Optical vs electronic gap of hafnia by ab initio Bethe-Salpeter equation

Abstract

We present first-principles many-body perturbation theory calculations of the quasiparticle electronic structure and the optical response of HfO2 polymorphs. We use the GW approximation including core electrons by the projector augmented wave method and performing a quasiparticle self-consistency also on wavefunctions. In addition, we solve the Bethe-Salpeter equation on top of GW to calculate optical properties including excitonic effects. For monoclinic HfO2, we find a fundamental bandgap of Eg = 6.33 eV (with the direct bandgap at Egd=6.41 eV) and an exciton binding energy of 0.57 eV, which situates the optical gap at Ego=5.85 eV. The latter is in the range of spectroscopic ellipsometry (SE) experimental estimates (5.5–6 eV), whereas our electronic bandgap is well beyond experimental photoemission (PE) estimates (<6 eV) and previous GW works. Our calculated density of states and optical absorption spectra compare well to raw PE and SE spectra. This suggests that our predictions of both optical and electronic gaps are close to, or at least lower bounds of, the real values.