Scanning X-ray fluorescence microspectroscopy of metallic impurities in solar-grade silicon

A rapid scanning synchrotron-based X-ray fluorescence microprobe technique is applied to relatively impure crystalline silicon feedstock for solar cells. The results reveal the distributions of metallic impurities in the material over regions several millimetres in size, allowing scans across several grains. Relatively high concentrations of Fe, Cu and Zn were observed, with traces of Mn and Ni. The metals were mostly present as discrete particles up to 60mm in size, while Cu was more uniformly distributed. More than 50% of the detected Fe was present as large particles at the grain boundaries, probably due to diffusion and precipitation during cooling. In contrast, less than 5% of the Cu resided in such large particles. The particles contained multiple metallic elements, with strongly varying proportions of their metal constituents.