Ion Substitution Effect on Defect Formation in Two-Dimensional Transition Metal Nitride Semiconductors, AETiN₂ (AE =...

A layered semiconductor, SrTiN₂, has an interesting crystal structure as a two-dimensional (2D) electron system embedded in a three-dimensional bulk periodic structure because it has alternate stacking of a SrN blocking layer and a TiN conduction layer, in which the Ti 3d_xy orbital forms the conduction band minimum (CBM) similar to the SrTiO₃-based thin-film heterostructure. However, SrTiN₂ has been reported to exhibit nearly degenerate conduction, but we reported that it would be due to the easy formation of nitrogen vacancies and oxygen impurities from air. In this paper, we extend the materials to family compounds, alkaline earth (AE) ion-substituted, AETiN₂ (AE = Ca, Sr, and Ba), and investigated how we can suppress the defect formation by (hybrid) density functional theory calculations. All AETiN₂ compounds possess thermodynamic stability in the wide nitrogen (N) chemical potential window. Especially, CaTiN₂ is the most stable even against N-poor conditions. Unintentional carrier generation occurs due to the nitrogen vacancies (V_N), oxygen substitution (O_N), and hydrogen anion substitution (H_N) at the nitrogen sites. The V_N and H_N impurities can be suppressed under N-moderate and N-rich conditions. The O_N defect is easily formed in SrTiN₂ and also in BaTiN₂ under N-rich conditions, but its formation can be suppressed in CaTiN₂. Present results suggest that high-purity CaTiN₂ can be obtained under wider N chemical conditions, which would lead to the realization of the novel functional properties originating from Ti 3d_xy 2D bands embedded in the bulk crystal structure.