Flat Shoulder Congruent Lithium Niobate Crystals Grown by the Czochralski Method

Abstract

The shoulder of the crystals grown by the Czochralski method are generally inclined. The quality of the inclined shoulder is poor and the processing is difficult, which result in the low utilization rate of the crystal. This problem can be solved by growing flat shoulder crystals. However, polycrystalline and inclusion defects are easy to occur in the stage of shouldering, so the requirement of thermal field and growth process for the flat shoulder crystal growth is very high. As a multifunctional crystal, lithium niobate crystals have been widely used in the fields of electronics, optical communication, laser, and integrated photonics. Take congruent lithium niobate (CLN) crystal as an example, this study used numerical simulation and experiment method to investigate the thermal field and growth process of flat shoulder crystal growth by the Czochralski method. The result shows that the shape of the solid-liquid interface should be convex toward melt at the stage of shouldering. The temperature gradient near the solid-liquid interface can be reduced by lowering the after-heater position (10 mm) to avoid the formation of polycrystalline. The control of the speed of shouldering is monitored mainly and heating power is adjusted slightly to ensure the trend of shouldering.Increasing the speed at the initial stage of shouldering (ϕ≤30 mm) and decreasing the speed at the middle and later stage of shouldering (ϕ≥35 mm) can shorten the period of shouldering and avoid 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 pulling rate and power with a small amplitude (Δt=10 min, ∆v= 0.2 mm/h). Using the appropriate 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 grown.