Recently, significant attention has been paid to methods and techniques for growing sapphire crystals for use in the fabrication of GaN-based LED devices.  We have investigated the growth of sapphire crystal using the traditional vertical Bridgman (VB) method, in which sapphire crystals were grown in a Mo or W crucible with rotation and translation in a hot zone to achieve an appropriate temperature distribution. Different types of seed crystals and the seeding processes using them were examined from the viewpoint of achieving single crystal growth.

The upper diagrams in Fig. 1 are schematic drawings showing the features of the crucibles and seed crystals used in the experiments. The full diameter, tapered and thin seed crystals and crucibles are shown in the upper parts of Fig. 1(a), Fig. 1(b), and Fig. 1(c) respectively.  The full diameter portions of each crucible have a structure with a taper of a few degrees so that the grown crystals can be released easily from the crucible.  Reproducible seeding processes have been established by using techniques which include precise temperature measurement and control by adapting a newly developed thermo-couple structure.  Photographs of c-axis sapphire crystals grown using the three kinds of seed crystal, and using Mo or W crucibles of the suitable shape, are shown in the respective lower portions of Fig 1.

It is concluded that: (1) the newly developed thermo-couple enabled temperature measurement with accuracy of ±0.5 °C at about 2000 °C near the crucible bottom; (2) a seeding process which provides acceptable reproducibility in the very low temperature gradient of about 5 °C /cm at a high temperature near the melting temperature of sapphire has been established; (3) c-axis sapphire single crystals with a diameter of about 2 inches have been successfully grown from the three kinds of seeds, full diameter, tapered and thin seeds; and (4) it was found in the growth using thin seed crystals that a seed diameter of more than 20 millimeters was necessary in order for it to be possible to release the crystal easily from the crucible without breaking the grown crystal ¹⁾.

(1) C. Miyagawa et. al: Demonstration of crack-free c-axis sapphire crystal growth using vertical Bridgman method, Journal of Crystal Growth. (in printing)

1. POSTER: Vertical Bridgman Growth of Sapphire - Seed crystal shapes and seeding processes-, JPEG image data, JFIF standard 1.01, 0.1MB

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