It is well established that crystals grown without contact with a container have far superior quality to otherwise similar crystals grown in direct contact with a container. In addition to float-zone processing, detached-Bridgman growth is often cited as a promising tool to improve crystal quality, without the limitations of float zoning. Detached growth has been found to occur quite often during microgravity experiments and considerable improvements of crystal quality have been reported for those cases. However, no thorough understanding of the process or quantitative assessment of the quality improvements exists so far. This project will determine the means to reproducibly grow Ge-Si alloys in the detached mode. Specific objectives include: (1) measurement of the relevant material parameters such as contact angle, growth angle, surface tension, and wetting behavior of the GeSi-melt on potential crucible materials; (2) determination of the mechanism of detached growth including the role of convection; (3) quantitative determination of the differences of defects and impurities among normal Bridgman, detached Bridgman, and floating zone (FZ) growth; (4) investigation of the influence of defined azimuthal or meridional flow due to rotating magnetic fields on the characteristics of detached growth; (5) control time-dependent Marangoni convection in the case of FZ-growth by the use of a rotating magnetic field to examine the influence on the curvature of the solid-liquid interface and the heat and mass transport; and (6) grow high quality GeSi-single crystals with Si-concentration up to 10 at% and diameters up to 20 mm.

The behaviour of dislocations in GaSb crystals grown in space both from a stoichiometric melt (floating zone method, FZ) and a Bi solution (floating solution zone, FSZ) respectively, is studied. Predominantly straight 60 degrees dislocations with Burgers vectors of the type b = a/2 < 110 > in (111) glide planes are identified. In the 20 mm long FZ single crystal the linear growing out of the dislocations is observed which reduces the dislocation density in the centre of the crystal to values below 300 cm(-2). The Bi incorporation in the FSZ crystal results in a misfit between seed and grown crystal and in a network of misfit dislocations at the interface. Thermocapillary convection during growth as well as the surface tension may be the reasons for the presence of curved dislocations and the higher dislocation density within a 1 - 2 mm border region at the edges of both of the crystals.

Results on the properties of the known impurities, Ge, Sn, V and Bi, and the lattice imperfections, VCd and TeCd are summarized. We discuss their role in compensation, and in buffering the variations in shallow electronic levels in the grown ingot. We demonstrate that (∼2÷3 kT) variations of the Fermi energy increases carrier trapping to the deep levels. Trapping is manifest in a photoconductivity signal that can be studied by photoconductivity methods, thus allowing to monitor the spectroscopic‐grade material before fabricating the detectors. Our approach could be important in preventing the after‐glow effect and polarization. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Experiments of deposition of CdTe films have been carried out under microgravity in the Russian Foton M3 mission. The influence of gravity has been studied with these experiments and compared to the results of simulations. The measured deposition rate could be confirmed by the theoretical results for lower temperatures. For higher temperatures the measured thickness of the deposited films was larger compared to the theoretical data.

We present a numerical investigation of general flow features in a Ge floating zone, which was performed by using a commercial finite element program FIDAD™. The results demonstrate that for fluids with a very small Pr number the influence of buoyancy forces cannot be ignored in a numerical model. The transition of a steady axisymmetric flow to a time-dependent flow in the Ge floating zone (Ma = 125 and Pr = 0.007) can be clearly attributed to the interaction between the buoyancy flow and the surface flow. The time-dependent buoyancy–thermocapillary convection was purely hydrodynamical. The thermocapillary convection causes an increase of the average flow velocity from 4.07 cm/s to 5.07 cm/s. Due to the small Marangoni and Prandtl numbers as well as the stabilization effect of the buoyancy convection, the temperature fluctuations were smaller than 1 °C in the melt and 2 °C at the free surface.

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 5T. 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⩾0.5T and B⩽4T 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 2T. These magnetic field induced inhomogeneities are damped with higher magnetic fields and can nearly be eliminated with a magnetic field of 5T. Due to their coupling to the static magnetic field and their specific shape, they can be attributed to TEMC.

