Optical properties and new vibrational modes in carbon films

The optical properties and vibrational modes of amorphous carbon (a-C) films were studied by spectroscopic ellipsometry (SE) and Raman spectroscopy. The films were prepared by sputtering, at various energies E-i (low to high), of the argon species bombarding the film surface during deposition. The SE spectra were analysed by the Tauc-Lorentz model to describe the dielectric function is an element of(omega) of the bulk films and to determine their optical gaps. The calculated is an element of(omega) revealed fundamental differences in the electronic structure and C-C bonding of the a-C films and can be described, in general, with two oscillators: one at similar to 4 eV, attributed to the pi-->pi(*) (sp(2) bonds), and one in the range 10-15.5 eV, attributed to the average of sigma-->sigma' (sp(3) bonds) and high-energy (sp2 bonds) absorption. A strong optical absorption at similar to 1.2 eV appears only in films developed with high E-i (greater than or equal to 130 eV). The Raman spectra of the a-C films show the following behavior with increasing E-i: (1) the strengths of disordered 'D' and graphite'G' peaks are gradually reduced and finally disappear; (2) three peaks (at similar to 1250, 1295 and 1330 cm(-1)) appear, reinforced at high E-i; and (3) at high E-i two well-defined peaks at similar to 1556 and 2330 cm(-1) are also detected. All of the above indicate the formation of a more ordered amorphous network with increasing Ei, and a new crystalline structure is formed at high E-i. (C) 2000 Elsevier Science S.A. All rights reserved.