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The effect of thermoelectromagnetic convection (TEMC) was investigated in the system germanium–silicon, grown by the vertical Bridgman method under axial static magnetic fields of up to 5 T. TEMC is generated, if thermocurrents, caused by concentration and/or temperature gradients, are running non-parallel to magnetic field lines. Under the influence of strong axial magnetic fields, the macrosegregation along the growth axis changed from a profile typical for normal freezing toward a concentration profile described by diffusive mass transport. At the same time, the segregation pattern on the microscale (i.e. the non-steady distribution of the silicon incorporation perpendicular to the growth axis) changed significantly. Without magnetic field, no evidence of short-range compositional fluctuations has been detected. Growth under static magnetic fields of B ⩾ 0.5 T and B ⩽ 4 T resulted in strong microsegregation. These compositional fluctuations are in the range of a few micrometers up to several hundred micrometers. The strength increased with the field strength and reached a maximum at a magnetic induction of 2 T. These magnetic field induced inhomogeneities are damped with higher magnetic fields and can nearly be eliminated with a magnetic field of 5 T. Due to their coupling to the static magnetic field and their specific shape, they can be attributed to TEMC.

Germanium (111)-oriented crystals have been grown by the vertical Bridgman technique, in both detached and attached configurations. Microstructural characterization of these crystals has been performed using synchrotron white beam x-ray topography (SWBXT) and double axis x-ray diffraction. Dislocation densities were measured from x-ray topographs obtained using the reflection geometry. For detached-grown crystals, the dislocation density is 4-6 x 10(exp 4) per square centimeter in the seed region, and decreases in the direction of growth to less than 10(exp 3) per square centimeter, and in some crystals reaches less than 10(exp 2) per square centimeter. For crystals grown in the attached configuration, dislocation densities were on the order of 10(exp 4) per square centimeter in the middle of the crystals, increasing to greater than 10(exp 5) per square centimeter near the edge. The measured dislocation densities are in excellent agreement with etch pit density results. The rocking curve linewidths were relatively insensitive to the dislocation densities. However, broadening and splitting of the rocking curves were observed in the vicinity of subgrain boundaries identified by x-ray topography in some of the attached-grown crystals.

The progress in advanced single crystal growth demands versatile, advanced equipment. Multizone furnaces are able to fulfill unusual technological requirements providing versatile and reliable all-in-one tools for laboratories on the ground or in space. The concept of the Universal Multizone Crystallizator (UMC), its hardware and software components along with several application examples are demonstrated.

A series of (1 0 0)-oriented gallium-doped germanium crystals has been grown by the vertical Bridgman method and under the influence of a rotating magnetic field (RMF). Time-dependent flow instabilities occur when the critical magnetic Taylor number ( T m c ) is exceeded, and this can be observed by noting the appearance of striations in the grown crystals. T m c decreases as the aspect ratio of the melt increases, and approaches the theoretical limit expected for an infinite cylinder. Intentional interface demarcations are introduced by pulsing the RMF on and off. The RMF has a marked effect on the interface shape, changing it from concave to nearly flat as the RMF strength is increased.

The conditions of detached solidification under controlled pressure differential across the meniscus were investigated. Uncoated and graphite- or BN-coated silica and pBN crucibles were used. Detached and partly detached growth was achieved in pBN and BN-coated crucibles, respectively. The results of the experiments are discussed based on the theory of Duffar et al.

Bridgman growth experiments were performed on Hg(0.89)Mn(0.11)Te (MMT) to determine the extent of radial Manganese segregation during directional solidification. MMT crystals were directionally solidified at rates of 0.09 and 0.18 p d s and in axial thermal gradients of 83 and 68"C/cm. Wavelength Dispersive Spectroscopy (WDS) and Fourier Transform Infra-Red (FTIR) analytical techniques were used to determine the radial homogeneity in all boules and the deflection of the solid-liquid interface (SLI) in two boules that were rapidly quenched after 5 to 6 cm of directional solidification. For all growth runs, the measured radial coinpositional variations were on the order of 0.01 molar percent MnTe in the steady state region of growth. Comparison of the measured radial compositional results of the crystals to predicted values in the diffusion-limited regime indicate a strong influence of convection near the solid-liquid interface. This conclusion is supported by the weak influence of the translation rates and axial thermal gradients utilized in this study upon radial compositional homogeneity.

