Influence of point defects on the optical properties of La3Nb0.5Ga5.5O14 crystals

Abstract

Point defects critically influence the optical performance of langasite-type La₃Nb_0.5Ga_5.5O₁₄ (LGN) crystals, yet their nature and evolution under different growth conditions remain unclear. Here, we systematically investigated the defect structure in LGN crystals grown under N₂ and N₂ + 2% O₂ atmospheres and after subsequent vacuum annealing. The results revealed that both gallium vacancies and oxygen vacancies intrinsically existed in LGN, while their relative concentrations strongly depended on the growth atmosphere. In LGN grown under an N₂ + O₂ atmosphere, excess oxygen vacancies formed F and F⁺ centers, giving rise to characteristic absorption bands at ∼480 nm and ∼335 nm, respectively, whereas these color centers were largely suppressed in crystals grown under a pure N₂ atmosphere. A distinct absorption band at ∼1860 nm was attributed to the electronic transitions involving gallium vacancy-induced shallow acceptor levels, as supported by first-principles calculations. Vacuum annealing selectively eliminated the F centers but preserved the F⁺ centers, which was attributed to their deeper defect levels and stronger electron localization. Based on these results, a defect formation mechanism governed by the competitive evaporation of Ga₂O and O₂ is proposed. Furthermore, dislocation analysis revealed that screw dislocations dominated on the (2−10) surface, while edge dislocations prevailed on the (001) surface, with a higher dislocation density observed in crystals grown under an oxygen-containing atmosphere.