Influence of vapor transport equilibration on surface composition of MgO:LiNbO₃ crystal

The surface composition of 5 mol% (in growth melt) MgO-doped z-cut LiNbO3 crystal was modified using Li-poor vapor transport equilibration (VTE) process. The VTE-induced surface Li₂O-content reduction ΔC was evaluated from the measured birefringence, and the ΔC dependence on VTE temperature T and time t were studied. The results show that the effect of T on ΔC follows the Arrhenius law, with a surface Li₂O-content reduction constant of (1.3 ± 0.1) × 10¹¹ mol% and an activation energy of 3.0 ± 0.1 eV, and the effect of t on ΔC is a square-root dependence, with ΔC = 0.17t0.51. Thus, a generalized empirical expression can be obtained to show the dependence of ΔC on T and t. A comparative study on undoped pure LN shows that the MgO doping has slight effect on ΔC. Based on the obtained dependence of ΔC on t, the solution to the Li⁺ out-diffusion equation is shown and verified experimentally to be an integral of complimentary error function. The results indicate that Li-deficientMgO-doped LiNbO₃ material with desired surface Li₂O-content can be produced by controlling the VTE temperature and duration.