First-principles study of structural, vibrational, and electronic properties of trigonally bonded II-IV-N2

Abstract

With the first-principles calculations, we probe into the structural, vibrational, and electronic properties of trigonally bonded planar II-IV-N2 (II = Zn, Cd; IV = Si, Ge, Sn), in contrast to their well-known tetrahedrally bonded counterparts. A detailed analysis is presented for the deformed honeycomb-like structures of planar II-IV-N2 in light of atomic radius and electronegativity of different group-II and group-IV elements. Cohesive energy and phonon property are calculated to investigate the stability of monolayer II-IV-N2. Among the six monolayer II-IV-N2 studied, five of them are dynamically stable except for monolayer CdSnN2 which has imaginary phonon frequencies. The band curvatures of trigonally bonded II-IV-N2 reveal distinct difference comparing to their tetrahedrally bonded counterparts and show connections to atomic transmutation and structural distortions. The different variation trends of band gap from tetrahedrally bonded structure to trigonally bonded structure are explored through density of states analysis. The highest valence band energies of monolayer ZnGeN2 and ZnSnN2 at Γ and X point are very close, resulting in a quasi-direct band gap. Tuning of band gap characteristics by uniaxial strain is investigated for these ternary nitrides and it is shown that a slightly change less than 0.1% on lattice constant b can induce indirect-to-direct band gap transition for monolayer ZnSnN2.