Double stacking faults (3C lamellae) formed by thermal processing of heavily (similar to3x10(19) cm(-3) n-type) doped 4H-SiC substrates, with or without lightly n-doped epilayers, are characterized by low temperature photoluminescence (PL), Raman scattering, secondary electron imaging (SEI), and electrostatic force microscopy (EFM). Electric fields are evident in the SEI and EFM images where the faults intersect the surface. Self-consistent simulations including spontaneous polarization explain several features observed in PL and Raman spectra.