Intrinsic point defects and charge carrier trapping in monolayer BiOX (X = I, Br, and Cl)

Abstract Two-dimensional (2D) layered bismuth oxyhalides, BiOX (X = I, Br, and Cl), have great potential in optoelectronics and photocatalysis applications. The intrinsic point defects are crucial for carrier conductivity and transport. However, the understanding for defect physics of 2D atomic-scale BiOX are still unclear. Herein, through the first-principles calculations, we investigate the formation of intrinsic point defects and their effect on charge carrier trapping in 2D monolayer BiOX. Under a O-poor condition, the donor defects, such as the Biad, BiX, VO, and VX, can form spontaneously and induce a high n-type conductivity. The VX shows a shallow transition level and has no defect states. In contrast, the Biad, BiX, and VO display deep transition levels and obvious localized defect states that are responsible for the charge carrier trapping. As O becomes richer, the concentration of acceptor defects increases. Nevertheless, the donor and the acceptor defects can strongly compensate each other, pinning the Fermi energy in the band gap. The dominant acceptor defects, such as the BrBi, OBr, and OCl, show the deep transition levels and serve as carrier traps due to the charge localized around the defect sites. Our work gives an insight into the defect physics of atomic-scale 2D BiOX and provides a guidance for their optoelectronics and photocatalysis applications.