Ex situ variable angle spectroscopic ellipsometry studies on chemical vapor deposited boron-doped diamond films: Layered structure and modeling aspects

We report the optical property measurements on boron-doped diamond (BDD) films which were synthesized by microwave plasma-assisted chemical vapor deposition technique on Si (100) using methane in high hydrogen dilution and trimethylboron as precursors with varying boron concentration such that $[B]∕[C]gas=100$⁠, 500, 1000, 2000, 4000, and $6467ppm$⁠. These BDD films were investigated using a rotating analyzer variable angle spectroscopic ellipsometry (SE) from the near IR to UV range $(830–193nm)$⁠. By applying the conventional Bruggeman effective medium approximation and linear regression analyses to the raw SE data that is, $[ψ(λi),Δ(λi)]$ and pseudodielectric function $(⟨εr(λi)⟩,⟨εi(λi)⟩)$⁠, we determined the most appropriate model fit. The SE modeling was performed through the normal and point-by-point fit methods combined with the coupled and uncoupled bulk and surface layer approaches providing the details about the thin films’ microstructure in terms of the (a) multilayer (component and surface) structure and component layer thickness of the films, (b)volume fraction of constituents [$fsp3C$⁠, $fsp2C$ and void $(fv)$ in the component layer], (c) inhomogeneity of the structure along the growth axis and its variation with boron concentration, and (iv) surface roughness layer thickness $(ds)$ with dimensions less than the optical wavelength that is not otherwise available. A simplified three-layer structural model consisting of an interfacial layer, an intermediate (or bulk) layer, and a top surface roughness layer has been proposed, which simulates the ellipsometry data reasonably well with coupled point-by-point method. An estimator, i.e., mean squared error $(χ2)$⁠, is used to assess the accuracy of the model fit. The results (surface roughness and constituents’ fraction) obtained through SE modeling are also compared with those from atomic force microscopy (AFM) and Raman spectroscopy to validate the layered model employed. Typically, high surface roughness values around $6nm$ were found for films grown under different boron concentrations which is almost five times smaller than determined from AFM. In this context, we determined an approximate linear relationship between these two variables. The relatively smaller surface roughness for BDD films indicates the combined role of boron-hydrogen (B, H) in diamond (C) while keeping the substrate temperature constant. We also discussed the variation of (⁠$fv$ and $fnd$⁠) for the bulk and surface layers with increasing boron concentration.