  We studied native point defects in CaS by DFT+ Hubbard U method. The effect
of the localization of the d orbitals of Ca pseudopotential has been included.
The Hubbard U corrected d-orbital for Ca sites are playing a role assisting the
charge transfer from p orbitals of S site to the Ca site, giving both localized
electron and hole states within the band gap, with an energy interval of
1.2~1.4 eV. This corresponds to the localized excitonic levels below the
conduction band edge for the optical absorption. The electronic properties and
formation energies of native point defects have been discussed. We found the
neutral S vacancy has the lowest energy of 0.62 eV under Ca-rich limit. The
Schottky defect pair defect is another dominant defect with cost of 1.51 eV per
defect site from S-rich to Ca-rich chemical potential limits. The defect
formation energy further summarized that the doubly positive S vacancy and
doubly negative Ca vacancy are both the most stable donor-type and
acceptor-type defects respectively coexisting in CaS. We also summarized a
narrow doping limit energy which has been determined as 1.33 eV constantly in
CaS independent to different chemical potential limits. But such doping allowed
range entirely shifts from valence band vicinity toward the conduction band
edge from S-rich to Ca-rich limits. Under S-rich limit, the dopable range for
EF shifting corresponds to Eu2+ doping experiments, the luminescence wavelength
limit is predicted to be 659 nm (1.88 eV) remarkably close to the reported
results 650 nm. This gives a solid theoretical reference for the lanthanide
ions doping experiments in CaS. We conclude that the defect levels combined
with formation energy is an accessible way to suggest the doping energy range
and an assistant to explain the activation stage of photostimulated
luminescence (PSL) mechanism in lanthanide ions doped crystal materials.
