With regard to microscale defects in sapphire crystals, many research studies reporting small bubbles of the size of 2 - 10 μm in EFG- [1] or CZ-grown crystals [2] have been published. However, there have been few reports about inclusions in sapphire crystals. Danko et al. reported hexagonal-shaped Al3O4 inclusions in sapphire crystals grown by the horizontally oriented crystallization method [3]. Recently, we reported the vertical Bridgman (VB) technique for growing c-axis sapphire crystal using a tungsten (W) or a molybdenum (Mo) crucible [4]. This paper reports the morphology and formation mechanism of inclusions observed in VB-grown sapphire crystals.
   Sapphire boules grown by the VB process using a W or a Mo crucible in an Ar atmosphere at 1 atm were cut into wafers, which were lapped and polished. The wafers were evaluated using a laser scattering observation technique to confirm the distribution of microscale light-scattering defects. Inclusions in the sapphire wafers were characterized using an optical microscope and a field emission scanning electron microscope (FE-SEM). The compositions of the inclusions were evaluated by energy dispersive X-ray spectrometry (EDS) working in the FE-SEM.
   Cubic, pentagonal or hexagonal-shaped inclusions with a size of 2 - 3 μm were observed in sapphire crystals grown using a W crucible. Figure 1 (a) shows a back-scattered electron image of an inclusion, and Figs. 1 (b) to (d) show the EDS mapping images of related elements Al, O and W, respectively. It was found that such inclusions consisted of W, which was the material used for the crucible. Similar-shaped Mo inclusions with the size of about 20 μm were observed in crystals grown using a Mo crucible.
   We concluded that the morphology of inclusions reflects a rhombic dodecahedron, which is based on the cubic structure of W and Mo and is surrounded by {110} faces.

[1] K. Wada et al., J. Cryst. Growth 50 (1980) 151-159.
[2] H. Li et al., Optical Mater. in press.
[3] A. Ya. Danko et al., Functional Mater. 10 (2003) 217-223.
[4] C. Miyagawa et al., J. Cryst. Growth in press.

1. PRESENTATION: Morphology and formation mechanism of metallic inclusions in VB-grown sapphire crystals, JPEG image data, JFIF standard 1.01, 0.1MB

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