Optical Second Harmonic Generation on Ferroelectric Polarization Reversal for Ferroelectric Hf_0.5Zr_0.5O₂ Capacitors

Optical second harmonic generation (SHG) is observed in materials with broken spatial inversion symmetry; thus, it can be used for the characterization of ferroelectrics with broken spatial inversion symmetry. In this study, we performed SHG measurements on the polarization reversal of ferroelectric capacitors consisting of a Hf0.5Zr0.5O2 (HZO) layer and indium tin oxide (ITO) electrodes. The SHG signals for the ITO/HZO/ITO capacitors were measured by applying a sequence of unipolar triangular voltage pulses, and SHG signal-voltage hysteresis loops were obtained. We qualitatively analyzed the results using models assuming the coexistence of switchable ferroelectric polarization in the HZO layer and nonswitchable apparent polarizations (fixed polarizations) at the surface, interfaces, and in the HZO layer of the capacitors. The analyses revealed that the SHG signals from the apparent polarizations of the capacitors were larger than those from ferroelectric polarization in the HZO layer, and the observed hysteretic SHG signal-voltage loops were attributable to polarization reversal (switching) in the HZO layer. These findings demonstrate that SHG measurements can reveal the existence of apparent polarizations in ferroelectric capacitors, as well as evaluate the impact of apparent polarizations on SHG signals compared to that of ferroelectric polarizations. This is an advantage of the SHG measurement over other electrical methods, which only address switchable ferroelectric polarization. Thus, SHG measurements can be used to detect polarizations at the surface and interfaces; the results of the study provide useful insights for improving the device performance of HfO2-based ferroelectrics.