Behind the scenes of sapphire: influence of titanium doping on bubble distribution and propagation in sapphire grown by micro-pulling-down

Abstract

Undoped and titanium-doped sapphire (Al₂O₃:Ti³⁺) single-crystal rods were grown using the micro-pulling-down (µ-PD) technique to investigate the effect of Ti incorporation on bubble defect formation. Crystals with Ti concentrations of 0, 500, and 1000 ppm were obtained with uniform geometry and good optical quality. Microstructural analysis revealed a strong dependence of bubble distribution and morphology on Ti dopant. Undoped sapphire exhibited only a thin peripheral layer of small spherical bubbles (0-20 µm), whereas Ti-doped crystals showed a significant increase in bubble layer thickness (up to ~370 µm), accompanied by the appearance of elongated and irregular bubble shapes. A quasi-steady-state numerical model, including heat transfer, melt convection, and thermocapillary (Marangoni) effects, was developed to clarify the governing mechanisms. Simulations indicate that increasing Ti concentration enhances the meniscus height and intensifies Marangoni convection, with melt velocities rising from ~5.6 to 20 mm•s⁻¹. The resulting flow structure promotes bubble transport toward the crystal periphery and contributes to their deformation under strong shear near the meniscus. These results provide a consistent interpretation of the relationship between titanium doping, melt flow, and bubble distribution during µ-PD sapphire growth.