Macroscopic Defects of Large Bi12GeO20 Crystals Grown Using Vertical Bridgman Method

As a multifunctional opto-electro material, Bi1(12)GeO(20) crystal shows high-speed photorefractive response in visible range, excellent piezoelectric, acousto-optic, magneto-optic, optical rotation, and electro-optic properties, etc. Presently, Czochralski (Cz) method, which is commonly used to grow Bi12GeO20 crystals, has several bottle-necks, such as high growth cost, irregular crystal boule shapes, low growth yield, poor optical quality in large crystals, and small effective crystal cross-sectional area. In this study, large Bi12GeO20 crystals were firstly grown by using modified vertical Bridgman method in platinum crucibles and air atmosphere. Morphology, distribution, and constitutes of main macroscopic defects in as-grown Bi12GeO20 crystals were investigated, and the formation process and causes of the main macroscopic defects during the crystal growth were studied. Dendrite and tubular inclusions are two types of main macroscopic defects existed in as-grown Bi12GeO20 crystals. The formation of dendrite inclusions is closely related to the platinum corrosion, while the formation of tubular inclusions is associated with precipitation of platinum, a mismatch in the stacking of growth units due to instability of the seeding interface, and instability temperature field. Technical approaches to eliminate or reduce these two types of macroscopic defects during the growth using vertical Bridgman method were proposed. High optical quality, large Bi12GeO20 crystals with sizes up to 55 mmx55 mmx80 mm and significantly improved optical transmittance were grown reproducibly by reducing control temperature, decreasing period of melt preserved at high temperature, and selecting seed crystals with better quality.