Mirror Furnace Research Articles

Numerical simulation of the influence of the orbiters attitude on the μg growth of InP : S crystals from an In solution during the EURECA-1 flight

S-doped InP crystals were grown from an In-solution in a THM arrangement during the EURECA-1 mission using the automatic mirror furnace (AMF) under microgravity conditions. The InP-crystals showed growth rate oscillations in the range of 0.8–2.0 μm/min with a period of 45 min, which is half the time of an orbit of the EURECA-1 satellite. The fluid dynamic conditions during this experiment are modelled using a 3D model with the commercial software FIDAP. The time-dependent simulation, considering thermal as well as solutal buoyancy effects, indicates a weak laminar flow in this material system. Due to the fixed attitude of the satellite with respect to the sun the residual gravity vector rotates relative to the sample axis. This rotation results in a significant change of the residual convection level. Its maximum is obtained, when the gravity is parallel to the growth interface (two times during each orbit). Due to a high Schmidt number Sc≈25 a time-dependent influence of the convection on the concentration field can also be observed.

Modulations in incommensurate (Ca 1-x Sr x ) 2 MgSi 2 O 7 single crystals

Single crystals of (Ca1–xSrx)2MgSi2O7 slightly doped with 1000 ppm Mn2+ and with x ranging from 0.04 to 0.32 were grown from the melt in a mirror furnace applying the Czochalski technique. Transmission electron microscopy (TEM) revealed incommensurately modulated structures at room-temperature for all compositions in accordance with earlier studies by electron paramagnetic resonance (EPR). Electron diffraction patterns clearly show satellite reflections typical for two-dimensional modulation, and their successive destabilization with increasing Sr content. The modulation is of tartan-like appearance. Beyond a Sr/(Sr+Ca) ratio of about 0.32 the synthesis of stable solid solution akermanite type crystals was proved not to be feasible, indicating the existence of a miscibility gap in the Sr akermanite system. As presumed from the diffuse scattering around the satellite reflections, and suggested more conclusively by crystallographic processing of high resolution EM images the Sr ions incorporated into the incommensurate crystal phase are distributed in an ordered fashion and are partly adapted to the displacive modulation of the pure akermanite. This means, occupational modulation even makes a contribution to the overall modulation characteristics in (Ca1–xSrx)2 MgSi2O7.

Compositional dependence of REE partitioning between diopside and melt at 1 atmosphere

The effects of composition on pyroxene-melt partitioning of several REE (rare earth elements), Y, and Sr were experimentally evaluated. Using the synthetic model systems anorthite–diopside, diopside–titanite and anorthite–diopside–titanite different diopsides were grown at atmospheric conditions in a double-ellipsoid mirror furnace. The single samples were melted and crystallised in a Pt/Au crucible with compositions corresponding to the invariant points of these systems. Rotational motion with approximately 25 rpm around the longitudinal axis of the crucible increases the prevailing convection flows. By this means, the exclusively diffusional transport of assembly groups onto the growing crystals is avoided. Quenching is achieved by dropping the crucible into water. Crystals up to 2 mm were obtained and analysed by electron microprobe. No inhomogeneities or compositional zonation, either in the diopsides or in the coexisting melts, were observed within the analytical uncertainty of the electron microprobe. The crystallised diopsides occur as both euhedral single crystals and large symplectitic lamellar intergrowths with anorthite or titanite. The chemical homogeneity and the texture indicate near-equilibrium conditions. The analyses show strong positive correlations between DREE and tetrahedrally coordinated Al in diopside but are not affected by octahedral Al or Ti-concentration. By means of correlations and mass balances the incorporation of REE can be described by 2 different coupled substitutions: $$$$ $$$$

Growth and characterization of Ge 1− xSi x ( x⩽10 at%) single crystals

GeSi single crystals in the range Si⩽10 at% have been grown on Ge 〈1 1 1〉-seed crystals by the vertical Bridgman method in a radiation heated mirror furnace. The diameter of the crystals was 9 mm, their length about 30–40 mm. Axial and radial macrosegregation have been measured by energy dispersive analysis by X-ray (EDAX), microsegregation was investigated using interference contrast microscopy. The average EPD is in the range 6×10 4–1×10 5 cm −2 and is enhanced at the seed/crystal interface and in the part grown in the vicinity of the container wall. Measurements with a Bartels five crystal diffractometer of the 〈1 1 1〉 peak resulted in FWHM (full-width at half-maximum) values of 20 to 40 arcsec, compared to the theoretical value of 17 arcsec. The FWHM values of the bound excitons, determined by photoluminescence, are in the range 2.5–4.5 meV, which are the best values reported hitherto, and show a linear enlargement with increasing silicon concentration. The segregation coefficients of Ga, In, P, As, and Sb fit well to a linear interpolation between the Ge and Si values.

