Boron Nitride (BN)

The optical functions of the hexagonal and cubic phases of BN (hBN and cBN, respectively) are not nearly as well defined as those of other group III nitrides. This is due in large part to the fact that single-crystal samples either do not exist (which is the case for hexagonal boron nitride) or are extremely rare (as in the case of cubic boron nitride) [1]. Optical studies on the optically anisotropic hBN have been performed on preferentially oriented, polycrystalline samples with E → ⊥ c ^ and E → ∥ c ^ polarization. In these samples, it is difficult to obtain a complete separation of the two polarizations because the typical mosaic spread of the c-axes is greater than 20°, and because pure a-faces are not easily obtained. In this critique, we have not included experiments performed on BN films. The reasons for this are that the optical constants of these films can vary greatly with film properties such as stoichiometry, homogeneity, crystallinity, and purity, to name a few. For the most part, these types of film properties have not been correlated with optical properties that have been reported. As we shall see, the optical properties of bulk BN materials are not that well defined.