Interplay between intrinsic defects and optoelectronic properties of semi-Heusler gapped metals

Semi-Heusler compounds hold significant research interest with applications ranging from electronic and optical to thermoelectric devices. Intrinsic defects play a vital role in tailoring the properties of these compounds. For instance, deviations from the 18-valence electron configuration may result in an excess of holes or electrons, causing the Fermi level to shift into the valence or conduction bands, thereby showing their electronic properties akin to those of gapped metals. In this study, we use NbCoSb semi-Heusler compound exhibiting gapped metal behavior to systematically explore the origin of spontaneous intrinsic defects. First-principles defect calculations combined with crystal orbital Hamilton population bonding analysis reveal that Nb vacancies introduce acceptor states within the internal gap by capturing free electrons from the conduction band, thereby promoting and stabilizing the formation of Nb vacancies. Furthermore, these intrinsic defects stabilize various non-stoichiometric compositions with different carrier concentrations, which can provide a pathway for desirable tuning of electronic and optical properties. Our results show that the real and imaginary parts of dielectric constants, as well as optical absorption of the NbCoSb gapped metal, can be significantly tuned with intrinsic defects. This tailoring strategy can play a significant role in the development of optical devices with desirable functionality, particularly involving the plasmonic and epsilon near-zero materials. Our predictions can provide fundamental insight for selecting compositions and controlling synthesis conditions across a wider family of semi-Heusler compounds.