Control of scandium diffusion in LiNbO₃ single crystal by co-diffusion of titanium

Abstract

The mutual influence of co-diffused Ti⁴⁺ and Sc³⁺ in LiNbO₃ single-crystal was studied. Sc³⁺/Ti⁴⁺ co-doped LN plates were prepared by annealing (at 1060 °C in wet O₂) Z-cut congruent LiNbO₃ substrates coated with stacked Sc₂O₃ and Ti films, which have different thicknesses and coating sequences. After the annealing, the refractive index and composition at the undoped and Sc³⁺-only doped surface parts were characterized by prism coupling technique. The Sc³⁺ and Ti⁴⁺ profiles were analyzed by secondary ion mass spectrometry. The results show that the Sc³⁺-doping has little contribution to the LiNbO₃ refractive index. Li₂O out-diffusion is not measurable. In the only diffusion case, the Sc³⁺ diffuses five times slower than the Ti⁴⁺. In the Sc³⁺/Ti⁴⁺ co-diffusion case, the Ti⁴⁺ assists and controls the Sc³⁺ diffusion. The Sc³⁺ diffusivity increases linearly with the initial Ti-metal thickness. In contrast, the Sc³⁺ affects the Ti⁴⁺ diffusion little. Neither the Sc₂O₃ film thickness nor the coating sequence influences the diffusivity of the two ions. The co-diffusion features are explained. A model for the co-diffusion system is suggested and verified experimentally. The Ti:Sc:LiNbO₃ planar and strip waveguide are optically characterized. The results show that the Sc³⁺ presence does not affect the waveguide characteristics and the waveguide is optical-damage-resistant.