Optical properties of layered non-equilibrium ZnO from first principles*

The boron-nitride (BN) phase has been reported for zinc oxide (ZnO) nano-structures and thin films. With the layered graphitic-like structure it exhibits, BN-ZnO has very different optical properties than the equilibrium wurtzite (WZ) phase of bulk ZnO. We use density functional theory to obtain the equilibrium structure of both polymorphs and we solve the Bethe-Salpeter equation to obtain accurate optical spectra. We found a larger band gap and larger optical anisotropy for BN-ZnO, compared to WZ-ZnO. By performing Maxwell's equations simulations, we show that the anisotropy in BN-ZnO leads to clear differences at the absorption onset for thin films. Our results indicate that possible existence of BN-ZnO in nano-structures leads to increased transmission in ultraviolet region, that can be used to optically distinguish both phases. By comparing to experiment, we show that for ultra-thin films, better agreement can be obtained when accounting for BN-ZnO, rather than pure WZ-ZnO.