Characterization of Pulsed-Laser Deposited Amorphous Diamond Films by Spectroscopic Ellipsometry

Spectroscopic ellipsometry is used to characterize amorphous diamond, also known as tetrahedral amorphous carbon, (ta-C) films grown by pulsed laser ablation. The ellipsometry data is collected with the two-modulator generalized ellipsometer, which measures all three parameters required to characterize isotropic samples, as well as additional parameters used to characterize strain-induced birefringence of the focusing optics. Lenses are used to focus the light spot to an ellipse 0.7 × 2.0 mm2, allowing us to perform several ellipsometric measurements across the profile of ta-C films grown on 7.5 cm diameter Si wafers. The spectroscopic ellipsometry data are fit using a model of the ta-C dielectric function based on the Tauc band edge and the Lorentz expression of the dielectric function for an ensemble of atoms. These fits are used to determine the thicknesses of the rough surface layer, the ta-C film, and the interface layer, as well as the energy gap of the film. Comparisons are made with fits to an earlier formulation due to Forouhi and Bloomer [Phys. Rev. B 34, 7018 (1986).]. In addition to being Kramers-Kronig consistent, the Tauc-Lorentz formulation fits the ta-C data better.