The role of silicon nitride crucible materials on silicon ingots contamination
MetadataShow full item record
This work focuses on the characterisation of the solid diffusion mechanism of iron and titanium impurities in a silicon nitride crucible, as a substitute for silica crucibles currently used for solar cells production. This is done by heat-treating diffusion couples, with samples of a slip-cast Si3N4 crucible, behaving as the impurity source, and Czochralski silicon. Glow discharge mass spectrometry (GD-MS) is employed to obtain concentration vs depth profiles of the impurities. By this means, the intent is to figure out indirectly how the impurities diffuse inside the crucible material. The requisites for future similar diffusion couple studies are underlined, followed by a better understanding of the titanium diffusion in this Si3N4 crucible. A finite difference method, simulating the effect of the treatment conditions in the diffusion couple, solves Fick s 2nd law until the resulting amount of titanium found in the silicon sample matches the contamination estimated through the sputtering technique. Challenges regarding crucible cracking and eventually unaccounted contamination call for a reproducibility analysis, preferably comprising annealing periods longer than 1 hour at 1200 ºC or higher. Relying on deductions from the manufacturer s specifications for the crucible s composition, three scenarios are proposed to achieve diffusivity estimates for titanium in the crucible material. A first low limit scenario, considering the average titanium concentration in silicon, based on the GD-MS measurements and assuming non-detected regions as zeros, suggests diffusion coefficients among, at least, 10-15 and 10-13 m2/s, in the range of 1200 to 1350 ºC. A second scenario uses the detected value nearest to the interface to describe the missing data near the surface, being conservative regarding the increasing profile suggested by Fick s 2nd law. Finally, the third resources to curve fitting of the GD-MS profiles with the particular solution of this law for an inexhaustible source. Both lead to diffusivities of 10-14 and 10-12 m2/s, for the same temperature interval.