Enhanced Ferroelectric Properties and Insulator–Metal Transition-Induced Shift of Polarization-Voltage Hysteresis Loop in VO<i>_x</i>-Capped Hf_0.5Zr_0.5O₂ Thin Films

Abstract

A capping layer is known to be critical for stabilizing the ferroelectric (FE) orthorhombic phase (o-phase) in a HfO₂-based thin film. Here, vanadium oxide (VO<i>_x</i>), a functional oxide exhibiting the insulator-metal transition, is used as a novel type of a capping layer for the Hf_0.5Zr_0.5O₂ (HZO) thin film. It is demonstrated that the VO<i>_x</i> capping layer (VCL) can enhance the FE properties of the HZO thin film comprehensively. Specifically, the HZO thin film with a VCL shows large remanent polarization (2<i>P</i>_r ≈36.9 μC/cm²), relatively small coercive field (<i>E</i>_c ≈1.09 MV/cm), high endurance (up to 10⁹ cycles), and long retention (>10⁵ seconds). The enhanced FE properties may be attributed to the VCL-induced stabilization of the FE o-phase and suppression of oxygen vacancies at the interface. Furthermore, the HZO thin film with a VCL exhibits a successive rightward shift of polarization-voltage (<i>P-V</i>) hysteresis loop as the temperature increases. This is well correlated with the insulator-metal transition in a VCL, which can modulate the interfacial built-in field and thus cause the <i>P-V</i> loop shift. It is therefore demonstrated that a VCL not only enhances the FE properties of HZO thin films but also provides a temperature degree of freedom to modulate the FE properties, which may open up a new pathway to develop HfO₂-based FE memories with high performance and novel functionalities.