Investigation of the wake-up process and time-dependent imprint of Hf0.5Zr0.5O2 film through the direct piezoelectric response

Ferroelectric HfO2-based thin films, which have been attracting a great deal attention because of their potential use in various applications, are known for their unique properties, such as a large time-dependent imprint and wake-up effect, which differentiate them from conventional ferroelectric materials. In this study, direct piezoelectric measurement was employed to investigate the state of polarization during the retention and wake-up process without applying an electric field. The polarization-electric field hysteresis loop of a sputtered Hf0.5Zr0.5O2 (HZO) film with a thickness of 10 nm showed a time-dependent imprint at room temperature during polarization retention, and the internal electric field that generated the imprint gradually increased from 0.05 to 0.6 MV/cm. While a space charge density of more than 1 μC/cm2 is required to form such an internal electric field, it was found that the magnitude of the direct piezoelectric response did not change at all during polarization retention. On the other hand, both the remanent polarization and direct piezoelectric response increased during the wake-up process. Based on the difference in the variation over time of these two characteristics, we concluded that the non-ferroelectric layer exists at the interface between the HZO film and TaN electrode and gradually transitions to ferroelectric phases through the electric field cycle.