Archibald L. Fripp

A procedure to visualize the melt-solid interface in Bridgman grown germanium and lead tin telluride

Abstract An X-ray and gamma-ray technique has been developed that enables the melt-solid interface to be observed during crystal growth in a Bridgman furnace. This technique has been used to observe the interface movement and shape in germanium and in lead tin telluride.

Thermal convection during Bridgman crystal growth

Abstract Numerical experiments are used to study thermally driven flows which occur during vertical Bridgman crystal growth of a single component fluid. The solid-liquid interface was specified as parabolic and flow patterns were calculated for various insulation thicknesses, Grashof, Prandtl and Biot numbers. When the melt is on top and the gravity vector is axially downward it was shown that flow persists as long as a radial temperature gradient is present. If the interface is convex, as viewed from the liquid, a single cell is observed. A concave interface exhibits multiple counterrotating cells. The insulation thickness and Grashof, Prandtl and Biot numbers influence the flow in a quantitative manner.

Properties of GaN grown on sapphire substrates

Epitaxial growth of GaN on sapphire substrates using an open-tube growth furnace has been carried out to study the effects of substrate orientation and transfer gas upon the properties of the layers. It has been found that for the (0001) substrates, surface appearance was virtually independent of carrier gas and of doping levels. For the (1 ¯102) substrates surface faceting was greatly reduced when He was used as a transfer gas as opposed to H2. Faceting was also reduced when the GaN was doped with Zn and the best surfaces for the (1 ¯102) substrates were obtained in a Zn-doped run using He as the transfer gas. The best sample in terms of electrical properties for the (1¯102) substrate had a mobility greater than 400 cm2 V−1 sec−1 and a carrier concentration of about 2 × 1017 cm−3. This sample was undoped and used He as the transfer gas. The best (0001) sample was also grown undoped with He as the transfer gas and had a mobility of 300cm2V−1 sec−1 and a carrier concentration of 1 × 1018 cm−3.

Magnetically damped convection and segregation in Bridgman growth of PbSnTe

Abstract The effects of an axially imposed magnetic field on convection and solute segregation during Bridgman growth of a non-dilute multicomponent system Pb 0.8 Sn 0.2 Te were studied using a finite-element model. The model considers heat and mass transport, fluid motion, solid/liquid-phase change and magnetic damping. The main objectives are to provide a quantitative understanding of the complex transport phenomena during solidification in a magnetic field, to provide estimates of the required magnetic field strength for low gravity growth, and to assess the role of magnetic damping for space and earth growth control. Numerical results for both vertical and horizontal growth configurations are presented. In addition to full-scale simulation, a revised scaling analysis is also presented.

Solidification behavior of low and high thermal conductivity materials in a Bridgman-Stockbarger furnace

Abstract The solid-liquid interface position and the temperature gradients in both the solid and liquid at the interface have been studied in a modified Bridgman-Stockbarger crystal growth furnace. These crystal growth factors have been studied as a function of ampoule translation rate, materials properties, and the size and temperature of a small auxillary heater placed at the edge of the furnace hot zone. It has been found that the interface position with respect to a furnace reference point is essentially constant during a run for a low thermal conductivity material whereas the interface position changes continuosly during a run with high thermal conductivity material. However, the ampoule translation rate and auxiliary heater conditions produce interface position changes in both high and low thermal conductivity materials.

Interface shapes during vertical bridgman growth of (Pb, Sn)Te crystals

Abstract Melt-solid interfaces obtained during vertical Bridgman growth of (Pb´, Sn)Te crystals were investigated with a quenching technique. The shapes of these interfaces, revealed by etching longitudinally cut sections, were correlated with the composition variations determined by electron microprobe analysis. These experiments demonstrated that the interface shape can be changed from concave to convex by moving its location from the edge of the cold zone into the hot zone. The metallography and microsegregation near the melt-solid interface were analyzed in detail. A sharp change in composition above the interface indicated the existence of a diffusion boundary layer of 40–90 μm thick. This small diffusion boundary layer is consistent with strong convective mixing in the (Pb, Sn)Te melt.

