We combine conventional metal oxide semiconductor (MOS) gated diode measurements and very sensitive electrically detected electron spin resonance (ESR) measurements to detect and identify negative bias temperature instability (NBTI) generated defect centers in fully processed HfO2 pMOS field effect transistors (pMOSFETs). The spectra were found to be quite different from those generated by NBTI in conventional Si/SiO2 based devices. The defect spectra generated by long term stressing differ from the short term stressing signals and are somewhat similar to those observed in plasma nitrided oxide Si/SiO2 based devices. These traces are similar in that their ESR g values are virtually identical. Our results strongly suggest that, in these HfO2 based devices, NBTI defects are located in the interfacial SiO2 layer.