Dopant segregations in oxide single-crystal fibers grown by the micro-pulling-down method

(Cr, Al)2O3 (k0 = 2), (Ga, Al)2O3 (k0 = 0.3), Gd3(Cr, Ga)5O12 (k0 > 1), (Gd, Yb)3Ga5O12 (k0 > 1) and (Yb, Y)3Al5O12 (k0 > 1) fibers have been grown by the micro-pulling-down method. k0 is the equilibrium distribution coefficient of the dopant with respect to the given host phase. The axial and radial dopant distribution was measured by electron probe microanalysis. The growth interface was frozen by pulling-down the fiber rapidly and quenching the molten zone. In the case of growth with a thin melt zone of about 30–70 μm between growth front and crucible nozzle radial dopant distributions in form of a homogeneous core and a peripheral rim of different composition were found in all fibers. The growth interface has a bump into the crucible nozzle. For (Yb, Y)3Al5O12, Gd3(Cr, Ga)5O12, (Gd, Yb)3Ga5O12 and (Ga, Al)2O3 mainly diffusional transport in the crucible nozzle was found. The dopant concentration is higher or lower at the rim than in the core for k0 < 1 or k0 > 1, respectively. This axial segregation corresponds to the bend interface. In the case of (Cr, Al)2O3 convective transport inside the crucible nozzle was found. Also, different to the diffusive case, the Cr2O3 concentration in the rim is higher than in the core region although k0 > 1. The growth with a higher melt zone of about 200 μm has the same axial segregation behaviour but the radial segregation is strongly suppressed. The interface is spherical. The experiments show, that the radial and axial concentration profiles that are the result from the micro-pulling-down growth of solid solutions are influenced by the distribution coefficient, the geometry of the crucible, the height of the molten zone and melt properties.