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

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 3dxy 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 (VN), oxygen substitution (ON), and hydrogen anion substitution (HN) at the nitrogen sites. The VN and HN impurities can be suppressed under N-moderate and N-rich conditions. The ON 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 3dxy 2D bands embedded in the bulk crystal structure.