Er3+ diffusion in congruent LiNb O3 crystal in Li-enriched atmosphere

The thermal diffusion of Er3+ into X - and Z -cut congruent LiNb O3 crystal in Li-enriched atmosphere [i.e., vapor transport equilibration (VTE)], created by Li3 Nb O4 -LiNb O3 two-phase powder at the temperature around 1130 °C, was attempted. Single-crystal x-ray diffraction, micro-Raman, photoluminescence spectroscopy, and secondary ion mass spectrometry (SIMS) were used to study the crystalline phase with respect to Er3+ ion and the Er3+ diffusivity. The results show that the thickness of the Er film coated should not be thicker than 10 nm for an X -cut plate and 15 nm for a Z -cut plate. In this case, the diffusion is complete if the duration is long enough (>150 h) and the Er3+ ions in the diffused layer still retain the LiNb O3 phase. On the other hand, if the initial thickness of the Er metal film is thicker than 10 nm for the X -cut plate and 15 nm for the Z -cut plate, the diffusion will be incomplete no matter how long the duration is. This is because the residual Er3+ ions form irremovable ErNb O4 grains on the surface of the crystal. SIMS analysis on an X -cut VTE (1130 °C192 h) and a Z -cut VTE (1129 °C158 h) crystal coated, respectively, with 10 and 15 nm thick Er film reveals that the Er diffusion shows obvious anisotropy with the mean diffusion coefficients of 0.0155 and 0.0957 μ m2 h, respectively. The surface concentrations are 1.5× 1020 and 1.0× 1020 at. cm3, respectively. The diffused Er3+ ions follow the stretched-exponential decay profile with a stretching factor of 1.85 and 3.5, respectively. The LiNb ratio in the Er-diffused layer is similar to 99.4% for the X -cut sample coated with 10 nm thick Er film and 99.3% for the Z -cut crystal coated with 15 nm thick Er film. The rms roughness of the diffused surface is better than 6 and 4 nm for the X -cut and Z -cut samples, respectively.