Effects of carbon-related oxide defects on the reliability of 4H-SiC MOSFETs

In this work, we use density functional theory-based calculations to study the hole trapping properties of single carbon-related defects in silicon dioxide. We show that such interstitials are stable in the carboxyl configuration, where the interstitial carbon atom remains three-fold coordinated with chemical bonds to two Si atoms and an oxygen atom (Si-[C=O]-Si). Using formation energy calculations, we observed a +2 to neutral charge transition level for carboxyl defect within the 4H-SiC bandgap. This leads us to propose that carboxyl defects are likely to act as switching oxide border hole traps in the oxide and contribute to threshold voltage instabilities in a 4H-SiC MOSFET. Thus, we provide an additional candidate to the traditional oxygen vacancy hole traps in 4H-SiC MOS systems. The atomic structures of the defect in various charge states are presented. The stability-providing mechanism for the carboxyl defect in the doubly positive state is found to be the puckering of the Si atom, as in the case of positively charged oxygen vacancy hole traps.