Trap Density Imaging of Silicon Wafers using a Lock-In Infrared Camera Technique

We apply a novel an imaging technique for non-recombination active minority-carrier trapping centres in silicon wafers based on lock-in infrared thermography. Measurements on Czochralski silicon wafers show that the trap density is highly inhomogenous and correlates with oxygen-induced striation patterns. A direct comparison of the trap density image with the corresponding recombination lifetime mapping reveals an anticorrelation of the two quantities. The application of the ITM technique to block-cast multicrystalline silicon wafers shows that the distribution of the trapping centres correlates with the dislocation density. Moreover, we find that areas with increased dislocation density often degrade during phosphorus gettering treatment. Finally, we demonstrate that one single spatially resolved measurement of the infrared emission signal of as-delivered multicrystalline silicon without surface passivation layers reveals already poorly-getterable areas, which decrease the solar cell efficiency. Hence, trap density imaging is a useful new instrument for assessing the efficiency potential of as-delivered mc-Si wafers