Defects present at the Si(100)/HfO2 interface are analyzed using a combination of electron spin resonance (ESR) and frequency-dependent impedance analysis. The 3.4 nm HfO2 layers were formed by injection metalorganic chemical vapor deposition on boron-doped silicon (100) substrates. ESR spectra indicate the presence of P-b0 defects ((5.0 +/- 0.4) x 10(12) cm(-2)), while analysis of the low-frequency (20 Hz) capacitance-voltage (CV) response indicates a defect density of (5.8 +/- 1.1) x 10(12) cm(-2), between 0.1 and 0.56 eV above the valence bandedge (E-v), with the peak density located at E-v + 0.28 eV. Analysis of conductance data reveals an interface state density of (8.4 +/- 2.1) x 10(12) cm(-2), with a peak density observed at E-v + 0.27 eV. These results provide a link between the density of P-b centers measured by ESR, and the electrical active defects measured from CV and conductance analysis. This provides an explanation for the nonideal, frequency-dependent features in the region between accumulation and strong inversion for high-kappa MOS structures. (C) 2004 The Electrochemical Society.