First-principles study of dopant stability and related optical properties in CdSiP2 crystal

CdSiP₂ (CSP) with outstanding mid-infrared nonlinear optical properties in the 3–5 μm and 8–12 μm windows has always been expected for application requirement, whereas the inadequate of absorption losses cannot be ignored as practical usage. The defect formation energies, electronic structure characteristics, structure distortion and related optical properties of CSP with some traditional and basic dopants M= (Mg, Al, Cu, Fe, and Mn) substituting Cd or Si sites are investigated by employing density functional theory (DFT). The calculations show that the energetically and thermally stable states in CSP crystals are Mg⁰, Al¹⁺, Cu⁴⁺, Fe⁴⁺, Mn⁴⁺ at Cd sites and Fe_Si¹⁺ at SiP₂ reference states. Fe and Mn dopants at Cd sites bring abundant valences, including 0, +1, +2 and +4 charged states, and result in a mean absorption spectrum increase. New peaks appear in the density of states as the Cu_Cd⁴⁺, Fe_Cd⁴⁺, and Mn_Cd⁴⁺ take shape, and thus affect the absorption spectrum. According to the calculation results, highly localized d orbitals of dopants should be mainly responsible for the absorption aberrance, because they not only lead to a mean absorption spectrum increase, but also bring new absorption peaks affecting the conversion efficiency along X/Y or Z directions.