Er ³⁺ diffusion into MgO (5 mol%)-doped LiNbO ₃ crystal under Li-poor atmosphere

Under Li-poor atmosphere, local Er ³⁺ diffusion into highly MgO-doped LiNbO ₃ (LN) crystal has been investigated. Three Z-cut 5 mol% (in growth melt) MgO-doped LN substrates locally coated with 44-60 nm thick Er-metal films underwent the respective Li-poor vapor transport equilibration (VTE) treatment at 1060°C, 1100°C, and 1130°C for the same 90 h duration. After the VTE, the reductions of Li ₂O content at the Er-free and Er ³⁺-doped parts of the surface of the three samples, determined from the measured birefringence, have a similar value of 1.0 ± 0.1, 2.0 ± 0.1, and 2.3 ± 0.1 mol%, respectively. Secondary ion mass spectrometry was used to study the Er ³⁺ diffusion properties and the VTE effect on Mg ²⁺ profile. From the measured Er ³⁺ profiles, characteristic diffusion parameters such as 1/e diffusion depth, diffusivity, diffusion constant, activation energy, surface concentration, solubility, solubility constant, and heat of solution are determined. A comparison with the related results previously reported allows to conclude that heavy MgO-doping results in substantial decrease of both the diffusivity and solubility, and under the same diffusion temperature the diffusivity and solubility increase by as much as one order of magnitude and two times, respectively, as the diffusion atmosphere goes from the traditional argon to the Li-poor atmosphere. The Mg ²⁺ profiles reveal that the VTE may induce MgO diffusion and hence an inhomogeneous profile in the Er ³⁺- diffused layer. This undesired effect is strong for the 1130°C VTE temperature whereas is relatively much weak for 1060°C. It is tentatively explained in terms of the Er ³⁺ diffusion and Li-poor VTE mechanisms.