Non-equilibrium BN-ZnO: Optical properties and excitonic effects from first principles

TL;DR

This study employs first-principles calculations to investigate the electronic and optical properties of non-equilibrium BN-ZnO, revealing insights into its band structure, excitonic effects, and optical anisotropy, which can help distinguish it from wurtzite ZnO.

Contribution

The paper provides a detailed first-principles analysis of BN-ZnO's optical and electronic properties, clarifying discrepancies with experimental data and highlighting its anisotropic optical behavior.

Findings

Band-gap of BN-ZnO matches experiment with theoretical geometry

Optical anisotropy differs significantly from WZ-ZnO

Potential for tuning optical properties via thin film design

Abstract

The non-equilibrium boron nitride (BN) phase of zinc oxide (ZnO) has been reported for thin films and nanostructures, however, its properties are not well understood due to a persistent controversy that prevents reconciling experimental and first-principles results for its atomic coordinates. We use first-principles theoretical spectroscopy to accurately compute electronic and optical properties, including single-quasiparticle and excitonic effects: Band structures and densities of states are computed using density functional theory, hybrid functionals, and the $GW$ approximation. Accurate optical absorption spectra and exciton binding energies are computed by solving the Bethe-Salpeter equation for the optical polarization function. Using this data we show that the band-gap difference between BN-ZnO and wurtzite (WZ) ZnO agrees very well with experiment when the theoretical lattice geometry is used, but significantly disagrees for the experimental atomic coordinates. We also show that the optical anisotropy of BN-ZnO differs significantly from that of WZ-ZnO, allowing to optically distinguish both polymorphs. By using the transfer-matrix method to solve Maxwell's equations for thin films composed of both polymorphs, we illustrate that this opens up a promising route for tuning optical properties.