Walk-out phenomena in 6H-SiC mesa diodes with SiO2/Si3N4 passivation and charge trapping in dry and wet oxides on N-type 6H-SiC

The drift or “walk-out” of the breakdown voltage in 6H-SiC mesa diodes passivated by a double layer of 1000 Å SiO₂ and 3000 Å Si₃N₄ was studied and related to the charge trapping in the oxide. The first-order trapping kinetics using four distinct electron traps with trapping cross-sections in the range 10⁻¹⁶ to 10⁻¹⁹ cm² were found to best describe the breakdown voltage drift curves. The wet oxide trapping cross-sections are 2 to 10 times larger compared to the dry oxide ones, resulting in about one order of magnitude faster charging of the traps. No significant differences in the amount of drift and saturation level of breakdown voltage were found between the different passivations. The influence of UV illumination, supplied by a HeCd laser with wavelength 325 nm, on the walk-out characteristics and on the reverse current was also investigated. The build-up of the surface states was observed in wet oxide under UV illumination and DC stress. The results are consistent with the coexistence of large concentrations of positive charge and acceptor type deep interface electron traps. The walk-out is a result of the acceptor states being filled by hot electrons supplied by the mechanism of avalanche injection. The suitability of the walk-out measurements as a tool for characterisation of the charge trapping properties of the passivation is demonstrated.