David J. Larson

Revised form of Jackson–Hunt theory: application to directional solidification of MnBi/Bi eutectics

A generalized eutectic theory, which defaults to the prior theories by Jackson–Hunt and Trivedi et al. under their boundary conditions, has been developed to consider phase relations, solidification temperature and interface velocity over the entire parametric range, and for both lamellar and rod eutectic morphologies. The analytical solution reveals that λ2V and λΔT, though varying with the Peclet number, are virtually constant at very low Peclet number (crystal growth conditions). These constant values, however, may vary with the phase reaction and the degree of undercooling.

Thermoelectric effects on interface demarcation and directional solidification of bismuth

This paper investigates thermoelectric effects on interface demarcation during directional solidification of bismuth. A complete description of thermo-electric effects is presented and calculations of related thermo-electric coefficients are elucidated. Numerical simulations for directional solidifications in bismuth were carried out by using an in-house computer code - MASTRAPP, in which a non-orthogonal curvilinear coordinate system is adopted and a multi-zone adaptive grid generation scheme is used. Numerical results with and without current pulsing are presented.

Characterization of Zn-alloyed CdTe compound semiconductors processed in microgravity on USML-1 and USML-2

Abstract Three of the four CdZnTe crystals grown on USML-1 and 2 have been analyzed with respect to hydrostatic and buoyant gravitational influences. Characterization was carried out using Synchrotron White Beam X-ray Topography, Fourier Transfer Infrared (FTIR) Spectroscopy and chemical etching. It was found that in the absence of hydrostatic pressure the liquid separated from the ampoule walls, depending on influences such as: volumetric fill-factor, level of constraint, residual g-vector, ampoule geometry and growth conditions. Regions solidified without wall contact were found to be virtually free from twinning, suggesting that these pervasive defects are surface nucleated. Further, these regions showed dramatic reductions in the density of dislocations, from 800,000 (l-g) to 800 (μ-g). This was attributed to thermo-mechanical stress reduction within the flight samples. Regions of partial wall contact showed defect gradients, high on the wall side and low on the free surface side. These results are consistent with our thermo and thermo-mechanical stress model.

Synchrotron white beam x-ray topography characterization of structural defects in microgravity and ground-based CdZnTe crystals

In a microgravity environment, gravity-dependent effects such as buoyancy, convection and hydrostatic pressure are minimized, providing an ideal environment for investigating diffusion-controlled, nonwetting crystal growth processes. To evaluate the influence of microgravity on the resultant crystal quality, Synchrotron white beam x-ray topography is applied to characterize defect structures in both flight and ground-based CdZnTe single crystals. Transmission x-ray topographs recorded from one flight sample revealed regions of very low dislocation density with individual dislocations clearly resolved. Dislocations of very high density arrayed na mosaic pattern were observed in all ground-base samples grown under identical growth conditions except for the gravity conditions. This observation indicates that the flight samples have much higher structural perfection than the ground-based samples. On the other hand, studies of defect configurations in a different flight sample revealed that structural defects and distributions can be strongly influenced by rapid cooling, thermal gradients, and constrained growth. Large thermal stresses induced by rapid cooling can be multiplied by wall contact leading to the formation of extensive slip bands and small angle tilt boundaries starting at the crystal periphery and propagating into the interior of the sample. It is concluded that an optimization of post solidification cooling rate is important to minimize the occurrence of slip.

Thermomechanical analysis in directional solidification of CdTe

Thermoelastic calculations for CdTe grown by the vertical Bridgman method are presented. Finite element calculations are verified by some experimental data. Solidification interface velocity, charge temperature and stress distributions are computed for prescribed ampoule withdrawal rates and several ampoule support systems. The support systems include various materials and seed-wafer transition zone geometries. Crystal stress in excess of the critical resolved shear stress is used as the figure of merit to judge the performance of a particular system. Emphasis is focused on the transition region between the seed and wafer. A processing strategy is proposed and desirable support system characteristics are presented.

The Role of Thermal Stress In Vertical Bridgman Growth of CdZnTe Crystals

Computational studies of thermal fields and resulting thermoelastic stress fields were undertaken for the vertical Bridgman-Stockbarger growth of CdZnTe crystals. Companion experimental studies included the growth of crystals grown with the same process parameters and the same geometry as the process modeled in the computations. Characteristics of the crystals grown were compared with the computational predictions. Predictions of growth ampoule outer wall temperatures agree well with thermocouple data taken during the growth experiment. Additionally, the computed excess stress distribution resulting from the thermoelastic stress history in the solid is seen to agree qualitatively with synchrotron contour topography on a slice taken from the grown ingot. The computational models are shown to provide a good tool for the study of the influence of process parameters on the quality of crystals grown by this method, at least as far as thermal stress influences the defect distribution. The influence of low-g and high-g environments on growth is discussed.