Impact of Depth-Wise Inhomogeneous Iron Distributions on the Accuracy of Lifetime-Based Interstitial Iron Measurements on Silicon Wafers

This article studies the impact of depth-wise inhomogeneous iron distributions in silicon wafers, and the magnitude of the average iron concentrations, on the accuracy of interstitial iron concentrations extracted from effective lifetime measurements on crystalline silicon wafers. The depth-wise inhomogeneous interstitial iron profiles are generated from simulations of gettering of iron to the wafer surfaces, and are therefore practically relevant for silicon solar cells. Our analysis shows that a considerable amount of error can be introduced into the interstitial iron concentration measurement if the iron profiles are highly inhomogeneous, such as in the early stages of gettering. The absolute interstitial iron concentration also has a determinant role on the magnitude of the error. A 'threshold' interstitial iron concentration of 1013 cm-3 or below (<1013 cm-3) is recommended for defect studies to avoid significant measurement errors. Other effects such as surface passivation, light source spectrum and wafer thickness are found to also affect the accuracy of the interstitial iron concentration measurement by affecting the uniformity of the carrier profiles.