This paper treats the axisymmetric flow and mass transport in a cylindrical liquid bridge between the melting end of a feed rod and the solidifying end of an alloyed semiconductor crystal. There is a strong, uniform, steady, axial magnetic field. The surface tension depends on the temperature and the concentration of the species, while variations of the concentration occur because one species is rejected into the liquid during solidification. The thermocapillary and solutocapillary convections tend to cancel- over part of the liquid bridge. For certain parameter ranges, there are two different stable steady solutions: one where the concentration gradient along the free surface leads to dominance by the solutocapillary convection and one where the mass transport due to the thermocapillary convection makes the concentration gradient along the free surface small, so that the thermocapillary convection is dominant.

Several undoped and Ga-doped germanium single crystals were grown by the vertical Bridgman method using a translating furnace and a multizone furnace, respectively. In both cases it was possible to exert influence on the contact between the growing crystal and the wall of the container. This allows growing nearly completely detached crystals as well as attached crystals in pyrolytic boron nitride containers. In detached-grown crystals the gap thickness between the container wall and the crystal, determined by profilometer measurements, varies from 5 to 50 μm. Observed fluctuations of the detachment gap up to 8 μm along the crystal axis in one of the crystals can be explained by a kind of stiction of the melt/crucible interface, which causes a variation of the meniscus shape.

The growth of detached crystals by the Bridgman technique, in which the growing crystal is not in contact with the crucible wall, has been observed both on earth and in microgravity conditions. Such detachment has particularly been in evidence in microgravity experiments, where the pressure head of the molten sample is absent. At present, the mechanisms contributing to the detachment are not completely understood and until recently detachment has not been reproducibly obtained. Key parameters which must be considered are the contact angle between the melt and the crucible and the growth angle. Another essential parameter is the pressure difference between the annular gap around the solid below the melt and the volume above the melt. Here we present results of the growth of Ge(0.98)Si(0.02) using adjustments in the applied temperature profile to control the pressure difference between the bottom and top of the melt. The technique is less susceptible to sample contamination than controlling the pressure by connecting the crucible to external gas sources. Using this technique, a pressure difference is created by decreasing the temperature in the volume above the melt while the sample is molten but prior to growth. A maximum pressure difference approximately equal to the pressure head of the molten sample can thus be obtained. Several GeSi crystals were grown in pyrolitic boron nitride crucibles. 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 pass 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 radius of the samples with a profilometer and by observations of the sample surfaces with optical and electron microscopy. The gap thickness between the crucible and detached crystal was on the order of 10 micron. The surfaces of the attached areas of the crystals had the same shape as the interior crucible wall whereas in the detached areas the crystal facets could usually be observed.

Several (111)-oriented, Ga-doped germanium crystals were grown in pyrolytic boron nitride (pBN) containers by the Bridgman and the detached Bridgman growth techniques. Growth experiments in closed-bottom pBN containers resulted in nearly completely detached-grown crystals, because the gas pressure below the melt can build up to a higher pressure than above the melt. With open-bottom tubes the gas pressure above and below the melt is balanced during the experiment, and thus no additional force supports the detachment. In this case the crystals grew attached to the wall. Etch pit density (EPD) measurements along the axial growth direction indicated a strong improvement of the crystal quality of the detached-grown samples compared to the attached samples. Starting in the seed with an EPD of 6-8 x 10(exp 3)/square cm it decreased in the detached-grown crystals continuously to about 200-500/square cm . No significant radial difference between the EPD on the edge and the middle of the crystal exists. In the attached grown samples the EPD increases up to a value of about 2-4 x 10(exp 4)/square cm (near the edge) and up to 1 x 10(exp 4)/square cm in the middle of the sample. Thus the difference between the detached- and the attached-grown crystals with respect to the EPD is approximately two orders of magnitude.

An Hg(0.84)Zn(0.16)Te alloy crystal was back-melted and partially re-solidified during the first United States Microgravity Laboratory mission in the Marshall Space Flight Center's Crystal Growth Furnace. The experiment was inadvertently terminated at about 30% of planned completion. Nonetheless, it was successfully demonstrated that a HgZnTe alloy ingot partially grown and quenched on the ground can be back-melted and re-grown in space under nearly steady-state growth conditions. An identical "ground-truth" experiment was performed following the mission and a comparison between the properties of the crystals is described. The results indicate the importance of residual microgravity acceleration (less than or approx. equal to 0.4 micro-grams) even in the sub-microgravity range for the slow solidification velocities and large density gradients.

