Ultrathin films of hafnium oxide (HfO2) and hafnium silicate (HfO2)(x)(SiO2)(1-x) gate stacks (similar to 3 nm) have been subjected to localized electrical stress with a conductive atomic force microscope (C-AFM) in ultrahigh vacuum. The nanoscale current-voltage (I-V) characteristics, prebreakdown temperature dependent I-V measurements on large area metal-insulator-semiconductor capacitors, postbreakdown (BD) topography, current maps, and AFM tip-surface contact force are used to interpret the progressive degradation of the oxide under electrical stress. For the pre-BD phase, trap-assisted tunneling and Fowler-Nordheim tunneling were found to be dominant current transport mechanisms in Hf-based gate stacks contributing to oxide leakage current. For the post-BD phase, an overall effect of barrier limited tunneling current on the charge propagation is confirmed and related to post-BD conductivity features observed by constant voltage scanning. A critical trap density required to trigger a BD event of the ultrathin (HfO2)(x)(SiO2)(1-x)/SiO2 gate stacks is postulated. (C) 2009 American Vacuum Society. [DOI: 10.1116/1.3025822]