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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 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.

Experimental results of vapor pressure measurement are presented for the whole temperature and pressure range for the existence of Cd0.95Zn0.05Te17d. Vapor pressure scanning approach was used to outline the maximum nonstoichiometry 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 Cd0.95Zn0.05Te17d solidus in P–T–X phase space is represented in T–X and P(i)–T projections (i ¼ 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 X S, which, on the one hand, constituted the thermodynamic basis of crystal growth of Cd0.95Zn0.05Te17d with fixed compositions, and on the other hand, led to evaluation of the partial thermodynamic functions for sublimation of Cd0.95Zn0.05Te17d. Thermodynamic aspects of Cd0.95Zn0.05Te17d sublimation are 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.

Vapour pressure measurement in the Cd–Zn–Te system is reported for T=700–1300 K and P≤760 mm 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 (50 at.% Te). Zn-boundary of ZnTe solidus is 50.004–50.005 at.% Te in composition at 826–1153 K. 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.

Knudsen cell mass spectrometry has been applied to study the quasi-binary system CdTe–ZnTe at 900 and 780 K. Composition of the vapours, vapour pressures, activities, and free energy of mixing were determined at 900 K. Activities of CdTe were shown to have a negative deviation from ideal behaviour, whereas those of ZnTe exhibited positive deviation. The free energy of mixing in CdTe–ZnTe is more negative than that of the ideal solution. P – X section of the P – T – X phase diagram was constructed, and the relationship between the composition of the crystal, vapour pressure and composition of the vapour was determined in the whole concentration range. At 900 K, CdTe–ZnTe was found to be a continuous solid solution with no phase separation and without congruent sublimation. The shape of the CdTe activity isotherm at 780 K might suggest phase separation in Cd 1− x Zn x Te at this temperature.

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.

CdTe crystals have been grown by the Vertical Bridgman method with diameters of 25, 45 and 75 mm. Improvements of the crystallinity could be achieved by reduction of twins and growth of large single crystalline grains up to 40×40 mm2. CdTe:Ge has been grown with resistivities up to 2×1010 Ω cm . The samples showed a mobility-lifetime product of 5.7×10−5 for holes.

At the new synchrotron light source ANKA (Angstrom Karlsruhe, Germany) one beamline is optimised for white beam synchrotron topography. First topographs taken at various geometries and materials demonstrate the easy operation and the high resolution. Details about the experimental set-up are given. Large area and section topography in transmission allow a quantitative analysis of the type of dislocations and the dislocation density up to 106 cm−2 which is shown for GaSb:Te and InP:S, respectively. For highly absorbing materials like CdTe the back reflection geometry is adequate to analyse dislocation networks, twins and small angle grain boundaries. The grazing incidence method is used for the characterisation of strain and defects as a function of depth by varying the tilt of the sample which is helpful for processed devices like CdTe strip detectors for X-rays.

The heat capacity of ZnTe was measured in an adiabatic calorimeter in the temperature range from 15 K to 330 K. The following standard thermodynamic properties of ZnTe were derived: C p, m ° (298.15 K )=(49.52±0.10) J · K −1 · mol −1 ; S ° m (298.15 K )=(81.94±0.17) J · K −1 · mol −1 ; H ° m (298.15 K )−H ° m (0)=(10.98±0.02) kJ · mol −1 ; and Φ ° m (298.15 K )=(45.12±0.10) J · K −1 · mol −1 . The Debye temperature, calculated from the heat capacity, is Θ D =252 K .

A complex investigation of defect structure of high-resistivity Ge-doped CdTe by a number of optical, photoelectrical and electrical methods was performed. It was found that material properties are strongly influenced by the presence of centers of strong recombination (S-centers) and photosensitivity (R-centers). A model of energy levels dominating the recombination processes in the material was elaborated, where the role of Ge, Fe and Cu related as well as native defects (Cd vacancy) is discussed.

This paper presents efficient numerical methods—the “inverse modeling” method and the adaptive finite element method—for optimizing the heat transport as well as for investigating the heat and mass transport under the influence of convection during crystal growth, especially near the liquid/solid interface. These methods have been applied to industrial Bridgman-furnaces for the growth of 65–75 mm diameter (Cd,Zn)Te crystals.

Knudsen cell mass spectrometry has been applied to study the quasi-binary system CdTe–ZnTe at 900 K. Composition of the vapors, vapor pressures, activities, and free energy of mixing were determined. Activities of CdTe were shown to have negative deviation from ideal behavior, whereas those of ZnTe exhibited positive deviation. Free energy of mixing in CdTe–ZnTe is more negative than that of the ideal solution. Pressure–composition P – X section of the Pressure–temperature–composition P – T – X phase diagram was constructed, and relationship between the composition of the crystal, vapor pressure and composition of the vapor was determined in the whole concentration range. At 900 K, CdTe–ZnTe was found to be a continuous solid solution with no phase separation and without congruent sublimation.

