The adsorption and diffusion behaviors of nitrogen impurities in h-BN by first-principles study

Due to the nitrogen atmosphere during the preparation of h-BN, N atoms are inevitably introduced into h-BN crystals to form impurities. An in-depth understanding of the adsorption and diffusion behaviors of N impurities in h-BN will be conductive to further improve and formulate effective crystal growth strategies. In this study, the effects of N impurities on the structural stability, electronic structure and optical properties of h-BN were systematically studied by first-principles calculations. The results show that N impurities tend to be adsorbed at the top site of surface N atoms (TN) to form stable covalent bonds with host N atoms. The covalent N–N bond is along the inter-layer direction with a bond length of 1.57 Å, and the two N atoms are equidistantly distributed on both sides of the atomic layer. Within the h-BN lattice, the top site above the B–N bond (M) shares the same formation energy as the TN sites, which provides another stable adsorption site for N impurities. The diffusion of N impurities on the h-BN surface needs to overcome a large energy barrier of 2.64 eV. Relatively, N impurities are more prone to inter-layer permeability diffusion, which overcome a much lower diffusion barrier of 1.74 eV. In this case, the impurity N atoms do not diffuse directly through the atomic layers of h-BN, but the host N atoms with which they form covalent bonds leave the equilibrium lattice site and diffuse into the inner layers. When N impurities are adsorbed on the surface or inside of the h-BN lattice, the impurity energy levels are clearly introduced into the electronic band gap. Fortunately, these defect levels do not impair the optical absorption performance of the h-BN system. In contrast, in the crystal plane direction, N impurities can significantly enhance the optical response intensity of h-BN to ultraviolet light.