Optical and defect properties of mid-IR laser crystal Dy3+: PbGa2S4: a DFT and XPS study

Abstract

The Dy³⁺:PbGa₂S₄ crystal with low phonon energy has been used to achieve direct lasing of 4.3–4.7 μm lasers. High quality PbGa₂S₄ crystals are difficult to fabricate due to component volatilization. In this work, the influence of component condition on the intrinsic defects and optical absorption in the Dy³⁺:PbGa₂S₄ crystal was investigated through first-principles calculations and crystal growth experiments. We found that PbGa₂S₄ was prone to decompose into Ga₂S₃ at its growth temperature, usually resulting in PbS-rich (Ga₂S₃-deficient) growth conditions. The calculated results of defect formation energy indicate that the Pb_Ga and V_Ga defects play a dominant role in PbGa₂S₄ under PbS-rich growth conditions. The Ga₂S₃-rich growth conditions can increase the defect formation energy and may lower the defect density. Our post-growth annealing and crystal growth experiments proved that the Ga₂S₃-rich conditions did improve the optical quality of the Dy³⁺:PbGa₂S₄ crystal. The structure and composition of PGS were analyzed by XRD and EDS. The XRC result showed the crystal has a high crystalline quality. The changes of bond length were studied through XPS characterization and DFT calculations. The V_Ga and Pb_Ga defects result in different XPS peaks of PbGa₂S₄ crystals fabricated under PbS-rich and Ga₂S₃-rich growth conditions.