Optical properties and new vibrational modes in carbon films

The optical properties and vibrational modes of amorphous carbon (a-C) film s were studied by spectroscopic ellipsometry (SE) and Raman spectroscopy. T he films were prepared by sputtering, at various energies E-i (low to high) , of the argon species bombarding the film surface during deposition. The S E spectra were analysed by the Tauc-Lorentz model to describe the dielectri c 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 an d can be described, in general, with two oscillators: one at similar to 4 e V, 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-ene rgy (sp2 bonds) absorption. A strong optical absorption at similar to 1.2 e V appears only in films developed with high E-i (greater than or equal to 1 30 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 hig h E-i two well-defined peaks at similar to 1556 and 2330 cm(-1) are also de tected. All of the above indicate the formation of a more ordered amorphous network with increasing Ei, and a new crystalline structure is formed at h igh E-i. (C) 2000 Elsevier Science S.A. All rights reserved.