Investigations of highly doped 4H-SiC single crystals grown by physical vapour transport (PVT) show that already some percent of N-2 in the Ar atmosphere result in one order of magnitude higher values of nitrogen chemically incorporated in the crystal in contrast to the usual background level. The crystals seem to be saturated with nitrogen at a concentration of about 4x10(19) cm(-3) for our growth conditions. The chemical nitrogen concentration generally exceeds the net donor concentration. It is supposed that there exists an increased probability that nitrogen atoms occupy other than carbon sites as well as to form complexes. In the photoluminescence (PL) spectra measured at room temperature the dominating bandgap emission (394 nm) decreases whereas the intensiy of a PL band at about 500 nm increases in crystals containing N above about 2x10(19) cm(-3). For these samples, scanning electron microscopy combined with cathodoluminescence indicate a drastically enhanced stacking fault formation. This behaviour correlates with the increased wafer warp.