Hole polarons and p -type doping in boron nitride polymorphs

Abstract

Boron nitride polymorphs hold great promise for integration into electronic and optoelectronic devices requiring ultrawide band gaps. We use first-principles calculations to examine the prospects for $p$-type doping of hexagonal ($h\ensuremath{-}\mathrm{BN}$), wurtzite ($wz\ensuremath{-}\mathrm{BN}$), and cubic ($c\ensuremath{-}\mathrm{BN}$) boron nitride. Group-IV elements (C, Si) substituting on the N site result in a deep acceptor, as the atomic levels of the impurity species lie above the BN valence-band maximum. On the other hand, group-II elements (Be, Mg) substituting on the B site do not give impurity states in the band gap; however, these dopants lead to the formation of small hole polarons. The tendency for polaron formation is far more pronounced in $h\ensuremath{-}\mathrm{BN}$ compared to $wz\ensuremath{-}\mathrm{BN}$ or $c\ensuremath{-}\mathrm{BN}$. Despite forming small hole polarons, Be acceptors enable $p$-type doping, with ionization energies of 0.31 eV for $wz\ensuremath{-}\mathrm{BN}$ and 0.24 eV for $c\ensuremath{-}\mathrm{BN}$; these values are comparable to the Mg ionization energy in GaN.