GROWTH OF LI2B4O7 SINGLE-CRYSTALS BY A PU LLING-DOWN METHOD

Bubble-free, transparent Li2B4O7 single crystals were grown by a newly developed pulling-down method, The use of a two-zone vertical furnace with a Pt-crucible realized a relatively steep temperature gradient o f about 50 degrees C/cm around the growth interface and a moderate gra dient of about 13 degrees C/cm below the interface, contributing to pr event grown crystals from cracking. Even at a pulling-down rate of 0.7 5 mm/h, which is higher than that in the Czochralski growth, the forma tion of bubbles was effectively suppressed at a rotation rate of 25 rp m. Any crystals grown along [110] direction involved no low-angle grai n boundaries, but tu inning was occasionally recognized, The dislocati on density in the initial part of the crystal growth was found to be a bout 5 x 10(3)/cm(2) from an etch pit pattern, while X-ray topography of a vertical cross section revealed that the number of dislocation li nes decreased as the growth proceeded.