Abstract
Real-time spectroscopic ellipsometry (RTSE) has been applied to characterize composition depth-profiles in compositionally-graded amorphous silicon–carbon alloy (a-Si 1− x C x :H) thin films, prepared using continuous variations in the flow ratio z( t)=[CH 4]/{[SiH 4]+[CH 4]} during r.f. plasma-enhanced chemical vapor deposition (PECVD). In order to calculate the dielectric functions of a-Si 1− x C x :H alloys for any value of x, a new parameterization of the measured dielectric functions was established using the recently-derived Tauc–Lorentz (T-L) model. This model is found to provide improved fitting of the measured dielectric functions of a-Si 1− x C x :H over the analyzed energy region of 2.2<E<4.2 eV, in comparison to previous parameterization schemes. Applying the T-L parameterization, the depth-profiles in the C-content were then analyzed using a virtual interface approximation. For several a-Si 1− x C x :H graded layers having triangular variations in the C-content (0< x<0.24) over 25–130 Å thick layers, we found very good agreement between the analyzed depth-profiles and those predicted from z( t) based on individually-deposited samples. In the depth-profile of the thinnest C-graded layer (∼25 Å), monolayer depth resolution was evident with an average compositional uncertainty of±0.009.
Published Version
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