidence of further long range order or microcrystallinity. A model for atomic structure is proposed which includes both sp2 and sp3 bond configurations and direct comparisons are made with data obtained from sputtered carbon films, graphite and diamond.