Optical constants of tetrahedral amorphous carbon films in the infrared region and at a wavelength of 633 nm

Abstract

The optical constants of tetrahedral amorphous carbon (ta-C) films in the infrared region and at a wavelength of 633 nm were investigated. The ta-C films were prepared by the filtered arc deposition technique under different carbon ion energies of 22–320 eV. The ta-C films contained high sp3 bonding of 82%–93% and had very smooth surfaces. The optical band gap was found to be controlled essentially by the presence and arrangement of the sp2 sites in the ta-C films. The real and imaginary parts, ε1 and ε2, of the dielectric constant, refractive index, n, and extinction coefficient, k, of ta-C films were determined from measurements of infrared reflection and spectroscopic ellipsometry. The results indicated that in our region of investigation ta-C films exhibited a fine transparency at a wavelength of 633 nm, especially in the infrared region. Both n and k, and consequently, ε1 and ε2, showed considerable variation in ion energy, and had minimum values approaching those of diamond for film prepared at an ion energy around 220 eV. The dependence of the optical constants on the sp3 content of ta-C films indicated that with increased sp3 content, ta-C film possessed a dielectric constant, refractive index and extinction coefficient much closer to that of diamond.