**5. Hydrogen passivation**

Wet oxidation of 4H-SiC also yields a small reduction of interface states with energies away from the semiconductor band edges when compared to SiO2 formation in dry oxygen [2, 26, 55, 90, 130]. It correlates well with the effects of H2 POA. While at silicon interfaces hydrogen annealing yields a *Dit* from about 10<sup>11</sup> to 1010 cm−2*eV*−<sup>1</sup> in the middle of the gap and a mobility close to half the one of the bulk [20, 21], its impact at SiC interfaces is much less efficient, highlighting the differences between the two semiconductors [27, 58, 96]. Molecular hydrogen can indeed passivate Si- or C- dangling bonds, and insert long Si-Si bonds [28]. But it does not significantly affect split carbon interstitials and slow near interface states which populate the majority of the *Dit* at the 4H-SiC band edges. Like Ar annealing, wet oxidation and/or H2 POA can be used together with other annealing techniques to optimize the trap density throughout the semiconductor band gap.
