Field-Induced Ferroelectric Phase Evolution During Polarization "Wake-Up" in Hf0.5Zr0.5O2 Thin Film Capacitors

As an emerging nonvolatile memory technology, HfO2-based ferroelectrics exhibit excellent compatibility with silicon CMOS process flows; however, the reliability of polarization switching in these materials remains a major challenge. During repeated field programming and erase of the polarization state of initially pristine HfO2-based ferroelectric capacitors, the magnitude of the measured polarization increases, a phenomenon known as "wake-up". In this study, the authors attempt to understand what causes the wake-up effect in Hf0.5Zr0.5O2 (HZO) capacitors using nondestructive methods that probe statistically significant sample volumes. Synchrotron X-ray diffraction reveals a concerted shift in HZO Bragg peak position as a function of polarization switching cycle number in films prepared under conditions such that they exhibit extremely large (approximate to 3000%) wake-up. In contrast, a control sample with insignificant wake-up shows no such peak shift. Capacitance - voltage measurements show evolution in the capacitance loop with switching cycle number for the wake-up sample and no change for the control sample. Piezoresponse force microscopy measurements are utilized to visualize the domain switching with wake-up. The combination of these observations clearly demonstrates that wake-up is caused by a field-driven phase transformation of the tetragonal phase to the metastable ferroelectric orthorhombic phase during polarization switching of HZO capacitors.