2. Fabrication of Lithium Tantalate Crystal

2.1 Fabrication of same composition lithium tantalate crystal

The same composition Lithium tantalate (CLT) crystals are often fabricated by mixing high-purity tantalum pentoxide with high-purity lithium carbonate at a stoichiometric ratio of 0.95:1 (molar ratio), and are prepared by the crucible pulling method. The quality of LiTaO₃ crystal (www.wisoptic.com) is generally affected by factors such as raw material ratio, pulling speed, seed crystal quality, crucible shape and type. LiTaO₃ crystal block can be sliced, blackened, ground, chamfered and cleaned to obtain LiTaO₃ wafers. The Czochralski method is currently the most commonly used method for preparing CLT.

When preparing CLT by Czochralski method, the raw materials contained in the crucible are melted by heating. Above the crucible, there is a rotating and lifting rod with a chuck at its lower end, and a seed crystal is installed on the chuck. Lower the lifting rod to make the rotating seed crystal touch and weld the melt surface, then slowly pull up the rod when keeping rotating, and complete the crystal preparation through steps including neck craning, neck drawing back, shoulder release, constant diameter control and ending, etc. During the crystal growth process, parameters such as temperature and thermal field, pulling speed and rotation speed, gas type and pressure in the furnace need to be strictly controlled.

Ohno Yutaka et al. studied the cracking process of 36°Y LiTaO₃ single crystal in the Czochralski process and found that cracks appeared at the intersection of twin crystal flakes. When the diameter of the ingot suddenly decreased, cracks appeared in the twin crystals. The advantages of the Czochralski method are simple equipment, easy operation and doping. The Kyropoulos method, the EFG method, and the temperature-gradient method are rarely used to grow CLN crystal due to the higher cost, lower crystal quality, and more process difficulty, even though they can also produce CLN crystal.