ZnO crystals are manufactured by various methods ? they can be grown from the melt, or produced by vapour deposition, or by hydrothermal method. The hydrothermal technology is now preferred by most manufactufacturers. Its first and foremost advantage is the size of the grown crystals. By using this method we can not only reproduce the bait wafers, we can make them bigger, which is a real breakthrough, especially if we consider that natural zinkite crystals are always quite small. The second advantage lies in the quality of the product. Growing ZnO by hydrothermal method is quite a slow process, which allows to achieve a very high quality of the crystal, without block marks, twins and dislocations.
Hydrothermal ZnO single crystals are grown in high pressure autoclaves by means of direct temperature drop in aqueous solutions of KOH + LiOH at crystallization temperature 320- 400 ^oC and pressure 20-70 MPa. Inside the autoclaves there are special vessels made of corrosion-resistant alloys ? they serve to protect autoclaves from corrosion. The charge, the bait and the chemical solution are put into the vessel, then the vessel is hermetically sealed and placed into the autoclave. Finally the autoclave is filled with water and is also sealed hermetically. After that the autoclave is heated to the fixed temperature. The whole production cycle takes 100-150 days.