Donald C. Gillies

One-dimensional analysis of segregation in directionally solidified HgCdTe

Abstract A one-dimensional model for solute redistribution is developed that can be applied to solid solution alloys with widely separated liquidus and solidus phase diagrams. The model is applied to the HgCdTe system and accounts for the variation in interface temperature, segregation coefficient, and growth velocity with composition. The model considers a finite length ampoule, treating both initial and final transient segregation. Agreement between the model and experimental results is good, indicating that the axial solute redistribution in directionally solidified HgCdTe crystals is dominated by diffusion. Comparison of the model to experiment in the final transient region gives values for the effective diffusion constant of 5×10 -5 to 7×10 -5 cm 2 /s.

Numerical simulation of THM growth of CdTe in presence of rotating magnetic fields (RMF)

Abstract The influence of rotating magnetic fields (RMF) on flow pattern and compositional uniformity in the solution zone of a traveling heater method (THM) system for growth of CdTe is numerically investigated. The analysis is conducted at the 10 −6 and 10 −1 g 0 as representative of space and ground processing conditions. It is shown that under microgravity conditions application of RMF can be used to overwhelm residual buoyancy-induced convection and to control the uniformity of solution-zone composition at the growth front without appreciable modification of the growth interface shape. At high-gravity levels, RMF is found not to be able to completely dominate buoyancy-induced convection. In this regime, for the range of field strengths studied, RMF is found to result in (a) complex flow structures in the solution zone, (b) enhancement of compositional nonuniformities at the growth front, and (c) increased convexity of the growth interface. A scaling analysis of convection in the solution zone is used to generate a nondimensional map delineating the RMF- and gravity-dominated flow regimes.

Growth of ZnTe by physical vapor transport and traveling heater method

ZnTe crystals were grown by horizontal physical vapor transport (PVT) and a Te-solution vertical traveling heater method (THM). The grown crystals were examined by X-ray Laue diffraction technique and Hall measurements to determine the growth orientation and the electrical properties of the crystals. They were also characterized by low temperature infrared (IR) absorption measurements. Several sets of distinct peaks were observed in the IR absorption spectra for the THM samples and were identified as resulting from Cu 2+ impurities. Similar measurements on vapor grown ZnTe showed featureless absorption spectra. Chemical analyses were carried out to measure the impurity content in various ZnTe samples and synchrotron radiation topography was used to study crystalline microstructure of the (111) ZnTe single crystals grown by PVT.

Effect of residual accelerations during microgravity directional solidification of mercury cadmium telluride on the USMP-2 mission

Abstract Directional solidification of mercury cadmium telluride (MCT) requires that the temperature gradient to growth rate ratio be high to avoid constitutional supercooling. With the optimum gradient condition for solidifying MCT in NASAs advanced automated directional solidification furnace (AADSF), it is necessary to use translation rates as low as 0.2 μm/s. The result is that any fluid flow with a velocity comparable to or higher than this will dominate the solidification characteristics, particularly the compositional distribution in an alloy such as this which has a large solidus-liquidus separation. In an effort to reduce fluid flow velocities a space experiment was performed. On the second United States Microgravity Payload Mission (USMP-2), the AADSF made its maiden flight and successfully completed growth of a MCT boule 16 cm long. The furnace was located approximately 3 m away from the center of gravity of the space shuttle, and this combined with the drag component of residual acceleration present during flight, resulted in quasisteady residual accelerations of the order of 1 μg0 where g0 is the earths natural gravity. Of more importance is that different orbiter attitudes during the mission produced significant differences in the resultant residual acceleration vector, in both magnitude and direction and that these differences caused large compositional variations both across the radii of the boule and along the surfaces of the boule. Comparison will be made with examples grown on the ground and in magnetic fields.

Photoluminescence of vapor and solution grown ZnTe single crystals

Abstract ZnTe single crystals grown by horizontal physical vapor transport (PVT) and by vertical traveling heater method (THM) from a Te solution were characterized by photoluminescence (PL) at 10.6 K and by atomic force microscopy (AFM). Copper was identified by PL as a major impurity existing in both crystals, forming a substitutional acceptor, Cu Zn . The THM ZnTe crystals were found to contain more Cu impurity than the PVT ZnTe crystals. The formation of Cu Zn -V Te complexes and the effects of annealing, oxygen contamination and intentional Cu doping were also studied. Finally, the surface morphology analyzed by AFM was correlated to the PL results.

