Diffusivity of Er3+ into LiNbO3 as a function of stoichiometry.

Diffusivity of Er³⁺ into LiNbO₃ crystal has been studied as a function of Li₂O content and crystal orientation. The Li₂O content in single-crystal LiNbO₃ plates was adjusted by Li-rich vapor transport equilibration (VTE) technique. Five X-cut and five Z-cut congruent substrates were VTE treated at first at 1100 °C for different durations ranging from 6 to 74 h, and then were subjected to a special post-VTE anneal procedure at the same temperature for 60 h to obtain homogeneous Li₂O content. The Li₂O content was determined from the measured fundamental optical absorption edge. Its homogeneity is verified by measuring the optical absorption edge as a function of the crystal thickness. After the composition characterization, the substrates were coated with 45 nm thick Er film and annealed at 1130 °C for 40 h in argon atmosphere. The Er³⁺ profile was analyzed by secondary ion mass spectrometry. The results show that the Er³⁺ diffusivity decreases monoexponentially with the increase in the Li₂O content in both cases of X- and Z-cut. As the Li₂O content is increased from the congruent point (48.4 mol %) to the near-stoichiometric composition (∼49.9 mol %), the diffusivity decreases from 0.107 (0.072) to 0.017 (0.018) μm²/h for the Z (X)-cut substrate. The diffusion shows considerable anisotropy for the lower Li₂O content and the diffusivity in a Z-cut substrate is larger. The anisotropy weakens gradually with the increase in the Li₂O content, and the diffusivities in the X- and Z-cut substrates ultimately tend to a same value (∼0.02 μm²/h) at the Li-rich phase boundary. After the Er diffusion procedure, endface coupling experiment was carried out to check if a planar waveguide layer was formed near the Er-diffused surface due to the Li outdiffusion. The results show that the Li outdiffusion did not accompany the Er diffusion procedure. [ABSTRACT FROM AUTHOR]