The sessile drop technique has been used to measure the wetting angle and the surface tension of molten germanium (Ge) on various substrate materials. Sapphire, fused silica, glassy carbon, graphite, SiC, carbon-based aerogel, pyrolytic boron nitride (pBN), AlN, Si3N4, and CVD diamond were used as substrate materials. In addition, the effects of different cleaning procedures and surface treatments on the wetting behavior were investigated. The highest wetting angles with values around 170 deg. were found for pBN substrates under active vacuum or with a slight overpressure of 5N Argon or forming gas (2% Hydrogen in 5N Argon). The measurement of the surface tension and its temperature dependence for Ge under a forming gas atmosphere resulted in gamma(T) = 591 - 0.077 (T-T(sub m).

Thermoelectromagnetic convection (TEMC) is the melt motion driven by the interaction of an externally applied magnetic field and the thermoelectric current due to variations of the absolute thermoelectric power and of the temperature. This paper presents numerical solutions for the three-dimensional TEMC in a simplified model of the floating zone process with a nonaxisymmetric temperature variation over the crystal–melt interface. The axial magnetic field is sufficiently strong that inertial effects are negligible, so that each azimuthal Fourier component of the TEMC can be treated independently. Results are presented for the cos θ and cos(2 θ ) components of the TEMC. The results show that a very small deviation from axisymmetry in the temperature produces a very large deviation from axisymmetry in the radial and axial components of the TEMC velocity.

During the detached Bridgman growth of semiconductor crystals, the melt has a short free surface which is detached from the ampoule wall near the crystal-melt interface, thus eliminating the crystal defects caused by contact with the ampoule wall. Recent modelling has indicated that initiation and continuation of detached growth depends on the rate of transport of dissolved gas from the crystal-melt interface, where gas is rejected into the melt, to the detached free surface, where evaporating gas maintains the pressure on the free surface. Here we use numerical modelling to investigate whether the application of a rotating magnetic field increases or decreases the transport of rejected gas to the detached free surface. Unfortunately the results show that a rotating magnetic field almost always decreases the evaporation rate at the detached free surface. The exception is an insignificant increase for a short period at the beginning of crystal growth for a few circumstances. The evaporation rate decreases as the strength of the rotating magnetic field is increased.

This paper treats a steady, axisymmetric melt motion in a cylindrical ampoule with a uniform, axial magnetic field and with an electric current due to a radial temperature variation along the crystal-melt interface, where the values of the absolute thermoelectric power for the crystal and melt are different. The radial component of the thermoelectric current in the melt produces an azimuthal body force, and the axial variation of the centrifugal force due to the azimuthal motion drives a meridional circulation with radial and axial velocities. For moderate magnetic field strengths, the azimuthal velocity and magnetic field produce a radial induced electric field which partially cancels the Seebeck electromotive force in the melt, so that the thermoelectric current and the melt motion are coupled. For weak magnetic fields, the thermoelectric current is decoupled from the melt motion, which is an ordinary hydrodynamic flow driven by a known azimuthal body force. The results show how the flow varies with the strength of the magnetic field and with the magnitude of the temperature variation along the crystal-melt interface. They also define the parameter ranges for which the simpler weak-field decoupled analysis gives accurate predictions.

Application of a low-intensity axial magnetic field can promote significant convection during Bridgman growth of GeSi when resident thermoelectric currents at the growth interface are large due to the difference of thermoelectric powers of the melt and of the crystal and the tangential temperature gradient at the interface. Thermoelectromagnetic convection (TEMC) in the GeSi melt is characterized by a meridional flow driven by the rotation of the fluid due to the azimuthal Lorentz force from currents in the radial direction concentrated near the interface and an axial magnetic field. In this work, we developed a computational model to study convection of the GeSi melt in varying g and magnetic field intensity levels.

This paper treats a liquid-metal flow inside an electrically insulating cylinder with electrically conducting solids above and below the liquid region. There is a uniform axial magnetic field, and there is an electric current through the liquid and both solids. Since the lower liquid-solid interface is concave into the solid and since the liquid is a better electrical conductor than the adjacent solid, the electric current is locally concentrated near the centerline. The return to a uniform current distribution involves a radial electric current which interacts with the axial magnetic field to drive an azimuthal flow. The axial variation of the centrifugal force due to the azimuthal velocity drives a meridional circulation with radial and axial velocities. This problem models the effects of Peltier marking during the vertical Bridgman growth of semiconductor crystals with an externally applied magnetic field, where the meridional circulation due to the Peltier current may produce important mixing in the molten semiconductor.

