The energy distribution of (1 0 0)Si/HfO2 interface states and their passivation by hydrogen are studied for different levels of nitrogen incorporation using different technological methods. The results are compared to those of N-free samples. The nitrogen in the (1 0 0)Si/HfO2 entity is found to increase the trap density in the upper part of the Si band gap and to hinder the passivation of traps in molecular hydrogen in this energy range. At the same time, the passivation of fast interface traps in the lower part of the band gap proceeds efficiently, provided the thickness of the grown Si3N4 interlayer is kept minimal. However, the lowest achievable interface trap density below midgap is set by the presence of slow N-related states, likely related to traps in the insulator. (c) 2004 Elsevier Ltd. All rights reserved.