Strong interlayer coupling and unusual antisite defect-mediated p-type conductivity in GeP _x ( x = 1, 2)

As an emerging candidate for anisotropic two-dimensional materials, the group IV-V family (e.g. GeP, GeP₂) has appealing applications in photoelectronics. However, their intrinsic point defect properties, which largely determine the device performance and optimization, are still poorly explored. In our study, through density functional theory (DFT) calculations, antisite defects were affirmed to be dominant with the lowest formation energies in 2D GeP_x semiconductors because of the similar atomic size and electronegativity of elemental components, which is in contrast to previous calculations and experimental speculation. These antisite defects could introduce relatively shallow states within the bandgap in bulk cases. The transition energy levels and electronic structures of defects reveal that Ge_P and P_Ge antisites act as dominant acceptors and donors, respectively. Strong interlayer coupling between anions results in a significant upshift of the valence band maximum (VBM) and shallower acceptor behaviors of GeP_x. Together with the dominant Ge_P antisite defect, the large upshift of the VBM in GeP leads to a remarkable transition of conductivity from intrinsic in the monolayer to p-type in the bulk. Such a synergistic effect in GeP₂ is rather weak due to the strong inherent intralayer coupling of anions. Our research provides deep insights into the strong anion coupling effects on the electronic structures and defect properties of GeP and GeP₂, which sheds light on defect engineering and electronic applications of GeP_x based semiconductors.