Dangling Bonds in Hexagonal Boron Nitride as Single-Photon Emitters

Hexagonal boron nitride has been found to host color centers that exhibit single-photon emission, but the microscopic origin of these emitters is unknown. We propose boron dangling bonds as the likely source of the observed single-photon emission around 2 eV. An optical transition where an electron is excited from a doubly occupied boron dangling bond to a localized B $p_z$ state gives rise to a zero-phonon line of 2.06 eV and emission with a Huang-Rhys factor of 2.3. This transition is linearly polarized with the absorptive and emissive dipole aligned. Because of the energetic position of the states within the band gap, indirect excitation through the conduction band will occur for sufficiently large excitation energies, leading to the misalignment of the absorptive and emissive dipoles seen in experiment. Our calculations predict a singlet ground state and the existence of a metastable triplet state, in agreement with experiment.

Figure 1

(a) Calculated configuration coordinate diagram for the ${{}^{1}A}_1\to {{}^{1}B}_1$ internal transition of the doubly occupied B DB. (b) Charge density isosurface of the B $p_z$ state that becomes localized upon excitation. (c) Charge density isosurface of the B DB state that the electron occupies in the ground state. The isosurfaces correspond to 10% of the maximal charge density and are colored by the sign of the wave function, with red and blue indicating opposite signs.

Figure 2

Equilibrium structures for the (a) boron and (b) nitrogen DBs. Boron atoms are shown in green, nitrogen in gray, and hydrogen in white. (c) Calculated thermodynamic transition levels for both DBs. The valence and conduction band are highlighted in blue and orange.

Figure 3

Schematic depiction of the KS states for (a) the N DB in the negative charge state, and (b) the B DB in the positive, neutral, and negative charge states. The valence and conduction band are highlighted in blue and orange. Solid arrows depict occupied states, and open arrows depict unoccupied states.

Figure 4

Intersystem crossing of the doubly occupied B DB. The solid blue line indicates an optical transition, and dashed orange lines indicate nonradiative crossings between spin channels. Alternative labels under $C_s$ symmetry are given in parentheses. Each transition is labeled with the transition energy and the Huang-Rhys factor $S$.