Mass flux of ZnSe by physical vapor transport

Abstract Mass fluxes of ZnSe by physical vapor transport (PVT) were measured in the temperature range of 1050 to 1160°C using an in-situ dynamic technique. The starting materials were either baked out or distilled under vacuum to obtain near-congruently subliming compositions. Using an optical absorption technique Zn and Se 2 were found to be the dominant vapor species. Partial pressures of Zn and Se 2 over the starting materials at temperatures between 960 and 1140°C were obtained by measuring the optical densities of the vapor phase at the wavelengths of 2138, 3405, 3508, 3613, and 3792 A. The amount and composition of the residual gas inside the experimental ampoules were measured after the run using a total pressure gauge. For the first time, the experimentally determined partial pressures of Zn and Se 2 and the amount and composition of the residual gas were used in a one-dimensional diffusion limited analysis of the mass transport rates for a PVT system. Reasonable agreement between the experimental and theoretical results was observed.

Growth of cadmium-zinc telluride crystals by controlled seeding contactless physical vapor transport

Bulk crystals of cadmium-zinc telluride, 23 mm in diameter and up to 45 grams in weight were grown. Controlled seed formation procedure was used to limit the number of grains in the crystal. Most uniform distribution of ZnTe in the crystals was obtained using excess (Cd + Zn) pressure in the ampoule.

Selenium precipitation in ZnSe crystals grown by physical vapor transport

Abstract The morphology of freshly cleaved ZnSe surfaces was investigated by atomic force microscopy and the results were correlated with differential scanning calorimetry (DSC) data. Selenium precipitates in undoped ZnSe crystals grown by the physical vapor transport method were determined. The Se inclusions have a size of about 20 nm. A transition temperature at 221°C in the DSC measurement is interpreted as the eutectic temperature of Se-saturated ZnSe. The total amount of the ZnSe Se-rich second phase was 0.8 wt%, and some segregation effect along low angle grain boundaries was evident.

Synchrotron topography characterization of ZnTe single crystals

Abstract (111) Wafers sliced from a single-crystal boule of ZnTe grown by horizontal physical vapor transport have been characterized using synchrotron white-beam X-ray topography. The presence of dislocation slip bands, subgrain structures, and long, thin 180° rotational twin lamellae was revealed. Distorted regions exhibiting radial lattice rotation were also observed near the crystal edges. One of these regions is clearly the source for the generation of some of the slip bands observed in the crystals. The slip bands were observed to break up the ordered dislocation cell structures comprising the subgrain boundaries, indicating that the dislocation cell structure appears earlier during the growth process, and that the slip dislocations appear later, perhaps during the postgrowth cooling process. This may also be an indication that modification of the cooling rate could have a significant effect on the final defect microstructure, with for example more rapid cooling giving rise to more slip which could obscure the true growth-defect microstructure. The initiation of slip at regions of stress concentration, such as at lateral twin boundaries, and at the junctions of subgrain boundaries and twin boundaries, was also revealed. The asymmetrical distribution of slip bands either side of the twinned region of crystal suggests that twin boundaries can act as barriers for slip. Several types of detailed twin boundary configuration were determined from the topographs. Using a combination of white-beam X-ray topography and Nomarski interference microscopy, the three-dimensional shapes of the twin boundaries were determined. Approximate atomic structures at these boundaries are presented. Possible mechanisms for the production of a long thin twin lamella, oriented at a small angle to the growth axis, are discussed.

Characterization of cadmium-zinc telluride crystals grown by ‘contactless’ PVT using synchrotron white beam topography

Abstract Crystals of Cd 1 − x Zn x Te grown by PVT using self-seeding ‘contactless’ technique were characterized using synchrotron radiation (reflection, transmission, and Laue back-reflection X-ray topography). Crystals of low ( x = 0.04) and high (up to x ≈ 0.4) ZnTe content were investigated. Twins and defects such as dislocations, precipitates, and slip bands were identified. Extensive inhomogeneous strains present in some samples were found to be generated by interaction (sticking) with the pedestal and by composition gradients in the crystals. Large (up to about 5 mm) oval strain fields were observed around some Te precipitates. Low angle grain boundaries were found only in higher ZnTe content ( x ⩾ 0.2) samples.