Quasi-static capacitance measurements and deep level transient spectroscopy measurements were combined to analyze the density of interface states in N2O oxynitrides after constant-current Fowler-Nordheim injection from the silicon. Two defect levels are found, around E-t1 = E-c - 0.33 eV and E-t2 = E-c - 0.2 eV, having capture cross sections of 2 x 10(-17) cm(2) and 3 x 10(-18) cm(2), respectively. The stress creates a very high density of E-t1 in the control sample. Stress-induced E-t1 are associated with P-b centers, and their density decreases with increasing N2O treatment temperature from 850 to 1050 degrees C. E-t2 are detected in the oxynitrided layers before and after the stress and are much less affected by electrical stress than E-t1.