Numerical investigation of an experimental Kyropoulos process to grow silicon ingots for photovoltaic application

Kyropoulos crystal growth seeding process for silicon was studied with 3D modeling of the full furnace. The process had three heating zones (top, bottom and side) to improve the control of the horizontal and vertical thermal gradients in the melt; and, a square crucible to reshape the final crystal into a square horizontal section. Modeling of the starting configuration, where crystal growth from the seed was not possible experimentally, explained that the ascendant melt flow in the center is bringing hot melt to the seed. The addition of a ceramic tube as a radiation shield around the seeding zone and the increase of side heating were modeled numerically as solutions to reverse the melt flow direction. The applied solutions on the experimental furnace allowed the growth from the seed. The process was sensitive to symmetry loss. The asymmetric growth of the crystal towards one preferential direction was explained by a coupling between the sudden increase of emissivity due to solidification of silicon and the local decrease of convection on the side of the preferred growth direction. A symmetric growth was obtained in the model by lowering radiative cooling at the top of the melt surface during the first stage of the growth, and by homogenizing the melt using crystal rotation. As a result, symmetrical circular crystals were obtained experimentally using the same growth conditions without contact with the crucible and with no upward pulling.