Implications of grain boundaries on quasi-steady-state photoconductance measurements in multicrystalline and cast-mono silicon

Quasi-steady-state photoconductance is commonly used for measuring the effective lifetime of excess carriers in silicon wafers. A known artifact, unrelated to the sample's lifetime in eddy current based photoconductance measurements, is a strong increase in lifetime at low carrier densities. It is commonly observed in multicrystalline and cast-mono crystalline silicon. This artifact is often attributed to bulk defects changing their charge state and is referred to as trapping. In this work, we investigate an alternative interpretation for materials with crystallographic defects. In this interpretation, the trapping-like behavior is caused by local changes in the Fermi energy level at grain boundaries (compared to the bulk crystal), along with the substantial movement of the majority carriers' quasi Fermi energy level during the photoconductance measurement. Using cast-mono wafers, we observe a variation in trapping signals and dark conductance that correlates with grain boundaries. We show that the trapping-like behavior in these regions can be explained by a change in electrostatic potential barrier heights at different illumination intensities and in the dark, impacting eddy currents. This should be considered when using eddy current based methods for characterization of multicrystalline and cast-mono silicon wafers.

Automated summary

This study provides an alternative interpretation for the trapping-like behavior observed in materials with crystallographic defects. It suggests that the changes in the Fermi energy level at grain boundaries and the movement of majority carriers' quasi Fermi energy level during photoconductance measurement contribute to this behavior. Experimental results using cast-mono silicon wafers show variations in trapping signals and dark conductance that correlate with grain boundaries.