Effect of stacking orientation on the electronic and optical properties of polar 2D III-nitride bilayers

Given the successful synthesis of 2D GaN and investigations into the properties of freestanding 2D nitrides, bilayers of these materials are now of particular interest. Extreme quantum confinement is a viable method to shift light emission to shorter wavelengths, but in 2D nitrides this is counteracted by the quantum-confined Stark shift due to the strong inherent polarization perpendicular to the 2D plane. We report the electronic and optical properties of 2D BN, GaN, AlN, and InN in various stacking orientations, such that the electric fields are either aligned or anti-parallel in two possible configurations. We employ density functional theory and quasiparticle corrections with the GW method, as well as the Bethe-Salpeter Equation, to derive accurate band structures, exciton binding energies, and luminescence energies. Our results demonstrate that the stacking orientation acts as a degree of freedom to tune the band gap of polar 2D bilayers over several eV, as well as to control the direct or interlayer nature of excitons, giving critical insight in how to improve 2D III-nitride-based optoelectronics.