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.

Hg(1-x)Zn(x)Se alloys have been studied as an alternative to Hg(1-x)Cd(x)Te for the detection of electromagnetic radiation, because the shorter ZnSe and HgSe bonds have been predicted to improve lattice stability. Several ingots with x = 0.1 were directionally solidified using a modified Bridgman-Stockbarger method; one was grown in an applied magnetic field, which greatly reduced radial compositional variations. A method was developed to reduce wetting. This, combined with the convex liquid-solid interface shape, produced boules that were single crystalline after growing about 3.5 cm. Observed surface features indicated ampoule wetting was eliminated using a graphite getter. Microstructural characteristics were greatly improved over HgCdTe alloys. In six boules, a total of only one twin was observed. A method for polishing and producing dislocation etch pits was developed for these alloys, revealing dislocation etch pit densities one to two orders of magnitude less than HgTe-based alloys. A kink in the thermal profile during processing of one boule generated more dislocations than did lattice mismatch due to compositional variations. This alloy has improved microstructural properties and resistance to dislocation formation compared with similar II-VI alloys.

Heat and mass transfer in float-zone processing are strongly influenced by convective flows in the zone. They are caused by buoyancy convection, thermocapillary (Marangoni) convection, or artificial sources such as rotation and radio-frequency heating. Flows in conducting melts can be controlled by the use of magnetic fields, either by damping fluid motion with static fields or by generating a defined flow with rotating fields. The possibilities of using static and rotating magnetic fields in silicon floating-zone growth have been investigated by experiments in axial static fields up to 5 T and in transverse rotating magnetic fields up to 7.5 mT. Static fields of a few 100 mT already suppress most striations but are detrimental to the radial segregation by introducing a coring effect. A complete suppression of dopant striations caused by time-dependent thermocapillary convection and a reduction of the coring to insignificant values, combined with a shift of the axial segregation profile toward a more diffusion-limited case, is possible with static fields greater than or equal to 1 T. However, under certain conditions the use of high axial magnetic fields can lead to the appearance of a new type of pronounced dopant striations, caused by thermoelectromagnetic convection. The use of a transverse rotating magnetic field influences the microscopic segregation at quite low inductions, of the order of a few millitesla. The field shifts time- dependent flows and the resulting striation patterns from a broad range of low frequencies at high amplitudes to a few high frequencies at low amplitudes.

Understanding the experimental conditions necessary for the development of radial and axial compositional homogeneity in directionally solidified Hg(0.89)Mn(0.11)Te(MMT) crystals has been difficult due to the lack of segregation coefficient data on the Hg(1-x)Mn(x)Te alloy system in the X = 0 to 0.3 composition range. Determining segregation coefficient data from the available Hg(1-x)Mn(x)Te alloy phase equilibria data is not practical due to discrepancies in the shape of the reported solidus and liquidus curves in the X = 0 to 0.3 range. To resolve these discrepancies and to obtain segregation coefficient data which can be used to understand homogeneity in directionally solidified MMT crystals, the solidus and liquidus temperatures of seven Hg(1-x)Mn(x)Te alloys in the X = 0 to 0.3 range were determined using differential thermal analysis (DTA). The Hg(1-x)Mn(x)Te phase diagram constructed for the X = 0 to 0.3 range of this alloy system from the DTA measurements clarifies the shape of the solidus and liquidus curves in this range. The segregation coefficient for the Hg(1-x)Mn(x)Te system was found to vary from 5 to 4.4 as the solidus composition increased from 0-30 atomic percent MnTe. This information will be useful in the analysis of axial and radial homogeneity of directionally solidified MMT crystals.

Two boules of Hg(0.89)Mn(0.11)Te(MMT) were solidified using the vertical Bridgman-Stockbarger method. Translation rates of 0.09 and 0. 18 microns/s were used. The influence of growth rate on axial compositional homogeneity in the MMT boules was evaluated experimentally by conducting precision density measurements on radial slices taken from each boule. In addition, Plane Front Solidification theory and segregation coefficient (k) data for the Hg(1-x)Mn(x)Te system were used to fit theoretical composition profiles to the measured MMT axial composition profiles. The strong correlation between the measured and calculated MMT axial composition profiles indicates diffusion dominated axial solute redistribution in the boules under the applied growth conditions. The analysis of the MMT axial composition profiles by Plane Front Solidification theory allowed the calculation of the effective diffusion coefficient (D(eff) = 3.5 x l0(exp -5) sq cm/s). The k-values for the Hg(1-x)Mn(x)Te system and the D(sub eff) - value were then used to verify that both boules were solidified under conditions which did not exceed the Constitutional Supercooling Criteria under ideal conditions. Finally, a preliminary examination of the radial compositional variation in each MMT was made using Fourier Transform Infra-Red Spectroscopy (FTIR). The radial homogeneity in the MMT boules was found to be comparable for both translation rates.

