First principles theory of the nitrogen interstitial in hBN: a plausible model for the blue emitter

Color centers in hexagonal boron nitride (hBN) have attracted attention due to their remarkable optical properties enabling robust room temperature photonics and quantum optics applications in the visible spectral range. However, identification of the origin of color centers in hBN has turned out to be a great challenge. This is also true for the blue emitter, which is an irradiation damage in hBN emitting at 436 nm wavelengths. We propose the negatively charged nitrogen split interstitial defect in hBN as a plausible microscopic model for the blue emitter. We carefully analyzed the accuracy of first principles methods and show that the commonly used HSE hybrid exchange-correlation functional fails to describe the electronic structure of this defect. Using the generalized Koopman's theorem, we fine tune the functional and obtain a zero-phonon photoluminescence (ZPL) energy in the blue spectral range. We show that the defect exhibits high emission rate in the ZPL line and features a characteristic phonon side band that resembles the blue emitter's spectrum. Furthermore, we study the electric field dependence of the ZPL and numerically show that the defect exhibits a quadratic Stark shift for perpendicular to plane electric fields, making the emitter insensitive to electric field fluctuations in first order. Our work emphasize the need for assessing the accuracy of common first principles methods in hBN and exemplifies a workaround methodology.