Ferroelectric Switching in x-cut MgO-doped Thin-Film Lithium Niobate: Implications for Periodic Poling and Domain-Wall Electronics
Thin-film lithium niobate (TFLN) revives a mature ferroelectric as a wafer-scale platform for integrated photonics and, increasingly, for domain-wall electronics such as nonvolatile memories and memristors. Both applications rely on controlled ferroelectric switching, which is poorly understood in TFLN: periodic poling consistently requires electric fields above 30~kV/mm, far above the coercive fields of about 21~kV/mm and 4.5~kV/mm of bulk congruent and 5~mol\% MgO-doped lithium niobate, respectively. Here, we present systematic ferroelectric hysteresis measurements on x-cut, 5~mol\% MgO-doped TFLN with coplanar electrodes as used for periodic poling, combining the positive-up-negative-down (PUND) method with second-harmonic microscopy (SHM). We obtain a spontaneous polarization of $(0.72\pm0.02)$~C/m$^2$, consistent with bulk values, while the coercive field varies between 24 and 40~kV/mm with the voltage ramp rate and depends on the electrode geometry. We identify three contributions to the high coercive fields: (i) fast voltage ramps, (ii) finger electrodes, and (iii) the coplanar surface-electrode geometry. Together with bulk data, the coercive field follows a single power law of the ramp rate over five orders of magnitude, showing that the high fields are dominated by switching dynamics rather than by intrinsic thin-film properties. Moreover, the apparent internal field depends on the electrode geometry, and reverse switching leaves buried, electrically isolated domains. These results provide guidelines for scalable, fab-compatible periodic poling---one of the principal bottlenecks of TFLN device integration---and for the write and erase operation of TFLN domain-wall devices.