Er 3 + diffusion in congruent LiNbO3 crystal in Li-enriched atmosphere

Abstract

The thermal diffusion of Er3+ into X- and Z-cut congruent LiNbO3 crystal in Li-enriched atmosphere [i.e., vapor transport equilibration (VTE)], created by Li3NbO4–LiNbO3 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 10nm for an X-cut plate and 15nm for a Z-cut plate. In this case, the diffusion is complete if the duration is long enough (>150h) and the Er3+ ions in the diffused layer still retain the LiNbO3 phase. On the other hand, if the initial thickness of the Er metal film is thicker than 10nm for the X-cut plate and 15nm 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 ErNbO4 grains on the surface of the crystal. SIMS analysis on an X-cut VTE (1130°C∕192h) and a Z-cut VTE (1129°C∕158h) crystal coated, respectively, with 10 and 15nm thick Er film reveals that the Er diffusion shows obvious anisotropy with the mean diffusion coefficients of 0.0155 and 0.0957μm2∕h, respectively. The surface concentrations are 1.5×1020 and 1.0×1020at.∕cm3, respectively. The diffused Er3+ 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 10nm thick Er film and 99.3% for the Z-cut crystal coated with 15nm thick Er film. The rms roughness of the diffused surface is better than 6 and 4nm for the X-cut and Z-cut samples, respectively.