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Thermophysical properties, namely, density, viscosity, and electrical conductivity of HgTe and Hg 0.9 Cd 0.1 Te melts were measured as a function of temperature. A pycnometric method was used to measure the melt density in the temperature range of 948 to 1073 K for the HgTe melt and 1011 to 1131 K for the Hg 0.9 Cd 0.1 Te melt. The density results show a maximum at 1023 and 1020 K, respectively, for the HgTe and Hg 0.9 Cd 0.1 Te melts. The viscosity and electrical conductivity were simultaneously determined using a transient torque method from 944 to 1098 K for the HgTe melt and from 1016 to 1127 K for the Hg 0.9 Cd 0.1 Te melt. The measured electrical conductivity decreases as the mole fraction of CdTe, x, increases at a specific temperature, and increases as a function of temperature, which shows a semiconductor-like behavior. The measured viscosity decreases as the mole fraction of CdTe, x, increases at a specific temperature and decreases as the temperature increased. The analysis of the electrical conductivity of the melts and the relationship between the kinematic viscosity and density implied a structural transition in these melts. No relaxation phenomena were observed in the density, electrical conductivity, and viscosity of the HgTe and Hg 0.9 Cd 0.1 Te melts.

In this study, Cd1−xZnxTe crystals, with x=0.20, were grown unseeded by closed-ampoule vertical directional solidification (Bridgman) technique under controlled Cd over-pressures. CdZnTe crystals of high electrical resistivity have been reproducibly obtained with the In dopant concentration of 4–6 ppm, atomic and a Cd reservoir temperature between 785 and 825 °C. A combination of three process modifications have contributed to the accomplishment: (1) homogenizing the ampoules of starting materials under the vacuum environment to minimize contamination, (2) performing growth under controlled Cd over-pressures with the optimal In dopant, and (3) controlling post-growth cooling procedures to maintain uniformity of electrical properties over the crystal.

Carbon nanotubes (CNT) were synthesized on graphite fibers by thermal CVD. On the fiber surface, iron nanoparticles are coated and act as catalysts for CNT growth. The growth temperature ranges from 550 to 1000 °C at an ambient pressure. Methane and hydrogen gases with methane contents of 10–100% are used for the CNT synthesis. At high growth temperatures (>800 °C), the rapid inter-diffusion of the transition metal iron on the graphite surface results in a rough fiber surface with no CNT grown on the surface. When the growth temperature is relatively low (650–800 °C), CNT are fabricated on the graphite surface with catalytic particles on the nanotube top ends. Using micro Raman spectroscopy in the breath mode region, single-walled or multi-walled CNT can be determined, depending on methane concentrations.

Optical transmission measurements were performed on GaN bulk single-crystal platelet at temperatures between 293 and 1237 K. The energy bandgaps were determined from the corresponding optical absorption spectra. The bandgaps can be fit well as a function of temperature using the Varshni expression as E g ( eV )=3.556−9.9×10 −4 T 2 (T+600). The shapes of the measured absorption edges were found to be dependent on the thermal treatments of the sample.

Carbon nanotubes (CNT) were synthesized using thermal chemical vapor deposition. The diameters of these CNT were quite uniform with the length of the CNT up to several tens of micrometers. With the substrate surface normal either along or against the gravity vector, different growth orientations of CNT were observed by scanning electron microscopy. The Raman spectra were similar for samples synthesized at different locations. These results suggest that gravity affects the growth of long CNT with small diameters.

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.

In the reactive off-axis sputtering deposition of ZnO films, three transport regimes from a ballistic motion to a diffusive drift of the ejected species were observed when the growth pressures were varied from 5 to 150 mTorr. The distribution of the film thickness at different substrate locations was measured. When the substrate surface was parallel to the gravity vector, a film thickness variation along the direction of gravity was detected in the region of diffusive transport deposition. This phenomenon depends strongly on the growth pressures while only slightly on the growth temperatures. Based on experimental results, a gravity-related motion of the sputtered species is proposed in film growth under relative low growth pressures.

