Er 3 + diffusion in congruent LiNbO 3 crystal doped with 4.5 mol% MgOJudd-Ofelt analysis of spectroscopic property of Er 3 + in congruent and near-stoichiometric Zn ∕ Er -codoped Li Nb O 3 crystalsThe thermal diffusion of Er 3+ into X-and Z-cut congruent LiNbO 3 crystal in Li-enriched atmosphere ͓i.e., vapor transport equilibration ͑VTE͔͒, created by Li 3 NbO 4 -LiNbO 3 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 ͑SIMS͒ were used to study the crystalline phase with respect to Er 3+ ion and the Er 3+ 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 ͑Ͼ150 h͒ and the Er 3+ ions in the diffused layer still retain the LiNbO 3 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 Er 3+ ions form irremovable ErNbO 4 grains on the surface of the crystal. SIMS analysis on an X-cut VTE ͑1130°C / 192 h͒ and a Z-cut VTE ͑1129°C / 158 h͒ 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 m 2 / h, respectively. The surface concentrations are 1.5ϫ 10 20 and 1.0ϫ 10 20 at./ cm 3 , respectively. The diffused Er 3+ ions follow the stretched-exponential decay profile with a stretching factor of 1.85 and 3.5, respectively. The Li/ Nb 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.

The feasibility of preparing locally Er‐doped near‐stoichiometric (NS) LiNbO3 crystals for integrated optics applications is demonstrated by a two‐step process with standard diffusion (1130°C/154 h) of Er metal followed by vapor transport equilibration (VTE) treatment under three different conditions of 1135°C/22 h, 1115°C/50 h, and 1125°C/60 h. Detailed studies on the crystalline phase, Li composition, diffused surface roughness, and emission characteristics of Er3+ ions indicate that there is an upper limit on the initial Er metal film thickness: ∼20 nm for an X‐cut crystal and ∼30 nm for a Z‐cut crystal. When the initial Er film thickness is below this limit, the post‐VTE does not induce formation of ErNbO4 precipitate and the diffused surface retains high quality with a root mean square roughness <3 nm. Depending on the VTE condition adopted, the VTE results in the increase of ([Li]+[Er])/[Nb] ratio in the diffused layer from congruent point (94.5%) to 97.4%–99.4%. Secondary ion mass spectrometry study shows that the post‐VTE does not affect the Gauss nature of the Er profile, but leads to the increase of diffusion depth by as much as 1.6 μm. In comparison with the standard Er diffusion, the post‐VTE results in the decrease of Er diffusivity by three to nine times. The higher the VTE temperature is, the lower the Er diffusivity is. In addition, the post‐VTE also results in definite reduction of OH− content in crystal, slight lengthening of lifetime and slight narrowing of linewidth of Er3+ emission at 1530 nm.