ANALYSIS OF THE FORMATION OF SiC AND Si_3N_4 PRECIPITATES DURING DIRECTIONAL SOLIDIFICATION OF MULTICRYSTALLINE SILICON FOR SOLAR CELLS

摘要

Directional solidification by the Vertical Gradient Freeze method (VGF) and related technologies isthe most important process for the industrial production of multi-crystalline silicon for photovoltaic applications. Aproblem in this crystallization process is the formation of SiO_2, SiC and Si_3N_4 precipitates. These defects arereducing the efficiency of solar cells and should therefore be avoided. The origin of the precipitates is assumed to becontamination of the silicon melt from graphite parts in the furnace and from the Si_3N_4 coating of the SiO_2 crucible.However, the mechanisms of the formation of the precipitates are not well understood.The goal of this work is to study the C-, N-, and O-distributions in the silicon ingots and to correlate it to theoccurrence of precipitates as well as to the growth conditions. Therefore, multi-crystalline silicon ingots with adiameter of 6 cm and a length of 4-5 cm were grown in Si3N4 coated silica crucibles in a laboratory-scale VGFfacility. The experimentally obtained distributions of the impurities will be explained by the aid of numericalsimulations.