Point defects in hexagonal boron nitride. II. Theoretical studies

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Abstract
Substitutional and displaced carbon impurities and an isolated nitrogen vacancy in hexagonal boron nitride are theoretically investigated by the "small periodic cluster" approach. The perfect-solid band structure is calculated from the solution of the eigenvalues of a finite and periodic cluster of atoms arranged according to the known crystal structure. The linear combination of atomic orbitals representation of the crystal orbitals is adopted and semiempirical MO (molecular orbital) methods (extended Huckel, iterative extended Huckel) are used for the solution of the electronic eigenvalue problem. Point-defect problems are then treated by introducing the impurity atom or the vacant site into the otherwise perfect periodic cluster, and repeating the solution. Lattice relaxations are introduced around the defect site and charge redistribution among the cluster atoms is allowed for via self-consistent MO treatment. For a substitutional carbon impurity defect, it is observed that two deep defect levels, mainly localized on the carbon atom, appear (3.2-4.9 eV below the conduction band). Another level splits from the conduction band as the carbon atom is raised from the layer plane in a perpendicular direction. This level, 1.0-1.3 eV below the conduction band, has a symmetrical charge distribution on the three boron atoms surrounding the impurity site. As the distance of the carbon atom from the layer plane is increased to infinity, a nitrogen vacancy is formed. It is characterized by a defect level 1.1-1.4 eV apart from the conduction-band edge which also possesses a three-boron character. Lattice relaxations were shown to stabilize these defects. The findings agree semiquantitatively with the experimental results on these defects.