Control of Er³⁺ diffusion by co-diffused Ti⁴⁺ in LiNbO₃ crystal

Er³⁺/Ti⁴⁺-co-doped LiNbO₃ plates were prepared by annealing congruent substrates coated with stacked Er- and Ti-metal films having different thicknesses and coating sequences. The crystalline phase, OH^- absorption, Er³⁺-doping effect on LiNbO₃ refractive index and Li⁺-composition in the Er³⁺/Ti⁴⁺-co-doped layer were studied. The results show that the LiNbO₃ phase dominates in the co-doped layer, the Er³⁺-doping affects little the LiNbO₃ refractive index, slight Li₂O out-diffusion occurred in the diffusion procedure, and the annealing procedure does not affect the OH^- absorption band noticeably. The Er³⁺/Ti⁴⁺-co-diffusion characteristics were studied by secondary ion mass spectrometry. The study shows that Ti⁴⁺ diffuses much faster than Er³⁺. The Ti⁴⁺ co-diffusion assists and controls the Er³⁺-diffusion and Er³⁺ diffusivity increases linearly with initial Ti-metal thickness. In contrast, neither Er-metal thickness nor coating sequence of Ti- and Er-metal films influences the Ti⁴⁺ or Er³⁺ diffusivity. The co-diffusion features are explained. A co-diffusion model is proposed.