Huge excitonic effects in layered hexagonal boron nitride

The all-electron GW approximation energy band gap of bulk hexagonal boron nitride is shown to be of indirect type. The resulting computed in-plane polarized optical spectrum, obtained by solving the Bethe-Salpeter equation for the electron-hole two-particle Green function, is in excellent agreement with experiment and has a strong anisotropy compared to out-of-plane polarized spectrum. A detailed analysis of the excitonic structures within the band gap shows that the low-lying excitons belong to the Frenkel class and are tightly confined within the layers. The calculated exciton binding energy is much larger than that obtained by Watanabe et al. Nat. Mater. 3, 404 (2004).] based on a Wannier model assuming h-BN to be a direct-band-gap semiconductor.