A high density of planar defects is observed by scanning and transmission electron microscopy in wafers cut from 4H-SiC crystals containing nitrogen above about 2 × 1019 cm−3 and heat treated at 1100 °C. All of the planar defects observed by high-resolution transmission electron microscopy have the same structure comprising six Si-C bilayers in cubic stacking sequence. Such a lamella can originate from two stacking faults in neighbouring basal planes and is therefore called double stacking fault. The recently proposed quantum well model of the electronic structure of the double stacking faults is used to explain the characteristic luminescence band at about 500 nm and the strong anisotropy of the electrical resistivity in the heavily doped, heat treated wafers.