Fluid imprint and inertial switching in ferroelectric La:HfO2 capacitors.

Ferroelectric HfO2-based thin films, which are considered as one of the most promising material systems for memory device applications, exhibit an adverse tendency for strong imprint. Manifestation of imprint is a shift of the polarization-voltage (P-V) loops along the voltage axis due to the development of an internal electric bias, which can lead to the failure of the writing and retention functions. Here, to gain insight into the mechanism of the imprint effect in La-doped HfO2 (La:HfO2) capacitors, we combine the pulse switching technique with high-resolution domain imaging by means of Piezoresponse Force Microscopy (PFM). This approach allows us to establish a correlation between the macroscopic switching characteristics and domain time-voltage-dependent behavior. It has been shown that the La:HfO2 capacitors exhibit a much more pronounced imprint compared to Pb(Zr,Ti)O3-based ferroelectric capacitors. Also, in addition to conventional imprint, which evolves with time in the poled capacitors, an easily changeable imprint, termed as "fluid imprint", with a strong dependence on the switching pre-history and measurement conditions has been observed. Visualization of the domain structure reveals a specific signature of fluid imprint - continuous switching of polarization in the same direction as the previously applied field that continues a long time after the field was turned off. This effect, termed as "inertial switching", is attributed to charge injection and subsequent trapping at defect sites at the film-electrode interface.