Ferroelectric devices such as capacitors, tunnel junctions and field-effect transistors rely on the reversible switching of polarisation under an electric field, which strongly depends on the screening charges at the interfaces. Despite the crucial role of charge trapping and detrapping on the performance of ferroelectric devices, current understanding relies heavily on electrical measurements of the whole device and/or local analysis of the atomic polarisation and structure. Here, we show how the internal electric fields can be measured within a ferroelectric Hf0.5Zr0.5O2 /Al2O3 tunnel junction using in situ electrical biasing electron holography. The charge densities at internal interfaces are quantitatively determined. Moreover, the polarisation switching of the ferroelectric film is mapped as the voltage gradually increases to the coercive voltage, revealing that switching occurs via both the nucleation and lateral growth of domains. This approach, complementary to existing techniques, opens new avenues for engineering the interfaces in ferroelectric devices.
