Ferroelectric tunnel junctions are key devices studied in FIXIT for emulating synapses. Interfaces play a major role in their electrical performance. Scientists from CEMES-CNRS and Helmholtz-Zentrum Berlin provide the first direct and quantitative determination of how electric fields and charges evolve inside hafnia-based ferroelectric devices under operation.

CEMES-CNRS and Helmholtz-Zentrum Berlin have collaborated for the operando study of ferroelectric devices by electron holography. The results have been published in Nature Communications (Mapping electric fields and observation of ferroelectric domain switching in hafnia zirconia devices by electron holography).

https://www.nature.com/articles/s41467-025-66807-4

Using operando electron holography, internal electric fields and interface charge densities have been mapped in a ferroelectric Hf₀.₅Zr₀.₅O₂/Al₂O₃ bilayer tunnel junction while it is electrically biased inside a transmission electron microscope. This unique capability allows to visualize polarization switching over hundreds of nanometers of field of view. It is shown that the switching proceeds not only by domain nucleation but by both nucleation and lateral domain expansion.

Beyond imaging, the measurements allow to quantify charge screening with unprecedented accuracy. The authors show that screening charges are concentrated at ferroelectric/dielectric interfaces and that even an ultrathin unintended interfacial layer can strongly reshape the internal field landscape.

Achieving this level of insight required major methodological advances to overcome long-standing experimental limitations of in situ holography. With sub-nanometer spatial resolution and a potential sensitivity of 15 mV, this work establishes operando electron holography as a powerful tool for engineering interfaces and optimizing the performance of next-generation ferroelectric devices.