Chong E. Chang

Control of interface shape in the vertical bridgman-stockbarger technique

Abstract At very slow travel rates the solid-liquid interface shape (from the viewpoint of the solid) is convex when it lies within the heater and concave when it lies within the cooler. The dependence of interface position on travel rate, geometry of the ampoule, and on the relative temperatures of the heater and cooler increases as the effectiveness of heat transfer between ampoule and the surroundings diminishes and as the thermal conductivity of the material increases (at least for low travel rates).

Inhomogeneities due to thermocapillary flow in floating zone melting

Abstract Theoretical calculations have shown that radial inhomogeneities are produced by thermocapillary flow at low driving forces unless the zone travel rate is very low or the distribution coefficient is near unity. At intermediate driving forces the flow is probably oscillatory, which would produce striations. At large driving forces the flow is turbulent. For floating zone melting of silicon, the thermocapillary flow is not appreciably influenced by buoyancy at earths gravity or by motion of the zone at the usual rates. Thermocapillary flow is probably also important in Czochralski growth of silicon and in floating zone melting with induction heating, unless effective heat shields are employed.

Analysis of surface tension driven flow in floating zone melting

Abstract Surface tension driven flow in a cylindrical melt suspended between two rods was investigated by numerical solution of the steady state differential equations for heat and momentum transfer. Radiation heating and electron beam heating were considered approximately. For small values of the driving force, one rotating ring was formed in the top half of the zone, and its mirror image in the bottom half. At larger driving forces, secondary cells form which probably would undergo oscillatory motion. The influence of Prandtl number, zone movement, and buoyancy on the convection was also studied. The primary resistance to mass transfer in the laminar regime was in the center of the zone rather than at the solid-liquid interfaces.

Vitreous boron oxide: Drying and moisture absorption

Abstract Frothing of reagent-grade boron oxide during heating can be avoided by a preliminary vacuum bake out. The rate of removal of trace water is much larger when dry nitrogen is bubbled through the melt than when a vacuum is applied. The moisture absorbed by vitreous boron oxide when exposed to air at room temperature is confined to the neighborhood of the surface. Eventually boric acid is formed. This surface moisture can largely be removed by a vacuum at room temperature. The kinetics of water removal and absorption were measured.

Vertical gradient freeze growth of gallium arsenide and naphthalene: Theory and practice

Abstract Gallium arsenide and naphthalene single crystals were produced by programming down furnaces constructed such that the temperature increased with height. In naphthalene, grain selection was enhanced by producing a convex interface shape. Both experiments and theory showed that the interface shape depended on the furnace temperature gradient, the cooling rate, additional cooling of the ampoule bottom, tapering of the crucible, the dimensions of the crucible, and the heat transfer coefficient between crucible and furnace.

Thermocapillary convection in floating zone melting: Influence of zone geometry and prandtl number at zero gravity

Abstract For silicon, changes in zone size and shape altered only slightly the appearance of the streamlines for a fixed value of the dimensionless thermocapillary flow parameter. However significant changes were noted in the heat and mass transfer and in the magnitude of the dimensionless vorticity. Steady state solutions were also obtained for sapphire with electron beam heating, but could not be obtained with radiant heating. This suggests that oscillatory flow might easily be produced with radiant heating of high Prandtl number materials.

Heat and mass transfer in the travelling heater method of crystal growth

Abstract Constitutional supercooling always occurs in the usual isothermal solution growth, but can be avoided in the travelling heater method. Travelling heater solution growth experiments with napthalene-benzoic acid and gallium arsenide-gallium confirmed that constitutional supercooling usually limits the maximum zone travel rate, and that the maximum rate increases as heater power increases. With gallium arsenide it is necessary to have a seed length greater than about one crystal diameter and a feed length greater than about five crystal diameters in order to avoid shifts in zone position relative to the heater and resultant changes in interface shapes during growth.

Computer simulation of convection in floating zone melting: I. Pure rotation driven flows

Abstract Forced convection induced by rotation of the crystals in a floating zone is theoretically analysed by using a computer for silicon and for aluminum oxide. Vortex cells circulating in the radial-axial direction are generated. The number of vortex cells depends on the mode of rotation and on the rotation rates. Co or counter rotation produces a stagnant surface or stagnant core, respectively in the zone and separates the zone into two regions of flow field that are not mixed. This tendency is eliminated by single rotation, or by counter rotation — by rotating one crystal slower than the other in the opposite direction. The maximum mixing is accomplished by the latter method. The influence of viscosity on the convection patterns is also discussed, especially for materials of high viscosity such as aluminum oxide.

Localized interface breakdown in zone melting and the travelling heater method

Abstract The crystallizing interface of naphthalene was observed in the vertical zone melting configuration both with and without added solvent. For stepwise-increased travel rates, breakdown began near the center of the tube for convex interfaces. A concave interface first broke down on the periphery, but at steady state was only broken down at the center for moderate travel rates. These results were attributed to variations in temperature gradient along the interface and to convection currents in the zone which cause solute variations along the interface.

Composition variations in directionally solidified InSb-GaSb alloys

Abstract Melts containing 10%, 30% and 50% InSb were directionally solidified horizontally, vertically and in Skylab 3 and 4. The resulting concentration profiles were much more uniform in the ingots solidified horizontally. Fluctuations in composition occurred sooner as the InSb content increased and later in the horizontally processed ingots. In the 10% InSb ingot processed in SL-3 a large orientation-dependent segregation was observed at about 2 cm from the initial position of the interface. A radial variation in composition was also observed, and tentatively attributed to surface-tension driven convection.