This article has been retracted at the request of the editor. Reason: This paper is a virtual duplication of a paper by the same author, Shen Zhu, entitled “Polarity effects of substrate surface in homoepitaxial ZnO film growth” (J. Crystal Growth 219 (2000) 361–367). Publication of the same paper twice is a violation of the basic precepts of all major journals and is considered by the community to be highly unprofessional.

Low-temperature photoluminescence (PL) spectra were measured on ZnSe starting materials provided by various vendors and on bulk crystals grown from these starting materials by physical vapor transport (PVT) to study the effects of purification and contamination during PVT process. The impurity levels in one set of starting material/grown crystal were also measured by glow discharge mass spectroscopy (GDMS). The purification effect of pre-growth heat treatments and the PVT process is evidenced from the GDMS results which showed orders of magnitude reduction in the Li and Na concentration and a factor of 3 reduction in the O content after growth. The PL spectra showed that the strong emissions associated with Li (or Na) in one of the starting materials disappeared after growth. The GDMS results also showed increases in the Al and Si contents by orders of magnitude after growth. To evaluate the contamination of the crystal during the high temperature growth process, three growth runs were processed using similar growth parameters but different furnace environments. The PL spectra suggest that the Al contamination originated from the fused silica ampoule and that the Inconel cartridge might have been the cause of the broad Cu green and Cu red bands observed in one of the grown crystals.

Carbon nanoparticles and nanotubes have been synthesized on Si substrates using pulsed laser vaporization. Two kinds of targets were used to grow the nanostructural materials. One was a pure carbon target and the other was carbon target containing Ni and Co catalysts. The growth temperatures were 600–1000°C and the argon pressure was 500 torr. Samples were characterized by scanning electron microscopy, X-ray diffraction, and Raman spectroscopy. Materials containing carbon nanocrystal particles could be observed using a pure carbon target when a thin Ni film was deposited on (100) Si substrates prior to growing carbon materials. The carbon nanotubes (CNT) were obtained in carbon soot when the target contained catalysts. By comparing the Raman radial breathing modes of the CNT grown at various temperatures, it was found that the density of the CNT was lower and the strongest Raman peak was slightly blue-shifted in samples grown at lower temperatures.

Epitaxial ZnO "lms were grown on two polar surfaces (O-surface and Zn-surface) of (0 0 0 1) ZnO single-crystal substrates using o!-axis magnetron sputtering deposition. It was found that the two polar ZnO surfaces have di!erent surface structure and morphology, which strongly in#uence epitaxial "lm growth. An interesting result shows that high-temperature annealing of ZnO single crystals will improve the surface structure on the O-terminated surface rather than the opposite surface. The morphology and structure of homoepitaxial "lms grown on the ZnO substrates were di!erent from heteroepitaxial "lms grown on the Al 2 O 3 . The measurements of low-angle incident X-ray di!raction and atomic force microscopy of ZnO "lms indicate that the O-terminated surface is better for ZnO epitaxial "lm growth. ( 2000 Published by Elsevier Science B.V.

ZnSe crystals were grown by self-seeded physical vapor transport (PVT) technique in the horizontal configuration. The source materials were heat treated by H2 reduction to remove the oxide followed by baking under dynamic vacuum to adjust the source composition toward that of congruent sublimation. Contactless growth of ZnSe single crystals have been performed consistently using three different source materials. The crystals grew away from the wall during the later stage of the growth with large (110) facets tend to align parallel to the gravity direction. The Scanning Electron Micrography (SEM) micrographs and the Atomic Force Microscope (AFM) images showed that large (110) terraces and steps dominate the as-grown facets. The measured residual gas pressures in the processed ampoules agree well among various source materials and the major components were CO and H2. No preferred growth direction was found. The one-dimensional diffusion model on the mass flux of a multi-species PVT system was employed to analyze the conditions for contactless growth. The calculated thermal profile for supersaturation is very close to the thermal profile measured inside the empty furnace bore in the region of contactless growth. The effects of convective flows in the vapor phase inside the ampoule on the growth processes are discussed.

