The influence of nitrogen incorporation on the energy distribution of interface states in the (100) SiHf O2 system and their passivation by hydrogen has been studied as compared to N-free samples. The nitrogen in the (100) SiHf O2 entity is found to increase the trap density, most significantly, in the upper part of the Si band gap, in which energy range N prevents passivation of interface traps by hydrogen. At the same time, passivation of fast interface traps in the lower part of the band gap proceeds efficiently, provided the thickness of the nitrogen-containing interlayer is kept within a few monolayers. The minimal interface trap density below the midgap achieved after passivation in H2 is determined by the presence of slow N-related states, likely located in the insulator. As inferred from capacitance-voltage and ac conductance analysis, the lowest density of electrically active defects [(8-9) × 1010 eV-1 cm-2 at 0.4-0.5 eV from the top of the Si valence band edge] is achieved both in the N-free and N-containing (100) SiHf O2 structures after post-deposition anneal at 800 °C in N2 +5% O2 followed by passivation in molecular hydrogen at 400 °C for 30 min.