Morphology and formation mechanism of metallic inclusions in VB-grown sapphire crystals

• Morphologies of inclusions in VB-grown sapphire using W crucible were investigated.
• Square- or hexagonal-shaped inclusions 2–5 μm in size were observed.
• Inclusions observed consisted of W metal used for the crucible.
• Inclusions observed are a quite different composition to those previously reported.
• Based on experimental results, a possible mechanism for the formation of inclusions is discussed.

Morphologies of metallic inclusions observed in sapphire crystals grown by the vertical Bridgman (VB) technique using a tungsten (W) crucible were investigated. Square- or hexagonal-shaped inclusions 2–5 μm in size were observed in sapphire crystals around the interface between the seed and the grown crystal. It was found that such inclusions consisted of W metal used for the crucible. The morphology of some of the inclusions reflects a rhombic dodecahedron which is based on the cubic structure of W and is surrounded by {110} faces. It is probable that inclusions form in the sapphire melt during the crystal growth process, and then sink in the melt to the growth interface due to the high density of W.