Effect of thermal annealing on the defects and electrical properties of semi-insulating 6H-SiC

•The property of SiC doesn’t obviously change after annealing at 950 °C and 1350 °C.•The resistivity drops by 5–6 orders of magnitude after annealing at 1650 °C.•The silicon vacancy defects disappear after annealing at 1650 °C.•Both vanadium dopants and intrinsic defects promote semi-insulating property.