Six ZnSe(1-x)Te(x) crystals were grown by the physical vapor transport technique. For each of the source material compositions, x = 0.10, 0.20 and 0.30, two crystals were grown - one under the horizontal and the other under the vertical stabilized configurations. The axial and radial compositional uniformity were measured by precision density measurements, wavelength dispersive X-ray spectroscopy (WDS) and optical transmission mappings. The measured radial ZnTe content was quite uniform for all the grown crystals except the horizontally grown crystal for x = 0.30. The WDS results on this crystal indicated a core with uniform ZnTe content, about 0.38, surrounded by a thin region of high ZnTe content with x = 0.8. This feature was confirmed by the SEM back scattering electron images. For the three source compositions the axial compositional variations for the vertically grown crystals were more uniform than that for the horizontally grown crystals. The measured compositions in the crystals grown from source composition of x = 0.10 suggest that the transport mechanism in the system can not be interpreted by a simple one-dimensional diffusion limited model.

ZnO films are deposited on (0001) sapphire, (001) Si and quartz substrates using the off-axis reactive magnetron sputtering deposition. Based on film thickness measurements, three transport regions of sputtered particles are observed when films are deposited in the pressure regions of 5 mtorr - 150 mtorr. X-ray diffraction, scanning probes microscopy, and electrical measurements are also used to characterize these films. The full width at half maximum of theta rocking curves for epitaxial films is less than 0.5 deg. In textured films, it rises to several degrees. The epitaxial films deposited at high pressure reveal a flat surface with some hexagonal facets. The density of hexagonal facets decreases when the growth pressure is reduced. The resistivity of these epitaxial films also depends on the growth pressures. A relationship between the pressure effects and film properties are discussed.

An in-situ monitoring furnace was constructed with side windows to perform partial pressure measurements by optical absorption and visual observation of the growing crystal. A fused silica -rowth ampoule with a 4.5 cm long square tube between the source and the seed was prepared for the optical absorption measurements. A ZnSe crystal was grown by the seeded physical vapor transport (PVT) technique in the horizontal configuration. The growth temperature was 1120 C and the furnace translation rate was 3nmVday. Partial pressures of Se2, P(sub Se2), at three locations along the length of the growth ampoule were measured at 90 min intervals during the growth process. The measured P (sub Se2) were in the range of 2.0 to 6.5 x 10(exp -3) atm. The P(sub Se2) results indicated that the partial pressure profile was inconsistent with the results of the one-dimensional diffusion mass transport model and that the source composition shifted toward Se-rich during the run, i.e. the grown crystal was more Zn-rich than the source. The visual observation showed that the seed crystal first etched back, with greater thermal etching occurring along the edges of the seed crystal. Once the growth started, the crystal crew in a predominately contactless mode and facets were evident during growth. The crystal did not grow symmetrically which is believed to be due to the unintentional asymmetry of the radial thermal profile in the furnace.

ZnSe bulk crystals were grown by self-seeded physical vapor transport technique in horizontal and vertical configurations. The impurities and defects in the grown crystals were studied by glow discharge mass spectroscopy (GDMS) and low temperature photoluminescence (PL) measurements. The PL results on the starting material and the grown crystals are consistent with the low impurity levels measured by GDMS. The crystalline quality of the grown crystals were examined by synchrotron white beam X-ray topography (SWBXT) and high resolution triple X-ray diffraction (HRTXD). The SWBXT shows that, aside from twins, the overall crystalline quality of the vapor grown ZnSe crystals, especially in the contactless grown region, was quite high. The HRTXD results are in line with the SWBXT findings. The comparison between the HRTXD on a chemical-mechanically polished and a cleaved surface seems to indicate that polishing damage can obscure the true microstructure in the as-grown ZnSe crystals.

