Hard carbon coatings with low optical absorption

Abstract

Hydrogenated amorphous (a-C:H) films were prepared by rf-plasma deposition from benzene vapor. Complete optical absorption spectra from the UV to the IR (0.2–20.0 μm) have been measured. The optical gap depends linearly on hydrogen content and Eopt can be varied between 0.8 and 1.8 eV. For energies below Eopt the films are almost transparent and absorption is especially low in the 2–6-μm region (e.g., α=15 cm−1 at λ=2.8 μm). Sharp C–H stretch absorption bands occur near 3.4 μm, giving insight into the microstructure of the films. A newly reported weak band at 3.03 μm is first evidence for C–C triple bonds (sp1 hybridization) in a-C:H, where sp3 (single) and sp2 (double) C–C bonds dominate.