Shariar Motakef

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.

Numerical simulation of THM growth of CdTe in presence of rotating magnetic fields (RMF)

Abstract The influence of rotating magnetic fields (RMF) on flow pattern and compositional uniformity in the solution zone of a traveling heater method (THM) system for growth of CdTe is numerically investigated. The analysis is conducted at the 10 −6 and 10 −1 g 0 as representative of space and ground processing conditions. It is shown that under microgravity conditions application of RMF can be used to overwhelm residual buoyancy-induced convection and to control the uniformity of solution-zone composition at the growth front without appreciable modification of the growth interface shape. At high-gravity levels, RMF is found not to be able to completely dominate buoyancy-induced convection. In this regime, for the range of field strengths studied, RMF is found to result in (a) complex flow structures in the solution zone, (b) enhancement of compositional nonuniformities at the growth front, and (c) increased convexity of the growth interface. A scaling analysis of convection in the solution zone is used to generate a nondimensional map delineating the RMF- and gravity-dominated flow regimes.

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.

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.

The influence of thermoelectromagnetic convection (TEMC) on the Bridgman growth of semiconductors

Application of a low-intensity axial magnetic field can promote significant convection during Bridgman growth of GeSi when resident thermoelectric currents at the growth interface are large due to the difference of thermoelectric powers of the melt and of the crystal and the tangential temperature gradient at the interface. Thermoelectromagnetic convection (TEMC) in the GeSi melt is characterized by a meridional flow driven by the rotation of the fluid due to the azimuthal Lorentz force from currents in the radial direction concentrated near the interface and an axial magnetic field. In this work, we developed a computational model to study convection of the GeSi melt in varying g and magnetic field intensity levels.

Model-based calibration of NASA CGF furnace

Two novel approaches to calibration of high-temperature furnaces are presented. Both of these techniques use a high-fidelity model of heat transfer in the furnace and calibration cartridges to reduce the measured data and calculate the drift in furnace control thermocouples. The two calibration techniques and model-based data reduction are presented using the NASA crystal growth furnace (CGF) as the model furnace.

Effects of a Rotating Magnetic Field on Gas Transport During Detached Crystal Growth in Space

During the detached Bridgman growth of semiconductor crystals, the melt has a short free surface, which is detached from the ampul wall near the crystal–melt interface, thus eliminating the crystal defects caused by contact with the ampul wall. Recent modeling has indicated that initiation and continuation of detached growth depends on the rate of transport of dissolved gas from the crystal–melt interface, where gas is rejected into the melt, to the detached free surface, where evaporating gas maintains the pressure on the free surface. Here we use numerical modeling to investigate whether the application of a rotating magnetic field increases or decreases the transport of rejected gas to the detached free surface. Unfortunately, the results show that a rotating magnetic field almost always decreases the evaporation rate at the detached free surface. The exception is an insignificant increase for a short period at the beginning of crystal growth for a few circumstances. The evaporation rate decreases as the strength of the rotating magnetic field is increased.

Electromagnetic control of convection in semiconductor melts: thermoelectromagnetic convection (TEMC) and rotating magnetic fields

Application of a low intensity axial magnetic field can promote significant convection during Bridgman growth of GeSi when resident thermoelectric currents at the growth interface are large due o difference of thermoelectric powers of the melt and of the crystal and the tangential temperature gradient at the interface. Thermoelectromagnetic convection (TEMC) in the GeSi melt is characterized by a meridional flow driven by the rotation of the fluid due to the azimuthal Lorentz force from currents in the radial direction, concentrated near the interface, and the axial magnetic field. A similar flow is caused by a rotating magnetic field (RMF). When the field is rotating sufficiently fast, a time-averaging azimuthal Lorentz force (almost uniform axially) causes a steady rotation of the melt, and an associated meridional convection (Ekman cells) near the interface. In this work, we developed a computational model to study convection of the GeSi melt in a microgravity environment in the presence of low intensity magnetic fields.

Novel approaches to calibration of high temperature furnaces

This paper describes two novel approaches to calibration of high temperature furnaces are presented. Both of these techniques use a high-fidelity model of heat transfer in the furnace and calibration cartridges to reduce the data and calculate the drift in furnace control thermocouples. The two calibration techniques and model-based data reduction are presented using the NASA Crystal Growth Furnace (CGF) as the model furnace.