Characterization of fast interface states in nitrogen- and phosphorus-treated 4H-SiC MOS capacitors

We investigate 'fast interface states' at semiconductor-dielectric interfaces in SiC MOS capacitors that underwent three different interface passivation processes: two nitrogen-based annealing techniques (NO and nitrogen-plasma) and phosphosilicate glass (PSG). 'Fast interface' states in this case refer to interface states with response times <1 μs typically used in standard admittance based MOS characterization methods. In order to appropriately characterize the density of interface states (D_it) taking into account these fast states, conductance and high-low frequency C-V methods were used from room temperature down to 100 K. Measuring at lower temperature shifts the response of the fast interface states into the accessible measurement frequency range. The key finding of this work is that while fast interface states were detected in the nitrided samples, such states were not observed in PSG-passivated samples. On the other hand, conventional interface states with time constants similar to those found in silicon samples were detected at room temperature in the PSG samples. The capture cross-section of fast interface states is larger than that of conventional interface states and demonstrates a different energy dependence. These results strongly indicate that the significantly lower density of fast states at the PSG-SiC interface is one of the main reasons for higher channel mobility in PSG MOSFETs.