In recent years, c-plane (0001) sapphire substrates have been used for the fabrication of GaN-based LED devices. We have applied the vertical Bridgman (VB) method to c-axis 3-inch diameter sapphire crystal growth, and crystals were successfully grown [1]. However, low-angle boundaries propagating from the seed-crystal interface were observed at the periphery of the crystal.
     The quality of a crystal is affected by the shape of the crystal-melt interface. In order to control this interface shape, two crucibles were prepared for crystal growth by VB method, each with a different shape in order to control the change in heat flux. The crystal-melt interface shapes and resulting low-angle grain boundaries in grown crystals were investigated experimentally and numerically.
     As-grown sapphire boules were cut into wafers. Polished sapphire wafers were evaluated using crossed polarizers and were also examined by X-ray topography. In addition, the influence of heat transfer using these crucibles on the crystal-melt interface shapes was numerically estimated using a 2D axisymmetric approach, performed using crystal growth simulator (CGSim) software.
     Photographs using crossed polarizers and X-ray topographic images showed that the shape of the crystal-melt interface and the occurrence of low-angle grain boundaries in the crystals were influenced by the heat transfer characteristics of the different crucible structures. The numerical predictions shown in Fig.1 (a) and (b) are consistent with available experimental data.

[1] C. Miyagawa et.al, Demonstration of crack-free c-axis sapphire crystal growth using the vertical Bridgman method, J. Cryst. Growth (in press).

1. PRESENTATION: Effect of heat transfer on the crystal-melt interface shape of sapphire crystal grown using the vertical Bridgman method, PNG image data, 1230 x 996, 8-bit/color RGBA, non-interlaced, 0.3MB

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