K. Iliev

Relationship between the hydrodynamics in the melt and the shape of the crystal/melt interface during Czochralski growth of oxide single crystals: I. Determination of the critical crystal rotation rate from physical simulation data on free and forced convections

Abstract Physical simulation with model glycerol-water solutions imitating oxide melts was used to investigate free and forced convections in the liquid under the conditions of Czochralski single crystal growth. The investigations were carried out with a wide range of physical and geometrical parameter values of the crucible-liquid-crystal system. Equations were obtained for determining the rates of the down-flow of free convention and the up-flow of forced convection. A relationship between critical crystal rotation rate ensuring a flat crystal/liquid interface and the system parameters was found assuming equality of the flow rates during the transition from a convex to a flat interface.

Physicochemical properties of high-temperature solutions of the K2O−P2O5−TiO2−WO3 system, suitable for the growth of KTiOPO4 (KTP) single crystals

High-temperature solutions of the K2O-P2O5-TiO2 system have been investigated and the concentration region and temperatures of spontaneous crystallization of KTiOPO4 (KTP) are determined. The dynamic viscosity, density, volumetric expansion coefficient and volatility of a series of solutions in the crystallization region of KTP are measured. On the basis of the data obtained on these properties, a new composition of the solvent for the preparation of KTP single crystals by crystallization from high-temperature solutions is proposed. In comparison with the usual K2O-P2O5 solvents, the new composition allows the preparation of solutions with a much higher concentration of KTP and a lower volatility.

Conditions of maintenance of a flat crystal/melt interface during Czochralski growth of bismuth germanium oxide single crystals

Abstract Investigations of Czochralski growth of Bi 12 GeO 20 and Bi 4 Ge 3 O 12 single crystals in crucibles with different diameters and a constant crystal diameter/crucible diameter ratio d D = 0.42 have been carried out. The effect of the change in height of the melt layer in the crucible on the temperature distribution in the melt and on the critical crystal rotation rate at which a flat crystal/melt interface is formed, is determined. The temperature dependences of the viscosity and density of the melts of the two bismuth germanium oxides and the values of their volumetric expansion coefficients are established from separate measurements. After a suitable processing of the experimental results it is shown that they satisfy the general correlation proposed earlier on the basis of model studies for the critical crystal rotation rate and the geometrical and physical parameters of the system crucible-melt-crystal.

On the conditions of formation of a flat crystal/melt interface during Czochralski growth of single crystals

The effect of some geometrical and physical parameters of the crucible-liquid-crystal system on the shape of the crystal/melt interface during the growth of single crystals by the Czochralski method is studied by physical simulation. The critical rotation rates at which a flat interface is obtained are established for different crystal diameters, model liquid viscosities and temperatures difference between the crucible walls and the crystal. The data obtained are generalized by a relationshhip of dimensionless numbers. This relationship is checked in Czochralski growth of bismuth germanium oxide (Bi12GeO20) single crystals is found to be satisfied in experiments with a high-temperature melt.

Simulation studies of the hydrodynamics in high-temperature solutions for crystal growth by the TSSG method

Abstract Investigations with model liquids of the convection flow rate under the crystal during its growth from a high-temperature solution according to the TSSG method have been carried out in the cases of: (a) forced convection, (b) free convection, and (c) simultaneous free and forced convection. Dependences between the rate of convection flow and the physicochemical properties of the liquid have been found and presented in their explicit form. For the case of simultaneous free and forced convection, which mostly occurs, intervals of convection flow rates are found for different crystal diameters, within which a flat crystal/liquid interface is formed. A dependence is established between the flow rate and the character and amplitude of temperature fluctuations on the interface. The conditions have been determined under which the formation of a flat interface in the absence of temperature fluctuations may be expected.

