Optical properties and new carbon forms of sputtered amorphous carbon films

Amorphous carbon films were deposited by r.f. magnetron sputtering at various bias voltages V-b applied on Si substrate. We studied the optical properties of the films using in situ spectroscopic ellipsometry (SE) measurements in the energy region 1.5-5.5 eV. From the SE data analysis the dielectric function epsilon(omega) of the a-C films was obtained, providing information about the electronic structure and the bonding configuration of a-C films. Based on the SE data the films are classified in three categories. In Category I and II belong the films developed with V-b greater than or equal to 0 V (rich in sp(2) bonds) and -100 less than or equal to V-b < 0 V (rich in sp(3) bonds), respectively. The dielectric function of the films belonging in these two categories can be described with two Lorentz oscillators located in the energy range 2.5-5 eV (pi-pi*,) and 9-12 eV (sigma-sigma*). A correlation was found between the oscillator strength and the sp(2) and sp(3) contents. The latter were calculated by analyzing the epsilon(omega) with the Bruggeman effective medium theory. In films deposited with V-b < -100 V (Category III), the formation of a new and dense carbon phase was detected which exhibits a semimetallic optical behavior and the epsilon(omega) can be described with two oscillators located at similar to 1.2 and similar to 5.5 eV. (C) 1999 Elsevier Science S.A. All rights reserved.