Evaluation and improvement of optical-grade LiTaO ₃ single crystals by the LFB ultrasonic material characterization system

This paper describes the first demonstration for feeding back the results obtained by the line-focus-beam ultrasonic material characterization (LFB-UMC) system to the crystal growth conditions for optical-grade LiTaO ₃ crystals and for achieving much improved homogeneity of chemical composition. We evaluated a commercially available optical-grade LiTaO ₃ single crystal with a nominally congruent composition in detail, by measuring distributions of the velocities of leaky surface acoustic waves (LSAW) along the Y-axis direction for a Z-cut specimen plate prepared from the crystal grown in the Y-axis direction. We detected an increment of 0.66 m/s in LSAW velocity along the pulling axis direction corresponding to 0.024 mol% in Li ₂O content, and the compositional gradient was +0.346 × 10 ^-3 (Li ₂O-mol%)/mm. By experimentally obtaining the starting material composition dependence of the gradients, we developed a method of estimating the proper composition ratio that would lead to a more homogeneous crystal. We grew a new crystal with a Li ₂O content of 48.47 mol%, resulting in a very small compositional gradient of +0.046 × 10 ^-3 (Li ₂O-mol%)/mm and a compositional homogeneity of less than 0.012 Li ₂O-mol% in a Z-cut area of 50 mm × 50 mm used for device substrates.