Germanium-rich SiGe bulk single crystals grown by the vertical Bridgman method and by zone melting

Silicon–germanium single crystals with a Si content up to 10 at. % have been grown by the vertical Bridgman technique in a monoellipsoid mirror furnace. Single crystal regions up to 50 at. % have been realized by zone melting in a double ellipsoid mirror furnace. The typical high temperature gradients of radiation heated facilities (grad T up to 100 K/cm or even higher, depending on the sample geometry) reduce the occurrence of constitutional supercooling and stabilize the growth interface. Photoluminescence measurements as well as rocking curves and etch pit densities reveal a high crystal quality. Effective segregation coefficients of Al, Ga, In, P, As, and Sb in Ge1−xSix (x<13 at. %) were determined.

Floating-zone growth of silicon in magnetic fields. I. Weak static axial fields

Silicon floating-zone experiments, employing P- and Sb-doped crystals of 8 mm diameter and zones of 10–12 mm length, were carried out in a mirror furnace under static axial magnetic fields (magnetic induction B ⩽ 500 mT). A strong influence of the magnetic field on the formation of dopant striations could be detected: Whereas crystals grown without field show an irregular striation pattern implying a broad frequency range of the melt flows (0.1–5 Hz), fields as small as 60 mT already show an influence by reducing the frequency spectrum to a range of 0.1–1.5 Hz. Higher fields reduce the spectrum even further and at fields of 220 mT only one frequency remains, producing a periodic striation pattern. Above 240 mT, nearly striation-free crystals could be obtained. In addition to changes in the microsegregation, the transient part of the axial macrosegregation is steeper because of the reduced mixing of the melt. The radial segregation is changed by a flattening of the interface curvature and the formation of a core region in the crystal. The core diameter is directly dependent on the magnetic induction.

Modeling of ellipsoid mirror furnace for floating-zone crystal growth

Because of its compactness and high efficiency, the image furnace using ellipsoid mirrors to focus infrared heating from lamps has widely been used in floating-zone (FZ) crystal growth. However, little research has been reported on modeling energy distribution in such a system. In this paper, a simple model considering direct and indirect radiation from the lamps is presented. This model is consistent with the point-source model as the lamp size is diminished. The effects of lamp size, shape, orientation, and defocusing are discussed. The model is further integrated with a three-dimensional thermal-capillary model. Therefore, the temperature distribution and the zone shape of the sample during FZ growth in the furnace can be estimated. Temperature measurements of a graphite rod in a mirror furnace were also conducted to validate the global model.

On the kinetics of carbide precipitation during reaction between graphite and Fe-Ti liquids under microgravity

Experiments on the reaction between graphite and liquid Fe-Ti alloys were performed with a mirror furnace on board an airplane during parabolic flights. Small Fe-Ti alloy samples were melted in contact with graphite and held for some seconds at a temperature of 1550 °C. The samples were melted and solidified during a microgravity period. Carbon and titanium atoms reacted in the melt and titanium carbides were formed. In the experiments, a precipitation zone with faceted titanium carbide crystals dispersed in high carbon Fe-C-Ti alloy matrix was obtained near the graphite/alloy interface. The thicknesses of the carbide precipitation zones were measured and effects of alloy composition on the growth rates of the carbide zones were revealed by experiments and calculations. It was shown that the process was controlled by the diffusion of titanium in the liquid at low titanium concentrations and by diffusion of carbon through the precipitation layer at high titanium concentrations in the melt. Supersaturation of the carbide in front of the reaction interface was predicted from the calculations. The analysis showed that homogeneous nucleation of titanium carbide can readily occur in the alloys. Carbide morphologies were analyzed, and the mechanisms which lead to their formation are discussed.

