Diffusion Properties of Scandium in Lithium Niobate Crystal

Diffusion properties of Sc³⁺ in LiNbO₃ crystal were studied, together with other two related issues: Sc³⁺-doping effect on LiNbO₃ refractive index and Li₂O out-diffusion arising from Sc³⁺ in-diffusion. To reach the goal, some Sc³⁺-doped LiNbO₃ crystal plates were prepared by in-diffusion of Sc₂O₃ film coated onto Z-cut congruent substrates in air at different temperatures ranging from 1030C to 1130C. After diffusion, the refractive indices at the doped and undoped parts of surface were measured by prism coupling technique and the surface composition was evaluated from the measured index. The results show that the Sc³⁺ doping has little contribution to the substrate index and the Li₂O out-diffusion is not measurable. The Sc³⁺ profile was analyzed by secondary ion mass spectrometry. Some characteristic parameters such as temperature-dependent diffusivity, diffusion constant, activation energy, and surface concentration are obtained. The Sc³⁺/Ti⁴⁺ co-diffusion characteristics were also studied. Comparison shows that in the single-diffusion case, the Sc³⁺ diffusivity is considerably smaller than the Ti⁴⁺ diffusivity. In the Sc³⁺/Ti⁴⁺ co-diffusion case, the Ti⁴⁺ co-diffusion results in substantial increase in Sc³⁺ diffusivity while the Ti⁴⁺ diffusivity changing little. The diffusion characteristics are qualitatively explained. The Ti⁴⁺-assisted Sc³⁺ diffusion would be utilizable in shortening the fabrication period of an optical-damage-resistant Ti-diffused LiNbO₃ waveguide doped with Sc³⁺, and some considerations for the fabrication are given.