Numerical investigations have been performed for modeling the global temperature field of an industrial liquid phase epitaxy (LPE) facility and to estimate the temperature fluctuations in a Te-rich solution during the LPE growth. The numerical results agreed well with experimental data and therefore provide reliable reference points for experimenters for further improvements of the growth conditions. (C) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

This paper presents a numerical study of Marangoni flows in a floating zone of germanium‐silicon crystals, which was performed by using a commercial finite element program FIDAD TM . The numerical results point out that for fluids with a small Pr number the influence of buoyancy forces cannot be ignored in the numerical model. Furthermore, the competition between the thermocapillary (TC) and solutocapillary (SC) flows in the floating zones was qualitatively examined. If the TC flow is as strong as that in the Si‐rich floating zone, the SC flow may be restricted to the bottom area near the free surface. Otherwise, the SC flow may overcome the TC flow and induce a surface transfer of species. The numerical predictions agree well with the previous experiment results. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Experimental results of vapor pressure measurement are presented for the whole temperature and pressure range for the existence of Cd0.95Zn0.05Te1±δ. Vapor pressure scanning approach was used to outline the maximum non-stoichiometry range and pressure–temperature–composition (P–T–X) phase arrangement of the solidus. Pronounced retrograde solubility on the Te side and rather weak temperature dependence of the metal solubility was registered. Position of the Cd0.95Zn0.05Te1±δ solidus in P–T–X phase space is represented in T–X and P(i)–T projections (i=Cd, Zn and Te2). Isothermal sections of the sublimation region are presented in partial pressures, and from these, temperature dependence of the crystal composition along the vapor pressure minimum curve is derived. Cd(g), Zn(g) and Te2(g) partial pressures as a function of temperature were deduced for constant crystal compositions XS, which, on the one hand, constituted the thermodynamic basis of crystal growth of Cd0.95Zn0.05Te1±δ with fixed compositions, and on the other hand, led to evaluation of the partial thermodynamic functions for sublimation of Cd0.95Zn0.05Te1±δ. Thermodynamic aspects of Cd0.95Zn0.05Te1±δ sublimation are discussed.

High resistive and photosensitive CdTe doped with In aimed for fabrication of X- and gamma-ray detectors was produced by vertical gradient freeze method. A complex investigation of defects and compensation by a number of optical and photoelectrical mapping methods was performed. A model of energy levels dominating the recombination processes in the material was elaborated, where the role of In, and related complexes as well as native defects (Cd vacancy and its competes) is discussed.

Two different X-ray tube accelerating voltages (60 and 70 kV) are used for diagnosis of front teeth and molars. Different energy ranges are necessary as function of tooth thickness to obtain similar contrast for imaging. This technique drives the costs for the X-ray tube up and allows for just two optimized settings. Energy range selection for the detection of the penetrating X-rays would overcome these severe setbacks. The single photon counting chip MEDIPIX2 http://www.cern.ch/medipix exhibits exactly this feature.First simulations and measurements have been carried out using a dental X-ray source. As a demonstrator a real tooth has been used with different cavities and filling materials. Simulations showed in general larger improvements as compared to measurements regarding SNR and contrast: A beneficial factor of 4% wrt SNR and 25% for contrast, measurements showed factors of 2.5 and up to 10%, respectively. (C) 2004 Elsevier B.V. All rights reserved.

We report a substantial reduction in the impurity concentration of semi-insulating CdTe:Ge grown by the vertical Bridgman method by using sublimation of the feed material. Specific resistivity (ρ_dark) values of up to 3 × 10^9 Ω cm were obtained for samples with a relatively high photosensitivity (PS) value and optimal compensation. Concentrations of impurities in the feed and as-grown crystals were determined by the glow discharge mass spectroscopy (GDMS) method. The energy levels in the band-gap were studied by photoluminescence (PL), and the data were correlated with the GDMS measurements. The highest values of ρ_dark and PS were observed in the regions where the PL bands via the deep levels of Ge and Te antisite were present.

We present a review of the vapor phase growth of cadmium telluride under conditions of reduced gravity in space. Particular emphasis is paid to the growth in a closed system by sublimation THM. Structural and electrical properties of the material grown under microgravity (μ-g) and earth gravity (1-g) conditions are reviewed and compared. A positive influence of μ-g conditions on CdTe crystal quality is reported. A brief literature survey of the vapor growth of other semiconductors (GeTe, HgI2, CdHgTe, ZnSe) in a μ-gravity environment is also presented.

