Spontaneous formation of stacking faults in highly doped 4H–SiC during annealing

Abstract

4H–SiC samples doped with nitrogen at ∼3×1019 cm−3 were annealed in Ar for 90 min at 1150 °C. Transmission electron microscopy revealed stacking faults at a density of approximately 80 μm−1 where faults were not found to exist prior to annealing. All faults examined were double layer Shockley faults formed by shear on two neighboring basal planes. The structural transformation was interpreted as due to quantum well action, a mechanism where electrons in highly n-type 4H–SiC enter stacking fault-induced quantum well states to lower the system energy. The net energy gain was calculated as a function of temperature and nitrogen doping concentration through solution of the charge neutrality equation. Calculations showed that doping levels in excess of ∼3×1019 cm−3 should result in double layer stacking faults forming spontaneously at device processing temperatures, in agreement with our observations. Single layer faults are not expected to be stable in 4H–SiC at concentrations below 1×1020 cm−3, but are expected to form at doping concentrations above ∼2×1019 cm−3 in 6H–SiC. Charge buildup in the stacking fault was shown to produce an electrostatic potential that exceeds 90% of the energy difference between the Fermi level position and lowest energy state in the fault-related quantum well. This potential barrier is one of the factors leading to increase of the forward voltage drop in SiC pin diodes.