A relationship between growth rates and void alignments in sapphire crystals grown by the vertical Bridgman method is experimentally investigated and the alignments and shapes of voids are discussed in the context of the growth rate and temperature gradient in the growth process.

Sapphire crystals of about 50mm in diameter and about 100mm in length were grown in a sealed chamber including a hot-zone constructed with a graphite heater, carbon felt heat-shields and a crucible made of Mo or W. The shape, size and distribution of voids in three crystals grown with crucible translation speeds of 3, 5 and 10 mm/h respectively were characterized by optical microscope and laser scattering observation techniques.

Slight voids of the size of several or several tens of micro-meters were detected with irregular distributions in the crystals grown with translation speeds of 3 and 5 mm/h. On the other hand, a high density of voids with characteristic alignments was observed in the crystal grown of 10mm/h. The alignments consisted of three-fold symmetries of void distributions in the c-plane, perpendicular to the growth axis. The shapes of voids showed the characteristic facet corresponding to crystal symmetry and temperature gradient in the growth processes.

It was concluded that the generation of high density voids and their alignment was strongly related to the cellular growth and to the volumetric shrinkage when the sapphire melt solidified to crystal¹⁾.

<References>

 (1) K. Wada et. al: Jpn. J. Appl. Phys. Vol.17 No.2 (1978) pp.449-450.

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