Directed crystallization of multicrystalline silicon under weak melt convection and gas exchange

Abstract

For the first time, the multicrystalline silicon is grown via directed crystallization using a heater submerged into the melt. The interaction of the heater casing material with molten silicon is studied on a model heater in the form of a graphite plate coated with a special structure SiC protective layer. During the crystallization, the plate has been kept on the melt surface and has almost completely overlaid the melt mirror, which favored a significant decrease in the rate of gas exchange between the melt and the atmosphere in the furnace. Marangoni convection was not observed in the absence of the free surface of the melt and the crystal was grown on the reduced melt convection, especially at the final stages of crystallization, when the thickness of the melt layer was inferior to the cross size of the crucible. The crystal is established to have a strongly pronounced columnar structure. The measured specific resistance varies over the ingot height from 1 to 1.3 Ω cm, and the lifetime of minority carriers is about 3.7 μs. The carbon and oxygen content in the ingot has been measured via FTIR spectroscopy, and the carbon concentration through the ingot height is shown to strongly differ from the linear dependence typical of directed crystallization.