Origin of scattering defect observed in large diameter Ti:Al2O3 crystals grown by the Kyropoulos technique.

Summary Large diameter Ti-doped sapphire single crystals grown by the Kyropoulos technique along the A-axis, show detrimental light scattering close to the central C-plane. Thermo-luminescence measurements evidence that this defect is associated to a high level of oxygen vacancies. X-ray topography and Rocking Curve Imaging were performed. They show that the dislocation number is very small where scattering occurs, compared to the rest of the crystal, in agreement with results of numerical modelling of thermo-elastic stresses during crystal growth. A model is proposed in order to explain the scattering effect, based on the precipitation of nano-voids from vacancies in the absence of dislocations.