Optical properties and new carbon forms of sputtered amorphous carbon films

Amorphous carbon films were deposited by r.f. magnetron sputtering at vario us bias voltages V-b applied on Si substrate. We studied the optical proper ties of the films using in situ spectroscopic ellipsometry (SE) measurement s 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 informati on about the electronic structure and the bonding configuration of a-C film s. Based on the SE data the films are classified in three categories. In Ca tegory I and II belong the films developed with V-b greater than or equal t o 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 belongi ng in these two categories can be described with two Lorentz oscillators lo cated 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) wit h 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 detect ed which exhibits a semimetallic optical behavior and the epsilon(omega) ca n be described with two oscillators located at similar to 1.2 and similar t o 5.5 eV. (C) 1999 Elsevier Science S.A. All rights reserved.