During the crystallization of multi-crystalline silicon the melt is contaminated by nitrogen, from the crucible coating as well as by carbon already present in the initial feedstock or transported via the gas atmosphere to the melt surface, where it is then dissolved. When the solubility limit is exceeded, SiC and Si3N4 particles are formed. Such particles cause problems during the wire sawing process due to their hardness. They can also influence the grain growth and size negatively, leading to the small grain “grit” structure and the formation of dislocations. Finally, SiC-precipitates can act as shunts reducing the performance of the solar cells. Therefore, the incorporation of such deleterious particles has to be avoided.
The occurrence of such foreign phases in the solid silicon can be correlated to melt convection. However, the interaction of such particles with the moving crystallization front and their incorporation into solid silicon are not fully understood today. Under the assumption that the particle would form in the melt, a critical growth velocity vcr should exist depending on the density, size and morphology of the particles. Below vcr a particle will be pushed in front of the growth interface, whereas above vcr, the particle will be captured and frozen into the growing crystal. This particle engulfment depends on the so-called interface-, drag-, lift-, and gravity-forces acting on the particle in the vicinity of the interface. It has been theoretically shown that only mm-sized particles should be captured for typical growth velocities used in crystallization of multi-crystalline silicon if only drag and interface forces are considered. This result is in full contradiction to the experimental observations that the particles detected in silicon are µm-sized. Preliminary investigations where SiC-particles were added to the melt have shown that gravity and lift forces (due to melt convection) are most likely determining the transport of the particles under terrestrial conditions. Depending on the size and density of the particles and on the convection intensity they can either sink to the bottom of the melt or float on the melt surface. Since gravity affects melt movement, solute distribution, and sedimentation of the particles, microgravity offers the unique opportunity to study particle nucleation, transport, and incorporation under purely diffusive conditions.
Therefore, it is one of the major goals of the ParSiWal-project to understand in detail the interaction of foreign phase particles with the moving solid-liquid interface during directional solidification of silicon under diffusion controlled growth conditions in the ELLI furnace during the TEXUS 51 mission scheduled for April 2013.
In the presentation the results of terrestric experiments and first results of the flight experiment will be shown. During all experiments single crystalline silicon rods with 8mm diameter containing a reservoir of SiC particles with particle size varying from 7µm to 300µm will be zone melted at different growth rates of 0.2-10 mm/min. In order to avoid Marangoni convection the rods are covered by a 5 µm thick oxide skin. Terrestric experiments with and without magnetic stirring before and during crystallization were done. The samples were analyzed by infrared transmission microscopy to determine the distribution of the particles in the silicon after processing, to gain information about the particle incorporation depending on the growth rate. |