M.P. Volz

A numerical investigation of the effect of thermoelectromagnetic convection (TEMC) on the Bridgman growth of Ge1−xSix

Thermoelectric currents at the growth interface of GeSi during Bridgman growth are shown to promote convection when a low-intensity axial magnetic field is applied. TEMC, typically, is characterized by a meridional flow driven by the rotation of the fluid; meridional convection alters the composition of the melt, and shape of the growth interface substantially. TEMC effect is more important in micro-gravity environment than the terrestrial one, and can be used to control convection during directional solidification of GeSi. In this work, we report on the numerical simulation of the effect of TEMC on the growth of GeSi.

Bridgman Growth of Detached GeSi Crystals

Ge1xSix (0oxo0:12) has been grown by the vertical Bridgman technique using adjustments in the applied temperature profile to control the pressure difference between the bottom and top of the melt. Using this technique, a pressure difference is created by decreasing the temperature in the gas volume above the melt while the sample is molten but prior to growth. A maximum pressure difference approximately equal to the hydrostatic pressure of the molten sample can thus be obtained. Several GeSi crystals were grown in pyrolitic boron nitride ampoules. When a pressure difference was applied, samples were reproducibly grown mostly detached. For comparison, samples were also grown in a configuration in which gas could flow freely between the gap below the melt and the volume above the melt and no pressure difference could be established. These samples were initially attached. Existence of detachment was determined both by measuring the surface roughness of the samples with a profilometer and by observations of the sample surfaces with optical and electron microscopy. r 2002 Elsevier Science B.V. All rights reserved.

Growth of ZnTe by physical vapor transport and traveling heater method

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.

An experimental study of the influence of a rotating magnetic field on Rayleigh–Bénard convection

A destabilizing vertical temperature gradient and a rotating magnetic field have been applied to a cylindrical column of liquid gallium. The convective flows which arise as a function of these parameters are identified. For small magnetic field strengths, a regime of stationary flow is observed. This regime is bounded by critical values of the Rayleigh and magnetic Taylor numbers. As the rotating magnetic field is increased, the critical Rayleigh number can increase by more than a factor of 10. The rotating magnetic field itself induces an instability at a critical value of the magnetic Taylor number independent of the Rayleigh number. The nature of the bifurcations (whether subcritical or supercritical) and the convective flows occurring at the critical Rayleigh numbers are dependent upon the magnetic Taylor number

Thermoconvective instability in a rotating magnetic field

The effect of a rotating magnetic field (RMF) on the stability of a fluid contained in a cylindrical column and heated from below is investigated. The RMF increases the critical Rayleigh number for asymmetric flow modes but does not affect the onset of instability for axisymmetric modes. The critical Rayleigh number is dependent upon the relative penetration of the magnetic field into the cylinder and the Prandtl number of the fluid. Instability first develops in the form of a single asymmetric meridional roll rotating around the axis of the cylinder, driven by the azimuthal component of the magnetic field.

Stability of detached-grown germanium single crystals

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

Flow transitions in a rotating magnetic field

Critical Rayleigh numbers have been measured in a liquid metal cylinder of finite height in the presence of a rotating magnetic field. Several different stability regimes were observed, which were determined by the values of the Rayleigh and Hartmann numbers. For weak rotating magnetic fields and small Rayleigh numbers, the experimental observations can be explained by the existence of a single non-axisymmetric meridional roll rotating around the cylinder, driven by the azimuthal component of the magnetic field. The measured dependence of rotational velocity on magnetic field strength is consistent with the existence of laminar flow in this regime.

Frequency effects of a rotating magnetic field on fluid flow in vertical cylinders

The body force generated by a rotating magnetic field applied to a cylindrical column of liquid metal of finite height is investigated theoretically. Although an exact analytical formula has not been found, the proposed approach leads to a good approximate solution. It is demonstrated that the force field is significantly affected by the angular frequency of the rotating magnetic field. In the low-frequency limit, only the azimuthal component is present, while in the high frequency regime, a complex force field is induced which is composed of both azimuthal and meridional components. The resulting azimuthal flow in the Stokesian regime has been numerically obtained and a counter-rotating profile has been demonstrated. The calculated results can be used to determine the fluid flow behavior during crystal growth in a weak rotating magnetic field.

Mass flux of ZnSe by physical vapor transport

Abstract 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.

Solutocapillary convection in the float-zone process with a strong magnetic field

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