Bulk growth of wide band gap II-VI semiconductors by physical vapor transport (PVT) has been developed and refined over the past several years at NASA Marshall Space Flight Center. Results from a modeling study of PVT crystal growth of ZnSe are reported in this paper. The PVT process is numerically investigated using a two-dimensional formulation of the governing equations and associated boundary conditions. Both the incompressible Boussinesq approximation and a compressible model are tested to determine the influence of gravity on the process and to discern the differences between the two approaches. The influence of a residual gas is included in the models. The results show that both the incompressible and compressible approximations provide comparable results and the presence of a residual gas tends to measurably reduce the mass flux in the system. Detailed flow, thermal and concentration profiles are provided. The simulations show that the Stefan flux dominates the system flow field and the subtle gravitational effects can be gauged by subtracting this flux from the calculated profiles. Shear flows, due to solutal buoyancy, of the order of 50 microns/s for the liorizont,-d growth orientation and 10 microns/s for the vertical orientation are predicted. Whether these flows can fully account for the observed gravity related growth morphological effects and inhomogeneous solute and dopant distributions is a matter of conjecture. A template for future modeling efforts in this area is suggested which incorporates a mathematical approach to the tracking of the growth front based on energy of formation concepts.

Cr-doped ZnSe single crystals were grown by self-seeded physical vapor transport technique under both vertical (stabilized) and horizontal configurations. The source materials were mixtures of ZnSe and CrSe. The growth temperatures were in the range of 1140 to 1150C and the furnace translation rates were 1.9 to 2.2 mm/day. The surface morphology of the as-grown crystals was examined by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The different features on the as-grown surface of the vertically and horizontally grown crystals suggests that different growth mechanisms were involved for the two growth configurations. The [Cr] doping levels were determined to be in the range of 1.8 to 8.3 x 10(exp 19)/cubic centimeter from optical absorption measurements. The crystalline quality of the grown crystals were examined by high resolution triple crystal X-ray diffraction (HRTXD) analysis.

ZnSe crystals were grown by the physical vapor transport technique under horizontal and vertical (stabilized and destabilized) configurations. Secondary ion mass spectroscopy and photoluminescence measurements were performed on the ZnSe samples to map the distributions of [Si], [Fe], [Cu], [Al] and [Li or Na] impurities as well as Zn vacancy, [V(zn)]. The annealings of ZnSe under controlled Zn pressures were studied to correlate the measured photoluminescence emission intensity to the equilibrium Zn partial pressure. In the horizontal grown crystals the segregations of [Si], [Fe], [Al] and [V(zn)] along the gravity vector direction were observed whereas in the vertically stabilized grown crystal the segregations of these point defects were radially symmetrical. No apparent pattern was observed on the measured distributions in the vertically destabilized grown crystal. The observed segregations in the three growth configurations were interpreted based on the possible buoyancy-driven convection in the vapor phase.

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.

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.

The effect of different heat treatments on stoichiometry and residual gas pressure in ZnSe physical vapor transport system was investigated. The dependence of the amount and composition of the residual gas on various heat treatment procedures is reported. Heat treatment of ZnSe starting materials by baking under the condition of dynamic vacuum to adjust its stoichiometry was performed and the effectiveness of the treatment was confirmed by the measurements of the partial pressure of Se2, P(sub Se2), in equilibrium with the heat treated samples. Optimum heat treatment procedures on the ZnSe starting material for the physical vapor transport process are discussed and verified experimentally.

Six batches of CdTe, having total amounts of material from 99 to 203 g and gross mole fraction of Te, X(sub Te), 0.499954-0.500138, were synthesized from pure Cd and Te elements. The vapor-phase stoichiometry of the assynthesized CdTe batches was determined from the partial pressure of Te2, P(sub Te2) using an optical absorption technique. The measured vapor compositions at 870 C were Te-rich for all of the batches with partial pressure ratios of Cd to Te2, P(sub Cd)/P(sub Te2), ranging from 0.00742 to 1.92. After the heat treatment of baking under dynamic vacuum at 870 C for 8 min, the vapor-phase compositions moved toward that of the congruent sublimation, i.e. P(sub Cd)/P(sub Te2) = 2.0, with the measured P(sub Cd)/P(sub Te2) varying from 1.84 to 3.47. The partial pressure measurements on one of the heat-treated samples also showed that the sample remained close to the congruent sublimation condition over the temperature range 800-880 C.

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.

