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Density functional theory study on effect of NO annealing for SiC(0001) surface with atomic-scale steps

Density functional theory calculations for the electronic structures of the 4H-SiC(0001)/SiO$_2$ interface with atomic-scale steps are carried out to investigate the effect of NO annealing. The characteristic behavior of the conduction band edge states of SiC is strongly affected over a wide area of the interface by the Coulomb interaction of the O atoms in the SiO$_2$ region as well as the step structure of the interface, resulting in the discontinuity of the inversion layers at the step edges under the gate bias. The spatially discontinued band only allows the very limited conduction paths in the inversion layer, leading to the significantly decreased mobile carrier density. It is found that the Coulomb interaction of the O atoms is screened and the inversion layers become continuous when the nitrided layers are inserted at the interface by NO annealing. This result is in good agreement with experimental findings that the improvement of the performance of SiC metal-oxide-semiconductor field-effect-transistors by NO annealing is attributed to an increase in the mobile electron density rather than an increase in the mobility of electrons in the inversion layer.