Silicon floating-zone experiments in axial magnetic fields up to 5 T have been performed and the macroscopic and microscopic segregation have been determined. Although the axial segregation is shifted towards the diffusive case with the application of higher fields, a pure diffusion-controlled regime cannot be attained even with fields of 5 T. The deterioration of the radial profiles experienced with smaller fields, due to the separation of the flow field in a quiescent center and a periphery mixed by thermocapillary (Marangoni) convection, can be reduced considerably with higher fields. A complete suppression of dopant striations caused by time-dependent thermocapillary convection is possible with fields of several Tesla. However, the use of high axial magnetic fields can sometimes lead to the appearance of a new type of pronounced dopant striations of oscillatory nature. These striations can be attributed to thermoelectromagnetic convection, caused by the interaction of thermoelectric currents with the magnetic field, and often suggest an annular flow pattern. The fact, that the direction and magnitude of thermocurrents depend on the interface shape, temperature field, and composition, explains the variety of striation patterns obtained, as well as the possibility to grow striation-free crystals.

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 NASA's 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 earth's 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.

A theoretical model for the diffusive mass transport of the ternary compound cadmium-zinc telluride by physical vapor transport (PVT) is presented. Considerable differences between the composition of the source and deposited material, and a steep decrease in the mass flux with an increase in ZnTe content in Cd1−XZnXTe crystals have been found for X < 0.1. The difference in composition between the crystal and the source decreases with a decrease in undercooling ΔT and/or with an increase in the excess of metal constituents (Cd + Zn) in the vapor phase.

Experimental studies on mass transport of ternary compound, Cd(1-x)Zn(x)Te by physical vapor transport (PVT) for source compositions up to X = 0.21 are presented. The effect of thermochemical (temperatures, vapor composition) and other factors (preparation of the source, crystal growth rate, temperature gradient) on composition and composition profiles of the grown crystals were investigated. A steep decrease in the mass flux with an increase in X(crystal) for X less than 0.1, and a difference in composition between the source and the deposited material have been observed. The composition profiles of the crystals were found to depend on the density and pretreatment of the source, and on the temperature gradient in the source zone. The homogeneity of the crystals improves at low undercoolings and/or when an appropriate excess of metal constituents is present in the vapor phase. The experimental results are in good agreement with our thermochemical model of this system.

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.

We have measured the electrical conductivity of molten germanium-silicon Ge(0.95)Si(0.05) from the liquidus temperature (1050 C) up to 1220 C. The data were acquired with a unique apparatus which utilizes the standard four-probe technique. The basic unit consists of a fused silica enclosure that contains hermetic glass-to-tungsten seals that can support vacuum pressures down to 10(exp -7) Torr. With calibration, the measurement error for the low vapor pressure materials of this study was typically less than 7%. The temperature dependence of the electrical conductivity of a Ge(0.95)Si(0.05) melt was found to vary from approximately 22,300/ohm/cm at the liquidus temperature down to approximately 16,000/ohm/cm at 1220 C. The negative temperature coefficient as well as the magnitude of these data clearly indicate that the molten material is metallic in nature despite the semiconducting properties of the solid.

(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.

The residual gas pressure and composition in sealed silica glass ampoules as a function of different treatment procedures has been investigated. The dependence of the residual gas on the outgassing and annealing parameters has been determined. The effects of the fused silica brand, of the ampoule fabrication, and of post-outgassing procedures have been evaluated.

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.

