Small Electron Polaron in Carbon-Doped Cubic Boron Nitride

Controllable and reliable doping of cubic boron nitride (cBN) is a critical challenge in its widespread application in power electronics. Recently, progress was made in this regard when an experiment reported successful n-doping of cBN with carbon, reaching carbon concentrations as high as 5%, which is beyond the thermodynamic solubility limit. However, the nature of carbon-based defects introduced within the cBN matrix remains unknown thus far. Here, we explore the electronic structure of carbon-doped cBN under nitrogen-rich conditions, which are conducive to the formation of donor-type defects (such as carbon substituents replacing boron), and predict several possible arrangements of carbon in clusters at experimentally relevant concentrations of 1.5–7.8%. Our theoretical calculations show that carbon dopants prefer to aggregate into small clusters with local ordering rather than distributing randomly in the bulk. Along with defects that result in easily ionizable defect states, we also report a distribution of carbon dopants, where the structure exhibits a defect-bound small electron polaron. The polaron can be identified via a split-off localized dopant band near the valence band edge. These findings not only shed light on identities of possible carbon-based defects but also predict additional carriers─defect-bound small polarons.