Regions of existence of the SrTiO3 phase in the system SrOTiO2Li2OB2O3

Abstract High-temperature solutions of the system SrOTiO 2 Li 2 OB 2 O 3 are investigated. The concentration and temperature ranges within which spontaneous crystallization of SrTiO 3 proceeds with component ratios of the solvents of 35:65, 50:50 and 65:35 mol%, respectively, are determined. It is established that with decreasing B 2 O 3 content in the solvent, the solubility of SrTiO 3 considerably decreases. It is shown that the region of existence of SrTiO 3 in the phase diagram is in all cases shifted with respect to the stoichiometric composition in the direction of the zone with excess SrO.

Conditions of existence and character of the temperature fluctuations during Czochralski growth of oxide single crystals

Abstract The temperature fluctuations in the liquid phase and, above all, below the crystal/liquid interface, are studied by physical simulation of the crucible-melt-crystal system during the growth of oxide crystals by the Czochralski technique, varying the geometrical and physical parameters of the system over wide ranges. The change in character of the fluctuations with varying crystal rotation rate is followed. It is established that the appearance of periodic temperature fluctuations is due to interaction between the free and forced convection flows in the liquid. These fluctuations are observed mainly in the boundary region between the two flows. The site of this region is determined by the values of the dimensionless numbers Re and Gr. It is shown that the maximum amplitude of the fluctuations depends mainly on the temperature difference between the crucible wall and the crystal, while the maximum fluctuation period is determined by the crucible diameter and the value of the Grashof number (Gr). Comparison of the results obtained in simulation studies with the results from real growth of Bi 12 GeO 20 and Bi 4 Ge 3 O 12 single crystals shows good agreement.

Simulation studies of the hydrodynamics in high-temperature solutions for crystal growth. I. Forced convection

Abstract Investigations have been carried out on the free convection in model liquids (glycerol, glycerol-water solutions, silicone oil and liquid paraffin). All possible heating conditions and temperature profiles for growing crystals from high-temperature solutions have been considered. Dependences between the rate of the up (down) convection flow and the parameters of the liquid have been established and the explicit form of these dependences has been found.

Effect of the hydrodynamics in high-temperature solutions on the quality of pure and substituted YIG single crystals grown by the TSSG method

Abstract Single crystals of Y 3 Fe 5 O 12 , Y 3 Fe 5− x Al x O 12 and Y 3 Fe 5- x Ga x O 12 ( x = 0.5 and 1.0) have been grown from high-temperature solutions by the top-seeded solution growth (TSSG) method. The experiments were carried out under different conditions which, according to previous simulation studies on the hydrodynamics of high-temperature solutions [Peshev et al., J. Crystal Growth 65 (1983) 173], formated convex, flat or concave crystal/solution interfaces in the presence or absence of temperature fluctuations on these interfaces. The experimental results have confirmed those predicted by the simulation studies. The hydrodynamic conditions in the solution during the growth are shown to substantially affect the quality of the single crystals obtained. Crystals grown with a flat crystal/solution interface and in the absence of temperature fluctuations have demonstrated minimum levels of striations, inclusions and dislocations.

Stability region and growth conditions of Al- and Ga-substituted Y3Fe5O12 single crystals in solvents from the BaO-B2O3-BaF2 system

Abstract The crystallization region of Y 3 Fe 4 AlO 12 and Y 3 Fe 4 GaO 12 was investigated in BaO-B 2 O 3 -BaF 2 solvents. The effect of each of the six components in the system on the concentration and temperature limits of the garnet phase stability region was shown. The experimental data indicated that solvents containing 0.60–0.77 moles of B 2 O 3 and 0.00–0.46 moles of BaF 2 per 1 mole of BaO are most suitable for the growth of the two garnets by slow cooling. When a gradient transport technique is used, the best sovents are those containing 0.00–0.60 moles of BaF 2 and 0.70–1.00 or 0.60–1.00 moles of B 2 O 3 per 1 mole of BaO in the cases of Y 3 Fe 4 AlO 12 and Y 3 Fe 4 GaO 12 , respectively.