The growth of n-type 4H-SiC crystals has been performed by physical vapor transport (PVT) growth method, with nitrogen and boron (N-B) co-doping. It was revealed that, in the growth of 4H-SiC with N-B co-doping, the generation of double Shockley-type stacking faults (DSSFs) was suppressed, as in the case of the nitrogen and aluminum co-doping. The resistivity of the N-B co-doped 4H-SiC was as low as 5.5 mOcm. In addition, we investigated the expansion velocities of DSSFs, which were intentionally caused by indention of crystal surfaces with a diamond indenter, while the crystals were annealed for 2 h at 1000 degrees C. The DSSF expansion velocities of the N-B co-doped crystals were found to be lower than the velocities of nitrogen doped crystals. The difference in expansion velocities is discussed in terms of the quantum well action model.