We have investigated the effect of doping on absorption for various SiC polytypes, i.e., n-type (N) 6H-SiC, 4H-SiC, and 15R-SiC, p-type (Al) 6H-SiC, and 4H-SiC, and p-type (B) 6H-SiC. For these polytypes the band-gap narrowing with higher doping concentration is observed. In addition, for n-type doping below band-gap absorption bands at 464 nm for 4H-SiC, at 623 nm for 6H-SiC, and at 422 and 734 nm for 15R-SiC are observed. The peak intensities of these absorption bands show a linear relation to the charge carrier concentration obtained from Hall measurements. The corresponding calibration factors are given. As an application a purely optical wafer mapping of the spatial variation of the charge carrier concentration is demonstrated.
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