Interfaces in heterostructures play a major role in the functionality of electronic devices. Phenomena such as charge trapping/detrapping at interfaces under electric field affect the dynamics of metal/oxide/metal capacitors and metal/oxide/semiconductor transistors used for memory and logic applications. Charge traps are also key for the stabilization of a ferroelectric polarization and its ability to switch in ferroelectric devices such as ferroelectric tunnel junctions (FTJs). However, electric-field induced charging phenomena remain unclear even in conventional dielectric heterostructures due to a lack of direct measurement methods. Here, it is shown how operando off-axis electron holography can be used to quantify the charges trapped at the dielectric/dielectric interfaces as well as metal/dielectric interfaces in HfO2- and Al2O3-based nanocapacitors. By mapping the electrostatic potential at sub-nanometer spatial resolution while applying a bias, it is demonstrated that these interfaces present a high density of trapped charges, which strongly influence the electric field distribution within the device. The unprecedented sensitivity of the electron holography experiments coupled with numerical simulations highlights for the first time the linear relationship between the trapped charges at each interface and the applied bias, and the effect of the trapped charges on the local electrical behavior.
