Dielectric functions of CVD–grown boron nitride from 1.1 to 9.0 eV by spectroscopic ellipsometry

The optical properties of CVD-grown hexagonal boron nitride (h-BN) and turbostratic BN (t-BN) layers on Al2O3 substrates are investigated by spectroscopic ellipsometry in the range from 1.1 to 9.0 eV at room temperature. The absorption spectra and dielectric functions (DFs) are characterized for analyzing critical points from near and above the bandgap (Eg) by analyzing the multilayer optical model. The real part e1 and imaginary part e2 of DFs for the BN layers are derived from the B–spline function under the Kramers–Kronig relation. The DFs show a significant difference between the h-BN and t-BN layers. The h-BN layer presents stronger and sharper spectra of DFs compared to the t-BN layer. Judging from extended Tauc's plot, the indirect Eg value of the h-BN layer is determined to be 5.97 eV, which is 0.15 eV larger than that of the t-BN layer.