Seed orientation and pulling rate effects on bubbles and strain distribution on a sapphire crystal grown by the micro-pulling down method

Undoped sapphire rod crystals (φ = 3 mm, length ≈ 170 mm) along different crystallographic orientations (a [1120], m [1010], c [0001] and c [0001] shifted 30° off axis) were successfully grown by the micro-pulling down (μ-PD) method. The bubble defect distribution was investigated as a function of the thermal gradient and pulling rates. It is observed that sapphire rods grown at a low pulling rate (v < 200 μm min^-1) were bubble-free. A homogeneous temperature distribution around the capillary die will limit the bubble propagation. Pulling sapphire rods along the c-axis is a good way to minimize the bubble propagation. The effects of growth parameters on bubble and strain distribution in sapphire rods grown from μ-PD rods were discussed.