Influence of Point Defects on the Optical Properties of La3Nb0.5Ga5.5O14 crystals

Abstract

Point defects critically influence the optical performance of langasite-type La3Nb0.5Ga5.5O14 (LGN) crystals, yet their nature and evolution under different growth conditions remain unclear. Here, we systematically investigate the defect structure in LGN crystals grown under N2 and N2+2%O2 atmospheres, as well as after subsequent vacuum annealing. The results reveal that both gallium vacancies and oxygen vacan-cies intrinsically exist in LGN, while their relative concentrations strongly depend on the growth atmosphere. In LGN grown under N2+O2, excess oxygen vacancies form F and F⁺ centers, giving rise to characteristic absorption bands at ~480 nm and ~335 nm, respectively, whereas these color centers are largely suppressed in crystals grown under pure N2. A distinct absorption band at ~1860 nm is attributed to electronic transi-tions involving gallium vacancy-induced shallow acceptor levels, as supported by first-principles calculations. Vacuum annealing selectively eliminates F centers but preserves F⁺ centers, which is 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 reveals that screw dislocations dominate on the (2-10) surface, while edge dislocations prevail on the (001) surface, with higher dislocation density observed in crystals grown under oxygen-containing atmosphere.