Structural changes during wake-up and polarization switching in a ferroelectric Hf0.5Zr0.5O2 film

Ferroelectric polycrystalline hafnium oxide films hold great promise for the electronics industry, though an understanding of ferroelectricity in this unconventional material is still lacking. Here, by an in situ synchrotron X-ray microdiffraction experiment, we reveal an important role of reversible and irreversible ferroelastic switching in the mechanism of polarization reversal in a 10 nm thick Hf$_0.5$Zr$_0.5$O$_2$ film and in the origin of the so called wake-up effect consisting in a gradual increase in the remanent polarization of as-prepared memory structures. During the wake-up, the oblique polar axis irreversibly rotates in some ferroelectric orthorhombic grains and becomes more vertical on average in the film, which is the mechanism of the increase in measured polarization. This effect originates from the tensile stress emerging in the film during crystallization annealing and its gradual decrease via the rearrangement of the crystal lattice, which is consistent with first-principles calculations. In the woken-up structures, the polar axis also rotates during polarization switching, but reversibly, which means a different crystal structure of Hf$_0.5$Zr$_0.5$O$_2$ in the upward and downward polarization states and breaking the inversion symmetry. The results provide insight into fundamentals of ferroelectric hafnium oxide and points the way for intelligent material engineering in the field of ferroelectrics-based devices.