Accuracy of Interstitial Iron Measurements on P-Type Multicrystalline Silicon Blocks by Quasi-Steady-State Photoconductance

A detailed knowledge of the distributions of carrier lifetimes, impurities, and crystal defects in silicon ingots is key for understanding and improving wafer quality, as well as solar cell processing steps. In this work, we have validated the use of the quasi-steady-state photoconductance method on p-type multicrystalline silicon blocks to determine the interstitial iron concentration. The extracted iron concentrations along a silicon block were compared with the interstitial iron concentrations measured on wafers from different heights of an adjacent block. The lifetime measurements were performed on the block before and after flashing to break the iron-boron pairs. The impact of nonuniform carrier profiles during the block measurements on the extraction of the Fe profiles is discussed and quantified based on simulations of the quasi-steady-state measurement conditions. The simulation results reveal a slight error in the extracted interstitial iron concentration along the block. However, this error is generally less than 20% for iron concentrations below 10¹¹ cm^-3, which is typical in the central region of an ingot, and in any case, can be corrected for based on the modeling results.