Asymmetric Polarization in FE/AFE Bilayer HZO Memcapacitors via Composition-Induced Built-in Bias

A ferroelectric/antiferroelectric (FE/AFE) bilayer Hf_xZr_1-xO₂(HZO) memcapacitor was fabricated to investigate asymmetric polarization and capacitance modulation governed by a built-in bias arising from the compositional gradient. This built-in bias facilitates polarization switching under negative bias while significantly suppressing forward switching under positive bias. Furthermore, it stabilizes the negatively poled state, leading to delayed relaxation and forming a wide capacitance memory window. Comparative AC and pulse analyses revealed the coexistence of a fast dipole component and a slow trapped-charge component. The AC read exhibited nonvolatile characteristics, while the pulse read showed volatile behavior with time-dependent charge decay. These differences explain the distinct retention and nondestructive read-out (NDRO) behaviors depending on the measurement scheme. The device demonstrated symmetric, highly linear long-term potentiation and depression, confirming its potential as a synaptic element for next-generation neuromorphic computing.