ANALYSIS OF THE ELLIPSOMETRIC SPECTRA OF AMORPHOUS-CARBON THIN-FILMS FOR EVALUATION OF THE SP(3)-BONDED CARBON CONTENT

Using spectroscopic ellipsometry (SE), we have measured the optical pr operties and optical gaps of a series of amorphous carbon (a-C) films similar to 100-300 Angstrom thick, prepared using a filtered beam of C + ions from a cathodic are. Such films exhibit a wide range of sp(3)-b onded carbon contents from 20 to 76 at.%, as measured by electron ener gy loss spectroscopy (EELS). The Tauc optical gaps of the a-C films in crease monotonically from 0.65 eV for 20 at.% sp(3) C to 2.25 eV for 7 6 at.% sp(3) C. Spectra in the ellipsometric angles (1.5-5 eV) have be en analyzed using different effective medium theories (EMTs) applying a simplified optical model for the dielectric function of a-C, assumin g a composite material with sp(2) C and sp(3) C components. The most w idely used EMT, namely that of Bruggeman (with three-dimensionally iso tropic screening), yields atomic fractions of sp(3) C that correlate m onotonically with those obtained from EELS. The results of the SE anal ysis, however, range from 10 to 25 at.% higher than those from EELS. I n fact, we have found that the volume percent sp(3) C from SE using th e Bruggeman EMT shows good numerical agreement with the atomic percent sp(3) C from EELS. The SE-EELS discrepancy has been reduced by using an optical model in which the dielectric function of the a-C is determ ined as a volume-fraction-weighted average of the dielectric functions of the sp(2) C and sp(3) C components. (C) 1998 Elsevier Science S.A.