Demonstration of crack-free c-axis sapphire crystal growth using the vertical Bridgman method

• We have applied the vertical Bridgman technique to c-axis sapphire crystal growth.
• 3-in. diameter sapphire crystals were grown using molybdenum and tungsten crucibles.
• As-grown crystals were easily released from respective crucibles nondestructively.
• Crack-free sapphire crystals were grown reproducibly using the same tungsten crucible.
• Growth mechanism of the stress-free crystal from the crucible wall was investigated.

We have applied the vertical Bridgman (VB) technique to c-axis sapphire crystal growth. Molybdenum (Mo) and tungsten (W) were used as the crucible materials. In both cases, c-axis, 3-in. diameter sapphire single crystals were successfully grown, and as-grown crystals were easily released from the respective crucibles nondestructively. In the case of the W crucible, crack-free sapphire crystals, up to 120 mm in length, were grown reproducibly, and the same W crucible could be reused repeatedly. However in crystals grown in the Mo crucible a few cracks were often observed. Dimensional variations during the cooling process, calculated from the linear thermal expansions of Mo, W, and sapphire, were investigated. It was found that a sapphire crystal grown in a W crucible is stress-free from the inner crucible wall during the cooling process, because the gap between the periphery of the crystal and the inner crucible wall expands gradually to several hundred micrometers for 3-in. diameter sapphire growth.