On the precipitation of TiC in liquid iron by reactions between different phases

In order to study the in situ formation of carbide-reinforced metal matrix composites and the inoculation reactions of some low alloy steels, titanium carbide precipitation in liquid iron was investigated. TiC particles were formed directly in liquid iron solution through reactions between different phases. The high-temperature reactions studied in the present work are reactions between (a) solid graphite and liquid Fe-Ti alloys, (b) Fe-Ti and Fe-C liquid alloys, and (c) pure solid titanium and liquid Fe-C alloys. Small samples for these reactions have been processed using a mirror furnace facility in a precisely controlled manner. Samples of Fe-Ti alloys reacted with graphite were also processed by a high frequency induction furnace and other facilities. The precipitation behavior of titanium carbide in various reactions and the distribution of carbide particles in iron melts were investigated. Different growth morphologies of the carbide were observed under various conditions. The effects of convection on the reactions were observed and discussed, and by processing the samples with various orientations relative to gravity, various patterns of carbide distribution were obtained.

Radiative exchange between a cylindrical crystal and a monoellipsoidal mirror furnace

The heating of a cylindrical sample in a monoellipsoidal mirror furnace is considered. A radiation model is formulated for slender samples where the radiative exchange between the sample and the mirror is studied analytically. To analyse the behavior of the model, the temperature field in the sample is calculated using a one-dimensional conduction radiation model. The differences with respect to a model where the radiative exchange is not considered are not only quantitative but qualitative. The model predicts a strongly asymmetric heating, which may cause asymmetries in the grown crystal as observed experimentally. Comparison with experimental data is made.

In situ Observation Setup for Semiconductor Growth Interface from Solution in a Magnetic Field

AbstractA setup for in situ observation was developed which makes possible the optical visualization of a solid‐liquid interface during solution growth of semiconductors in an external magnetic field. This setup consisted of an optimized monoellipsoidal mirror furnace, a near‐infrared optical system, and static or rotating electromagnetic facility. The target crystals were GaP, GaAs and CdTe.

Determination and modification of the thermal conditions in a double ellipsoid mirror furnace

A crucible-less double ellipsoid mirror furnace was used for growing beryllium single crystals with certain defect structures. The temperature field is not rotationally symmetrical in the double ellipsoid mirror, hence inhomogeneous configurations of dislocations and even subgrains can be formed. Investigation of the temperature fields during solidification and their manipulation was therefore necessary. By controlled defocusing of the lamps, a significant reduction of the temperature gradients in azimuthal and axial direction can be achieved.

Effect of axisymmetric magnetic fields on radial dopant segregation of floating-zone silicon growth in a mirror furnace

The effect of axisymmetric magnetic fields on radial dopant segregation of floating-zone silicon growth in a mirror furnace is studied through computer simulation. The mirror furnace is modeled by a point lamp model, and the magnetic fields are generated by wire coils carrying different currents. Heat and mass transfer, fluid flow, melt/solid interfaces, and the free surface are solved globally by a robust finite-volume/Newton's method. Both axial and cusp fields are considered. It is observed that the magnetic fields suppress the convection in the core region effectively, and squeeze the thermocapillary flow toward the free surface. However, due to poor mixing in the core region, large radial segregation is induced by strong magnetic fields, particularly by the cusp field. With a higher heat input, multiple steady states are also observed in weak magnetic fields.

Long‐term Crystal Growth under Microgravity during the EURECA‐1 Mission (I) THM Growth of AlxGa1−xSb

AbstractLong‐term crystal growth experiments were successfully performed under microgravity conditions during the first flight of the unmanned mission EURECA‐1 in the automatic mirror furnace (AMF). Two crystals of AlxGa1−xSb with [001] and [111] orientation respectively were grown from gallium solution by the travelling heater method. The grown length of the single crystals is 4.0 mm and 4.2 mm respectively. The space‐grown samples show a high microscopic homogeneity which indicates the absence of time dependent convection. From pulse markers a constant growth rate of 0.6 ± 0.1 μm/min is measured which is lower than 0.8 ± 0.1 μm/min obtained in earth grown reference samples. Details about the experiment performance and the growth results are given.

Long‐term Crystal Growth under Microgravity during the EURECA‐1 Mission (II) THM Growth of Sulphur‐doped InP

AbstractLong‐term crystal growth experiments were successfully performed under microgravity conditions during the first flight of the unmanned EURECA‐1 mission in the automatic mirror furnace (AMF). Two crystals of sulphur‐doped InP with [001] and [111] orientation respectively were grown from indium solution by the travelling heater method (THM). The absence of time dependent buoyancy‐driven convection is documented by the lack of type I striations in the space‐grown crystals. The sulphur concentration is measured by spatially resolved photoluminescence. As expected, the macrosegregation can be described by a pure diffusion‐controlled model which is in good agreement with the findings from the first German spacelab mission D1. Compared to the earth‐grown reference samples, both of the space‐grown InP crystals show strong disturbances such as inclusions and type II striations. The morphological instabilities are similar to growth disturbances already known from the space‐grown MD‐ELI‐01 from the D1 mission.

