In this work, tunneling atomic force microscopy (TUNA) is used to describe the charge trapping in high-k ZrO2 dielectric stacks at nanoscale dimensions by analyzing the alteration of the I-V curves and the I-V hysteresis phenomena with repeated measurements (up to 100 curves) at a single spot of only several nm2 in area. TUNA is also suggested as a powerful technique to correlate the electrical characteristics to the physical properties of the stacks. In particular, the influence of the thin SiO2 interfacial layer thickness and its modification with annealing conditions on the electrical properties is demonstrated. Furthermore, the appearance of an I-V hysteresis and its relation to degradation mechanisms in high-k dielectric stacks are explained. Trapping at pre-existing traps is evidenced.