Low-Angle Grain-Boundary Macrostructure in Large-Diameter Czochralski White Sapphire

Large-diameter Czochralski sapphire crystals grown in several orientations were occasionally found to contain complex networks of low-angle grain boundaries. The subboundaries lie in major low-index crystallographic planes which are uniquely determined by the gowth orientation. The major subboundary planes were the (11¯00) and (011¯0) for crystals gro_wn_in the direction normal to the ( r )-(1¯012) plane and the (1¯21¯0) and (21¯1¯0) planes for crystals grown in the 90°-[1120] direction. The formation of the subboundary macrostructure was attributed to an excessive temperature gradient during seeding and is not related to growth rate, impurity content, or annealing.