Semi-insulating (SI) 4H-SiC substrates doped with vanadium (V) in the range 5.5×1015
–1.1×1017 cm–3 were studied by electron paramagnetic resonance. We show that only in heavily
V-doped 4H-SiC vanadium is responsible for the SI behavior, whereas in moderate V-doped
substrates with the V concentration comparable or slightly higher than that of the shallow N donor or
B acceptor, the SI properties are thermally unstable and determined by intrinsic defects. The results
show that the commonly observed thermal activation energy Ea~1.1 eV in V-doped 4H-SiC, which
was previously assigned to the single acceptor V4+/3+ level, may be related to deep levels of the carbon
vacancy. Carrier compensation processes involving deep levels of V and intrinsic defects are
discussed and possible thermal activation energies are suggested.