The role which microgravity plays for the understanding of the growth of semiconducting materials is reviewed. Main emphasis is placed on the interaction of fluid flow and mass transport phenomena during different growth processes, the character and strength of surface tension driven flows, the limitations of diffusive transport regimes, and possibilities provided by magnetic fields with respect to support or replace microgravity conditions. Some of the most relevant topics to be investigated during the next decade on board the International Space Station are outlined. The first results of experiments carried out on the ISS are presented.

In this study we investigate AlxGa1−xN/AlN heterostructure layers grown on c-plane sapphire by RF-plasma enhanced molecular beam epitaxy. The Al mole fraction was varied through the entire range of compositions (0≤x≤1), and was determined from Rutherford backscattering (RBS) analysis and X-ray diffraction (XRD). The thicknesses of the AlxGa1−xN layers ranged from 650 nm to ≈2 μm, and those of the AlN buffer layers from ≈100 nm to ≈400 nm. Reflection and cathodoluminescence measurements were carried out in the photon energy range between 3.0 and 6.2 eV at various cryogenic temperatures down to liquid helium temperature. The reflection measurements yield a bowing parameter b=(0.91±0.14) eV for the dependence of the band-gap energy on the alloy composition.

High-temperature heat capacities were measured using the Setaram DSC 111 in the temperature range 290–925 K. Experimental data were smoothed jointly with earlier determined low-temperature heat capacities to calculate thermodynamic properties in temperature range 15–925 K. Extrapolation of heat capacity curve to 1500 K allows to calculate Gibbs free energy at high temperatures.

Several earlier μg experiments have shown that time-dependent thermocapillary (Marangoni) convection is the major cause for the formation of dopant striations in floating-zone grown semiconductor crystals, at least in small-scale systems not employing RF heating. To quantify this correlation, a silicon floating-zone experiment was performed on the TEXUS36 flight (February 7, 1998) in a monoellipsoid mirror furnace to measure temperature fluctuations in the melt zone and the microscopic growth rate simultaneously. Fluctuations of 0.5K – 0.7K with main frequencies between 0.1Hz and 0.3Hz were detectable. The microscopic growth rate fluctuated considerably around the average growth rate of 1mm/min: rates from 4mm/min to negative values (backmelting) were observed. Dopant striations are clearly visible in the Sb-doped crystal. The frequencies associated with the dopant inhomogeneities correspond quite well with those of the temperature fluctuations and microscopic growth rates. 3D numerical simulations were performed to predict the optimum position of the temperature sensor, to evaluate characteristic temperature amplitudes and frequencies, and to give insight into the instability mechanisms of Marangoni convection in this configuration. The simulations were in good agreement with the experimental values, showing temperature fluctuations with frequencies f ≤ 0.25Hz and amplitudes up to 1.8K at a position equivalent to that of the sensor tip in the experiment. Future microgravity S-FZ experiments, currently scheduled for a MAXUS flight in 2001, will try to influence the formation of dopant striations by rotating magnetic fields or by vibrational convection.

Synchrotron topography characterization of vapor grown ZnTe crystals showed that the available data in this system are not sufficient to control the growth parameters. First P–T–X phase equilibrium studies confirm that the stoichiometric plane X=50 at.% Te is completely outside of the single-phase volume of ZnTe. The Te boundary exhibits retrograde solubility. The maximum Te non-stoichiometry of 0.0186 at.% was found at T=1344 K.

To study the influence of the growth parameters on structural properties and crystal quality epitaxial layers of AlN and Al x Ga 1-x N (x = 0-1) were grown in an RF-plasma enhanced MBE-system on Si-terminated (00.1) SiC, (00.1) sapphire and (111) Si substrates by varying substrate temperature, growth rate and III/V ratio. The alloy composition of the Al x Ga 1-x N layers was studied by RBS-measurements and the results were compared with X-ray diffraction and with the optical properties obtained from cathodoluminescence (CL). The formation of an Al x Ga 1-x N superstructure is observed with ordered Al and Ga distribution. Increasing degree of chemical ordering is observed with inereasing growth temperature. The CL-spectra at 77 K show relatively sharp near-hand-edge transitions for pure AlN and Ga-rich compositions indicating high material quality. The near-band-edge transition follows the composition of the solid solution connected with a broadening of the peaks and evident optical bowing for samples grown at low growth rates (

The optical properties of epitaxial aluminum nitride (AlN) films grown on sapphire, SiC, and Si substrates have been investigated in the (2–6.3) eV photon energy range using cathodoluminescence (CL) at liquid nitrogen temperature. Besides a broad luminescence band at 3.2 eV the CL spectra shows multiple, relatively narrow excitonic features in the near band-edge region at approximately 6.1 eV. The nature of the excitonic transitions is discussed and an estimate of the band-gap energy is given.