Hg 0.84 Zn 0.16 Te crystals were grown vertically by back-melting a series of precast segments followed by directional solidification. The predetermined composition profiles of these segments simulate the distribution of a diffusion boundary layer in the melt ahead of the solid-melt interface during the directional solidification experiment. Composition analysis of the grown crystals confirm that steady-state growth were achieved from the beginning of the growth process.

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.

Hg0.84Zn0.16Te crystals were grown vertically by directional solidification in a 5 T axial magnetic field. Growth in an identical thermal environment but without the presence of magnetic field was also performed. The ingots were partially grown and quenched to preserve the growth interface shapes. The effects of magnetic damping on the buoyancy-driven convective flows in the melt were investigated by examining the quenched-in interface shapes and 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.

Mass fluxes of ZnSeS and ZnSeTe during physical vapor transport (PVT) were measured in the temperature range 1060–1160°C using an in situ dynamic technique. The starting binary materials were either baked out or distilled under vacuum to obtain near-congruently subliming compositions. The amount and composition of the residual gas inside the experimental ampoules were measured after each run using a total pressure gauge. A one-dimensional diffusion limited analysis of the mass transport rate for the PVT system was performed including the residual gas species. Four diffusion equations were solved simultaneously using a numerical method and the mass transport rates as well as the alloy compositions of the deposited materials were predicted as functions of the thermal field, the stoichiometry of the source material, and the pressure of the residual gas. Reasonable agreement between the experimental and theoretical results was observed.

In order to quantify the effects of convection on segregation, 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 of 50 kG. 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 decrease radial segregation to the diffusion-limited regime. It was also found that the suppression of the convective cell near the solid-liquid interface results in an increase in the slope of the diffusion-controlled solute boundary layer, which can lead to constitutional supercooling.

In this study, the photoluminescent and electric properties of as-grown CdS crystals and the effects observed after post-growth annealing in a cadmium, sulphur, and selenium overpressure were investigated. Bulk CdS crystals were grown by physical vapor transport (PVT) from high-purity starting materials, and two types of as-grown crystals, one "dark" and the other "clear" were obtained both having a medium resistivity of 106-108 l)-cm. The "dark"-type crystals are cadmium rich, as evidenced by an emission peak centered around 2.07 eV, which may be attributed to sulphur vacancies, while in the "clear" crystals, this emission is less pronounced. Heat treatment under a cadmium-vapor atmosphere has the effect of decreasing the resistivity by approximately 5 orders of magnitude, while sulphur annealing at 500°C, increases the resistivity of the as-grown crystals by 4 orders of magnitude and creates a new emission peak centered around 620 nm (2 eV). 1. Introduction Cadmium sulphide (CdS) is an important and useful material for optoelectronic applications. The growth of CdS by physical vapor transport (PVT) has been intensively studied for many years [1-3]. A relatively high concentration of defects resulting from the high growth temperature required by previous methods led to a low reproducibility of the results. Recently, a modification of the PVT method was * Corresponding author. Fax: +1 615 329 8634; E-mail: aburger@dubois.fisk.edu. I Present address: Universities Space Research Association, Huntsville, Alabama, USA. reported [4] which produces, at lower growth temper- atures, large crystals having both low dislocation density and second-phase inclusion density. Undoped CdS is always n-type, and the photoluminescent (PL) properties of CdS crystals were extensively studied in the past with respect to its conversion type. Most of the PL properties were studied in the context of intentionally doped crystals [5-7]. Other authors found [8-10] that post-growth heat and illumination treatments affect the general features of their lumi- nescence spectra. CdS, a good model material for the investigation of optical and electronic properties at high pressure, became the focus of several studies [11,12], and renewed interest has been triggered by recent reports on the development of electron-beam- pumped devices [13,14]. 0022-0248/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved

