F.R. Szofran

A numerical investigation of the effect of thermoelectromagnetic convection (TEMC) on the Bridgman growth of Ge1−xSix

Thermoelectric currents at the growth interface of GeSi during Bridgman growth are shown to promote convection when a low-intensity axial magnetic field is applied. TEMC, typically, is characterized by a meridional flow driven by the rotation of the fluid; meridional convection alters the composition of the melt, and shape of the growth interface substantially. TEMC effect is more important in micro-gravity environment than the terrestrial one, and can be used to control convection during directional solidification of GeSi. In this work, we report on the numerical simulation of the effect of TEMC on the growth of GeSi.

A method for interface shape control during Bridgman type crystal growth of HgCdTe alloys

A method is described for interface shape control of Hg1 - xCdxTe alloys during Bridgman-type crystal growth. The authors have used this method for several years to obtain the flat or convex interface shapes required for simultaneously reducing radial compositional variations and crystal defects. The method turns to advantage the thick-walled, fused-silica ampules required for growing these alloys. A proper combination of furnace geometry (two isothermal zones seperated by a thin barrier), upper- and lower-zone temperatures, and growth rate is required. Examples of results are reported.

Bridgman Growth of Detached GeSi Crystals

Ge1xSix (0oxo0:12) has been grown by the vertical Bridgman technique using adjustments in the applied temperature profile to control the pressure difference between the bottom and top of the melt. Using this technique, a pressure difference is created by decreasing the temperature in the gas volume above the melt while the sample is molten but prior to growth. A maximum pressure difference approximately equal to the hydrostatic pressure of the molten sample can thus be obtained. Several GeSi crystals were grown in pyrolitic boron nitride ampoules. When a pressure difference was applied, samples were reproducibly grown mostly detached. For comparison, samples were also grown in a configuration in which gas could flow freely between the gap below the melt and the volume above the melt and no pressure difference could be established. These samples were initially attached. Existence of detachment was determined both by measuring the surface roughness of the samples with a profilometer and by observations of the sample surfaces with optical and electron microscopy. r 2002 Elsevier Science B.V. All rights reserved.

Growth of HgZnTe alloy crystals by directional solidification

Abstract A series of Hg1-xZnxTe crystals were grown by directional solidification with x ranging from 0.15 to 0.22. The axial and radial compositional variations were determined by precision density measurements, infrared (IR) transmission spectra, and energy dispersive X-ray spectroscopy (EDX). Comparison between the axial compositional profiles and the calculated results of a one dimensional diffusion model gives a value for the effective diffusion coefficient in the HgZnTe melt of 2 to 4x10-6 cm2/s.

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 growth parameters on compositional variations in directionally solidified HgCdTe alloys

Abstract A series of Hg 1 - x Cd x Te alloy crystals was grown by directional solidification with compositions ranging from x = 0.2 to x = 0.4. The measured axial compositional profiles were interpreted in terms of an exact numerical solution of the appropriate diffusion equation that takes into account both the variations of the segregation coefficient and solidification rate with composition. The solutions for all growth rates agree generally well with the experimental data and confirm earlier observations that employed approximate analytical solutions. The effective mass diffusion coefficients showed no significant composition or melt temperature dependence. Slightly higher diffusion coefficients were obtained, however, for the highest growth rates.

Detached growth of gallium doped germanium

Detached Bridgman growth of gallium-doped germanium itself as well as the transition from detached to attached growth was observed in-situ for the first time, using a quartz-glass ampoule in a mirror furnace. Crystal diameter was 9 mm, the growth length 41 mm, and the growth velocity 0.5 mm/min. Undoped 1 1 1-oriented germanium served as seed material; the melt was doped with gallium (C0=8.2×1018 at/cm3). Detachment took place after a growth length of 7 mm and continued for 27 mm; the remaining 7 mm grew with wall contact again. The wall-free growth could be observed around the entire circumference except for some small bridges (width: a few tens of micrometers, length: some hundreds of micrometers), where the crystal grew in contact with the wall. In the detached-grown part of the crystal, the 1 1 1-related growth lines are clearly visible. The transition from attached to detached growth and vice versa did not take place along a straight line but transitioned as islands over a length of about 1 mm. The gap between the growing crystal and the container wall varied between 10 and 80 μm, as measured by a profilometer. The etch pit density is greatly reduced in the part of the crystal that grew free of the wall. An increase in the EPD is seen in the area where the crystal had contact with the ampoule wall by the bridges described above.

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.

Numerical simulation of heat transfer during the crystal growth of HgCdTe alloys

The effects of growth parameters on the thermal distribution during a Bridgman-Stockbarger-type crystal growth of HgCdTe alloys using short gradient zones are analyzed numerically. The analysis takes into account the change in the thermophysical properties upon freezing and considers both the temperature and composition dependences of the properties as well as translation rate effects. The calculated results compare favorably with previously published empirical results.