Understanding and control of the flow structures in metallic fluids is important
for the development of optimal crystal growth processes. One of the techniques used
to control flow structures is the application of a rotating magnetic field (RMF) in
the plane perpendicular to the growth direction, which induces two magnetic body
force components; one in the radial direction and the other one in the circumferential
direction. These two body force components alter the fluid flow in the growth system,
leading to enhanced mixing, flatter growth interface, and more homogeneous crystal
composition. The application of RFM was therefore considered in three separate
projects: 1) the zone refining of cadmium and tellurium, 2) the synthesis of cadmium
telluride (CdTe) by the travelling heater method (THM), and 3) the THM growth of
gallium antimonide (GaSb).
In the zone refining of tellurium, the objective was to enhance the transport
of selenium in the melt since the selenium segregation coefficient is close to unity.
A magnetic field with intensity of 0.6 mT and frequency of 100 Hz was selected
based on the results of earlier numerical simulations. Due to the very low electrical
conductivity of tellurium, the numerical simulations predicted a very small effect of
RMF on selenium transport. The designed zone refining experiments for the tellurium
system have verified this numerical simulation result. On the other hand, cadmium is
an electric conductor, and thus the numerical simulations predicted a notable effect
of RMF. However, experiments on the cadmium system could not be carried out
because of the instability of molten zones caused by cadmium’s very high thermal
conductivity.
The commercial synthesis of CdTe is presently done by THM, which produces
materials with much better stoichiometry than other techniques, but very slow process
speeds make THM very costly. An application of RMF was considered in order
to improve the speed of the process. A 1.3 mT, 0.5 Hz field was applied during the
THM synthesis of CdTe. Under the experimental conditions employed, the examination
of samples has shown that the application of RMF did not increase the maximum
synthesis speed. The use of higher intensity RMF was not possible with the present
system, but it is thought that higher fields might worsen the mixing of Cd and Te to
produce non-stoichiometry.
The objective of the third project was to carry out preliminary THM growth experiments
for GaSb under RMF in order to prepare a basis for future THM growth
experiments aimed at reducing the cost of THM by using higher growth rates and
smaller seeds with tapered ampoules. The substantially redesigned THM furnace permits
rotation of the growth ampoule, better control of the experimental environment,
and a stronger temperature gradient at the growth interface. Two crystals have been
grown at 25 mm diameter with and without the application of a magnetic field of
0.6-mT intensity and 100-Hz frequency. These preliminary experiments have shown
that the system can be used for the planned THM experiments; however, further
experiments are required to attribute any effect to RMF.
| Identifer | oai:union.ndltd.org:uvic.ca/oai:dspace.library.uvic.ca:1828/1356 |
| Date | 01 April 2009 |
| Creators | Roszmann, Jordan D. |
| Contributors | Dost, Sadik |
| Source Sets | University of Victoria |
| Language | English, English |
| Detected Language | English |
| Type | Thesis |
| Rights | Available to the World Wide Web |
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