Butterfly-shaped distribution of SiNx precipitates in multi-crystalline Si for solar cells

• Butterfly-shaped distribution region of precipitates was observed in cast silicon wafer.
• The precipitates show the same SiNx chemical composition but different morphologies.
• Periodic growth interruption of precipitates indicates the fluctuation of convection.
• We propose a non-uniform convection flow model that contains two vortexes.
• Model explains the mechanism of precipitate growth and butterfly formation.

This paper concerns the precipitation mechanism of SiNx impurities in multi-crystalline silicon grown with a directional solidification system. A butterfly-shaped precipitate region was found across the middle part of ingot. These precipitates showed filament-like and fiber-like shapes of several micrometers in diameter and up to several millimeters in length. These two types of precipitates exist in different areas, forming three distribution zones in the butterfly region. The precipitate growth periodically stopped at the last stage of the butterfly formation. A non-uniform convection flow model was proposed to explain the precipitation behavior at the butterfly.