Modeling the effect of mechanical stress on bipolar degradation in 4H-SiC power devices

Bipolar degradation, which is caused by the expansion of stacking faults (SFs) during operation, has been a serious issue in 4H-SiC power devices. To evaluate the threshold minority carrier density of SF expansion, ρ th, Maeda et al. proposed a theoretical model based on quantum well action and dislocation theory. This model includes SF energy variations, electronic energy lowering due to carrier trapping, and resolved shear stress applied to partial dislocations, τ rss. Though the SF energy and the electric energy lowering were quantitatively established, the effect of τ rss has not been discussed well yet. In this study, we first conducted theoretical predictions of the effect of τ rss on ρ th. Then, based on our previous experiment on the dependence of threshold current density on mechanical external stress, we investigated the dependence of ρ th on τ rss. We conducted submodeling finite element analysis to obtain τ rss induced by both residual stress due to the fabrication process and experimentally applied external stress. Finally, we obtained ρ th at the origin of SF expansion from the experimentally measured threshold current density using device simulation. It was found that the dependence of ρ th on τ rss was almost linear. Its gradient was −0.04 ± 0.01 × 1016 cm−3/MPa, which well agrees with the theoretical prediction of −0.03 ± 0.02 × 1016 cm−3/MPa. Our study makes possible a comprehensive evaluation of the critical condition of bipolar degradation.