Defect properties in Yb³⁺-doped CaF₂ from first-principles calculations: A route to defect engineering for up- and down-conversion photoluminescence

Calcium fluoride has been widely used for light up-/down-conversion luminescence by accommodating lanthanide ions as sensitizers or activators. Especially, Yb-doped CaF₂ exhibits unique defect physics, causing various effects on the luminescence. This makes it vital for high efficiency of devices to control the defect-clustering, but methodological guidelines for this are rarely provided. Here we perform a first-principles study on defect physics in Yb-doped CaF₂ to reveal the thermodynamic transition levels and formation energies of possible defects. We suggest that a fluorine-rich growth condition can play a key role in enhancing the luminescence efficiency by facilitating the Yb-clustering and suppressing the defect quenchers in the bulk. The detailed energetics of defect aggregation not only well explains the experimentally favored Yb-clustering but also presents n- or p-type doping methods for cluster control.