The purpose of this study was to examine several microstructural features in Bridgman-grown CdTe and determine whether the use of a tranverse, 4.6 kG magnetic field during solidification had any effect on these features. Any variation in these microstructural features due to the use of the magnetic field could be attributed to changes in the connective state of the melt and the resulting disruption or stabilization of the solid-liquid interface. In order to accomplish this objective, samples of CdTe were directionally solidified by the vertical Bridgman-Stockbarger method and a static magnetic field of 4.6 kG was applied transverse to the growth direction during the solidification of selected samples. The microstructural features examined included the preferred crystallographic growth direction of primary gains, grain and twin boundary length per unit area and etch-pit and subgrain boundary densities on a (111)-type plane. Measurements of these features were performed using the Laue X-ray backreflection technique and quantitative optical microscopy. No measurable difference in the microstructural features occurred as a result of using the 4.6 kG transverse magnetic field. The lack of any effect is attributed to either an insufficient magnetic field intensity or the possibility that the effect of a 4.6 kG transverse magnetic field on the fluid flow may not play a significant role in the mechanisms generating the microstructural features analyzed by the characterization techniques used in this study. Subgrain boundary densities were found to vary along the length of each boule and this variation was attributed to post-growth annealing.

A comprehensive thermo-chemical analysis of the HgZnTe-HgI2 chemical vapor transport (CVT) system was performed. The equilibrium partial pressures of vapor species were calculated and the mass flux equations of the four major species solved simultaneously using numerical methods. The mass transport rates and the alloy compositions of the deposited material as a function of transport agent (HgI2) pressure and of undercooling temperature are predicted for fixed source temperature (600°C) and composition, Hg0.84Zn0.16Te. The reasonably good agreement between the experimental mass transport results and the theoretical calculations confirms the validity of the analytical model applied to the CVT process of this system.

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.

Infrared-absorption spectra of substitutional Cu(2+) ions in ZnTe have been measured at 4.6 K. Several distinct absorption peaks are observed between 800 and 2000/cm. Absorption peaks at 1002 and 1069/cm are identified as zero-phonon lines arising from 2T2-2E transitions. Between 1069 and 2000/cm, several sets of sharp absorption lines are seen to recur regularly at an interval of 210/cm, corresponding to the LO phonon energy. Within each set distinct vibronic sidebands that cannot be identified with critical-point energies of TA, LA, TO or LO phonon modes are observed. A dynamic Jahn-Teller effect, involving coupling between a single-phonon mode and the electronic states of the 2E level, is proposed to account for the observed spectra.

Hg(1-x)Zn(x)Se alloys with composition between x = 0.08 and 0.115 were synthesized from elemental constituents and were resolidified using a Bridgman-Stockbarger growth technique. By performing precision mass-density measurements on selected wafers cut perpendicular to the growth axis, it was shown that the axial compositional profiles fit a numerical solution to the 1D diffusion equation which takes into account the variation of interface velocity with time. Infrared transmission-edge measurements performed on selected transverse slices from each ingot showed that the relative radial variations in composition decreased with decreasing growth rate. Van der Pauw measurements on selected wafers showed that the 10 exp 18/cu cm electron concentration, typical of as-grown crystals, could be reduced by approximately an order of magnitude by annealing in Se vapor.

Hg(0.80)Cd(0.20)Te crystals were grown vertically in a transverse magnetic field by directional solidification. The effect of a magnetic field on the nature of fluid flow in the melts was investigated by measuring compositional variations along the axial and radial directions of the grown ingots. Magnetic field effects were shown to be significant over the entire field range employed (i.e., 2 to 5 kG). The axial compositional profiles (determined by precision density measurements) showed an abrupt decrease in the mole fraction of CdTe when the field was applied. Radial compositional mapping by IR transmission and X-ray energy dispersion spectrometry indicated that the solid-liquid interface evolved through three stages when the field was applied (i.e., from a radially symmetric concave interface to an off-center concave shape when the field was initially applied, then to a tilted plane, and, finally, to an off-center concave interface). The axial compositional profile of an Hg(0.84)Zn(0.16)Te ingot showed similar field effects.

Far infrared reflection spectra of Hg 1− x Zn x Te with x = 0 to 1 have been measured at 295 K in the spectral region 40 to 500 cm −1 . The spectra exhibited two transverse optical phonon modes characteristic of II–VI mixed crystals. For x −1 . The spectra were analyzed by Kramers-Kronig integrations and by fitting them with a dynamic dielectric function. Random element isodisplacement (REI) model calculations of the dependence of the long-wavelength optic phonon frequencies on composition agreed well with experimental results. The REI model calculations were compared with previous results on HgCdTe and suggest a stronger HgTe force constant exists in HgZnTe than in HgCdTe.

A growth method for the physical vapor transport of compound semiconductors in closed ampoules is described. With the unique techniques applied in the heat treatment of the starting materials and the temperature profiles provided by the three-zone translational furnace, large crystals of CdS have been grown successfully by the method at lower temperatures than previously used. Both unseeded and seeded growth have been investigated. The CdS crystals were examined using optical and scanning electron microscopies (SEM) to study the microstructure and the dislocation etch-pits. The crystals were further characterized by infrared (IR) and ultraviolet (UV) transmission measurements.