This paper presents data on experimentally and theoretically deduced total energies and energy level positions in the band-gap for eight elementary defects in CdTe. Based on these data, the total energy dependence on the Fermi-level position in the band-gap is graphically analysed. Two types of defect reactions, which are responsible for cadmium vacancy (VCd) creation and transformation, are discussed. It is shown that the most probable candidate for the native deep-level donor is a tellurium antisite.

The growth of CdTe from the vapour phase offers several improvements in crystal quality and homogeneity. CdTe and (Cd, Zn)Te were grown by the modified Markov technique. The transport properties and the detector performance are given and compared to melt grown material.

Aluminum nitride layers were grown on Si-terminated SiC (0 0 0 1) at various substrate temperatures, growth rates, and V/III-ratios in a RF-plasma enhanced MBE system. Surface morphology detected by AFM is strongly influenced by V/III-ratio. Smooth surfaces with a RMS-roughness below 1 nm are obtained for stoichiometric growth conditions giving the smallest FWHM of the XRD-rocking curves of 190 arcsec. For the growth of a more rough AlN-surface layer (RMS 2–24 nm) atomically smooth areas have been detected around micropipes by AFM indicating changed growth conditions. Lateral extension of this area is increased by reducing the growth rate or the V/III-ratio.

CdTe and (Cd,Zn)Te crystals were compared by their material properties. The compensation mechanism of (Cd,Zn)Te and the influence of deep levels are compared to CdTe. The detector grade of (Cd,Zn)Te is obtained with a peak-to-valley ratio of 3.5 for a 57 Co spectrum. The homogeneity of electrical properties is discussed by photoluminescence and TDCM mappings. Recent applications of (Cd,Zn)Te detectors are presented.

CdTe is one of the most encouraging semiconductor materials in the field of room temperature γ- and X-ray spectroscopy. To improve the detector properties, the ternary systems (Cd,Zn)Te and Cd(Te,Se), and CdTe were grown by vertical Bridgman technique. To achieve low noise detectors, the resistivity of all materials was increased by chlorine doping. The crystals were characterized by electrical (Hall measurements, photoinduced current transient spectroscopy) methods. The numbers of deep levels influencing the resistivity were reduced by introducing Se into the CdTe system. A common deep level close to the middle of the bandgap has been identified, responsible for the compensation effect in all three systems. In addition high resistivity and n-type conductivity were achieved in CdTeSe materials for the first time

CdTe and CdTe 0.9 Se 0.1 crystals were grown by the travelling heater method under different material transport regimes. The different transport regimes in the liquid zone were either caused by diffusion (and residual convection), or by 1 g convection, or by forced convection. The forced convection was produced by a B=2 mT rotating magnetic field (400 Hz) under μg conditions. The equations for describing the model of the rotating magnetic field are derived, showing that the magnetic force term in the Navier-Stokes equation can be calculated separately in the case of μg conditions and small magnetic field. The rotating magnetic field generates a stable steady flow in the solution zone, which improves the radial and axial distribution of μτ products

Semi-insulating GaAs is suitable as detector for X-rays and particles. Compared to silicon, charge carriers in GaAs have less effective mass, so they gain a higher acceleration. The detectors operate as Schottky diodes. Design and processing were carried out at the IAF in Freiburg. The layout contains quadratic diodes of various sizes (2x2 square millimetre, 3x3 square millimetre, etc.) and also microstrip detectors. Test measurements were made with Alpha-, Beta- and Gamma-rays. For the first time X-rays down to 20 keV have been observed with room temperature GaAs. With Gamma-radiation of high57Co, 122 keV, an energy resolution of 18 % FWHM was obtained. It is planned to integrate signal processing fast electronics on the same wafer. Also discussed is a data transfer by optoelectronical means, as for example laser diodes and MSM photodiodes.

High purity lead iodide (PbI2) is prepared by precipitation of lead nitrate (Pb(NO3)) and potassium iodide (KI) solutions. Thoroughly dried PbI2 is then zone refined. The main impurity causing a strong adherence of solidified melt to the glass ampoule wall is identified to be lead-hydroxyl-iodate (Pb(OH)I). In order to decrease the OH− concentration and consequently the deviation from stoichiometry, molten PbI2 is treated with a gaseous mixture of methylene-iodide (CH2I2) and iodethane (C2H5I). The criterion for the efficiency of this scavenging process is the removal of the adhesive effect; thus, the solidified PbI2 ingot can move freely in the ampoule. A wet chemical analytic method to determine the deviations from stoichiometry is described.