Growth rates and interface shapes in germanium and lead tin telluride observed in-situ, real-time in vertical Bridgman furnaces

Using the advanced technology developed to visualize the melt-solid interface in low Prandtl number materials, crystal growth rates and interface shapes have been measured in germanium and lead tin telluride semiconductors grown in vertical Bridgman furnaces. The experimental importance of using in-situ, real time observations to determine interface shapes, to measure crystal growth rates, and to improve furnace and ampoule designs is demonstrated. The interface shapes observed in-situ, in real-time were verified by quenching and mechanically induced interface demarcation, and they were also confirmed using machined models to ascertain the absence of geometric distortions. Interface shapes depended upon the interface position in the furnace insulation zone, varied with the nature of the crystal being grown, and were dependent on the extent of transition zones at the ends of the ampoule. Actual growth rates varied significantly from the constant translation rate in response to the thermophysical properties of the crystal and its melt and the thermal conditions existing in the furnace at the interface. In the elemental semiconductor germanium the observed rates of crystal growth exceeded the imposed translation rate, but in the compound semiconductor lead tin telluride the observed rates of growth were less than the translation rate. Finally, the extent of ampoule thermal loading influenced the interface positions, the shapes, and the growth rates.

Effects of supercooling in the initial solidification of PbTe-SnTe solid solutions

Abstract Deviations from compositions anticipated by the thermal equilibrium phase diagram have been observed in Bridgman-grown crystals of Pb 1− x Sn x Te, in the first to freeze region of the boule. A set of experiments were conducted to determine the extent of thermal supercooling of Pb 1− x Sn x Te in a Bridgman-like configuration. The results of the compositional profiles and the supercooling measurements are consistent with a diffusionless transformation occurring at the onset of solidification and the length of uncontrolled growth is inversely related to the temperature gradient of the furnace.

The microgravity environment: its prediction, measurement, and importance to materials processing

Abstract One of the primary benefits of conducting scientific research in space is to take advantage of the low acceleration environment. For experimenters conducting space research in the field of materials science the quality of the science return is contingent upon the extremely low frequency acceleration environment (⪡ 1 Hz) aboard the spacecraft. Primary contributors to this low frequency acceleration environment (commonly referred to as the steady-state acceleration environment) include aerodynamic drag, gravity-gradient, and rotational effects. The space shuttle was used on the STS-75 mission as a microgravity platform for conducting a material science experiment in which a lead tin telluride alloy was melted and regrown in the Advanced Automated Directional Solidification Furnace under different steady-state acceleration environment conditions by placing the shuttle in particular fixed orientations during sample processing. The two different shuttle orientations employed during sample processing were a bay to Earth, tail into the velocity vector shuttle orientation and a tail to Earth, belly into the velocity vector shuttle orientation. Scientists have shown, through modeling techniques, the effects of various residual acceleration vector orientations to the micro-buoyant flows during the growth of compound semiconductors. The signatures imposed by these temporally dependent flows are manifested in the axial and radial segregation or composition along the crystal.

Oscillation phase relations in a Bridgman system

The thermal conditions under which convection in a tin melt undergoes the transition from steady flow to time-dependent flow are investigated. Previously, it has been shown that crystals that are directionally solidified from a time-dependent melt will have a larger defect density and increased chance of compositional striations. Thus, it is important for this transition to be well characterized. The experimental results to be presented were obtained from a two-zone Bridgman furnace with a middle insulation zone. Thermocouples were placed on the axis and on the outer wall of a cylindrical vitreous silica tube which contained molten tin. The phase relations between temperature oscillations measured at different positions in the cell are discussed. Fourier transforms are used to investigate the increasing complexity of convection as the temperature gradient is increased.