Growth of 20 mm diameter GaAs crystals by the floating-zone technique with controlled As-vapour pressure under microgravity

Five GaAs single crystals with diameters of 20 mm were grown by the floating-zone technique (FZ) under microgravity during the second German Spacelab Mission D-2. Both Si-doped and undoped semi-insulating GaAs feed material was used for studying a variety of crystal growth phenomena and material properties of relevance for GaAs melt growth. The GaAs rods were sealed in silica ampoules which contained an integrated As source to provide controlled stoichiometry conditions. The heating system consisted of a specially designed mirror furnace (PARELLI). In this paper we present results of the microgravity experiments concerning technical aspects, such as the control of interface curvature and crystal diameter, as well as studies of the density and distribution of dislocations. The formation of dopant striations is used as an indicator for the state of the Marangoni convection in the GaAs melt zone and the influence of a magnetic field. Results of photoluminescence studies show an improvement with respect to the dislocation network formation by a strong increase of the cell size. Both oxygen and boron contents of the space-grown crystals are very low. Results of numerical calculations of the curvature of the interface, the distribution of temperature and stress in the crystal are shown to be in good agreement with the experiments.

Thermogalvanomagnetic properties of Sn-doped Bi95Sb5 and its application for solid state cooling

Measurements of the complete set of thermogalvanomagnetic coefficients of Sn-doped Bi95Sb5 have been carried out from liquid nitrogen up to room temperature as a function of magnetic field strength and crystallographic direction (parallel and perpendicular to the trigonal axes). The dependences of the transport coefficients on temperature, magnetic field strength, and impurity density as well as their complex mutual correlations are explained, qualitatively. These investigations were performed in order to establish the optimum tin concentration that will maximize the thermomagnetic and thermoelectric figures of merit at a given temperature and magnetic field. As calculated theoretically a 2.5-fold enhancement of the thermomagnetic figure of merit was achieved. The crystals were grown by the horizontal zone-leveling technique under a hydrogen atmosphere. To achieve high homogeneity and to avoid constitutional supercooling a traveling double-ellipsoid mirror furnace was built up. This method enables us to produce a small liquid zone and a steep temperature gradient at the solid–liquid interface.

A neutron powder investigation of the high-temperature structure and phase transition in LiNbO3

The structure of LiNbO 3 has been determined around the phase transition at T c ≃1430 K by neutron powder diffraction using a mirror furnace in air up to 1505 K. The transition is of the order-disorder type with some displacive components. In the paraelectric high-temperature phase, Li is randomly distributed over two sites on both sides of the O plane and Nb assumes a centrosyminetric position within its O 6 octahedron. There are indications of more complicated disorder and a high mobility of Li

CdTe crystal growth by a sublimation traveling heater method

Both undoped and halogen-doped CdTe bulk crystals were grown in a monoellipsoid mirror furnace using a vapor zone transport method in closed ampoules (sublimation traveling heater method, STHM). Partial pressure gradients in the vapor affect the growth front stability and thus the maximum pulling rate significantly. A thermodynamic consideration leading to a “critical” growth rate J crit for sublimation growth of undoped CdTe in closed systems as well as a comparison with experimental results for halogen-doped material are introduced. In addition to chlorine, bromine and iodine were used as dopants for the first time in CdTe sublimation material. Resistivity measurements and photoluminescence investigations were performed. Resistivities in the range of 1.2 × 10 6 to 8 × 10 8 ω cm were found; the photoluminescence (PL) spectra of doped materials show a band within the defect region that may be assigned to a complex (V 2- Cd; halogen + Te) -, the A-center previously found for Cl doping.

Structure refinements of β-Si3N4 at temperatures up to 1360°C by X-ray powder investigation

Abstract The structure of β-Si3N4 was reinvestigated with X-ray powder diffraction and Rietveld refinement to determine if the space group should be centrosymmetric, P63/m, or acentric, P63. Displacement for one sort of the nitrogen atoms N(1) away from the high-symmetry positions are found to be even larger than reported before and this behaviour is retained to higher temperatures. R-factor ratio tests favour P63 at a 95% significance level. No exceptional axial thermal expansion was observed. Above 900°C a significant growth of a disordered high-cristobalite phase was observed. It is shown that structure refinements with the X-ray powder technique can be done even at high temperature with high confidence. A newly developed X-ray mirror furnace reaching temperatures up to 1360°C is described.