Analysis of amorphous carbon thin films by spectroscopic ellipsometry

Using spectroscopic ellipsometry (SE), we have measured the optical properties of amorphous carbon (a-C) films similar to 10-30 nm thick prepared using a filtered beam of C(+) ions from a cathodic are. Such films exhibit a wide range of sp(3)-bonded carbon contents from 20 to 76 at.% as measured by electron energy loss spectroscopy (EELS), and a range of optical gaps from 0.65 eV (20 at.% sp(3) C) to 2.25 eV (76 at.% sp(3) C) as measured by SE. SE data from 1.5 to 5 eV have been analyzed by applying the most widely used effective medium theory (EMT) namely that of Bruggeman with isotropic screening, assuming a model of the material as a composite with sp(2) C and sp(3) C components. Although the atomic fractions of sp(3) C deduced by SE with the Bruggeman EMT correlate monotonically with those obtained by EELS, the SE results range from 10 to 25 at.% higher. The possible origins of this discrepancy are discussed within the framework of an optical composite. Improved agreement between SE and EELS is obtained by employing a simple form for the EMT, in which the effective dielectric function is determined as a volume-fraction-weighted average of the dielectric functions of the two components. (C) 1998 Elsevier Science B.V. All rights reserved.