The Vickers microhardness was measured as a function of composition in four II-VI semiconducting alloy systems: Hg 1- x Cd x Te, Hg 1- x Zn x Te, Hg 1- x Cd x Se, and Hg 1- x Zn x Se. The hardness data for Hg 1- x Cd x Te are in agreement with those previously reported and show a maximum in the hardness in the composition range 0.6 x x 1- x Cd x Se system as was observed for Hg 1- x Cd x Te. Hardening in the Se-based system increases more rapidly with x than in the Te-based system. Zinc additions to the Se-based system greatly increase the hardness of the alloy. A maximum in microhardness appears to occur in the composition range 0.7 x 1- x Zn x Te the highest in terms of structural integrity.

The effect of applied axial and radial thermal gradients and an axial static magnetic field on the macrosegregation profiles of Bridgman-grown GeSi alloy crystals has been assessed. The axial thermal gradients were adjusted by changing the control setpoints of a seven-zone vertical Bridgman furnace. The radial thermal gradients were affected by growing samples in ampoules with different thermal conductivities, namely graphite, hot-pressed boron nitride (BN), and pyrolytic boron nitride (PBN). Those samples grown in a graphite ampoule exhibited radial profiles consistent with a highly concave interface and axial profiles indicative of complete mixing in the melt. The samples grown in BN and PBN ampoules had less radial variation. Axial macrosegregation profiles of these samples fell between the predictions for a completely mixed melt and one where solute transport is dominated by diffusion. All of the samples were grown on Ge seeds. This resulted in a period of free growth until the Si concentration in the solid was in equilibrium with the Si concentration in the liquid. The length of crystal grown during this period was inversely proportional to the applied axial thermal gradient. Several samples were grown in an axial 5 Tesla magnetic field. Measured macroscopic segregation profiles on these samples indicate that the magnetic field did not, in general, reduce the melt flow velocities to below the growth velocities.

The solidification of liquids to produce materials is almost invariably affected by natural convection occurring in the liquid, due to concentration or temperature gradients. Fundamental studies of solidification studies of solidification are hampered by the existence of this convection and such studies have been facilitated by experiments in the microgravity of an orbiting spacecraft, where convection can be nearly eliminated. The paper describes early results from an alternative approach where buoyancy forces can be countered by a magnetic body force that arises from the temperature dependence of the magnetic susceptibility of the liquid. Velocities in a liquid volume, across which a temperature difference was imposed, have been measured by particle image velocimetry. The velocity were found to be significantly reduced when a magnetic field was imposed on the liquid using a superconducting magnet.

The growth process of silicon crystals grown with the float- zone method is strongly influenced by convection in the melt zone. Crystal growth under microgravity (μg)offers the advantage to reduce the buoyancy related convection. The size restriction which we have under terrestrial conditions due to the hydrostatic pressure is also avoided. But due to the low Prandtl number of silicon (Pr=0.02) and the comparatively high temperature dependence of surface tension (δy/δT=-.28x10-3 N/mK) even small free melt zones exhibit time-dependent thermocapillary (Marangoni) convection. Thus irregular dopant inhomogeneities occurred in doped crystals grown under 1g conditions as well as in crystals grown under (mu) g. Attempts to enhance the dopant distribution by applying static magnetic fields resulted in most cases in either a reduced radial homogeneity, or in time-dependent thermoelectromagnetic convection (TEMC). A promising alternative are transverse rotating magnetic fields: the time-dependent thermocapillary convection is not damped as in the case of static fields, but a 2D-axisymmetric, azimuthal flow is superimposed. Several experiments performed under 1g demonstrated the positive effect of rotating magnetic fields on the microscopic dopant distribution as well as on the radial dopant profile homogeneity.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Infrared detector materials, such as Hg(1-x)Cd(x)Te, Hg(1-x)Zn(x)Te have energy gaps almost linearly proportional to its composition. Due to the wide separation of liquidus and solidus curves of their phase diagram, there are compositional segregations in both of axial and radial directions of these crystals grown in the Bridgman system unidirectionally with constant growth rate. It is important to understand the mechanisms which affect lateral segregation such that large uniform radial composition crystal is possible. Following Coriell, etc's treatment, we have developed a theory to study the effect of a curved melt-solid interface shape on the lateral composition distribution. The system is considered to be cylindrical system with azimuthal symmetric with a curved melt-solid interface shape which can be expressed as a linear combination of a series of Bessell's functions. The results show that melt-solid interface shape has a dominate effect on lateral composition distribution of these systems. For small values of b, the solute concentration at the melt-solid interface scales linearly with interface shape with a proportional constant of the product of b and (1 - k), where b = VR/D, with V as growth velocity, R as sample radius, D as diffusion constant and k as distribution constant. A detailed theory will be presented. A computer code has been developed and simulations have been performed and compared with experimental results. These will be published in another paper.

