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.

The semiconducting compounds CdTe and CdZnTe have important applications in high-energy radiation detectors and as substrates for infrared devices. The materials offer large band gaps, high resistivity, and excellent charge transport properties; however all of these properties rely on very precise control of the material composition. Growing bulk crystals by the travelling heater method (THM) offers excellent compositional control and fewer defects compared to gradient freezing, but it is also much slower and more expensive. A particular challenge is the current need to grow new crystals onto existing seeds of similar size and quality. Simulations and experiments are used in this work to investigate the feasibility of growing these materials by THM without the use of large seed crystals. A new fixed-grid, multiphase finite element model was developed based on the level set method and used to calculate the mass transport regime and interface shapes inside the growth ampoule. The diffusivity of CdTe in liquid tellurium was measured through dissolution experiments, which also served to validate the model. Simulations of tapered THM growth find conditions similar to untapered growth with interface shapes that are sensitive to strong thermosolutal convection. Favourable growth conditions are achievable only if convection can be controlled. In preliminary experiments, tapered GaSb crystals were successfully grown by THM and large CdTe grains were produced by gradient freezing. Beginning with this seed material, 25 mm diameter CdTe and CdZnTe crystals were grown on 10 mm diameter seeds, and 65 mm diameter CdTe on 25 mm seeds. Unseeded THM growth was also investigated, as well as ampoule rotation and a range of thermal conditions and ampoule surface coatings. Outward growth beyond one or two centimeters was achieved only at small diameters and included secondary grains and twin defects; however, limited outward growth of larger seeds and agreement between experimental and numerical results suggest that tapered growth may be achievable in the future. This would require active temperature control at the base of the crystal and reduction of convection through thermal design or by rotation of the ampoule or applied magnetic fields. / Graduate / 0346 / 0794 / 0548 / jordan.roszmann@gmail.com