Flat Shoulder Congruent Lithium Niobate Crystals Grown by the Czochralski Method

The shoulder of the crystals grown by the Czochralski method is generally inclined, leading to poor quality and difficult processing, which then result in low utilization rate of the grown crystal. Take congruent lithium niobate（CLN） crystal as an example, this study used numerical simulation and experimental method to investigate the thermal field and growth process of flat shoulder crystal growth by the Czochralski method which can well acceptedly overcome the above problems. The result shows that the shape of the solid-liquid interface should be convex toward melt at the stage of shouldering. Temperature gradient near solid-liquid interface can be reduced by lowering the after-heater position（10 mm） to avoid the formation of polycrystalline. Control of the shouldering speed is the main way while control heating power is the accurate way to ensure the trend of shouldering. Accelerating the speed at the initial stage of shouldering（φ≤30 mm） and slowdown the speed at the middle and later stage of shouldering（φ≥35 mm） can shorten the period of shouldering and avoid defect inclusion. The pulling rate（0-1.5 mm/h） can be changed rapidly（1.5-2 h） without affecting the trend and quality of shouldering by adjusting power with a small amplitude（Δt=10 min, ?v= 0.2 mm/h）. By using these adjusting ways to thermal field and growth process, a series of 3-inch（1 inch=25.4 mm） flat shoulder CLN crystals with good optical homogeneity have been successfully grown.