SPECTROSCOPIC ELLIPSOMETRY STUDIES OF TETRAHEDRAL AMORPHOUS-CARBON PREPARED BY FILTERED CATHODIC VACUUM-ARC TECHNIQUE

The optical property of the tetrahedral amorphous carbon (ta-C) films deposited by the filtered cathodic vacuum are (FCVA) technique has bee n investigated using a spectroscopic ellipsometer. The structure of th ese films deposited on silicon wafer was simulated by a four-layer mod el consisting of a roughness layer, a ta-C layer a graded ta-C:Si laye r and the silicon substrate. The optical properly of the ta-C layer wa s derived based on Forouhi and Bloomer amorphous semiconductor model. The graded layer consisting of the mixture of ta-C anti silicon simula tes the carbon ion impinging/diffusion into the surface of the silicon substrate. The dielectric constants of the random mixture microstruct ure were modeled by the effective medium approximation. From the fitti ng, the F.B. parameters for the ta-C film have been obtained following their physical significance. The complex refraction index, N = n - ik over the range of 1.5-4.2 eV, for ta-C has been determined. The Tauc (optical) gap of 2.5-2.7 eV is obtained from ta-C with 79-88% sp(3) fr action in the film. Similarity in trend is observed from the Tauc gap, F.B., optical band gap, parameter B and sp(3) fraction plotted agains t the car bon ion energy. The change of the slope of the optical absor ption edge is observed when the ion energy is varied. (C) 1998 Elsevie r Science S.A.