Modelling iron distribution in the crucible-coating-silicon ingot system and the impact of diffusion barrier layers

It is known that iron is one of the main metallic impurities with a significantly detrimental effect on the silicon ingot' s electrical properties. The main sources of the Fe contamination in the silicon ingot are the quartz crucibles and silicon nitride powder used for the coating. Therefore, in this study we aim at investigating the distribution of Fe in the crucible-coating-ingot system. A model has been developed, which considers both the solid-state diffusion and segregation mechanisms. The predicted results have been compared with the measurements on an ingot made in a pilot directional solidification furnace. The concentration of Fe is estimated with the numeral model and compared with the chemical analyses of the experimental materials by glow discharge mass spectrometry (GDMS). The analysis indicates the importance of mixing conditions in the melt (effective partition coefficient) and in-diffusion from the crucible and coating. The model has also been used to assess the impact of introducing a diffusion barrier layer made of a high purity quartz. The computations show that thicker layers can serve as a significant diffusion barrier and form the basis for further crystallisation trials.