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In this work 7 nm thin Ta2O5/SiOxNy insulating stacks on Si were studied by a combination of C-V If and leakage current (I-V) measurements, based on the comprehensive model involving different conduction mechanisms in Ta2O5 and the interfacial SiOxNy layer. Equivalent oxide thickness (EOT) was determined using the extrapolation method for leaky high-k dielectrics proposed by Kar. Interface state densities were determined from the hf C-V characteristics by a standard method. Degradation after constant current stress (10 mA/cm2, 100 s) was analyzed in terms of oxide charge build-up, interface states density (Dit) generation and consumption of the interfacial layer. No significant variation of Dit with the stress was observed - midgap values (Ditm) for both fresh and stressed samples are close to 5times1012 eV-1cm-2. The observed decrease in interface charge (QA is very small and could be attributed to the interfacial layer consumption. Fixed positive oxide charge increases for a small but significant amount of 4times1011 cm-2. The most important variation with the stress is observed in the effective interfacial layer thickness (dIL); it decreased for 0.12 nm as a result of the stress, leading to a decrease ofthe EOT for 0.10 nm, having that the dielectric constant of the interfacial SiOxNy layer is higher than that of the SiO2 layer. Hence, the interfacial layer consumption is found to be the dominant wearout mode.