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 (phi = 3 mm, length approximate to 170 mm) along different crystallographic orientations (a [1120], m [1010], c [0001] and c [0001] shifted 30 degrees off axis) were successfully grown by the micro-pulling down (mu-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 mu m 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 mu-PD rods were discussed.