Heavily nitrogen-doped n(+) 4H-SiC single crystals were grown by the physical vapor transport (PVT) method. The nitrogen incorporation kinetics in a heavily doped regime was studied in terms of growth temperature dependence, and it was revealed that the growth temperature substantially influenced the amount of nitrogen incorporated into the crystals and their surface step structures on the (0 0 0 (1) over bar )C facet plane. The structural quality of heavily nitrogen-doped 4H-SiC crystals was examined by X-ray rocking curve measurements and defect selective etching by molten KOH at around 500 degrees C. The crystals contained an extremely low density of 3C-SiC inclusions and stacking faults and showed a comparable crystalline quality to conventionally doped 4H-SiC substrates. Furthermore the structural stability of the heavily nitrogen-doped 4H-SiC substrates during high-temperature treatments has been investigated. The substrates with a large {0 0 0 1} surface roughness showed a resistivity increase after annealing at 1100 degrees C for 2 h, which was confirmed to be caused by the formation and expansion of double Shockley-type basal plane stacking faults in the substrates. The occurrence of the stacking faults largely depended on the surface preparation conditions of substrates, which indicate that the primary nucleation sites of stacking faults exist in the near-surface regions of substrates. (C) 2009 Elsevier B.V. All rights reserved.