In this communication, the latest experimental results are presented on a composition of Cd 1−x Zn x Te 1±δ , both x and δ, determined by the high-precision, vapor-pressure scanning method. The space arrangement of the single-phase solidus volume of Cd 1−x Zn x Te 1±δ in the pressure-temperature-composition (P-T-X) phase space has been derived from direct vapor-pressure measurements in the temperature range up to 1,375 K. From the experimental data for x=0.0, 0.05, 0.1, 0.15, 0.25, 0.5, 0.75, 0.8, 0.9, and 1, the complete phase diagram of Cd 1−x Zn x Te 1±δ was constructed. Transformation of the diagram was traced for the whole range of existence of the Cd 1−x Zn x Te 1±δ solid solution as a function of ZnTe concentration. Increase of ZnTe in the solid solution results in the extension of the homogeneity range and a shift of the solidus toward Te, so that already for x≥0.15 the solidus does not contain the stoichiometric, (n Cd +n Zn ):n Te =50 mol.% plane. From detailed experimental studies, maximum non-stoichiometry as a function of the temperature was determined in a wide temperature range for Te and metal solubility. Temperature dependencies of the partial pressures of vapor-phase species for different crystal compositions, X S =const, were deduced from the vapor-pressure experiment for the x=0.05 section of the diagram. From these results, relationships are derived between crystal composition, vapor pressure, and composition of the conjugated vapor.

Undoped and Ge-doped CdTe crystals have been grown using the dewetting phenomenon on the earth. A gap effected by the dewetting between the crystal and ampoule was created with a thickness up to 60 μm, but it was not stable for the complete growth process. The dewetting was stable for the first 25 mm of the growth, but than it got unstable. The main deteriorating factor was a change of a liquid–solid interface shape from the convex to concave. Structural, electrical, photoelectrical and optical characteristics of the studied samples showed better material quality in dewetted areas.

Vapour pressure measurement in the Cd–Zn–Te system is reported for T=700–1300K and P≤760mm Hg. From the experimental data for x=0.05, 0.10, 0.15, 0.25, 0.5, 0.75, 0.80, 0.90 and 1.0, complete P–T projection of the phase diagram for the quasi-binary Cd1−xZnxTe system has been constructed. Transformation of the P–T projection was traced for the whole range of the Cd1−xZnxTe solid solution as a function of the ZnTe concentration. From detailed studies of sections x=0.05, 0.1, 0.15 and 1.0, maximum non-stoichiometry as a function of temperature was determined both for Te and metal solubility. For ZnTe the solidus volume is on the Te side of the stoichiometric plane (50at.% Te). Zn-boundary of ZnTe solidus is 50.004–50.005at.% Te in composition at 826–1153K. Formation of the Cd1−xZnxTe solid solution leads to extension of the homogeneity range, especially at high temperatures. Increase of ZnTe content in the solid solution results in a shift of the solidus toward Te, so that already for x=0.15 the solidus does not contain the stoichiometric plane.

Two CdTe crystals had been grown in microgravity during the STS-95 mission. The growth configuration was dedicated to obtain dewetting of the crystals and to achieve high quality material. Background for the performed experiments was based on the theory of the dewetting and previous experience. The after flight characterization of the crystals has demonstrated existance of the dewetting areas of the crystals and their improved quality regarding the earth grown reference sample. The samples had been characterized by EDAX, Synchrotron X-ray topography, Photoluminescence and Optical and IR microscopy. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The homogeneity of the semi-insulating CdTe/CZT crystals will determine the capacity of these semiconductors to be used in pixelized imaging applications using gamma- and X-rays. It appears that the smaller the pixel unit size, the better the material uniformity has to be. Fast characterization methods become therefore, imperative to keep the production costs in industry acceptable. The goal of the present study is to evaluate two experimental methods in their ability to fulfill in a quick and routine manner the material properties: RESISTIVITY MAPPING: a contactless time dependent charge measurement (TDCM) method was used for mapping large scale wafers with spatial resolution of 1.0 mm/sup 2/ in an automatic set-up. It gives very good reproductibility for resistivities higher than 10/sup 6/ Ohm cm and displays full scale image of the resistivity distribution. - INFRARED TRANSMISSION MAPPING: allows the analysis of Te inclusions and an adapted software gives directly the distribution of the precipitate sizes. Samples of THM grown CdTe crystals have been investigated as grown and after various thermal treatments and the pixelized detectors produced from these wafers were tested using a /sup 57/Co radiation source.