Total Ionizing Dose Effects on Strained ${\rm HfO}_{2}$ -Based nMOSFETs

Radiation-induced charge trapping and mobility degradation are measured on uniaxially stressed HfO₂-based nMOSFETs. Controlled external mechanical stress is applied via a four-point bending jig while the samples are irradiated using 10-keV X-rays. Positive charge trapping is observed for unstressed devices, and for devices irradiated under both compressive and tensile stress. Reduced trapped charge is measured as the uniaxial stress level increases. These results suggest that the increased stress leads to a reconfiguration of defect microstructure, as compared to the unstressed devices. The reconfiguration consists of changes in bond lengths and angles and a corresponding change in trap energy levels, which can (1) reduce the probability that a defect can capture holes, (2) increase the probability for electron and trapped-hole recombination, and/or (3) increase the mobility of the transporting holes in the oxides of the devices that are irradiated under stress. These results show that increased mechanical stress in high-k dielectrics does not degrade their radiation hardness. We also observe that the post-irradiation channel mobility degrades less in uniaxially stressed devices than unstressed devices.