Mass fluxes of ZnSe by physical vapor transport (PVT) were measured in the temperature range of 1050–1140°C using an in situ dynamic technique. The starting materials were heat treated by a hydrogen reduction process followed by the dynamic bake-out method. The amount and composition of the residual gas inside the experimental ampoules were measured after the transport experiments using a total-pressure gauge. The total residual gas pressures measured were about one order of magnitude lower than the values reported on similar processed ZnSe ampoules which were not treated with a hydrogen reduction process. As a result of the reduction in the residual gas pressure, the measured fluxes were 3–7 times higher than previously reported and correspond to a growth rate higher than 10 mm/day at 1120°C. The simultaneous measurements of partial pressures of transport species and the mass fluxes were also performed on a ZnSe optical ampoule. The partial pressures of Zn and Se 2 were obtained by measuring the optical densities of the vapor at the wavelengths of 2138, 3405, 3508, 3613, and 3792 A. For the first time, the experimentally obtained mass fluxes were compared with those calculated, without any adjustable parameters, from a one-dimensional diffusion model which uses all the measured parameters (thermal field, partial pressures of transport species, and residual gas) as inputs, and good agreement was observed.

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.

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.

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.

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.

A comparative study of thermosolutal convection was carried out by growing lead bromide by silver bromide and studied the transparent Bridgeman furnace. We doped the high purity lead bromide by silver bromide and studied the microsegregation of silver in the matrix of the crystal. Theories based on two extremes: complete diffusive or convective transport did not agree with the experimental data. X-ray rocking curves and contour scans showed that increasing solutal convection deteriorated the crystal quality.

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.

Epitaxial layers of Hg1−xCdx Te were grown on CdTe substrates by the chemical vapor transport technique using Hgl2 as a transport agent. The epilayers were of nearly uniform composition both laterally and to a depth of about one-half of the layer thickness. By comparison, the composition varied continuously throughout the depth of the layer for epilayers grown by the physical vapor transport technique. Layers were grown both p- and n-type with carrier concentrations on the order of 1017 cm−3. Low-temperature annealing was used to convert the p-type layers into n-type. The room-temperature carrier mobilities of as-grown and converted n-type layers ranged from 103 to 104 cm2/V-s depending on the composition and are comparable to previous literature values for undoped Hg1−xCdxTe crystals.

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.

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.

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.

A detailed evaluation of the influence of growth conditions on the radial and axial compositional variations in directionally solidified Hg(1-x)Cd(x)Se alloys was performed. The measured axial compositional profiles were fitted to theoretical profiles to determine the effective solute (CdSe) diffusion coefficient (D) for the Hg(1-x)Cd(x)Se system. The value for D was estimated to be 3.0 + or - 0.5 x 10 to the -5th sq cm/s and did not appear to be significantly affected by the rate of crystal growth. The axial crystal uniformity was shown to be highly growth-rate dependent, with the faster growth rates producing crystals of more uniform composition in the axial direction. The magnitude of the radial variations was also shown to be highly growth-rate dependent, with the slower growth rates producing crystals of greater radial uniformity. This translation rate dependence of the radial uniformity is discussed in terms of lateral solute diffusion and convective interfacial fluid flows.

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.

Instrumentation and software are described, which are used to determine maps of properties of materials based on optical transmission spectra. In addition to a Fourier transform spectrometer, the hardware includes an xy-stage containing a small stationary aperture and two computer controlled stepper motors for positioning the sample. This idea has been used for years but this implementation is highly automated compared to those previously described. All mapping has been in the 2-15 micron wavelength range but this is not an inherent limitation of the technique. In this spectral range, useful results may be obtained with apertures as small as 25 microns in diameter. The software is described including the algorithm for obtaining the cut-on wavelength from a semiconductor spectrum. Data from both transverse slices and longitudinal slabs of narrow bandgap alloy semiconductors are shown including composition maps for materials solidified with convex and concave growth interfaces.

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.

Interrupted growth experiments on Hg(0.8)Cd(0.2)Se alloys were performed in an attempt to estimate the actual internal temperature gradient in the melt during directional solidification. The results have been analyzed in terms of a theoretical model which assumes an axial composition profile characteristic of diffusion controlled solute redistribution during growth. A comparison of the calculated and measured values suggests a reduction in the applied (empty furnace) temperature gradient by about a factor of three. Values of the interface segregation coefficient (k) determined from the phase diagram, predict values of solute concentration build up in the solid during growth interruptions which are inconsistent with measured results. It appears that the k values used tend to underestimate the actual k values for higher alloy compositions and overestimate k for the lower alloy compositions.