Theory of substitutional carbon defects in hexagonal boron nitride

Carbon is a frequent contaminant of 2D hexagonal boron nitride (hBN), which is responsible for various magnetic- and optical-features of the material. In this contribution, we investigate the role of substitutional carbon defects in the development of color centers in hBN. We show that a neutral substitutional carbon defect at the B-site (C_B) is responsible for the single spin ODMR center, known as D1 center. We also provide an overview of the properties of C_B-C_N donor-acceptor pairs (DAP). Here, we explain how a large Coulomb binding energy in the ground-state enables to tune the emission wavelength from deep-UV to NIR by increasing the distance between DAPs. Finally, we examine the properties of seventeen larger carbon clusters. Here, we show that even the extended chain-like arrangements still can exhibit low formation energies due to the development of energetically-favorable carbon-carbon bonds. Among those, we have identified a 6C ring as the most stable defect configuration and propose it as an alternative model for the famous 4.1-eV single photon emitter in hBN.