Locally Er-doped high-solubility LiNbO₃ crystal prepared by Li-poor vapor transport equilibration and Er codiffusion

We demonstrate the feasibility of preparing locally Er-doped high-solubility LiNbO₃ (LN) crystal for the use of integrated optics by simultaneous work of Er in-diffusion and Li-poor vapor transport equilibration (VTE) treatment. Four X-cut and four Z-cut congruent LN substrates locally coated with 30-44 nm thick Er-metal films underwent a Li-poor VTE treatment of 1030/240°, 1050/180°, 1070/180°, and 1050/180°C/h, respectively. After the VTE procedure, the Li₂O content in the crystal, determined from the measured birefringence, decreases from original 48.5±0.1 mol% to (48.0-48.2)±0.1 mol% with an approximately homogeneous profile and shows less effect of the cut of plate. Secondary ion mass spectrometry was used to study the Er diffusion properties. The results show that all of the Er diffusion reservoirs were not exhausted. The diffused Er ions in all of the samples studied follow the complementary error function profile. From the measured Er profiles, characteristic diffusion parameters such as diffusion depth, diffusivity, diffusion constant, activation energy, and solubility were determined and discussed in comparison with the case of the traditional diffusion in the argon or O₂ atmosphere. It is found that the diffusion in the Li-poor atmosphere shows an anisotropy similar to that in the traditional argon or O₂ atmosphere. Under the same temperature, the Er diffusivity in the Li-poor atmosphere is considerably larger than in the O₂. From the logarithmic plot of diffusivity versus 1/T and the Arrhenius law, the diffusion constant and activation energy are determined to be (1.2±0.2) × 10⁷/(2.2±0.3) × 10 ⁴μm²/h and (2.32±0.02)/(1.51±0.02) eV for the diffusion in the X-/Z-cut plate. The solubility in the X-/Z-cut plate is evaluated to be (1.42±0.24)/(1.14±0.15), (2.18±0.27)/(2. 01±0.19), (2.62±0.26)/(2.68±0.19), and (2.54±0.20)/ (2.30±0.16) × 10²⁰ ions/cm³ for the VTE temperature of 1030°, 1050°, 1070°, and 1100°C, respectively. As expected, it depends on both the VTE temperature and the Li₂O content in crystal. Under the same VTE temperature, the solubilities in the X-and Z-cut plates can be regarded as same within the error range. Under the same temperature, the solubility in the Li-poor atmosphere case is considerably higher than that in either the argon or the O₂ atmosphere case.