Electronic structure and core exciton of hexagonal boron nitride

A new tight-binding band calculation is presented for the layer compound boron nitride which correctly describes the widths of both $\sigma$ and $\pi$ bands. The binding energy and wave function of the B $1s\to 2p$ core exciton are studied. The core exciton is found to be small and bound by the medium-range part of the core-hole potential. Its binding energy far exceeds the interlayer band dispersion at the conduction-band minimum. Its incomplete $\pi$-like polarization is found to be due to a small admixture of B $\sigma$ states from adjacent layers into the $A_1^{-}$ final state.