Electronic structure and defect properties of B6O from hybrid functional and many-body perturbation theory calculations: A possible ambipolar transparent conductor

Abstract

${\mathrm{B}}_{6}\mathrm{O}$ is a member of icosahedral boron-rich solids known for their physical hardness and stability under irradiation bombardment, but it has also recently emerged as a promising high mobility $p\ensuremath{-}$type transparent conducting oxide. Using a combination of hybrid functional and many-body perturbation theory calculations, we report on the electronic structure and defect properties of this material. Our calculations identify ${\mathrm{B}}_{6}\mathrm{O}$ has a direct band gap in excess of 3.0 eV and possesses largely isotropic and low effective masses for both holes and electrons. Of the native defects, we identify no intrinsic origin to the reported $p\ensuremath{-}$type conductivity and confirm that $p\ensuremath{-}$type doping is not prevented by intrinsic defects such as oxygen vacancies, which we find act exclusively as neutral defects rather than hole-killing donors. We also investigate a number of common impurities and plausible dopants, finding that isolated acceptor candidates tend to yield deep states within the band gap or act instead as donors, and cannot account for $p\ensuremath{-}$type conductivity. Our calculations identify the only shallow acceptor candidate to be a complex consisting of interstitial H bonded to C substituting on the O site ${(\mathrm{CH})}_{\mathrm{O}}$. We therefore attribute the origins of $p\ensuremath{-}$type conductivity to these complexes formed during growth or more likely via isolated ${\mathrm{C}}_{\mathrm{O}}$ which later binds with H within the crystal. Lastly, we identify Si as a plausible $n\ensuremath{-}$type dopant, as it favorably acts as a shallow donor and does not suffer from self-compensation as may the C-related defects. Thus, in addition to the observed $p\ensuremath{-}$type conductivity, ${\mathrm{B}}_{6}\mathrm{O}$ exhibits promise of $n\ensuremath{-}$type dopability if the stoichiometry and both native and extrinsic sources of compensation can be sufficiently controlled.