Anisotropic index of refraction and structural properties of hexagonal boron nitride epilayers probed by spectroscopic ellipsometry

The anisotropic index of refraction of 200 mu m thick boron-10 enriched hexagonal boron nitride (h-BN) freestanding epilayers grown by metalorganic chemical vapor deposition has been measured using spectroscopic ellipsometry in the UV (4.0-5.1 eV) spectral range. It was found that the index of refraction for the polarization mode with an electric field perpendicular to the c-axis (ordinary, n(o)) is much higher than that with an electric field parallel to the c-axis (extraordinary, n(e)). By inclusion of turbostratic- (t-) phase layers within h-BN having an average inclination angle (theta) with respect to the ideal c-plane, a simple method for quantifying theta has been deduced. Our results revealed that the presence of t-phase layers decreases the optical anisotropy of h-BN and that a signature of improved crystalline quality is an increase in the ordinary index of refraction (n(o)) as a result of the average incline angle theta approaching 0 degrees and predicted that n(o) = 2.7 and n(e) = 1.5 at 280 nm for single crystalline h-BN epilayers. More importantly, our results demonstrated that spectroscopic ellipsometry is an effective technique for characterizing the crystalline quality of h-BN epilayers with the advantages of being noninvasive and highly sensitive.