Electronic properties of intrinsic vacancies in single-layer CaF2 and its heterostructure with monolayer MoS2

Abstract

Exploring gate insulator materials for 2D transistors and their defect properties is of importance for device performance optimization. In this work, the structural and electronic properties of intrinsic vacancies in the CaFcom.elsevier.xml.ani.Math@271443c8 single layer and its heterostructures with monolayer MoScom.elsevier.xml.ani.Math@74f82638 are investigated from first-principles calculations. Vcom.elsevier.xml.ani.Math@46be901c introduces a shallow defect level close to the VBM, whereas Vcom.elsevier.xml.ani.Math@6a5bc347 introduces a deep level below the CBM. In both cases, spin polarization is observed. Overall, Vcom.elsevier.xml.ani.Math@6fd4d7c7 has a relatively lower formation energy than Vcom.elsevier.xml.ani.Math@5aa8f5e3, except for the extreme Ca-rich case. Thus, Vcom.elsevier.xml.ani.Math@68a4a0da should be dominant in CaFcom.elsevier.xml.ani.Math@5cd10772. The band offset between CaFcom.elsevier.xml.ani.Math@157299c0 and MoScom.elsevier.xml.ani.Math@8e87160 is determined to be type-I, with large offsets at both the conduction band and valence band. With the presence of vacancies in CaFcom.elsevier.xml.ani.Math@4587b2d7, the type-I band offset is preserved. The electron or hole on the defect states will transfer from CaFcom.elsevier.xml.ani.Math@1139c782 to MoScom.elsevier.xml.ani.Math@1004e0fc due to the large band offset, and spin polarization vanishes. Nevertheless, there are no defect states inside the gap or around the band edge of MoScom.elsevier.xml.ani.Math@5bace0ed, and the electronic properties of MoScom.elsevier.xml.ani.Math@23c5f045 are almost intact. Compared with com.elsevier.xml.ani.Math@7c1f2668-BN that has a small valence band offset with MoScom.elsevier.xml.ani.Math@4bab3a17 and could introduce in-gap defect states, CaFcom.elsevier.xml.ani.Math@773c1007 can be a good candidate to serve as the dielectric layer of MoScom.elsevier.xml.ani.Math@7b77817d-based transistors.