As a solid solution semiconductor having, a large separation between liquidus and solidus, mercury cadmium telluride (MCT) presents a formidable challenge to crystal growers desiring an alloy of high compositional uniformity. To avoid constitutional supercooling during Bridgman crystal growth it is necessary to solidify slowly in a high temperature gradient region. The necessary translation rate of less than 1 mm/hr results in a situation where fluid flow induced by gravity on earth is a significant factor in material transport. The Advanced Automated Directional Solidification Furnace (AADSF) is equipped to provide the stable thermal environment with a high gradient, and the required slow translation rate needed. Ground based experiments in AADSF show clearly the dominance of flow driven transport. The first flight of AADSF in low gravity on USMP-2 provided an opportunity to test theories of fluid flow in MCT and showed several solidification regimes which are very different from those observed on earth. Residual acceleration vectors in the orbiter during the mission were measured by the Orbital Acceleration Research Experiment (OARE), and correlated well with observed compositional differences in the samples.

In order to simulate the space environment for basic research into the crystal growth mechanism, Hg(0.8)Cd(0.2)Te crystals were grown by the vertical Bridgman-Stockbarger method in the presence of an applied axial magnetic field. The influence of convection, by magneto hydrodynamic damping, on mass transfer in the melt and segregation at the solid-liquid interface was investigated by measuring the axial and radial compositional variations in the grown samples. The reduction of convective mixing in the melt through the application of the magnetic field is found to have a large effect on radial segregation and interface morphology in the grown crystals. Direct comparisons are made with a Hg(0.8)Cd(0.2)Te crystal grown without field and also in the microgravity environment of space during the second United States Microgravity Payload Mission (USMP-2).

Hgo.g4Zno.igTe crystals were grown vertically by directional solidification in a 5 Tesla axial magnetic field. The effects of magnetic damping on the buoyancy-driven convective flows in the melt were investigated by measuring the composition variations along the axial and radial directions of the grown ingots and comparing them with those of ingots grown without a field.

A Hg 0.84 Zn 0.16 Te alloy crystal was back-melted and partially solidified during the first United States Microgravity Laboratory (USML-1) mission in the Marshall Space Flight Center's Crystal Growth Furnace. The experiment was inadvertently terminated at about 30% of planned completion. Nonetheless, it was successfully demonstrated that HgZnTe alloy ingots partially grown and quenched on the ground can be back-melted and regrown in space under nearly steady state growth conditions. An identical “ground-truth” experiment was performed following the mission. Preliminary results are presented for both crystals, as well as for a series of other crystals grown prior to the mission for the purposes of optimizing in-flight growth conditions.

(111) wafers sliced from a boule of ZnTe grown by horizontal physical vapor transport (PVT) have been characterized using synchrotron white beam X-ray topography. The growth axis was about 6° off [311]. The presence of dislocation slip bands, subgrain structures and [111] axis 180° rotational twins were revealed. The slip bands were observed to break up the ordered dislocation cell structures comprising the subgrain boundaries. The initiation of slip at regions of stress concentration at the junctions of subgrain boundaries and twin boundaries was observed. The asymmetric 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.

The first United States Microgravity Laboratory Mission (USML- 1) flew in June 1992 on the Space Shuttle Columbia. An important part of this SpaceLab mission was the debut of the Crystal Growth Furnace (CGF). Of the seven samples grown in the furnace, three were bulk grown 2-6 compounds, two of a cadmium zinc telluride alloy, and one of a mercury zinc telluride alloy. Ground based results are presented, together with the results of computer simulated growths of these experimental conditions. Preliminary characterization results for the three USML-1 growth runs are also presented and the flight sample characteristics are compared to the equivalent ground truth samples. Of particular interest are the effect of the containment vessel on surface features, and especially on the nucleation, and the effect of the gravity vector on radial and axial compositional variations and stress and defect levels.